Mineral Potential Modelling of 

Gold and Silver Mineralization 

in the Nevada Great Basin 

• 

A GIS-Based Analysis Using 

Weights of Evidence 

By: 

Mark J. Mihalasky1

Open-File Report: 01-291 

This report is preliminary and has not been reviewed for conformity with U.S. Geological 

Survey editorial standards or with the North American Stratigraphic Code. Any use of 

trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 

1

Formerly at: U.S. Geological Survey, Reno Field Office, Mackay School of Mines, MS-176, 

University of Nevada, Reno, Nevada, 89557. Currently at: Richard Stockton College, Faculty of 

Natural Sciences and Mathematics, PO Box 195, Pomona, New Jersey, 08240. 
ii

ABSTRACT

The distribution of 2,690 gold-silver-bearing occurrences in the Nevada Great Basin was

examined in terms of spatial association with various geological phenomena. Analysis of these

relationships, using GIS and weights of evidence modelling techniques, has predicted areas of

high mineral potential where little or no mining activity exists. Mineral potential maps for

sedimentary (“disseminated”) and volcanic (“epithermal”) rock-hosted gold-silver mineralization

revealed two distinct patterns that highlight two sets of crustal-scale geologic features that likely

control the regional distribution of these deposit types.

The weights of evidence method is a probability-based technique for mapping mineral potential

using the spatial distribution of known mineral occurrences. Mineral potential maps predicting

the distribution of gold-silver-bearing occurrences were generated from structural, geochemical,

geomagnetic, gravimetric, lithologic, and lithotectonic-related deposit-indicator factors. The

maps successfully predicted nearly 70% of the total number of known occurrences, including

~83% of sedimentary and ~60% of volcanic rock-hosted types. Sedimentary and volcanic rockhosted mineral potential maps showed high spatial correlation (an area cross-tabulation

agreement of 85% and 73%, respectively) with expert-delineated mineral permissive tracts. In

blind tests, the sedimentary and volcanic rock-hosted mineral potential maps predicted 10 out

of 12 and 5 out of 5 occurrences, respectively. The key mineral predictor factors, in order of

importance, were determined to be: geology (including lithology, structure, and lithotectonic

terrane), geochemistry (indication of alteration), and geophysics.

Areas of elevated sedimentary rock-hosted mineral potential are generally confined to central,

north-central, and north-eastern Nevada. These areas form a conspicuous “V”-shape pattern that

is coincident with the Battle Mountain-Eureka (Cortez) and Carlin mineral trends and a segment

of the Roberts Mountain thrust front, which bridges the southern ends of the trends. This pattern

appears to delineate two well-defined, sub-parallel, northwest–southeast-trending crustal-scale

structural zones. These features, here termed the “Carlin” and “Cortez” structural zones, are

believed to control the regional-scale distribution of the sedimentary rock-hosted occurrences.

Mineralizing processes were focused along these structural zones and significant ore deposits

exist where they intersect other tectonic zones, favorable host rock-types, and (or) where

appropriate physio-chemical conditions were present. The origin and age of the Carlin and

Cortez structural zones are not well constrained, however, they are considered to be transcurrent

features representing a long-lived, deep-crustal or mantle-rooted zone of weakness.

Areas of elevated volcanic rock-hosted mineral potential are principally distributed along two

broad and diffuse belts that trend (1) northwest-southeast across southwestern Nevada, parallel

to the Sierra Nevada, and (2) northeast-southwest across northern Nevada, extending diagonally

from the Sierra Nevada to southern Idaho. The first belt corresponds to the Walker Lane shear

zone, a wide region of complex strike-slip faulting. The second, here termed the “Humboldt

shear(?) zone”, may represent a structural zone of transcurrent movement. Together, the Walker

Lane and Humboldt shear(?) zones are believed to control the regional-scale distribution of

volcanic rock-hosted occurrences. Volcanic rock-hosted mineralization was closely tied to the

southward and westward migration of Tertiary magmatism across the region (which may have

been mantle plume-driven). Both magmatic and mineralizing processes were localized and

concentrated along these structural zones. The Humboldt shear(?) zone may have also affected
iii

the distribution of sedimentary rock-hosted mineralization along the Battle Mountain–Eureka

(Cortez) and Carlin mineral trends. The Getchell trend and Independence group deposits are

believed to be the northeastward-displaced northern extensions of these mineral trends

(respectively). Displacement was achieved by post-mineralization right-lateral movement along

crustal segments within the Humboldt shear(?) zone. Latest movement along the Humboldt

shear(?) zone is constrained between ~42-30 Ma (sedimentary rock-hosted mineralization) and

~17-14 Ma (most recent igneous activity along the northern Nevada rift zone). However, this

structure likely has origins relating to the mid-Proterozoic assembly of the Laurentian

protocraton and/or late Proterozoic rifting.
 iv

TABLE OF CONTENTS

1. Introduction ...........................................................1

1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Purpose and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4 Methodology ..........................................................2

1.5 Spatial Datasets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.6 Similar Studies ..................... ...................................5

1.7 Layout . . ...... ......................................................6

2. Geology of the Great Basin ...............................................8

2.1 Introduction ...........................................................8

2.2 Physiographic Setting ...................................................8

2.3 Tectonic Setting and Geologic History ......................................9

2.4 Craton Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4.1 Assembly and Margin Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4.2 Western Edge and Crustal Provinces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.5 Cenozoic Extensional Tectonism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.5.1 Early Stage (Mid-Eocene to Late-Lower Miocene) . . . . . . . . . . . . . . . . . . . . . . . 14

2.5.1.1 Magmatic Activity—Distribution, Age, and Composition . . . . . . . . . . . . . 16

2.5.1.2 Extensional Activity—Distribution, Age, Character, Magnitude . . . . . . . . 18

2.5.2 Late Stage (Late-Lower Miocene to Present) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.5.2.1 Magmatic Activity—Distribution, Age, and Composition . . . . . . . . . . . . . 18

2.5.2.2 Extensional Activity—Distribution, Age, Character, Magnitude . . . . . . . . 19

2.6 Strike-Slip and Related Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.7 Characteristics of the Present-Day Crust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.7.1 Lithotectonic Framework and Tectonostratigraphy . . . . . . . . . . . . . . . . . . . . . . . 25

2.7.2 Crustal Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.7.3 Gravity Anomaly and Crustal Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.7.4 Geomagnetic Anomaly and Crustal-Scale Structures . . . . . . . . . . . . . . . . . . . . . 28

2.7.5 Heat Flow and Crustal Fluid Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.7.6 Electrical Conductivity and Crustal Permeability . . . . . . . . . . . . . . . . . . . . . . . . 29

2.7.7 Seismicity and Seismic Velocities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3. Characteristics and Distribution of

Sedimentary and Volcanic Rock-Hosted Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2 Sedimentary Rock-Hosted Gold Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.1 Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.2 Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.3 Volcanic Rock-Hosted Gold-Silver Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3.1 Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
 v

3.3.2 Distribution ..................................................... 35

3.4 Regional-Scale Distribution with Respect to Mineral Trends and Belts . . . . . . . . . . . 37

3.5 Regional-Scale Distribution with Respect to Crustal Terranes . . . . . . . . . . . . . . . . . . 40

3.6 Quantitative Studies on Mineral Trends and Crustal Terranes . . . . . . . . . . . . . . . . . . . 42

4. Analysis and Modelling Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.2 Preliminary Spatial Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3 Weights of Evidence Mineral Potential Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3.2 Theoretical Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.3.3 Conditional Independence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.3.4 Posterior Probability Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.3.5 Practical Implementation of the Modelling Procedures . . . . . . . . . . . . . . . . . . . . 48

5. Spatial Datasets ....................................................... 49

5.1 Introduction .......................................................... 49

5.2 GIS Study Area Parameters ............................................. 49

5.2.1 Projection ....................................................... 49

5.2.2 Extents ......................................................... 50

5.2.3 Resolution ...................................................... 50

5.3 Spatial Datasets—Selection Criteria and Terminology ....................... 50

5.3.1 Data Sources .................................................... 50

5.3.2 Gold-Silver-Bearing Mineral Occurrences (Training Datasets) ............. 53

5.3.3 Mineral Potential Modelling Evidence Maps ........................... 54

5.4 Error, Data Accuracy and Limitations ..................................... 56

5.4.1 Error Sources .................................................... 56

5.4.2 Data Limitations .................................................. 57

6. Single Map Analysis, Interpretation, 

and Mineral Predictor Map Generation ................................... 58

6.1 Introduction .......................................................... 58

6.2 Gold-Silver-Bearing Occurrences ......................................... 59

6.2.1 Introduction and Summary of Findings ................................ 59

6.2.2 Distribution of Gold-Silver-Bearing Occurrences ........................ 59

6.2.3 Correlations and Interpretive Synthesis ................................ 61

6.3 Lithology ............................................................ 63

6.3.1 Introduction and Summary of Findings ................................ 63

6.3.2 Distribution and Spatial Association of Gold-Silver-Bearing Occurrences .... 64

6.3.3 Correlations and Interpretive Synthesis ................................ 66

6.4 Lithologic Diversity and Lithotectonic Terranes ............................. 69

6.4.1 Introduction and Summary of Findings ................................ 69

6.4.2 Distribution and Spatial Association of Gold-Silver-Bearing Occurrences .... 70

6.4.3 Correlations and Interpretive Synthesis ................................ 71
vi

6.5 Distance Buffers Around Plutons and Faults ................................ 74

6.5.1 Introduction and Summary of Findings ................................ 74

6.5.2 Distribution and Spatial Association of Gold-Silver-Bearing Occurrences .... 75

6.5.3 Correlations and Interpretive Synthesis ................................ 77

6.6 Geophysics—Geomagnetic and Gravity Anomalies ........................... 78

6.6.1 Introduction and Summary of Findings ................................ 78

6.6.2 Distribution and Spatial Association of Gold-Silver-Bearing Occurrences .... 79

6.6.3 Correlations and Interpretive Synthesis ................................ 81

6.6.3.1 Sedimentary Rock-Hosted Occurrences:

Geomagnetic Lows—Isostatic Gravity Highs ....................... 81

6.6.3.2 Volcanic Rock-Hosted Occurrences:

Geomagnetic Highs—Isostatic Gravity Lows ....................... 83

6.7 Geochemistry—K/Na and Ba/Na Anomalies ................................ 85

6.7.1 Introduction and Summary of Findings ................................ 85

6.7.2 Distribution and Spatial Association of Gold-Silver-Bearing Occurrences .... 87

6.7.3 Correlations and Interpretive Synthesis ................................ 88

7. Multi-Map Modelling and Gold-Silver Mineral Potential .................... 90

7.1 Introduction .......................................................... 90

7.2 Combination of Mineral Potential Evidence ................................. 91

7.2.1 Introduction ..................................................... 91

7.2.2 Combination Weighting Factors ..................................... 91

7.3 Sedimentary and Volcanic Rock-Hosted Mineral Potential Maps ................ 92

7.3.1 Introduction ..................................................... 92

7.3.2 Favorability for Sedimentary Rock-Hosted Occurrences .................. 92

7.3.3 Favorability for Volcanic Rock-Hosted Occurrences ..................... 93

7.4 Validation of Mineral Potential Models .................................... 93

7.4.1 Introduction ..................................................... 93

7.4.2 Conditional Independence and Uncertainty of the Mineral Potential Maps .... 94

7.4.3 Favorability at Known Gold-Silver-Bearing Occurrence Areas ............. 95

7.4.4 Comparison to Expert-Delineated Mineral Potential Areas ................ 96

7.4.5 Blind Test of Mineral Potential Model Predictability ..................... 97

7.5 Geologic Characterization and Delineation of Regional-Scale

Sedimentary and Volcanic Rock-Hosted Occurrence Exploration Targets ......... 98

7.5.1 Introduction ..................................................... 98

7.5.2 Sedimentary Rock-Hosted Mineral Potential Regions ................... 100

7.5.3 Volcanic Rock-Hosted Mineral Potential Regions ...................... 103

8. Controls on Sedimentary and

Volcanic Rock-Hosted Occurrence Distribution ........................... 107

8.1 Introduction ......................................................... 107

8.2 Mineral Potential Evidence Associated

with the Gold-Silver-Bearing Occurrences ................................. 107

8.2.1 Sedimentary-Rock Hosted Occurrences .............................. 107

8.2.2 Volcanic Rock-Hosted Occurrences ................................. 108
vii

8.3 Delineation of the Regional-Scale Control Structures ........................ 110

8.3.1 Sedimentary Rock-Hosted Mineralization ............................. 110

8.3.2 Volcanic Rock-Hosted Mineralization ............................... 110

8.4 The Humboldt Shear(?) Zone ........................................... 112

8.4.1 Geologic and Other Evidence ...................................... 112

8.4.2 Topographic Expression .......................................... 112

8.4.3 Origin, Age, and Character of the Humboldt Shear(?) Zone ............... 113

8.5 The Carlin and Cortez Structural Zones ................................... 116

8.5.1 Geophysical and Geochemical Evidence .............................. 116

8.5.2 Age and Origin of the Carlin and Cortez Structural Zones ................ 118

8.5.3 Transcurrent Movement ........................................... 120

8.5.4 Truncation of the Battle Mountain–Eureka (Cortez)

and Carlin Mineral Trends by the Humboldt Shear(?) Zone ............... 121

8.6 A Mantle Plume ..................................................... 124

References .............................................................. 126

Appendix A Stratigraphy and Description of Lithologic Units. . . . . . . . . . . . . . . . . . . . . A1

Appendix B Weights of Evidence Mineral Potential Modelling

Theory, Implementation, and FORTRAN Utilities . . . . . . . . . . . . . . . . . . . B1

Appendix C Mineral Potential Map Generation,

Conditional Independence, and Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . C1

Appendix D Mineral Potential at

Known Mineral Occurrence Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D1
viii

LIST OF TABLES

Table 1.1 Types of Models Used in the Geosciences . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

Table 1.2 Types of Mineral Potential Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

Table 1.3 Gold-Silver-Bearing Occurrences Examined in this Study . . . . . . . . . . . . . . 182

Table 3.1 Sedimentary Rock-Hosted Deposit-Type Synoptic Model . . . . . . . . . . . . . . . 183

Table 3.2 Volcanic Rock-Hosted Deposit-Type Synoptic Model . . . . . . . . . . . . . . . . . . 186

Table 5.1 Geoscience Datasets Composing the Navada and Great Basin GIS

Database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

Table 5.2 Total Number of Observations in All Gold-Silver-Bearing

Occurrence-type Samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Table 7.1 Weights of Spatial Association and Related Data for the Primary,

Sedimentary, and Volcanic Rock-Hosted Occurrence-Type Mineral

Potential Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

Table A.1 Correlation Chart of Geological Map

Units with Accompanying Table of Unit Descriptions . . . . . . . . . . . . . . . . . . A2

Table A.2 Key to GIS Geological Map Legend and

Lithologic Unit Abbreviations Appearing on Digital Maps . . . . . . . . . . . . . A15

Table A.3 Key to Lithologic Units Composing Assemblage Map Units . . . . . . . . . . . . A17

Table B.1 Example of an Attribute Table Used for Map Reclassification . . . . . . . . . . B26

Table B.2 Example of a Unique Conditions Map Attribute Output from SPANS

GIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B27

Table B.3 Map Area Analysis Table Output from SPANS GIS . . . . . . . . . . . . . . . . . . B28

Table B.4 Point-in-Polygon Analysis Table Output from SPANS GIS . . . . . . . . . . . . B29

Table B.5 Contingency Table for Testing Conditional

Independence, Based on Cells Containing Only Deposits . . . . . . . . . . . . . . B15

Table B.6 Output from the FORTRAN Utility WTS.EXE

for the Weights of Spatial Association Estimation

Component of Weights of Evidence Mineral Potential Modelling . . . . . . . . B30

Table B.7 Example of Weights of Evidence Bayesian

Map Overlay Modelling Equation Implemented in SPANS GIS . . . . . . . . . B31

Table B.8 Output from the FORTRAN Utility

PREDICT.EXE for the Bayesian Multi-Map Overlay

Component of Weights of Evidence Mineral Potential Modelling . . . . . . . . B32

Table B.9 Output from FORTRAN Utility CI.EXE for the Error Analysis

Component of Weights of Evidence Mineral Potential Modelling . . . . . . . . B33

Table C.1 Pairwise Conditional Independence (CI) test 2

Scores for the Primary Occurrence-type Model . . . . . . . . . . . . . . . . . . . . . . C10

Table C.2 Pairwise Conditional Independence (CI) test 2

Scores for the Sedimentary Rock-Hosted Occurrence-type Model . . . . . . . C11

Table C.3 Pairwise Conditional Independence (CI) test 2

Scores for the Volcanic Rock-Hosted Occurrence-type Model . . . . . . . . . . C12

Table C.4 Summary Information for the Overall Conditional

Independence Test for the Various Mineral Potential Models . . . . . . . . . . . C13

Table C.5 Comparison of Expected Versus Observed 
ix

Occurrences for the Weights of Evidence (WOE) and Weighted 

Logistic Regression (WLR) Methods of Evidence Combination . . . . . . . . . C14

Table D.1 Primary Gold-silver-bearing Occurrences of

All Sizes and Types Having Posterior Probabilities 0.1000, 

as Determined with the Primary 7-layer Mineral Potential Model . . . . . . . . . D4

Table D.2 Posterior Probabilities Associated with Gold-silver-bearing

Sedimentary Rock-hosted Occurrences of All Sizes, as Determined

with the Sedimentary Rock-hosted 8-layer Mineral Potential Model . . . . . . D12

Table D.3 Posterior Probabilities Associated with Gold-silver-bearing 

Volcanic Rock-hosted Occurrences of All Sizes, as Determined 

with the Volcanic Rock-hosted 7-layer Mineral Potential Model . . . . . . . . D14
 x

LIST OF FIGURES

Figure 1.1 Great Basin Physiographic Province, Southwestern United States . . . . . . . 202

Figure 1.2 “Basin and Range” Topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

Figure 1.3 Principal Metallogenic Features of the Great Basin . . . . . . . . . . . . . . . . . . 204

Figure 2.1 Cordilleran Orogenic Belt, North America . . . . . . . . . . . . . . . . . . . . . . . . . 205

Figure 2.2 The Great Basin and Environs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

Figure 2.3 Plate Tectonic Setting and Selected Tectonomagmatic Elements of

the Western United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Figure 2.4 Primary Tectonic Subdivisions of the Central Cordilleran Orogen . . . . . . . 208

Figure 2.5 Major Orogenic Belts and Thrusts of the Central Cordilleran

Interior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

Figure 2.6 Time Chart of Orogenic Events Affecting the Great Basin . . . . . . . . . . . . . 211

Figure 2.7 Phases of the Cordilleran Orogeny— Space-Time Relationships . . . . . . . . 212

Figure 2.8 Geologic Events Affecting the Great Basin and Environs During the

Cenozoic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

Figure 2.9 Depositional Framework of the Cordilleran Geosyncline . . . . . . . . . . . . . . 214

Figure 2.10 The Western Edge of Precambrian North America . . . . . . . . . . . . . . . . . . . 215

Figure 2.11 Crustal Formation Provinces of the Western United States . . . . . . . . . . . . . 216

Figure 2.12 Variability of Selected Geologic Attributes across the Southern

Great Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

Figure 2.13 Phanerozoic Tectonostratigraphy of the Great Basin . . . . . . . . . . . . . . . . . . 218

Figure 3.1 Distribution of Sedimentary Rock-Hosted and Volcanic RockHosted Gold-Silver Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

Figure 3.2 Sedimentary and Volcanic Rock-Hosted Deposit Age Boundary . . . . . . . . 220

Figure 3.3 Mineral Trends and Belts of Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

Figure 4.1 Weights of Evidence Mineral Potential Modelling Method . . . . . . . . . . . . 222

Figure 6.1 Distribution of Gold-Silver-Bearing Occurrences in the Nevada

Great Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

Figure 6.2 Distribution of Sedimentary Rock-Hosted and Volcanic RockHosted Gold-Silver-Bearing Occurrences in the Nevada Great Basin

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

Figure 6.3 Density Map of Gold-Silver-Bearing Occurrences and All Metallic

Mineral Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

Figure 6.4 Time-Slice Maps of Cenozoic Igneous Rocks in Nevada . . . . . . . . . . . . . . 226

Figure 6.5 Map Patterns of Various Datasets Reflecting the “U-Shaped” or

“Horse-Shoe Shaped” Distribution Pattern of the Precious and

Non-Precious Metal Occurrences in Nevada . . . . . . . . . . . . . . . . . . . . . . . . 227

Figure 6.6 Shaded-Relief Image of Topography in the Southwestern United

States Highlighting Regional-Scale Linear Trends and Features . . . . . . . . . 228

Figure 6.7 Geological Map of Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

Figure 6.8 Lithologic Assemblage Map of Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

Figure 6.9 Frequency of Primary Occurrences by Lithologic Assemblage . . . . . . . . . . 231

Figure 6.10 Lithologic Units Hosting Over 90% of Primary Occurrences . . . . . . . . . . . 232

Figure 6.11 Lithologic Units Having a Strong Spatial Association with Primary
 xi

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

Figure 6.12 Frequency of Sedimentary Rock-Hosted Occurrences by Lithologic

Assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Figure 6.13 Lithologic Units Hosting 100% of Sedimentary Rock-Hosted

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

Figure 6.14 Lithologic Units Having a Strong Spatial Association with

Sedimentary Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

Figure 6.15 Frequency Volcanic Rock-Hosted Occurrences by Lithologic

Assemblage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

Figure 6.16 Lithologic Units Hosting 100% of Volcanic Rock-Hosted

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

Figure 6.17 Lithologic Units Having a Strong Spatial Association with Volcanic

Roc-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239

Figure 6.18 Lithology Ternary-Class Predictor Map for Gold-Silver-Bearing

Occurrences of all sizes and deposit types . . . . . . . . . . . . . . . . . . . . . . . . . . 240

Figure 6.19 Lithology Binary-Class Predictor Maps for Sedimentary and

Volcanic Rock-Hosted Gold-Silver-Bearing Occurrences . . . . . . . . . . . . . 241

Figure 6.20 Lithologic Units Reclassified by Posterior Probability of Hosting

Small, Medium, and Large Primary-Commodity Gold-SilverBearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

Figure 6.21 Lithologic Units Having Elevated Posterior Probabilities for

Hosting Small, Medium, and Large Primary-Commodity GoldSilver-Bearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Figure 6.22 Lithologic Units Having Elevated Posterior Probabilities for

Hosting Small and Big Primary-Commodity Gold-Silver-Bearing

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

Figure 6.23 Comparison Between the Extent and Trend of the Cordilleran

Geosyncline “Transitional Assemblage” and Lithologic Units

Having Elevated Posterior Probability for Hosting Small Size GoldSilver-Bearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

Figure 6.24 Lithologic Units Having Elevated Posterior Probabilities for

Hosting Sedimentary Rock-Hosted or Volcanic Rock-Hosted GoldSilver-Bearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Figure 6.25 Lithologic Units Having “Substantial” Posterior Probabilities for

Hosting Volcanic Rock-Hosted Gold-Silver-Bearing Occurrences,

Compared to a Map Illustrating the Space-Time Distribution of

Volcanic Rocks in Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Figure 6.26 Diversity of Lithology and Diagrammatic Representation of the

Diversity Neighborhood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

Figure 6.27 Lithotectonic Terranes of Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

Figure 6.28 Frequency of Primary Occurrences by Lithologic Diversity Unit . . . . . . . . 250

Figure 6.29 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

by Lithologic Diversity Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

Figure 6.30 Strength of Spatial Association Between Primary Occurrences and

Lithologic Diversity Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

Figure 6.31 Frequency of Primary Occurrences by Lithologic Terranes . . . . . . . . . . . . . 253

Figure 6.32 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences
xii

by Lithologic Terranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

Figure 6.33 Strength of Spatial Association Between Primary Occurrences and

Lithotectonic Terranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

Figure 6.34 Diversity of Lithology Map and Diversity Binary-Class Predictor

Map for Gold-Silver-Bearing Occurrences of All Sizes and Types,

Sedimentary Rock-Hosted Occurrences, and Volcanic Rock-Hosted

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

Figure 6.35 Lithotectonic Terranes Binary-Class Predictor Maps for GoldSilver-Bearing Occurrences of All Sizes and Types, Sedimentary

Rock-Hosted Occurrences, and Volcanic Rock-Hosted Occurrences

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

Figure 6.36 Plutonic Rocks of Nevada, Buffered Plutons, Pluton Density, and

the Density of All Gold-Silver-Bearing Occurrences . . . . . . . . . . . . . . . . . 258

Figure 6.37 Faults from the Geological Map of Nevada, Faults Distance

Buffers, and Density of Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

Figure 6.38 Frequency of Primary Occurrences Within Buffer Zones

Surrounding Plutonic Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

Figure 6.39 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Within Buffer Zones Surrounding Plutonic Bodies . . . . . . . . . . . . . . . . . . . 261

Figure 6.40 Variation of Contrast with Successive Area-Cumulative Distance

Intervals from Plutonic Bodies for Primary Occurrences . . . . . . . . . . . . . . 262

Figure 6.41 Frequency of Primary Occurrences Within Buffer Zones

Surrounding Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

Figure 6.42 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Within Buffer Zones Surrounding Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

Figure 6.43 Variation of Contrast with Successive Area-Cumulative Distance

Intervals from Faults for Gold-Silver-Bearing Occurrences . . . . . . . . . . . . 265

Figure 6.44 Pluton Buffer and Fault Buffer Binary-Class Predictor Maps for

Primary, Sedimentary and Volcanic Rock-Hosted Gold-SilverBearing occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

Figure 6.45 The Total Residual Field Geomagnetic Anomaly, the Isostatic

Residual Gravity Anomaly, and the Bouguer Gravity Anomaly of

Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

Figure 6.46 Frequency of Primary Occurrences Relative to the Strength of the

Total Residual Field Geomagnetic Anomaly . . . . . . . . . . . . . . . . . . . . . . . . 268

Figure 6.47 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Relative to the Strength of Total Residual Geomagnetic Anomaly . . . . . . . 269

Figure 6.48 Variation in Contrast with Successive Area-Cumulative Total

Residual Field Geomagnetic Anomaly Values for Primary

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

Figure 6.49 Frequency of Primary Occurrences Relative to Strength of Isostatic

Gravity Anomaly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

Figure 6.50 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Relative to Strength of Isostatic Residual Gravity Anomaly . . . . . . . . . . . . 272

Figure 6.51 Variation in Contrast with Successive Area-Cumulative Isostatic

Residual Gravity Anomaly Intervals for Primary Occurrences . . . . . . . . . . 273

Figure 6.52 Geophysical Binary-Class Predictor Maps, Derived from Total
xiii

Field Geomagnetic and Isostatic Gravity Geophysical Anomaly

Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

Figure 6.53 The Total Residual Field Geomagnetic Anomaly Intervals

Determined to Have a Strong Spatial Association with Sedimentary

and Volcanic Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . 275

Figure 6.54 The Isostatic Residual Gravity Anomaly Intervals that are Spatially

Associated with Sedimentary and Volcanic Rock-Hosted

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

Figure 6.55 Composition-Slice Maps of Cenozoic Igneous Rocks in Nevada . . . . . . . . 277

Figure 6.56 Sedimentary Rocks and Cenozoic Volcanic, Subvolcanic, and

Related Intrusive Rocks of Nevada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

Figure 6.57 K/Na and Ba/Na Geochemical Ratio Maps derived from National

Uranium Resource Evaluation Program Data . . . . . . . . . . . . . . . . . . . . . . . 279

Figure 6.58 Frequency of Primary Occurrences Relative to K/Na Anomaly . . . . . . . . . 280

Figure 6.59 Frequency of Primary Occurrences Relative to Ba/Na Anomaly . . . . . . . . . 281

Figure 6.60 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Relative to K/Na Anomaly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

Figure 6.61 Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Relative to Ba/Na Anomaly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

Figure 6.62 Variation in Contrast Across Successive Area-Cumulative K/Na

Anomaly Intervals for Primary Occurrences . . . . . . . . . . . . . . . . . . . . . . . . 284

Figure 6.63 Variation in Contrast Across Successive Area-Cumulative Ba/Na

Anomaly Intervals for Primary Occurrences . . . . . . . . . . . . . . . . . . . . . . . . 285

Figure 6.64 Geochemical Binary-Class Predictor Maps Derived from K/Na and

Ba/Na Geochemical Anomaly Data for Primary, Sedimentary and

Volcanic Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

Figure 6.65 Spatial Distribution of the K/Na and Ba/Na Geochemical

Anomalies Relative to the Distribution of the Sedimentary RockHosted Occurrences Along the Battle Mountain–Eureka (Cortez)

and Carlin Mineral Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

Figure 7.1 Relative Influence of Evidence: Primary Mineral Potential Models . . . . . . 288

Figure 7.2 Relative Influence of Evidence: Sedimentary Rock-Hosted Mineral

Potential Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

Figure 7.3 Relative Influence of Evidence: Volcanic Rock-Hosted Mineral

Potential Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

Figure 7.4 Sedimentary Rock-Hosted Occurrence-Type Posterior Probability

Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

Figure 7.5 Volcanic Rock-Hosted Occurrence-Type Posterior Probability Map . . . . . 292

Figure 7.6 Difference Between Mineral Potential Maps . . . . . . . . . . . . . . . . . . . . . . . . 293

Figure 7.7 Sedimentary Rock-Hosted Occurrence-Type Mineral Potential Model

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

Figure 7.8 Volcanic Rock-Hosted Occurrence-Type Mineral Potential Model

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

Figure 7.9 Uncertainty of Posterior Probabilities Used to Generate the

Sedimentary Rock-Hosted Occurrence-Type 8-Layer Mineral

Potential Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

Figure 7.10 Uncertainty of Posterior Probabilities Used to Generate the
 xiv

Volcanic Rock-Hosted Occurrence-Type 8-Layer Mineral Potential

Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

Figure 7.11 Posterior Probabilities Appended to Sedimentary Rock-Hosted

Gold-Silver-Bearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

Figure 7.12 Posterior Probabilities Appended to Volcanic Rock-Hosted GoldSilver-Bearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

Figure 7.13 Sedimentary Rock-Hosted Occurrence-Type 8-Layer Mineral

Potential Model—The Distribution of Elevated Favorability

Mineral Potential Areas in Comparison to the Distribution of

Known Sedimentary Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . 300

Figure 7.14 Volcanic Rock-Hosted Occurrence-Type 7-Layer Mineral Potential

Model—The Distribution of Elevated Favorability Mineral

Potential Areas in Comparison to the Distribution of Known

Volcanic Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

Figure 7.15 Comparison Between Sedimentary Rock-Hosted Mineral Potential

Areas Delineated in This Study and Tracts Permissible for CarlinStyle Sediment-Hosted Gold Mineralization as Delineated by

Teams of Experts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

Figure 7.16 Comparison Between Volcanic Rock-Hosted Mineral Potential

Areas Delineated in This Study and Tracts Permissible for

Epithermal Gold-Silver Mineralization as Delineated by Teams of

Experts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

Figure 7.17 Blind Test of the Weights of Evidence Sedimentary Rock-Hosted

8-Layer Mineral Potential Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

Figure 7.18 Blind Test of the Weights of Evidence Volcanic Rock-Hosted 7-

Layer Mineral Potential Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

Figure 7.19 Regional-Scale Exploration Target Areas for Sedimentary RockHosted Occurrence-Types, as Predicted by the Weights of Evidence

8-Layer Mineral Potential Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

Figure 7.20 Sedimentary Rock-Hosted Occurrence-Type Regional Exploration

Target Areas #1 and #2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Figure 7.21 Sedimentary Rock-Hosted Occurrence-Type Regional Exploration

Target Areas #3 and #4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

Figure 7.22 Sedimentary Rock-Hosted Occurrence-Type Regional Exploration

Target Area #5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

Figure 7.23 Regional-Scale Exploration Target Areas for Volcanic Rock-Hosted

Occurrence-Types, as Predicted by the Weights of Evidence 7-

Layer Mineral Potential Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

Figure 7.24 Volcanic Rock-Hosted Occurrence-Type Regional Exploration

Target Area #1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

Figure 7.25 Volcanic Rock-Hosted Occurrence-Type Regional Exploration

Target Areas #2 and #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

Figure 7.26 Volcanic Rock-Hosted Occurrence-Type Regional Exploration

Target Area #4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

Figure 7.27 Volcanic Rock-Hosted Occurrence-Type Regional Exploration

Target Area #5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

Figure 8.1 Linear Trends Highlighting Possible Regional-Scale Crustal
 xv

Structures that May Have Been Important to the Localization of

Sedimentary Rock-hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

Figure 8.2 Broad Regional Crustal Structures that May Have Been Important to

the Localization of Volcanic Rock-Hosted Occurrences . . . . . . . . . . . . . . . 316

Figure 8.3 Spatial Distribution Relationships Among All Gold-Silver-Bearing

Occurrences, K/Na Geochemical Anomaly, Geomagnetic Anomaly,

K/Na Anomaly Layered on Shaded Relief of Geomagnetic Anomaly,

Isostatic Gravity Anomaly, and Bouguer Gravity Anomaly . . . . . . . . . . . . 317

Figure 8.4 Kriged Surface Map of RADB Radiometric Age Dates for Cenozoic

Igneous Rocks in the Great Basin Region . . . . . . . . . . . . . . . . . . . . . . . . . . 318

Figure A.1 Stratigraphy of Nevada: Correlation of Geological Map Units . . . . . . . . . . A3

Figure B.1 Standard and Quadtree Raster Data Structures . . . . . . . . . . . . . . . . . . . . . B34

Figure B.2 Two- and Multi-Map Overlay and Combination Methods . . . . . . . . . . . . B35

Figure B.3 Entity and Unique Conditions Map Overlays and Linked Attribute

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B36

Figure B.4 Weights of Evidence Multi-Map Overlay Modelling Method . . . . . . . . . . B38

Figure B.5 Spatial Overlap Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B39

Figure B.6 Graph of Area-Cumulative Contrast Versus Buffer Zone Distances or

Anomaly Intensity Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B22

Figure C.1 Primary Occurrence-Type Mineral Potential Model Results . . . . . . . . . . . C15

Figure C.2 Sedimentary Rock-Hosted Occurrence-Type Mineral Potential

Model Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C16

Figure C.3 Volcanic Rock-Hosted Occurrence-Type Mineral Potential Model

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C17

Figure C.4 Overall Goodness-of-Fit Test for All Size and Type Occurrences,

Using the K-S Statistic: Primary 11-Layer Model; Primary 7-Layer

Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C18

Figure C.5 Overall Goodness-of-Fit Test for All Size and Type Occurrences, 

Using the K-S Statistic: Sedimentary Rock-Hosted 9-Layer Model; 

Sedimentary Rock-Hosted 8-Layer Model . . . . . . . . . . . . . . . . . . . . . . . . . C19

Figure C.6 Overall Goodness-of-Fit Test for All Size and Type Occurrences,

Using the K-S Statistic: Volcanic Rock-Hosted 9-Layer Model;

Volcanic Rock-Hosted 7-Layer Model . . . . . . . . . . . . . . . . . . . . . . . . . . . C20

Figure C.7 Difference Between Mineral Potential Favorability Maps . . . . . . . . . . . . C21

Figure C.8 Posterior Probability Estimates Derived from WOE and WLR

Methods for the Primary 11-Layer Mineral Potential Model . . . . . . . . . . . C22

Figure C.9 Posterior Probability Estimates Derived from WOE and WLR

Methods for the Sedimentary Rock-Hosted 9-Layer Mineral

Potential Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C23

Figure C.10 Posterior Probability Estimates Derived from WOE and WLR

Methods for the Volcanic Rock-Hosted 9-Layer Mineral Potential

Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C24

Figure C.11 Examples of the Various Sources and Types of Uncertainty in the

Posterior Probability Estimates Used to Generate the Primary

Occurrence-Type 11-Layer Mineral Potential Map . . . . . . . . . . . . . . . . . . C25

Figure C.12 Uncertainty of Posterior Probabilities Used to Generate the Primary

Occurrence-Type 7-Layer Mineral Potential Map . . . . . . . . . . . . . . . . . . . C26
xvi

Figure C.13 Uncertainty of Posterior Probabilities Used to Generate the

Sedimentary Rock-Hosted Occurrence-Type 8-Layer Mineral

Potential Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C27

Figure C.14 Uncertainty of Posterior Probabilities Used to Generate the

Volcanic Rock-Hosted Occurrence-Type 8-Layer Mineral Potential

Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C28

Figure C.15 Three-Dimensional Rendering of Relative Uncertainty of Posterior

Probabilities Used to Generate the Sedimentary Rock-Hosted

Occurrence-Type 8-Layer Mineral Potential Map . . . . . . . . . . . . . . . . . . . C29

Figure D.1 Distribution of Posterior Probabilities associated with Big (Large

and Medium), Small, and Unknown Size Primary Occurrences . . . . . . . . D23

Figure D.2 Primary Occurrence-Type 7-Layer Mineral Potential Model—The

Distribution of Elevated Favorability Mineral Potential Areas in

Comparison to the Distribution of Known Primary Gold-SilverBearing Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D24

Figure D.3 Distribution of Posterior Probabilities associated with Big (Large

and Medium), Small, and Unknown Size Sedimentary RockHosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D25

Figure D.4 Sedimentary Rock-Hosted Occurrence-Type 8-Layer Mineral

Potential Model—The Distribution of Elevated Favorability

Mineral Potential Areas in Comparison to the Distribution of

Known Sedimentary Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . D26

Figure D.5 Distribution of Posterior Probabilities associated with Big (Large

and Medium), Small, and Unknown Size Volcanic Rock-Hosted

Occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D27

Figure D.6 Volcanic Rock-Hosted Occurrence-Type 7-Layer Mineral

Potential Model—The Distribution of Elevated Favorability

Mineral Potential Areas in Comparison to the Distribution of

Known Volcanic Rock-Hosted Occurrences . . . . . . . . . . . . . . . . . . . . . . . D28
xvii

ACKNOWLEDGEMENTS

This open file report is based on dissertation research, which was supervised and supported

by Dr. Eion M. Cameron (Geological Survey of Canada, primary advisor), Dr. Graeme F.

Bonham-Carter (Geological Survey of Canada), and Dr. Anthony D. Fowler

(University of Ottawa). Dr. Gary L. Raines (U.S. Geological Survey, Reno Field Station,

Reno, Nevada) contributed much in the way of resources, encouragement, and discussion.

Numerous scientists and staff at the USGS Reno Field Station and the MacKay School of

Mines (University of Nevada) also contributed to my understanding of Great Basin geology.

Dr. Leigh A. Readdy (President, GAEA Consulting) provided valuable insights and support.

Reviews by Byron R. Berger (U.S. Geological Survey, Denver) contributed to a much more

focused manuscript.

Assistance with technical and statistical matters was provided by Qiuming Cheng (York

University), Bahram Daneshfar, Brian Eddy, Brad Sim, Kevin Telmer (University of

Victoria), and Danny Wright (Geological Survey of Canada). TYDAC Technologies Inc.

and Newmont Exploration Ltd. provided summer employment and assistance with various

technical and geological matters.

Special acknowledgements are given to the numerous workers in the Great Basin who have

contributed data, both public-domain and private (the citations in Table 5.1 should be read as

an extended acknowledgement). Data and assistance were also provided by scientists and

staff at the Geological Survey of Canada in Ottawa, NOAA National Geophysical Data

Center in Denver, the U.S. Bureau of Mines in Spokane (now, unfortunately, defunct), U.S.

Geological Survey Office of Mineral Resources in Washington D.C., and the U.S. Geological

Survey EROS Data Center in Sioux Falls.
1

Chapter 1. Introduction

1.1 Overview

The Great Basin is a “classic” modern extensional tectonic regime located in the central

Cordilleran interior of the southwestern United States. It has a long and complex geologic

history of crustal rifting, shortening, and accretion that can be traced back to the Archean. The

Great Basin is host to a large number and a variety of base- and precious-metal deposits. In

recent years, it has emerged as one of the more important gold-producing areas in the world,

especially since the start-up of the Carlin open-pit mine in 1965 and the rise in gold price to more

than $200US per ounce in 1978 (Mohide, 1981; Coope, 1991). The greater and richest part of

the Great Basin lies within the State of Nevada, and it is here that the well-known precious-metal

deposits occur. These include low-grade–high-tonnage sedimentary rock-hosted (Carlin-type)

deposits, such as Carlin and Getchell, and high-grade–low-tonnage volcanic rock-hosted

(epithermal) deposits, such as Comstock and Goldfield. 

A high level of economic interest has stimulated much research into the genesis of preciousmetal mineralization in Nevada. As a result, a comprehensive collection of geological,

geophysical, and geochemical spatial data has been generated. In the mid-1980's, Babcock

(1984) and Cook (1986) predicted that the search for new gold environments would likely

accelerate with emphasis on more sophisticated application of databases and conceptual models,

such as inference networks (artificial intelligence programs) used for computer assisted

prospecting. Since the early 1990’s, computer-based Geographical Information Systems (GIS)

applications have become an integral part of many mineral resource exploration programs. A GIS

is an integrated system of hardware, software, and methodologies for the management of spatial

(georeferenced) data. It facilitates data compilation and synthesis, and permits exploratory data

analysis and modelling. Evaluation of geoscience data with GIS can provide support for various

geological investigations and aid decision making processes, such as determining successful and

cost-effective exploration or management strategies.

The use of GIS-based techniques to explore geoscientific data may reveal insights not readily

obtained by more traditional means of data analysis or representation. In this study, quantitative

mineral potential modelling with a GIS has been used to investigate the regional-scale

distribution of precious-metal mineralization in the Nevada Great Basin. Weights of evidence,

a recently developed mineral potential modelling method, has been applied to this task. Weights

of evidence is a data-driven, discrete multivariate statistical method that uses conditional

probabilities to determine the relative importance of mineralization “evidence” and Bayesian

principles for integrating multiple “layers of evidence”.

1.2 Study Area

The area of interest in the Great Basin is confined to the State of Nevada (Fig. 1.1). Nevada,

situated near the geographic center of the Great Basin, contains most of the basin's area and

precious-metal mineral occurrences. The political constraints of the study area extent is due to

the digital geological data available at the commencement of this study. A sound geology base
2

map is essential to a study of this type, and at the time, Nevada was the only region within the

confines of the Great Basin for which digital geology of suitable resolution and accuracy was

publicly available.

Basin-range structure is wide-spread and highly-developed across the region, and consists of

roughly north–south-trending, evenly spaced parallel mountain ranges with intervening broad,

flat, alluviated desert basins (Fig. 1.2). On a regional-scale, the Great Basin is characterized by:

Uplift and extension—mean elevation of ~ 1.5 km and with an average extension of 100%, in excess of 300-

400 % in some areas (Stewart, 1978; Dewey, 1988; Wernicke, 1992; also see Harry et al., 1993).

Thinned crust—less than 30 km over much of the region, in comparison to 40-50 km for surrounding regions

(Allenby and Schnetzler, 1983; Allmendinger et al., 1987; also see Harry et al., 1993).

Anomalous upper mantle—regional Bouguer gravity low (Eaton et al., 1978), low seismic mantle velocities

(Stewart, 1978; Dewey, 1988; Smith et al., 1989; also see Harry et al., 1993), high heat flow (“reduced” HFU

values 50 to 100% and up to 300% greater than in stable regions) (Blackwell, 1978; Morgan and Gosnold,

1989).

Modern seismic activity—seismicity is concentrated around the margins of the region (Christiansen and Yeats,

1992).

The Great Basin region has a long and complex geologic history, involving major episodes of

crustal accretion, sedimentation, igneous activity, compressional deformation, and continental

rifting (Stewart, 1980). This includes at least three orogenies in the Precambrian, two

compressional orogenies in the Paleozoic, three compressional orogenic phases in the Mesozoic

(to earliest Cenozoic), two extensional events in the middle and late Cenozoic, and the presentday continued basin-range development (Stewart, 1980; Hoffman, 1989; Berger and Bonham,

1990; Burchfiel et al., 1992; Miller et al., 1992). The longevity, diversity, and intensity of

tectonomagmatic activity in this region has resulted in the formation of a distinctly unique and

rich geologic-metallogenic province.

1.3 Purpose and Objectives

The purpose of this study was to produce maps of mineral potential that predict the distribution

of sedimentary and volcanic rock-hosted gold-silver-bearing occurrences across Nevada.

Mineral potential modelling was carried out using weights of evidence method (Bonham-Carter

et al., 1989). The objectives were to:

& Determine and evaluate the spatial associations between the gold-silver-bearing occurrences and a variety of

regional-scale geoscientific data.

& Produce predictive maps of mineral potential (favorability) for sedimentary and volcanic rock-hosted

occurrences, which include analyses of error and uncertainty associated with the mineral potential maps.

& Delineate promising regional-scale exploration targets for sedimentary and volcanic rock-hosted occurrences,

and determine the important mineral potential evidence in these areas.

Determine the first-order geologic factors controlling the regional-scale spatial distribution of the sedimentary

and volcanic rock-hosted occurrences.

1.4 Methodology

An important goal of mineral potential modelling is to discover new deposits. As such, mineral
3

potential mapping involves the use of predictive models, as opposed to prescriptive models,

which are based on a set of criteria that represent sound engineering practices, and/or some blend

of economic or social factors (Bonham-Carter, 1994a). 

In the geosciences, models can be classified into three types based upon the kinds of

relationships they represent: (1) theoretical, (2) hybrid, and (3) empirical (Table 1.1). Mineral

potential models based on statistical or heuristic relationships, such as weights of evidence, are

empirical models. Such models are usually augmented or constrained by relationships

formulated as conceptual models, as the geologic conditions and processes that lead to the

formation of mineral deposits is too complex to express mathematically (Bonham-Carter, 1994a).

Empirical mineral potential models can be further subdivided on the basis of how the relative

importance of a deposit-indicator (or predictor) map pattern is determined: (1) data-driven or (2)

expert-driven (Table 1.2).Weights of evidence is data-driven. In weights of evidence, mineral

potential is calculated by weighting and combining multiple sources of evidence, which typically

includes multi-class “evidence maps” of geologic, geochemical, geophysical, or other

geoscientific phenomena. The estimation of mineral potential consists of two main procedures:

(1) the application of conditional probabilities to measure the spatial associations between known

mineral occurrences and various evidence maps; and (2) the use of Bayesian updating techniques

to combine the evidence and produce a posterior probability map. In the first step, multi-class

evidence maps are typically reduced to binary-class deposit-indicator or “predictor maps” (the

“layers of evidence”) in order to maximize the spatial association (the weights) between the

evidence and mineral occurrences and to simplify map combination carried out in step two.

Weights of evidence attempts to explain the spatial distribution of mineral occurrences in terms

of the spatial distribution of evidence map patterns. The ultimate intent is to produce a map of

mineral potential that accurately models mineral occurrence distribution. Ideally, the mineral

potential map will highlight areas of yet undiscovered mineral occurrences.

In general, model building consists of three main stages (Chatfield, 1988):

1. Model formulation or specification.

2. Parameter estimation, or model fitting.

3. Model validation.

As applied to weights of evidence mineral potential modelling conducted in this study, these

stages involved:

1. (a) Establishing and measuring the spatial associations between the gold-silver-bearing occurrences and

multi-class mineralization “evidence maps”.

(b) Determining the significance of these relationships.

(c) The creation of binary-class mineral “predictor maps” from the mineralization-favorable units

composing multi-class evidence maps.

2. Generating mineral potential maps for sedimentary and volcanic rock-hosted occurrences by combining the

predictor maps in a multi-map overlay using a loglinear formulation of Bayes’ Rule.

3. Validation of the models, including testing for conditional independence among the evidence layers, analysis

of mineral potential estimation uncertainty, mineral favorability at known occurrence locations, and blind

tests.
4

1.5 Spatial Datasets

The geoscientific spatial data compiled for this study include: 

1. Mineral deposits—various metallic mineral occurrence datasets, and mineral belts and trends.

2. Regional geology—bedrock and surficial geology, metamorphic rocks and core complex locations, volcanic

centers and cinder cone locations, Mesozoic pluton distribution, geosyncline facies boundaries, regions of

strong upper crustal extension, numerous fault and thrust fronts datasets, deep-seated fracture zones,

lithotectonic terranes, and Tertiary rock attitudes.

3. Physical geography—30 arc-second and 5 arc-minute gridded elevation, mountain peak heights, generalized

and detailed Great Basin physiographic province boundaries.

4. Geophysical—various gravity anomaly data (observed, isostatic, Bouguer, free air), geomagnetism,

geothermal heat flow, geothermal conductivity, geothermal heat production, geothermal well/hot spring

temperatures, and paleothermal anomaly.

5. Seismic—depth to reflection Moho, earthquake depth and magnitude, and crustal stress data.

6. Geochemical—igneous rock radiometric age dates, base- and precious-metal mineralization radiometric age

dates, 87Sr/86Sr initial values and ISr=0.706 and 0.708 isopleths, and major and minor element data.

7. Remote sensing imagery—LANDSAT linear features, AVHRR, and SLAR radar.

8. Hydrology—drainage divides, streams and water bodies.

9. Human/cultural-features—major cities, administrative boundaries, roads, highways, and railways. 

A series of mineral potential evidence maps was prepared from these datasets. Some were used

for weights of evidence analysis and modelling while others served as supplementary material

for interpretation, exemplification, and referencing (see section 5.3.3).

The Mineral Resource Data System mineral occurrence database (MRDS; U.S. Geological

Survey, 1993) was used to model the distribution of precious-metal mineralization in Nevada.

From a population of 5572 metallic and semi-metal mineral occurrences listed in MRDS, 2690

gold-silver-bearing occurrences (containing gold and/or silver as the primary commodity listed

in MRDS) were selected and subdivided into samples as indicated in Table 1.3:

The mineral occurrences in MRDS are classified according to the scheme of Cox and Singer

(1986). Occurrence size designation is based on precious-metal content (production plus

reserves), and is derived from the Metallogenic Map of North America (Guild, 1968). Modelling

was carried out using the three principal occurrence-type samples (training datasets). In some

instances, analysis and modelling was also performed using the large, medium, and/or small size

sub-samples.

The sedimentary rock-hosted occurrences, which include “Carlin-type”, “carbonate-hosted”, or

“disseminated” deposit types, are largely situated in north-central and north-eastern Nevada. The

most important and greatest number are distributed along two regional-scale deposit alignments

known as the Carlin and the Battle Mountain–Eureka (Cortez) mineral trends (Fig. 1.3), which

are both oriented at acute angles to the regional structural grain (compare Figs. 1.3 and 1.2).

Sedimentary rock-hosted deposits are typically low-grade–high-tonnage open-pit operations, and

generally defined as stratiform occurrences of sub-microscopic to microscopic gold coating

disseminated sulfide minerals in carbonaceous calcareous sedimentary rocks (Bagby and Berger,

1985; Cox and Singer, 1986). The age of these deposits was poorly constrained until recently,

with ages ranging between 8-35 to ~120 Ma (see Arehart et al., 1993, 1995; Christensen, 1995;
5

Kuehn, 1989), but new research suggests that they formed between Late Eocene and Early

Oligocene time (42-30 Ma) (Maher et al., 1993; Emsbo et al., 1996; Groff et al., 1997; Hall et

al., 1997; Hofstra, 1997).

Most of the volcanic rock-hosted occurrences, which include “epithermal”, “hot springs”,

“vein/stockworks ± disseminations” deposit types, are located in southwestern and western

Nevada, and occur within a broad belt that parallels the Nevada-California border (Fig. 1.3).

This belt, known as the Walker Lane, is a zone of strike-slip movement that trends northwest at

an acute angle to the regional structural grain of the central Great Basin (compare Figs. 1.3 and

1.2). Volcanic rock-hosted deposits are usually high-grade–low-tonnage underground operations

(open-pit in the case of some disseminated mineralization). They are generally defined as

“volcanic centered” vein and/or disseminated occurrences (often co-occurring) of gold±silverbearing minerals in, or associated with, a variety of brittle deformation structures hosted by

volcanic and associated rock types (Cox and Singer, 1986; Panteleyev, 1986; Berger and Henley,

1989; Hedenquist et al., 1996). The volcanic rock-hosted deposits appear to be Upper

Oligocene, to Early to Middle Miocene and younger in age (most forming between 27 and 5 Ma,

making them about 20 to 100 Ma younger than the sedimentary rock-hosted deposits) (Dreier,

1984; Cox et al., 1991; Hutchinson and Albers, 1992; Ludington et al., 1993).

1.6 Similar Studies and Original Contribution

The application of GIS to minerals exploration, specifically mineral potential modelling, is

gaining an increasing amount of attention and has been shown to yield favorable results (see

Bonham-Carter et al., 1988; Bonham-Carter et al., 1989; George and Bonham-Carter, 1989;

Watson and Rencz, 1989; Agterberg et al., 1990; Braux et al., 1990; Moon, 1990; An et al.,

1991; Bonham-Carter, 1994a; Cheng et al., 1994). Much of this work was done in Canada, and

until recently, GIS-based quantitative mineral potential modelling studies in the United States,

specifically data-driven statistical approaches, have been limited in number. As of September

1997, nothing had been published on mineral potential modelling in the Great Basin, however,

the U.S. Geological Survey and various mining companies and private consultants are now

actively engaged in such research.

Gary L. Raines, of the U.S. Geological Survey, Reno Field Office, began weights of evidence

modelling in 1991. He produced a posterior probability mineral potential map for hot springrelated gold, based upon proximity to volcanic rocks and vents, hydrothermal alteration, placer

deposits, faults, linear features, and anomalous U, Ag, As, Mn, Se and aeromagnetic values. Dr

Raines used expert-delineated maps of hot spring mineral permissibility to validate his model,

which showed a high degree of agreement (74%). 

Dean D. Turner, a private consultant from Reno and former employee of the Newmont and FMC

gold companies, began working on an M.Sc. thesis at the Colorado School of Mines in 1991 (a

paper summarizing his results, Turner, 1997, was presented at the Exploration ‘97 symposium

in Toronto, which was held in October of 1997). Mr. Turner produced a gold favorability map,

based on weights of spatial association with non-placer gold-bearing occurrences, using buffered

host-rock lithology and granitoid pluton domains, structural domains (lineament intensity, based

on interpretation of topography and aeromagnetic, isostatic gravity, and radiometric anomalies),
6

and geochemical domains (based on As-Sb-Zn-Ag-W and Cr-V-Fe-Co-Sc pathfinder element

groupings). The model was validated using 52 major sediment-hosted gold deposits, 88.5% of

which were identified by his favorability map.

The study undertaken here differs substantially from, and extends beyond, those conducted by

Raines and Turner. A fully-implemented application of weights of evidence mineral potential

modelling was undertaken, complete with weights of spatial association analysis and assessment,

generation of posterior probability maps of mineral potential (favorability), analyses of modelling

error and uncertainty, and blind testing of the models. Also, this study is broader in scope,

dealing not only with hot spring-related deposits and non-placer gold-bearing occurrences in

north-central Nevada, but also with other epithermal mineralization (collectively modelled as

“volcanic rock-hosted”), sedimentary rock-hosted occurrences State-wide, and with gold-silverbearing occurrences in general. For the mineral potential models produced here, the training

datasets and combinations of mineralization evidence are substantially different from those used

by Dr. Raines and Mr. Turner (as well as having been utilized and processed differently). In

addition, a large number of supplementary datasets were used in this study for interpreting the

output of the models.

The original contribution of this research to the study of economic geology in the Nevada Great

Basin includes:

 Predictive maps of mineral potential favorability for sedimentary and volcanic-rock hosted mineralization,

which highlight areas of elevated potential but no known deposits. Associated maps of error and uncertainty.

 Maps delineating regional-scale exploration targets for sedimentary and volcanic rock-hosted occurrences,

including a review of the important mineral potential evidence within the target area and a listing of specific

high mineral potential sites (at the mountain-range-scale) for further investigation.

 New, revised, and supporting hypotheses regarding the regional-scale controls over the distribution of

sedimentary and volcanic rock-hosted mineralization. It has been demonstrated that a data-driven approach

to mineral potential mapping can illuminate the understanding of mineral deposit distribution.

In addition, the work carried out here identifies new, potentially mineral-rich, regions for

research opportunity, as well as demonstrating the capacity to facilitate mineral resource

assessment in the Great Basin.

1.7 Layout

This study is subdivided into eight chapters and five appendices. The appendices are provided

as digital files on a CD-ROM located in the back cover pocket, and are available in Adobe

Acrobat (.pdf) version 4.

Chapter Contents

1. Introduction—An overview and of this study. Includes the purpose, scope, and objectives, as well as

a review of the datasets and the analysis and modelling techniques.

2. Geology of the Great Basin—An overview of the geology of the Great Basin and its environs, with

emphasis placed on the features and time periods pertinent to the interpretation of the sedimentary and

volcanic rock-hosted weights of evidence mineral potential maps. This is a background chapter that

represents a summary literature review.
7

3. Characteristics and Distribution of Sedimentary and Volcanic Rock-Hosted Deposits—An overview

of sedimentary and volcanic rock-hosted deposit types, with emphasis placed on their spatial

distribution. This is a background chapter that represents a summary literature review.

4. Analysis and Modelling Techniques—An overview of preliminary GIS spatial data analysis techniques

and weights of evidence mineral potential modelling (an extended and detailed discussion is given in

Appendix B). This is a background chapter that represents a summary literature review.

5. Spatial Datasets—An overview of the GIS study area, training datasets, and mineral potential evidence.

Included is a discussion on gold-silver-bearing mineral occurrence selection criteria, evidence map

selection, and reviews on data coverage, collection, accuracy, and error issues. 

6. Single Map Analysis, Interpretation, and Mineral Predictor Map Generation—Analysis of mineral

potential evidence. Each evidence map is examined in relation to the distribution of gold-silver-bearing

occurrences (this chapter represents the first of two procedures that constitute the weights of evidence

modelling method; see Chapter 4). The distribution and basic statistical nature of the datasets are

described, and the spatial associations between the occurrences and the evidence maps are measured.

The significance of the spatial relationships is determined and an interpretive synthesis in light of

geology and metallogeny is given. Also discussed are generation of binary-class mineral predictor maps

and various assumptions, definitions, and selection criteria for the gold-silver-bearing occurrences.

7. Multi-Map Modelling and Gold-Silver Mineral Potential—Summary and discussion of the results and

output of mineral potential modelling. The mineral predictor maps are combined to produce the mineral

potential maps (this chapter represents the second of two procedures which constitute the weights of

evidence method; see Chapter 4). Analyses of mineral potential model conditional independence, error,

and uncertainty are reviewed. Validation of the models, including mineral favorability at known

occurrence locations and blind tests are discussed. Regional-scale sedimentary and volcanic rock-hosted

occurrence exploration targets are delineated.

8. Controls on Sedimentary and Volcanic Rock-Hosted Occurrence Distribution—Discussion and

summary of the geologic and metallogenic interpretation of the mineral potential maps. Important

mineral potential evidence within each target region is reviewed. Possible factors controlling the

regional-scale distribution of sedimentary and volcanic rock-hosted occurrences are delineated and

discussed.

Appendix Contents

A. Stratigraphy and Description of Lithologic Units—A correlation chart of geological map units with an

accompanying table of unit descriptions.

B. Weights of Evidence Mineral Potential Modelling Theory, Implementation, and FORTRAN

Utilities—A detailed theoretical discussion of the weights of evidence modelling procedure,

implementation of the procedure, and DOS and OS/2 executable and source code for the stand-alone

command-line FORTRAN utilities (used to calculate the spatial weights of association, the posterior

probabilities, and the weights errors and uncertainty factors). Also included is a review of various GIS

spatial analysis techniques and tools. The DOS and OS/2 utilities are included on this CD-ROM in the

\appd_b folder.

C. Mineral Potential Map Generation, Conditional Independence, and Uncertainty—A detailed account

and analysis of mineral potential maps generation, conditional independence testing, interpretation and

mitigation and of conditional dependence, and mineral potential map uncertainty.

D. Mineral Potential at Known Mineral Occurrence Areas—A detailed examination of how well the

mineral potential models predict data from which they were built. Includes three tables containing the

posterior probability value associated with each of the occurrences.
8

Chapter 2. Geology of the Great Basin

2.1 Introduction

The Great Basin is one of the most intensely studied regions on Earth. This chapter presents a

review of the geology of the Great Basin and its environs, with emphasis placed on the features

and time periods pertinent to the interpretation of the sedimentary and volcanic rock-hosted

weights of evidence mineral potential maps. An overview of physiographic and tectonic setting

and an outline of the geologic history are given in sections 2.2 and 2.3. The Precambrian and

Cenozoic time periods are reviewed in greater detail in sections 2.4 and 2.5. Strike-slip

tectonism and related regional-scale crustal features are discussed in section 2.6. The

characteristics and structure of the present-day crust are reviewed in section 2.7. 

Comprehensive reviews of the geologic, tectonic, magmatic, and geophysical nature of Great

Basin can be found in Atwater (1970, 1989), Axen et al. (1993), Bally and Palmer (1989), Best

and Christiansen (1991), Burchfiel et al., (1992), Coward et al., (1987), Crittenden et al. (1980),

Eaton (1982), Ernst (1988), Pakiser and Mooney (1989), Raines et al. (1991), Smith and Eaton

(1978), Stewart (1980), and Zoback et al. (1981).

2.2 Physiographic Setting

The Basin and Range Province of the southwestern United States and northern Mexico is an

extensional tectonic regime set in the southern North American Cordillera, part of the larger

Cordilleran orogenic belt that stretches from southwestern Alaska in North American to Chile

and Argentina at the southern tip of South America (Fig. 2.1). It is part of a region known as the

“continental interior” of the Cordilleran orogen (Christiansen and Yeats, 1992), also referred to

as the Intermontane System (Anderson, 1989). The Intermontane System covers a vast region

in the western United States—it extends from the United States-Mexico border northward to

southeastern Oregon and southern Idaho; it is bound to the east by the Rocky Mountain System

and to the west by the Pacific Mountain System; and it encompasses the Basin and Range

Province, the Colorado Plateaus, and the high lava plains of the Snake River Plateau (Anderson,

1989; Thelin and Pike, 1991) (Fig. 2.2). The Cordilleran orogen in the western United States is

unusually wide, measuring approximately 1,500 km across (Guild, 1985), as compared to the

modern-day Andean orogen with a width of 400-800 km (Dickinson and Snyder, 1978).

The Basin and Range Province of the southwestern United States may be subdivided into

northern, central, and southern regions, based upon contrasting geologic histories and structural

styles (Wernicke, 1992), and is composed of five individual physiographic sub-provinces (Fig.

2.2) (Thelin and Pike, 1991):

1. Great Basin

2. Sonoran Desert, or the Mojave Block (Wilkins, 1984)

3. Salton Trough

4. Mexican Highland (which includes the Rio Grande Rift zone), or the Porphyry Copper Block (Wilkins, 1984)

5. Sacramento section
9

The Great Basin, coincident in name and in expanse with the northern Basin and Range

Province, is the largest of the five physiographic provinces, covering ~452,000 km2

 (as

circumscribed by Thelin and Pike, 1991) and varying from 500 to 1000 km in width (see Harry

et al., 1993). It occupies nearly all of the State of Nevada (~95%), much of western Utah, small

parts of southern Idaho and Oregon, and California east of the Sierra Nevada Range. The Great

Basin is not a basin proper, but rather is a collection of over 200 small basins that form a large

region of interior drainage that is bound to the east (Wasatch front, Utah) and to the west (Sierra

Nevada, California) by marginal highlands rising 2-3 km above SML, approximately one

kilometer above the surrounding regions (Eaton et al., 1978; Mayer, 1986; Hendricks and

Plescia, 1991; Sherrod and Tosdal, 1991; Christiansen and Yeats, 1992; Wernicke, 1992). The

area within the Great Basin is uniformly above one kilometer SML and has an average elevation

of 1.5 km, comparable to that of the Colorado Plateau (Christiansen and Yeats, 1992; also see

Wilkins, 1984; Harry et al., 1993). Basin–range-style topography is characteristic of the Basin

and Range Province as a whole, but is best developed in the Great Basin sub-province (Fig. 1.2).

2.3 Tectonic Setting and Geologic History

The Cordilleran orogen is one of the longest-lived and extensive orogenic belts in the world

(Burchfiel et al., 1992). The major tectonic elements within and surrounding the Great Basin

date from the Archean through the present, and with the exception of the Precambrian elements,

are products of the interaction between the Pacific oceanic and the North American continental

tectonic plates (Atwater, 1970, 1989; Mutschler et al., 1987). The tectonic environments of the

western United States and the Great Basin are illustrated in Figures 2.3 and 2.4, respectively.

The major orogenic belts in the Great Basin are shown in Figure 2.5.

The geologic history of the Great Basin region is complex, spanning at least 600 to >800 Ma, and

involving major episodes of crustal accretion, sedimentation, igneous activity, compressional

deformation, and continental rifting and extension. This history includes at least three orogenies

in the Precambrian (Yavapai-Mazatzal, episodic Middle Proterozoic intracontinental rifting,

polyphase Late Proterozoic marginal rifting; Burchfiel et al., 1992; Hoffman, 1989), two in the

Paleozoic (Antler and Sonoma; three if the “Humboldt event” is included, see Stewart, 1980;

Miller et al., 1992), three in the Mesozoic (Nevadan-Elko, Sevier, and start of the Laramide; or

one, if considered as three pulses of the “Cordilleran orogeny”, see Miller et al., 1992), one in

the Cenozoic (initial local crustal extension and basin-range development; Laramide activity

wanes), and the present-day continued basin and range development (Stewart, 1980; Berger and

Bonham, 1990). The current stage of basin-range development represents only about 5% of its

total evolution (Stewart, 1980). Precambrian and Phanerozoic orogenic history are illustrated

in Figure 2.6, mid-Mesozoic through early Cenozoic in Figure 2.7, and latest Cretaceous through

present in Figure 2.8.

In broadest terms, the Phanerozoic geologic setting and history of the Great Basin region can be

summarized in terms of three contrasting regimes:

1. Latest Proterozoic to late Devonian passive margin regime—A tectonically quiescent stable Precambrian

continental passive margin along which a two-fold northerly-trending miogeoclinal-eugeoclinal depositional

regime developed (Stewart, 1980; Suppe, 1985).

2. Late Devonian to early Tertiary accretionary and compressional regime—Island-arc and oceanic terranes
10

were accreted to the western edge of the continental margin. Eastward-directed compressional orogenic

events emplaced areally-extensive thrust sheets along and east of the margin, resulting in large-magnitude

crustal shortening and the juxtaposition of the coeval but contrasting miogeoclinal and eugeoclinal

sequences. Geosynclinal sedimentation was terminated in the Early Triassic. By Late Triassic or Early

Jurassic an “Andean-type” continental-margin subduction regime was established. Widespread plutonism

and upper crustal thrusting followed, resulting in the development of a regionally extensive magmatic arc,

foreland fold and thrust belt and associated hinterland (Coney, 1978; Stewart, 1978, 1980; Suppe, 1985;

Mutschler et al., 1987; Berger and Bonham, 1990; Thorman et al., 1991).

3. Early Tertiary to Present extensional regime—The modern-day Great Basin developed east of a waning

Mesozoic magmatic arc in an intra-arc and back-arc setting within thrust belts formed from Paleozoic

miogeocline and accreted terranes. A transition from compressional to extensional tectonics occurred around

45-40 Ma, and after about 36 Ma, widespread volcanism and crustal extension prevailed (Stewart, 1978,

1980; Eaton, 1983; Lipman, 1983; Wernicke et al., 1987; Hamilton, 1988; Berger and Bonham, 1990).

The geologic events that shaped these three regimes, as well as the pre-Cordilleran settings, may

be subdivided into seven stages:

1. Archean to Latest Proterozoic—Nucleation and configuration of the core of the North American crystalline

craton, including numerous episodes of collisional and extensional deformation; polyphase rifting gives rise

to approximate shape of present-day North American continent and truncates Precambrian tectonic grain.

2. Latest Proterozoic to Late Devonian—Passive-divergent subsiding margin and sedimentation; inception and

main-stage deposition of the Cordilleran geosyncline; depositional framework consisted of a western

siliceous assemblage (the eugeosynclinal facies) and the eastern carbonate assemblage (the miogeoclinal

facies), separated by a transitional zone (the transitional facies).

3. Late Devonian to Late Triassic—Passive, sub-island arc subduction, accretionary margin; addition of crust

to the western margin of the Precambrian crystalline craton core; initial deformation of the Cordilleran

geosyncline; two island-arc system's and their associated accretionary prism sequence's are welded to the

craton in two separate episodes during the Antler and Sonoma orogenies (emplacement of the Roberts

Mountain and Golconda allochthons, respectively); sedimentation in the Cordilleran geosyncline is

terminated at the start of the Sonoma orogeny in the Early Triassic; global plate reorganization and tectonic

truncation of Paleozoic orogenic grain by Late Triassic; arc polarity reversal.

4. Late Triassic to Upper Cretaceous—Active margin sub-continental subduction (“Andean-type” steep angle

plate descent mode); passive-to-active margin transition, and initiation of sub-continental subduction; crustal

thickening and compressional deformation; development of continental magmatic arc (Nevadan-Elko phase)

and “thin-skinned” fold and thrust belt and associated hinterland (Sevier phase); region of present-day Great

Basin undergoes contraction between a continental arc on the west and the craton in central Utah to the east.

5. Upper Cretaceous to Mid-Eocene—Active margin sub-continental subduction (shallow angle plate descent

mode, high rate of plate convergence); Cordilleran geosyncline depositional environment is wiped out;

intraplate deformation; compressional orogenesis migrates inland toward the craton during Laramide activity;

arc magmatism shifts progressively eastward and wanes; transition from compressional to extensional

orogenesis occurs as active margin sub-continental subduction wanes.

6. Mid-Eocene to Late-Lower Miocene—Active margin sub-continental subduction rapidly ending (steep angle

subduction plate roll-backs from east to west, decreasing plate convergence rate); Pacific oceanic plate closes

on North American continental plate; large-magnitude, pre-basin–range-style extension (early extensional

phase, ~36-17 Ma), characterized by low angle shallow listric and detachment faulting in the upper crust and

in some places the emergence of mid-crustal metamorphic core complexes; regional uplift; magmatism

resumes and sweeps generally west and southward (predominantly felsic-intermediate calc-alkaline volcanic

flows and silicic ash-flow tuffs); inception and growth of San Andreas transform system; cessation of active

margin sub-continental subduction and transition to passive transform margin.

7. Late-Lower Miocene to Present—Transform margin between North American continental plate and Pacific

oceanic plate develops as Mendocino triple junction migrates north; earlier extensional period wanes and

is superseded by wide-spread basin–range-style extension (later extensional phase, 17-6 Ma), characterized

by high angle deep-penetrating block faulting that produces the modern Basin and Range Province structural

grain and topography; brief lull in felsic-intermediate and silicic calc-alkaline magmatism (20-17 Ma)
11

followed by transition to bimodal rhyolite-basalt volcanism which is characterized by increasing proportions

of basalt; intensity of silicic volcanism progressively decreased during last 10 m.y.

These seven stages are based upon plate boundary interactions and orogenic styles, and reflect

coherent periods in the development of the Great Basin region and its environs. They are not

meant to suggest any implicit genetic relationship between the events within any one stage,

though in many instances this may be the case.

2.4 Craton Development

The Precambrian terrain of the western Cordillera of the United States is a poorly understood

collage of collided oceanic island arcs, overprinted by continental magmatic arcs, complicated

by rifts, and perhaps affected by anorogenic magmatism (Hamilton, 1987). For the most part,

the evolution and tectonic activity of the Precambrian have not been delineated in detail

(Dickinson, 1981), but recent work by Hoffman (1989) has added greatly to the understanding

of this era in North America.

2.4.1 Assembly and Margin Formation

The evolutionary stages of the North American craton overlap in time, but can be roughly

outlined as: (1) 1.98-1.83 Ga initial assembly of the Archean core of the Laurentian protocraton;

(2) 1.91-1.65 growth of the Laurentian protocraton by accretion of mainly proterozoic material;

(3) 1.65- 0.8 Laurentian supercontinental tectonic environment; and (4) 0.8-0.5 marginal rifting

of Laurentia (ages generalized from Hoffman, 1989). The fourth stage was particularly

important. During this stage, the edges of Laurentia were carved away, forming the crystalline

cratonic core of North America and establishing the cratonic margin in the southwestern United

States (Stewart, 1980; Hoffman, 1989). This episode of continental margin rifting was to have

far reaching effects on the orientation, location, and style of subsequent Phanerozoic orogenic

elements that developed in the southwestern Cordillera and northern Basin and Range Province.

The initial assembly of Laurentian protocraton took place in the Early Proterozoic (1.98-1.83 Ga)

when seven former microcontinents were welded together during collisional orogenesis, which

was characterized by deformed passive margin and foredeep sedimentary prisms, “Andean-type”

magmatic arcs, hinterlands with compression-related fault tectonism, and foreland thrust-fold

belts (Hoffman, 1989). These microcontinents (or crustal provinces, in post-accretionary

parlance) are Late Archean crustal aggregates that contain variable proportions of Early and/or

Middle Archean rocks, and include the Superior, Wyoming, Slave, Nain, Hearne, Rae, and

Burwell provinces (Hoffman, 1989). In the Early Proterozoic, growth continued as crust

containing little or no Archean material was accreted along the western, southern, and

southeastern margins of the Archean core of the protocraton between about 1.91 and 1.65 Ga

(Hoffman, 1989). Along the southern and southwestern United States, more than 1,200 km of

juvenile crust was accreted between 1.80 and 1.65 Ga during the Central Plains and YavapaiMazatzal orogenies, respectively (Hoffman, 1989). The construction of the Laurentian

supercontinent, within which the North American craton was embedded, was coherent by 1.7 Ga

and assembled by 1.65 Ga (Dickinson, 1981; Hoffman, 1989; Burchfiel et al., 1992).

In the late Middle Proterozoic, sedimentary rocks now within the Cordilleran orogen were
12

probably deposited during at least three pre-Cordilleran intracontinental extensional events that

took place around 1.5-1.3 Ga, 1.15-1.07 Ga, and 0.9-0.8 Ga, the first being the most extreme of

the three (Burchfiel et al., 1992). Between 0.8 and 0.5 Ga, polyphase rifting took place along the

margins of Laurentia, giving rise to the approximate shape of present-day North American craton

(Hoffman, 1989). Rifting along the present-day western margin of North America was

accompanied by wide-spread mafic igneous activity and sedimentation, and marked the initiation

of the throughgoing Cordilleran geosyncline, which was considered by Burchfiel et al. (1992)

to be the inception of the Cordillera orogen. Main-stage deposition of the Cordilleran

geosyncline dominated geologic activity from latest Precambrian through mid-Paleozoic. By

about 650-600 Ma, major rifting had subsided and the western edge of the North American

craton resembled a passive Atlantic-type margin (Stewart, 1980; Burchfiel et al., 1992). Strata

were deposited along and inland from the continental margin, resting unconformably on

structurally complex crystalline basement rocks consisting of gneiss, schist, quartzite, marble,

greenstone, and granite ranging in age from about 2.5 to 1.41 Ga, and on younger supracrustal

sedimentary and volcanic rocks ranging in age from about 1.45 to 0.9-0.8 Ga (Poole et al., 1992).

The pattern of deposition that was to characterize the early Paleozoic was well defined in Nevada

by latest Precambrian time (Stewart, 1980) (Fig. 2.9). Clastic shelf sedimentation continued

through the Early Cambrian, but as the continent moved into lower latitudes, it was gradually

superseded in the Middle Cambrian by younger shallow-water carbonate strata that transgressed

farther inland onto the cratonic basement (Hamilton, 1987). These conditions continued with

only minor changes in sedimentation patterns and with little disruption through the lower and

middle Paleozoic until stable-margin sedimentation ended in the late Devonian-early

Mississippian with the Antler compressional orogeny, during which the Roberts Mountain and

Golconda allochthons were emplaced, respectively (Stewart, 1980; Dickinson, 1981; Speed,

1983; Suppe, 1985;Hamilton, 1987; Hutchinson and Albers, 1992).

2.4.2 Western Edge and Crustal Provinces

The western edge of the Precambrian craton in the western United States is commonly believed

to correspond closely to an initial value of 87Sr/86Sr = 0.706 in Mesozoic and Tertiary granitoids

(Fig. 2.10), a value considered to represent the boundary between magmas related to oceanic or

continental lithosphere (Kistler and Peterman, 1973; Stewart, 1980). In the Great Basin, the

87Sr/86Sr ISr = 0.706 isopleth trends from the central Sierra Nevada to western Idaho in a variable

north or northeast orientation, which is dependant upon the datasets and interpretations used to

place the line (see Kistler and Peterman, 1973; Fleck and Criss, 1985; Kistler, 1990; Carlson et

al., 1991). Burchfiel et al. (1992) noted that the 87Sr/86Sr ISr = 0.706 isopleth is a composite line

with a location throughout the Cordillera controlled by different events of different ages. Strikeslip activity (late Paleozoic, mid- to late Mesozoic, and Cenozoic) has been the controlling factor

in the position of the line from the Mexican border to the central Sierra Nevada (Burchfiel et al.,

1992), hence the northwestward “spike” in the line along the California-Nevada border. In

central Nevada, its location may be close to the original site of the latest Precambrian rifting,

albeit modified by the effects of Paleozoic and Mesozoic shorting and Cenozoic extension

(Burchfiel et al., 1992; see Levy and Christie-Blick, 1989, for a Pre-Mesozoic palinspastically

restored 87Sr/86Sr ISr = 0.706 isopleth). An alternative to the 87Sr/86Sr ISr = 0.706 isopleth is the

87Sr/86Sr ISr = 0.708 (Nd = -7) isopleth (Farmer and DePaolo, 1983), located as much as 100 km

to the east of the 87Sr/86Sr ISr = 0.706 isopleth (Levy and Christie-Blick, 1989). The 87Sr/86Sr ISr

= 0.708 (Nd = -7) isopleth, however, appears to be inconsistent with the distribution of
13

Proterozoic and Lower Cambrian sedimentary rocks that occur near the 87Sr/86Sr ISr = 0.706

isopleth (Levy and Christie-Blick, 1989). The western edge of the craton may also be delineated

by a late Paleozoic-early Mesozoic regional paleothermal anomaly that trends roughly northsouth across Nevada (Cunningham, 1988) (Fig. 2.10).

In addition to the craton edge, three or four Precambrian crustal formation provinces that

underlie the Great Basin have been delineated using (1) initial Sr and Nd isotopic ratios in Late

Proterozoic, Mesozoic, and Tertiary granitoids and mafic rocks, (2) Pb isotopic compositions of

ores and Mesozoic and Cenozoic igneous rocks, and (3) granitoid crystallization ages (Zartman,

1974; Farmer, 1988; Livaccari and Perry, 1993; and especially Bennett and DePaolo, 1987,

figures 3 and 6) (Fig. 2.11). These Precambrian crustal formation provinces trend roughly westsouthwest—east-northeast, at a high angle to the western edge of the craton, and decrease in age

from northwest to southeast, reflecting a basement structure which suggests successive accretion

of increasingly younger Proterozoic-aged crust.

The tectonic grain of the Precambrian North American craton in the southwestern United States

generally trends east-west to northeast-southwest, at a high angle to the generally north-south to

northwest-southeast tectonic grain of Cordilleran orogen, and was most probably truncated by

the Late Proterozoic (0.8-0.5 Ga) polyphase rifting event (Hamilton, 1987; Burchfiel et al.,

1992). The youngest rocks having the older tectonic grain include the Belt Supergroup in the

United States, and its Canadian correlative, the Purcell Supergroup. Estimates for the age of the

Belt/Purcell rocks range from a low of 1.5-0.9 Ga to a high of 1.6-1.3 Ga, the better of the two

ranges being the latter (Burchfiel et al., 1992). In Nevada, Precambrian basement rocks occur

in the southernmost tip of the State and compose less than one-half of one percent (~0.4%) of

the total unit area mapped on the geological map of Nevada (Stewart and Carlson, 1978; Stewart,

1980). These units include Precambrian Y (~1.5-1.4 Ga) granitic rocks and Precambrian X

(~1.74-1.70 Ga) metamorphic rocks, and are part of a metamorphic terrain developed from

stratified and volcanic geosynclinal rocks that is perforated by anorogenic porphyritic rapakivi

granites (Stewart, 1980). The several known Precambrian tectonic events subsequent to the

formation of the crystalline basement rocks in the western United States are not recognized in

Nevada. Rocks that would show the effects of these events (i.e.—rocks equivalent in age to the

Belt/Purcell Supergroup) do not crop out in the State (Stewart, 1980).

2.5 Cenozoic Extensional Tectonism

The end of the Laramide orogenesis in the Cordillera of the western United States marked a

fundamental change in the style from compressional tectonism, which had dominated since the

Late Triassic, to local and regional extension and local strike-slip tectonism, which dominated

during the middle and late Cenozoic and is still ongoing today (Figs. 2.6, 2.7, and 2.8). Middle

and late Cenozoic extensional orogenesis and associated magmatism are responsible for the

present-day tectonomagmatic and structural configuration of the Great Basin, and developed in

two main stages: (1) an earlier (middle Tertiary, ~36-17 Ma) stage of calc-alkaline-dominated

magmatic activity, and rapid, extreme, and localized extension characterized by “thin-skinnedstyle” low-angle fault structures, including Cordilleran metamorphic core complexes; and (2) a

later (late Tertiary, ~17-0 Ma) stage of predominantly basaltic magmatism, and slow, moderate,

and more widespread extension characterized by high-angle block-fault structures (Zoback et al.,
14

1981; Eaton, 1982, 1984; Stewart, 1983; Wernicke, 1985; Rehrig, 1986; Hamilton, 1987; PatinoDouce and Humphreys, 1987). In the hinterland of the Sevier orogen, where the Great Basin was

to develop, magmatism and extension were time-transgressive, migrating southward and

westward across the region during the early through subsequent late stage of extensional

tectonism (Armstrong et al., 1969; Stewart 1980; Silberman 1985; White 1985; Gans et al, 1989;

Best and Christiansen, 1991; Seedorff, 1991).

2.5.1 Early Stage (Mid-Eocene to Late-Lower Miocene)

The early stage of extensional orogenesis is characterized by: (1) the renewal of vigorous,

predominantly silicic to intermediate calc-alkaline tuff and lava volcanism, which swept

generally west and southward; and (2) crustal thinning in the form of discrete, relatively narrow

and elongate, localized domains of large-magnitude extension in the ductile lower crust, which

include low-angle shallow listric and detachment faults in the upper crust, and locally the

exhumation of mid-crustal rocks in the form of metamorphic core complexes (Stewart, 1983;

Wernicke et al., 1987; Gans et al., 1989; Best and Christiansen, 1991; Seedorff, 1991; Catchings,

1992; also see Harry et al., 1993). The end of this stage of extensional orogenesis is

characterized by the waning of calc-alkaline magmatic activity, a decline in large-magnitude

extension, and by the transition to a passive transform continental margin, brought about by the

collision of the Pacific and North American plates in middle Oligocene and the initiation of San

Andreas transform system.

2.5.1.1 Magmatic Activity—Distribution, Age, and Composition

The Sevier hinterland had been magmatically and tectonically quiescent throughout most of the

late Paleocene and early Eogene. Arc magmatism in this region, which had thrived until about

80 Ma, had been extinguished, and a gap in the continuity of the arc created (Laramide magmatic

gap) as Laramide magmatism and deformation migrated eastward well onto the craton in

response to subhorizontal inboard subduction tectonics (Coney and Reynolds, 1977; Coney,

1978; Dickinson and Snyder, 1978; Keith, 1978; Dickinson, 1981; Berger and Bonham, 1990;

Armstrong and Ward, 1991; Hutchinson and Albers, 1992). Magmatism resumed in Idaho and

Montana north of the Sevier hinterland in earliest Eocene time, swept southward after 45 Ma

narrowing the Laramide magmatic gap, and entered the northern part of the Sevier hinterland in

Eocene time, about 43 Ma (Gans et al, 1989; Armstrong and Ward, 1991; Best and Christiansen,

1991). Magmatic activity continued a southward transgression during the Oligocene into a

southern part of the region along an arcuate roughly east-west front that was at a high angle to

the continental margin and on the order of hundred kilometers long (Elston, 1983; Wernicke et

al., 1987; Best and Christiansen, 1991). Coeval with the southernly sweep of magmatism

through Nevada and Utah, volcanism swept west and north out of southwest New Mexico across

southern Arizona, and by the beginning of Miocene time (~22 Ma), the northern and southern

magmatic fronts had merged, closing the Laramide magmatic gap, and a continuous arc trend

extended once again along the flank of the whole western Cordillera (Dickinson, 1981;

Dickinson and Snyder, 1978; Anderson, 1989; Armstrong and Ward, 1991). In southernmost

Nevada, the landward portions of the southerly- and northerly-migrating volcanic fronts

converged, but did not completely merge, resulting in an “amagmatic zone” extending from ~37o

to ~36o latitude trending nearly east-west (McKee, 1971; Blakely and Jachens, 1991). 
15

In the Sevier hinterland, magmatism was concentrated within the core and interior regions of the

present-day Great Basin, first appearing in east-central Nevada, or the north-central part of the

basin in Late Eocene to Early Oligocene time (~43-35 Ma) then generally swept south and

westward toward the subduction zone (Armstrong et al., 1969; Noble, 1972; Stewart and

Carlson, 1976; McKee, 1977; Smith, 1978; Tweto, 1979; Leeman and Fitton, 1989; Zoback et

al., 1981; Elston, 1983; Stewart, 1983; Schermer, 1985; Best et al., 1989). The initial phases of

magmatic activity overlapped the final period of waning subduction (Leeman, 1982; McKee,

1995) and increased in intensity throughout the latest Eocene and Oligocene (Berger and

Bonham, 1990; Leeman and Harry, 1993) (Fig. 2.8). Magmatic activity peaked between 31-20

Ma (Best and Christiansen, 1991) in a regional event known as the “great ignimbrite flareup”

(Noble, 1972), which was characterized by a widespread and dramatic increase in the volume

of ash-flow tuffs and pyroclastics extruded (Best and Christiansen, 1991). During this interval,

great thicknesses of volcanic materials erupted from nested calderas and formed northwest- to

west-northwest-trending troughs or belts in central Nevada (Anderson, 1989). Magmatism

waned after the ignimbrite flareup and a short lull in igneous activity was experienced in the

Early Miocene (~20-17) (McKee, 1977; Wernicke et al., 1987).

Magmatism was predominantly intermediate calc-alkaline in chemistry (McKee, 1995), also

referred to as silicic to intermediate in composition (Stewart and Carlson, 1978; Stewart, 1980;

Wernicke et al., 1987; Glazer and Ussler III, 1989; Hamilton, 1989; Best and Christiansen, 1991;

Leeman and Harry, 1993). Minor basaltic variants occurred locally (Leeman, 1982; Leeman and

Harry, 1993). Stewart and Carlson (1976, 1978) and Stewart (1980) categorized the Cenozoic

igneous rocks in Nevada into four broad temporal groups or “time-slices”:

43-34 Ma Characterized by andesitic to rhyolitic lava flows and shallow intrusives, local

voluminous silicic ash-flow tuffs, and granitic plutons. These rocks are

distributed across the cental, north-cental, and east-central parts of the State.

The 34 Ma lower boundary marks the approximate middle of the transition

from dominantly andesitic and rhyolitic lavas of the 43-34 Ma time period to

dominantly silicic tuffs of 34-17 Ma period (Stewart, 1980; Stewart and

Carlson, 1976; Thorman and Christensen, 1991). 

34-17 Ma Characterized by voluminous quartz latitic to rhyolitic ash-flow tuffs, sparse

basaltic lavas, and intermediate to granitic plutons. These rocks are distributed

in a broad arc, trending east-west across southern Nevada (just north of the

amagmatic zone) and southeast-northwest across central Nevada parallel to

the northeastern margin of the Walker Lane. The 17 Ma lower boundary falls

within a period of relatively little volcanic activity and marks a time of

significant tectonic change that eventually led to the development present-day

basin-range structure (Stewart, 1980; Stewart and Carlson, 1976; Thorman and

Christensen, 1991). 

17-6 Ma Characterized by basalt, bimodal rhyolite-basalt, high silica rhyolite, and

andesite volcanism and the emplacement of intermediate to granitic plutons.

These rocks are distributed in a crescent-shaped pattern that surrounds central

and east-central Nevada, trending east-west across southern Nevada (just north

of the amagmatic zone), southeast-northwest along (within) the Walker Lane,

and southwest-northeast in a board belt across northern Nevada. The 6 Ma

lower boundary marks the approximate end of widespread ash-flow activity

(Stewart, 1980; Stewart and Carlson, 1976; Thorman and Christensen, 1991).

6-0 Ma Characterized by scattered basaltic cinder cones and lava flows, and minor

rhyolite and andesite flows (Stewart, 1980; Stewart and Carlson, 1976;
16

Thorman and Christensen, 1991). These rocks occur locally along the Walker

Lane and in northwest, north-central, and south-central Nevada, but are

primarily distributed outside the borders of Nevada, around the margins of the

Great Basin (see Stewart and Carlson, 1976).

This scheme is based upon the gross composition, spatial distribution, and the time and tectonic

environment in which the igneous rocks were emplaced or extruded (see Stewart, 1980). It is

a useful way of representing the space-time and geologic aspects of these rocks in a GIS

environment.

2.5.1.2 Extensional Activity—Distribution, Age, Character, Magnitude

Extensional activity in the Cordillera began during the waning stages of the Laramide phase, well

before Laramide calc-alkaline intermediate arc volcanism had ended (Eaton, 1983; Elston, 1986;

Wernicke et al., 1987). Contemporaneous with the renewal of magmatism (Gans et al., 1989),

or shortly thereafter (Best and Christiansen, 1991), a pattern of relatively localized tectonic

extension began to develop throughout parts of the region in response to relaxation of Laramide

contractional stresses (Mutschler et al., 1987; Dewey, 1988; Christiansen and Yeats, 1992).

Extension began in the Pacific Northwest, and probably swept southward in the wake of the

southern sweep of Eocene-Oligocene magmatism (Best et al., 1989) where it became best

developed within four subregions, that include from north to south: (1) the Omineca extended

belt in northeastern Washington State and southern British Columbia; (2) the Rocky Mountains

Basin and Range in southwestern Montana, eastern Idaho, and northwestern Wyoming; (3) the

northern and southern Basin and Range provinces; and (4) the Rio Grande rift zone, which

extends north-south across central New Mexico north into Colorado (Wernicke, 1992, see figure

1).

Extension can be traced from early Eocene time in southern British Columbia as it migrated

south into the northern Cordillera of Washington, Idaho, and Montana, and then as it continued

through the central Cordillera and further south (Tweto, 1979; Zoback et al., 1981; Mutschler

et al., 1987; Wernicke et al., 1987; Gans et al., 1989). Extensional tectonism is first evident in

the geologic record at ~52 Ma in southern British Columbia, northern Washington, Idaho and

Montana (Eaton, 1983 ,Wernicke et al., 1987). In the northern Basin and Range, extensional

tectonism is recognized as early as the latest Eocene in southern Oregon and northern Nevada,

between about 38-35 Ma in the Great Basin, and at 29 Ma in the Rio Grande Rift (Eaton,1983;

Gans and Mahood, 1987; Wernicke et al., 1987; Gans et al, 1989). Extension in the Sevier

hinterland during the early stage of extension was concentrated within the core and interior

regions of the present-day Great Basin. The initial phases started in the middle to late Eocene

(~39 to 36 Ma) in east-central Nevada, or the north-central part of the present-day basin, and

migrated southwestward and slightly eastward into western Utah and central Nevada in early

Oligocene (Eaton, 1980, 1984; Rehrig, 1986; Wernicke et al., 1987; Gans et al., 1989; Hodges

and Walker, 1992). Extensional activity appears to have proceeded in two phases that were

separated by the great ignimbrite flareup, the period of peak volcanic activity that took place

from about 31 to 22-20 Ma (Best and Christiansen, 1991).

Extension was not uniformly distributed, rather, it occurred in relatively localized, elongate, and

narrow corridors or domains of high-magnitude rapid extension (Wernicke et al., 1987; Gans et

al., 1989; Seedorff, 1991; Catchings , 1992; Christiansen and Yeats, 1992; Wernicke, 1992).
17

The domains are arranged in “patchwork pattern” of belts, trending roughly north to northnortheast (Axen et al., 1993; Wernicke, 1990, 1992), and formed initially in a restricted region

within the Sevier fold-thrust belt, along the eastern margin of the then-developing Great Basin

(Eaton, 1982; Elston, 1984; Hamilton, 1987; Wernicke et al., 1987; Hodges and Walker, 1992;

Miller et al., 1992). Known extensional domains occur primarily in the eastern half of the

present-day Great Basin, however, they are probably much more widespread than is generally

accepted, especially in the north-central and western regions (Seedorff, 1991). The extensional

domains at present are roughly 30-100 km wide (Seedorff, 1991), and are structurally complex,

composed of fields of low-angle normal and listric faults, surficial detachment faults, and

include, but are not limited to, Cordilleran metamorphic core complexes (Dickinson, 1981;

Eaton, 1982; Stewart, 1983; Seedorff, 1991). Metamorphic core complexes are considered to

be among the most extreme and visible effects of crustal attenuation, and may represent local

sites where thermal weakening of the crust and an unusually shallow brittle-ductile transition,

the result of intense magmatic activity and/or upwelling of subcrustal mafic material, allowed

the middle and lower continental crust to be dragged out from beneath a fracturing, extending

upper crust (Rehrig, 1986; Lister and Davis, 1989; Armstrong and Ward, 1991).

Widening of the Cordilleran orogen in the western United States, which had begun during

Laramide- time when orogenic activity migrated east onto the craton, continued throughout the

early and late stages of Cenozoic extensional orogenesis. Crustal extension occurred primarily

in the Basin and Range and Rio Grande provinces (Elston, 1986). As a whole, the Basin and

Range Province approximately doubling in width (Hamilton, 1989), with the most dramatic

widening taking place in the Great Basin. Estimates of the average increase in width across the

present-day Great Basin over that of the original width of the region prior to extension varies

greatly: 10-35% (Stewart, 1978); 50% (Proffett, 1977); 55% (Smith et al., 1991); 50-85%

(Hauser and Lundy, 1989); 65-75% (Oldow et al., 1989; Wernicke et al., 1982); and as great as

100% (Hamilton and Myers, 1966; Elston, 1986, 1987; Rehrig, 1986). An average amount of

extension for the Great Basin as a whole, however, is difficult to determine because of the

differences in extensional style between the early or late stage of Cenozoic extensional

orogenesis (localized thin-skinned versus widespread brittle extensional structures), and because

of complications created by the structural overprint of late Tertiary extension. Leeman and Harry

(1993) noted that the amount of extension decreases from south to north, from about 100% at

the latitude of Las Vegas, Nevada, to 75% in central Nevada, to probably less than 40% in

southern Oregon. In domains of extreme extension (“hyperextended” regions), the amount of

extension can be in excess of 250% to 300-400% (Hamilton, 1987; Gans et al., 1989; Oldow et

al., 1989; Spencer and Chase, 1989; Wernicke, 1992). In absolute distance, the above estimates

translate to extension at the surface ranging from a low of in excess of 10 km (Wernicke, 1990)

to 50-100 km (Hamilton and Myers, 1966), and up to 250 km in hyperextended domains

(Wernicke, 1992). Bogen and Schweickert (1985) estimated the total east-west extension across

the Great Basin at 40o

 latitude to be 178±33 km.

Isolating the amount of extension in the Great Basin due specifically to either the early or late

stage of Cenozoic extensional orogenesis is also problematic (for the same reasons as stated

above). However, with the exception of the westernmost and southern Great Basin, the majority

of extension probably took place prior to the middle Miocene, during earlier “pre-basin and

range” extension, and was concentrated in local extensional domains, which include

metamorphic core complexes (Zoback et al., 1981; Patino-Douce and Humphreys, 1987;
18

Seedorff, 1991). Total middle Tertiary extension is estimated to be from 50%-80% to 100% on

average, and up to 200%, presumably in domains of hyperextension (Proffett, 1977; Wernicke,

1981; Zoback et al., 1981; Stewart, 1983; Speed et al., 1988). Speed et al. (1988) concluded that

most of the extension in the Great Basin took place in the eastern half, estimating 0-20%

extension for the western half of the basin, based upon an average elongation of 80%-100% for

the eastern half reported by Wernicke (1981) and corroborated by Phillip B. Gans in a 1986

personal communication (see Speed et al., 1988).

Strain rates were extremely high, on the order of 10-14/s and locally higher (Zoback et al., 1981;

Eaton, 1982; Gans et al., 1989). Maximum strain rates occurred at about 22-17 Ma and have

declined exponentially through time since the Miocene, probably as a result of gravitational

potential due to crustal thinning (Gans et al., 1989; Armstrong and Ward, 1991). The direction

of strain was generally normal to Laramide or older Cordilleran structural trends (Dickinson,

1981). Data based on preferentially orientated dike swarms and fault slip vectors indicate a

strikingly uniform west-southwest–east-northeast least principal stress orientation (the direction

of extensional movement) during the early stage of extension (Zoback et al., 1981).

2.5.2 Late Stage (Late-Lower Miocene to Present)

The late stage of Cenozoic extensional orogenesis (17-0 Ma) affected a much broader area than

did the earlier stage of orogenesis, developing over regions previously unaffected by extension

(Eaton, 1982; Stewart, 1983; Wernicke et al., 1987). During this extensional stage, the presentday Great Basin region experienced further widening and modern basin-range rift topography

(Fig. 1.2) was attained as the province was broken into blocks, generally 30 km across, where

continental sediments collected in the intermontane graben structures (Stewart, 1980 Eaton,

1982; Elston, 1986; Hamilton, 1987; Wernicke et al., 1987). The late stage of extensional

orogenesis is characterized by: (1) widespread and increasingly basalt-dominated bimodal

rhyolite-basalt lava volcanism (magmatism overall, however, was on the decline); (2) continued

regional uplift; and (3) crustal thinning in the form of upper crust brittle deformation by highangle, deep-penetrating block faulting, having moderate heaves and occurring at relatively slow

strain rates (Stewart, 1983, 1978; Zoback et al., 1981; Lipman, 1983; Eaton, 1982, 1984;

Mutschler et al., 1987; Wernicke, 1985, 1987; Hamilton, 1988). High-angle structures were

superimposed in part upon earlier low-angle extensional structures, strongly distorting,

dismembering, and rotating many preexisting geologic features (Stewart, 1983; Wernicke et al.,

1987; Hamilton, 1988; Seedorff, 1991). Large-magnitude low-angle thin-skinned faulting,

though considered typical of the earlier stage of extensional orogenesis and not the later, was

nonetheless not unusual (Proffett, 1977, 1989; Seedorff, 1991). Strike-slip faulting was locally,

and possibly regionally, most active during the late Tertiary than at any other time in the

Phanerozoic (see section 2.7 below) (Stewart, 1983; Stewart and Crowell, 1992). In the latter

phase of this extensional stage, basalt-dominated volcanism became more prevalent, although

magmatic activity as a whole declined to the point of magmatic quiescence. Extensional blockfaulting continued, but at greatly reduced strain rates (Eaton, 1982; Wernicke et al., 1987).

2.5.2.1 Magmatic Activity—Distribution, Age, and Composition

In the early Miocene (~22 Ma) the Laramide magmatic gap, which had been a prominent feature

in the central Cordillera since about 80-70 Ma, had closed as calc-alkaline arc activity was
19

renewed and volcanic fields to the north and south merged (Armstrong and Ward, 1991). Arc

volcanism was short lived, however, as the Mendocino triple junction migrated north and the San

Andreas transform evolved along the continental margin, progressively ending subduction

tectonics and extinguishing arc activity inland (Dickinson, 1981). By the Middle Miocene, the

northern and southern volcanic arcs were once again receding, as the southern end of the

Cascades arc retreated northward and the northern end of the Mexican arc withdrew southward

(Dickinson, 1981). At present, arc volcanism in the region of the central Cordillera is inactive,

but continues to the north in the Cascades range of Oregon and Washington states, flanking the

Juan de Fuca subduction trench, and to the south in the Trans-Mexico volcanic belt of the Sierra

Madre Del Sur in the southern Cordillera of Mexico, stretching along the middle America

subduction trench (Stewart, 1978; Dickinson, 1981). 

Late Tertiary magmatism in the Great Basin was generally concentrated along the outer regions

of the present-day basin, surrounding a core of more felsic-intermediate calc-alkaline volcanic

rocks (Smith and Luedke, 1984; Wernicke et al., 1987). The transition to fundamentally basaltic

and bimodal volcanism in the Great Basin took place between approximately 22-15 Ma (Mellott

and Hart, 1987), beginning in the southeast, near the axis of the province in areas undergoing

active extension, then moved northwest (Snyder et al., 1976; Wernicke et al., 1987). Widespread

ash-flow volcanism had ceased by about 20 Ma throughout most of the Great Basin (Best and

Christiansen, 1991), except in westernmost Nevada, where andesitic volcanism was voluminous

(Stewart, 1980), and was followed by a brief lull in magmatic activity between about 20-17 Ma

(McKee, 1977; Smith, 1978; Wernicke et al., 1987). By about 15 Ma volcanism had resumed,

but with a distinct switch to bimodal basalt-rhyolite compositions, and marked bimodal

tendencies after 14 Ma (McKee, 1977; Smith, 1978; Stewart, 1980; Elston, 1986, 1987;

Wernicke et al., 1987; Axen et al., 1993). A well-developed bimodal compositional spectrum

and “true basalt” (IUGS classification) did not appear, however, until after the latest Miocene,

about 8-6 Ma (Best and Christiansen, 1991).

Late Tertiary magmatism in the western Cordillera was most intense during its initial phases, and

was most profound in the Great Basin and its immediate environs, but relative to earlier middle

Tertiary activity, magmatism as a whole was on the decline (Eaton, 1982, 1984; Mutschler et al.,

1987; Armstrong and Ward, 1991; Leeman and Harry, 1993). During the last 10 m.y., the

intensity of silicic volcanism progressively decreased (Armstrong et al., 1969), and since about

6-0 Ma, volcanic activity in general has fallen sharply (Seedorff, 1991). Quaternary and present

day (1.5-0 Ma) magmatic activity is largely localized along the eastern and western edges of the

Great Basin, and in a north-northeast-trending belt into central Nevada (Mutschler et al., 1987;

Wernicke et al., 1987; Christiansen and Yeats, 1992). At the eastern edge of the basin, basaltic

volcanism has migrated onto the margin of the Colorado Plateau (Christiansen and Yeats, 1992).

Along the western margin of the basin, within the zone at the Sierran front, a long-lived volcanic

field climaxed in the Pleistocene with voluminous rhyolitic ash flows to form the Long Valley

caldera (Stewart, 1980; Christiansen and Yeats, 1992). In Nevada, only scattered volcanism has

occurred, mainly as basaltic cinder cones and lava flows with many local sources (Stewart,

1980). The present-day Great Basin is by and large magmatically quiescent (Seedorff, 1991).

2.5.2.2 Extensional Activity—Distribution, Age, Character, Magnitude

Basin-range block faulting began to develop during the earlier stage of localized large-magnitude
20

thin-skinned extensional activity when extension and uplift rates were at their maximum in the

Great Basin (22-17 Ma), and continued as strain rates declined after 17 Ma (Eaton et al., 1978;

Elston, 1986). Modern block-faulting first appeared around 22 Ma in the Rio Grande and Basin

and Range provinces (Elston, 1986) and probably entered the Great Basin around 20-17 Ma

(Eaton et al., 1978). Development of block-faulting in the Great Basin was erratic in space and

time, and probably not abrupt (Christiansen and Lipman, 1972; Zoback et al., 1981). Locally,

both large-magnitude low-angle thin-skinned faulting and high-angle block-faulting occurred

concurrently, and it is uncertain as to what degree the two structural styles represent differences

in, or a continuum of, middle through late Cenozoic crustal extension processes (Proffett, 1977,

1989; Zoback et al., 1981; Seedorff, 1991)

Basin-range block-faulting was largely confined to, and best developed in, the Great Basin

(Hamilton, 1988). Extensional deformation spread over a much broader region than it had

during middle Tertiary time, moving into regions that had not previously been affected by

extension, and widening areas that had already undergone extension (Stewart, 1983; Wernicke

et al., 1987). Basin-range-style faulting in the Great Basin began between 17-15 Ma with the

creation of local basins in which erosional debris collected (Eaton, 1982; Wernicke et al., 1987

Best and Christiansen, 1991; Bohannon and Parsons, 1995). Block-faulting and regional uplift

were most active from about 14-12 to 6-4 Ma (Eaton, 1986, 1987; Elston, 1986), but the presentday well-developed basin-range physiography and pronounced north-northeast–south southwest

structural and topographic grain (Fig. 1.2) was not attained before 10 Ma, at the earliest (Stewart,

1978; Zoback et al., 1981). Since about 6-5 Ma, faulting has proceeded at a much slower rate

and continues to the present-day (Zoback et al., 1981; Christiansen and Yeats, 1992). Quaternary

and present-day activity are largely localized along the eastern and western edges of the Great

Basin, in positions corresponding to the Walker Lane tectonic belt of western Nevada and the

Colorado Plateau transition zone (Smith, 1978; Mutschler et al., 1987; Wernicke et al., 1987;

Gans et al., 1989). On the western edge of the basin, the youngest initiation of extension was

westward from Death Valley to the Sierra Nevada front, where a Pliocene erosion surface is now

broken by five major ranges (Christiansen and Yeats, 1992). The Great Basin may be expanding

eastward and westward at the expense of the Colorado Plateau and the Sierra Nevada (Mutschler

et al., 1987; Wernicke et al., 1987) into the thicker and gravitationally unstable crust (Gans et

al., 1989).

Relative to the earlier middle Tertiary episode of extension, basin-range extension was much

more uniform in its distribution and fault structure. Late Tertiary crustal extension broke the

province into gently tilted blocks—horsts, grabens, and other tilted, rotated, and dismembered

units, generally 30 km across—bounded by steeply-dipping normal faults (Stewart, 1978; Zoback

et al., 1981). Block-faulting probably occurred in the upper 5-15 km of the crust (Eaton, 1980;

Zoback et al., 1981; Wernicke, 1985; Hamilton, 1987; Fitton et al., 1988) and took place on a

complex hierarchy of faults, including high-angle and listric faults (Zoback et al., 1981). Basinrange “mountain blocks” were created by the vertical movement of adjacent blocks along major

high-angle faults on one or both sides of the block, but faulting was not confined only to the

sides of mountain blocks, rather it was distributed throughout the mountain areas and in

suballuvial rocks of the valleys as well (Stewart, 1983). Major crustal blocks are aggregate

structural units, moving more or less in a uniform manner relative to adjacent structural units

(Stewart, 1983). Fault blocks may also have developed in groups of blocks. Shawe (1965)

observed that, surface faulting, related to seven major earthquakes in the past 60 years, is
21

distributed along a coherent arcuate linear zone known as the “Churchill arc”, located in westcentral Nevada (see Shawe's figure 1). The Churchill arc transgresses several mountain ranges,

demonstrating that the basin-range fault-block mountains in the Great Basin have probably not

grown independently of one another (Shawe, 1965).

Most of the widening of the Great Basin region took place during the later episode of slower

basin-range extension (Wernicke et al., 1987; Bohannon and Parsons, 1995), as opposed to

crustal extension, the majority of which was achieved during earlier middle Tertiary rapid

extension, and affected the whole of the Basin and Range Province (Zoback et al., 1981;

Seedorff, 1991). The amount of extension in the Great Basin due to late Tertiary basin-range

block-faulting has been difficult to determine because of the effects of earlier attenuation.

Stewart (1980) explained that, estimates on the amount of extension necessary to produce basinrange structure are generally of order of 10 to 35 percent of the original width of the region, but

varies considerably because of disagreements about the geometry of the high-angle normal faults

at depth. If the structure is considered to be composed of relatively simple horsts and grabens,

then estimates of the amount of extension suggest about 10- to 20-percent increase in the width

of the region, whereas if the structure is considered to be composed of blocks tilted along

downward-flattening faults, estimates range from 30 to 50 percent, to as much as 100 percent

locally. In absolute ground distance, conservative estimates of extension range from 72 km

(13%) to 50-100 km (8-18%) (Hamilton and Myers, 1966; Stewart, 1978).

The rate of strain at which time basin-range deformation developed was generally much lower

than during the earlier stage of extensional orogenesis, and has been declining since the Miocene

(Eaton, 1982; Armstrong and Ward, 1991). The regional stress field orientation in which basinrange strain initially took place (~17-15 Ma) was the same as that which had prevailed during

earlier extension (west-southwest–east-northeast least principal stress orientation) (Zoback et al.,

1981). By about 10 Ma, however, a 45o

 clockwise swing in the orientation of least principal

stress to west-westnorthwest–east-eastsoutheast occurred, at which point present-day “classic”

basin-range block-faulting began and continues to develop (Zoback et al., 1981). The Late

Miocene rotation of the regional stress field was most likely in response to the northward

migration of the Pacific plate and growth of the San Andreas transform boundary along the

western margin of the continent (Atwater, 1970, 1989; Zoback et al., 1981; Eaton, 1982; Ward,

1991).

2.6 Strike-Slip and Related Features

Wrench tectonism has been a major element in the development of the western Cordillera of the

United States (Stewart and Crowell, 1992), and has probably played a significant role in the

evolution of the Great Basin. Regional- and local-scale northwest-trending right-lateral and

northeast-trending left-lateral strike-slip faults are common in certain parts of the Great Basin,

and have offsets on individual faults as great as 80-100 km (Stewart and Crowell, 1992; also see

Stewart, 1983). Dip-slip faulting appears to be the predominant mode of deformation in the

Great Basin throughout the middle and late Tertiary stages of Cenozoic extensional orogenesis,

and is relatively well mapped and understood. Strike-slip faulting, however, is neither well

understood nor sufficiently mapped across the whole of the Great Basin (Stewart and Crowell,

1992).
22

Precambrian origins have been proposed for a number of regional-scale strike-slip tectonic zones

in the western Cordillera (see Fig. 2.3), including the Great Falls tectonic zone (see Hoffman,

1989), the Louis and Clark zone (Marshak and Paulsen, 1996, see figure 3; also see Stewart and

Crowell, 1992), the Snake River plain (Christiansen and Yeats, 1992; Poole et al., 1992), the

Walker-Texas lineament (Brookfield, 1993; Marshak and Paulsen, 1996, see figure 3), the

Mojave-Sonara megashear (Anderson, 1983; Dickinson, 1981; Marshak and Paulsen, 1996, see

figure 3, also see Stewart and Crowell, 1992, p. 624), and the Colorado lineament (Warner,

1978). However, evidence for such ancestry is usually circumstantial and conjectural, with the

possible exception of the Colorado lineament (Stewart and Crowell, 1992).

Paleozoic strike-slip systems in the western Cordillera are not well known, but those that have

been proposed are mostly right-lateral northwest-trending faults that are likely inland effects of

the collision of North America with South America-Africa during the Paleozoic OuachitaMarathon orogeny (see Stewart and Crowell, 1992). In the Rocky Mountains, 200 km of leftlateral Paleozoic offset has been proposed on a fault along the trend of the Snake River plain in

Idaho (see Fig. 2.3, however, Mesozoic displacement has also been suggested along the same

trend by other workers (Poole et al., 1977; Poole and Sandberg, 1977).

In the Upper Paleozoic and Early Mesozoic, a regional-scale strike-slip system is thought to have

been in operation along the western and southwestern margin of North America, which had

formed in response to a change in global patterns of continental drift and seafloor spreading that

accompanied the breakup of Pangea—the splitting of North America-Europe from South

America-Africa, and the opening of the North Atlantic between Africa and North America and

the Gulf of Mexico (Caribbean ocean basin) between North and South America (Dickinson,

1981; Suppe, 1985). The transition from Paleozoic sub-island arc compressional orogenesis

(Antler and Sonoma orogenies) to sub-continental Andean-style compressional orogenesis

(Cordilleran orogeny) is distinguished by the tectonic truncation of the Paleozoic structural and

stratigraphic trends of the southwestern Cordillera against the newly developing Mesozoic

continental arc (Saleeby and Busby-Spera, 1992; see Fig. 2.6). This second and last major

tectonic truncation episode in the history of the Cordilleran region (the first being preCordilleran, occurring between 0.8-0.5 Ga) is known as the “Early Mesozoic truncation event”

(Hoffman, 1989; Suppe, 1985). This event represents the reorientation of continental margin

deformation along a northwest-southeast trend that acutely crosscuts the roughly northeastsouthwest Paleozoic orogenic grain, and marks the initiation Cordilleran tectonic activity that

was dominated by continental margin volcanic and plutonic activity (Suppe, 1985). Paleozoic

orogenic belts that might reasonably have been expected to continue on the western side (the

Pacific side) of the Cordilleran orogenic belt are nowhere to be found. They are postulated to

have been displaced southward across Mexico along the Mojave-Sonora megashear (Silver and

Anderson, 1974), a controversial strike-slip system extending between southern California and

the Gulf of Mexico, roughly along the U.S.-Mexican border (Dickinson, 1981, see figures 4 and

5; also see Saleeby and Busby-Spera, 1992; Suppe, 1985). Truncation, and presumably

movement on the megashear, appears to have begun in Pennsylvanian time, when a northwesttrending left-lateral strike-slip zone formed across the Antler orogenic belt and adjacent

miogeocline along east-central and southern California. Rocks of the Antler belt were displaced

southward toward the Mojave region during Pennsylvanian-Permian time along a transcurrent

zone that may have extended through the Mojave region and into Sonora, Mexico, where other

miogeoclinal strata are suspected to have been displaced southward (see Saleeby and Busby-
23

Spera, 1992, figures 1 and 2). Movement of the megashear continued throughout mid-Early and

mid-Late Jurassic time, and by the late Jurassic, truncation of the Paleozoic tectonic grain was

well established (Dickinson, 1981; Suppe, 1985). Up to 800 km of left-lateral movement has

been postulated along the Mojave-Sonora megashear (Burchfiel et al., 1992).

A few Mesozoic or early Cenozoic strike-slip systems have been recognized inland of the

Mesozoic western margin of North American, the most significant in the Great Basin being the

Wells fault of Nevada, along which 65 to 80 km of right-lateral offset has been proposed, and

the Tintic Prince, Inez, and Leamington right-lateral faults in western Utah (see Stewart and

Crowell, 1992). North of the Great Basin, the Olympic-Wallowa lineament (see Fig. 2.3),

manifest as a zone of discontinuous northwest-trending faults and folds in the overlying late

Cenozoic cover (i.e.—Columbia River Plateau basalts), may be early Mesozoic to early Cenozoic

in age, although it might have genetic connections with a Proterozoic or Mesozoic cratonal

margin (Mann and Meyer, 1993; also see Stewart and Crowell, 1992).

Most of the recognized strike-slip activity in the Great Basin appears to be late Cenozoic in age

(see Stewart and Crowell, 1992). After about 20 Ma, the central Cordilleran region became

progressively influenced by a right-lateral stress regime as the plate tectonic setting changed from

a convergent margin to a complex margin characterized by convergence in its northern part and

transform faulting along the San Andreas system in its southern part (Atwater, 1970, 1989; also

see Stewart and Crowell, 1992). By 12.5-10 Ma, the old plate margin fell apart with the onset

of major strike-slip faulting and the opening of the Basin and Range (Ward, 1991). Five late

Cenozoic strike-slip systems have been recognized in the Great Basin (Stewart and Crowell,

1992, see references therein if not otherwise noted):

1. Strike- or oblique-slip reactivation—Inactive preexisting faults (or systems) that were reactivated with a

strike-slip component in response to changes in the orientation of local or regional stress fields.

2. Oblique-slip components on dip-slip faults—Oblique-slip faulting related to variations in strike of

extensional faults, producing normal movement on segments perpendicular to a regional extension directions

and oblique-slip on segments oblique to extension direction.

3. Strike-slip accommodation-zone systems—Strike-slip faulting orientated parallel to extension direction that

occurred along a boundary between areas that differ in the amount, rate, or sense of normal faulting. Such

systems represent “zones of accommodation” between regions differing in kinematic behavior, and should

typically be expected to form along transverse zones separating regions with different tilt directions of basinrange blocks.

4. Walker Lane belt—A northwest-trending structural zone 700 km long and from 100 to 300 km wide

containing both right-lateral and left-lateral faults. Also known as the Walker Lane shear zone, this regionalscale strike-slip system is a complex zone composed of structural sections (large structural blocks as well as

relatively narrow zones) that have undergone either right-lateral shear, left-lateral shear, or remained

relatively undeformed by shear. The Walker Lane separates the Sierra Nevada region to the west from areas

of typical basin-range topography on the east, and is far and away the largest and most significant known

strike-slip feature in the Great Basin (see Fig. 2.3). The Walker Lane may have origins dating back to the

Mesozoic (Stewart, 1988), perhaps related to the Mojave-Sonora megashear (see Saleeby and Busby-Spera,

1992, figures 1 and 2), or to the Precambrian, perhaps related to late Proterozoic rifting (Blakely and

Jachens, 1991, also see section 2.4). It is currently active.

5. Regional conjugate system—A widespread system of conjugate north- to northwest-trending right-lateral

faults and northeast-trending left-lateral faults. Most are in the western, southern, and eastern regions of the

Great Basin, and, at least in southwestern Utah, the strike-slip faults are great in number, but generally short

in length. This system of strike-slip faults may be related to the same stress field that has produced

extensional basin-range structures, suggesting that normal faulting and strike-slip faulting are intermixed and
24

contemporaneous. Shawe (1965) proposed that this broad conjugate strike-slip system provides the

fundamental structural framework of the Great Basin, and that these strike-slip structures in general probably

lie within the upper mantle, penetrating clear through the crust, although some might be blind. When

movement along these structures took place, near-surface rocks (the upper part of the crust) responded by

faulting into horsts and grabens, en echelon to the underlying or nearby shears. The orientation of the crustal

blocks were controlled by the established “grain” of northerly oriented folds and foliations, developed at

deeper structural level during earlier periods of deformation (Shawe, 1965). Carey (1955, 1958, 1976, 1988)

and Livaccari (1979) have made similar proposals from the perspective of regional-scale tectonism. A

review of structural patterns and basin formation in relation to strike-slip faults by Christie-Blick and Biddle

(1985) details how such a mechanism could produce basin-range structure as observed in the Great Basin.

Putnam and Henriques (1991) have interpreted structural relationships observed in the Pinon Range in terms

of strike-slip tectonism, and have extended this interpretation to the regional-scale suggesting the presence

of northwest-oriented wrench-fault system in northeastern Nevada.

In addition to these five systems, the circumference of the Great Basin is bound by what appear

to be late Cenozoic strike-slip zones that act to accommodate differential extension in a manner

analogous to oceanic transform faults (Christiansen and Yeats, 1992). The faults composing

these zones vary in trend and history, and their positioning may have been guided by older

preexisting structures (Christiansen and Yeats, 1992). The northern boundary is wide, diffuse,

and not well defined, and is mainly composed of structures related to the eastern Snake River

plain volcanic trend and Brothers fault zone (Fitton et al., 1991; Christiansen and Yeats, 1992).

Evidence for strike-slip movement on the fault, inferred to be under Miocene Snake River

volcanic cover, can be demonstrated by offset of the Sevier thrust system, and indicates left- and

right-lateral movements (Poole et al., 1977; Poole and Sandberg, 1977; Poole et al., 1992). The

western strike-slip boundary of the Great Basin can be considered to be Walker Lane shear zone

(Stewart and Crowell, 1992). The southern Great Basin is bound by left-lateral features that

include the Lake Mead fault zone in Nevada and the Garlock fault in California, where the trace

of the Garlock fault may in part follow an old rifted margin of Precambrian continental crust

(Kistler and Peterman, 1978). The eastern margin of the Great Basin is not well delineated by

strike-slip features, although a conjugate system composed of many short strike-slip faults does

occur in southwestern Utah (Stewart and Crowell, 1992).

2.7 Characteristics of the Present-Day Crust

The crust in the Great Basin is a complex collage of sedimentary and tectonomagmatic provinces

that were tectonically assembled, transformed, and molded together into a single geologic and

physiologic entity by a nearly continuous cycle of orogenesis dating from the Archean through

the Holocene. Virtually every type of sedimentary, igneous, and metamorphic rock is

represented in the lithologic assemblages of crust in the Great Basin and its environs (King,

1969). The crust in Nevada and the Great Basin is relatively thin as compared to surrounding

regions, and is characterized by (1) low regional Bouguer gravity anomaly, (2) a roughly even

distribution of shallow- and deep-source magnetic anomalies, (3) high regional heat flow, (4)

high regional electrical conductivity, (5) low seismic velocities in the crust and upper mantle,

and (6) ongoing seismic activity (Blackwell, 1978; Eaton et al., 1978; Lachenbruch and Sass,

1978; Stewart, 1978; Eaton, 1982; Allmendinger et al., 1987; Dewey, 1988; Speed et al., 1988;

Wernicke et al., 1988; Morgan and Gosnold, 1989; Blakely and Jachens, 1991; Christiansen and

Yeats, 1992; Wernicke, 1992; also see Harry et al., 1993) (Fig. 2.12).
25

2.7.1 Lithotectonic Framework and Tectonostratigraphy

The Great Basin straddles the edge of the Precambrian craton, and as such, can be subdivided

into two principal crustal provinces: (1) an eastern region of latest Proterozoic to Late Devonian

and younger sequences that are underlain by older supracrustal rocks and North American

Precambrian crystalline craton; and (2) a western Paleozoic and Mesozoic accretionary belt

(Burchfiel et al., 1992; Poole et al., 1992) (Figs. 2.4 and 2.10).

In the eastern region of the Great Basin, the Precambrian basement consists of two main crustal

blocks: (1) the Archean (>2.7 Ga) Wyoming province (a “core” province), underlying the

northeasternmost part of the basin, and (2) juvenile 2.3-2.0 to 1.8-1.7 Ga accreted terranes of the

Yavapai-Mazatzal orogen, underlying the most of the eastern, southeastern, and southern areas

of the basin (Bennett and DePaolo, 1987; Hoffman, 1989) (Fig. 2.11). Precambrian basement

in the Great Basin extends as far west as central Nevada. Supracrustal sedimentary and volcanic

rocks ranging in age from about 1.4 to 0.9-0.8 Ga, presumably related in part to Middle-Late

Proterozoic rifting that formed the western margin of the North American craton, occur locally,

and unconformably overly the crystalline basement rocks (Poole et al., 1992, see figure 2).

Latest Proterozoic to Late Devonian and younger miogeoclinal successions unconformably onlap

both the supracrustal and basement rocks. In the Sevier hinterland and foreland fold-thrust belt

of east-central Nevada and west-central Utah, mountain ranges are underlain mostly by an upper

Precambrian to Triassic sequence of sandstones, shales, and carbonates, which include about 20

distinctive and regionally extensive formations with an aggregate thickness in excess of 13 km

(Stewart and Poole, 1974; also see Poole et al., 1992, figure 2).

The western accretionary belt (Fig. 2.4) is composed of material that was successively obducted

on and accreted to the Precambrian craton margin from the Late Devonian through Middle-Late

Jurassic time, mainly during the Antler, Sonoma, and Nevadan orogenies (Fig. 2.5). This

material includes eugeoclinal-miogeoclinal sequences flanking the continental margin, oceanic

island arc terranes and related forearc sequences, scraps of oceanic basins with ophiolitic

basement, fragments of continental margins (some with Precambrian basement), and other

adjoining oceanic and continental environments (Silberling, 1991; Burchfiel et al., 1992; Jones

et al., 1982). The terranes are mainly of oceanic affinity and most have somewhat uncertain

origins (Burchfiel et al., 1992). A few western accreted terranes have been identified as “native”

and can be tied directly to North America, while a small number of others appear to be “exotic”

and have travelled far from other parts of the Pacific Basin (Burchfiel et al., 1992). These

terranes were rafted into place, welded to the laterally expanding continental margin, modified

during younger accretionary events, and reworked in nonaccretionary plate-boundary systems

(Burchfiel et al., 1992).

The Foothills suture in the Sierra Nevada is a major accretionary structure that separates terranes

that attached to the passive margin of North America before Middle-Late Triassic time

(i.e.—Antler and Sonoma accretionary activity) from those that arrived during the later active

margin phase (i.e.—Nevadan and later accretions) (see Speed, 1983; Speed et al., 1988). East

of the foothills suture in the Great Basin, three major accretionary terranes are exposed (Speed,

1983):

1. Roberts Mountains allochthon—Accretionary prism of the Antler magmatic arc (formed as it converged on
26

North America); consists of a tectonic assemblage of pelagic, hemipelagic, turbiditic, and volcanic rocks

of early Paleozoic age and probable oceanic derivation; laps over lower Paleozoic strata of the North

American continental shelf at least 130 km from the sialic edge and was almost certainly emplaced from the

west early in Mississippian time.

2. Golconda allochthon—Accretionary prism of the Sonomia magmatic arc (formed as it converged on North

America); possesses similar rocks and architecture to the Roberts Mountains allochthon, except that the rocks

are Mississippian to Permian age; emplaced in Early Triassic time at least 100 km inboard of the sialic edge

and above the earlier Roberts Mountains allochthon and the late Paleozoic and Early Triassic cover to the

Roberts Mountain.

3. Sonomia—Sonoma magmatic arc; believed to be a lithospheric fragment of Paleozoic arc-related

lithotectonic units, surmounted by a Permian magmatic arc; collided with the edge of sialic North America

early in the Triassic; the central regions of Sonomia are deeply buried below thick Triassic flysch and

continental arc volcanics that succeeded Sonomia.

A cryptic fourth terrane (Antleria) has been postulated, but is nowhere exposed (Dickinson,

1981; Speed, 1983).

In the western Great Basin, between the Sierra Nevada and Nevada-Utah border, rocks at the

surface can be assigned seven major lithotectonic units (Fig. 2.13), two of which are comprised

by the Roberts Mountains and Golconda allochthons, and Sonomia (Speed et al., 1988):

1. Precambrian crust plus parautochthonous cover—Cover may be as young as Triassic in the eastern Great

Basin; this unit has probably maintained near-coherency through Phanerozoic time although probably

deformed by major extension in late Precambrian, contraction in Mesozoic, and extension in Cenozoic time.

2. Displaced Paleozoic oceanic terranes—Composed of the Roberts Mountains and Golconda allochthons;

overlies the western ~150 km of Precambrian sialic North America; Early Triassic and older attachment.

3. Displaced Paleozoic terranes—Sonomia; probably of arc and oceanic origin; probable Early Triassic

attachment to North America; overlain and intruded by Mesozoic arc rocks.

4. Parautochthonous Mesozoic cover to and intrusions in displaced Paleozoic terranes—Triassic and Jurassic

strata of mainly basinal but locally shelfal affiliation; Mesozoic sedimentary and igneous rocks that were

deposited or intruded near their present sites; magmatic rocks, including the Sierra Nevada batholith, of

probably arc affinity and latest Triassic through Late Cretaceous ages; everywhere allochthonous with respect

to North America on the Fencemaker and Luning thrusts.

5. Mainly autochthonous Mesozoic cover to and intrusions in Precambrian sial, sial cover, and older displaced

terranes— Mainly autochthonous Mesozoic rocks that probably or certainly formed above or in the

Precambrian crust or its parautochthonous cover; chiefly contains shelfal strata and rhyolite of Triassic ages

that were deposited on the Golconda allochthon.

6. Other displaced terranes—Consists of displaced and suture-related terranes of Mesozoic melange or of arc

affiliation; Late Triassic or younger attachment.

7. Quaternary to upper Cretaceous cover—Cover to all other units; consists of sedimentary and volcanic rocks

that are as old as Late Cretaceous in California and Oregon, and Tertiary farther east.

The terranes of unit 6 are separated from those of units 1-5 by the Sierra Nevadan Foothills

suture. The Sonoma magmatic arc terrane (lithotectonic unit 3) has recently been further

subdivided by Silberling (1991) into six or more subterranes based on various temporal,

compositional, depositional, deformational, and stratigraphic relationships (see Silberling, 1991,

figure 1).

2.7.2 Crustal Thickness

The present-day thickness of the lithosphere (crust plus upper mantle) is at least 65 km (probably
27

closer to 80 to 100 km thick), as compared to 100-110 km thick beneath the Sierra Nevada and

Colorado Plateau, and approximately 80 km thick beneath the Colorado Plateau margin transition

zone (Thompson and Zoback, 1979; Eaton, 1980; Smith et al., 1989; Hendricks and Plescia,

1991; also see Leeman and Harry, 1993). At 40 Ma, prior to extension, the lithosphere may have

been considerably thicker, possibly as thick as 115 to 175 km (assuming a uniform vertical strain

distribution of a net 75% extension) (Leeman and Harry, 1993). The present-day crustal

thickness in the Great Basin ranges from 25 to 35 km in thickness (Fig. 2.12), averaging 30 km,

as compared to the 40-50 km thick crust beneath the adjacent Colorado Plateau and Sierra

Nevada, and especially in comparison to its thickness in the Cretaceous, when the crust may have

exceeded 60 km as a result of crustal shortening and thickening during Sevier and Laramide

compressional orogenies (Allenby and Schnetzler, 1983; Allmendinger et al., 1987; Dewey,

1988; Benz et al., 1990; Catchings and Mooney, 1991).

The crust is thickest in the center of the Great Basin, where the reflection Moho is encountered

at approximately 34 km (Allenby and Schnetzler, 1983; Allmendinger et al., 1987). Crustal

thickness generally decreases away from the central Great Basin, thinning eastward to

approximately 25 km at the Wasatch front in Utah (Allenby and Schnetzler, 1983), and westward

to < 30 km at the eastern front of the Sierra Nevada (Allenby and Schnetzler, 1983;

Allmendinger et al., 1987). The largest area of thin crust (~20-22 km) is in the northwestern

Great Basin, in the region of the “Battle Mountain heat high anomaly” (see section 2.7.5), near

the Carson Sink (Allenby and Schnetzler, 1983). Beyond the western and eastern margins of the

Great Basin, crustal thickness increases rapidly to about 50 km under the Sierra Nevada and >

40 km under the Wasatch range (Allenby and Schnetzler, 1983; Allmendinger et al., 1987).

Most recent data, however, indicates that the thin crust of the southern Great Basin in the region

of the Sierra Nevada eastern front may actually extend under the eastern High Sierra as well,

where it is estimated to be 30-40 km thick (Wernicke et al., 1996). North and northeast of the

Great Basin the crust thickens to >40 km towards the Idaho Batholith. In the northwestern

United States, an axis of crustal thinness (<30 or 25 km) extends roughly north from the Carson

Sink area to central Washington State (Allenby and Schnetzler, 1983). The axis of crustal

thinness also trends southeast from the Carson Sink area, following along the Walker Lane shear

zone where the crust is <30 km, and extends into southernmost Nevada (just northwest of Las

Vegas) where the crust may approach only ~20-22 km thick (Allenby and Schnetzler, 1983). The

~20-22 km thick area is relatively small in geographic extent and occurs in the “amagmatic zone”

(see McKee, 1971; Blakely and Jachens, 1991).

2.7.3 Gravity Anomaly and Crustal Structure

The Great Basin is situated in a regional gravity low (Eaton et al., 1978) (Fig. 2.12). Regional

Bouguer gravity at wavelengths greater than 1000 km indicate that the dominant, first-order

feature in the Great Basin and adjacent regions to the east, is an enormous anomalous low (less

than -200 mGals), and reflect sources within the pre-Tertiary basement (Kane and Godson, 1989,

see plate in back pocket; Blakely and Jachens, 1991).

At residual Bouguer gravity wavelengths of less than 250 km, the Great Basin is characterized

by predominantly northwest trending high-low-alternating anomalies, which extend roughly the

length of the State of Nevada, and are approximately 100 km wide with an approximate

amplitude of 200 km (Kane and Godson, 1989, see plate in back pocket). A series of four,
28

possibly four and a half or five, high-low anomaly pairs are visible as standing-wave-sets that

“roll” across California, where they are very well defined, through Nevada, where they become

increasingly less defined as the anomalies enter the Great Basin, and into Utah, where the

anomalies break up and end abruptly against the Wasatch Range (eastern margin of the Great

Basin). The progressive degradation in the definition of these anomalies across the Great Basin

is probably due to late Lower Miocene basin-range extension, characterized by block-faulting,

and may indicate that the anomalies were established at least before the onset of regional-scale

block-faulting (~10 Ma). On the scale of Nevada, in the center of the Great Basin, one and a half

cycles of the 250 km wavelength anomaly has been referred to as an “axis of bilateral symmetry”

or “the butterfly”, reflecting higher topography and thicker crust in this region (Eaton et al.,

1978). 

Simple Bouguer gravity features in Nevada are bounded by an orthogonal system of mostly

northeast- and northwest-trending lines, suggesting that major crustal blocks of differing density

may be bounded by northeasterly and northwesterly structural zones. Some of these hypothetical

structural zones are also visible in total field magnetic anomaly data in Nevada (Shawe, 1991,

see figures 2 and 3). The major negative anomalies of the 250-km-filtered and 1000-km-filtered

Bouguer gravity data suggest that these anomalies represent low density material at depths

extending from the crust-mantle boundary to >125 km (Mutschler et al., 1992; Parsons et al.,

1994).

2.7.4 Geomagnetic Anomaly and Crustal-Scale Structures

The magnetic character of the Great Basin is variable. On a continent-wide scale, the magnetic

anomaly is relatively fine-grained to flat. There are no conspicuous anomalies in comparison to

areas north (Cascades Range, Oregon Plateaus, Snake River Plain), west (Sierra Nevada),

southeast (Colorado Plateau), or east (Laramide uplifts), with the exception of the Walker Lane

anomaly(-ies), which range from about 400 to -100 nT (Kane and Godson, 1989, see plate in

back pocket). On a regional-scale (Great Basin), the northern Nevada rift zone and the Walker

Lane are the most prominent features.

The northern Nevada rift is visible as long, narrow, and well defined magnetic anomaly “splays”

trending north-northwest, the longest of which (the Northern Nevada rift proper) extends at least

280 km through the north-central part of the State (see Fig. 2.3). The rift is characterized by a

linear swarm of dikes and associated lavas (basalt and basaltic andesite) of 17 to 14 Ma age, by

a fault-bounded trough, and by a northwest-southeast topographic grain (most visible in shaded

relief) which cuts obliquely across the north-northeast–south-southwest basin-range structural

grain (see Fig. 1.2) (Christiansen and McKee, 1978; Zoback and Thompson, 1978; and Zoback

et al., 1994; also see Christiansen and Yeats, 1992). The anomaly source-depth of northern

Nevada rift zone appears to be both shallow and deep, possibly extending as deep as 15 km

(Blakely and Jachens, 1991; also see Christiansen and Yeats, 1992). Stewart et al. (1975)

suggested that this feature might represent the southern extension of the Brothers fault zone (see

Fig. 2.3), which is a 240 km long zone of short en echelon shears younger than 6 Ma (Stewart

et al., 1975; Christiansen and Yeats, 1992). Together, they form the Oregon-Nevada lineament

(or the Orvada rift), which has been interpreted to be the surface expression of a deep-seated

fracture zone that may have had a complex history of strike-slip and transcurrent movement

(Stewart et al., 1975; Smith, 1978; Livaccari, 1979). The northern Nevada rift, and the other
29

mega-scale linear features in the western Cordillera (see Fig. 2.3), may be the surface

manifestations of deep crustal or mantle-rooted zones of weakness, and have been inferred by

Carey (1976, 1988), Shawe (1965), and Livaccari (1979) to be shear-related features.

The Walker Lane is marked by arcuate, northwesterly-trending anomalies. The width of the

anomaly pattern as a whole is considerably wider than that of the belt, and extends in some

places over 150 km north-northeast of the Walker Lane into topography with north to northeast

trends more typical of the Basin and Range province (Blakely and Jachens, 1991).

2.7.5 Heat Flow and Crustal Fluid Circulation

The Great Basin has anomalous heat flow (Fig. 2.12). The “reduced” heat flow values for the

Great Basin (those for which radiogenic heat production in the crust has been accounted for) are

greater than those for stable regions by as much as 50% to 100%, and in warmer regions of the

basin, by as much as 300% (Eaton, 1982). An average for the whole of Basin and Range

Province is given by Morgan and Gosnold (1989) as 2.7 HFU and is likely higher in the Great

Basin when taken alone. Mean heat flow values in the Sierra Nevada (1.3 HFU) and the

Colorado Plateau (1.6 HFU) provinces, which are marginal to the Great Basin (see Fig. 2.2), are

more typical of stable continental lithosphere, indicating an unusually high geothermal gradient

beneath the basin (Morgan and Sass, 1984; Morgan and Gosnold, 1989). These heat flow

observations, together with the widespread and voluminous occurrence of Mesozoic and

Cenozoic igneous rocks, suggests that elevated temperatures have persisted within the crust for

at least 230 m.y. (Eaton, 1982; Elston, 1984). These conditions probably facilitated fluid

circulation at many crustal levels in the Great Basin region, creating an environment favorable

for wide-spread mineralization.

Much of the present-day anomalous heat is believed to have been transferred into the crust by

convection from below (Eaton, 1982). Heat flow appears to be highest in north-central and westcentral Nevada, in a region known as the “Battle Mountain heat flow high”, coincident with an

area of thin (20 km) crust. In northern regions, heat flow averages about 2.8 HFU as opposed

to an average of approximately <1.7 HFU for the southeastern part of the State (Hittelman et al.,

1990; David D. Blackwell, 1994, personal communication). The temperature of geothermal well

and spring waters is also higher in the northern regions, averaging 60o

C, as compared to an

average of 30o

C in the southeastern part of the State (Trexler et al., 1983).

2.7.6 Electrical Conductivity and Crustal Permeability

Electrical conductivity is anomalously high in the Basin and Range Province in contrast to

regions east and west. The extent of a probable regional conductivity high is nearly centered on

the Great Basin (see Keller, 1989, figure 35), and is coextensive with the Great Basin regional

gravity low. Anomalously high values of conductivity are observed at depths as shallow as 10

km in the crust, and profound lateral changes in the electrical properties of the upper mantle

(very strong anomalies) are present at depths as much as 350 km—the roots of the Rocky

Mountains and the Wasatch Front in particular appear to be reflected by important electrical

structures extending to more than a depth of 300 km (Keller, 1989). These conductive zones

may be caused either by partial melting of crustal rocks in regions of high heat flow, or by the

presence of significant amounts of fluid in fractured rock (Keller, 1989). Partial melting is
30

probable for the deep-source anomalies whereas deeply circulating fluid is likely for the shallow

conductors in the crust. In the upper crust, permeability is relatively high and fluids are normally

at or near hydrostatic pressure (McCaig, 1988). Calculations indicate that with permeabilities

of 10-17 m2

 or greater, fluids can freely circulate under a normal geothermal gradient, suggesting

that fluid movement through upper crustal fault zones is common (Nesbitt and Muehlenbachs,

1989). Permeabilities on the order of 10-16 m2

 have been observed to depths of 10 km in the Kola

deep drill hole in the Soviet Union (Kozlovsky, 1987). Permeabilities in excess of 10-17 m2

 are

typical in the upper crust, especially in areas that have undergone even minor amounts of strain,

such as the Great Basin (Brace, 1984). An increase in strain from 1% to 6% is capable of

producing an increase in permeability of five orders of magnitude or more (Spiers and Peach,

1989), suggesting deep circulating fluids in the crust of the Great Basin are especially common

place. This was probably critical to the metallogenic development of Great Basin.

2.7.7 Seismicity and Seismic Velocities

Seismic activity is ongoing in the Great Basin. It is generally restricted to the eastern and

western margins of the basin, and to an arcuate linear zone known as the “Churchill arc” located

in west-central Nevada (Shawe, 1965, see figure 1; Smith, 1978; Christiansen and Yeats, 1992).

Seismic data compiled by Hittelman et al. (1990), detailing earthquake activity over the past

century in the southwestern United States, indicate that the greatest amount of activity occurs

along the southwestern border of the State, which corresponds to the Walker Lane shear zone.

Although greatly concentrated, this activity is of relatively low magnitude (typically 3.5-3.0

Richter magnitude) and shallow depth (8 km). The higher magnitude (4.0 Richter magnitude)

and deeper (20 km) earthquakes occur in the north-central and southeasternmost part of Nevada

where activity is less frequent and not as concentrated (or data are sparse).

As a whole, the crust and upper mantle in the Great Basin are characterized by relatively low

average seismic velocities as compared to the eastern and mid-continental U.S., the northwestern

U.S., and the adjacent Sierra Nevada to the west (Beghoul and Barazangi, 1989; Braile et al.,

1989, see figures 4 and 5; Iyer and Hitchcock, 1989; Benz et al., 1990; Catchings and Mooney,

1991; Hearn et al., 1991). Seismic velocities show strong dependencies on temperature,

lithostatic pressure, and pore pressure (Christensen and Wepfer, 1989; Morgan and Gosnold,

1989). Velocities increase with increasing lithostatic pressure and decrease with increasing

temperature, pore pressure, and porosity. The greatest increases occur over the first 100 MPa

and are attributed to the closure of microcracks (Christensen, 1989; Christensen and Wepfer,

1989). Upper crustal seismic velocities are often lower than those predicted because the upper

crust is penetrated by regional-scale fractures. Data from the Soviet deep Kola hole suggest that,

at least locally, fractures and fluids may be pervasive throughout much of the upper crust,

indicating that velocities through such regions will tend to be lower than theoretical velocities

(Fountain and Christensen, 1989). As such, the low seismic velocities in the crust of the Great

Basin (for example, ~7.5 km/sec P-wave; Smith et al., 1989) may be due to pervasive fluid-filled

fracture or shear systems at depth (see Catchings, 1992), which most likely are products of

crustal extension.
31

Chapter 3. Characteristics and Distribution of

Sedimentary and Volcanic Rock-Hosted

Deposits

3.1 Introduction

Nevada is known as the “Silver State”, but is a major contributor to world gold production.

Nevada has a long and varied history of gold mining. Much of the early gold production (pre1965) was related to volcanic rock-hosted “epithermal”, high-grade silver-gold vein deposits.

Since 1965, sedimentary rock-hosted “Carlin-type”, bulk-mined, lower-grade disseminated

deposits have become the principal gold producers. Nevada has a total historic gold production

(1849-1996) of approximately 95.4 million Troy ounces (derived from Elevatorski, 1981, and

Meeuwig, 1995, 1997). In 1995, the United States produced 10,170,000 troy ounces of gold,

second only to South Africa, which produced 16,800,000 ounces (Anonymous, 1997). Nevada

produced 6.8 million troy ounces of gold worth $2.6 billion in 1995, which amounted to 65%

of all gold produced in the United States and about 10% of all gold produced in the world

(Meeuwig, 1996). Just over 56% of this gold came from a small number of sedimentary rockhosted open-pit mines that occur along the well known Carlin mineral trend (see Fig. 1.3). Total

published gold resources for Nevada, including mineable reserves and perhaps sub-economic

deposits, totalled 144,000,000 ounces at the end of 1995 (Meeuwig, 1996). 

The Great Basin is a Tertiary-age, sedimentary rock-hosted plus volcanic rock-hosted, preciousmetal deposit-type metallogenic province (Wilkins, 1984). Precious metals are recovered from

a wide variety of deposit-types that occur in nearly every rock type of any age (Koschmann and

Bergendahl, 1968; Thorman and Christensen, 1991; Ludington et al., 1993). The greatest

amount of gold and silver in Nevada is recovered as primary-products from sedimentary rockhosted disseminated, volcanic rock-hosted disseminated and vein, and a number of other

epithermal vein and breccia deposit-types. By-product gold and silver are principally recovered

from porphyry-related, skarn- and mantos-related, and a few volcanogenic massive sulfide

deposit types (Wilkins, 1984; Thorman and Christensen, 1991; Ludington et al., 1993). The age

of mineralization ranges from about middle Mesozoic to late Tertiary, with the bulk of the

primary- or co-product gold-silver mineralization having occurred in the Tertiary (Wilkins, 1984;

Cox et al., 1991; Hutchinson and Albers, 1992; Ludington et al., 1993). The abundance of

precious metal occurrences in the Great Basin is attributed to its complex pre-Tertiary tectonic

history and unique Tertiary tectonomagmatic setting, both of which produced a variety of

favorable sedimentary and volcanic host lithologies, a high degree of structural permeability, and

widespread hydrothermal activity (Shawe and Stewart, 1976; Eaton, 1984; Barton, 1990; Berger

and Bonham, 1990; Seedorff, 1991; Hutchinson and Albers, 1992).

This chapter presents a summary of the sedimentary “Carlin-type” and volcanic “epithermal”

rock-hosted deposits, with emphasis placed on their spatial distribution. The characteristics and

distribution of sedimentary and volcanic rock-hosted occurrences are given in sections 3.2 and

3.3, respectively. The regional-scale distribution of mineral occurrences with respect to mineral

trends and crustal terranes is reviewed in sections 3.4 and 3.5, respectively. A number of

quantitative studies dealing with regional-scale distribution of mineral occurrences in relation
32

to mineral trends and crustal terranes are summarized in section 3.6.

Comprehensive reviews on mineral deposits and metallogenesis in the Great Basin can be found

in Berger and Bethke (1985, and papers therein), Berger and Bonham (1990), Berger and Eimon

(1982), Berger and Henley (1989), Cox et al. (1991), Cunningham (1988), Eaton (1984), Heald

et al. (1987), Hedenquist et al. (1996), Hutchinson and Albers (1992), Ludington et al. (1993),

Percival et al. (1988), Raines et al. (1991, and papers therein), Schafer et al. (1988, and papers

therein), Seedorff (1991), Singer (1996), Tooker (1985, and papers therein), Vikre et al. (1997,

and papers therein), and Wilkins (1984).

3.2 Sedimentary Rock-Hosted Gold Deposits

3.2.1 Characteristics

The sedimentary rock-hosted deposits are variable in character and not well understood, but can

generally be defined as sub-microscopic or microscopic gold, generally much less than 1 micron

in size, that occurs within the crystal structure of pyrite, which is disseminated in variably

silicified, argillized, and decalcified thin-bedded, flaggy, mixed carbonate-siliciclastic rocks of

Paleozoic age (Cox and Singer, 1986; Peters et al., 1996; Romberger, 1986; Tooker, 1985).

Most deposits contain both unoxidized (primary) and oxidized ores. Primary ores are subdivided

into five gradational sub-types and constitute the bulk of ore at most of the deposits

(e.g.—Carlin, Jerritt Canyon, Mercur, Getchell). Oxidized ores are economically more desirable

than unoxidized ores and are mined at most deposits, but of principal importance at only a few

deposits (e.g.—Alligator Ridge) (for more information on ore types, see Radtke et al., 1980;

Bagby and Berger, 1985; Romberger, 1986; Percival et al., 1988). The salient geologic features

of sedimentary rock-hosted deposits are summarized as a synoptic model presented in Table 3.1.

3.2.2 Distribution

The sedimentary rock-hosted deposits are predominantly located in the northeastern quadrant of

Nevada, and are distributed in a roughly equant circular or ovoid pattern (Fig. 3.1). They are

conspicuously absent from the Walker Lane belt (see Figs. 1.3 and 2.3), which is host to many

volcanic rock-hosted deposits, and from the southern, western, and easternmost regions of

Nevada as well. As a deposit-type group, the sedimentary rock-hosted deposits form a

distribution “core” that lies inward of a semicircular “crescent-shaped” distribution of various

volcanic rock-hosted deposits, referred to by Ludington et al. (1993) as the “epithermal crescent”

(compare distribution patterns in Fig. 3.1). As a group, the sedimentary rock-hosted deposits are

generally considered to be older than the volcanic rock-hosted (Fig. 3.2; Ludington et al., 1993).

Most of the sedimentary rock-hosted deposits in Nevada occur in six groups or trends: the Carlin

trend, the Battle Mountain–Eureka (Cortez) trend, the Getchell trend, the Humboldt trend, the

Independence group, and the Alligator group (Fig. 3.3) (Roberts, 1966; Shawe and Stewart,

1976; Percival et al., 1988; Thorman and Christensen, 1991). A small number of other deposits

occur as isolated mineral concentrations. Well developed trends are geologically characterized

by alignments of igneous rocks, geophysical anomalies (or boundaries), and various structural

features (Bagby and Berger, 1985; Percival et al., 1988; Shawe, 1991). The two most prominent
33

trends are the Carlin and the Battle Mountain–Eureka (Cortez), which are defined by the greatest

number and the largest deposits. These trends are oriented north-northwest, oblique to the

northerly to north-northeasterly grain of Paleozoic, Mesozoic and Cenozoic structures (Fig. 3.3).

The Carlin trend is 75 km long, and marked by sedimentary rock-hosted deposits (the “Carlin”

type deposits) (Fig. 3.3). It extends from the Dee deposit in the northwest to the Emigrant

Springs, Gnome, and Trout Creek deposits in the southeast (Thorman and Christensen, 1991).

The trend is geologically characterized by an alignment of (1) structural windows (or “fenster”)

eroded through overlying allochthonous units, (2) intrusive rocks (although they are relatively

sparse), (3) positive aeromagnetic anomalies, and (3) broad antiforms or synforms adjacent to

some deposits (Bagby and Berger, 1985; Madrid and Bagby, 1986; Shawe, 1991). 

The Battle Mountain–Eureka belt (Cortez trend) is 235 km long, and marked by gold skarn

deposits associated with hypabyssal stocks, vein and stockwork deposits, and disseminated types

(Fig. 3.3; Shawe, 1991). The trend was once identified by the alignment of the Gold Acres,

Cortez, Horse Canyon, and Tonkin Springs deposits (Bagby and Berger, 1985), but now extends

northwest to include the Lone Tree and Stonehouse deposits, and to the southeast to include the

Easy Junior-Night Hawk and Green Springs deposits (Thorman and Christensen, 1991; Singer,

1996). The trend was originally identified by Roberts (1966) as part of the Battle

Mountain–Eureka mineral belt. Its trace is highlighted by a distinct linear feature visible on

LANDSAT imagery of north-central Nevada, and it is geologically characterized by an alignment

of (1) several sets of northwest-striking en echelon high-angle faults, (2) structural windows in

the Roberts Mountain allochthon, (3) Mesozoic and Tertiary intrusive rocks, (4) aeromagnetic

anomalies, and (4) northwest-directed regional folds at a number of deposits (Bagby and Berger,

1985; Shawe and Stewart, 1976; also see Percival et al., 1988). The Battle Mountain–Eureka

(Cortez) trend is also subparallel to a long, narrow, and well-defined geomagnetic anomaly high

that is believed to delineate the regional-scale northern Nevada rift zone (see Blakely and

Jachens, 1991; Christiansen and Yeats, 1992; also see section 2.7.4; Fig. 2.3).

The Getchell trend is 50 km long, but unlike the Carlin and Battle Mountain–Eureka (Cortez)

trends, it is oriented northeast-southwest, approximately parallel to the structural grain of the

Great Basin (Fig. 3.3). It is largely localized along the eastern margin of the Osgood Mountains

and marked by disseminated and vein gold deposits. It was originally defined by the alignment

of Getchell, Pinson, and Preble deposits (Bagby and Berger, 1985), but now extends to the

northeast to include the Twin Creeks (Chimney Creek and Rabbit Creek) deposit, and to the

southwest to include the Kramer Hill deposit (Thorman and Christensen, 1991). The

southwestern end of the trend coincides with northwestern end of the Battle Mountain–Eureka

(Cortez) trend, at approximately the Marigold and Kramer Hill deposits. The deposits along the

trend are characterized by intrusive rocks, and north–northeast-trending faults and anti- and synforms (Bagby and Berger, 1985; Kretschmer, 1991; Hall et al., 1997; also see Tooker, 1985

Percival et al., 1988).

The Humboldt “trend” is 35 km long, and is delineated by the Standard and Relief Canyon

sedimentary rock-hosted deposits (Percival et al., 1988; Percival also included the Florida

Canyon deposit just north of the Standard deposit (Fig. 3.3). In the version of MRDS used in this

study, Florida Canyon is classified as a sedimentary rock-hosted deposit, but Singer et al., 1996,

classify it as a hot spring deposit in pre-Tertiary sedimentary rocks). If the Fondaway Canyon
34

deposits to the south in the Stillwater Range are included, the length of the trend increases to 80

km. Percival et al. (1988) commented that the Humboldt trend is neither a well defined nor a

recognized linear feature, but is a convenient way to refer to the deposits that occur along the

Humboldt Range. The trend is characterized by thrusting, and antiformal structures occur at

some deposits, both of which are controls at a number of the deposits (Percival et al., 1988). In

addition, the Humboldt Range is dissected by numerous, predominantly north-trending, highangle faults, which are significant structural features with respect to the localization of

mineralization (Percival et al., 1988).

The Independence group is 40 km long, and marked by sedimentary rock-hosted deposits (Daly

et al., 1991) occurring throughout the Independence Mountains (Fig. 3.3). The group is defined

by the rough alignment of deposits extending from the Jerritt Canyon (Saval Canyon) and Burns

Basin deposits in the southern part of the range to the Wood Gulch deposit in the northern part

of the range (Thorman and Christensen, 1991). East–west- through north–south-trending faults,

the Roberts Mountain thrust fault, and regional east-west trending antiforms occur at some

deposits (Bagby and Berger, 1985). The deposits do not appear to have an obvious linear

controlling structure (Seedorff, 1991), and there are not enough data to determine how, or if, they

are related (Thorman and Christensen, 1991). It has been suggested by Clark et al. (1985) that

the late Eocene Bull Run normal fault may extend under the Independence range.

The Alligator group is a broad region of sedimentary rock-hosted deposits that extends 60 km

northeast-southwest from the Bald Mountain district in the north to the Illipah district in the

south, and extends even further (up to 90) if the group overlaps with the deposits at the

southeastern end of the Battle Mountain–Eureka (Cortez) trend (Fig. 3.3; Thorman and

Christensen, 1991). There is no known or apparent structure that collectively controls the

distribution of these deposits. However, some of these deposits, particularly the Alligator Ridge

deposit, are associated with gentle north–northwest-trending antiformal structures and strike-slip

faults (Romberger, 1986; Nutt, 1997; also see Percival et al., 1988). Plutonic rocks occur in the

Bald Mountain district (Hitchborn et al., 1996; Nutt, 1997).

3.3 Volcanic Rock-Hosted Gold-Silver Deposits

3.3.1 Characteristics

Volcanic rock-hosted deposits have been extensively studied and are generally better understood

in comparison to sedimentary rock-hosted deposits (e.g.—Buchanan, 1981; Berger and Eimon,

1982; Dreier, 1984; Berger and Bethke, 1985, and papers therein; Hayba et al., 1985; Tooker,

1985, and papers therein; Mosier et al., 1986; Panteleyev, 1986; Bonham, 1988; Berger and

Henley, 1989; Berger and Bonham, 1990; Hedenquist et al., 1996). These deposits can be

generally defined as “volcanic centered” vein and/or disseminated gold-silver deposits that

formed at relatively shallow (within ~1 km) crustal levels. Volcanic rock-hosted deposits can

be broadly subdivided into two types: low-sulfidation (LS; “Adularia-sericite”) and highsulfidation (HS; “Acid-sulfate”). Hedenquist et al. (1996) and Hedenquist and Lowenstern

(1994) equate LS deposits to geothermal environments and HS to volcanic-hydrothermal. LS

deposits significantly outnumber and are generally more important than the HS in Nevada.

Examples of important LS deposits in Nevada include Comstock, Tonopah, and Round
35

Mountain; HS include Paradise Peak and Goldfield. The salient geologic features of volcanic

rock-hosted deposits are summarized in Table 3.2.

3.3.2 Distribution

The volcanic rock-hosted deposits are distributed across Nevada in a distinctive semi-circular

“crescent-shaped” pattern, dubbed the “epithermal crescent” by Ludington et al. (1993), which

encompasses the roughly circular distribution pattern of the sedimentary rock-hosted occurrences

(Fig. 3.1; Ludington et al., 1993). The “epithermal crescent” trends northeast–southwest across

northern Nevada, arcs to the southeast in the vicinity of west-central Nevada, and extends

northwest–southeast parallel to the southwestern boarder of Nevada. The majority of the

volcanic rock-hosted deposits occur within the northwesterly-trending portion of the “epithermal

crescent”, which is coincident with the Walker Lane shear zone (Blakely and Jachens, 1991; Cox

et al., 1991; Hutchinson and Albers, 1992). The volcanic rock-hosted deposits are conspicuously

absent from the roughly circular area in the northeast quadrant of Nevada that hosts nearly all

of the sedimentary rock-hosted deposits, and, with a few notable exceptions in north-central and

northeastern Nevada (i.e.—Getchell trend and Independence group deposits), the sedimentary

rock-hosted deposits do not occur within the region of the volcanic rock-hosted deposit

distribution pattern (Cox et al., 1991). As a group, the volcanic rock-hosted deposits are

generally considered to be younger than the sedimentary rock-hosted (Fig. 3.2; Ludington et al.,

1993).

In contrast to the well-defined linear mineral trends formed by the sedimentary rock-hosted

deposits, the volcanic rock-hosted deposits show only weak regional-scale continuous

alignments, and have been grouped together into broad mineral belts (Fig. 3.3; also see Roberts,

1966; Shawe and Stewart, 1976). These volcanic rock-hosted deposit mineral belts are generally

not as clearly defined by geophysical, geochemical, or other geologic features as the sedimentary

rock-hosted mineral trends. These belts commonly contain deposit types and districts other than

volcanic rock-hosted (e.g.—polymetallic). With respect to mineral belts and trends in general,

the best defined deposit alignments occur in eastern Nevada, and principally consist of

sedimentary rock-hosted deposits, whereas the least well-defined are all farther west, and these

are mainly defined by volcanic rock-hosted deposits (Shawe and Stewart, 1976). Like most of

the sedimentary rock-hosted deposit trends, the volcanic rock-hosted deposit belts are oriented

oblique to the structural grain of the Great Basin (Fig. 3.3). The main volcanic rock-hosted

mineral belts include the Beatty-Searchlight, Aurora, Walker Lane (Virginia City-Tonopah),

Fallon-Manhattan, Lovelock-Austin, and the northern extent of the Lynn-Railroad. 

The Beatty-Searchlight belt is 325 km long, and marked by polymetallic-vein, Comstock-vein,

and quartz-vein epithermal gold-silver-bearing deposits (Fig. 3.3). It extends from about the

Tokop-Hornsilver district areas in the northwest to the Searchlight district in the southeast. The

belt lies en echelon to, and is subparallel with, the Walker Lane shear zone and presumably

follows a related fracture system (Roberts, 1966).

The Aurora belt is 75 km long, and marked by hot spring and Comstock-vein deposits (Fig. 3.3).

It extends from the Borealis and Aurora district areas in the northwest to the Tip Top deposit in

the southeast. The belt is situated in and parallels the structure of Walker Lane shear zone, and

is associated with a northwest–southeast-trending alignment of geomagnetic highs.
36

The Walker Lane (Virginia City-Tonopah) belt is 350 km long, and is marked by quartz-alunitevein, Comstock-vein, and hot spring deposits (Fig. 3.3). It extends from Peavine-WedekindComstock district areas in the northwest, through the Camp Douglas and Gilbert districts, to the

Tonopah, Divide, and Goldfield districts in the southeast (Fig. 3.1). This belt is rather broad, but

it is one of the more productive and better defined volcanic rock-hosted deposit belts. The

mineral belt lies within the Walker Lane shear zone, and it is collinear with a northwestsoutheast alignment of geomagnetic highs, a broad isostatic gravity high, and a Bouguer gravity

gradient “ridge”.

The Fallon-Manhattan belt is 200 km long, and is marked primarily by Comstock-vein deposits

(Fig. 3.3). It extends from the Sandsprings-Fairview district areas in the northwest, through the

Broken Hills district, to the Round Mountain, Manhattan, and Tybo districts in the southeast.

The mineral belt is associated with a topographic discontinuity that coincides with the

northeastern margin of the Walker Lane shear zone (Shawe and Stewart, 1976; see Fig. 3.3).

Many of the ore deposits in this belt have been localized by west–northwest-trending faults, and

regional aeromagnetic anomalies suggest the presence of west–northwest-oriented geologic

trends (Shawe and Stewart, 1976).

The Lovelock-Austin belt is 230 km long, and is marked by Comstock-vein and low-sulfide

quartz-gold-vein deposits to the north, and polymetallic-vein and sedimentary rock-hosted

deposits to the south (Fig. 3.3). It extends from about the Rosebud and Sulfur districts in the

northwest, past the Farrel, Seven Troughs, and Trinity districts, through the Rochester district,

to the Reese River polymetallic district and the Birch Creek district in the southeast. The Birch

Creek district is host to the Austin Gold Ventures sedimentary rock-hosted deposit. This mineral

belt is sub-parallel with two long, narrow, and relatively well-defined geomagnetic anomaly

highs that flank to the west the geomagnetic expression of the Northern Nevada rift zone (see

Blakely and Jachens, 1991; Christiansen and Yeats, 1992; also see section 2.7.4; Fig. 2.3).

Lynn-Railroad belt is 200 km long, and is marked by the Comstock-vein and hot spring deposits

to the north, and by the Carlin trend sedimentary rock-hosted deposits to the south (Fig. 3.3).

It extends from about the National district in the northwest, through the Spring City, Gold Circle,

and Hollister-Ivanhoe deposits, to the southeastern end of the Carlin trend deposits (Emigrant

Springs-Gnome-Trout Creek area). The northern part of the belt is sub-parallel to a regionalscale northwest–southeast-trending Bouguer gravity gradient that separates an area of low gravity

to the east from an area of higher gravity to the west. The southern portion of the Lynn-Railroad

belt is addressed above in section 3.2.2 as the sedimentary rock-hosted deposit “Carlin trend”.

The Shoshone-Jarbidge and Tuscarora-Spruce Mountain mineral belts are very weak alignments

based on few widespread deposits. The Shoshone-Jarbidge belt, which is the only mineral belt

that trends roughly parallel to the structural grain of the Great Basin, is 220 km long, and extends

from the Jarbidge Comstock-vein district in the northeast to the Shoshone Range southeast of

the town of Battle Mountain, Nevada. This belt is host to major deposits of barite, and lies 15

km southeast of the northeast-striking Cambrian-Devonian Cordilleran geosynclinal depositional

trend (as identified by Roberts, 1966, p. 50). The Shoshone-Jarbidge belt also hosts a number

of deposits where it intersects other northwest–southeast-trending mineral belts—the northern

Carlin trend deposits at the intersection of the Lynn-Railroad belt; the Lewis, Bullion, and Hill

Top district deposits (various deposit-types) at the intersection of the Battle Mountain-Eureka
37

(Cortez) trend. The Tuscarora-Spruce Mountain mineral belt is 180 km long, and extends from

the Good Hope and Tuscarora Comstock-vein districts in the northwest, through several silver

and base-metal districts, to the Spruce Mountain polymetallic replacement deposit in the

southeast (Roberts, 1966).

3.4 Regional-Scale Distribution with Respect to Mineral

Trends and Belts

Mineral trends are commonly defined as district-scale linear alignments of similar type and age

ore deposits, whereas mineral belts are broad regional-scale deposit alignments, which may

include deposits of various types and ages (Seedorff, 1991; also see Roberts, 1966; Shawe and

Stewart, 1976). In general, the sedimentary rock-hosted deposits are regarded in terms of trends

whereas volcanic rock-hosted deposits occur in belts (see section 3.2.2 and 3.3.2). For

sedimentary rock-hosted deposits, it is observed that:

& The mineral trends are generally well developed and defined, and are geologically characterized by alignments

of igneous rocks, geophysical anomalies (or discontinuities), and various high- and low-angle dip-slip and

strike-slip structural features.

& The deposits occur in six trends or groups: the Carlin trend, the Battle Mountain–Eureka (Cortez) trend, the

Getchell trend, the Humboldt trend, the Independence group, and the Alligator group.

& The Carlin and the Battle Mountain–Eureka (Cortez) are the two most prominent trends, hosting the greatest

number and the largest deposits.

The Carlin and the Battle Mountain–Eureka (Cortez) trends are oriented north-northwest, oblique to the

northerly to north-northeasterly grain of Paleozoic, Mesozoic and Cenozoic structures.

For volcanic rock-hosted deposits, it is observed that:

& The mineral belts are broad, show only weak regional-scale continuous linear alignments, and are generally

lacking in distinct alignments of geologic and geophysical features (with the exception of the Walker Lane shear

zone as a whole, which is characterized by northwest–southeast-trending strike-slip and normal faults, and by

arcuate, northwesterly-trending aeromagnetic anomalies.

& The deposits occur mainly in seven belts: Beatty-Searchlight, Aurora, Walker Lane (Virginia City-Tonopah),

Fallon-Manhattan, Lovelock-Austin, and Lynn-Railroad (northern extension).

& The principal deposits occur in the Walker Lane shear zone (a structural and magmatic belt), which comprises

the Beatty-Searchlight, Aurora, Walker Lane (Virginia City-Tonopah), and Fallon-Manhattan mineral belts.

Most of the mineral belts are oriented oblique to the structural grain of the Great Basin

The term “trend” will be used from here on as a general reference to alignments of any mineral

deposit types.

The mineral trends are not well understood. Some hypotheses suggest that the trends follow

zones of crustal weakness, such as deep-seated fracture, shear, or suture zones, which in some

cases might be tectonically reactivated structures inherited from the underlying Precambrian

basement (Roberts, 1966; Shawe and Stewart, 1976; Guild, 1985; Madrid and Bagby, 1986;

Hutchinson and Albers, 1992; Arehart et al., 1993; Maher et al., 1993; Grauch et al., 1995;

Rodriguez, 1997; Wooden et al., 1997). Others have speculated that deposit alignments parallel

major lithotectonic and tectonomagmatic structures, such as terrane or facies boundaries and

volcanic fronts (Roberts, 1966; Shawe and Stewart, 1976; Woodward, 1984; Bryant and Nichols,

1988; Hutchinson and Albers, 1992; Grauch et al., 1995). Wallace (1991) proposed that some
38

mineral trends in the Great Basin may be products of post-mineralization deformation. He

suggested that the Getchell trend deposits, for example, may have originally formed along

north–northwest-striking structures (parallel to the Battle Mountain-Eureka (Cortez) and Carlin

trends), but that the present-day northeast-southwest distribution of the deposits reflects exposure

by post-mineralization extension and movement along faults that strike oblique to the mineral

trend.

Although there has been much interest in mineral trends, there has only recently been a concerted

effort in northeastern Nevada to determine the physical nature of these apparent deposit

alignments (see numerous papers in Vikre et al., 1997). The paucity of studies on the origins of

mineral trends in the Great Basin, or a consensus as to what these features actually represent, has

not discouraged continued speculation on the subject. Indeed, many researchers have argued for

the existence of deep-seated regional-scale crustal features, and frequently include such

structures as an integral part of their metallogenic models to explain the distribution and genesis

of volcanic, and in particular, sedimentary rock-hosted deposits in the Nevada Great Basin. 

Hutchinson and Albers (1992) noted that many latter Tertiary (22-0 Ma) gold and silver

epithermal veins appear to be lineament-controlled by regional cryptic fracture systems, which

is reflected by the linear alignment of numerous deposits or intrusions, often marked by parallel

veins systems within individual deposits, and is strongly indicative of throughgoing fracture

systems at the time of mineralization (also see Dreier, 1984; Berger and Bonham, 1990). This

is probably especially true for mineralization within the Walker Lane belt, where Cox et al.

(1991) suggested that mid-Tertiary and later shear zone deformation may have provided the

tectonic conditions that permitted the formation of volcanic-hosted epithermal deposits. 

The Carlin trend sedimentary rock-hosted deposits are coincident with alignments of igneous

bodies and structural windows, and as a mineral trend, it transects the northerly trend of the

Paleozoic tectonic fabric, suggesting that a regional, probably deep-seated, structural control was

important for the formation and spatial distribution of the these deposits (Madrid and Bagby,

1986; Shawe, 1991). Cunningham (1988) recognized that many of the major sedimentary rockhosted deposits are located along the eastern margin of the older regional paleothermal anomaly,

in close proximity to the western margin of the buried Precambrian craton and the Roberts

Mountain Thrust (see Cunningham, 1988, figure 1, 2, and 4). In addition, the Carlin trend and

the western limit of strongly extended domains in the Great Basin largely coincide with the

western margin of the buried Precambrian craton, and Seedorff (1991) commented that many

deposits, although lying outside of domains of extreme extension, are proximal to these areas.

Evidence for deep-crustal regional-scale structural control for the alignment of sedimentary rockhosted and other types of deposits along the Battle Mountain-Eureka (Cortez) mineral trend

appears to be much stronger than it is for the Carlin trend (Christensen, 1995; also see Maher et

al., 1993; Grauch et al., 1995; Wooden et al., 1997). Percival et al. (1988) noted that, like the

Carlin trend, the Battle Mountain-Eureka (Cortez) trend is in part defined by an alignment of

structural windows developed within the Roberts Mountain allochthon and by Mesozoic and

Tertiary intrusive rocks, but in addition, it is further defined by aeromagnetic anomalies, several

sets of north–northwest-striking, en echelon, high-angle faults, which define a zone that

obliquely transects the pervasive north-northeast Basin-Range structural fabric, and by a distinct

linear feature that is visible in Landsat imagery of north-central Nevada. Furthermore, the Battle
39

Mountain-Eureka (Cortez) mineral trend lies near-parallel and in close proximity to the central

northern Nevada Rift magnetic anomaly and to a gradient ridge (or a geophysical discontinuity)

observed in the regional Bouguer and isostatic gravity anomalies (Shawe, 1991; Grauch et al.,

1995; also see Blakely and Jachens, 1991; Christiansen and Yeats, 1992; sections 2.7.3 and

2.7.4; Fig. 2.3). Collectively, these features have been interpreted to delineate a major crustal

discontinuity—a fundamental regional-scale crustal structure—which controls the linear

distribution of deposits along the trend (Shawe, 1991; Arehart et al., 1993; Grauch et al., 1995).

Arehart et al. (1993) suggested that if such structures are responsible for the alignment of

sedimentary rock-hosted deposits along the Battle Mountain–Eureka (Cortez) and Carlin mineral

trends, then they must represent fairly long-lived and deep features in the crust (possibly related

to the Precambrian craton margin) which controlled thermal and mineralizing processes on a

regional scale through geological time (also see Roberts, 1966; Shawe, 1977; Speed, 1983;

Woodward, 1984; Madrid and Bagby, 1986; Bryant and Nichols, 1988; Armstrong and Ward,

1991; Hutchinson and Albers, 1992).

Given the complex pre-Cenozoic tectonomagmatic history of the southwestern United States,

the development of an extensional regime in the Cenozoic, and widespread occurrence of

mineralizing processes that led to the formation of precious- and base- metal deposits across the

province, it is not unreasonable to consider that the geometries and component structures of the

Great Basin crust and its boundary regions may have had a bearing on the origin of the trends,

or apparent trends (if the structures do indeed exist) (Shawe and Stewart, 1976; Cunningham,

1988; Seedorff, 1991; Hutchinson and Albers, 1992; Arehart et al., 1993). The presence of

regional-scale crustal structures could facilitate ore deposit formation in a number of ways (see

sections 2.7.6 and 2.7.7), some of which include:

Increasing throughgoing fluid pathways, thereby promoting whole-crustal circulation of nearsurface and deep fluids (increasing lower crust/upper mantle permeability and allowing the

release and mixing of deep crustal fluids with upper crustal fluids).

Assisting in the vertical and lateral transference of heat, whereby warmer lower crustal fluids

and melts could more easily penetrate and migrate to higher crustal levels.

Increasing the country-rock surface area on which circulating fluids could react and leach

(scavenge) metals; occurs at many crustal levels.

Contributing to shallow crustal ground preparation, whereby physical and chemical changes

increase the suitability of potential host rock for mineralization (an increase in permeability,

wallrock area, ore traps, etc.).

Focusing, concentrating, and directing ore-bearing hydrothermal fluids, ultimately serving

to control the localization of intrusions, volcanic fields, high-level thermal anomalies,

meteoric hydrothermal cells, and individual deposits.

Shawe (1991) commented that the great linear extent of the Battle Mountain-Eureka (Cortez) and

Carlin sedimentary rock-hosted mineral trends, the large number of deposits in each trend, and

the evidence for a large vertical range of gold deposition, indicate that extremely large

hydrothermal systems were concentrated along these trends (also see Thorman and Christensen,

1991). Shawe (1991) suggested that regional-scale (much larger than individual deposits),

deeply-penetrating crustal structures controlled emplacement of magmas into the upper crust,

guided dispersal of hydrothermal solutions and gases derived from the intrusions or formed from

heated ground waters, and shattered upper crustal rocks along the extent of the trends, providing
40

local permeable zones favorable for solution flow and precipitation of gold ores (also see

Cunningham, 1988; Berger and Henley, 1989; Arehart et al., 1993; Maher et al., 1993; Grauch

et al., 1995). The same is probably also true with regard to the formation of volcanic rock-hosted

deposits. Berger and Bonham (1990) noted that deeply rooted rifting is a key element in the

genesis of epithermal systems within any volcanic terrane. In the western United States, they

suggested that Mesozoic and Cenozoic epithermal deposits were formed in the near-surface zone

above through-going, deep-seated fracture systems, which were formed or reactivated in an

evolving plate tectonic framework, and that these fracture systems controlled the emplacement

of magmas and served as conduits for magmatic and meteoric fluids. It has been suggested by

Rowley (1996) that regional transverse zones of strike- and oblique-slip in the Great Basin

controlled the localization of some Oligocene to Early Miocene calderas (e.g.–Caliente; also Big

Ten Peak, Jefferson (Toquima), and Round Mountain; Byron R. Berger, 1998, personal

communication), and in turn, the formation of epithermal gold districts (also see Shawe, 1995).

In addition, it is likely that the role which such crustal structures may have played in controlling

mineralizing processes and/or the distribution of ore deposits was enhanced by the Cenozoic

extensional setting of the Great Basin (Guild, 1985).

3.5 Regional-Scale Distribution with Respect to Crustal

Terranes

Roberts (1966) subdivided the Great Basin into two metallogenic provinces, consisting of an

eastern base-metal province, with peripheral gold-silver-bearing deposits, and a western

precious-metal province, with primary precious-metal producing deposits (see Fig. 1.3). The

east-west partiality observed in the distribution of precious- and base-metal deposits, which is

defined best by the preference of primary-producing precious-metal deposits for the western half

of the basin, may in part be related to the lithotectonic makeup of the crust, as well as differences

in mantle composition (Roberts, 1966; Shawe and Stewart, 1976). The eastern base-metal subprovince coincides with the eastern carbonate assemblage (the miogeoclinal facies) of the

Cordilleran Geosyncline, and overlies the western flank and margin of the Precambrian

crystalline basement (see Figs. 2.4, 2.9, 2.10, and 2.11). The western precious-metal subprovince coincides with the western siliceous assemblage (the eugeoclinal facies), and is

associated with predominantly Mesozoic basement crust composed of volcanic rocks and

accreted arc and oceanic terranes (Roberts, 1966; also see Shawe and Stewart, 1976; Albers,

1983; Tooker, 1985; Titley, 1991). A transitional zone, trending approximately north-south,

contains both base- and precious-metal occurrences, and roughly coincides with the Cordilleran

geosyncline facies boundary (see Fig. 2.9).

The spatial correlation of broad kinds of ore deposit-types (e.g.—precious- versus base-metal)

with crustal segments of grossly different lithotectonic composition, such as observed across the

Great Basin metallogenic province, appears not to be merely coincidental. Indeed, Hodgson

(1995) commented that it is very difficult to explain most metal provinces without evoking a

control by chemical heterogeneity on the scale of the crust. Broad correlations between the metal

contents of particular deposit types and the lithotectonic composition of the crust in which the

deposits formed, as well as spatial clustering within particular crustal segments of deposits of

differing ages and types that contain the same relative gold-silver content, suggest that the

makeup of the crust at the mining district and regional scales may exercise a significant control
41

on the formation and distribution of specific metallic deposit types (Wilkins, 1984; Hodgson,

1995; also see Guild, 1985; Tooker, 1985; Hutchinson and Albers, 1992).

A number of studies have suggested that there is an association between mineral deposit metal

content and upper crustal lithotectonic composition, and for the Cordilleran region of the western

United States, these include Titley (1987, 1989, 1991), Albers (1981, 1983), and Campa and

Coney (1983).

Titley (1987, 1989, 1991) documented the ratios of silver to gold produced from epigenetic ore

districts in the southwestern United States and observed that, in general, ores relatively enriched

in gold lie above, or within, a Proterozoic basement dominated by mafic-felsic volcanic (arc)

successions (largely submarine volcanic and volcaniclastic strata, and devoid of Paleozoic

strata), whereas ores relatively enriched in silver occur in terranes floored by thick Proterozoic

clastic and Paleozoic marine successions (schists and metasedimentary rocks, formed from a

protolith of mostly clastic sedimentary rocks). It was concluded that the metallogenic signatures

of ore districts (regardless of the deposit type) are fundamentally related to the crust in which the

ores occur, and do not appear to be related to subduction processes, wherein metals and magmas

are directly derived from partial melting of a descending slab of oceanic rocks (Titley, 1987).

Albers (1981, 1983) examined the distribution of mineral deposits in the metallogenic provinces

of California, and in accreted terranes and cratonal rocks of the western United States. Albers

determined that: (1) most of the large deposits are in the craton (a few occur in accreted islandarc terranes); (2) miogeoclinal terranes underlain by craton are characterized by replacement and

vein-type Pb-Zn-Ag, skarn W deposits, Mo, and Sn, and in Nevada by Carlin-type disseminated

gold deposits; and (3) accreted terranes (mainly of Mesozoic age) contain all the known

volcanogenic massive sulfide deposits, all chromite and chert-associated Mn, all the large gold

quartz-vein deposits (except Goldfield, Nevada), and the greatest proportion of Hg and Sb

deposits (Sb also forms important deposits in cratonal rocks). Hutchinson and Albers (1992),

in a review of the metallogenic evolution of the Cordilleran region of the western United States,

concluded that lode gold-producing mines and districts occur preferentially in accreted oceanic

and island-arc terranes, whereas silver-rich deposits exhibit an even stronger preference for the

craton, and that the terrane and cratonal rocks could have provided different suites of metals for

the deposits they host. 

Campa and Coney (1983) examined the distribution of the principal gold-silver and Pb-Zn

producing mines in Mexico relative to the distribution of lithotectonic crustal terranes. They

determined that the distribution of these two broad types of metallic ore deposits was a function

of the distribution of basement terranes: (1) most of Mexico's important gold and silver mines

(over 70%, perhaps up to 84%) are located within Late Jurassic (or older) to late middle

Cretaceous accreted basement terranes, which consists of submarine to partly continental

andesitic, and volcanic and associated volcaniclastic sequences of magmatic arc aspect; and (2)

most of the productive mines for Pb and Zn (over 61%) occur in late Precambrian through

Paleozoic age terranes, which are underlain by autochthonous Precambrian North American

craton, and are composed of thick sequences of sandstones, shales, and limestones that appear

to be correlative with miogeoclinal sequences in southwestern Nevada and southern California

as well as well known sequences in Arizona and New Mexico. Overall, Campa and Coney

(1983) concluded that the distribution pattern of gold-silver and Pb-Zn deposits in Mexico
42

appears to have a significant element of basement [terrane] control.

3.6 Quantitative Studies on Mineral Trends and Crustal

Terranes

Few studies have used quantitative methods to specifically examine the spatial distribution of

ore deposits with respect to mineral trends or crustal terranes, but the results of these suggest that

regional-scale crustal structures and/or lithotectonic terranes do exercise some degree of control.

Carlson (1991) used fractal modelling techniques to examine the spatial distribution of 4,775

hydrothermal precious-metal deposits in the Great Basin. He observed that faults and fractures,

and probably hydrothermal systems and igneous and tectonic heat sources as well, appear to be

similar at many different scales, which suggests that the features and/or processes controlling

mineral deposit distribution operate at scales other than those of a mining district. The spatial

clustering of ore deposits was determined to be persistent at all scales between 1 and 1,000 km,

reflecting geologically significant controls on mineralization acting at all of these scales

(Carlson, 1991, p. 113 and 114). Carlson (1991) concluded that local-scale geologic controls,

such as calderas, local faults, and the availability of ground water, are not sufficient to explain

deposit clustering at the regional scale, and suggested that clustering on the scale of metallogenic

provinces, lithotectonic terranes, or major zones of crustal weakness take on additional

credibility (i.e.—major zones of crustal weakness, or regional scale crustal structures, may be

in part responsible for the localization/distribution of deposits along mineral deposit trends; Carl

A. Carlson, 1997, personal communication).

In the Indian Shield, Talapatra and Mukhopadhyay (1993) have performed a multi-variate cluster

analysis on Cu-Pb-Zn base-metal sulfide deposits occurring along three mineral trends in order

to locate undiscovered or concealed mineralization. They compiled between 20 to 23 variables

relating to lithological, structural, and ore-mineral attributes for known deposits. These attributes

were examined over a 10 by 10 km cell-size grid that was superimposed on the study areas,

where each grid cell was evaluated for the “presence” or “absence” of each attribute. The

analysis revealed that a number of barren cells (regions thought to be devoid of mineralization)

clustered with control cells (regions containing known deposits) (see Talapatra and

Mukhopadhyay’s figures 3, 5, and 7). The barren cells that were predicted to have some mineral

potential occurred within the trends, bridging gaps between spatial clusters of deposits, and at

the ends of the trends, extending outward along trend (see Talapatra and Mukhopadhyay’s

figures 2, 3, and 6; particularly figure 2). These results suggest that the lithological, structural,

and ore-mineral attributes examined may exercise some control over the linear distribution of

the deposits in these trends.

In the northwestern United States (Oregon and environs), Tooker (1983) used an areal-pattern

recognition technique to investigate the spatial distribution relationships between lithotectonic

terranes and important metal occurrences (Co, Cr, Cu, F, Au, Fe, Pb, Mn, Hg, Ni, Pt-group, Sn,

Ti, V, Zn). He calculated and contoured the relative areal-abundance of metal districts, deposits,

or subeconomic occurrences within a 15-minute quadrangle unit cell. Tooker found that (1) Pb,

Zn, Sn, and W metals are concentrated locally in the boundary zones along the edge and in the

interior of the craton, (2) Ni, Co, Cr, and the Pt-group metals predominate in accreted terranes,
43

and (3) Cu and Au appear to be abundant in both accreted and cratonic terranes (read cratonunderlain terranes). He observed that the occurrence of metals on the regional-scale is not

random. Metals tend to occur along broad, linear, persistent zones that trend at sub-normal

angles to one another, parallel to the Cordilleran orogenic fabric and the fabric of the

Precambrian craton (see section 2.4; see Tooker, 1983, figure 1 and 2). West of the craton

margin, the metal-distribution patterns trend roughly northwest–north and seem to parallel axes

of accretion of lithotectonic terranes, with a notable correlation to Mesozoic terrane assemblages.

East of the craton margin (on the craton), metal-distribution patterns trend northeast-southwest,

which was interpreted by Tooker (1983) to coincide generally with boundaries between Archean

and Proterozoic crustal terranes or structural zones (this interpretation is consistent with the

orientation of a number of Precambrian tectonic zones and craton crustal province boundaries

to the south; see Fig. 2.11 and section 2.4). Highest metal occurrence density appears to be

coincident with intersections between the craton basement features and prominent, east-west to

northwest-trending, near-surface structural zones (e.g.—the Lewis and Clark line) or uplifts

(e.g.—Uinta trend). Tooker (1983) concluded overall that the metal occurrence distribution

patterns reflect primary sources of metals in the deep crust, and that the patterns are controlled

by the location of fundamental channels (crustal flaws) which collected and distributed orebearing fluids into receptive lithosphere structural traps and lithologies where the metals were

deposited in a variety of occurrence types.

The purpose of the above studies was to investigate the spatial distribution of ore deposits in

terms of mineral trends and/or crustal terranes. The research conducted in this study also

considers these relationships (among other geologic associations), but not in such a direct

manner. The primary objective of weights of evidence mineral potential modelling is to generate

a map that shows areas favorable for ore deposit formation. The favorability patterns on these

maps carry a large amount of geological information, representing a synthesis or distillation of

various geologic phenomena associated with known areas of mineralization. The utility of the

mineral potential maps can be extended beyond just exploration target delineation by examining

the spatial distribution of the favorability patterns for trends, and interpreting these trends with

respect to regional-scale geologic and metallogenic features. The trends can be inspected more

closely to determine the specific, or combination of geologic factors that contribute to the

favorability pattern, which in turn can shed some light on whether the favorability trend is

controlled by regional-scale structural zones or lithotectonic associations.
44

Chapter 4. Analysis and Modelling Techniques

4.1 Introduction 

A GIS can facilitate data analysis and interpretation by revealing spatial associations which

might otherwise be obscure. Spatial data analysis can be preliminary in nature, involving nonstatistical or summary statistical approaches (e.g.—visual pattern recognition and simple map

reclassification, or, histogram generation and area analysis, respectively), or it can be more

sophisticated, involving statistical approaches with probabilistic components (e.g.—regression

analysis, weights of evidence).

This chapter presents a summary and overview of the preliminary and more sophisticated

analysis methods that were used to identify and measure the spatial associations between the

gold-silver-bearing occurrences and the various mineral potential evidence maps. Preliminary

methods are briefly mentioned in section 4.2. Section 4.3 covers the basics of the weights of

evidence mineral potential modelling method. The theoretical framework, the assumption of

conditional independence, and error and uncertainty issues are reviewed. An extended and

detailed discussion of preliminary data analysis techniques and weights of evidence, including

equations and practical implementation of the method, is presented in Appendix B.

4.2 Preliminary Spatial Data Analysis

The modelling conducted in this study made use of many spatial data manipulation and analysis

tools available in a standard GIS environment. The preliminary spatial data manipulation and

analysis techniques used here include: (1) map reclassification/generalization; (2) map

overlay/combination; (3) spatial and topological modelling; and (4) visual pattern recognition

and spatial query. All of these methods assist in clarifying and defining trends and patterns. A

comprehensive examination of these tools is beyond the scope of this study. GIS spatial data

manipulation and analysis methods are well known. For a thorough and detailed treatment of

spatial modelling and GIS in general, see Bonham-Carter (1994a).

4.3 Weights of Evidence Mineral Potential Modelling

4.3.1 Introduction

Weights of evidence (WOE) provides a measure of spatial association (a “weight”) between

multi- and/or binary-class map patterns and known point or polygon objects, and uses a loglinear

formulation of Bayes’ probability theorem to combine the map patterns to predict the distribution

of the point or polygon objects. As applied to mineral potential modelling, the point objects

represent known mineral deposits. The multi-class map patterns are typically maps of particular

geologic phenomena, such as geology, geochemistry, geophysics, etc., which are likely to be

useful mineral deposit predictors. These maps are referred to as “evidence maps”, representing

geo-spatial evidence for the occurrence of mineralization. In order to facilitate combination, the

evidence maps are usually reduced to deposit-indicator or mineral “predictor maps” of a few

discrete states (typically binary) where the spatial association between mineralization-favorable
45

evidence and the occurrences is optimized. The mineral predictor maps collectively constitute

the “layers of evidence” for the mineral potential model.

A layer of evidence need not completely cover the study area, as the WOE modelling method can

easily accommodate missing data (incomplete coverage). This is an important and distinct

advantage of this method. Other advantages include uncomplicated calculations, an objective

procedure for weighting the evidence layers, relatively straightforward interpretation of the

weights, direct user involvement with optimization of the evidence maps, which is an important

inductive process that provides insight into the spatial data relationships, and the ability to model

the error and uncertainty of the mineral potential map (Bonham-Carter, 1994a, 1994b).

4.3.2 Theoretical Framework

The weights of evidence mineral potential modelling method has been described by Agterberg

(1989a), Agterberg et al. (1990), Bonham-Carter (1994a, 1994b), Bonham-Carter (1991),

Bonham-Carter et al. (1988), Bonham-Carter et al. (1989), Wright and Bonham-Carter (1996),

and Wright (1996). WOE is a data-driven method, requiring data about the distribution of

known mineral deposits to estimate weights of spatial association for the mineralization evidence

layers. It is a discrete, multivariate statistical method based on a technique originally developed

in a non-spatial context for medical diagnosis (Spiegelhalter and Knill-Jones, 1984) and has been

modified by Bonham-Carter et al. (1989) to deal with spatial prediction—“diagnosing” mineral

deposits using the “symptoms” of various geologic phenomena (Bonham-Carter, 1994b). WOE

evaluates the spatial distribution of known mineral deposits (the response variable) relative to

multi- or binary-class map patterns (the predictor variables), calculates weights of spatial

association (W+

 and W–) for each pattern, and produces a multi-map signature for mineralization

(Bonham-Carter et al., 1989). The evidence layers are combined using a loglinear formulation

of Bayes’ Rule in a multi-map overlay operation where the prior probability of an

occurrence—the probability of an occurrence given no information (random), equal to the

average density of known occurrences in the study area, and held constant over the whole

area—is updated by the addition of predictor variables and their weights to produce a single

posterior probability map of occurrence (Bonham-Carter et al., 1989; Bonham-Carter, 1991).

In this case, the posterior probability map is a map of mineral potential, which reflects the

distribution of known occurrences and predicts the distribution of yet unidentified occurrences

(Bonham-Carter et al., 1988). The whole process is similar to that of an exploration geologist

manually integrating information and combining maps in order to delineate favorable areas of

mineralization (Agterberg et al., 1990).

Weights of evidence mineral potential modelling can be subdivided into two main procedures,

as illustrated in Figure 4.1:

i. Conditional probabilities that involve area proportions are used to calculate two weights of spatial

association between the occurrences and each individual evidence map class: W+

 for a particular map class

present, W–

 for not present (the occurrence points are not themselves classified or weighted, and each point

is treated as equally important). Weights are relative, dimensionless values, which depend on the ratio of

occurrences that fall on a particular map class to the total number of occurrences, against, the ratio of the

particular map class area to the total map area. A positive correlation between occurrences and a map class

is represented by W+

 > 0 and W– < 0 (there are more occurrences in a particular map class than would be

expected due to chance); a negative correlation by W+

 < 0 and W– > 0. Where no spatial association exists
46

1 As used here, the term “training area” is defined as a sub-region of, or a region outside of, the study area that

is designated an experimental control, where (1) the geologic conditions are similar to the prospecting region,

(2) all the mineral deposits are known, and no more are expected to be discovered, (3) all of the mineralization

evidence is present (i.e.—full evidence map coverage), and (4) the quantitative relationships (the spatial

weights of association) between the mineral deposits and the evidence are established (Chung, 1995; Chung

and Moon, 1990).

(i.e.—the two ratios are equal), the weights are both zero. Where data are unknown or missing (incomplete

evidence map coverage), the weights are assigned the value zero (Bonham-Carter, 1991). The weights can

be combined into a single coefficient called the contrast (C = W+ – W–

), which provides a useful measure

of the strength of the spatial association between the occurrences and a particular map class. The weights

and C provide a guide for reducing the multi-class evidence map to a binary-class deposit-indicator (or

predictor) map pattern. For each evidence map, an individual map class that is highly correlated (spatially)

with the mineral occurrences may be selected as a deposit-indicator pattern, or multiple map classes may be

grouped in such a way as to maximize the spatial association between the occurrences and the map (see

Appendix B for additional details).

ii. The binary-class mineral predictor maps (layers of evidence) are combined in a multi-map overlay operation

where a loglinear formulation of Bayes' Rule (based on an assumption of conditional independence) is used

to sum and update the weights associated with each of the predictor map classes that come into coincidence,

producing a posterior probability mineral potential map which closely exhibits the distribution of known

deposits and indicates areas where more deposits are expected than are observed. In summary, the prior

probability of an occurrence for a unit area is successively “updated” by the addition of each new layer of

evidence (information) to produce a posterior probability. The prior probability is equal to the probability

of an occurrence within a unit area given no further information, which for this study is taken to be equal

to the density of known occurrences in the study area (total number of occurrences divided by the total area

of the study region, assuming a 1 km2

 unit area of measure and where an occurrence point is represented by

one unit area). Bayes' Rule effectively revises the prior probability by incorporating the new evidence into

the model (Mendenhall and Reinmuth, 1974). The posterior probability reflects both the prior and the new

evidence, and with each subsequent addition of new evidence, the posterior is treated as the prior, thus

providing a more efficient model for prediction (Mendenhall and Reinmuth, 1974; Bonham-Carter, 1994a).

The posterior probability calculated after the addition of new evidence may be larger or smaller than the

prior probability, depending on the overlap combination of predictor maps and their weights (i.e.—if

evidence of factors favorable to mineralization are added, the posterior probability rises, and vice versa)

(Agterberg, 1989a; Bonham-Carter et al, 1989). The mineral potential map is generated by grouping into

intervals the calculated posterior probabilities according to a user-defined classification scheme (“density

slicing”; often based on quantiles, an area-based percentiles classification where the posterior probability

interval break-points are determined in such a way that each interval is roughly equal in area). See Appendix

B for additional details.

In WOE modelling, no “training area1” is used to select mineralization evidence or to calculate

“initial baseline” weights (i.e.—to establish the “initial conditions”). The “initial conditions”

of the mineral potential model are established using the mineral predictor maps, as indicated

above in procedure “i”. A model may be further calibrated using other factors such as mineral

deposit size, where different schemes for weighting the layers of evidence are calculated for each

deposit size subset (Wright and Bonham-Carter, 1996). The selection of evidence maps is

largely guided by an accepted or proven deposit or exploration conceptual model (this is the

standard mode of implementation; see Bonham-Carter et al., 1989; Bonham-Carter, 1994a;

Wright, 1996; Wright and Bonham-Carter, 1996). The choice of evidence should reflect current

understanding on the genesis of the particular deposit type being modelled as well as the geologic

features believed to control its spatial distribution. An evidence map should ideally provide

either universal coverage or coverage over the majority of the study area (Bonham-Carter et al.,

1989). 
47

4.3.3 Conditional Independence

An important assumption made in WOE modelling is that the mineralization evidence layers

included in a model be conditionally independent (CI) of one another with respect to the mineral

deposits (see Bonham-Carter, 1994a, pp. 312-317). The mineral potential map is adversely

affected if, at the locations of the known mineral occurrences, the presence of a mineralizationfavorable map pattern in one layer of evidence is dependent on the presence of a mineralizationfavorable map pattern in another layer of evidence. Violation of CI results in either the overestimation or under-estimation of posterior probabilities during the combination of predictor

maps, and the expected mineral deposit frequencies either notably exceed or fall short of the

observed deposit frequencies in the most and least favorable areas of the mineral potential map

(Agterberg et al., 1990). In practice, CI is probably always violated to some degree, and the

possibility of the occurrence of CI generally increases with an increase in the number of evidence

layers included in a model (Bonham-Carter, 1991; Bonham-Carter, 1994a). Because of the CI

assumption, calculation of the spatial weights of association are carried out independently

between the mineral deposits and each evidence layer, and as a result, WOE has the opportunity

to examine bivariate relationships in some depth (Bonham-Carter, 1994b). The assumption of

CI leads to a model that, like most models, does not fit the data perfectly, but provides a

simplification that is useful for prediction when applied carefully, and gives insight into the

relative contributions of the separate sources of evidence (Bonham-Carter, 1994a).

It is important to understand how serious the CI violation is so that the appropriate action can

be taken to minimize the problem and so that proper judgments can be made when evaluating

areas of high mineral potential. Conditional independence can be checked visually or tested for

using pairwise and overall goodness-of-fit methods. If a predictor map is found to be in serious

violation of the assumption of CI, it can then be (1) rejected from the model, (2) combined with

another map in order to minimize the dependency, or (3) modified in some way to reduce the

problem (see Agterberg, 1989; Agterberg et al., 1990; Bonham-Carter, 1994a).

Both the pairwise and overall CI tests make use of the observed versus the expected number of

observations (mineral occurrences). The pairwise goodness-of-fit test measures conditional

independence (CI) between all possible pairings of binary-class predictor maps (with respect to

the mineral deposits) by calculating the 2

 (chi-square) statistic for each map pair, and comparing

the calculated value of 2

 to the tabled value of 2 having one degree of freedom (Bonham-Carter,

1994a). CI may be present due to three-way or multi-way interactions, and testing for these cases

is also possible, but for practical purposes, pairwise testing reveals the most serious CI violations

(Bonham-Carter, 1994a). The results of the test are typically presented in the form of pairwise

cross-tabulation tables. The overall goodness-of-fit test is a measure of the conditional

independence (CI) among all of the layers of evidence in a model as a whole. The overall

method is a simple procedure involving the relative comparison of predicted versus observed

occurrences—if the total predicted number of occurrences is much larger than the total observed

number (greater than ~10-15%), it suggests that CI is being violated, and may warrant a check

of the pairwise tests and some sort of remedial action (Bonham-Carter, 1994a). The overall test

can be evaluated using the Kolmogorov-Smirnov (K-S) statistic. The K-S test is based on the

maximum deviation of the observed number of occurrences from the predicted number, and the

results are typically presented in graphic form. See Appendix B for additional details.
48

4.3.4 Posterior Probability Uncertainty

An important aspect to interpreting a mineral potential map is recognizing and quantifying the

uncertainty inherent to the posterior probabilities. The two primary sources of uncertainty are:

(1) the uncertainty due to variances in weight estimates (W+ and W–); and (2) the uncertainty due

to one or more of the binary-class predictor maps having incomplete coverage (i.e.—missing

data) (Bonham-Carter et al., 1989). The uncertainties due to weights and due to missing data

may be examined separately, or combined to produce a total uncertainty for a given unique

overlap combination of binary-class predictor maps, which is calculated as the variance due to

weights, plus, the sum of variances due to missing data (Bonham-Carter et al., 1989). The

uncertainty due to the weights, which includes the uncertainty of the prior probability, is in

general correlated to the posterior probability, and therefore maps of variance of weights have

the same trends as the posterior probability maps. 

In addition to the uncertainties due to weights variances and missing data, a relative certainty

(variance) of the posterior probability can be determined by dividing the posterior probability by

its standard deviation (i.e.—a “studentized” posterior probability), which, in effect, applies a

student t-test (based on a normal distribution) to determine whether the posterior probability is

greater than zero for a given level of statistical significance (i.e.—compared to a tabled t-value)

(Bonham-Carter et al., 1989; Agterberg et al., 1993). The larger the t-value over the critical

tabled t-value cut-off, 1.645 for a significance of 95% for example, the greater the certainty of

the posterior probability. The relative certainty is often more useful than the weights variances

or missing data uncertainties because it indicates the degree of confidence to which the posterior

probabilities are “real”, as opposed to being an artifact of “chance” effects (or due to chance).

As compared to the uncertainty due to the weights variances or missing data, relative certainty

is generally not as highly correlated to the posterior probability.

Ideally, the four uncertainty factors (weight variances, missing data, total, and relative) may be

used to create classified uncertainty maps for comparison to the posterior probability mineral

potential map, or the uncertainty factors may be combined in various ways and reclassified to

a binary-class map which can be used to “mask-out” areas of the mineral potential map that are

deemed to be too uncertain (Bonham-Carter et al., 1989; Bonham-Carter, 1994a).

4.3.5 Practical Implementation of the Modelling Procedures

Weights of evidence modelling was implemented in a geographical information system (GIS)

environment. SPANS GIS (TYDAC Technologies Inc., 1993) was used to compile, prepare, and

manage the spatial datasets, as well as perform most of the spatial data analysis and modelling

procedures. Preliminary procedures, such as visual appraisal and pattern recognition, distance

calculation, map reclassification and overlay operations, summary statistical analysis

(histograms), and spatial and topological modelling (point and line buffering, point-in-polygon,

area, and other coincidence analysis, and surface contouring and interpolation) were carried out

using tools commonly available in a GIS. The calculation of W+

, W–, weights variances, and

posterior probabilities for weights of evidence modelling was performed external to the GIS

using custom-made command-line FORTRAN utilities. The output of these utilities was

imported into the GIS as reclassification templates and used to generate posterior probability

mineral potential maps, as well as various maps of posterior probability uncertainty (error maps).
49

Chapter 5. Spatial Datasets

5.1 Introduction

The initial steps in developing any GIS database involve data collection and input. This phase

typically accounts for 70% to 80% of the time spent on a project. The data used for this study

are diverse, and characterize the nature of the lithosphere in the Nevada Great Basin from the

earth's surface to the upper mantle. The data are subdivided according to regional geology,

physical geography, geophysics, seismology, geochemistry, remote sensing imagery, economic

geology, hydrology, and human features. They are listed in Table 5.1, at the end of this chapter.

This chapter provides an overview of the GIS study area, training datasets, and mineral potential

evidence. The GIS study area parameters are given in section 5.2. In section 5.3.2, the criteria

used to select the gold-silver-bearing occurrences from the Mineral Resource Data System

(MRDS) database are reviewed. Definitions for “gold-silver-bearing occurrence” and various

occurrence-type samples and sub-samples (i.e.—“small”, “medium”, “large”, and “sedimentary

rock-hosted occurrences” and “volcanic rock-hosted occurrences”) are also given. The initial

examination and selection of the evidence maps is reviewed in section 5.3.3. An overview of

data accuracy, limitations, and error issues is given in section 5.4.

5.2 GIS Study Area Parameters

5.2.1 Projection

The projection was adopted from Stewart and Carlson’s (1978) geological map of Nevada:

Projection: Lambert Conformal Conic

Ellipsoid: Clarke 1866

First Parallel: 33.0o

Second Parallel: 45.0o

Longitude of Origin: -117.0o

Latitude of Origin: 38.5o

X-Coordinate False Origin: 0.0

Y-Coordinate False Origin: 0.0

Coordinates west of the prime meridian (the Greenwich meridian) and south of the equator are

represented as negative values.
50

5.2.2 Extents

The geographical coverage of the study area includes the region of the Great Basin and environs:

45o

 N

122 Study Area... o

 W 111o W

34o

 N

5.2.3 Resolution

The geological map of Nevada (Stewart and Carlson, 1978), which is the geological “base map”

layer for this study, was compiled at a scale of 1:500,000. The effective resolution of a

1:500,000 scale map is 250 m (~820.2 ft.), assuming a 0.5 mm line thickness (a conservative

thickness estimate) (Goodchild, 1995). The line width of the geological contacts on the

geological map of Nevada (Stewart and Carlson, 1978) is nearer 0.2 mm, yielding an effective

paper map resolution of 100 m (~328.1 ft.) for the position of the unit boundaries.

The resolution of the SPANS GIS study area for this analysis is 83.4 m (~273.6 ft.). In the

southwestern United States this translates to longitude-latitude positional accuracy of

approximately 3 seconds of arc, or 0.0008 decimal degrees. This therefore, is the theoretical

limit of positional accuracy for this study, although as stated above, the practical effective limit

to the accuracy of positional data ranges between ~4 and ~9 arc-seconds of longitude (~97-219

m) or latitude (~123-277 m), assuming that spatial objects were located and/or digitized in a

reliable manner.

5.3 Spatial Datasets—Selection Criteria and Terminology

5.3.1 Data Sources

Public domain data comprises nearly all of the information compiled here and comes from the

U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration

(NOAA), or “the literature”. Many of the digital datasets were obtained from the USGS and

NOAA while a significant portion of the analog data, especially point and attribute data, were

compiled from published documents. Most of the digital data and the analog data in map form

are available from the USGS Earth Science Information Centers (ESIC), the USGS EROS Data

Center, and the NOAA National Geophysical Data Center (NGDC). A small number of the

digital datasets obtained from the USGS and other academic institutions are currently in

preparation and are not yet available for general public consumption. These datasets were

acquired through personal communications with the authors and were provided for the purposes

of academic research.
51

5.3.2 Gold-Silver-Bearing Mineral Occurrences (Training Datasets)

A “gold-silver-bearing occurrence” is a mineral concentration having particular attributes that

allow it to be classified as a specific deposit type, or can be said to have an anomalous

concentration of gold or silver (Ludington et al., 1993). Specifically, for the purposes of this

investigation, a gold-silver-bearing occurrence is any lode metallic ore mineral, in any

concentration or deposit type, that is recorded in the Mineral Resource Data System mineral

occurrences database (MRDS, 1993 download) and meets with any one of the following criteria:

& Gold or silver is the primary commodity produced or that is in reserve (i.e.—the first listed in the

“commodities” field of the database).

& Gold or silver is a secondary commodity, where the primary commodity is a platinum group element, such as

Pt or Pd.

& Gold or silver is a secondary commodity, where the primary commodity is a known precious-metal

mineralization “path-finder” element (e.g.—Sb or As).

Gold or silver is a secondary commodity, where the primary commodity is a non-metal, such as gem-quality

semi-precious stones.

From a population of 5572 metallic and semi-metal mineral occurrences listed in MRDS, 2690

gold-silver-bearing occurrences were selected. Duplicate points (spatially the same) and

redundant points deemed to represent the same occurrence were discarded (e.g.—a district

having one large mine, but represented as a district point and a mine point). However, a large

deposit, geologically and areally speaking, may be represented by one or more mines (points).

The occurrences matching the above criteria include past-producing mines, currently producing

mines, and ore in reserve associated with producing mines, as well as lean or sub-economic

mineral “showings”. Woodall (1984) has suggested that the geologic conditions and processes

responsible for the formation of large economic ore deposits may differ substantially from those

that produce lean ore deposits and sub-economic mineral occurrences. The implication is that

not much emphasis should be placed on the lean and sub-economic occurrences. However, it

has been recommended by Skinner (1979, p. 1) that exploration should be equally concerned

with the occurrence and genesis of sub-economic mineralization as with economic deposits. The

rationale being that sub-economic mineralization indicates where deposit-forming processes,

albeit not economic ore-forming processes, have taken place (Ludington et al., 1993), and that

any precious-metal concentration therefore provides valuable spatial information useful for

delineating and refining mineral occurrence distribution patterns. The recognition of these

distribution patterns, the identification of geologic features associated with the patterns, and the

investigation into what factors might control them, can provide guidance in delineating

exploration targets for economically viable deposits.

Of the 2690 gold-silver-bearing occurrences selected, 1106 had a Cox and Singer (1986) or Bliss

(1992) deposit type classification designation. The occurrences were subdivided by deposit type

into three “occurrence-type samples” according to the following criteria:
52

Gold-Silver-Bearing Occurrences of All Types—Any type of mineralization that has gold and/or silver as its

primary commodity (or as defined above). Includes 1584 unclassified deposits and 1106 deposits that include

the following Cox and Singer (1986) and Bliss (1992) deposit types:

Deposit Model Name Model Number Count

W skarn 14a 7

Porphyry Cu 17 1

Cu skarn 18b 12

Zn-Pb skarn 18c 4

Au skarn 18f 2

Polymetallic replacement 19a 62

Replacement Mn 19b 5

Distal disseminated Ag-Au 19c 9

Porphyry Cu-Mo 21a 3

Porphyry Mo, low-F 21b 2

Polymetallic veins 22c 253

Besshi massive sulfide 24b 1

Hot spring Au-Ag 25a 49

Comstock epithermal veins 25c 328

Sado epithermal veins 25d 2

Epithermal quartz-alunite Au 25e 25

Epithermal Mn 25g 3

Sediment-hosted Au-Ag 26a 89

Hot spring Hg 27a 4

Simple Sb 27d 16

Kuroko massive sulfide 28a 2

Low-sulfide Au-quartz veins 36a 221

Au on flat faults 37b 4

Quartzite-hosted Au 37c 2

Sedimentary Rock-Hosted Occurrences—Disseminated mineralization hosted by carbonate and/or clastic

sedimentary rocks, commonly referred to as “Carlin-type” or “distal disseminated” deposits. Members of this

group include the following Cox and Singer (1986) and Bliss (1992) deposit types:

Deposit Model Name Model Number Count

Distal disseminated Ag-Au 19c 9

Sediment-hosted Au-Ag 26a 89

The "distal-disseminated" type deposits were not well documented or understood at the

commencement of this research, and were included under the general category of

“sedimentary rock-hosted” deposit type. Present understanding may possibly warrant the

separation of these two deposit types, but based upon the expertise of the mine geologists at

the Cove deposit, this is unclear. The Cove deposit, the largest known distal disseminated

type deposit in Nevada, is located along the Battle Mountain-Eureka mineral trend in the

Battle Mountain area. Currently operated by Echo Bay Minerals, the McCoy-Cove

exploration geologists believe that the distal-disseminated-style mineralization at Cove may

be Carlin-style mineralization overprinted on preexisting skarn/porphyry base-metal

mineralization (Dave McLean, 1997, personal communication). Such a scenario could

account for some of the important geochemical traits typically associated with distaldisseminated type deposits: (1) the high variability (Cox, 1992) or generally higher (Singer,

1996) Ag-Au ratio as compared to the Carlin-type deposits; and (2) Mn, Zn, Pb, As, Sb trace
53

element geochemistry for distal-disseminated-type versus As, Sb, Hg, Zn for Carlin-type

(Cox, 1992; Singer, 1996). With regard to mineral potential modelling, the inclusion of

distal disseminated occurrences in the sedimentary rock-hosted training sample, what is

being considered is the combined affect of the deposit types. Removal of the distal

disseminated occurrences has negligible to inconsequential affects on the model: the

difference in posterior probability values between a model trained with just “sediment-hosted

Au-Ag” and a model including “Distal disseminated Ag-Au” is less than the prior probability

(i.e.—indistinguishable from background noise and representing a probability of occurrence

no more than that due to chance). 

Volcanic Rock-Hosted Occurrences—Vein or disseminated epithermal mineralization hosted primarily by

volcanic rocks and less commonly by proximal volcaniclastic and sedimentary rocks. Members of this group

include the following Cox and Singer (1986) deposit types:

Deposit Model Name Model Number Count

Hot spring Au-Ag 25a 44

Comstock epithermal veins 25c 333*

Sado epithermal veins 25d 2

Epithermal quartz-alunite Au 25e 25

Epithermal Mn 25g 3

Hot spring Hg 27a 4

Au on flat faults 37b 4

* Note: Five occurrences in this category are also classified in part as 25a.

The occurrences were further broken down into a number of size categories (or “sub-samples”)

based upon primary commodity (gold or silver) metal content:

Large

Greater than 500 tonnes (16,075,373 Troy ounces) Au content (production plus reserves).

Greater than 10,000 tonnes (321,507,465 Troy ounces) Ag content (production plus reserves).

Medium

500-25 tonnes (16,075,373 to 803,768 Troy ounces) Au content (production plus reserves).

10,000-500 tonnes (321,507,465 to 16,075,375 Troy ounces) Ag content (production plus reserves).

Small

Less than 25 tonnes (803,768 Troy ounces) Au content (production plus reserves).

Less than 500 tonnes (16,075,375 Troy ounces) Ag content (production plus reserves).

Big

Large and medium size occurrences.

Unknown

Deposit size reported as “unknown” or not specified.

The size designations of large, medium, and small are coded into MRDS for 2,421 of the

occurrences, and are derived from Guild's (1968) Metallogenic Map of North America.

The gold-silver-bearing occurrences as subdivided above are summarized in Table 5.2:
54

The occurrence-type samples are referred to using the following terminology:

Primary occurrences—Includes all occurrences of any size or any type (i.e.—all 2,690

occurrences examined in this study), unless otherwise noted. Also referred to as "goldsilver-bearing occurrences" or just "occurrences" if no reference to either sedimentary or

volcanic rock-hosted mineralization is made.

Sedimentary rock-hosted occurrences—A subgroup of the primary occurrences. Includes all

sedimentary rock-hosted occurrence sizes, unless otherwise noted.

Volcanic rock-hosted occurrences—A subgroup of the primary occurrences. Includes all

volcanic rock-hosted occurrence sizes, unless otherwise noted.

Occurrence size—big, large, medium, small, unknown—is usually explicitly stated. If it is not

stated, then occurrences of all sizes should be assumed (as indicated in the points above).

5.3.3 Mineral Potential Modelling Evidence Maps

The following evidence maps were considered for mineral potential modelling:

1. Lithologic:

Lithologic units (all 101 units considered separately; see Appendix A).

Diversity of map lithologic units (per 2.5 km by 2.5 km neighborhood grid cell).

Lithologic assemblage units (101 units grouped into 20 as per general rock-type and age;

see Appendix A).

Cenozoic igneous rock unit distance buffers.

Cenozoic igneous rock time-slices.

Cenozoic igneous rock composition-slices.

Mesozoic pluton distance buffers.

Mesozoic pluton density.

Clastic and carbonate rock units.

Clastic and carbonate rock unit distance buffers.

2. Structural/tectonic:

Cenozoic fault distance buffers.

Cenozoic fault density.

Thrust-front distance buffers (Luning-Fencemaker, Golconda, Roberts Mountain,

Sevier).

Strike-slip fault distance buffers.

LANDSAT linear-features distance buffers

LANDSAT linear-features density

Highly extended upper crustal terrain.

Deep-seated basement fracture system buffers.

Lithotectonic terrane.

Crustal thickness.

Tertiary rock dip angle and direction.

Late-Paleozoic–early Mesozoic paleothermal anomaly.

3. Geophysical:

Bouguer gravity anomaly.

Isostatic residual gravity anomaly.
55

Total residual field geomagnetic anomaly.

Geothermal conductivity.

Geothermal gradient.

Crustal heat flow.

Geothermal wells and springs temperature.

4. Geochemical (and related):

K/Na, Ba/Na, U/Th, Fe/Al, La/K, Sc/Fe, Sc/V, K, Al, As, geochemical anomalies (NURE

data).

Igneous rock major element analyses (PETROS).

Igneous rock radiometric age dates (RADB data).

Mineralization (metallic deposits) radiometric age dates.

5. Geographic:

Topographic elevation.

Shaded relief of topography.

Degree of topographic slope.

The suitability of each map as mineralization evidence was determined in two steps. In the first,

initial evaluation was carried out using standard GIS analysis techniques, such as point-inpolygon analysis, interactive query and visual map inspection. In some instances, multi-map area

cross-tabulation overlays and histogram generation were performed to investigate spatial

associations in greater detail (see Bonham-Carter, 1994a, p. 238-264, for a discussion of these

techniques). In the second step, the evidence maps that were found to be promising deposit

indicators in the first step were measured for their spatial associations with respect to the

occurrences (as described in chapter 4 and detailed in Appendix B). The maps that displayed

clear and strong spatial associations were selected for formal analysis and modelling, and

include:

Lithologic.

Diversity of lithologic units.

Lithotectonic terrane.

Mesozoic pluton distance buffers.

Cenozoic fault distance buffers.

Isostatic residual gravity anomaly.

Total residual field geomagnetic anomaly.

K/Na geochemical anomaly.

Ba/Na geochemical anomaly.

The maps that were found unsuitable for weights of evidence mineral potential modelling were

rejected for one or more reasons:

1. Spatial correlations with the occurrences were either nonexistent (or not evident), poorly

developed, or not readily interpreted. Many maps were consequently rejected, including

the LANDSAT linear-features maps, the highly-extended terrains map, the deep-seated

basement fracture system distance buffer map, the thrust-front distance buffer maps, and

the Bouguer gravity anomaly map. Distance buffers around certain lithologic units seemed

a logical choice for modelling, but most of these maps had poorly developed or ambiguous

spatial correlations.
56

2. Some layers of evidence carried broadly similar types of deposit predictor/indicator

information. These include the igneous composition-slice and time-slice maps, the

Mesozoic pluton density map, the mineralization radiometric age date map, and the

topographic slope map (which interestingly is spatially correlated with the fault distance

buffer and lithologic diversity maps).

3. The spatial data density was insufficient or the distribution too irregular to produce a wellconstrained evidence map. In such instances, it was not possible to determine if the

inadequate data distribution was due to sampling bias or some other unknown controls.

Much of the geophysical data were rejected on this basis, in particular the heat flow and

other maps of geothermal measure, and the crustal thickness map. The PETROS major

element igneous rock geochemical data were also rejected because, while being fairly

uniform in distribution for the western United States, were not nearly dense enough.

4. Integrity problems with the source data. Most of the NURE geochemical data were

unuseable or rejected on this basis (see footnotes for section 6.7.1).

The evidence maps considered unsuitable for formal analysis and modelling provided essential

support for interpreting model output, developing metallogenic hypotheses, and illustrating

various geologic spatial associations. In particular, the shaded relief topographic map played a

critical role in delineating the crustal structures proposed in chapter 8 to control the regionalscale distribution of sedimentary and volcanic rock-hosted occurrences in the Nevada Great

Basin.

5.4 Error, Data Accuracy and Limitations 

5.4.1 Error Sources

Uncertainty of various types are inherent to spatial data, as well as its accompanying attribute

data. Error associated with spatial data may be subdivided into two broad categories: (1)

cartographic or positional error, and; (2) thematic or attribute error (Veregin, 1989; Chrisman,

1991). Aronoff (1989) cited six common sources of error encountered in using a GIS:

1. Data collection—errors in field data collection, in existing maps used as source data, and

in the analysis of remotely sensed data.

2. Data input—inaccuracies in digitizing, and fuzziness inherent in the edges of geographic

features.

3. Data storage—insufficient numerical precision and spatial precision.

4. Data manipulation—inappropriate class intervals, boundary errors, error propagation as

multiple overlays are combined, and slivers due to polygon edge-matching problems.

5. Data output—scaling inaccuracies, error caused by inaccuracy of the output device and

caused in medium instability.

6. Use of results—the information may be incorrectly understood or inappropriately used.

Thematic, or attribute data are complementary to positional data, and describe something about

the spatial object or characterize some phenomena occurring at that location (i.e.—an anomaly

in the geomagnetic field) . These data may be discrete or continuous in nature (Aronoff, 1989).

Error associated with attribute data can include (1) the misclassification of nominal data such

as a geological unit, (2) incorrectly ranked ordinal data such as metamorphic grade, (3)
57

incorrectly determined ratio data such as the length of a fault, (4) or the misrepresentation of

continuous data by imposing a classification scheme having too many intervals, thereby giving

the impression that the data are more numerous, well distributed, or robust than they might be

(Flowerdew, 1991). Many errors of this type may result from logical inconsistencies, such as in

the misclassification of a geological unit by a field mapper, or from interpolation and estimation

processes (Chrisman, 1991). For example, Morgan and Gosnold (1989) pointed out that making

geophysical heat flow determinations is not a routine procedure. Many local factors can affect

a measurement, such as topography, variations in ground-surface temperature, lateral variations

in thermal conductivity and conductivity anisotropy, ground-water flow in the measurement

interval, and a host of other conditions. With regard to gold-silver content of mineral deposits,

Wilkins (1984) indicated that the past methods and practices of determining and recording

production and reserves impact significantly on the accuracy of the figures. 

5.4.2 Data Limitations

Data quality and accuracy problems of the nature outlined above are not unusual to many of the

datasets compiled for this study. Quality and accuracy metadata of some form should exist for

every GIS dataset, but, unfortunately, not every dataset compiled for this analysis was

accompanied with metadata. A good deal of caution was exercised when collecting datasets.

Error detection and correction was performed throughout all aspects of assembling and

compiling both digital and analog data. Where necessary, hard copy of the data or the dataset

author was consulted. The consequences of mismatch between point and polygon locational

accuracy were also assessed (e.g—the proportion of volcanic or sedimentary rock-hosted

occurrences not falling within their respective rock-type map units; see discussion at the bottom

of section 6.4.3). It was not practical or possible to test the integrity of every aspect of every

dataset to the fullest extent. At some point in the quality control process, the data had to be

trusted and its overall quality understood. Despite such limitations and inherent errors, these

datasets present a reasonable representation of the various geologic characteristics of this region.
58

Chapter 6. Single Map Analysis, Interpretation, and

Mineral Predictor Map Generation

6.1 Introduction

Chapter 6 is an analysis of the mineral potential evidence. It represents the first of three stages

of model building, the model formulation or specification stage (Chatfield, 1988; see section

1.4). The material reviewed in this chapter constitutes the first of two procedures that compose

the weights of evidence mineral potential modelling method, graphically illustrated in Figure 4.1

as procedure “i”. The spatial associations between the gold-silver-bearing occurrences and the

multi-class mineralization evidence maps were established and measured, and the significance

of the relationships was determined. Binary-class mineral predictor maps (“layers of evidence”)

were created from the mineralization-favorable units composing the multi-class evidence maps.

The second and third stages of model building, parameter estimation or model fitting and model

validation, are addressed in chapter 7, where, in procedure ii of weights of evidence modelling,

the predictor maps were combined in a multi-map overlay using a loglinear formulation of

Bayes’ Rule to generate various mineral potential maps. In chapter 8, the results of the

modelling exercises carried out in chapters 6 and 7 are discussed and interpreted with respect to

geology and metallogenesis.

Most of the material presented in chapter 6 follows a standardized format. Datasets of like

character are grouped together and discussed in tandem. The general distribution of the goldsilver-bearing occurrences is examined in section 6.2. In sections 6.3 through 6.7, the

distribution of the occurrences is analysed with respect to lithology, lithologic diversity and

lithotectonic terranes, proximity to faults and plutons, and geophysical and geochemical

anomalies. With the exception of 6.2, each section is consists of:

1. Introduction

Summary overview of the dataset(s), its relevance to gold-silver-bearing occurrence

genesis and/or distribution.

A short statement highlighting the main points of the analysis carried out in that section.

2. Distribution and Spatial Association of gold-silver-bearing Occurrences

Examination and analysis of occurrence distribution with respect to the evidence maps.

Examination and analysis of the spatial associations between the occurrences and the

evidence maps.

Reclassification of multi-class evidence map to binary-class mineral potential predictor

map.

3. Interpretive Synthesis

Results from the previous section are interpreted and discussed in context of regionaland local-scale geology, tectonism, and magmatism.

Significance of the evidence as a mineral occurrence predictor is assessed.

Under each of these headings, analysis and discussion usually proceeds sequentially—firstly

from “primary occurrences” (all sizes and types, as a group), secondly to “sedimentary rockhosted occurrences”, and thirdly to “volcanic rock-hosted occurrences”.
59

The overall conclusion of the analyses carried out in this chapter is that the gold-silver-bearing

occurrences in Nevada can be subdivided into two broad mineral deposit types—sedimentary and

volcanic rock-hosted—as demonstrated by very different behavior in terms of their spatial

distributions and geologic associations. Collectively, the geologic features spatially associated

with the occurrences suggest that their respective regional-scale distributions are controlled in

large part by two separate and distinct fundamental crustal-scale structures (or sets of structures).

These structures—the Walker Lane shear zone and presumed northeast–southwest-trending

regional-scale structures in northern Nevada (appearing to control the distribution of volcanic

rock-hosted occurrences) and presumed northwest–southeast-trending regional-scale structures

in central–north-central Nevada (appearing to control the distribution of sedimentary rock-hosted

occurrences)—are introduced in this chapter. In chapter 8, these are further developed into firstorder constraints on the distribution of sedimentary and volcanic rock-hosted occurrences.

6.2 Gold-Silver-Bearing Occurrences

6.2.1 Introduction and Summary of Findings

Section 6.2 serves as a general introduction to the regional-scale distribution of the gold-silverbearing occurrences in space. No formal analyses or comparisons are made with respect to

mineral potential evidence. 

The principal findings of the analyses carried out in this section include: (1) gold-silver-bearing

occurrences in general have an affinity for the western half of the Great Basin (the central and

western portions of Nevada); (2) the distribution of volcanic rock-hosted occurrences forms a

semi-circular pattern which stretches across northern, northwestern, western, and southwestern

Nevada; and (3) the sedimentary rock-hosted occurrences form a roughly ovoid distribution

pattern in the central and east-central region of northern Nevada, which “cores” the semicircular distribution pattern of the volcanic rock-hosted occurrences. In addition, spatial density

maps of the precious and base metal occurrences, as well as map patterns of a number of other

geological datasets, show an overall “U”-shaped pattern to the distribution of precious metal

mineralization. Distributions (2), (3), and the “U”-shaped pattern may reflect regional-scale

lithotectonic features and/or crustal structural zones.

6.2.2 Distribution of Gold-Silver-Bearing Occurrences

The spatial point distribution of the primary occurrences is shown in Figure 6.1. The point

distributions of the sedimentary and volcanic rock-hosted occurrences are shown in Figure 6.2.

The occurrences appear to be preferentially concentrated in the western half of the Great Basin

(western and central Nevada), especially the large and medium size occurrences. The volcanic

rock-hosted occurrences form a semi-circular distribution pattern along the northern, western,

and southwestern borders of Nevada. The distribution of the sedimentary rock-hosted

occurrences forms a roughly ovoid pattern, which, by and large, is encompassed by the volcanic

rock-hosted occurrences, forming a sedimentary rock-hosted occurrence “core” region in

relation to the semi-circular volcanic rock-hosted occurrence distribution. Sedimentary rockhosted occurrences are also notably scarce in the region of the Walker Lane shear zone (see Figs.

1.3 and 6.2), where volcanic rock-hosted occurrences tend to be concentrated.
60

The density of gold-silver-bearing occurrences is compared and contrasted to the density of all

types of metallic mineral occurrences in Figure 6.3. The first-order density distribution pattern

confirms the affinity of the occurrences for the western half of the Great Basin (central and

western Nevada), as observed in Figures 6.1 and 6.3. Many of the occurrences appearing in the

eastern half of the Great Basin (eastern Nevada and western Utah), in particular those in the

southeast along the border of Utah, represent base-metal deposits (e.g.—Pb-Zn polymetallic

replacements) that are silver-rich, having “Ag” listed as their primary commodity in MRDS. If

these were removed from the gold-silver-bearing occurrences density map, the western affinity

would be even stronger. A western affinity is also recognized for occurrences having:

“Lighter metals” listed as the primary commodity—“lighter” defined here as having a

density generally much less than 7 g/cm3

, and “metal” as classified on the periodic table of

elements. These include (in ascending order of density in g/cm3

): Li, Na, Mg, Be, A., Ba,

Ti, As (a semi-metal), V, and Sb.

Mo or W listed as the primary commodity.

U or Th listed as the primary commodity (although this association does not appear to be

particularly strong).

The eastern half of the state is dominated by occurrences having base metals, in particular Pb and

Zn, as the primary listed commodity. The occurrences include polymetallic replacements, which

have a strong eastern affinity, and Pb-Zn skarns, which as a deposit type are not well represented,

but appear to have a weak eastern affinity as well. While it appears that occurrences having base

metals listed as their primary commodity may be more common in the eastern half of the Great

Basin (eastern Nevada and western Utah) than those having precious or “lighter” metals listed

as primary commodities, the density map for all metallic mineral occurrences (Fig. 6.3) indicates

that, in general, the western half of the Great Basin (central and western Nevada) is host to a

greater number.

Second-order density distribution patterns show that gold-silver-bearing occurrences are

noticeably concentrated in the central part of the Walker Lane shear zone region, and in a region

in the northwestern quadrant of Nevada known as the “Battle Mountain heat high” (Fig. 6.3; see

sections 2.6 and 2.7). Based on the distributions of occurrences in Figure 6.3, the elevated

density in the Walker Lane region probably represents a concentration of predominantly volcanic

rock-hosted occurrences, while sedimentary rock-hosted occurrences contribute more to the

higher density regions in northern Nevada (Fig. 6.2). Hot-spring-type Au-Ag and Hg mineral

occurrences have strong affinity for the “heat high” region. Gold-silver-bearing occurrences are

distinctly lacking along an axis trending northwest-southeast through the central part of the state,

extending from a most conspicuous region in southernmost Nevada known as the “amagmatic

zone” (Fig. 6.4), a feature which is related to the closure of the “Laramide magmatic gap”

around 22 Ma and represents the southern termination of the southwestward sweep of Tertiary

magmatism in the Great Basin (McKee, 1971; Blakely and Jachens, 1991; also see section 2.5.1).

This axis of occurrence-scarcity is better resolved in the density map of all metallic mineral

occurrences. The metallic occurrences form somewhat of a “U”-shaped or “horse-shoe-shaped”

distribution around this axis, as illustrated in the inset in Figure 6.3, entitled “Generalized

Distribution”.
61

6.2.3 Correlations and Interpretive Synthesis

The most prevalent occurrence distribution patterns that emerge from the observations above are:

(1) the eastern versus western affinity for Au-Ag primary commodity (western) and base-metal

primary commodity (eastern) occurrences, and sedimentary rock-hosted (eastern) and volcanic

rock-hosted (western) occurrences; (2) the roughly ovoid sedimentary rock-hosted occurrence

distribution that “cores” a semi-circular volcanic rock-hosted occurrence distribution, and; (3)

the generally “U”-shaped or “horse-shoe-shaped” overall distribution pattern of occurrences

(includes all types of mineralization).

The east-west distribution character observed in the precious-metal-bearing and non-preciousmetal occurrences is consistent with the observations of Roberts (1966), that Nevada contains

two distinct metallogenic provinces—an eastern base-metal province and a western preciousmetal province (see Fig. 1.3). The eastern province is characterized by deposits of Pb-Zn and

peripheral by-product precious metal producers, whereas the western province hosts W, Hg, Sb,

and primary precious metal producing deposits (Roberts, 1966, Shawe and Stewart, 1976). This

regional east-west distribution has generally been ascribed to the compositional makeup of the

crust—the eastern base-metal sub-province coincides with the eastern carbonate assemblage (the

miogeoclinal facies) of the Cordilleran Geosyncline, and may also be related to the presence of

Precambrian crystalline basement; the gold-silver occurrences of the western precious-metal subprovince are located in the western siliceous assemblage (the eugeosynclinal facies), and may

be related to crust composed of volcanic rocks and accreted arc and oceanic terranes (Roberts,

1966, Shawe and Stewart, 1976) (see section 3.5). In regional-scale investigations conducted

by Albers (1981, 1983) and Hutchinson and Albers (1992), it was found that non-placer goldproducing mines and districts in the Cordillera occur preferentially in accreted oceanic and

island-arc terranes, whereas silver-rich deposits exhibit an even stronger preference for the

craton, with the exception of northwestern Nevada, where deposits of both metals are common.

Tooker (1983, 1985) and Hutchinson and Albers (1992) concluded that the marked east-west

regional terrane preference of the precious-metal (Au) and base-metal (including Ag) deposits

is due to different source rock which provide different metals for the two deposit-type suites (for

additional information, see sections 3.5 and 3.6).

The sedimentary rock-hosted and volcanic rock-hosted occurrence point distributions observed

here (Fig. 6.2) are the same as those described by Cox et al. (1991) and Ludington et al. (1993)

(see Figs. 3.1 and 3.2). Ludington et al. (1993) referred to the semi-circular distribution of the

volcanic rock-hosted occurrences as the “epithermal crescent”, and noted that the sedimentary

rock-hosted occurrences are distributed in a roughly ovoid area inward from the crescent,

forming a circular “core” of sedimentary rock-hosted deposits in north-central to eastern

Nevada. This arrangement, wrote Cox et al. (1991), suggests that the tectonic environment in

which the volcanic rock-hosted occurrences formed appears to preclude the simultaneous or

subsequent formation of the sedimentary rock-hosted occurrences, irrespective of the presence

of suitable host rocks. This observation led Cox et al. (1991) and Ludington et al. (1993) to

conclude that the sedimentary rock-hosted occurrences, as a group, are probably older than the

volcanic rock-hosted occurrences (>27 Ma versus <27 Ma for the volcanic rock-hosted; see Fig.

3.2).

The “U”-shaped or “horse-shoe-shaped” distribution of the metallic occurrences (Fig. 6.3)
62

represents the superpositioning of sedimentary and volcanic rock-hosted distribution patterns.

The southwestern limb of the “U” is delineated by occurrences in the Walker Lane belt. The

northeastern limb is delineated by occurrences that lie primarily along the Battle MountainEureka (Cortez) and Carlin trends (sedimentary rock-hosted occurrences). The “hinge” region

of the “U” is delineated by occurrences that cluster around the region of the Battle Mountain

heat high and extend northeastward across northern Nevada (volcanic rock-hosted occurrences;

including sedimentary rock-hosted occurrences in north-central Nevada). The “U”-shaped

occurrence distribution pattern could be due to sampling bias, or it may be an artefact of the

techniques used to render the maps of the density distributions. However, this distribution

resembles (to varying degrees) map patterns that appear in a number of other regional-scale

geological datasets compiled for this study (Fig. 6.5), suggesting that the distribution may be

related to regional-scale lithotectonic and/or crustal features.

The “U”-shaped occurrence distribution appears to follow prominent northwest-southeast and

northeast-southwest structural trends visible in shaded relief of topography (Fig. 6.6; also see

Fig. 1.2). The “hinge” region occurrences coincide with the intersection of these trends (compare

Figs. 6.3, 6.5, and 6.6). Given these general observations, it is not unreasonable to consider that

the sedimentary rock-hosted occurrences may be controlled by regional-scale structural zones

that parallel the Battle Mountain-Eureka (Cortez) and Carlin mineral trends in north-central

Nevada, whereas the volcanic rock-hosted occurrences could be controlled by the

northwest–southeast-trending Walker Lane shear zone in western Nevada and possible regionalscale structures trending northeast-southwest across northern Nevada. Similar proposals,

particularly for the control of mineralization along the Battle Mountain-Eureka (Cortez) and

Carlin trends, have been made in various studies, as reviewed in chapter 3 (section 3.4 and 3.6).

The control of the Walker Lane shear zone over the distribution of volcanic rock-hosted

occurrences is well recognized (see Cox et al., 1991). Of special interest is the

northeast–southwest-trending distribution of volcanic rock-hosted occurrences across northern

Nevada. No structural zone is commonly cited or recognized as a control over this distribution.

The presence of a regional-scale structural feature may be reflected in the metallic mineral

occurrence density maps (see Fig. 6.3), where a broad but clearly linear distribution of

occurrences extends northeastward and southwestward away from the region of the Battle

Mountain heat high (compare Fig. 6.3 to 6.5g and 6.5h). Along this trend, in a belt similar in

width to the Walker Lane shear zone, sub-parallel linear features are visible in shaded relief of

topography (Figs. 6.6 and 1.2). The most readily recognized feature of comparable position,

orientation, and dimension is the “Humboldt gravity zone”, a broad and diffuse Bouguer and

isostatic gravity anomaly high (Mabey et al., 1983). The origin of this zone, or what it

represents, is controversial and uncertain (literature on this feature is sparse). If the Humboldt

zone marks some type of broad crustal structure, it might exercise some degree of control over

the regional-scale distribution of mineral occurrences in northern Nevada. The Humboldt zone,

and other possible controls over the regional-scale distribution sedimentary and volcanic rockhosted mineralization, are investigated in greater detail in subsequent sections in this chapter,

and chapters 7 and 8, where the occurrences are analysed with respect to various geologic

phenomena.
63

6.3 Lithology

6.3.1 Introduction and Summary of Findings

The distribution and spatial association of the gold-silver-bearing occurrences in relation to

lithology is examined in this section.

Lithology is represented in two maps, the geological map of Nevada (Stewart and Carlson, 1978;

Turner and Bawiec, 1991; Fig. 6.7) and a lithologic assemblage map (Fig. 6.8). The geological

map of Nevada contains 101 lithologic units. The assemblage map contains 20 units and is

generalized from the geological map (see Appendix A). Special treatment was given to the

lithology map. Single-layer-of-evidence (i.e.—lithology) mineral potential maps, highlighting

the lithologic units that have the highest posterior probability of hosting occurrences of various

sizes and types, were prepared to facilitate interpretation and illustrate the geographic component

of the distribution of the occurrences. Maps showing only the high-potential lithologic units

were created as well.

The principal results of the analyses conducted in this section indicate that: (1) most of the goldsilver-bearing occurrences are hosted by marine and/or shelf carbonates of late Precambrian to

late Paleozoic age, or by felsic to intermediate and intermediate to mafic extrusive and intrusive

rocks of early Mesozoic to late Cenozoic age; (2) most of the medium and large size sedimentary

rock-hosted occurrences are hosted by early to mid-Paleozoic marine and shelf assemblages,

whereas most of the medium and large size volcanic rock-hosted occurrences are hosted by late

Cenozoic felsic to intermediate extrusive and intrusive assemblages; (3) sedimentary rock-type

lithologies display a stronger spatial association with occurrences of all sizes and types than do

volcanic rock-type lithologies, whereas for large and medium size occurrences of any type,

sedimentary and volcanic rock-type lithologies have about the same degree of spatial association

with the occurrences; (4) most of the units composing the facies transitional between the eastern

carbonate assemblage and western siliceous assemblage (specifically units St, Ot, Ct, CZS, Zw,

and OCt) have strong spatial associations with the occurrences; (5) the sedimentary rock-hosted

occurrences have the strongest spatial association with lithologic units composing the western

siliceous and transitional assemblages (specifically units Os, Ch, Osv, St, and Ct); (6) the

volcanic rock units composing the 6-17 Ma time-slice of Stewart and Carlson (1976) are strongly

correlated with the volcanic rock-hosted occurrences, and these units are broadly distributed

along the Walker Lane shear zone and Humboldt zone; (7) the volcanic unit Tmi (Tertiary

intrusive rocks of intermediate to mafic composition) has the strongest weight of spatial

association with volcanic rock-hosted occurrences; (8) lithologic units strongly correlated with

small size occurrences of any type are generally more widely distributed across Nevada, and are

located in the eastern (carbonate) assemblage and western (siliceous) assemblage, in the Walker

Lane shear belt, and along the trend of the transitional assemblage; (9) lithologic units strongly

correlated with large size occurrences of any type are generally less widely distributed across

Nevada, and are located in the eastern (carbonate) assemblage, and particularly, in the Walker

Lane shear zone belt; (10) the distribution of lithologic units strongly correlated with medium

size occurrences of any type is weakly gradational between the distributions observed for units

correlated with small and large size occurrences, but bears slightly more similarity to the

distribution of the units correlated with large size occurrences; (11) spatial correlations suggest

that small size occurrences of all types are hosted by a wider variety of lithologies than big size
64

1 Very limited and local occurrences of serpentinite (unit Pzsp) occur within the Walker Lane belt and in

northern Nevada within the region of the Humboldt zone (as indicated on the 1:500,000 scale geological map

of Stewart and Carlson, 1978). The limited presence of serpentinite may be genetically significant, possibly

indicating mantle uplift and upward thrusting of mantle slices that were structurally emplaced at shallow crustal

levels as a result of alternating compressional-extensional transcurrent movement along the Walker Lane shear

zone (see Vielzeuf and Kornprobst, 1984; Cameron, 1993). Because of Pzsp's small areal coverage, however,

it is recognized as a highly rated unit but treated as an outlier.

occurrences, and overall, that big size occurrences in general should form in units that show a

different geographic distribution, and to some degree, a different composition than those units

that would host small size occurrences.

The most important result of this investigation with respect to training datasets and the analyses

carried out in the remainder of this chapter, is that (1) the large and medium size occurrences

exhibit similar lithologic affinities and map patterns and, therefore, may be treated as a group

(the sample of “big” occurrences), rather than as two separate samples, and (2) the sedimentary

rock-hosted and volcanic rock-hosted samples are better treated as two single samples composed

of all sizes of their respective occurrences, rather than being subdivided into their respective

large, medium, and small sub-samples (in part because of the low number of occurrences making

up both the large or medium samples).

6.3.2 Distribution and Spatial Association of Gold-Silver-Bearing

Occurrences

The review of occurrence distribution presented below excludes occurrences that are apparently

“hosted” by unconsolidated or semi-consolidated Quaternary cover units (valley fill), such as Qa

(alluvium), Qp (playa, etc.), Qls (landslide), Qm (moraine), and QToa (older alluvium) (see

Appendix A for a description of these units). The relatively high number of occurrences falling

within Quaternary cover units is discussed in the interpretations section below.

The most common group of like-lithologic assemblages hosting all sizes and types of

occurrences are the marine and/or shelf carbonates of late Precambrian to late Paleozoic age (Fig.

6.9, dashed box). Approximately 34% of all occurrences, and ~29 % of the medium and large

size occurrences, fall within these four assemblage units. Of these four units, the greatest

number of large and medium size occurrences (~20%) are hosted by the early to mid-Paleozoic

marine and shelf assemblage. The single lithologic assemblage unit with the greatest number

of occurrences is the late Cenozoic felsic to intermediate extrusive and intrusive assemblage unit,

hosting nearly 15% of all and nearly 32% of the large and medium size. The rank of individual

lithologies relative to the number of occurrences hosted is shown in Figure 6.10. Unit Tt2

(Tertiary silicic ash-flow tuffs) has the greatest number of occurrences, hosting over 11%. Most

of the large and medium size occurrences (almost 20%) are hosted by unit Ta3 (Tertiary andesite

and intermediate volcanic rocks). The lithologic unit having the highest spatial association with

the occurrences is Pzsp1

 (Paleozoic serpentinite), due to the fact that it covers only 6 km2 but,

hosts 8 occurrences (2 small, 2 medium, 4 of unreported size in the Candelaria district within the

Walker Lane shear zone; occurrence locations are believed to be accurate) (Fig. 6.11). The

majority of lithologic units with higher W+

 values (10th percentile and greater W+ values;

i.e.—W+

 values ranging from ~0.5 to 5) for spatial association with the occurrences are
65

predominantly sedimentary-type (~40% sedimentary; ~32% volcanic), and is even more so if the

units of “mixed” rock type are included (~56% sedimentary; 38% volcanic). The top five highest

rated units, excluding of Pzsp1

, consist of mainly sedimentary rock-type units. For large and

medium occurrences as a group, more volcanic lithologies have strong W+

 and the lithologies

are split nearly 50%-50% between sedimentary units and volcanic units.

The most common group of like-lithologic assemblages hosting the sedimentary rock-hosted

occurrence types are the marine and/or shelf carbonates of late Precambrian to late Paleozoic age

(Fig. 6.12, dashed box). Approximately 51% of all sedimentary rock-hosted occurrences and

~58 % of the medium and large size fall within these four assemblage units. Of these four units,

as well as any of the assemblage units, the greatest number of large and medium size sedimentary

rock-hosted occurrences are hosted by the early to mid-Paleozoic marine and shelf assemblage

(50%). The single lithologic assemblage unit having the greatest number of sedimentary rockhosted occurrences is the early to mid-Paleozoic marine and shelf assemblage, hosting nearly

34% of all and 50% of the large and medium size. The rank of individual lithologies relative to

the number of sedimentary rock-hosted occurrences hosted is shown in Figure 6.13. Units Os

(Ordovician clastics, metamorphics, carbonates) and St (Silurian limestone, limy clastics, cherts)

have the greatest number of sedimentary rock-hosted occurrences, hosting over 29%. The

lithologic unit having the highest weight of spatial association with sedimentary rock-hosted

occurrences is St (Silurian limestone, limy clastics, cherts) (Fig. 6.14), part of the facies

transitional between the eastern carbonate assemblage and western siliceous assemblage (seeFig.

2.9). Most of the other units composing the transitional assemblage (i.e.—Ot, Ct, CZs, Zw, and

OCt) also have strong associations with the gold-silver-bearing occurrences in general (see Fig.

6.11). 

The most common group of like-lithologic assemblages hosting the volcanic rock-hosted

occurrences are the felsic to intermediate and intermediate to mafic extrusive and intrusive rocks

of early Mesozoic to late Cenozoic age (Fig. 6.15, dashed box). Approximately 49% of all

volcanic rock-hosted occurrences and ~66 % of the medium and large size fall within these three

assemblage units. Of these three units, as well as any of the assemblage units, the greatest

number of large and medium size volcanic rock-hosted occurrences is hosted by the late

Cenozoic felsic to intermediate extrusive and intrusive assemblage (~60%). The single lithologic

assemblage having the greatest number of volcanic rock-hosted occurrences is the late Cenozoic

felsic to intermediate extrusive and intrusive assemblage, hosting 40% of all and over 60% of

the large and medium size. The rank of individual lithologies relative to the number of volcanic

rock-hosted occurrences hosted is shown in Figure 6.16. Units Tt2 (Tertiary silicic ash-flow

tuffs), Ta3 (Tertiary andesite, intermediate rocks), and Tr3 (Tertiary rhyolite flows and shallow

intrusives) have the greatest number of volcanic rock-hosted occurrences, hosting approximately

50% of all and over 50% of the large and medium size. The lithologic unit having the highest

weight of spatial association with volcanic rock-hosted occurrences is Tmi (Tertiary intrusive

rocks of intermediate to mafic composition) (Fig. 6.17). Other related intrusive rocks (Tri and

Ti) also have relatively strong associations with all occurrences, as well as some plutonic rocks

of Triassic (Tgr and TJgr) and upper Cretaceous age (Jgr, KJd, Mzgr, and Kgr).

Based on the above relationships, three lithology mineral potential predictor maps were

produced: (1) a ternary-class map composed of low, medium, and high units of favorability for

hosting primary occurrences (Fig. 6.18); (2) a binary-class map of favorable units for hosting
66

sedimentary rock-hosted occurrences (Fig. 6.19); and, (3) a binary-class map of favorable units

for hosting volcanic rock-hosted occurrences (Fig. 6.19). The high number of units composing

the lithology map (101), and the minuscule area covered by the most favorable units (e.g.—the

four highest rated units cover 202 km2

, ~0.07 % the area of Nevada), necessitated the

combination of groups of high favorability lithologic units (typically 7-15 units) into single

favorability units for the predictor maps.

6.3.3 Correlations and Interpretive Synthesis

The spatial associations for the occurrence samples discussed above were used to create posterior

probability maps for the small, medium, and large size occurrences, and are presented in Figure

6.20. These maps highlight in bulk lithologic units that are most highly correlated with the

occurrences, and those units which host more occurrences than would be expected due to chance

(that chance being the prior probability, see chapter 4 for more information). These single-layerof-evidence posterior probability maps are used to detail and interpret the geographic component

of the distribution of the spatial association data with regard to the lithologic units, which is

lacking in the histograms presented above. The maps subdivide the lithologic unit posterior

probabilities into six intervals—the lower values into larger and roughly equal areas (white and

grey), and the higher values into smaller and roughly equal areas (blue, green, yellow, and

red)—so as to highlight the distribution of those units with higher potential.

The distribution of lithologies having high posterior probabilities of hosting small, medium, and

large size occurrences forms a continuum in map pattern between the small and large occurrence

end members (Figs. 6.20a and 6.20c). Relative to the medium and large size occurrence

posterior probability maps, the lithologies having a high posterior probability of hosting small

size occurrences (Fig. 6.20a) are in general more widely distributed across the region, and

displays a posterior probability high (red) which is both smaller in area coverage and different

in geographical extent than the highs of the medium and large maps (Figs. 6.20b and 6.20c). The

small occurrences map highlights units of the eastern (carbonate) assemblage and western

(siliceous) assemblage, units in the Walker Lane shear belt, and especially units that parallel the

trend of the transitional assemblage (see Figs. 1.3, 2.3, and 2.9). Lithologies having a high

posterior probability of hosting large size occurrences(Fig. 6.20c) are generally less widespread

compared to the small occurrences map, and are preferentially located in the Walker Lane and

eastern (carbonate) assemblage. Lithologic units in the Walker Lane shear zone have particularly

high (red) posterior probabilities for hosting large size occurrences. The distribution of

lithologies having a high posterior probability of hosting medium size occurrences (Fig. 6.20b)

is weakly gradational between the distributions of the small and large occurrences maps (Figs.

6.20a and 6.20c), displaying highs in the Walker Lane (similar to large occurrences map), as well

as more highs in central and northwestern Nevada than does the large occurrences map (similar

to the small occurrences map). Overall, however, the medium occurrences posterior probability

map shares more characteristics with the large occurrences map than it does with the small. All

three small, medium, and large occurrence maps taken together, the lithologic units having the

highest posterior probabilities (red and yellow) are roughly coincident to the aforementioned

“U”-shaped distribution (see section 6.2) of the occurrences—the highest posterior probability

lithologic units in the medium and large occurrences map delineating the southwestern limb of

the “U”, the second highest (yellow) posterior probability lithologic units in the large and

(especially) and medium occurrences map delineating the northeastern limb, and the “hinge”
67

2 For a given occurrence type or size sample, an elevated posterior probability is defined as those values which

are greater than (and equal to) the value at the most pronounced upward inflection point on a sorted frequency

distribution curve of the posterior probabilities. If more than one inflection point was prominent, the value

closest to the upper ~20-25% fraction of the data sample was chosen.

region of the “U” delineated by the high posterior probability (red and yellow) units in the small

occurrences map (compare Fig. 6.20 to Figs. 6.3 and 6.6).

The specific lithologies making up the higher posterior probability units for the small, medium,

and large size occurrences are shown in Figure 6.21. These lithologic units represent those

having an “elevated” posterior probability.2

 These maps show essentially the same distribution

patterns as the posterior probability maps above (Fig. 6.20), except that they highlight the

lithologies with the highest posterior probabilities. As a result, the lithologies shown in these

maps correspond with those having the highest posterior probability (red) on the posterior

probabilitymaps (Fig. 6.20), but are probablymore representative of the units having anomalous

or significant spatial associations with the occurrences. The elevated posterior probability

lithologic unit maps (Fig. 6.21) suggest that the small size occurrences would tend to occur in

a greater number of lithologies (a wider variety) than the medium or larger deposits, but this

observation, though it may be true, is in part seen because of the far greater number of small

(n=2269) occurrences as compared to the medium (n=118) or large (n=59) occurrences.

Therefore, because of their greater number and wider distribution, the small size occurrences

would be expected to be hosted by a greater number (variety) of lithologies. One point which

is of interest is the relative proportion of sedimentary rock-type lithologies versus igneous rocktype lithologies making up the respective small-medium-large elevated posterior probability

units. The number of sedimentary rock-type lithologies units composing the elevated posterior

probability units hosting small occurrences is greater than the number of igneous rock-type units

(10 to 7), as opposed to the medium and large occurrence elevated posterior probability lithology

maps which have a greater number of igneous rock-type units (5 to 4 and 5 to 3, respectively;

these counts include unit Pzsp, serpentinite). This is probably due in part to the greater number

of large and medium occurrences represented in the volcanic rock-hosted sample as compared

to the sedimentary rock-hosted sample.

The distribution patterns for medium and large size occurrences displayed by both the posterior

probability (Fig. 6.20) and elevated posterior probability lithology (Fig. 6.21) maps suggest that

these two occurrence samples may be treated as one sample—the sample of “big” occurrences.

All subsequent analyses were carried out on this one big sample, rather than the two separate

medium and large samples. In addition the lower cutoff for medium-sized occurrences,

~803,768 Troy ounces Au content (and/or ~16,075,375 Troy ounces Ag content), is close to the

informal demarcation for “giant” “million ouncer” gold camps. The lithologies having elevated

posterior probabilities for hosting big occurrences is compared to those having elevated posterior

probabilities for hosting small occurrences in Figure 6.22. The distribution of lithologies

between these two samples is noticeably different. For the units having an elevated posterior

probability of hosting small size occurrences, the number of sedimentary versus igneous rocktype lithologies is 10 to 7 (respectively; ~60% versus ~40% of the total number of lithologies),

whereas for the units having an elevated posterior probability of hosting big size occurrences,

the proportions are equal (5 sedimentary and 5 igneous lithologies). Only four elevated posterior

probability lithologies are shared between the small and big occurrence samples (Tmi, Trk, Zw,
68

and Pzsp; all less than 420 km2 in area, and together totalling ~700 km2, or <0.25% of the total

area of Nevada). The lithologies having elevated posterior probability of hosting small

occurrences form a pattern trending roughly north-northeast, and correspond to the trend and

some of the lithologies of the transitional assemblage (Fig. 6.23). Note also that the lithologies

and the trend correspond to the general north-south occurrence density trend, which is positioned

along the border separating the eastern base metal metallogenic province from the western

precious metal metallogenic province (see Figs. 1.3, 6.1, and 6.3). The distribution of lithologic

units having an elevated posterior probability for hosting big occurrences is correlative to

regions known to host larger deposits, such as the Carlin and Battle Mountain-Eureka trends, and

in the Independence group in northeastern and north central Nevada, and along the Walker Lane

(Virginia City-Tonopah), Aurora, and Beatty-Search Light trends (see Fig. 3.3). Overall, the

elevated posterior probability lithology maps suggest that the lithologies that host big

occurrences are different in their geographic distribution, and possibly in their composition, than

those that host smaller size occurrences.

The particular lithologies having elevated posterior probabilities for hosting sedimentary rockand volcanic rock-hosted occurrences are shown in Figure 6.24. The distribution patterns for

both of these elevated posterior probability lithologic unit maps reflect the distribution pattern

of the occurrences (compare Fig. 6.2) and represent suitable host-rocks. For the sedimentary

rock-hosted occurrences, the lithologic units of elevated posterior probability (Fig. 6.24a) are

composed of nearly equal proportions of western (siliceous) and eastern (carbonate) assemblage

lithologies (~45% and ~48% respectively; compare Fig. 6.24a to 6.23a). However, as indicated

above in section 6.3.2, the sedimentary rock-hosted occurrences have the strongest spatial

association with lithologic units composing the western siliceous and transitional assemblages

(specifically units Os, Ch, Osv, St, and Ct). The western and transitional assemblages are

coincident with the western edge of the Precambrian craton and the Roberts Mountain and

Golconda thrust fronts, which are regional-scale crustal features that may have served to focus

and concentrate mineralizing processes, and is further discussed in sections 6.4.3, 6.5.3, 6.6.3,

and 6.7.3.

For the volcanic rock-hosted occurrences, the lithologic units of elevated posterior probability

(Fig. 6.24b) are particularly correlative to the volcanic units making up the 34-17 Ma and 17-6

Ma age units in Stewart and Carlson's (1976) time-slice maps, as compared in Figure 6.25. The

elevated posterior probability lithologies map for volcanic rock-hosted occurrences presented in

Figure 6.25a is modified (lower threshold W+

 value decreased) to include more volcanic units.

This was done for the purpose of highlighting the correlation, but even without this modification,

Figure 6.24b can still be seen to correlate well with the 17-6 Ma time-slice volcanic unit of

Figure 6.25b (also see Fig. 6.4). This correlation indicates that the volcanic rock-hosted

occurrences were closely tied to the space-time migration of magmatism across Nevada (as

depicted in time-slice maps of Stewart and Carlson, 1976; Fig. 6.4). The regional distribution

trend of the 17-6 Ma time-slice volcanic units parallel the Walker Lane shear zone and the

Humboldt zone (compare Fig. 6.4 to 1.3 and 6.6). As indicated above and in section 6.2, the

Walker Lane belt and the region of the Humboldt zone region are host to a large number of

important volcanic rock-hosted occurrences (see Figs. 1.3, 3.3, and 6.2). Cox et al. (1991)

suggested that Walker Lane-style (strike-slip) deformation provided the tectonic conditions that

permitted the formation of volcanic rock-hosted deposits. In the Humboldt zone, the broad,

linear distribution of volcanic rocks and their associated mineral occurrences suggests that
69

3 A total of 226 occurrences, including 5 large and 10 medium size occurrences. Of the total number of

sedimentary rock-hosted deposits, ~13% fall in alluvium and 8% in igneous rock types. Of the total number

of volcanic rock-hosted occurrences, ~9% fall in alluvium and 13% in sedimentary rock types.

similar conditions and regional-scale structures may have focused magmatic and mineralizing

activity. The high degree of spatial association between the volcanic rock-hosted occurrences

and the volcanic units of the 34-17 Ma and 17-6 Ma time-slice implies that mineralization along

the Walker Lane belt and Humboldt zone took place between 34-6 Ma, with the possibility that

many (most?) formed between 17-6 Ma. The age of volcanic rock-hosted mineralization is

generally within one to several million years of that of the host rocks (see Hutchinson and

Albers, 1992) or less (possibly as little as 50,000 years at Round Mountain; see Henry et al.,

1997).

The Quaternary cover units, especially unit Qa, were “apparently” host to a significant number

of occurrences3. Because of the relatively large areal coverage of these units (139,103 km2

combined, or ~49% of the total area of Nevada), they (particularly Qa) were rated as having a

low spatial association with the occurrences (with the exception of those cover units having

relatively small areas, such as the landslide or moraine units). Occurrences hosted by Quaternary

cover or by units other than expected (e.g.—volcanic rock-hosted mineralization in sedimentary

rocks, or vice versa) can be explained by (1) mineralization in pediments that are covered by

Quaternary erosional materials, the real host rock possibly being that which lies range-ward of

the valley-fill sequence, (2) mineralization in sedimentary host-rocks that were later covered by

volcanic sequences (e.g.—a mineralized sedimentary unit is stratigraphically below a postmineralization volcanic unit, or vice versa), or (3) volcanic rock-hosted mineralization in

sediments or gravels associated with the geothermal environment (see Table 3.2). Alternatively,

the occurrence points and/or the mapped unit boundaries might be misplotted.

6.4 Lithologic Diversity and Lithotectonic Terranes

6.4.1 Introduction and Summary of Findings

The distribution and spatial association of the gold-silver-bearing occurrences in relation to the

diversity of lithology (Fig. 6.26) and lithotectonic terranes (Fig. 6.27) is reviewed in this section.

The diversity map measures the spatial complexity of lithology, and was prepared from the

geological map of Nevada (Stewart and Carlson, 1978). The lithotectonic terranes map is a

representation of tectonically juxtaposed accreted terranes, and was prepared primarily from the

maps of Silberling et al. (1987) and Silberling (1991). 

In the Great Basin, lithologic complexity, at least in the plan view of a map, is in part a product

of the structural, stratigraphic, and intrusive relationships within a given neighborhood area of

measurement. For example, faults distort, dismember, and rotate structural blocks, disrupt the

continuity of units, juxtapose unrelated rocks, and expose, end-up, once-horizontal stratigraphic

units. The greater the number and more intricate these relationships, the more spatially complex

an area should appear. The purpose of creating a lithologic diversity map is to determine the

degree of spatial association between mineral occurrences and such areas, the rationale being that

structure, stratigraphy, and intrusive activity are important factors contolling mineralization
70

and/or exposing ore bodies (see sections 3.4 and 3.6). It is likely, therefore, that lithologically

diverse regions should show a high degree of spatial association with the occurrences. This

proposition is supported by the research of Griffiths and Smith (1992), where it is demonstrated

that there is a simple linear relationship between geologic diversity and the mineral-resource

diversity. They found that sample units with relatively high diversity are favorable hosts for

metallic ores, as demonstrated by most of the counties in Nevada (12 of 17) which have a high

diversity and are prolific producers of base- and precious-metals. Geologically, Griffiths and

Smith (1992) interpreted this as “reflecting the complex mixture of igneous, metamorphic, and

sedimentary rocks that accompany metallic ores which, in turn, serves to emphasize the

complexity of processes necessary to provide a good potential for metallic ores and a high

geologic diversity.”

The lithotectonic terranes in Nevada are composed of Paleozoic and lower Mesozoic rocks, and

each have a distinctive geologic history (see Silberling, 1991). The Mesozoic rocks represent

fringing-arc, back-arc basin, and shelf environments with respect to North America (see Fig.

2.4). The Paleozoic rocks are incorporated and remobilized in the Mesozoic terranes, some of

these having pre-Mesozoic emplacement histories (i.e.—Roberts Mountain and Golconda

allochthons) (Silberling, 1991). The terranes are fault-bounded, and the present-day map pattern

is primarily the result of Middle Jurassic to Early Cretaceous thrust and strike-slip faulting,

further modified by faulting, rotation, and tilting in response to Cenozoic extensional tectonics

(Silberling, 1991). See Figure 6.27 for more information.

The principal results of the analyses conducted in this section indicate that occurrences in

Nevada have a strong spatial association with: (1) areas of moderate lithologic complexity,

where the strength of the spatial association generally increases with increasing lithologic

diversity, but declines sharply in areas of moderately-high to high lithologic complexity (an

effect related to lack of occurrences in very small areas of high complexity); and (2) accreted

lithotectonic terranes that may possibly have an intrinsically high gold content (i.e.—potential

gold source rocks) and that may also be proximal to possible deep-seated regional-scale crustal

tectonic zones, which could have facilitated the movement of mineralizing fluids and formation

of ore deposits (e.g.—the Walker Lane shear zone, the north–northeast-trending mid-Paleozoic

to mid-Mesozoic overthrust zones of the Roberts Mountain, Golconda, and Luning-Fencemaker

allochthons, and the western margin of the North American Precambrian craton).

6.4.2 Distribution and Spatial Association of Gold-Silver-Bearing

Occurrences

Diversity units #2, #3, and #4 contain the most occurrences (76.5%), and the greatest number of

sedimentary rock-hosted occurrences (nearly 83%) and volcanic rock-hosted occurrences (over

80%) (Figs. 6.28 and 6.29). This is because they collectively represent 65% of the total area of

Nevada (see solid and dashed lines in Figs. 6.28 and 6.29). Of these three units, #3 contains the

greatest number of primary occurrences (nearly 31%) and the largest number of big occurrences

(nearly 33%). Unit #4 contains the greatest number of sedimentary rock-hosted occurrences

(nearly 32%). Units #2 and #3 contain most of the volcanic-rock-hosted occurrences (32%

each). Overall, sedimentary rock-hosted occurrences tend toward areas of slightly greater

lithologic diversity than do the big, small, or volcanic rock-hosted occurrences. The spatial

association between occurrences and lithologic diversity increases with increasing diversity.
71

Figure 6.30 shows the strength of spatial association (contrast of W+ and W–) between all

occurrences and successive (non-area-cumulative) units of lithologic diversity. Collectively,

diversity units #4-#7 account for ~40% of all occurrences, ~35% of big occurrences, ~44% of

sedimentary rock-hosted occurrences, and ~26% of volcanic rock-hosted occurrences, but have

a combined area of only ~15% of the total area of Nevada. At higher degrees of lithologic

diversity, units #8 to #11, the strength in spatial association between the occurrences and

lithologic diversity tapers off quickly, which is probably due to the fact that these diversity units,

representing ~0.06% of the total area of Nevada, are highly scattered and account for only 3

occurrences (see Figs. 6.26 and 6.28).

The North American miogeocline (NAm) lithotectonic terrane contains the greatest number of

occurrences (35%), primarily by virtue of its large extent (~48% of the total area of Nevada),

followed by the Roberts Mountain allochthon (RM) hosting 16%, and the Jungo (JO, includes

the Fencemaker allochthon) and the Golconda allochthon (GC), which collectively host about

10% of the occurrences each (Fig. 6.31). Most of the big size occurrences are hosted by the

NAm (~27%) and RM (~26%) terranes, RM consisting of only ~11% of the total area of Nevada.

The largest number of sedimentary rock-hosted occurrences is hosted by the RM terrane (~62%)

and by NAm (~23.5%) (Fig. 6.32). The largest number of volcanic rock-hosted occurrences are

hosted by the Cenozoic cover deposits (Cz), JO, and NAm terranes, containing each about 20%

of the occurrences (Fig. 6.32). The lithotectonic units having the overall greatest spatial

association with the primary occurrences sample are the Paradise (PD), Pine Nut (PN), RM, and

Sand Springs (SS) terranes (Fig. 6.33). Big size occurrences show a strong spatial association

for SS and RM, and in particular the PN terrane (accounting of ~13%). The RM terrane shows

the strongest spatial association for sedimentary rock-hosted occurrences, hosting ~62%, while

volcanic rock-hosted occurrences are most strongly associated with the JO (~19%), PN (~14%),

and SS (~8%) terranes.

Based on the above relationships, four binary mineral potential predictor maps were produced;

one from the diversity of lithology map, and three from the lithotectonic terranes map (Figs. 6.34

and 6.35). The lithologic diversity binary predictor map is composed of diversity units #4-#11,

representing units favorable for hosting primary occurrences, sedimentary rock-hosted

occurrences, and volcanic rock-hosted occurrences. The lithotectonic terranes map was

reclassified into three separate binary predictor maps, one for each of the three occurrence

samples (primary, sedimentary rock-hosted, and volcanic rock-hosted). All three of the maps

are combinations of two or more lithotectonic terranes.

6.4.3 Correlations and Interpretive Synthesis

The increase in spatial association between occurrences and lithologic diversity with increasing

diversity may reflect a higher density of structures that are thought to control, focus, localize, or

expose mineralization (as expressed at the present-day erosional surface). If the proposition

stated in the introduction to section 6.4 is reasonable—that the lithologic diversity of a map is

dependent on the degree to which various structural, stratigraphic, and intrusive features are

present at the surface within a region—then areas of higher lithologic diversity should represent

(1) areas of post-mineralization exposure, where faulting and other tectonic activity have

revealed crustal sections which host mineral occurrences, and/or (2) regions of “structurally

prepared” host rocks that might facilitate ore formation if mineralizing processes have taken
72

place. The degree to which either one (or a combination) of these possibilities is responsible for

the positive spatial association between lithologic diversity and mineral occurrences would likely

be contingent on the age of mineralization relative to the features or events resulting in higher

diversities. Where mineralization post- (or syn-) dates the events and features responsible for

lithologic diversity, a higher density of faults and intrusives could provide “plumbing” for

mineralizing fluids. If the faults (fault zones?) are presumed to continue at depth, they might

promote upper crust permeability, acting as conduits and pathways for ascending deep-crustal

fluids and magmas, which could contribute heat, metals, and/or agents for gold complexing and

transport (also see sections 2.7.6, 2.7.7, and 3.4). Regions of “structurally prepared” ground

might also expose geochemically favorable stratigraphic units. If mineralization pre-dates

diversity, then the spatial correlation might largely exist by virtue of exposure and exhumation

of ore-bearing units (see for example Wallace, 1991).

In the western United States and Mexico, a number of studies have found correlations between

the lithotectonic composition of the crust and ore deposit type and metal content (see Roberts,

1966; Albers, 1981, 1983; Campa and Coney, 1983; Tooker, 1983; Titley, 1987, 1989, 1991;

Hutchinson and Albers, 1992; see reviews in sections 3.5 and 3.6). In summary, these studies

indicate that base-metal, silver, and ferro-alloy mineralization is more commonly related to crust

composed of, or underlain by, Precambrian craton, whereas precious-metal mineralization

(particularly gold) is more often associated with geosynclinal terranes accreted to the craton. In

Nevada, the lithotectonic terranes that have stronger spatial associations with the occurrences

may be composed of country-rocks that could have gold content above that of typical

“background” values. Country-rocks (and rocks at depth) with higher than normal gold content

may be potential source rocks for gold and other ore fluid components, and include mafic

extrusive and intrusive igneous rocks and, in particular, pelitic geosynclinal sequences. This is

investigated below.

The RM, GC, and JO terranes (spatially associated with the sedimentary rock-hosted

occurrences) are composed of deep oceanic pelitic (RM and GC) and terrigenous-clastic (JO)

sedimentary sequences (Silberling, 1991). The gold content of unaltered (sulfide-free or sulfidepoor) sedimentary rocks is reported to be restricted to a relatively narrow range, rarely exceeding

5 ppb and generally below 2 ppb (see Korobeynikov and Goncharenko, 1986; Krauskopf, 1979;

Rose et al., 1979; Govett, 1983; Korobeynikov, 1985). In a geosynclinal depositional

environment, however, pelitic rocks and terrigenous sequences that have undergone any

alteration, even if only diagenetic, may have comparatively higher gold contents, and could

potentially be a source for gold, especially if any degree of pyritization has taken place.

Korobeynikov (1985) determined that the gold content of miogeoclinal-eugeoclinal carbonaceous

black-shales, with respect to associated carbonate, clastic, and volcanic sequences, is elevated

by a factor of 5-10 (6.7-65 ppb), having an estimated average gold content of 14.7 ppb. Pyrite

was found to be the main concentrator of gold in carbonaceous shales and other pelitic rocks,

containing 27-1360 ppb gold (Korobeynikov, 1985). Aureoles of elevated gold content were

controlled by jointing zones, which allowed for influx of diagenetic and other solutions. Given

the depositional and deformational history of the Cordilleran geosyncline, it is not unreasonable

that assume that the geosynclinal sequences, especially those at greater depths, were affected by

a certain amount of alteration, even if only diagenesis (in central Nevada, these units have

attained great thickness, perhaps as much as 15 km at time of deposition; Stewart, 1980;

Dickinson, 1981; also see Fig. 2.9). In Nevada, Nelson (1991) observed that many Carlin-type
73

deposits are spatially associated with rock units reported to contain metalliferous marine black

shales, particularly the Vinini, Valmy, Woodruff, and Comus formations. In this study, three of

these units—Vinini (Os), Valmy (Osv), and Chainman (MDs)—are found to host ~30% of the

sedimentary rock-hosted occurrences (Fig. 6.13) and have especially strong weights of spatial

association (Fig. 6.14; also see legend in Fig. 6.24a). The Vinini formation (Os), in particular,

is highly correlated with the sedimentary rock-hosted occurrences. The pelitic rocks of the

Cordilleran geosyncline may constitute potential pre-enriched source rocks from which, with the

aid of induced permeability (i.e.—faults, fractures, stockworks, breccias), gold and other metals

were scavenged. This is consistent with the findings of Hofstra (1997) and Hofstra et al. (1991),

where new and existing sulfur isotope data from various Carlin-type gold deposits were

examined (deposits include Jerritt Canyon, Getchell and Twin Creeks, Carlin, Post-Betze,

Alligator Ridge, Gold Pick, Chert Cliff, Mercur, Melco, and the Lone Tree distal-disseminated

deposit). Hofstra (1997) concluded that the bulk of 34S values indicate that H2S in the ore fluids

was derived from a variety of sedimentary sources (values from Jerritt Canyon, Carlin, and Gold

Pick are high enough to effectively exclude a magmatic source). Gold and sulfur may have been

scavenged from the Cordilleran geosyncline sedimentary pile by deeply circulating meteoric

fluids (Hofstra, 1994, 1997; Ilchik and Barton, 1997) or from ascending fluids derived from

metamorphic devolatilization at depth (Seedorff, 1991).

The PD, SS, and PN terranes (associated with the volcanic rock-hosted occurrences) are all

mainly volcanogenic, with varying degrees of intermediate-mafic rocks and basinal strata

(Silberling, 1991). Tilling et al. (1973) reported that unaltered igneous rocks, particularly rocks

of intermediate or silicic composition, have a rather restricted range in gold content, rarely

exceeding 10 ppb and generally well below 5 ppb, but that mafic rocks tend to have more gold

than felsic and intermediate rocks in both plutonic and volcanic suites (also see Gottfried et al.,

1972; Korobeynikov, 1986; Korobeynikov and Goncharenko, 1986). The presence of basalt or

gabbro might have acted as a pre-enriched source rock, from which circulating hydrothermal

fluids could have scavenged gold. However, Tilling et al. (1973) concluded that the

concentration of gold in a potential source rock is a trivial factor, and that the differences in gold

content among common rock types is too small to consider any particular rock type a more

favorable source of gold than another (also see Keays and Scott, 1976).

The lithotectonic terranes that have stronger spatial associations with the occurrences are also

coincident with deep-seated regional-scale crustal tectonic zones. As previously discussed in

chapter 3 (section 3.4) and earlier in this section, zones of structural deformation could facilitate

vertical and lateral fluid movement, as well as promote fluid-wall rock interactions. The RM,

GC, and JO terranes represent north–northeast-trending mid-Paleozoic to mid-Mesozoic

allochthons, emplaced along the Roberts Mountain, Golconda, and Luning-Fencemaker thrust

faults, respectively (see Figs. 2.4, 2.5, and 2.13; section 2.7.1; also see Hardyman and Oldow,

1991; Silberling, 1991). The orientation and location of the overthrusts coincide with the Late

Proterozoic rifted margin of the North American craton, as delineated by 87Sr/86Sr ISr = 0.706 and

a late Paleozoic or early Mesozoic regional-scale paleothermal anomaly (Figs. 2.10 and 2.11).

The rifted craton margin, a zone of crustal weakness and a suture along which terrane accretion

took place, could have focused and localized mineralizing processes (see Cunningham, 1988;

also see sections 2.4 and 3.4). The PD, SS, and PN terranes are located within the Walker Lane

shear zone (see section 2.6). The Walker Lane crosscuts the older northeast-southwest Paleozoic

orogenic grain that is tectonically truncated against the younger northwest-southeast Mesozoic
74

orogenic grain, and may be related to rifting and/or translational activity which took place during

the early Mesozoic (the “Early Mesozoic truncation event”; Silver and Anderson, 1974;

Dickinson, 1981; Suppe, 1985; also see section 2.6). The Walker Lane also has an extensive

history of Cenozoic strike-slip movement (see Stewart and Crowell, 1992) and may coincide

with a possible major crustal flaw zone, perhaps dating back to the Precambrian (Blakely and

Jachens, 1991). As noted above, such a regional-scale structure could have focused and

localized mineralizing processes and provided the tectonic conditions that permitted the

formation of volcanic rock-hosted deposits (Cox et al., 1991).

From the spatial analyses carried out in this section, it is not possible to isolate exactly why

lithotectonic terranes are important to the formation of gold-silver-bearing occurrences in

Nevada. However, it would seem that an association with lithotectonic terranes composed of

pre-enriched source rock could be a significant factor for sedimentary rock-hosted

mineralization, whereas it may not be critical for the formation of volcanic rock-hosted

occurrences. Ore depositing fluids in volcanic rock-hosted mineralizing systems appear to have

important magmatic-derived components (see Rock and Groves, 1988; Rock et al., 1989;

Giggenbach, 1992a, 1992b, 1992c; Muffler et al., 1992; Hedenquist and Lowenstern, 1994;

Hedenquist et al., 1996; Taylor, 1996; Norman et al., 1996). For both sedimentary and volcanic

rock-hosted mineralization, it seems that proximity to regional-scale structural features could

facilitate mineralization. Perhaps of more importance to the formation of gold-silver-bearing

occurrences in Nevada is the focusing of mineralizing processes by crustal structures associated

with the lithotectonic terranes, which serve only to provide physiochemically suitable host rocks

rather than pre-enriched source rocks.

6.5 Distance Buffers Around Plutons and Faults

6.5.1 Introduction and Summary of Findings

The distribution and spatial association of the gold-silver-bearing occurrences in relation to

distance from plutonic bodies (Fig. 6.36) and faults (Fig. 6.37) are reviewed in this section.

Maps of distance buffers from plutonic bodies (Fig. 6.36b) and faults (Fig. 6.37b) were prepared

using data from the geological map of Nevada (Stewart and Carlson, 1978). 

The plutonic rocks examined here range in age from Late Triassic to Late Miocene, but with

respect to total area covered, are predominantly Mesozoic (Fig. 6.36a). Strictly Mesozoic rocks

(Mzgr, Kgr, KJd, Jgr, Trgr) make up ~81% of the total area of all the plutonic rocks combined,

and range in composition from granitic to dioritic and quartz-monzonitic. Rocks transitional

between Cretaceous and Tertiary age include only one unit, TJgr, which makes up less than 3.6%

of the combined total area, and is granitic in composition. The Cenozoic rocks (Tgr, Tri, Tmi,

Ti) are Tertiary (~43-6 Ma) intrusions, ranging in composition from felsic to intermediate, with

some mafic units, and make up nearly 20% of the combined total area. Conventional wisdom

indicates that, for whatever the reason, genetic or otherwise, metallic mineralization is often

found proximal to plutonic and intrusive igneous rocks. A map of relative spatial-density of

plutons was constructed for Nevada (Fig. 6.36c), and the density distribution map pattern was

found to bear a close resemblance to the map pattern for the density distribution of occurrences

(Fig. 6.36d) (even more so to the density distribution map pattern for precious and base metal
75

4 Azimuthal filtering of the fault dataset by the application of a binomial probability distribution technique

(Donald. L. Sawatzky, 1994, personal communication) determined that statistically significant fault orientations

lie between N35W and N29E, forming an envelope of approximately 30o

 either side of north. A comparison

of the strength of spatial correlation (contrast, C) between the occurrences and the azimuthally filtered datasets

was found to be not as strong as the spatial relationship between the occurrences and the unfiltered dataset,

hence the filtered fault dataset was not used for this analysis.

mineralization; compare to Fig. 6.3). It is therefore reasonable to investigate the spatial

association between occurrences and plutonic rocks in the Great Basin. Plutonic and intrusive

bodies were buffered at intervals of 0.0–0.5, 0.5–1.0, 1.0–1.5, 1.5–2.5, 2.5–5.0, and 5.0–10.0 km.

The faults examined here are those which appear on the geological map of Nevada (Stewart and

Carlson, 1978) (Fig. 6.37a), and include faults that are mapped as solid lines, dashed lines, dotted

lines, or otherwise inferred (see Turner and Bawiec, 1991). The faults are of all ages, but

predominantly Cenozoic, and include thrust, strike-slip, normal and reverse. No differentiation

was made between fault type, age, or orientation4

. On the deposit and district level, faults are

known to be important ore controls and conduits for mineralizing fluids, and play a large part in

“ground preparation”, which make the potential host rocks more receptive or more reactive to

ore-bearing solutions (Guilbert and Park, 1986). Berger (1996) indicated that, empirically, it

may be inferred that epithermal orebodies form preferentially in constraining structural

environments within active faults. Fluid flow systems always show coupled mechanical,

thermal, and hydraulic phenomena (Byron R. Berger, 1998, personal communication). The rate

of change of synhydrothermal deformation may be a key aspect of focusing fluids and

establishing environments favorable to efficient ore-depositing processes (Berger, 1996). Given

the tectonic history of the Great Basin, it would therefore seem probable that a spatial

relationship between faults and occurrences should exist, and the coupling of stress and fluid

flow is the plausible argument (Byron R. Berger, 1998, personal communication). An analysis

of fault distance buffers with respect to the location of mineral occurrences is one manner by

which this relationship can be quantified in order to help predict precious metal mineralization.

The fault dataset was buffered at intervals of 0.0–0.5, 0.5–1.0, 1.0–1.5, 1.5–2.5, 2.5–5.0, and

5.0–10.0 km (Fig. 6.37b). The relative concentration of faults per unit area is shown in Figure

6.37c.

The most significant results of the analyses carried out in this section suggest that gold-silverbearing occurrences in Nevada have a strong spatial association with areas of close to moderate

proximity (1.0-2.5 km) to plutons, and close proximity (1.0-1.5 km) to faults. Sedimentary rockhosted occurrences also show a moderate spatial association with distance (2.5-5.0 km to 5.0-

10.0 km) from plutonic and intrusive bodies. These results indicate that fault zones and

structural deformation related to regional-scale plutonism may have facilitated formation of the

gold-silver-bearing occurrences.

6.5.2 Distribution and Spatial Association of Gold-Silver-Bearing

Occurrences

The distribution of occurrences, relative to pluton buffers, indicates that a nearly equal number

fall within 0.5 km of (or in) plutonic bodies (22.2%) as do outside the outermost buffer zone

(>10.0 km, 23.7%) (Fig. 6.38). However, only ~12% of the big occurrences fall within 0.5 km
76

of (or in) plutonic bodies, the majority (48.6%) lie 5-10 km (19.2%) and >10 km (29.4%) away.

Approximately 45% of all occurrences fall in or within 2.5 km of plutons, and just over 50% of

the big deposits lie in or within 5 km of plutonic bodies. The volcanic rock-hosted occurrences

have a distribution trend across the buffer zones that is somewhat similar to the primary

occurrences, however, the greatest number of occurrences (~41%) fall 5-10 km (~20%) and >10

km (~21%) away from plutonic bodies, where as only ~11% occur in or within 0.5 km of a

pluton (Fig. 6.39). Approximately 60% of all volcanic rock-hosted occurrences lie in or within

5 km of a pluton. The distribution of sedimentary rock-hosted occurrences differs substantially

from the distribution trends of both the primary occurrences and the volcanic rock-hosted

occurrences. The highest number of sedimentary rock-hosted occurrences are located at a

distance from plutonic bodies (Fig. 6.39). Approximately 23.5% of all occurrences lie in or

within 2.5 km of plutonic bodies, whereas ~76.5% lie further than 2.5 km away from a pluton,

the greatest number of occurrences (nearly 37%) lying >10 km away. The spatial association

between occurrences and the proximity to plutonic bodies is strongest for primary occurrences

within 1 km, strongest for sedimentary rock-hosted occurrences within 1.5 km, and strongest

for big size and volcanic rock-hosted occurrences within 2.5 km of a plutonic or intrusive body

(Fig. 6.40).

Approximately half of all the occurrences, of any of the occurrence type or big or small size

samples, fall within 1 km of a fault (or fault zone)—greater than 50% of the primary occurrences,

>64% of the big size occurrences, ~57% of the sedimentary rock-hosted occurrences, and >49%

of the volcanic rock-hosted occurrences (Fig. 6.41 and 6.42). The 1 km buffer for all of the

faults collectively represents ~27% of the total area of Nevada. The majority of occurrences in

each of the occurrence-type or size samples fall within 0.5 km of a fault—just over 32% of the

primary occurrences, >41% of the big size occurrences, >28% of the sedimentary rock-hosted

occurrences, and ~31% of the volcanic rock-hosted occurrences. The 0.5 km buffer for all of the

faults collectively represents ~16% of the total area of Nevada. There are also a relatively large

proportion of occurrences beyond 2.5 km of a fault (~10% for any occurrence-type or size

sample) and a significant number (about 8% for any occurrence-type or size sample) farther than

5 km from a fault. Outside of 10 km, only about 1% of the small size and volcanic rock-hosted

occurrences are found, but none of the big size or sedimentary rock-hosted occurrences. The

distribution of sedimentary versus volcanic rock-hosted occurrences shown in Figure 6.42

suggests that sedimentary rock-hosted occurrences (as a group) may be somewhat more closely

distributed around faults than are volcanic rock-hosted occurrences. The spatial association

between occurrences and the proximity to faults is strongest for primary occurrences within 5

km, strongest for big size, sedimentary and volcanic rock-hosted occurrences within 1.5 km (Fig.

6.43).

Based on the above relationships, two binary mineral potential predictor maps were produced,

one from the pluton buffer map and a second from the faults buffer map (Fig. 6.44). Both maps

are a combination of distance buffers that have high spatial associations with primary

occurrences, sedimentary rock-hosted occurrences, and volcanic rock-hosted occurrences. The

binary predictor map based on distance buffers from plutonic and intrusive bodies is a

combination of buffer zones 1 and 2 (0.0-0.5 and 0.5-1.0 km), a 1.0 km threshold. The binary

predictor map based on distance buffers from faults is a combination of buffer zones 1 and 2

(0.0-0.5 and 0.5-1.0 km), a 1.0 km threshold.
77

6.5.3 Correlations and Interpretive Synthesis

The strong spatial correlation between the occurrences and the plutonic and intrusive rocks may

be related to structural modifications made to the country rocks during emplacement, which may

have affected the crust at deeper levels and on a regional scale, and could be a product of older

pre-Cenozoic events. Most of the plutonic rocks in western Nevada were emplaced in the

Mesozoic during the Nevada-Elko phase of the Cordilleran orogeny when the continental

magmatic arc was developing. This was well before most of the sedimentary and volcanic rockhosted occurrences formed, 42-30 Ma (Hofstra, 1997) and 27-5 Ma (Ludington et al., 1993),

respectively (see Fig. 3.2).

The emplacement of plutonic rocks is often accompanied by a high degree of structural

modification to the surrounding country rocks (see Billings, 1972, chapter 17). At intermediate

crustal depths, surrounding material is heated and compressed and transported laterally away

from the zone of emplacement (Gastil, 1979; also see Paterson and Fowler, 1993). Upper crustal

rocks respond by undergoing lateral extension along brittle high- and low-angle and strike-slip

structures, while lower crustal rocks are thought to move laterally by ductile flow (Gastil, 1979;

also see Christie-Blick and Biddle, 1985; Sonder and England, 1986; Gans et al., 1989;

Thompson et al., 1989; Armstrong and Ward, 1991; Buck, 1991). Shearing at depth may also

act to accommodate mid-crustal mass transfer (see Allmendinger et al., 1987; Hauge et al., 1987;

Potter et al., 1987; Kusznir and Matthews, 1988; Malavieille and Taboada, 1991; Snyder et al.,

1997). Strain related to pluton emplacement can result in an overall increase in permeability at

both the upper and lower crustal levels (an increase in strain from 1% to 6% is capable of

producing a five order of magnitude or greater increase in permeability; Spiers and Peach, 1989;

see discussion in sections 2.7.6, 2.7.7, and 3.4). Hence, ductile flow and shear structures at depth

could facilitate the transfer of heat and fluids from the lower crust and increase fluid-wall rock

reaction-surface areas from which circulating fluids could scavenge metals. Brittle structures

in the shallow crust could promote near-surface fluid circulation, focusing and localizing

mineralizing processes, as well as creating favorable environments for mineral deposition

(e.g.—structural traps). Cenozoic magmas and deeply circulating ore fluids may have taken

advantage of the structural pathways and reaction-surface areas that the Mesozoic plutons could

have provided, thus perhaps explaining in part the strong spatial association between the plutons

and the occurrences.

The high degree of spatial association between the occurrences and close proximity to faults is

not unexpected. Fault and shear zones have long been recognized as important conduits for the

movement of fluids within the crust. This is well established from geochemical studies of

alteration and mineralization in ancient structures and from contemporary observations related

to seismic events (Kerrich, 1986). Crustal fluids play mechanical and chemical roles in a host

of geologic phenomena which include faulting and shearing, magma generation, metamorphism,

hydrocarbon migration, and transport of minerals (Kerrich, 1986; Oliver, 1986; Sibson, 1987).

In the North American Cordillera, Eaton (1984) suggested that the circulation of hydrothermal

fluids through extensional structures was the most important process responsible for Cenozoic

ore formation. Similar associations are observed in Precambrian mid-crustal mesothermal

deposits within compressional shear zones, such as those of the Canadian greenstone

metamorphic belts (e.g.—Yellowknife, Larder Lake, Timmins, and many other shear zone hosted

gold deposits). The degree and extent of mineralization associated with highly faulted and
78

sheared regions demonstrates how important these structures are to focusing fluid movement.

In the Great Basin, faults, fault zones, rift zones, and related structures are important to precious

metal mineralization at the province-wide, regional, and local levels. At the province-wide scale

(deep crustal), structural zones could provide throughgoing lower-crust–upper-mantle

permeability, promoting whole-crust fluid circulation, and assisting in the transference of heat

by allowing warmer lower-crustal fluids and melts to more easily penetrate and migrate to higher

crustal levels. At the regional scale (shallow crustal and mineral belt), structures may control

localization of intrusions, volcanic fields, high-level thermal anomalies, meteoric hydrothermal

convection cells, and individual gold deposits (see Rowley, 1996, for example; also see section

3.4). Linear alignments of igneous features and possible basement flaws are believed to be

present at several Cretaceous and Cenozoic gold deposits (see Bagby and Berger, 1985; Madrid

and Bagby, 1986; Berger and Bonham, 1990; Shawe, 1991; Rodriguez, 1997; also see sections

3.2.2, 3.3.2, and 3.4) . In many volcanic rock-hosted and pluton-centered deposits, intrusions

acted as heat engines driving hydrothermal convection. Brittle faulting of upper crust provides

“plumbing”, which promotes permeability and facilitates near surface and deep fluid circulation.

Regional-scale shear and fault zones form fluid pathways that serve to concentrate and direct

hydrothermal gold-bearing solutions (Bagby and Berger, 1985), as well as “structurally prepare”

potential source rocks by effectively increasing the fluid-wall rock reaction-surface area,

enhancing the leaching of metals from the rocks. At the local scale (deposit and ore body), faults

may control the loci of mineral deposition, as well as determine the orientation of various faultrelated structural traps, such as veins, fractures, stockworks, breccia pipes, and shatter zones.

Faults and related secondary permeability structures facilitate physiochemical reactions that may

induce mineral precipitation (e.g.—”fluid throttling”), and expose geochemically favorable

environments to precious metal-bearing fluids where mineral precipitation may take place.

Along the Carlin-trend, for example, sedimentary rock-hosted deposits and ore bodies are

localized within favorable stratigraphic intervals near controlling local faults (see Bagby and

Berger, 1985; Bakken and Einaudi, 1986; Percival et al., 1988).

6.6 Geophysics—Geomagnetic and Gravity Anomalies

6.6.1 Introduction and Summary of Findings

The distribution and spatial association of the gold-silver-bearing occurrences in relation to the

total residual field geomagnetic and isostatic residual gravity anomalies are reviewed in this

section (Fig. 6.45). The maps were prepared from data obtained from the National Oceanic and

Atmospheric Administration.

Gravity and geomagnetic studies are complementary methods that are very valuable to mineral

exploration (for comprehensive reviews, see Overstreet and Marsh, 1981; Blakely and Connard,

1989; Simpson and Jachens, 1989). They provide information about local- and regional-scale

subsurface crustal structures and igneous rocks important to mineralization and ore formation,

and contribute to the delineation and interpretation of the geologic development of a region.

Geomagnetic anomalies are produced by contrasts in the magnetic properties of various rock

types (primarily at shallow crustal levels, and in particular, igneous rocks). Magnetite (Fe3O4)

and its solid solutions with ulvöspinel (Fe2TiO4), and in some situations a few other Fe-bearing

minerals and compounds (usually hematite and pyrrhotite), are the most important magnetic
79

minerals, hence basalts and their intrusive equivalents are usually more magnetic than rhyolites

and their equivalents (Mabey, 1983; Blakely and Connard, 1989). Anomalies originating from

shallow (<1 km) sources (local) are higher in amplitude and shorter in wavelength, and appear

“sharp” and well-defined, whereas anomalies originating from deeper sources (regional) are

generally lower in amplitude and longer in wavelength, and appear broad and “smoother”

(Blakely and Jachens, 1991; Saltus, 1988). Gravity anomalies (Bouguer or isostatic) are the

result of lateral and vertical variations in rock density. Bouguer gravity anomaly maps

emphasize regional signatures, originating as deep as the lower crust and upper mantle, whereas

isostatic anomaly maps, the data being corrected for the regional effect of topographic

compensation, isolate and emphasize small but broad intracrustal gravity anomalies, more clearly

reflecting the shallow density distributions of the upper crust (Mabey et al., 1983; Saltus, 1988;

Blakely and Jachens, 1991). Some of the more prominent magnetic and gravity anomaly features

in Nevada are listed in Figure 6.45.

The analysis in this section focuses on the total residual field geomagnetic and isostatic residual

gravity anomalies, both of which emphasize somewhat different geologic structures and features

in the shallow and upper crust. The Bouguer anomaly map is used to facilitate interpretations

and for visual comparisons. Initial trials using the Bouguer and isostatic gravity datasets

revealed that the isostatic gravity anomaly possessed a significantly better overall spatial

correlation with the occurrences than did the Bouguer anomaly. In addition, the isostatic gravity

dataset is a derivation of the Bouguer dataset, and as such, displays spatial patterns similar to

those of the Bouguer anomaly. 

The most significant results of the analyses carried out in this section suggest that sedimentary

rock-hosted occurrences tend to occur in regions of lower intensity geomagnetic anomalies (0 nT

and less) and higher intensity isostatic gravity anomalies (0 mG and greater), whereas volcanic

rock-hosted occurrences tend to occur in regions of higher intensity geomagnetic anomalies

(greater than 50 nT) and lower intensity gravity anomalies (-10 mG and less). On a regional

scale, the geophysical character of these two deposit types suggest that: (1) sedimentary rockhosted occurrences are associated with tectonically over-thickened crust, which is predominantly

sedimentary in nature, generally lacking in late Cenozoic volcanism, and proximal to possible

deep-seated structural features (e.g.—northern Nevada rift zone); and (2) volcanic rock-hosted

occurrences are associated with crustal segments that lie along (or within) broad and diffuse

structural zones, which have been subjected to wide-spread felsic volcanic activity and intruded

by mafic and intermediate magmas (e.g.—Walker Lane belt; Humboldt zone). The occurrences

are associated with contrasts in magnetic and gravity anomaly intensities, which are transitional

between high and low values, suggesting a close spatial relationship to structural features that

might act to facilitate fluid movement and/or host ore deposits.

6.6.2 Distribution and Spatial Association of Gold-Silver-Bearing

Occurrences

The frequency of occurrences display a fairly normal distribution across the magnetic intensity

spectrum (Fig. 6.46), centered slightly to the negative side of the zero value, and correlate closely

with the map area represented by each anomaly interval. Most of the occurrences (~39%) fall

between -75 to +25 nanoteslas (nT), representing ~38% of the total area of Nevada, with the

greatest number of occurrences (~11%) falling in the -50 to -25 nT range, representing ~11% of
80

the total area of the Nevada. The sedimentary and volcanic rock-hosted occurrences also show

near-normal distributions (Fig. 6.47), but the distribution peaks of the occurrence samples are

significantly offset—the distribution peak of the volcanic rock-hosted occurrences is offset

toward the positive anomaly values (centered on 0 to +25 nT), whereas the sedimentary rockhosted occurrence sample is offset more toward the negative anomaly values (centered on -75

to -50). The volcanic rock-hosted occurrences are also found over a broader range of magnetic

anomaly values. This relationship indicates that sedimentary rock-hosted occurrences are

preferentially located in areas of generally lower intensity magnetic anomalies, while volcanic

rock-hosted occurrences are located in regions having generally higher values. The spatial

association between the occurrences and the magnetic anomaly reaches its maximum strength

in the low positive anomaly values (peaking at 0 nT) for the primary and volcanic rock-hosted

occurrences. The >+450 to -200 nT intensity range accounts for about 55% of the volcanic rockhosted and ~42.5% of the primary occurrences, and for both occurrence samples represents

~29% of the total area of Nevada. The big size and sedimentary rock-hosted occurrences reach

maximum strength of spatial association with the magnetic anomaly in the middle range of

anomaly intensity (peaking at -200 and -175 nT, respectively) (Fig. 6.48). The >+450 – -175 to

-200 nT intensity range accounts for virtually all (>98%) of the big size and sedimentary rockhosted occurrences, and represents nearly 95% of the total area of Nevada. The sedimentary

rock-hosted occurrences have a strong spatial association (non-area-cumulative) with a relatively

narrow range of lower intensity magnetic anomalies (-100 to 0 nT), as shown in Figure 6.48 by

the heavy black line segment of the non-area-cumulative contrast (C) curve.

The distribution of primary occurrences is near normal across the isostatic gravity intensity

spectrum (Fig. 6.49), the peak slightly offset toward the negative values (centered on -10 to 0

milligals), and correlates to the area of each anomaly interval. Most of the occurrences (~34%)

fall in the -10 to 0 milligal (mG) interval, representing ~28% of the total area of Nevada, with

the greatest number (nearly 29%) falling in the 0 to +10 mG interval, representing ~18% of the

total area of Nevada. The distributions of the sedimentary and volcanic rock-hosted occurrences

are skewed—the sedimentary rock-hosted occurrence toward positive values, and the volcanic

rock-hosted occurrences toward negative values (Fig. 6.50, see trend lines). Sedimentary rockhosted occurrences clearly have a preference for regions having higher isostatic gravity levels,

whereas the volcanic rock-hosted occurrences tend toward regions of lower isostatic gravity. The

non-area-cumulative C curves for the isostatic gravity anomaly (Fig. 6.51) indicate that the big

size and sedimentary rock-hosted occurrences have positive spatial associations with 0 to >+10

mG intensities, cumulatively representing ~25% of the total area of Nevada, and accounting for

~43.5% of the big size and ~69% of the sedimentary rock-hosted occurrences. The sedimentary

rock-hosted occurrences show an especially strong association with the highest isostatic gravity

anomaly intensity interval (>10 mG), representing ~7% of the total area of Nevada, but

accounting for nearly 41% of the occurrences—a rather notable correlation. The volcanic rockhosted occurrences show the highest degree of spatial association with anomaly intensity values

of -20 to -10, representing ~26% of the total area of Nevada and ~36% of these occurrences.

Once again, the volcanic rock-hosted occurrences display a partiality for the lower end of the

isostatic gravity anomaly spectrum, while the sedimentary rock-hosted occurrences show a bias

for the higher end of the spectrum.

Based on the above relationships, two sets of three binary-class mineral potential predictor maps

were produced from the magnetic and isostatic gravity geophysical anomaly maps, one for each
81

of the three occurrence samples (primary occurrences, sedimentary rock-hosted occurrences, and

volcanic rock-hosted occurrences) (Fig. 6.52). For the magnetic anomaly map, the 19 highest

intensity intervals were combined for the primary occurrences binary predictor map, and the 17

highest intervals were combined for the volcanic rock-hosted occurrence binary predictor map.

In both cases, the threshold intensity interval was determined using the area-cumulative contrast

(C) maximum. The volcanic rock-hosted occurrences binary predictor map excludes the two

intensity intervals with the highest C values (hence combining 17 rather than 19 map successive

classes) in order to emphasize the relation between the occurrences and the volcanic rocks that

the anomalies represent. The sedimentary rock-hosted binary predictor map was produced by

combining a group of four successive map classes that had the highest non-cumulative C values

(Fig. 6.48, black segment of dashed curve). The magnetic binary predictor map for primary

occurrences accounts for ~42.5% of the occurrences, and represents ~29% of the total area of

Nevada. For the isostatic gravity anomaly map, the three highest intensity intervals were

combined for the primary occurrences binary predictor map, and the highest intensity interval

was used for the sedimentary rock-hosted occurrence binary predictor map. The volcanic rockhosted occurrences binary predictor map is a combination of two successive map classes

(intensity intervals 0 to -10 and -10 to -20) that, in combination, displayed the strongest weights

of association with the occurrence, but especially important, yielded a more geologically

meaningful binary map pattern than did other combinations of gravity map classes.

6.6.3 Correlations and Interpretive Synthesis

The gold-silver-bearing occurrences display two distinct frequency distribution trends with

respect to the magnetic and isostatic gravity anomalies: (1) the sedimentary rock-hosted

occurrence sample distribution mode is offset from zero toward negative magnetic anomaly

values, and toward positive gravity anomaly values; and (2) the volcanic rock-hosted occurrence

sample distribution mode is offset from zero toward positive magnetic anomaly values, and

toward negative gravity anomaly values, an inverse relationship relative to the sedimentary rockhosted occurrences sample (see Figs. 6.47 and 6.50).

The following two sections discuss the spatial relationship between various regional-scale crustal

features interpreted from the geophysical data and the distribution of the sedimentary and

volcanic rock-hosted occurrences. This material is expanded upon and the ideas further

developed in chapter 8.

6.6.3.1 Sedimentary Rock-Hosted Occurrences: Geomagnetic Lows—Isostatic Gravity

Highs

The frequency distributions for sedimentary rock-hosted occurrences, skewed toward negative

magnetic values and positive isostatic gravity values, indicate that these occurrences tend to be

found in regions characterized by less magnetic and higher density rocks (i.e.—regions having

lower magnetic anomalies and higher isostatic gravity anomalies). Figures 6.53a and 6.54a show

the spatial distribution of magnetic and isostatic gravity anomalies associated with these

occurrences. The geographic regions that these geophysical anomalies cover, especially the

magnetic anomaly, are broadly correlative in extent with the circular-shape distribution pattern

of the sedimentary rock-hosted occurrences, which form a “core” lying inward of the crescentshape volcanic rock-hosted occurrences (compare to Fig. 6.2).
82

Magnetic anomaly lows in Nevada are generally associated with rhyolitic and related tuffaceous

rocks, granitic rocks, and other non-igneous rock-types which are deficient (relative to mafic

rocks) in magnetic minerals. The most prominent low-amplitude negative anomaly is the

“magnetic quiet zone” (Figs. 6.45a, feature #3, and 6.53a). The magnetic quiet zone forms a

north-trending zone along eastern Nevada that extends from a broad, low-amplitude, regionalscale magnetic low in the northeastern quadrant of Nevada to a smaller, circular region of very

low magnetic intensity in southernmost Nevada, known as the “amagmatic zone.” The

amagmatic zone marks the southern termination of the southwestward sweep of Tertiary

volcanism (McKee, 1971; see Fig. 6.4), and may be non-magnetic simply because it was not

affected by magmatism that occurred prior to 17 Ma (Blakely and Jachens, 1991). The broad

portion of the magnetic quiet zone in northern Nevada corresponds to widespread coverage of

thick sections of sedimentary rocks produced by late Paleozoic and Mesozoic overthrusting

(Berger and Henley, 1989; see Figs. 2.4, 2.5, and 2.6), which may in part be responsible for the

regional low (sedimentary rocks are commonly less magnetic than igneous rocks). The origin

of magnetic quiet zone in northern Nevada may be quite complex, and is perhaps a product of

regional-scale hydrothermal alteration, which effectively reduced the magnetic qualities of the

quiet zone rocks, or is related to a belt of granitoids of low magnetic susceptibilities and remnant

magnetizations, which are localized along the western edge of the craton margin (see Blakely,

1988, for full discussion).

Isostatic gravity anomaly highs in Nevada are related to rocks, or packages of rocks, of higher

density. On a local level, gravity highs generally correlate with ranges where pre-Tertiary

basement rocks are near the surface (Blakely and Jachens, 1991). On a much broader regionalscale, particularly in northern and northeastern Nevada, gravity highs correlate with the

tectonically over-thickened crust (Berger and Henley, 1989; see Figs. 2.4, 2.5, and 2.6), which

represents upward thrust emplacement of deeper-origin, higher density packages of rock (Blakely

and Jachens, 1991; Saltus, 1988). The two isostatic gravity anomalies of principal concern in

Nevada are a “gradient ridge” associated with the possible southern extension of the northern

Nevada rift and the Humboldt zone gravity high (Figs. 6.45b, feature #1; 6.45b, feature #5; and

6.54a).

The Humboldt zone isostatic gravity high stretches northeast-southwest across northern Nevada,

and is well distinguished in the northwestern quadrant of Nevada by a Bouguer gravity high, and,

of particular interest, is correlative in broad extent with the magmatic quiet zone in the

northeastern quadrant of Nevada (Figs. 6.45a, feature #3; 6.45b, feature #5; and 6.53a). As

indicated above, the northeastern portion of this anomaly is coincident with tectonically overthickened crust. The Humboldt zone is also correlative with a wide band of Cenozoic volcanism

that trends northeast-southwest across northern Nevada (compare Figs. 6.45b and 6.4). The

delineation, character, origin, and the relationship of the Humboldt zone to the distribution of

the sedimentary and volcanic rock-hosted occurrences is further discussed in chapter 8.

The northern Nevada rift is most readily recognized by its geomagnetic signature (Fig. 6.45a,

feature #1; also see section 2.7.4). Associated with the geomagnetic trace of the rift zone is a

well-defined isostatic gravity “gradient-ridge”, which borders the magnetic anomaly on the west

for ~250 km of its length, and continues south beyond the terminus of the magnetic anomaly,

extending nearly as far as the amagmatic zone (see Figs. 6.45b, feature #1, and 6.54). The

gravity gradient-ridge anomaly indicates that the northern Nevada rift may be at least 600 km
83

long in Nevada alone (Blakely, 1988; Blakely and Jachens, 1991). The origin of the anomaly

may be related to, for example, the fault juxtaposition of low-density volcanic and sedimentary

rocks filling down-thrown structures, and higher-density basement rocks composing up-thrown

blocks. The gradient ridge may also reflect density increases or decreases related to complex

structural features associated with the rift zone (e.g.—decreases in bulk rock density caused by

wide-spread fracturing related to fault or shear zones, or increases in bulk rock density caused

by igneous rocks that may have intruded along the rift zone). The isostatic gravity gradient ridge

is also coincident with an axis of bilateral symmetry in the Bouguer gravity anomaly (Fig. 6.45c,

axis of the “butterfly-shaped” low, centered just southeast under the number “1”), suggesting that

the isostatic gravity anomaly associated with the rift may originate in the basement (Blakely and

Jachens, 1991). The northern Nevada rift, including its southeastern extension as imaged by

gravity anomaly, runs subparallel to the Battle Mountain–Eureka (Cortez) and Carlin

sedimentary rock-hosted mineral trends, and is particularly well correlated to the southern half

of the Battle Mountain–Eureka (Cortez) trend (compare Figs. 1.3 and 6.2 to Figs. 6.53 and 6.54).

As indicated numerous times previously, proximity to a regional-scale structure may serve to

focus mineralizing processes.

In summary, the lower magnetic and higher isostatic gravity anomalies (Figs. 6.53a and 6.54a)

suggest that, on a regional scale, the sedimentary rock-hosted occurrences are associated with

shallow crust that is: 

Tectonically over-thickened (see Fig. 6.5d).

Predominantly sedimentary in character (Fig. 6.56a).

Relative to surrounding areas, generally lacking in late Cenozoic mafic volcanism and,

relative to other regions of Nevada, generally less affected by post–43-34 Ma volcanism

(Figs. 6.55, 6.56b, and 6.4).

Associated with numerous possible deep-seated and upper crustal structural features,

including the western craton margin (see Fig. 2.11), the northern Nevada rift zone and its

possible extension southeastward, and major thrust faults and thrust root zones (specifically

the Roberts Mountain and Golconda thrusts; see Fig. 2.5).

6.6.3.2 Volcanic Rock-Hosted Occurrences: Geomagnetic Highs—Isostatic Gravity Lows

The frequency distributions for volcanic rock-hosted occurrences, offset toward positive

magnetic values and skewed toward negative isostatic gravity values, indicate that they tend to

be found in regions characterized by more magnetic and less dense rocks. The spatial

distribution of the magnetic and isostatic gravity anomalies associated with these occurrences

is shown in Figures 6.53b and 6.54b. The regional distribution of these geophysical anomalies

share broadly similar patterns with the crescent-shaped distribution of the volcanic rock-hosted

occurrences, especially in the region of the Walker Lane shear zone (compare to Fig. 6.2).

Magnetic anomaly highs in Nevada are primarily associated with basaltic and gabbroic rocks,

which contain a relative abundance of magnetic minerals. Three most prominent high-amplitude

positive magnetic anomalies (or anomalous regions) are (1) the northern Nevada rift zone, (2)

the Walker Lane shear zone, and (3) northwestern Nevada, just northwest of a regional

geomagnetic low and isostatic gravity high associated with the Humboldt zone (Figs. 6.45a,

features #1, #1a, #2, and #5; 6.53, and 6.54). 
84

The northern Nevada rift zone is most prominently recognized as a long, narrow, and welldefined regional magnetic high, trending north-northwest for 280 km through north-central

Nevada, the source of which may extend to a depth of 15 km (Blakely and Jachens, 1991; also

see Christiansen and Yeats, 1992). The rift zone is characterized by linear dike swarms and

associated lavas (basalt and basaltic andesite) of 17 to 14 Ma age, by a fault-bounded trough, and

by a northwest-southeast topographic grain, and has been interpreted to be the surface expression

of a deep-seated fracture zone that may have had a complex history of transcurrent movement

(see section 2.7.4). The magnetic signature of the rift suggests that the structure extends only

as far south as central Nevada (~N 39o

 20' to N 39o 40'), but gravity data suggest that it may

extend further southeast into southern Nevada (see previous section). Blakely and Jachens

(1991) suggested that the basaltic and andesitic extrusive and intrusive rocks associated with the

northern portion of the rift zone might also be found along its southeastern extension, but if such

rocks are present, they remain concealed. 

The Walker Lane shear zone is a northwest-trending structural zone 700 km long and from 100

to 300 km wide containing both right-lateral and left-lateral faults. It is currently active, but may

have developed primarily during the late Cenozoic, though it may have older origins (see section

2.6). The Walker Lane is magnetically characterized by a pattern of arcuate anomalies that trend

northwestward, generally parallel to the shear zone. Blakely and Jachens (1991) noted that the

width of the magnetic anomalies is considerably wider than the structural or topographic

expression of the shear zone, extending in some places more than 150 km north-northeast of the

Walker Lane and into (under?) basin-range topography characterized by north to northeast

structural trends. Further, Blakely and Jachens (1991) suggested that the magnetic anomalies

may indicate an underlying tectonic fabric older than modern topography and exposed geology

that is perhaps related to the Precambrian breakup of North America. The greater number of

higher magnetic anomalies in the Walker Lane belt may also be related to intermediate and mafic

rocks, which are more common in western and southwestern Nevada, and more abundant around

the margin of the Great Basin in general (see Fig. 6.55). As with the possible southeastern

extension of the northern Nevada rift, mafic and intermediate rocks may intrude the Walker Lane

at depth, but are nowhere exposed at the surface.

In northwestern Nevada, a broad belt of patchy geomagnetic highs extends northeast-southwest

across the region (Figs. 6.45a, feature #5, and 6.53b). This band is delineated in part by a

similarly oriented band of magnetic anomaly lows to the southeast, which is prominently

transected by the northern Nevada rift anomaly and west-flanking rift anomalies (Fig. 6.45a,

features #1 and #1a). At the regional scale, the juxtaposition of the these two magnetic high-low

anomaly bands corresponds to the Humboldt zone gravity anomaly high, which stretches

northeast-southwest across northern Nevada (Fig. 6.54a). The magnetic anomaly highs across

northwestern Nevada probably reflect the outpourings of Tertiary and younger mafic volcanics

in the region and possibly their presence at depth (units such as QTb, Tb, Tba, Tbg, and Tob; see

Appendix A).

Gravity anomaly lows in Nevada are related to rocks of lower density, or attributes which

effectively lower the density of rocks, such as wide-spread fracturing related to fault and shear

zones (Telford et al., 1976; also see Jachens et al., 1989). In Nevada, isostatic gravity lows

generally correlate with sediment and volcanic rock-filled inter-range basins, and with the

presence of felsic intrusions (Mabey et al., 1983; Saltus, 1988; Blakely and Jachens, 1991). The
85

dominant feature, visible in both the isostatic and Bouguer anomalies, is a regional-size gravity

low that stretches from the Nevada-Utah border across the center of the state into the Walker

Lane region (Figs. 6.45b, 6.45c, 6,54b). This low, flanked to the north and south by gravity

highs, reflects sources in the pre-Tertiary basement, but its strongest correlation is with the

distribution of thick accumulations of Cenozoic volcanic rocks (Blakely and Jachens, 1991;

Mabey et al., 1983). The isostatic gravity lows are reasonably well correlated with (1) volcanic

rocks erupted during the 34-17 Ma time-slice intervals (see Fig. 6.54b and compare to Fig. 6.4),

and (2) a possible east–west-trending structural zone that extends across southern Nevada (the

south-central Nevada structural zone; see Kepper et al., 1991, figure 1). The isostatic gravity low

is also visible in the Bouguer gravity anomaly, where it is characterized by gross bilateral

symmetry that is best developed in east-central Nevada. The axis of symmetry trends northwest

and is generally coincident with the northern Nevada rift zone. The Nevada gravity low may

actually be part of a larger alternating pattern of northwest-southeast-trending high-low

anomalies that stretches from California, across the Great Basin and into Utah (see Jachens et

al., 1989, plate 1; Kane and Godson, 1989, plate in back pocket). This larger gravity pattern is

probably related to features at the crust-mantle boundary, and is reviewed in greater detail in

section 2.7.3.

In summary, the relationship between the volcanic rock-hosted occurrences and regional-scale

geologic features, as interpreted through the geophysical anomalies, is not as well-defined as it

is for the sedimentary rock-hosted occurrences, however, it appears likely that the volcanic rockhosted occurrences are associated with shallow crust that:

Has been affected by widespread felsic volcanic activity. The regional distribution of

magmatism between 34-6 Ma (particularly between 6-17 Ma) parallels (1) the distribution

of volcanic rock-hosted deposits (compare Fig. 6.4 to 6.2 and 6.3) and (2) the Walker Lane

shear zone and the Humboldt zone (compare Fig. 6.4 to 1.3, 6.6, and 6.45).

Is associated with broad and diffuse structural zones that visibly affect basin-range structure,

suggestive of regional-scale shear, and may have late Cenozoic movement (Walker Lane

shear zone; possibly the Humboldt zone also, as suggested in shaded relief of topography;

Figs. 6.6 and 1.2; the Humboldt zone is discussed and illustrated in greater detail in chapter

8).

Has been intruded by mafic and intermediate magmas, particularly along structural zones

(e.g.—figure 4.3 in Smith and Luedke, 1984). 

6.7 Geochemistry—K/Na and Ba/Na Anomalies

6.7.1 Introduction and Summary of Findings

The distribution and spatial association of the gold-silver-bearing occurrences in relation to K/Na

(Fig. 6.57b) and Ba/Na (Fig. 6.57c) anomalies are reviewed in this section. The maps were

prepared using geochemical data collected as part of the National Uranium Reconnaissance

Evaluation (NURE) program in the conterminous western United States between 1976 and 1980,

and are depicted as K/Na and Ba/Na ratios, rather than the absolute concentrations of K, Ba, or
86

5 Samples were selected that contained concentrations for Ba and Na, or K and Na—6,390 samples were chosen

for K/Na and 6,460 for Ba/Na, and include mostly (~90%) soil and other sediment samples. The NURE data

have a number of well-recognized errors, omissions, and biases, which made regional-scale analysis and

comparison of single elements prohibitive. The NURE geochemical data, as published on the USGS DDS-1

CD-ROM (1991), contains a number of inconsistencies which render the data difficult to work with for a

regional-scale analysis. Overall, the basic problem with the NURE data is the lack of quality control

procedures in both the sampling and laboratory analysis programs. In regional-scale compilations, this led to

various “boundary” effects between individual collection-analysis regions. Maps prepared directly from

uncorrected Ba, K, or Na concentrations produced maps showing erratic anomaly patterns with no continuity

of the patterns across 1o

 by 2o map sheet blocks. It was determined that by calculating ratios between elements

that were analysed using the same techniques, and from samples processed by the same laboratory, the relative

magnitude of the geochemical anomaly could be preserved.

Na 5.

Alteration of aluminosilicate wall-rocks accompanies precious metal mineralization in the

sedimentary and volcanic rock-hosted deposits of the Great Basin. In sedimentary rock-hosted

deposits, alteration assemblages are characterized by decarbonatization, silicification,

argillization and alteration of aluminosilicate minerals to a host of other clay minerals,

pyritization, deposition of alunite and barite, oxidation and acid leaching near the surface, as well

as the introduction and redistribution of carbon (Bagby and Berger, 1985; Radtke, 1985; Bakken

and Einaudi, 1986; Cox and Singer, 1986; Kuehn, 1989; Hofstra, 1994; also see Table 3.1).

Alteration facies do not usually form regional haloes (Christensen, 1995). In general, for

sedimentary rock-hosted mineralization, the ratios of K/Na and Ba/Na should increase as (1) Ba

is introduced, (2) Na is depleted, and (3) K is either immobile or not as commonly depleted as

Na (see Hofstra, 1994). Volcanic rock-hosted deposits may be subdivided into high- and lowsulfidation systems, associated with volcanic-hydrothermal and geothermal environments,

respectively (Hedenquist et al., 1996). High-sulfidation systems are characterized by acid-sulfate

alteration, whereas low-sulfidation systems are characterized by adularia-sericite alteration (see

Table 3.2). The acid-sulfate (high sulfidation) assemblage shows silicification and advanced

argillic alteration near ore bodies and grades outwards to an argillic zone. The adularia-sericite

(low sulfidation) assemblage shows K-metasomatism (adularia) and silicification near ore bodies

and grades outward to a sericitic-argillic zone. Both alteration assemblages are surrounded by

an outermost zone of propylitic alteration, which may form halos hundreds of meters wide

around veins (Buchanan, 1981; Hedenquist et al., 1996). In general, for volcanic rock-hosted

mineralization, the ratios of K/Na and Ba/Na should increase as (1) Ba (particularly in highsulfidation systems) and K are introduced, and (2) Na is removed (Sillitoe, 1995; Hedenquist et

al., 1996) (Barnes and Rose, 1998, indicate that Na is removed to become a major solute in

hydrothermal fluids).

The most significant results of the analyses carried out in this section suggest that occurrences

in Nevada, particularly the sedimentary rock-hosted occurrences found along the Battle

Mountain–Eureka (Cortez) and Carlin mineral trends, have a strong spatial association with high

K/Na and Ba/Na anomalies (no genetic link is suggested or implied, although one may exist).

As reviewed in chapter 3 (sections 3.4 and 3.6), a number of proposals have been put forth that

mineralization along the Battle Mountain-Eureka (Cortez) and Carlin trends is controlled by a

regional-scale crustal structure(s) (e.g.—Madrid and Bagby, 1986; Cunningham, 1988; Shawe,

1991; Arehart et al., 1993; Maher et al., 1993; Grauch et al., 1995). The distribution pattern of
87

the K/Na–Ba/Na anomaly clearly delineates the Battle Mountain–Eureka (Cortez) and Carlin

mineral trends, and it is suggested that the geochemical anomalies might also be related to

presumed crustal structure(s), which control the regional-scale distribution of sedimentary rockhosted occurrences along the trends.

6.7.2 Distribution and Spatial Association of Gold-Silver-Bearing

Occurrences

As shown in Figure 6.57, the geochemical evidence layers do not completely cover the study

area: they cover only about 50% of the total area of Nevada, however, nearly 70% of the goldsilver-bearing occurrences fall on the K/Na and Ba/Na anomaly maps. Because of the

distribution of these data, the results of spatial analysis performed on the sedimentary rockhosted occurrences is considerably more reliable and stable than for volcanic rock-hosted

(compare Fig. 6.57 to 6.2). With respect to weights of evidence modelling, the areas of missing

data contribute no weight (W+

 = W– = 0) and the mineral potential (the posterior probability

value) is neither up-weighted nor down-weighted (Bonham-Carter, 1994a, p. 327).

Of the total number of primary occurrences (2690), 1840 are located on the K/Na and Ba/Na

maps, and of these 1840, the greatest number fall within the lowest geochemical ratio interval

(~38%for the K/Na map; ~67% for the Ba/Na map) (Figs. 6.58 and 6.59). Of the total number

of sedimentary rock-hosted occurrences (98), 61 are located on the K/Na and Ba/Na maps, and

of these 61, over half fall in the top eight K/Na ratio intervals (“6-7” through “50”) and in the

top five Ba/Na ratio intervals (“0.30-0.35” through “0.50”) (Figs. 6.60 and 6.61). Of the total

number of volcanic rock-hosted occurrences (415), 270 are located on the K/Na and Ba/Na maps,

and of these 270, about 85% fall in the three lowest K/Na ratio intervals (“<1” through “2-3”)

and nearly 90% in the lowest Ba/Na ratio interval (“<0.10”) (Figs. 6.60 and 6.61). Keeping in

mind that the volcanic rock-hosted occurrences are not well represented with respect to the

coverage of the K/Na and Ba/Na geochemical anomaly maps, the most notable difference

between the sedimentary and volcanic rock-hosted occurrence frequency distributions is the

affinity of sedimentary rock-hosted occurrences for higher geochemical ratio values, especially

in the case of Ba/Na.

The spatial association between the K/Na anomaly and both the primary and volcanic rockhosted occurrences is strongest when the areas of the top five highest ratio intervals are

combined (“9-10” through “50”) (Fig. 6.62). The spatial association between the K/Na

anomaly and sedimentary rock-hosted occurrences is strongest when the areas of the top three

highest ratio intervals are combined (“15-25” through “50”) (Fig. 6.62). In general, the

sedimentary rock-hosted and big size primary occurrences show the strongest spatial association

with higher K/Na anomalies, while the volcanic rock-hosted occurrences show only a moderate

spatial association for mid-range K/Na values. The sedimentary rock-hosted occurrences show

the strongest spatial association with the higher Ba/Na anomaly values (Fig. 6.63). As with the

K/Na anomaly, the volcanic rock-hosted occurrences show a weak and even a negative spatial

association with Ba/Na anomalies. Again, these relationships reflect the poor distribution of the

NURE dataset with respect to the volcanic rock-hosted occurrences.

Three binary-class mineral potential predictor maps were produced from each of the K/Na and

Ba/Na geochemical anomaly maps, one for each of the three occurrence samples (primary,
88

6 The units Os and Osv, from the geological map of Nevada (Stewart and Carlson, 1978), have been identified

in this study as having a high spatial association with sedimentary rock-hosted occurrences, as illustrated in

Figures 6.14 and 6.24.

sedimentary and volcanic rock-hosted) (Fig. 6.64). For the K/Na anomaly maps, the six highest

ratio intervals (“8-9” through “50”) were combined for the primary occurrences binary

predictor map, the three highest intervals (“15-25” through “50”) were combined for the

sedimentary rock-hosted occurrence binary predictor map, and the five highest intervals (“9-10”

through “50”) were combined to create the volcanic rock-hosted binary predictor map. For the

Ba/Na anomaly maps, the eight highest ratio intervals (“0.15-0.20” through “0.50”) were

combined for the primary occurrences binary predictor map, the five highest intervals (“0.30-

0.35” through “0.50”) were combined to produce the binary predictor maps for both the

sedimentary and volcanic rock-hosted occurrence binary predictor maps.

6.7.3 Correlations and Interpretive Synthesis

The K/Na and Ba/Na anomalies are mainly associated with the sedimentary rock-hosted

occurrences, although geochemical coverage extends only over part of the Walker Lane, which

is the principal locale for volcanic rock-hosted occurrences. This restricted coverage reduces the

utility of the statistical correlations. The K/Na and Ba/Na anomaly maps display a distinctive

“V”-shaped pattern in north-central Nevada that correlate with the Battle Mountain–Eureka

(Cortez) and Carlin sedimentary rock-hosted mineral trends (Fig. 6.65). The western northwestsoutheast arm of the “V” delineates the Battle Mountain-Eureka (Cortez) trend. The eastern arm

of the “V” is north-south from the base of the “V”, then bends northwest along the Carlin trend.

The north-south eastern arm extends south of the Carlin trend, near the Emigrant Springs and

Rain mines, along a line of mainly small silver-bearing polymetallic occurrences to intersect the

western arm of the “V” near a large number of polymetallic replacement and some disseminated

deposits around the Eureka district. This north-south section of the eastern arm traces the

leading edge of the Roberts Mountain thrust (see Figs. 6.65 and 2.5), but it is not implied here

that this forms a connection (structural or otherwise) between the Carlin and Battle MountainEureka (Cortez) mineral trends.

The spatial correlation between Ba/Na and the sedimentary rock-hosted occurrences, which is

strongest for the Battle Mountain-Eureka (Cortez) trend, is related to the presence of vein and

bedded barite in the stratigraphic sequences of the rock package(s) hosting gold mineralization.

Most sedimentary rock-hosted occurrences along the Battle Mountain-Eureka (Cortez) and

Carlin trends are associated with structural windows in the upper plate of the Roberts Mountain

allochthon (Bagby and Berger, 1985). Paleozoic submarine exhalative bedded barite deposits

occur in the upper plate of the Roberts Mountain allochthon, particularly within the units Os and

Osv6

 (Papke, 1984), and barium is anomalous in the cherts and argillites of the upper plate

(Shawe et al., 1967; Papke, 1984; Byron R. Berger, 1998, personal communication). Structural

windows expose the units containing the bedded barite deposits, the barite-enriched cherts and

argillites, as well as the units hosting sedimentary rock-hosted gold mineralization. As such, the

Ba/Na anomaly should be spatially associated with the occurrences. However, it should also be

noted that geochemical anomalies of As, U/Th, Al, Sc/V, K, La/K, and Sc/Fe show varying

degrees of continuity with, and similarity to, the regional-scale extent of the sedimentary rockhosted mineral deposit trends and K/Na and Ba/Na anomaly patterns:
89

The northern Carlin trend (particularly the northern terminus) and the northern Battle Mountain–Eureka

(Cortez) trend are well delineated by a linear distribution of anomalous As, in comparison to the region

between the trends, which is characterized by less anomalous values.

The northern Carlin trend (particularly the northern terminus) and the southern Battle Mountain–Eureka

(Cortez) trend are relatively well delineated by a linear distribution of anomalous U/Th, in comparison to the

region between the trends, which is characterized by less anomalous values.

The Battle Mountain–Eureka (Cortez) and Carlin trends are delineated by anomalously low Al in comparison

to the region between the trends, which is characterized by anomalous values.

Sc/V and K show moderately-defined anomaly gradient changes along the Battle Mountain–Eureka (Cortez)

trend and along the Carlin trend (which is more weakly defined).

La/K and Sc/Fe show a weak anomaly between the Battle Mountain–Eureka (Cortez) and Carlin trends.

Most of these anomalies are linear and continuous over large distances (at least 240 km for the

western arm of the “V”) and trend across and oblique to most mountain ranges and drainages

(except along the northernmost portion of Battle Mountain–Eureka (Cortez) trend).

The spatial association of K/Na–Ba/Na anomaly to the Battle Mountain-Eureka (Cortez) and

Carlin mineral trends is clear. Any genetic link is, however, inferred. Many samples from

bedded barite deposits have been analysed for gold and none found (Byron R. Berger, 1998,

personal communication). One possible indirect link is regional-scale structural features

associated with the western margin of the Precambrian craton. Cunningham (1988) recognized

that a number of important sedimentary rock-hosted deposits are located near the edge of the

buried craton (compare Fig. 6.2 to 2.10; see figure 1 in Cunningham). He proposed that deep

crustal structures developed along older reactivated zones of weakness at the cratonic boundary,

which had a localizing effect on sedimentary rock-hosted mineralization. Papke (1984)

interpreted the barite deposits to be exhalative, favoring a magmatic source for the heat and the

barium. He suggested that the deposits formed in submarine basins scattered along a 500 km belt,

the length and linearity of which suggests a major zone of rifting parallel to the axis of the lower

Paleozoic eugeosyncline (see Fig. 2.9; Papke, 1984, figure 1). Structural zones associated with

the craton edge may have provided a conduit for the passage of fluids that contained barium, and

along different structures and at a later time, gold.

Regardless of the link between the K/Na and Ba/Na anomalies and the Battle Mountain–Eureka

(Cortez) and Carlin mineral trends, they are spatially associated with sedimentary rock-hosted

mineralization along the trends and do provide reasonably good exploration guides (given the

existing dataset). These matters are further discussed in chapter 8.
90

Chapter 7. Multi-Map Modelling and Gold-Silver

Mineral Potential 

7.1 Introduction

Chapter 7 is a summary and discussion of the results of mineral potential modelling. In part, the

material below represents the second and third of three stages of model building, the parameter

estimation or model fitting and model validation stages, respectively (Chatfield, 1988; see

section 1.4). It constitutes the second of the two procedures that compose the weights of

evidence mineral potential modelling method, graphically illustrated in Figure 4.1 as procedure

“ii”. The first stage of model building, model formulation or specification, was addressed in

chapter 6, where, in procedure “i” of weights of evidence modelling, (1) spatial associations

between the gold-silver-bearing occurrences and the multi-class mineralization evidence maps

were established and measured, (2) the significance of the relationships was determined, and (3)

the binary-class mineral predictor maps (“layers of evidence”) were created from the

mineralization-favorable units composing multi-class evidence maps. The implementation and

theoretical aspects of the modelling procedures carried out here are addressed in Appendix B

(summarized in chapter 4).

The former part of this chapter (section 7.2) reviews the combination of the binary-class mineral

predictor maps (“layers of evidence”) generated in chapter 6. The sedimentary and volcanic

rock-hosted mineral potential maps are presented in section 7.3. Validation of the mineral

potential models, including analysis of conditional independence and error, mineral favorability

at known occurrence locations, and blind tests, are discussed in section 7.4. In the latter portion

of this chapter (section 7.5), regional-scale sedimentary and volcanic rock-hosted occurrence

exploration targets are delineated, and the important mineral potential evidence within each

target region is reviewed. Specific sites for more detailed investigation and further exploration

are also suggested. In chapter 8, the mineral potential evidence presented here and the spatial

relationships and interpretations presented in chapter 6 are used to identify geologic features that

may control the first-order regional-scale distribution of sedimentary and volcanic rock-hosted

occurrences in the Great Basin.

The main conclusions of this chapter are that (1) the most significant mineral potential evidence

layers examined in this study for predicting sedimentary and volcanic rock-hosted are, in order

of importance, geology-related, geochemistry, and geophysics, (2) the sedimentary and volcanic

rock-hosted occurrence posterior probability maps have a high degree of statistical certainty and

are reliable indicators of mineral potential, despite known conditional dependence problems, and

(3) the mineral potential models are valid, as indicated by the prediction of approximately  to

3⁄4 of all occurrences for any given occurrence-type used to train the models, the successful

prediction of 15 out of 17 newly discovered occurrences, and by high area cross-correlation with

expert-delineated areas of mineral occurrence permissibility. Areas favorable for sedimentary

and volcanic rock-hosted mineralization were readily recognized on the mineral potential maps,

and promising regional-scale exploration targets were readily identified.
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7.2 Combination of Mineral Potential Evidence

7.2.1 Introduction

The posterior probabilities for predicting mineralization were estimated by combining the layers

of evidence for each of the primary, sedimentary and volcanic rock-hosted occurrence-type

mineral potential models in two ways: (1) a combination using all of the evidence layers, and

(2) a combination using fewer layers, where one or more predictor maps were removed from the

model in an attempt to mitigate conditional dependency (CI) violations and posterior probability

uncertainty. A total of six posterior probability mineral potential maps were produced. 

The weighting factors used to combine the layers of evidence are reviewed in section 7.2.2

below. A review of CI and uncertainty, with respect to mineral potential model validation, is

presented later in section 7.4.2. Appendix C gives detailed examinations of (1) the predictor map

combinations, (2) a pair-wise comparison between the two posterior probability maps created

for each occurrence-type mineral potential model, and (3) analysis of CI and uncertainty.

7.2.2 Combination Weighting Factors

The predictor maps for each of the three mineral occurrence-types were combined using the

spatial weights of association (W+ and W–) listed in Table 7.1. The relative significance,

correlation, or “influence”, that a particular layer of evidence has on a posterior probability

mineral potential map for a given mineral potential model is taken to be the contrast

(C=W+ – W–

), which is the measure of spatial association between the occurrences and a

predictor map pattern. The relative influence of the various evidence layers in the mineral

potential models is summarized in Figures. 7.1, 7.2, and 7.3. In each of these Figures, the

evidence is ordered from most influential to least influential layer (i.e.—sorted by “C”).

A collective examination of the Figures reveals that lithology or lithology-related

(e.g.—lithotectonic terranes and buffers around plutons) factors rank highly among the top three

or four most influential evidence layers, along with geochemical evidence (particularly in the

case of sedimentary rock-hosted occurrences). Overall, it appears that geology-related evidence

has the highest degree of spatial correlation to the gold-silver-bearing occurrences, followed by

geochemical-related evidence and then geophysical-related evidence. These correlations are

consistent with those observed by Wright (1996), in a study that compared the merits of four

different methods for combining geoscientific spatial data related to volcanic-hosted massive

sulfide (VMS) deposits (weights of evidence, weighted logistic regression, fuzzy logic, and

Dempster-Shafer belief theory). He found that, in general, stratigraphic evidence (read lithologic

here) has the highest degree of correlation to the VMS occurrences, followed by alteration and

geochemical evidence, heat-source evidence, and lastly, geophysical evidence (VLF

electromagnetics, gravity, and geomagnetics). Another interesting conclusion of Wright (1996)

is that the type of evidence used in a model is more critical than the method by which it is

combined (see Wright and Bonham-Carter, 1996, for further details).
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7.3 Sedimentary and Volcanic Rock-Hosted Mineral

Potential Maps

7.3.1 Introduction

The gold-silver mineral potential maps of Nevada represent areas that are ranked according to

their favorability for hosting gold-silver-bearing mineral occurrences (both economic and subeconomic). These areas are the result of unique combinations of overlap among the various

binary-class predictor maps. An area is ranked according to its associated posterior probability

value, which is based on the degree of spatial association between mineral occurrences and an

evidence layer, and on the overlap combinations between the evidence layers (i.e.—the

geological attribute binary-class predictor patterns). Areas having posterior probabilities greater

than the prior probability, as indicated in the legend for each map, represent a combination of

evidence layers that is favorable for the occurrence of gold-silver-bearing mineralization. In

these areas, the favorability of finding an occurrence is greater than that which is simply due to

chance (the “simply due to chance” condition is represented by the prior probability).

Conversely, areas having posterior probabilities equal to or less than the prior probability

represent evidence layer combinations that are not favorable for mineralization. In this study,

the favorable areas contain greater than 70% of the large and medium size primary, sedimentary

and volcanic rock-hosted gold-silver-bearing occurrences (see section 7.4.3 and Appendix D for

details). It is implicit in the weights of modelling technique that this is based upon the

distribution of known occurrences and a combination of geological, geophysical or geochemical

features spatially associated with these occurrences.

In this and the remaining sections of this study, only two of the six mineral potential maps

generated in this study are considered: (1) the 8-layer sedimentary rock-hosted and (2) the 7-layer

volcanic rock-hosted (the evidence layers making up these maps are given in Table 7.1; also see

Appendix C for a detailed discussion on these and other mineral potential maps). These maps

are presented in sections 7.3.2 and 7.3.3, respectively. The mineral potential maps reflect the

distribution of known occurrences, but more importantly, they identify areas where yet

undiscovered occurrences may exist. 

These maps serve as the basis for delineating regional-scale exploration targets, reviewed

separately in section 7.5, and for determining the geologic characteristics of high mineral

potential areas, which are interpreted with respect to geologic features that may control the

regional-scale distribution of the gold-silver-bearing occurrences (the subject of chapter 8).

7.3.2 Favorability for Sedimentary Rock-Hosted Occurrences

The sedimentary rock-hosted occurrence-type posterior probability map is shown in Figure 7.4.

This map is based on the 8-layer mineral potential model (see Table 7.1) and overlain on the

shaded relief of topography. Elevated mineral potential areas (light greens, yellow, orange, red)

are generally confined to central, north-central, and north-eastern Nevada. The areas of highest

mineral potential (yellow-orange-red) form a conspicuous “V”-shape trend of posterior

probability values that is largely coincident with the major sedimentary rock-hosted deposit

trends (Battle Mountain–Eureka (Cortez), Carlin, Getchell, and Independence group), and which
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may parallel possible regional-scale structures, as defined by geophysical (and possibly

geochemical) anomalies (see sections 3.4, 3.6, 6.6, and 6.7; further discussed in chapter 8).

These areas represent instances where nearly all of the layers of evidence come into spatial

agreement. A typical combination of evidence consists of lithology, the Ba/Na geochemical

anomaly, lithotectonic terrane, the geomagnetic anomaly, and distance buffer zones surrounding

faults (a more detailed review is presented in section 8.2.1).

7.3.3 Favorability for Volcanic Rock-Hosted Occurrences

The volcanic rock-hosted occurrence-type posterior probability map is shown in Figure 7.5. This

map is based on the 7-layer mineral potential model (see Table 7.1) and overlain on the shaded

relief of topography. Elevated mineral potential areas (light greens, yellow, orange, red) are

principally distributed along two broad belts: (1) a north-west-trending belt that extends along

the southwestern border of Nevada (the Walker Lane belt); and (2) a north-east-trending belt that

extends diagonally across northern and northwestern Nevada (correlative to Humboldt zone; see

chapter 8). The two belts intersect in the vicinity of Reno and Carson City, Nevada, between

approximately N 39o–40o and W 119o–120o, where the 9-layer posterior probability map shows

a relatively strong region of volcanic rock-hosted mineral potential (see Fig. 7.8a below). The

two broad belts of elevated volcanic rock-hosted mineral potential roughly envelop the main

region of elevated sedimentary rock-hosted mineral potential (compare Figs. 7.4 and 7.5 to the

point distribution of known occurrences in Fig. 6.2) In comparison to the sedimentary rockhosted elevated mineral potential areas, the volcanic rock-hosted elevated mineral potential areas

are patchy and discontinuous, and generally more evenly distributed across the state. Like the

sedimentary rock-hosted mineral potential “highs”, the volcanic rock-hosted “highs” represent

instances where nearly all of the layers of evidence come into spatial agreement. A typical

combination of evidence consists of lithology, distance buffer zones surrounding plutons, the

geomagnetic anomaly, distance buffer zones surrounding faults, lithologic diversity, and the

K/Na geochemical anomaly (a more detailed review is presented in section 8.2.2).

7.4 Validation of Mineral Potential Models

7.4.1 Introduction

Model validation is the third of the three stages of model building (Chatfield, 1988). Validation

may be carried out using the same data from which the models were built, where the output of

a model is typically analysed for goodness-of-fit and error, and is tested for the ability to predict

the distribution of the data from which it was built and specified (training point prediction).

Goodness-of-fit and error test are reviewed in section 7.4.2, “Conditional Independence and

Uncertainty of the Mineral Potential Maps”, and examined in detail in Appendix C. The

capacity of the mineral potential models to predict known occurrences is reviewed in section

7.4.3 and detailed in Appendix D. But perhaps more important is the ability of a model to

predict the distribution of data which were not used to construct the model. Validation of this

type is usually referred to as a “blind test”. A blind test of the mineral potential models built here

was performed using (1) expert-delineated tracts permissible for sedimentary-rock hosted and

epithermal mineralization (in essence an informal blind test; section 7.4.4), and (2) newly

discovered gold occurrences (a true blind test; section 7.4.5). 
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The results of the model validation analyses indicate that the posterior probability maps output

from the mineral potential models are within acceptable limits. Generally speaking, weights of

evidence mineral potential modelling is successful if the resulting mineral potential maps (1)

predict the distribution of known mineral occurrences, and/or (2) are useful for delineating or

targeting regions of elevated mineral potential for follow-up exploration (Bonham-Carter,

1994b). Both of these criteria were met. The first is discussed in section 7.4.3. The second is

discussed in section 7.4.4, and is the subject of section 7.5.

7.4.2 Conditional Independence and Uncertainty of the Mineral Potential

Maps

The results of CI testing suggest that the mineral potential maps should be considered in terms

of relative “favorability”, rather than strictly as “probabilistic” maps. Both the 2

 pairwise and

K-S overall tests indicate that the layers of evidence composing these mineral potential models

are not conditionally independent with respect to the mineral occurrences, and that this lack of

CI is real, in both the statistical and the geological sense. The violation of CI in the mineral

potential models is not unexpected, as CI in WOE modelling is always violated to some degree

(Bonham-Carter, 1994a; see chapter 4 and Appendix B for details). The greater the number of

evidence maps used in a model, especially if those maps have similar distribution patterns and

represent similar types of evidence, the greater the possibility is that conditional dependency will

exist. 

The sedimentary rock-hosted occurrence mineral potential models have a high discrepancy

between the number of observed and predicted occurrences. The volcanic rock-hosted

occurrences mineral potential models show a lesser amount of CI violation. The lack of CI

occurs most notably in the mineral potential models where all of the available binary-class

predictor maps were combined. To mitigate some of this CI, the evidence layers that caused the

greatest CI violations were rejected from the models. The sedimentary rock-hosted model was

reduced from 9 to 8 evidence layers by removing the pluton buffer binary-class predictor map

(rejection of 2 or 3 additional combination predictor maps had relatively little additional

corrective value). The volcanic rock-hosted model was reduced from 9 to 7 evidence layers by

removing the Ba/Na geochemical anomaly and lithotectonic terranes binary-class predictor maps

(while not remedying all of the CI problems, removal of these two maps reduced CI violations

substantially; see Appendix C for further explanation). In addition, it was determined that

increasingly large numbers of training points result in increasingly larger magnitude and a greater

number of CI violations. Therefore, the CI violation exhibited by these mineral potential models

may in part be related to the relatively large number of mineral occurrences modelled.

In order to determine whether the violations of CI severely affected the mineral potential maps,

comparisons were made between the CI- mitigated and -unmitigated maps, as well as between

mineral potential maps generated using the weights of evidence (WOE) and weighted logistic

regression (WLR) methods. Maps of absolute difference in posterior probability value between

CI- mitigated and -unmitigated mineral potential maps show relatively small discrepancies (Fig.

7.6). The comparison between the WOE-derived and WLR-derived posterior probability maps

for sedimentary and volcanic rock-hosted mineralization shows only minor differences in the

distribution of favorable mineral potential areas at the local-scale (Figs. 7.7 and 7.8). At the

regional scale, no significant differences are visible in the overall map patterns, although the
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WOE mineral potential maps appear to be somewhat “warmer”. The WOE–WLR comparison

was chiefly made because the WLR method does not require the assumption of CI. In general,

the results of the WLR method appear to be consistent with those of the WOE method, and

suggest that, for this study, the results of the WOE method are within acceptable and expected

bounds.

Analysis of the “certainty” of the posterior probability estimates also suggests that the mineral

potential maps are within acceptable limits. Maps of posterior probability uncertainty for the

sedimentary and volcanic rock-hosted mineral potential maps are shown in Figures 7.9 and 7.10,

respectively (see Appendix C for complete analysis). Uncertainty may be due to the variances

in weight estimates (W+ and W–) and/or to one or more of the predictor maps having incomplete

coverage (i.e.—missing data) (Bonham-Carter et al., 1989). Uncertainty due to variance in the

weights estimates is usually correlated to the posterior probability values, and when mapped,

results in an uncertainty map pattern similar to that of the posterior probability map. A measure

of relative certainty, consisting of the posterior probability divided by its standard deviation (in

effect, a student t-test), indicates the degree of confidence to which the posterior probability

mineral potential map patterns are “real”, as opposed to being an artifact of “chance” effects (or

due to chance). The t-test is useful for identifying gross violations of the model assumptions.

In general, the maps of uncertainty due to variance in weights estimates and due to missing data

reflect the large overall variation in the data (i.e.—”noise in the system”). However, the relative

certainty maps suggest that, despite this high degree of variability, the mineral potential map

favorability patterns are reliable (i.e.—areas of elevated t-value are spatially coincident with

areas of high mineral potential).

7.4.3 Favorability at Known Gold-Silver-Bearing Occurrence Areas

The mineral potential favorability at known primary, sedimentary and volcanic rock-hosted

occurrence locations was determined as follows:

1. Appending the probability calculated with WOE to each occurrence point.

2. Counting the number of occurrences (subdivided by size) having posterior probabilities greater than the prior

probability.

3. Comparing the distribution pattern of the point occurrences (as classified by their appended posterior

probability) to the distribution pattern of the mineral potential posterior probability map.

Approximately  to 3⁄4 of all occurrences, for any given occurrence-type, have posterior

probabilities greater than the prior probability (see Appendix D for complete listing of data). For

the primary occurrences, ~76% of the big (large and medium size) and ~69% of the small size

occurrences have posterior probabilities larger than the prior probability, with a total of ~69%

of all of the primary occurrences having a higher posterior probability. For the sedimentary rockhosted occurrences, ~84% of the big and ~82% of the small size occurrences have posterior

probabilities larger than the prior probability, with a total of ~83% of all of the sedimentary rockhosted occurrences having a higher posterior probability (Fig. 7.11). For the volcanic rockhosted occurrences, ~72% of the big and ~60% of the small size occurrences have posterior

probabilities larger than the prior probability, with a total of ~60% of all of the volcanic rockhosted occurrences having a higher posterior probability (Fig. 7.12). The proportion of

occurrences with posterior probabilities higher than the prior is roughly the same for the primary
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1 In “permissive” areas, the lithologic and/or structural features necessary for ore deposition are present. In

“favorable” areas, permissive lithologic and/or structural features are present, as well as other features

indicating that potential mineralizing processes have taken place. The term “favorable” used in this study

includes mainly exposed (not covered by Cenozoic alluvium) “permissive” areas delineated in OFR-96-96 and

OFR-96-2, and perhaps indicates a higher degree of mineral potential than “permissive”.

as it is for the volcanic rock-hosted occurrence-type. The sedimentary rock-hosted occurrences,

however, show markedly higher proportions, which is mainly due to the greater degree of

conditional dependence in the sedimentary rock-hosted occurrence-type mineral potential model.

In addition to a large proportion of the gold-silver-bearing occurrences having elevated posterior

probabilities, the spatial distribution of occurrences having elevated posterior probability values

is coincident with areas of elevated mineral potential favorability, as predicted by the posterior

probability maps. This correlation is particularly noticeable for the sedimentary and volcanic

rock-hosted occurrences, as shown in Figures 7.13 and 7.14. In both of these Figures, a

conspicuous pattern of occurrences is highlighted with a dashed line, and is shown to have a

spatially associated and corresponding pattern of areas of elevated posterior probability. The

dashed lines on these Figures merely delineate the spatial distribution of the occurrences, and do

not necessarily reflect any geologic, structural, or other genetic-related trends.

These observations indicate that (1) a relatively large proportion of occurrences have posterior

probability values higher than that of the prior probability, and (2) the distribution of this

relatively large proportion of occurrences is correlative to areas of elevated posterior probability.

Based upon Bonham-Carter’s (1994b) first criterion for successful mineral potential

modelling—the resulting mineral potential maps predict the distribution of known mineral

occurrences—the modelling carried out here, despite known CI problems, was successful.

7.4.4 Comparison to Expert-Delineated Mineral Potential Areas

In 1996 two regional-scale mineral resource assessment studies were published by the U.S.

Geological Survey: (1) OFR-96-96, “Data Base for a National Mineral Resource Assessment

of Undiscovered Deposits of Gold, Silver, Copper, Lead, and Zinc in the Conterminous United

States” (Ludington et al., 1996); and (2) OFR-96-2, “An Analysis of Nevada's Metal-bearing

Mineral Resources” (Singer, 1996). These assessments were conducted over a number of years

with the assistance and expert knowledge of a fairly large team of experts from various

geological sub-disciplines. The goal of these projects was to provide an analysis of the mineral

resources in Nevada and the United States for the purposes of facilitating economic

development, land-use and management, and minerals exploration. As part of these surveys, an

assessment for undiscovered gold and silver deposits for the Nevada Great Basin was made and

deposit “tract maps” delineating regions permissible1

 for various types of sedimentary and

volcanic rock-hosted mineralization were produced. For the sediment-hosted gold tract map,
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2 The deposits considered include: (1) an eastern group, exemplified by Bald Mountain, Golden Butte, Alligator

Ridge, Illipah, Night Hawk, and Green Springs; (2) a central group, exemplified by deposits in the Jerritt

Canyon (Burns Basin) district, the Carlin trend, Marigold, deposits around Cortez, Tonkin Springs, and

Northumberland; and (3) a western group, exemplified by deposits in the Getchell trend, Standard, and

Fondaway Canyon. Ludington et al. (1996) and Singer (1996) should be consulted for the complete list.

3 The deposit types considered include: (1) Comstock epithermal vein, exemplified by the Comstock, Tonopah,

Jarbidge, National, and Tuscarora districts, (2) quartz-alunite vein, exemplified by the Goldfield district, (3)

hot spring gold-silver, exemplified by the Round Mountain, Borealis, Paradise Peak, Hog Ranch, and Lewis

deposits, and other epithermal deposits, including hot spring Hg, simple Sb, epithermal Mn, and rhyolite-hosted

Sn. Ludington et al. (1996) and Singer (1996) should be consulted for the complete list.

only Carlin-style disseminated gold deposits were considered2. Tract delineation was based on

the distribution of the Roberts Mountain, Golconda, and several other allochthons, and, in the

Sevier orogenic belt in eastern Nevada, by patterns of folding and thrust faulting shown on the

Stewart and Carlson (1978) geological map of Nevada (Ludington et al., 1996; Singer, 1996).

For the epithermal gold-silver tract map, a number of different deposit types were considered3.

Tract delineation was based on distribution of volcanic rocks, of epithermal mineral deposits,

prospects, and occurrences, on the distribution of synvolcanic faults, and on the magnetic

anomaly patterns associated with the Walker Lane and northern Nevada rift (Ludington et al.,

1996; Singer, 1996). Ludington et al. (1996) and Singer (1996) should be consulted for further

details on the mineral assessment methodology, assumptions, and rational.

Using area cross-correlation analysis, comparisons were made between permissible tracts for

sedimentary-hosted gold and epithermal gold-silver mineralization as delineated by the expert

mineral assessments and the sedimentary and volcanic rock-hosted mineral potential maps as

delineated in this study with weights of evidence (Figs. 7.15 and 7.16). The analysis indicated

that:

Approximately 85% of the favorable and ~95% of the elevated favorable areas for sedimentary rock-hosted

gold mineralization lie within the expert-defined permissive tracts for such deposits.

Approximately 73% of the favorable and ~91% of the elevated favorable areas for volcanic rock-hosted

mineralization (epithermal), lie within the expert-defined permissive tracts for such deposits.

Both the sedimentary and volcanic rock-hosted occurrence posterior probability maps display

regions of strong mineral potential that are coincident with permissive tracts as delineated by the

experts, as well as coinciding with areas hosting important and/or many occurrences. But more

important, the mineral potential maps produced in this study highlight regions of strong mineral

potential where known occurrences are absent or few in number. Based upon Bonham-Carter’s

(1994b) second criterion for successful mineral potential modelling—the resulting mineral

potential maps are useful for delineating or targeting regions of elevated mineral potential for

follow-up exploration—the maps created here have successfully outlined areas that might be

interesting regional-scale exploration targets. These targets are presented in section 7.5.

7.4.5 Blind Test of Mineral Potential Model Predictability

The posterior probability maps produced in this study were built from mineral potential models

calibrated (trained) using a 1993 download of Mineral Resource Data System (MRDS) database
98

(with some modifications from a 1995 download; see chapter 5). A list of newly discovered

occurrences (or at least newly tabulated or reported since the 1993/95 MRDS occurrences used

for this study) was compiled from Ludington et al. (1996), the Nevada Mineral Industry reports

for 1994, 1995, and 1996 (Nevada Bureau of Mines and Geology staff, 1995-1997), and from

Joe V. Tingley of the Nevada Bureau of Mines and Geology (1998, personal communication).

These discoveries consist of 12 sediment-hosted occurrences (Gold Canyon, Golden Butte, Pan,

Pinon Range–Cord Ranch, Pipeline, Reona, Saddle, South Pipeline, Treasure Hill, TrentonValmy, Winters Creek, and Wright Window) and 5 epithermal occurrences (Gemfield, Golden

Arrow, Hydra-Hercules, Midway, Mule Canyon).

The blind test was carried out by appending the posterior probability value calculated using the

appropriate mineral potential model to the 12 new sedimentary and 5 new volcanic rock-hosted

occurrences. Ten of the twelve sedimentary rock-hosted occurrences were estimated to have

posterior probabilities higher than the prior probability, where four have notably high posterior

probabilities (Fig. 7.17). Five of five epithermal occurrences were estimated to have posterior

probabilities higher than the prior probability, where two have notably high posterior

probabilities (Fig. 7.18). These results clearly validate the sedimentary and volcanic rock-hosted

mineral potential models built in this study and indicate that they have a significant predictive

capacity.

7.5 Geologic Characterization and Delineation of RegionalScale Sedimentary and Volcanic Rock-Hosted

Occurrence Exploration Targets

7.5.1 Introduction

A couple of the main purposes of modelling mineral potential are (1) to predict areas where

geologic conditions conducive to mineralization exist and where processes resulting in economic

ore concentrations are likely to have taken place, and (2) to target regions of high mineral

potential for further investigation using the predicted areas as an exploration guide. 

The selection of favorable regions for exploration involves identifying areas of elevated posterior

probability that have few or no known gold-silver-bearing occurrences. This is most easily

accomplished by first superimposing a “mask” of known occurrences over the mineral potential

map, and locating those areas within a region of elevated posterior probability that show through

the mask. For this study, a mask consisting of a 10 km radius buffer around all known

occurrences was used. Ten kilometers is a subjective estimate representing the extent of a

mining lease, which may or may not include peripheral “backyard” claims, prospects, or other

exploration areas. A radius of 10 km around an ore deposit may also be a fair approximation for

the extent of (or at least encompasses) the mineralizing system (i.e.—the extent of district or

local-scale hydrothermal circulation; see Nesbitt and Muehlenbachs, 1989; Sillitoe and Bonham,

1990). This estimate may not be appropriate in all instances—10 km may be an overstatement

with regard to mineral “showings” or an understatement in cases such as the Comstock or Carlin

deposits—but, for all practical purposes, it is considered reasonable for revealing sites of interest.

Using the mineral potential models, promising target sites can be interactively queried “on-
99

screen” with the GIS in order to investigate which of the evidence layers (or combination of

layers) is responsible for the favorability highs in a given region. This provides a “feel” for what

geologic factors (according to the data-driven, weights of evidence mineral potential model)

might be important for mineralization at a particular site. Such information (supplementing

knowledge and experience) can be used to judge whether or not the site is viable. The ability to

isolate clearly and understandably which of the evidence layers produce a mineral potential high

is one of the important advantages that weights of evidence has over other methods for

combining geoscientific spatial data.

This section outlines regional-scale exploration targets for sedimentary and volcanic rock-hosted

occurrences. In total, 10 prospective target regions were selected (five for sedimentary rockhosted occurrences and five for volcanic rock-hosted occurrences). The target regions are

presented in as a series of maps:

1. Regional-Scale Exploration Target Index Maps—Two Figures that show the sedimentary and volcanic rockhosted occurrence posterior probability maps overlain with a mask consisting of all three occurrence-type

samples (primary, sedimentary and volcanic rock-hosted occurrences). The purpose of applying this mask,

rather than a mask consisting only of one or another occurrence-type, is to isolate those regions where there

is an absence of occurrences of any type.

2. Close-up Maps of Individual Regional-Scale Exploration Targets—Three or four Figures, where each Figure

consists of: (1) a map showing known occurrences relative to predicted favorable areas within the region

(some of the more prominent deposits are labelled); (2) the sedimentary or volcanic rock-hosted occurrences

posterior probability map shown with respect to regional- and local-scale structures, against shaded relief

of topography; and (3) the geological assemblage map for the target region shown with primary occurrences.

The occurrences shown on these maps are displayed with a radius of 2 km, rather than 10 km. This was done

to reveal more of the area of interest.

The prospective regional-scale exploration targets shown on the two index maps (item #1 above)

have a number code that is used to link them to a close-up map (item #2 above) and a detailed

description of the lithologic, structural, and tectonomagmatic mineral potential evidence each

target region (presented below in sections 7.5.2 and 7.5.3 below). For each target region, the

following material is reviewed:

General location.

Significant deposits hosted by the target region.

The main mineral potential evidence that qualifies the area as a target region.

Other significant geologic features conducive to mineralization.

Potential host lithologies.

Specific sites (at the mountain-range-scale) for further investigation.

The mineral potential maps presented below are intended to help define regional-scale

exploration targets for detailed data collection and further investigation—they are not intended

for delineating drilling targets. The scale of the data compiled here is too small (not “fine”

enough) to provide the resolution and information needed to delimit a drilling target. The

mineral potential maps are, however, effective for selecting areas that have the proper geologic

conditions favorable for gold-silver mineralization. The prospective regional-scale exploration

targets presented below are not ranked in any particular order of importance. Geographic

references (names of mountain ranges, valleys, creeks, highways, etc.) are from the geological
100

map of Nevada (Stewart and Carlson, 1978).

7.5.2 Sedimentary Rock-Hosted Mineral Potential Regions

The sedimentary rock-hosted occurrence-type regional-scale exploration target index map is

shown in Figure 7.19. The individual target regions are shown in Figures 7.20, 7.21, and 7.22.

They consist primarily of elevated mineral potential areas that are not immediately surrounded

by sedimentary rock-hosted or other types of occurrences, and areas that are proximal to a few

lone scattered occurrences (i.e.—target region #5). The target regions are mainly located along

a “V”-shaped trend located in northern Nevada. The individual target regions are described

below. A summary of the important geologic mineral potential evidence associated with areas

of elevated favorability in the target regions is given in section 8.2.1.

Region #1—Located in the northeastern corner of the state, there are no known sedimentary rock-hosted

occurrences in this target region (or at least no known occurrences listed in the MRDS database) (Fig. 7.20a). The

Jarbidge district (volcanic rock-hosted deposits) is the most important known gold-silver mineralization in the

region. The most significant mineral potential evidence for this region consists of lithology, lithotectonic terrane,

and the isostatic gravity anomaly, as well as possible deep-seated structures (Fig. 7.20b). This region is transected

by a number of crustal-scale structural features, which include the 87Sr/86Sr ISr = 0.706 isopleth (presumed to

delineate the western edge of the North American Precambrian craton, and may represent a zone of crustal

weakness), the Golconda and Roberts Mountain thrust fronts, and several possible deep-seated basement and/or

strike-slip features (see Shawe, 1965; Blakely, 1988; Putnam and Henriques, 1991; also see Figs. 2.5, 2.10, and

2.11). The northeastern segment of the Nevada paleothermal anomaly, trending roughly northwest–southeast, also

extends across this region (see Fig. 2.10). The areas of highest posterior probability are located proximal to the

intersection of many of these features, which could provide fluid pathways for mineralizing fluids at upper crustal

levels and facilitate the movement of deep-crustal fluids to higher crustal levels. The crust in this region is also

characterized by high present-day heat flow. Lithologies permissive for mineralization include the “Marine and

Shelf–Early-Mid PZ” and “Siliceous Rocks–Mid PZ-PC” lithologic assemblages (Fig. 7.20c). These assemblages

were most likely thrust into place, which effectively tectonically over-thickened the crust in this region (as

suggested by geophysical anomalies; see section 6.6.3). These assemblage units have the highest weights of spatial

association (W+

) with the sedimentary rock-hosted occurrences (see Fig. 6.12). While the lithology is favorable

for ore deposition, the relative fault density over most of the region is not particularly high, although the areas of

elevated mineral potential favorability do show moderate lithologic diversity. It is interesting to note that the area

of highest relative fault density corresponds the location of the Jarbidge district. Suitable sites for further

investigation might include: (1) areas east and southeast of the Arizona mine, along the northeast–southwesttrending length of Knoll Mountain, and the area just north of Jake's Creek, in the westward range across the valley

adjacent to (west of) the Arizona mine; (2) in the mountains surrounding the town of Melanoco (Antelope Peak

to the west, and Black Mountain to the east), in the vicinity of N 41.28o – W 114.85o

; (3) the northernmost Pequop

Mountains, just west of the town of Oasis, around N 41.06o – W 114.85o

 in the vicinity of the Pequop district; (4)

the area due-southeast of the Silver Giant Claim Group and Norman mines (Jarbidge district), between Marys River

Peak and the source of Anderson Creek; and (5) in the immediate vicinity south of the Coleman and Foss mines

(Island Mountain district area), in the mountains just northeast of the town of Wild Horse, and on Cornwall

Mountain.

Region #2—Located in northern-most central Nevada, just west of target region #1, this target region is host to

a number of world-class deposits that include the Independence group and Carlin-trend sedimentary rock-hosted

deposits, as well as the Tuscarora district (volcanic rock-hosted deposits), and the Aura and Edgemont polymetallic

vein districts (Fig. 7.20a). This is one of the richest precious metal producing mining regions in the world. The

most significant mineral potential evidence for this region consists of lithology, the Ba/Na and K/Na geochemical

anomalies, lithotectonic terrane, the isostatic gravity anomaly, as well as many deep-seated and crustal structures

(Fig. 7.20b). Many crustal-scale structural elements come together in this region, all of which may have provided

lateral and vertical pathways for fluid movement: the favorable areas are (1) adjacent to the northern Nevada rift

zone (see sections 2.7.4, 6.6.3, 8.2.1, and 8.5.1); (2) flanked to the W-NW by Roberts Mountain thrust and E-SE
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by the Golconda thrust fronts (see Fig. 2.5); (3) collateral with the eastern margin of the Nevada paleothermal

anomaly (see Fig. 2.10); (4) intersected by a number of possible deep-seated basement structures; and (5) they

straddle, at right-angles, the 87Sr/86Sr ISr = 0.706 isopleth, presumed to be the western edge of the North American

Precambrian craton (see Figs. 2.10 and 2.11). Other regional characteristics conducive to ore formation include

moderate lithologic diversity and a high relative fault density. This region is also characterized by moderately high

modern-day heat flow. As with target region #1, the two most permissive lithologic assemblages include “Marine

and Shelf–Early-Mid PZ” and “Siliceous Rocks–Mid PZ-PC” (Fig. 7.20c; also see Fig. 6.12). Promising sites for

further investigation include: (1) the southern Tuscarora Mountains, on a southerly traverse between the Carlin

deposit and the Marys Mountain/Carlin Peaks area (Carlin trend); (2) the hills on the northeastern side of Maggie

Creek (the stream), approximately northeast of the Tusc, Gold Quarry, and Maggie Creek deposits (Carlin trend);

(3) along a traverse northwest of the Rain and Emigrant Springs deposits to about N 40.70o – W 116.07o

; (4) the

region along a northeast-southwest traverse through the mountains connecting the northern Carlin trend deposits

(i.e.—Post and Meikle area) and the and the southern Independence group deposits (i.e.—Mill Creek and Burns

Basin), especially in the vicinity of Wheeler Mountain; (5) in the southern Independence Mountains, in the vicinity

surrounding Swales Mountain, especially areas to the northeast; (6) in the central Independence Mountains,

between the Generator Hill and Sammy Creek deposits (Independence group), in the areas west of McAfee and

Jacks peaks (possibly under some of the morainal deposits in this area); (7) in the northern (north central?)

Independence Mountains, the area immediately southeast of Rocky Bluff and north–northeast of the Sammy Creek

deposits (Independence group); and (8) isolated spots in the northern Tuscarora Mountains, in the region

surrounded by the Toe Jam Mountain–Dry Creek Mountain–Mount Blitzen–Castile Mountain, and especially the

area down-hill to the south of Toe Jam and Castile mountains. It seems that “backyard” prospecting might still be

very worthwhile in this region.

Region #3—Located in central Nevada, just south of target region #2, this target region is south of the Carlin

trend deposits and east of the Battle Mountain–Eureka (Cortez) trend deposits. The Rain, Emigrant Springs, Trout

Creek, and South Bullion deposits (Carlin trend) are located in the northern portion of the region, and the Tonkin

Springs, Gold Pick, and Gold Bar deposits (Battle Mountain–Eureka trend) are found in the south-southwestern

part of the region (Fig. 7.21a). Like region #2, the northern and south-southwestern areas of this target region are

very rich in gold. The most significant mineral potential evidence for this region consists of lithology, the Ba/Na

or K/Na geochemical anomalies, the isostatic gravity anomaly, as well as numerous crustal-scale structural features

(Fig. 7.21b). Of particular interest is a narrow zone of high mineral potential that trends north-south across the

target region, and a circular area in the south-central part of the region. The narrow north-south trending

favorability zone is collateral with the Roberts Mountain thrust, and is intersected at acute angles by a number of

deep-seated basement structures, which are parallel and proximal to the northern Nevada rift zone. The circular

zone of favorability is approximately located where the northern Nevada rift zone geomagnetic anomaly intersects

the trace of the Roberts Mountain thrust front. These structural arrangements, similar to those found in target

region #2, could facilitate fluid passage and mineralizing processes. Both of the favorable zones lie to the east of

the eastern margin of the Nevada paleothermal anomaly (see Fig. 2.10). The favorability zones are also

characterized by moderate relative fault density, moderate lithologic diversity, and they trend along the easternsoutheastern margin of the present-day Battle Mountain heat high. The two most permissive lithologic assemblage

units include “Marine and Shelf–Late PZ” and “Marine and Shelf–Early-Mid PZ” (Fig. 7.21c), slightly different

from those found in regions #1 and #2, but also highly rated with respect to their weights of spatial association (W+

)

with the sedimentary rock-hosted occurrences (see Fig. 6.12). Localities that might warrant further exploration

include: (1) areas in the northern Sulphur Spring Range (the Pinon Range), between and west of Buckskin, Spring

Canyon, and Pine mountains, proximal and west-northwest of the Rain and Emigrant Springs deposits (Carlin

trend); (2) areas in the Pinon Range, east of Pine Mountain, the region between the Rain and Emigrant Springs

deposits and the Trout Creek and South Bullion deposits (Carlin trend), in the vicinity of Web Foot (Cu-skarn), Last

Chance (polymetallic replacement), and Sylvania mines; (3) the entire length of the narrow north-south-trending

zone of mineral potential highs, along the Sulphur Springs Range, roughly extending along Pine Mountain–Coffin

Mountain–Union Mountain–Mineral Hill–Bald Mountain–Baily Pass (this target is especially interesting because

there is only one deposit of any significance, Mineral Hills, a large polymetallic replacement deposit, located along

this range); (4) areas in the Roberts Mountains, immediately surrounding, and to the northeast and southwest of,

the Gold Pick, Gold Ridge, and Goldstone deposits (Battle Mountain-Eureka trend), and especially north and east

of Roberts Creek Mountain.

Region #4—Located in central Nevada, west of region #3, the targets in this region are just southwest of, and
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parallel to, the Battle Mountain–Eureka (Cortez) trend deposits. The Cove mine is located in the northwest part

of this region, and the Hilltop mine, Gold Acres deposit, Cortez deposit, and Horse Canyon mine (Battle

Mountain–Eureka trend) occur in the northeastern portion of the region (Fig. 7.21a). The Tonkin Springs, Gold

Pick, Gold Bar, Gold Ridge, and Goldstone deposits (Battle Mountain–Eureka trend) are found in the southeastern

part of the region, and were addressed above in the description of region #3. This region, as are regions #2 and #3,

is rich in precious metals. The most significant mineral potential evidence in this region consists of lithology, the

Ba/Na geochemical anomaly, and lithotectonic terrane, in addition to some crustal-scale structures (Fig. 7.21b).

The mineral potential areas of interest are parallel to and bound by the northern Nevada rift zone to the northeast

and the Golconda thrust front to the southwest (see Figs. 2.3, 2.5, and 6.45). Further west, the favorable areas are

flanked by the 87Sr/86Sr ISr = 0.706 isopleth, presumed to be the western edge of the North American Precambrian

craton (see Figs. 2.10 and 2.11), and by a shorter and smaller geomagnetic anomaly related to the northern Nevada

rift, presumed to represent a crustal-scale fracture or structural zone (see Fig. 6.45a, feature #1, and sections 2.7.4,

6.6.3, and 8.5.1). In addition, the favorable areas lie largely within and along the eastern margin of the Nevada

paleothermal anomaly (see Fig. 2.10). The parallel alignment and proximity of these regional-scale features to one

another could have provided through going upper-crust–lower-crust permeability for mineralizing fluids and heat.

The areas of high mineral potential are also characterized by moderate relative fault density, moderate to

moderately-high lithologic diversity, and anomalously high present-day heat flow (this target region is situated

nearly in the center of the Battle Mountain heat high). The most permissive lithologic assemblages are once again

the “Marine and Shelf–Early-Mid PZ” and “Siliceous Rocks–Mid PZ-PC”, which have the highest weights of

spatial association (W+

) with the sedimentary rock-hosted occurrences (Fig. 7.21c; also see Fig. 6.12). Sites that

might be considered for further investigation include: (1) areas in the general vicinity of the Shoshone Range, near

Mt. Lewis and Goat Peak, especially around and immediately to the southwest of the Elder Creek mine (Battle

Mountain–Eureka trend); (2) areas further southwest of the Elder Creek mine in the Shoshone Range, just to the

northwest of the central and southern end of Carico Lake Valley, around N 40.05o – W 117.10o

; (3) areas in the

northern Toiyabe Range, along the eastern margin of Carico Lake Valley, west of Bald Mountain, around N 40.00o

– W 116.85o; (4) areas in the northern Toiyabe Range–southern Cortez Mountains, in the vicinity of Bald Mountain

along the northwestern margin of the northern end of Grass Valley, around N 40.04o – W 116.67o

; (5) areas in the

northern Toiyabe Range, immediately southwest of Mt. Callaghan; and (6) areas in the Simpson Park Mountains,

just southeast of Twin Peaks and parallel to the Battle Mountain–Eureka trend (northwest–southeast), and along

the ridge-line connecting McClusky Peak, Buck Mountain, and Fagin Mountain (trending approximately northnortheast–south-southwest). 

Region #5—Located in south-central and southwestern Nevada, the principal target areas in this region are

located near the Austin Gold Ventures (Quito) mine and the Northumberland mine, in the north-central portion of

the region, and near the Candelaria mine in the southwestern part of the region (Fig. 7.22a). Other targets are

located just south of the Ratto Canyon mine, in the extreme northeast corner of the region, and north of the Tonopah

district (volcanic rock-hosted deposits). With the exception of a few other widely scattered deposits, this target

region is generally lacking in sedimentary rock-hosted occurrences, although this region is host to a number of

medium- and large-size volcanic rock-hosted deposits and other types of mineralization (see Fig. 7.22a). The most

significant mineral potential evidence in this target region consists of the K/Na and Ba/Na geochemical anomalies

(where present), lithology, and lithotectonic terrane, as well as the presence of several regional-scale structural

features (Fig. 7.22b). The principal mineral potential favorability areas are crossed by the Golconda thrust front

and the 87Sr/86Sr ISr = 0.706 isopleth (presumed to represent the western edge of the North American Precambrian

craton; see Figs. 2.10 and 2.11), or are proximal to these features. The favorable areas also correspond to locations

along the Golconda thrust and 87Sr/86Sr ISr = 0.706 isopleth that are intersected at acute angles or parallel to

presumed deep-seated structural features, some of which may be related to transcurrent movement along the Walker

Lane belt. In addition, the principal high mineral potential areas lie along the eastern or western margin of the

Nevada paleothermal high. The spatial coincidence of these structural zones and the paleothermal heat high may

have promoted fluid circulation and facilitated precious metal mineralization. The relative density of faults across

the region is variable, but most of the favorable mineral potential targets lie within areas of moderate to slightly

higher relative fault densities, and moderate lithologic diversity. Modern-day crustal heat flow in this target region

is variable: very high in the western half, which corresponds to the Walker Lane shear zone, and very low in the

eastern half, which corresponds to the highlands of central–south-central Nevada. The most permissive lithologic

assemblages for hosting occurrences in this region are the “marine and shelf” assemblages, particularly the “Marine

and Shelf–Early-Mid PZ” assemblage, which has the second highest weight of spatial association (W+

) with

sedimentary rock-hosted occurrences (see Fig. 6.12). In general, the areas of interest in this region are fewer and
103

smaller in extent, as compared to target regions #1-#4, however, the following sites may warrant some

consideration: (1) the central Toiyabe Range, along Toiyabe Range Peak–Bunker Hill–Toiyabe Peak–Bob Scotts

Summit, especially north and west of Bunker Hill, and in the vicinity of the Austin Gold Ventures (Quito) mine;

(2) the central Toquima Range, around Petes Summit, Wildcat Peak, and especially south of Wildcat Peak in the

vicinity of the Northumberland deposit, but well short of Mount Jefferson; (3) the southernmost Fish Creek Range,

well south of Dave Keane Mountain (no known deposits in this area); (4) the northern Hot Creek Range, north of

Morey Peak and northwest of Morey district; (5) the Candelaria Hills, in southeastern Mineral County, north and

northeast of Miller Mountain, and especially the region west of the Lucky Hill mine and the Candelaria district;

and (6) the southwestern Monte Cristo Range, in central northwest Esmeralda county, southwest of the Charley

claims along the eastern margin of the Columbus Salt Marsh.

Other Regions—In addition to the targets noted above, a number of other promising exploration sites in the

Pershing–Lander–Humboldt counties region, which are not reviewed in detail here, include: (1) the northern Tobin

Range, north of the Needle Peak and Mount Tobin area, south of the Iron Hat polymetallic vein district and north

of the Mount Tobin hot springs deposit; (2) the western (northwestern) half of Battle Mountain, north and

northwest of Antler and North peaks; (3) the mountains northwest of the northeastern opening of Pumpernickel

Valley, especially in the area surrounding the Golconda Summit; (4) the northwestern Osgood Mountains, to the

west and north of the Getchell trend deposits; (5) the Hot Springs Range (west of the Osgood Mountains), the

whole of the range but particularly north of Hot Springs Peak in the vicinity of the fault-bound Paleozoic

serpentinite units (this lithologic unit, Pzsp, has a high spatial association with distal disseminated occurrences in

the Candelaria district in southeastern Mineral County). All of these sites have similar geologic characteristics and

attributes to those mentioned in the target regions above.

7.5.3 Volcanic Rock-Hosted Mineral Potential Regions

The volcanic rock-hosted occurrence-type regional-scale exploration target index map is shown

in Figure 7.23. The individual target regions are shown in Figures 7.24, 7.25, 7.26, and 7.27.

They consist primarily of elevated mineral potential areas that are not immediately surrounded

by volcanic rock-hosted or other types of occurrences. The target regions are mainly located

within the Walker Lane belt and the Humboldt zone, which trends northeast–southwest across

northern Nevada. The individual target regions are described below. A summary of the important

geologic mineral potential evidence associated with areas of elevated favorability in the target

regions is given in section 8.2.2.

Region #1—Located in the extreme northwestern corner of Nevada, the main targets in this region are associated

with a cluster of Au-Ag hot spring deposits (Virgin Valley district) that occur in the northern portion of this region,

and the Western Hog Ranch Au-Ag hot spring mine, which is located in the central part of the region (Fig. 7.24a).

The Crofoot and Lewis mines (Sulphur district) are found in the southeastern part of this target region. The

occurrences in this region are principally Au-Ag hot-spring-type. The most significant mineral potential evidence

for this region consists of lithology, the geomagnetic anomaly, fault distance buffers, and the isostatic gravity

anomaly, as well as numerous possible deep-seated basement structures that trend through the eastern and southern

halves of the region (Fig. 7.24b). In some cases the mineral potential regions of interest are aligned along some

of these deep-seated basement structures, while in other instances the favorable areas are proximal to the

intersection of the structures. The favorable areas are also characterized by moderate to high relative fault density,

and are flanked to the east, southeast, and south by Mesozoic plutons. These structural arrangements could

facilitate fluid flow, as well as provide structural traps and the appropriate physio-chemical conditions conducive

to ore deposition (see discussion in sections 3.4, 6.4.3, and 6.5.3). In addition, this region is marginal to the

present-day Battle Mountain heat high, and may have a relatively thin crust (~20 km; see Allenby and Schnetzler,

1983; Pakiser, 1989; and sections 2.7.2 and 2.7.5). If these conditions were present around 27-5 Ma, they most

likely contributed heat to the region, promoting magmatic activity and the convection of crustal fluids. The most

permissible lithologic assemblage in this region is “Felsic-Intermediate Extrusive & Intrusive–Late CZ”, which is

the volcanic assemblage having the highest weight of spatial association (W+

) with the volcanic rock-hosted

occurrences (Fig. 7.24c; also see Fig. 6.15). The two principal areas of interested are located in the north-central
104

and west-central portions of the region, with other areas located in the northwest and the east. The following sites

may warrant further investigation: (1) north and south of the Western Hog Ranch mine, in the vicinity and north

of Fox and Hog Ranch mountains; (2) the hills west and north of Cottonwood Creek (just west of the Western Hog

Ranch mine), and especially in the areas northwest and west of High Rock Canyon (no known mines in this area);

(3) the Hays Canyon Range, in the vicinity of Hays Canyon Peak (no known mines in this area); (4) the mountains

located in northwesternmost Nevada, around -118.80o

 longitude–41.40o latitude (no known mines in this area); (5)

the general area around the Virgin Valley district, in the vicinity of Catnip, Fish Creek, and Blow Out mountains;

(6) northeast of the Virgin Valley district area, in the vicinity of McGee and Big mountains (no known mines in

this area); and (7) the Pine Forest Range, along a traverse following Bartlett Peak, Duffer Peak, and extending to

the northern part of the range (this area has a large number of small low-sulfide gold-quartz vein deposits).

Region #2—Located in northernmost central Nevada, east of target region #1, the principal targets in this region

border the southwestern and southeastern margin of relatively broad late Tertiary volcanic plateau, consisting of

basalt and associated gravels, known as the Banbury formation (Fig. 7.25a and 7.25c). The most prominent area

of mineral potential favorability is located in the northwestern portion of this region, and trends roughly northwestsoutheast. It is host to the National district, the Golden Circle (Midas) district, and the Hollister (Ivanhoe) deposits

(Fig. 7.25a). Other large volcanic rock-hosted deposits occur in the eastern part of the region, and include the

deposits of the Good Hope and Tuscarora districts. This target region also hosts a number of very significant

sedimentary rock-hosted deposits, including the Getchell trend, Carlin trend, and Independence group occurrences,

making it one of the richest gold-producing areas in the world. The most significant mineral potential evidence for

this region consists of lithology, the geomagnetic anomaly, fault distance buffers, and lithologic diversity. In

addition, this target region, and in particular the prominent northwest–southeast-trending mineral potential

favorability area, is intersected by many crustal-scale lithotectonic features, including: (1) the northern Nevada rift

zone, which is roughly coincident with the most significant mineral potential “high” in this region; (2) numerous

proposed deep-seated basement structures (Fig. 7.25b); and (3) several strike-slip faults that parallel the Humboldt

zone (see Shawe, 1965; Putnam and Henriques, 1991). The main mineral potential favorability area also intersects

the 87Sr/86Sr ISr = 0.706 isopleth, presumed to represent the western edge of the North American Precambrian craton

(see Figs. 2.10 and 2.11), the Golconda thrust front, and the Nevada paleothermal high. Furthermore, the areas of

high mineral potential are characterized by relatively thin crust (~20-28 km), high present-day heat flow (marginal

to the Battle Mountain heat high), and moderate to high relative fault density. Like target region #1, as well as

sedimentary rock-hosted target regions #2-4, the configuration of tectonic elements observed in this region could

facilitate mineralization processes. The most permissible lithologic assemblages in this region include “FelsicIntermediate Extrusive & Intrusive–Late CZ” and “Intermediate-Mafic Extrusive & Intrusive–Late CZ”, the former

being the volcanic assemblage having the highest weight of spatial association (W+

) with the volcanic rock-hosted

occurrences, the latter having a moderate weight (Fig. 7.25c; also see Fig. 6.15). Possible exploration sites for

further investigation include: (1) the Santa Rosa Range, along a traverse connecting McConnell Peak–Buckskin

Mountain–Granite Peak–Santa Rosa Peak–Paradise Peak, but particularly in the vicinity and north of McConnell

Peak; (2) the mountains along the southwestern margin of the Banbury formation, along a traverse extending from

the McConnell Peak–Capital Peak vicinity to the Gold Circle district, and particularly south of the south fork of

the Little Humboldt River and north of the town of Midas; (3) directly east of the town of Red House, the

mountains on the far side of the valley, forming the eastern border of the valley; (4) the mountains along the

southeastern margin of the Banbury formation, along a traverse extending northeast from the Gold Circle district

area to the Cornucopia deposit; (5) the northern Tuscarora Mountains, a small area just southeast of Toe Jam

Mountain and southwest of Castile Mountain; and (6) the eastern margin of the Banbury formation, immediately

in the vicinity of Hat Peak.

Region #3—Located in northeastern corner of Nevada, east of target region #2, this target region is nearly the

same in extent as sedimentary rock-hosted target region #1 (Figs. 7.25 and 7.20). The Jarbidge district deposits

(mainly Comstock epithermal veins) occur in the northwestern portion of the region, and the principal areas of

favorable mineral potential are located in the west and east (Fig. 7.25a). The Independence group sedimentary

rock-hosted deposits also occur along the western boundary of the region. The most significant mineral potential

evidence for this region consists of lithology, and in varying degrees of frequency and combination, fault distance

buffers, pluton distance buffers, the geomagnetic anomaly, lithologic diversity, and the isostatic gravity anomaly,

in addition to a number of deep-seated and crustal-scale structures (Fig. 7.25b). This region is transected by several

crustal-scale structural features, as is noted for sedimentary rock-hosted exploration target region #1 (see discussion

above). The mineral potential favorability area in the western part of the region is located at the intersection of the
105

87Sr/86Sr ISr = 0.706 isopleth, the Golconda thrust front, a possible deep-seated basement structure, and the Nevada

paleothermal high. The two principal favorability areas are associated with crust that has moderate to high relative

fault density, and are proximal to Mesozoic plutons. The region is also characterized by high present-day heat flow

and a thick crust (30-38 km). The most permissive lithologic assemblage in this region is “Felsic-Intermediate

Extrusive & Intrusive–Late CZ”, which is the volcanic assemblage having the highest weight of spatial association

(W)+

 with volcanic rock-hosted occurrences (Fig. 7.25c; also see Fig. 6.15). Potential sites for further exploration

include: (1) northeast of the northern Independence Mountains, in “The Mahoganies”, along a traverse extending

northeast from the Maggie Summit–Rock Bluff vicinity, through to Haystack and Rough mountains, and up to Hot

Springs Butte; (2) the mountains along the eastern side of State Highway 43, between the towns of Wild Horse and

North Fork; (3) along the eastern fork of Beaver Creek, within the area of Wild Horse–Mason–Lookout mountains,

as well as the area immediately south of the confluence of Beaver Creek and the Humboldt River; (4) the Jarbidge

Mountains, in the area between Marys River–Gods Pocket peaks and Twin Buttes (to the northwest), the areas

immediately surrounding Marys River–Gods Pocket peaks, and areas further south of Marys River–Gods Pocket

peaks; (5) along the northeast–southwest-trending highlands southwest of L&D Mountain; (6) in the immediate

vicinity of Tijuana John Peak (in northeastern Nevada); (7) south and southwest of Gollaher Peak (in northeastern

Nevada); (8) in the immediate vicinity of White Rock Mountain, and immediately south of Bald Mountain (both

in northeastern Nevada); and (8) in the immediate vicinity of China Jim Mountain (in northeastern Nevada).

Region #4—Located in central western Nevada, this region is host to some of the largest volcanic rock-hosted

deposits in Nevada, including the Buster mines, the Comstock district, and the Como district in the western portion

of the region, the Aurora district in the south, and the Wonder district, the Sand Springs district, the Fairview

district, the Rawhide deposit, and the Paradise Peak mine in the eastern part of the region (most are Comstock-type

epithermal vein deposits) (Fig. 7.26a). The areas of elevated mineral potential are distributed in a semi-uniform

manner as a broad belt that trends northwest-southeast across the region. The most significant mineral potential

evidence for this region consists of lithology, pluton distance buffers, the geomagnetic anomaly, faults distance

buffers, lithologic diversity, and the isostatic gravity anomaly, in addition to numerous possible deep-seated and

strike-slip structures (Fig. 7.26b). Clearly, the most important structural feature in this region is the Walker Lane

shear zone, along which extends the belt of elevated mineral potential areas. Possibly equally important are a

number of linear features, trending northeast-southwest along the Humboldt zone (see Figs. 1.2 and 6.6), which

intersect the Walker Lane shear zone in the vicinity of Comstock district (the Humboldt zone and these linear

features are discussed in greater detail in sections 8.3.2 and 8.4). The region is in general transected by a number

of crustal-scale possible deep-seated basement structures and strike-slip faults (see Shawe, 1965 Blakely, 1988;

Putnam and Henriques, 1991). The crust is presumed to be thin (<30 km; see Allenby and Schnetzler, 1983;

Allmendinger et al., 1987; Wernicke et al., 1996, and section 2.7.2), and characterized by a variable relative fault

density (although the relative density of small linear features interpreted from LANDSAT imagery is moderate to

high) and by a density of plutons greater than any other area in Nevada. Present-day heat flow across the region

is variable, with a distinct high in the area around Reno, Nevada (Steamboat hot springs), approximately where

linear features that parallel the Humboldt zone intersect the Walker Lane shear zone. The structural and crustal

conditions in this region appear to have been near-optimal for ore deposition—enhanced fracture and fault

permeability, as well as structural traps for mineralization, provided by shearing along the Walker Lane

deformational belt, and heat input from widespread volcanism to drive hydrothermal convection. The most

permissive lithologic assemblages in this region include “Felsic-Intermediate Extrusive & Intrusive–Late CZ” and

“Breccia and Tuffs–Late CZ”, the former being the volcanic assemblage having the highest weight of spatial

association (W+

) with the volcanic rock-hosted occurrences, the latter having a relatively high weight as well (Fig.

7.26c; also see Fig. 6.15). Sites for further investigation in this region are numerous, and there are far more than

can be reviewed here. The following is a list of general areas or groups of sites that have good potential for hosting

volcanic rock-hosted precious metal occurrences: (1) the northern Carson Range, the area north of Mount Rose;

(2) the Virginia Range, the area north of the Comstock district and south of Interstate Highway 80; (3) the northern

Pine Nut Mountains, along a traverse connecting Table Mountain–Rawe Peak–Lyon Peak–Bismark Peak; (4) the

southern Pine Nut Mountains, along a traverse connecting Mount Como–Mount Siegel–Eagle Mountain–Wild Oat

Mountain; (5) the Wellington Hills, in the vicinity of Desert Creek Peak; (6) the Pine Grove Hills, along a traverse

extending north from Bald Mountain through Mount Etna–Mount Wilson–Shamrock Hill to Carson Hill; (7) the

Desert Mountains, between Cleaver Peak and Brown Knob; (8) the Gabbs Valley Range, from the northernmost

reaches southeast to Table Mountain and south to Pilot Peak; (9) the southernmost Clan Alpine Mountains, south

and east of Big Kasock and Slate mountains; (10) the southwestern, central, and northeastern Excelsior Mountains,

as well as southeast of the northern end of Huntoon Valley; and (11) the central Monte Cristo Range.
106

Region #5—Located in southeastern Nevada, this region is generally lacking in volcanic rock-hosted occurrences

as compared to the other target regions, but despite this, it does have a number of elevated mineral potential areas.

The notable deposits in the region include the Eagle Valley district deposits (Comstock epithermal veins) in the

northeast, the Delamar (Ferguson) district deposits (Comstock epithermal veins) in the east-central part of the

region, the Golden Arrow district deposits (Comstock epithermal veins) in the northwest, and in the southwest, the

Mother Lode mine, which is just east of the Bullfrog district, host to a number of larger Comstock epithermal vein

deposits (Fig. 7.27a). Other deposit types of interest in this region include the Pioche and Highland districts in the

northeast (polymetallic replacement deposits), and the Sterling mine in the southwest (sedimentary rock-hosted AuAg). The areas of elevated mineral potential are mainly distributed throughout the western half of this region, and

concentrated in the east. The most significant mineral potential evidence for this region consists of lithology, pluton

distance buffers, the geomagnetic anomaly, faults distance buffers, lithologic diversity, and to some degree, the

isostatic gravity anomaly, as well as a number of regional-scale structures (Fig. 7.27b). The area of elevated mineral

potential occurring in the eastern part of the region is located along the Sevier thrust front, where it is intersected

by several possible deep-seated basement structures and proximal to a number of strike-slip faults (see Shawe,

1965; Blakely, 1988; Putnam and Henriques, 1991). Mineral potential “highs” in the eastern part of the region

occur along possible deep-seated basement and/or strike-slip structures and in areas proximal to the intersection

of these features. The areas of elevated mineral potential in the east are also marginal to the Nevada paleothermal

anomaly. It is also interesting to note that the proposed southeastern extension of the northern Nevada rift zone,

as interpreted from the isostatic gravity anomaly (see section 6.6.3), roughly bisects this target region, the two main

areas of high mineral potential occurring to either side of the inferred rift structure (also see Figs. 6.45, 6.53, and

6.54). The crust in this region is characterized by moderate to low present-day heat flow, presumed thinning from

east to west (~32-34 km to < 30 km; see Allenby and Schnetzler, 1983; Allmendinger et al., 1987; Wernicke et al.,

1996, and sections 2.7.2 and 2.7.5), and by variable relative fault density (although the relative density of small

linear features interpreted from LANDSAT imagery is moderate to high in favorable areas). Such structural

conditions should be conducive to ore formation provided that a heat source(s) was available to drive hydrothermal

circulation. The most permissive lithologic assemblage in this region is “Breccia and Tuffs–Late CZ”, which has

a relatively high weight of spatial association (W+

) with the volcanic rock-hosted occurrences (Fig. 7.27c; also see

Fig. 6.15). A number of deposits are also hosted by the “Felsic-Intermediate Extrusive & Intrusive–Late CZ”

lithologic assemblage, which has the highest weight of spatial association (W+

) with the volcanic rock-hosted

occurrences. Possible sites for follow-up exploration include: (1) the whole of the Cactus Range, and east of the

southern end of the Cactus Range in the vicinity of Gold Mountain; (2) immediately in the vicinity of Black

Mountain on Pahute Mesa; (3) the Kawich Range, south of Kawich Peak to Quartzite Mountain; (4) Shoshone

Mountain, immediately northwest-west-southwest of Shoshone Peak; (5) the North Pahroc Range, from Pahroc

Summit Pass north to Silver King Mountain; (6) the central Delamar Mountains, just east and southeast of Delamar

(Ferguson) district area; (7) northeast of the Cedar Range, along a traverse extending to the Eagle Valley district

area; and (8) the eastern Clover Mountains, west and northwest of the Vigo district area.

Other Regions—In addition to the targets noted above, a number of other promising exploration sites in the

Pershing, northern Nye, and western Nye counties regions, which are not reviewed in detail here, include: (1) the

whole of the Seven Troughs Range and Trinity Range (Pershing county); (2) the southern Eugene Mountains

(Pershing county); (3) the northern East Range, particularly around Dun Glen Peak (Pershing county); (4) the

southern East Range–northern Stillwater Range, between Logan Peak and Granite Mountain (Pershing county);

(5) the southern Tobin Range, just south of Mount Tobin (Pershing county); (6) the central Monitor Range, south

of Savory Mountain (northern Nye county); (7) the Antelope Range, south of Nine Mile Peak (northern Nye

county); (8) Moody Mountain, around Moody Peak, and southeast of Moody Peak to the northern Pancake Range

(northern Nye county); (9) the whole of the San Antonio Mountains (western Nye county); and (10) the northern

Goldfield Hills, in the vicinity of Blackcap Mountain. The geologic characteristics of these sites vary from area

to area, but they are generally consistent with those described in the target regions above.
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Chapter 8. Controls on Sedimentary and Volcanic

Rock-Hosted Occurrence Distribution 

8.1 Introduction

Chapter 8 is a discussion and summary of the geologic and metallogenic interpretation of the

mineral potential maps. It brings together the lithologic, structural, and tectonomagmatic mineral

potential evidence observed in the target regions reviewed section 7.5 and the spatial

relationships and interpretations presented in chapter 6. 

The results of mineral potential modelling suggest that there are two distinct and separate sets

of crustal structures that control the first-order regional-scale distributions of the sedimentary and

volcanic rock-hosted occurrences in the Great Basin:

Sedimentary rock-hosted (Carlin-type) mineralization in northern Nevada appears to be constrained by two,

well-defined, sub-parallel, northwest–southeast-trending structural zones, here termed the “Carlin” and

“Cortez” structural zones (Fig. 8.1a, red lines).

Volcanic rock-hosted (epithermal) mineralization in northern, western, and southwestern Nevada appears to

be constrained by two broad and diffuse structural zones: the Walker Lane shear zone in southwestern Nevada,

and a wide belt of possible transcurrent movement that trends northeast–southwest across northern Nevada,

here termed the “Humboldt shear(?) zone” (Fig. 8.2a, red transparency).

The presence of these structural zones is in large part suggested by patterns of elevated mineral

potential delineated in the sedimentary and volcanic rock-hosted occurrence posterior probability

maps (Figs. 7.4, 7.5, 7.13, and 7.14). 

Important mineral potential evidence within the regional-scale exploration target areas is

summarized in section 8.2. This evidence helped to recognize and delineate the Humboldt

shear(?) zone and the Carlin and Cortez structural zones, which are reviewed section 8.3. The

remainder of this chapter discusses geologic evidence for the existence, character, and possible

origins of these features (sections 8.4 and 8.5).

8.2 Mineral Potential Evidence Associated with the GoldSilver-Bearing Occurrences

8.2.1 Sedimentary-Rock Hosted Occurrences

In the sedimentary rock-hosted occurrence regional-scale exploration targets (section 7.5.2) the

areas of elevated mineral potential are most commonly associated with the “Marine and

Shelf–Early-Mid PZ”, “Siliceous Rocks–Mid PZ-PC”, and “Marine and Shelf–Late PZ”

lithologic assemblages (see Fig. 6.12). Within these assemblages, the lithologic units hosting

most of the sedimentary rock-hosted occurrences include Os, St, Dc, and MDs (see Fig. 6.13;

note that most of the occurrences apparently “hosted” by the Quaternary cover unit are possibly

hosted by units Os, Dc, or MDs, which lie range-ward of the occurrences). The units Os, St, Dc,
108

and MDs are variably composed of shale (Os, MDs), limestone and dolostone (Os, St, Dc, MDs),

silty-limestones–limy-siltstones (St), siltstone (MDs, St), sandstone (Dc, MDs), chert (Os, St),

and minor amounts of other detrital and metamorphosed rocks (Os, Dc, MDs) (see Appendix A).

These units in part constitute the “Siliceous (Western) Assemblage” (Os), the “Transitional

Assemblage” (St), the “Carbonate (Eastern) Assemblage” (Dc), and the “Carbonate-Detrital Belt

Along Eastern Margin of Antler Orogenic Belt or Western Part of Foreland Basin” (MDs), as

classified by Stewart and Carlson (1978) (see Appendix A). The targets areas are also proximal

to, or related to the intersection of, a number of regional-scale structures, which include the

87Sr/86Sr ISr = 0.706 isopleth, presumed to represent the western edge of the Precambrian craton

(see Figs. 2.10 and 2.11), the northern Nevada rift zone (see sections 2.7.4 and 6.6.3; Fig. 6.45a,

feature #1), the Roberts Mountain and Golconda thrust fronts (Fig. 2.5), and possible deep-seated

basement structures interpreted from aeromagnetic data. The nature of the crust in the

sedimentary rock-hosted exploration target regions is variable, but in the vicinity of Battle

Mountain–Eureka (Cortez) and Carlin mineral trends, the present-day crust is characterized by:

Thicknesses of greater than 30 km, particularly to the east and northeast of the mineral trends (Fig. 6.5d).

Moderate to low relative fault density (Fig. 6.37c), although LANDSAT linear feature density is high to

moderate. and lithologic diversity is moderate to high (Fig. 6.34).

General lack of widespread magmatism in relation to surrounding regions (Figs. 6.4 and 6.56).

Variable heat flow (areas of moderate to high heat flow are proximal to the mineral trends; Fig. 6.5g). Heat

flow may have been higher in the Paleozoic or Mesozoic as evidenced by a paleothermal anomaly (Fig. 2.10).

The prominent association of elevated sedimentary rock-hosted mineral potential areas with the

lithologic and structural features indicated above suggests that, collectively, they probably

exercised strong controls on sedimentary rock-hosted precious metal mineralization in the Great

Basin. The lithologic associations suggest that certain sedimentary assemblages (1) could have

acted as potential source rocks for gold and/or (2) were more suitable host rocks for ore

deposition (see sections 3.5, 3.6, 6.3.3, and 6.4.3 for more detailed discussions). The structural

associations indicate the importance of local- and regional-scale deformational features to

mineralization. Such structures most likely facilitated mineralization by preparing potential host

and/or source rocks, concentrating and directing hydrothermal gold-bearing solutions, and

increasing permeability, thereby promoting near-surface and deep fluid circulation, vertical and

lateral heat transfer, as well as enhancing fluid-wall rock interactions (see discussion in sections

2.7.6, 2.7.7, 3.4, 3.6, 6.4.3, and 6.5.3).

8.2.2 Volcanic Rock-Hosted Occurrences

In the volcanic rock-hosted occurrence regional-scale exploration targets (section 7.5.3) the areas

of elevated volcanic rock-hosted precious metal mineral potential are most commonly associated

with the “Felsic-Intermediate Extrusive & Intrusive–Late CZ” and “Breccia and Tuffs–Late CZ”

lithologic assemblages (see Fig. 6.15). Within these assemblages, the lithologic units hosting

most of the volcanic rock-hosted occurrences include Tt2, Ta3, Tr3, Ta2, and Tba (see Fig. 6.16).

These units are primarily andesite (Ta3, Tba), silicic ash-flow tuffs (Tt2), rhyolite (Tr3, Ta2), and

other intermediate rocks, with varying amounts of basalt, and range in age from 34-6 Ma and

younger for some basalt units (see Appendix A). These rocks are generally distributed in semicircular fashion along the northern–western–southwestern borders of Nevada, and compose in

part the 34-17 Ma and 17-6 Ma age units of Stewart and Carlson's (1976) time-slice maps (see
109

Figs. 6.4, 6.24, and 6.25; section 6.3.3). As observed by Ludington et al. (1993), and shown in

Figures 3.1 and 6.2, the volcanic rock-hosted occurrences are distributed in a crescent-shape

pattern that coincides with the semi-circular distribution of these units. In particular, they have

a high degree of spatial correlation with igneous rocks comprising the 17-6 Ma time-slice (see

section 6.3.3; compare Fig. 6.2 to 6.4 and 6.24). Igneous rock composition-slice maps derived

from the geological map of Nevada (Stewart and Carlson, 1978) reveal that nearly all units of

mafic to intermediate composition are also distributed in a semi-circular manner (see Fig. 6.55),

which is coincident in space and time with the 34-17–17-6 Ma igneous rock time-slice ages and

with the volcanic rock-hosted occurrence distribution pattern (see Figs. 6.4 and 6.25). The

targets areas are also proximal to, aligned along, or located at the intersection of possible deepseated basement structures, strike-slip faults, thrust fronts, and crustal-scale features, including

the 87Sr/86Sr ISr = 0.706 isopleth (presumed to represent the western limit of the North American

Precambrian craton; see Figs. 2.10 and 2.11) and in some instances, the northern Nevada rift

zone or the Nevada paleothermal anomaly (see sections 2.7.4 and 6.6.3; Figs. 2.10 and 6.45a,

feature #1). The nature of the crust in the volcanic rock-hosted exploration target regions is

variable, but in regions known to host deposits of significant number and size, the present-day

crust is characterized by:

& Thicknesses of less than 30 km (Fig. 6.5d).

& Moderate to high relative fault density and/or LANDSAT linear feature density (Fig. 6.37c).

& High relative density of intrusives (primarily Mesozoic in age; Fig. 6.36c).

Moderate to high present-day heat flow (Fig. 6.5g).

The close spatial association of elevated mineral potential areas with the lithotectonic and crustal

features described above indicates that they probably exercised a great deal of control over the

distribution and development of volcanic rock-hosted precious metal mineralization in Nevada.

The strong spatial and temporal coincidence between the occurrences (formed mostly between

27 and 5 Ma; Ludington et al., 1993) and the volcanic units composing the 34-17–17-6 Ma timeslice age units indicates that mineralization was closely tied to the migration of the inception of

magmatism (and extension) across the Nevada Great Basin (see sections 2.5.1.1, 2.5.2.1, and

6.3.3). This coincidence suggests that the 34-17–17-6 Ma time-slice volcanic rocks could have

provided heat and contributed gold to volcanic rock-hosted mineral systems (either as source

rock to be scavenged or by the input of magmatic ore-bearing fluids into the mineralizing

hydrothermal system; see discussions in sections 3.5, 3.6, 6.4.3, and 6.5.3). There is also spatial

correlation between the volcanic rock-hosted occurrence distribution pattern and: (1) the

intermediate to mafic composition-slice units (see Fig. 6.55); (2) the Walker Lane, which may

be intruded by mafic and intermediate rocks at depth, as indicated by a high density of positive

geomagnetic anomalies along the Walker Lane (see Fig. 6.45a, feature #2); and (3) the Humboldt

shear(?) zone, which may also be intruded by mafic and intermediate rocks at depth, as indicated

by a broad belt of high density of positive geomagnetic anomalies north and northwest of the

zone (as defined here), as well as at least three prominent cross-cutting anomalies (northern

Nevada rift and subordinate rifts to the west) (see Fig. 6.53b and 6.45a, feature #5). These

intrusive rocks, which in the Walker Lane are nowhere exposed at the surface, could presumably

have contributed gold as well. The structural and crustal features associated with volcanic rockhosted mineralization could have facilitated the movement of mineralizing fluids and heat, as

stated in the preceding section on sedimentary rock-hosted deposits, and as summarized in

sections 3.4, 3.6, 6.4.3, and 6.5.3.
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8.3 Delineation of the Regional-Scale Control Structures

8.3.1 Sedimentary Rock-Hosted Mineralization

The regional-scale alignment of sedimentary rock-hosted occurrences along the Carlin and Battle

Mountain–Eureka (Cortez) mineral trends strongly suggests the presence of a sub-parallel pair

of northwest–southeast-trending crustal-scale structural zones (or segments of zones). Two

linear features, here termed the “Carlin” and “Cortez” structural zones (Fig. 8.1a, red lines), have

been delineated on the basis of:

& The distribution trend of elevated sedimentary rock-hosted mineral potential (Figs. 7.4 and 7.13).

& The linear distribution of known occurrences, including volcanic rock-hosted occurrences, along two welldefined mineral trends (Figs. 1.3, 3.3, 6.2, 6.65, 7.13b, 8.1b).

& The linear arrangement of structural elements visible in the shaded relief of topography (Figs. 1.2, 6.6, and 8.1;

also see Thelin and Pike, 1991).

& The distribution pattern of isostatic and Bouguer anomalies and geomagnetic anomalies (Figs. 6.45, feature

#1; 6.53, and 6.54).

& The distinctive “V”-shaped distribution trend of K/Na and Ba/Na geochemical anomalies, although any genetic

connection of these anomalies with the Carlin and Cortez structural zones is unclear (see discussion section

6.7.3; Figs. 6.57 and 6.65).

The Carlin and Cortez structural zones may have acted as fundamental controls over the

regional-scale distribution of sedimentary rock-hosted mineralization in northern Nevada. Along

these zones, ore-bearing fluids could have been focused and circulated, forming particularly

significant deposits at intersections with other structural zones where favorable host-rocks and/or

the appropriate physio-chemical conditions were present (Fig. 8.1). The extents of the Carlin and

Cortez structural zones, and the other intersecting linear features depicted in Figure 8.1 (blue

lines), are exaggerated in order to highlight structural and mineral trends (i.e.—presented for the

purposes of illustration). Assuming that these structural zones and linear features are real, they

most likely have a complex history of movement involving more intricate crosscutting

relationships than the simple non-offset linear intersections illustrated here. 

Sedimentary rock-hosted mineralization is considered to be 42-30 Ma in age (Hofstra, 1997).

The Carlin and Cortez structural zones are thought to be mid-Mesozoic or older in age, perhaps

dating back to late Proterozoic rifting along the western edge of the North American craton

(Grauch, 1995). The most significant recent geologic activity related to these structural zones

could be the formation of the northern Nevada rift zone in the mid-Miocene. These topics are

further discussed in section 8.5.

8.3.2 Volcanic Rock-Hosted Mineralization

The “crescent-shaped” pattern of volcanic rock-hosted occurrences in Nevada suggests that two

broad, crustal-scale, structural zones control the regional-scale distribution of epithermal

mineralization in northern and southwestern Nevada. The northwest–southeast-trending Walker

Lane shear zone in southwestern Nevada, and a belt trending northeast-southwest in northern

Nevada, here termed the “Humboldt shear(?) zone”, have been delineated on the basis of:

& The distribution trend of elevated volcanic rock-hosted mineral potential (Figs. 7.5 and 7.14).
111

& The distribution of known occurrences along two broad and diffuse belts across northern and southwestern

Nevada (Figs. 3.1, 6.2, 7.14b, and 8.2b).

& The linear arrangements of structural elements visible in the shaded relief of topography (Figs. 1.2, 6.6, 8.1,

and 8.2; also see Thelin and Pike, 1991; used particularly to identify the Humboldt shear(?) zone).

& The broad distributions of 6-17 Ma volcanic rocks (Figs. 6.4 and 6.25).

& The distribution of lithologic units having elevated posterior probabilities for volcanic rock-hosted occurrences

(Fig. 6.24b).

& The general distribution of warmer geothermal well and spring waters (Fig. 6.5h; used particularly to identify

the Humboldt shear(?) zone).

& The general distribution of areas of higher heat flow (Fig. 6.5g; used particularly to identify the Humboldt

shear(?) zone).

& The distribution trend of relative density of plutons (Fig. 6.36c; used particularly to identify the Humboldt

shear(?) zone).

& The distribution of higher isostatic and Bouguer gravity anomalies (Figs. 6.45b and 6.45c, feature #5, and

6.54a; used particularly to identify the Humboldt shear(?) zone).

& A moderately defined, bilaterally symmetric, northeast-southwest-trending corridor of geomagnetic lows,

juxtaposed along length by highs (Fig. 6.45a, feature #5; 6.53b).

The Walker Lane shear zone and the Humboldt shear(?) zone may have acted as fundamental

controls over the regional-scale distribution of the volcanic rock-hosted mineralization in

northern, western, and southwestern Nevada (Fig. 8.2; also see Fig. 6.6). The southward sweep

of Eocene-Oligocene magmatism across the Nevada Great Basin could have exploited these

zones, where heat and mineralizing processes were focused and ore deposits formed.

Furthermore, it is suggested that Cenozoic activity along the Humboldt shear(?) zone may have

had an incidental affect on the post-mineralization distribution sedimentary rock-hosted deposits

along the northern termini of the Battle Mountain–Eureka (Cortez) and Carlin trends (as

discussed below in section 8.5.4).

The majority of volcanic rock-hosted deposits are considered to have formed between 27 and 5

Ma (Ludington et al., 1993; also see Fig. 3.2). The Walker Lane is thought to be at least

Mesozoic in age (Stewart, 1988; Saleeby and Busby-Spera, 1992), or perhaps Precambrian

(Blakely and Jachens, 1991), with recent activity documented in the Oligocene or Miocene

(Dilles and Gans, 1995) and ongoing today. The Humboldt shear(?) zone is considered to be at

least mid-Paleozoic in age (Poole et al., 1992), with possible origins dating back to the midProterozoic assembly of the Laurentian protocraton and/or late Proterozoic rifting (see section

2.4). Latest activity along the Humboldt shear(?) zone is believed to be between mid-Oligocene

and mid-Miocene time. The Humboldt shear(?) zone is further discussed in section 8.4.

The Walker Lane shear zone has been the focus of much research (for a review, see sections 2.6

and 6.6.3). Correlations between the Walker Lane and the distribution pattern of epithermal

volcanic rock-hosted deposits and the Walker Lane have been proposed by Cox et al. (1991),

where they stated “...northwest-trending Walker Lane-style deformation began as early as 27 Ma

and provided the tectonic conditions that permitted the formation of volcanic-hosted deposits.”

Furthermore, the Walker Lane forms the southwestern portion of Ludington et al. (1993)’s

“epithermal crescent” (see Fig. 6.2). It seems well established that the Walker Lane shear zone

was an important control over the regional-scale distribution of volcanic rock-hosted occurrences

in western and southwestern Nevada. This is further verified by the observations and

correlations made in this study in previous sections (6.2, 6.3.3, 6.4.3, 6.5.3, and 6.6.3) and will

therefore not be further expanded upon here.
112

8.4 The Humboldt Shear(?) Zone

8.4.1 Geologic and Other Evidence

The modelling exercises conducted here suggest that the Humboldt shear(?) zone was potentially

as important to volcanic rock-hosted mineralization in the Great Basin as was the Walker Lane

shear zone. Control over the regional-scale distribution of volcanic rock-hosted occurrences by

the Humboldt shear(?) zone has not been discussed in the literature, other than a few references

made to possible northeast–southwest-oriented strike-slip movement across northern Nevada.

The Humboldt shear(?) zone is a controversial feature, and the few comments found in the

literature suggest that it may possibly be related to another debatable structure, the northeastsouthwest-trending Snake River fault in southern Idaho. The Snake River fault may be part of

a broader zone of transcurrent movement, which appears to show latest activity in the midMiocene, but could have origins dating back to the mid-Paleozoic or later (see Poole et al., 1977;

Poole and Sandberg, 1977; Christiansen and Yeats, 1992, p. 362; Poole et al., 1992, figure 1, text

p. 16). Such an interpretation is based on the premise that the northern Nevada rift, the western

Snake River graben structure, and the Chief Joseph dike swarm (see Fig. 2.3) are tectonically

related (Taubeneck, 1970; also see Zoback and Thompson, 1978), and once formed a continuous

700-plus km long rift zone. A 70 to 100 km right-lateral offset between the northern terminus

of the northern Nevada rift and western Snake River graben now exists, and a southward

extrapolation along the Humboldt shear(?) zone of the N55o

E Snake River Plain trend is an

appropriate orientation for a transcurrent structure (Christiansen and Yeats, 1992). In addition,

Mabey et al. (1983, p. 314) observed that, in northwest Nevada, the northeast structural grain of

the Humboldt gravity zone influenced the trend of the faults, which are generally northeasterly

rather than normal to the direction of extension, and thus involve considerable strike-slip

movement. Transcurrent movement across northern Nevada would be in accord with a

northeastern shift in trend or offset seen in a number of generally north-trending geologic

features, including (1) the 87Sr/86Sr ISr = 0.706 isopleth, (2) the Wasatch or hinge line, (3) the

eugeocline-miogeocline transitional facies, (4) the Roberts Mountain, Golconda, and Sevier

thrust fronts, and (5) northeast bend in the trend of the Cordilleran Mesozoic batholith belt (see

Figs. 2.3, 2.5, 2.9, 2.10, 2.11, and 6.23a). In addition, Smith and Luedke (1984) delineated a

northeast-southwest trend of 16-10 Ma volcanic loci that may define a broad structure extending

from southern Idaho across northern Nevada (see Smith and Luedke’s figure 4.4, and comments

on p. 58). Most recently, Wooden et al. (1997) have presented 208Pb/204Pb isotopic data that

suggest the existence of a major crustal fault or suture north of the Carlin trend (see figure 1 in

Wooden et al., 1997). They describe two isopleths (208Pb/204Pb>39 and 208Pb/204Pb<38.8) that are

southwest of, and parallel to, the Battle Mountain–Eureka (Cortez) trend. At the northern

termination of the mineral trend, the isopleths swing acutely to the northeast, which may reflect

the presence of the Humboldt shear(?) zone.

8.4.2 Topographic Expression

The Humboldt shear(?) zone is most readily recognized as a broad and diffuse Bouguer and

isostatic gravity high zone that stretches northeast-southwest across northern Nevada (Fig. 6.45b,

feature #5, and 6.54a). Of the many datasets compiled and examined for the study, the Humboldt
113

shear(?) zone is best resolved in shaded relief of topography (based on ~800 meter pixel

resolution) (see Figs. 1.2, 6.6, and 8.2; also see Thelin and Pike, 1991). The shaded relief of

topography reveals a number of unique structural features and some subtle, but clearly visible,

differences in the tectonic grain of the crust between the Humboldt shear(?) zone, the Walker

Lane shear zone, and the central–east-central interior of Nevada (these differences are primarily

visible northwest of a line that extends from the northeastern corner of the state to Reno, and

represents the approximate southeastern limit of the Humboldt shear(?) zone). Some of these

features and differences include: (1) the gross orientation of mountain ranges changes between

central and northwestern Nevada, from northeast-trending to north-trending in a relatively abrupt

manner as the southeastern limit of the Humboldt shear(?) zone is crossed (as delineated in Fig.

8.2a); (2) on closer inspection, some ranges display domino-style east-west lateral offsets

(e.g.—the Independence Mountains, the Osgood Mountains, the Selenite Range, among others);

(3) several continuous and segmented linear features traverse northeast-southwest across Nevada,

the most prominent of which are seen to cut the topographic expression of the northern Nevada

rift zone; and (4) the northern and/or southern ends of a few ranges appear to have a decidedly

“sheared” aspect to their morphology (the West Humboldt Range is the most prominent

example). All of the features pointed out above either trend roughly northeast-southwest, or are

indicative of movement along this direction.

An additional point of interest is the broad and open semi-continuous “S”-shaped curvilinear

alignment of ranges visible in the shaded relief (Figs. 1.2 and 8.2). The northern terminus of this

feature, which arcs to the northeast, is presumed to mark the southeastern limit of the Humboldt

shear(?) zone (as expressed in basin-range morphology), while the southern terminus, which arcs

to the southwest, could indicate the approximate northeastern limit of the Walker Lane shear

zone (Fig. 8.2a). This feature is further discussed in connection with the Carlin and Cortez

structural zones in section 8.5.3 below.

8.4.3 Origin, Age, and Character of the Humboldt Shear(?) Zone

In addition to the features delineated in shaded relief of topography, a variety of other compelling

geological observations and correlations suggest that a broad northeast–southwest-trending zone

of transcurrent movement, with possible ancestral heritage, extends across northern Nevada. The

following material, presented in point form, further details, characterizes, and demonstrates the

existence of the proposed Humboldt shear(?) zone:

Points that generally contend for the origin, orientation, and age of the Humboldt shear(?)

zone include:

Northern Nevada is characterized by an isostatic residual gravity anomaly high that stretches northeastsouthwest across the region, and by a Bouguer gravity anomaly high located in the northwestern part of the

State that appears to be elongated along a northeast–southwest-orientated axis (see Figs. 6.45b and 6.45c,

feature #5, and 6.54a; sections 2.7.3 and 6.6.3). Referred to as the Humboldt gravity anomaly or zone (see

Mabey et al., 1983), the anomaly is believed to roughly delineate the limits of the Humboldt shear(?) zone.

At residual Bouguer gravity wavelengths of less than 250 km, the Great Basin is characterized by

predominantly northwest trending high-low-alternating anomalies, which extend about the length of the State

of Nevada (Kane and Godson, 1989, see plate in back pocket). These anomalies are truncated and severely

disrupted by the Humboldt gravity anomaly in northern Nevada, forming a crustal gravity zone boundary with
114

anomalies of different character to the north. The boundary extends northeast-southwest from the

southeastern Idaho–central Wyoming border, across northern Nevada, to the Mendocino triple junction, and

may be interpreted as the juxtaposition of major crustal blocks of differing densities, along which a crustal

flaw zone or suture zone might exist.

& The Walker Lane is magnetically characterized by a pattern of arcuate anomalies that trend northwestward,

generally parallel to the shear zone (Blakely and Jachens, 1991; see section 6.6.3; Figs. 6.45a, feature #2,

and 6.53b). This pattern of anomalies is disrupted by a diffuse, northeast–southwest-trending, total residual

field geomagnetic low, forming a crustal geomagnetic zone boundary, northwest and north of which

geomagnetic highs once again resume (Kane and Godson, 1989, figure 2). The boundary, as delineated by

Kane and Godson (1989), parallels the southern limit of a group of magnetic highs that extend

southwestward along the southern Snake River Plain, into northern Nevada, and across California to the

Mendocino triple junction. This magnetic zone boundary, as well as the gravity zone boundary mentioned

above, may be the expression of the primary structural framework of the continental crust (Kane and

Godson, 1989), and as such, could represent a crustal flaw-zone or suture zone.

Neodymium isotopic data that delineate a Precambrian crustal province boundary, which separates the >2.7

Ga Wyoming province from 2.3-2.0 Ga crust, appears to coincide with the Humboldt shear(?) zone (see Fig.

2.11;section 2.4.2). During the Central Plains and Yavapai-Mazatzal orogenies, between 1.80 and 1.65 Ga,

juvenile crust was accreted to the Laurentian protocraton, and if the Humboldt shear(?) zone is related to this

suture, then its ultimate age and origin are Middle Proterozoic. Note also that the northeast-southwest trend

of the Humboldt shear(?) zone parallels the trend of the 87Sr/86Sr ISr = 0.706 isopleth (presumed to represent

the western margin of the North American Precambrian craton) in northern Nevada—either the Humboldt

shear(?) zone controls the trace of the 87Sr/86Sr ISr = 0.706 isopleth (has offset it), or the western margin of

the Precambrian craton controls the location of the shear zone. Whichever the case, an argument can be made

that, if a transcurrent structure is present, it might trend northeast-southwest parallel to the 87Sr/86Sr ISr = 0.706

isopleth in northeastern Nevada.

The southwestern segment of the Humboldt shear(?) zone may coincide with a region of crustal thinning (~20

km) in northwestern Nevada (Fig. 6.5d; also see Allenby and Schnetzler, 1983, figure 2, Braile et al., 1989,

figure 3, and Pakiser, 1989, figure 2). In addition, recent work by Wernicke et al. (1996) suggested that the

thin crust of the western Great Basin may extend under the southern Sierra Nevada region. Should this be

the case for the northern Sierra Nevada as well, then the zone of thin crust observed in northwestern Nevada

would extend farther southwestward along an axis parallel to the Humboldt shear(?) zone. COCORP studies

support limited Moho relief in the east-west direction in this area, but do not suggest that the northern Sierra

Nevada is much less than 35 km thick (Allmendinger et al., 1987; Knuepfer et al., 1987). There are no data

to suggest that the zone of crustal thinning extends northeast under northern Nevada into southern Idaho and

western Wyoming, although a ridge in the reflection-surface of the Moho does trend northeast–southwest

from northeastern Nevada to east-central Idaho (see Allenby and Schnetzler, 1983, figure 2; Braile et al.,

1989, figure 3).

Northern Nevada is thermally characterized by a northeast–southwest-trending zone of anomalously high

present-day heat flow, which extends from the region southwest of the Battle Mountain heat high in

northwestern Nevada, across northeastern-Nevada–southeastern-Idaho along the Snake River Plain, to the

Yellowstone hot spot (see Fig. 6.5g; also see Blakely, 1988, figure 2; Eaton et al., 1978, figure 3-12;

Lachenbruch and Sass, 1978, figure 9-1; Morgan and Gosnold, 1989, figure 12). Present-day anomalous

thermal activity along the northeast–southwest extent of the Humboldt shear(?) zone is also expressed by

elevated water temperatures in hot springs and geothermal wells (Fig. 6.5h). It is presumed that thermal

activity would be preferentially focused in regions of thinner crust and/or along crustal segments of higher

permeability, such as zones of induced permeability that would result from fault or shear activity.

In addition to the alignment of volcanic rock-hosted occurrences along the Humboldt shear(?) zone (Figs.

6.2, 6.3, 7.14b, and 8.2b), a map of the relative density of Mesozoic age plutons (see Fig. 6.36c) shows a

broad zone of high pluton density extending northeast-southwest across the northern part of the state.

Presumably, both mineralizing fluids and magmas would have taken advantage of the greater permeability
115

associated with a preexisting structure (i.e.—path of least resistance). Should this be the case, than the

Humboldt shear(?) zone could be at least Mesozoic in age.

Smith and Luedke (1984) delineated a northeast-southwest trend of 16-10 Ma volcanic loci (the Snake River

and Humboldt River zones) extending from southern Idaho across northern Nevada (see Smith and Luedke’s

figure 4.4, and comments on p. 58). The trend of these loci may define a broad zone of crustal weakness,

and as stated in the two previous points, it is presumed that heat flow and magmatism would have exploited

a preexisting structure such as the Humboldt shear(?) zone.

Points that generally contend for the transcurrent movement, orientation, and age of the

Humboldt shear(?) zone include:

The Bouguer gravity anomaly in central–east-central Nevada is characterized by gross bilateral symmetry,

the axis of bilateral symmetry trending northwest-southeast and being generally coincident with the northern

Nevada rift zone (see Fig. 6.45c, feature #1; sections 2.7.3 and 6.6.3; Eaton et al., 1978, figure 3-8). In

northern Nevada, the axis of symmetry disappears, but Eaton et al. (1978) proposed that the axis has been

offset eastward and may extend into central Idaho (see Eaton et al., 1978, figure 3-11). The implication is

that offset of the axis to the east was achieved along a broad structure characterized by lateral movement.

It is also interesting to note that, in Eaton et al.’s (1978) figure 3-11, the dotted line trending

northeast–southwest that offsets the axis in central Nevada is coincident to the lower boundary of the

Humboldt shear(?) zone, which has been delineated independently in this study from shaded relief of

topography (Figs. 8.2a).

The Humboldt shear(?) zone is parallel to the northeast-southwest-trending inferred Snake River fault in

southern Idaho, which is suspected to be a strike-slip fault (Poole et al., 1977; Poole and Sandberg, 1977;

Poole et al., 1992, figure 1). Poole et al. (1992, p. 16) noted that evidence for the Snake River fault can be

demonstrated by offset of the Sevier thrust system across the Snake River Plain (see Figs. 2.4 and 2.5, and

Poole et al., 1992, figure 1), and evidence for both left- and right-lateral movements can be demonstrated by

significantly different amounts of offset of thickness trends of Devonian and Mississippian rocks (Poole et

al., 1977, have proposed 200 km of Paleozoic left-lateral offset on a fault along the trend of the Snake River

plain). The Snake River fault is more than likely to be related to, or is an element of, the Humboldt shear(?)

zone, and assuming this to be true, the evidence provided by Poole et al. (1992) indicates that the Humboldt

shear(?) zone could be at least mid-Paleozoic in age.

The presence of a transcurrent feature extending along the trend of the Humboldt shear(?) zone can be

inferred based upon the premise that the northern Nevada rift, the western Snake River graben structure, and

the Chief Joseph dike swarm (see Fig. 2.3) are tectonically related, and once formed a continuous 700-plus

km long rift zone (Taubeneck, 1970; also see Zoback and Thompson, 1978). A 70 to 100 km right-lateral

offset between the northern terminus of the northern Nevada rift and western Snake River graben now exists,

and a southward extrapolation along the Humboldt shear(?) zone of the N55o

E Snake River Plain trend is an

appropriate orientation for a transcurrent structure (see Christiansen and Yeats, 1992, p. 362-363). Assuming

this premise is correct, and that the lateral displacement between these features is the result of transcurrent

movement along the Humboldt shear(?) zone, then the time of latest movement along the shear zone would

be at least mid-Miocene.

Several generally north-trending crustal-scale structures and features of varying ages and types show an

eastward deflection, right-lateral offset, or discontinuity across the Humboldt shear(?) zone, which would be

consistent with transcurrent movement along a northeast-southwest trend, and include: (1) the 87Sr/86Sr ISr

= 0.706 isopleth, presumed to represent the western margin of the North American Precambrian craton (see

Figs. 2.3, 2.10, 2.11); (2) the Wasatch or “hinge” line (see Fig. 2.3); (3) the Cordilleran Mesozoic batholith

belt (see Fig. 2.3); (4) the Roberts Mountain, Golconda, Luning-Fencemaker(?), and Sevier thrust fronts (see

Figs. 2.4 and 2.5; also see Poole et al., 1992, p. 16); (5) the Nevada paleothermal anomaly (see Fig. 2.10);

and (6) the northern Nevada rift zone and its postulated northern extension (either the Brothers fault zone or

the Snake River graben structure; see Fig. 2.3, and section 2.6; also see Christiansen and Yeats, 1992, p. 362-

363, 367).
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The material presented above argues strongly in favor of the existence of Humboldt shear(?)

zone, and that it may have exercised a fundamental control over the distribution and formation

of volcanic rock-hosted occurrences. The Humboldt shear(?) zone may also have had an

incidental affect on the distribution of the sedimentary rock-hosted occurrences. Right-lateral,

post-mineralization transcurrent movement along the Humboldt shear(?) zone may have

displaced the Getchell trend and Independence group deposits northeastward of the northern

termini of the Battle Mountain-Eureka (Cortez) and Carlin mineral trends (respectively). This

is suggested by a bend to the northeast of As and U/Th geochemical anomalies, believed to have

some spatial association with the mineral trends, and by eastward lateral offsets in Basin-Range

structure, that are visible in shaded relief of topography. This proposal is investigated in greater

detail below in section 8.5.4, where it is suggested that the Getchell trend represents the

northeastward-sheared extension of the Battle Mountain–Eureka (Cortez) trend and that the

Independence group deposits (trend) represent the northeastward-sheared extension of the Carlin

trend.

8.5 The Carlin and Cortez Structural Zones

8.5.1 Geophysical and Geochemical Evidence

The isostatic gravity and geomagnetic anomalies examined in section 6.6 provide some support

for the existence of the Carlin and Cortez structural zones. The K/Na and Ba/Na geochemical

anomalies show strong spatial association with the sedimentary rock-hosted occurrences and

clearly delineate the Battle Mountain–Eureka (Cortez) and Carlin mineral trends, but it is unclear

whether or not the anomalies have any genetic association with mineralization (see section

6.7.3). The geochemical anomalies, however, may provide some additional insight by

augmenting the interpretations made from the geophysical data. 

In section 6.6 (“Geophysics–Geomagnetic and Gravity Anomalies”), it was shown that the

frequency distribution of gold-silver-bearing occurrences with respect to the geophysical

anomalies is approximately normal, with the distribution modes located in the ranges that are

transitional between the high-end and low-end of the magnetic and gravity anomaly intensity

spectrums (mode lies slightly negative of the zero value, see Figs. 6.46, 6.47, 6.49, and 6.50).

The near-zero-centered distribution modes for the occurrences indicate that the majority are

located in regions that are transitional between rocks that are more highly and less highly

magnetized, and transitional between rocks of higher and lower density. In general, anomaly

gradient contrasts are thought to indicative of crustal structures, such as terrane boundaries,

structural trends, and fault and shear zones. Such structures could facilitate the convection of

hydrothermal fluids at shallow crustal levels and the vertical movement of heat and deep-seated

fluids from lower crustal levels (see sections 3.4, 3.6, 6.4.3, and 6.5.3 for additional details).

Kutina and Hildenbrand (1987) reported that some of the principal endogenic ore deposits of the

western United States cluster in or adjacent to regions of gravity lows (e.g.—the majority of

deposits of the Colorado mineral belt). Gold-silver-bearing occurrences in Nevada possess

similar characteristics not only in relation to gravity anomalies but also in relation to magnetic

anomalies. Grauch et al. (1995, p. 206) found an overall statistical tendency for gold occurrences

to be spatially related to major density boundaries, which they explained as either (1) a major

change in lithology of the crust associated with a terrane boundary or strike-slip fault, or (2) as

a regional-scale structure that juxtaposes high-density basement material against low-density.
117

Examples of large and medium size sedimentary rock-hosted occurrences within transition

regions between (1) low and high magnetic anomalies include the Goldstrike mine and the

northern Carlin trend deposits, (2) low and high gravity anomalies include the Atlanta, Cortez

Gold, Gold Acres, Florida Canyon, Gold Bar deposits/mines, and the deposits of the northern

Carlin trend, and (3) low and high magnetic and gravity anomalies include the Gold Bar mine

and northern Carlin trend deposits.

Shallow-source magnetic and pre-Tertiary basement gravity anomalies in north-central Nevada,

particularly along the Battle Mountain–Eureka (Cortez) mineral trend, and the distribution of the

K/Na–Ba/Na geochemical anomaly, may reflect the Cortez and Carlin structural zones. Figure

8.3 illustrates the spatial relationship between the sedimentary rock-hosted occurrences and these

geophysical and geochemical anomalies.

Figures 8.3e and 8.3f show the trend of the K/Na anomaly (and the occurrences) relative to the

isostatic and Bouguer gravity anomaly (respectively), which in Nevada, reflects sources in the

pre-Tertiary basement as well as representing the density distribution of late Cenozoic masses

in the upper (5-10 km) crust (Mabey et al., 1983; Blakely and Jachens, 1991; Shawe, 1991, figure

3; also see section 6.6.3). The gravity anomalies show close alignment with the mineral and

geochemical trends (the “V”), expressed by a relatively sharp and well-defined changes in

gradient between regions of gravity lows and adjacent areas of moderate to high intensities

(particularly for the isostatic gravity anomaly). The gravity gradient parallelling the EurekaBattle Mountain (Cortez) arm of the “V” is believed to be the northern extension of the gravitylow bilateral axis of symmetry in Nevada, and represents the northern segment of a linear

isostatic gravitational gradient ridge that may delineate the southern extension of the northern

Nevada rift zone (Blakely, 1988; also see sections 2.7.4 and 6.6.3; Figs. 6.45b, feature #1, and

6.54). 

Figures 8.3b and 8.3c show the trend of the K/Na anomaly relative to the total residual magnetic

anomaly, and Figure 8.3d shows the K/Na anomaly highs against a shaded relief rendition of the

magnetic data. The magnetic anomaly highs depicted in the Figure 8.3 (lighter, brighter colors)

are believed to represent shallow magnetic sources (~1 km, Blakely and Jachens, 1991; also see

section 6.6.3). The northern Nevada rift is the main magnetic anomaly feature in Figure 8.3 (a

long, narrow, arcuate, magnetic high trending roughly NW-SE; see sections 2.7.4 and 6.6.3), and

has been interpreted as a mid-Miocene (~17-14 Ma) rift zone (Zoback et al., 1994; also see

Mabey et al., 1983). Two smaller rift systems can also be seen flanking the northern Nevada rift

to the west. The northern Nevada rift zone magnetic anomaly is not as closely aligned with the

mineral and geochemical trends as the gravity anomalies, and can clearly be seen to extend subparallel along the Eureka-Battle Mountain (Cortez) arm of the trends between the K/Na

anomalies (Fig. 8.3d; also see 8.1, green line), forming an alternating geochemicalanomaly–magnetic-anomaly pattern. There appears to be a conspicuous lack of Ba/Na or K/Na

geochemical anomalies or mineral occurrences (either sedimentary or volcanic rock-hosted)

along much of the length of the northern Nevada rift—neither the Battle Mountain–Eureka

(Cortez) trend nor the Carlin trend deposits, or their associated K/Na–Ba/Na geochemical

anomaly, show close spatial coincidence with the axis of the rift zone. Rather, they envelop the

rift to the west and the east. However, this may be due to sampling bias, although, given the

large amount of mineral exploration activity in northern Nevada, this seems unlikely.

Alternatively, the absence of K/Na or Ba/Na anomalies may reflect the distribution of Cenozoic
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volcanic cover in the area, where the geochemical anomalies are concentrated along the eroded

or flow-edges of the units (see section 6.7.3)

The deeper basement features imaged by the gravity anomalies show much better spatial

correlation with the Battle Mountain–Eureka (Cortez) and Carlin mineral trends and the K/Na

anomaly pattern than does the shallow feature revealed by the geomagnetic anomaly. This

correlation, as well as the relative “sharpness” and linearity and of the gravity and geochemical

anomalies (and of the geomagnetic anomaly too), suggest the presence of a crustal-scale

structural feature, such as a suture zone, terrane boundary, or dip- and/or strike-slip fault zone.

This is supported by recent work by Tosdal and Wooden (1997) and Wooden et al. (1997), who

have been able to subdivide the northern Great Basin into four crustal provinces based on Pb

206/204 and 208/204 isotopic rations: western, central, and eastern provinces, flanked to the east

by Archean crust. The boundary between the central and eastern provinces (208Pb/204Pb >39.5

isopleth) is coincident with the Carlin trend. The sharpness of this boundary suggest to Tosdal

and Wooden (1997) and Wooden et al. (1997) that it represents a major crustal fault. In addition,

the results of a magnetotelluric survey across the Battle Mountain–Eureka (Cortez) and Carlin

mineral trends (Rodriguez, 1997) have revealed deep-penetrating regional structures which

appear to be major structural breaks in the crust. The presence of regional-scale structural zones

along the Battle Mountain–Eureka (Cortez) and Carlin mineral trends is also consistent with

proposals made by Arehart et al. (1993), Grauch et al. (1995), Madrid and Bagby (1986), Maher

et al. (1993), and Shawe (1965, 1991) (further discussed below, and previously in sections 3.4

and 3.6).

8.5.2 Age and Origin of the Carlin and Cortez Structural Zones

Assuming that the Carlin and Cortez structural zones do exist, and that they did exercise strong

control over the distribution of sedimentary rock-hosted deposits in north-central Nevada, then

the inferred crustal structures would had to have been in place prior to ore-deposition, or formed

as a result of (and during) the processes that led to mineralization. Sedimentary rock-hosted

mineralization along the Battle Mountain–Eureka (Cortez) and Carlin trends is generally

considered to be Upper Eocene–Early Oligocene (42-30 Ma) in age (Bagby and Berger, 1985;

Maher et al., 1993; Hofstra, 1994, 1997; Ilchik, 1995; Thorman et al., 1995; Emsbo et al., 1996;

Hofstra, 1994, 1997; Ilchik and Barton, 1997), but ages as old as Lower–Upper Cretaceous have

been suggested (see Kuehn, 1989; Arehart et al., 1993, 1995). The northern Nevada rift trends

sub-parallel to the Battle Mountain–Eureka (Cortez) trend (Fig. 8.3c). The age of the northern

Nevada rifting event has been determined by Zoback et al. (1994) to be ~17-14 Ma, which is

based on new radiometric dating of north-northwest-trending dikes and flows occurring within

the rift. The authors also pointed out that, the northern Nevada rift is equivalent in age, trend,

and composition to feeder dikes that fed the main eruptive pulse of the Columbia River flood

basalts in northern Oregon ~15.5-16.5 Ma (Zoback et al., 1994) (see Fig. 2.3). Assuming the age

estimates of mineralization and rifting are correct, the sedimentary rock-hosted deposits are some

20-100 m.y. or so older than the rift structure, suggesting that the rift, in its present

manifestation, did not play a key role in mineralization.

Alternatively, the dikes and flows associated with the northern Nevada rift, which are the source

of the magnetic anomaly delineating the rift zone, may be the product of magmas that were

focused along and ascended through a zone of preexisting weakness in the crust, in effect using
119

the crustal flaw zone as a conduit (see, for example, Cameron and Hattori, 1995). This,

contended Zoback et al. (1994), was not the case. Zoback et al. (1994) observed that the

orientations of the magnetic anomaly, exposed basaltic feeder dike swarms, and structural

troughs associated with the rift zone, all trend uniformly N20o

–25oW, and this consistency, they

argued, indicates that the northern Nevada rift zone developed in response to the prevailing least

principal stress at the time of formation (horizontal and oriented N65o

–70oW), rather than by

exploiting a pre-existing zone of crustal weakness. While latest activity may in fact have taken

place in this environment (i.e.—rifting), the observations and spatial relationships among the

geophysical and geochemical anomalies and the mineral trends presented in section 8.5.1 suggest

that a pre-existing crustal weakness may indeed exist. This proposal is in accord with Grauch

et al. (1995). They suggested that a pre-existing basement structure played a major role in the

formation and/or location of other nearby geologic features of similar trend (i.e.—the northern

Nevada rift), as well as having focused magmatism (heat) and the flow of deep-seated and/or

high-level convecting ore-related fluids, which resulted in the formation of Battle

Mountain–Eureka (Cortez) and Carlin mineral trends. Furthermore, Grauch et al. (1995)

suggested that the basement feature must have developed during or prior to Jurassic time, as

determined by the oldest alignment of geologic features along its trend (Jurassic intrusive rocks

and probable mid-Jurassic folds). The northerly orientation of the basement gravity gradient, and

its location proximal to tectonic activity directed at the continental margin, led Grauch et al.

(1995) to conclude that this feature may be associated with the rifted Precambrian continental

margin. This interpretation places the inferred structural feature in an acceptable geologic

environment and chronological order with respect to Upper Eocene–Early Oligocene

mineralization and mid-Miocene rifting:

1. Development of a Jurassic or older crustal-scale structural feature, which may represent (1) a normal fault

related to the Precambrian rifting event that acted as a buttress against later tectonic compression, producing

ramped thrust plates and parallel fold belts, or (2) just one manifestation of a general structural grain in the

Precambrian continent (Grauch et al., 1995).

2. Focusing of heat and mineralizing fluids along the crustal-scale structural feature to produce the alignment

of Upper Eocene–Early Oligocene age sedimentary rock-hosted occurrences along the Battle

Mountain–Eureka (Cortez) mineral trend and possibly the Carlin trend.

3. Formation of the northern Nevada rift zone proximal to the crustal-scale structural feature during the midMiocene in an extensional tectonic regime, where the preexisting Jurassic or older structural feature

influenced the location of rifting and associated magmatism.

These findings are also consistent with Maher et al. (1993), where it was suggested that the

northern Nevada rift is the most recent manifestation of activity related to an “older” Paleozoic

or pre-Late Cretaceous fundamental structure in the crust.

The observations made in this study, in addition to those mentioned above, further support the

possibility that mineralization along the Battle-Mountain–Eureka (Cortez) and Carlin trends

could be the result of heat and mineralizing fluids distributed laterally along regional-scale

crustal structural zones of mid-Jurassic or older origin.

8.5.3 Transcurrent Movement

The presence of sedimentary rock-hosted occurrences in east-central Nevada suggests that the

Carlin and Cortez structural zones may be extrapolated along trend to the southeast of the
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northerly-trending Roberts Mountain thrust front (see Figs. 8.1a, dashed red lines). This notion

is based on a number of structural elements visible in shaded relief of topography (derived from

~800 meter pixel resolution), as well as by isostatic gravity data, which shows a linear gradient

ridge running sub-parallel to, and south of, the northern Nevada rift zone, and may represent the

northern extension of the Bouguer gravity-low bilateral axis of symmetry in Nevada (Blakely,

1988; also see sections 6.6.3 and 2.7.3; Figs. 6.45b and 6.45c, feature #1, and 6.54). The Bald

Mountain district, Alligator area deposits, and Illipah deposit occur approximately along strike

of the southeastern extension of the Carlin mineral trend, and roughly on the eastern side of the

bilateral axis of symmetry. The Eureka district and the Ratto Canyon deposit occur along strike

of the southeastern extension of the Battle Mountain–Eureka (Cortez) mineral trend, and to the

west of the bilateral axis of symmetry. In between the Carlin and Battle Mountain–Eureka

(Cortez) mineral trends, and roughly within the bilateral axis zone, the Easy Junior-Night Hawk

and Green Springs deposits are found.

Shaded relief of topography may give some indications of transcurrent movement along (or

between) the Carlin and Cortez structural zones. The southeastern extensions of the Carlin and

Cortez structural zones, as delineated in Figures 8.1a (dashed red lines), roughly bisect a broad

and open semi-continuous “S”-shaped topographic curvature. This feature, which is clearly

visible in shaded relief (located just east of center in Fig. 8.1), is formed by the curvilinear

alignment of (from north to south) the Ruby Mountains and the White Pine–Horse–Grant–Quinn

Canyon ranges. Left- or right-lateral movement, widely distributed between the southeastern

extensions of the Carlin and Cortez structural zones, could have produced this topographic

feature:

& Assuming that this topographic feature was originally continuous, left-lateral movement could have displaced

the northeast–southwest-trending northern and southern segments, and during the process, realigned the ranges

in the zone of transcurrent movement along a more northerly orientation (central segment of the “S”, area

between dashed red lines in Fig. 8.1a).

& Assuming that this topographic feature was not originally continuous, right-lateral movement could have

realigned the ranges in the central portion of the “S” from roughly northeast–southwest-trending to

north–northwest—south–southeast-trending, effectively “joining” the northern and southern segments of the

“S”.

Evidence for both left- and right-lateral motion sense are found along the Carlin structural zone.

The Alligator group deposits occur near the southern extension of the Carlin structural zone, and

as a group, parallel the ranges in the central part of the “S”-shaped topographic structure

(compare Fig. 8.1 and 3.3). Recent mapping in the Alligator Ridge area has documented Eocene,

left-lateral, transpressive deformation structures (Nutt, 1997; Nutt and Good, 1998). In

sedimentary rock-hosted deposits along the northern Carlin trend, right-lateral strike-slip faulting

has been recognized by a number of Newmont Exploration Ltd. geologists (1990, personal

communication), Putnam and Enriquez (1991), Teal and Jackson (1997), and Byron R. Berger

(1998, personal communication; e.g.—Genesis and Post-Betze pits). Right-lateral movement

on a regional scale has been suggested by Carey (1955, 1958, 1976, 1988), Shawe (1965),

Atwater (1970), Livaccari (1979), and Ward (1988, 1991). In order to reconcile the conflicting

sense of kinematic motion, Tosdal and Nutt (1998) suggested that the Carlin mineral trend

(which includes the “southern extension” deposits of the Alligator group) delineates a transfer

or accommodation zone, linking geographic offset or steps in extending domains and thereby

allowing for opposing movement (see Faulds and Varga, 1998).
121

1 Or perhaps in the vicinity of the Preble deposit, if only the alignment of deposits with the Battle

Mountain–Eureka (Cortez) mineral trend is considered.

Transcurrent movement between the Carlin and Cortez structures may also be supported by the

trend of Pb isotope values of Jurassic, Cretaceous, and Tertiary granitic rocks. Wooden et al.

(1997) have identified a number of Pb isotopic boundaries in northern Nevada, an eastern

boundary (208Pb/204Pb >39.5) that coincides with the Carlin trend and a western boundary

(

208Pb/204Pb >39) that lies parallel but to the southwest of the Battle Mountain–Eureka (Cortez)

trend (see figure 1 in Wooden et al., 1997). They indicate that the westernmost Pb boundary

appears to swing northeast, possibly encountering the Humboldt shear(?) zone as suggested in

section 8.4.1, except at the Battle Mountain–Eureka (Cortez) trend, where there is a prominent

southeast deflection (Tosdal and Wooden, 1997). The boundary then bends back to the northeast

toward the Carlin trend where it appears to turn further northward (Wooden et al., 1997, figure 1;

Richard M. Tosdal, 1998, personal communication). The “jog” in the northeast-trending

208Pb/204Pb >39 isopleth across the Carlin and Battle Mountain–Eureka (Cortez) mineral trends

could have been the result of transcurrent movement along and/or between the Carlin and Cortez

structural zones.

Finally, the northern Nevada rift zone may provide some support for transcurrent activity along

the Carlin and Cortez structural zones. The rift zone trends between and sub-parallel to the

structural zones (Figs. 8.1a, green line; also see 8.3c), suggesting that it could be related to these

features (although they may differ in age; see discussion in sections 8.5.1 and 8.5.2). The

northern Nevada rift zone has been postulated to be the southern extension of the Brothers fault

zone (see sections 2.7.4 and 6.6.3; Fig. 2.3). Together, they form the Oregon-Nevada lineament

(Stewart et al., 1975), which has been interpreted to be the surface expression of a deep-seated

fracture zone (up 15 km deep) that may have had a complex history of strike-slip and

transcurrent movement (Stewart et al., 1975; Blakely and Jachens, 1991; also see Christiansen

and Yeats, 1992).

8.5.4 Truncation of the Battle Mountain–Eureka (Cortez) and Carlin

Mineral Trends by the Humboldt Shear(?) Zone

As discussed in section 8.4, broad northeast–southwest-trending zone of transcurrent movement,

with possible ancestral heritage, is believed to extend across northern Nevada and to be

responsible for controlling the regional-scale distribution of volcanic rock-hosted occurrences

in this area. Termed the Humboldt shear(?) zone, it has, to a large degree, been delineated in this

study on the basis of topographic expression. Within this zone, a number of northeast–southwesttrending linear features have been identified (see Fig. 8.1a, features “A”, “B”, and “C”). The

intersection of these linear features with the proposed Carlin and Cortez structural zones and the

northern Nevada rift zone correlate with (1) some degree of deposit clustering (see text in caption

of Fig. 8.1), (2) discontinuities along the geomagnetic signature of the northern Nevada rift zone

(particularly in the flanking rift signatures to the west; see Figs. 6.45a and 6.53b), and (3) lateral

offsets and discontinuities in the K/Na–Ba/Na geochemical anomaly pattern (see Fig. 6.65).

Associated with these linear features is the abrupt termination of the Carlin and

Mountain–Eureka (Cortez) mineral trends in the vicinity of the Dee, Bootstrap, and Capstone

deposits, and the Lone Tree and Stone House deposits1

, respectively. In both instances, a group
122

of sedimentary rock-hosted deposits are encountered to the northeast of the northern termini of

the mineral trends: the Independence group deposits to the northeast of the Carlin trend, and the

Getchell trend deposits to the northeast of the Battle Mountain–Eureka (Cortez) trend.

It is not unreasonable to assume that the Battle Mountain–Eureka (Cortez) and Carlin mineral

trends continue northwestward along the same approximate strike. As previously discussed

(section 6.7.3), the K/Na–Ba/Na geochemical anomaly is spatially, if not genetically, associated

with the sedimentary rock-hosted occurrences and clearly delineates the mineral trends. As such,

the K/Na–Ba/Na geochemical anomaly might also extend further northwestward. Unfortunately,

the NURE K/Na and Ba/Na geochemical datasets examined for this study do not significantly

extend north of the northern termini of the Battle Mountain–Eureka (Cortez) or Carlin trends.

However, the NURE U/Th and As datasets do extend further north, and they show anomaly

distribution patterns that are very similar to the K/Na–Ba/Na anomaly pattern and to the

distribution of deposits along the northern reaches of the mineral trends (see section 6.7.3). It

appears that the U/Th and As anomaly patterns do not extend northwestward, rather they appear

to bend to the northeast:

In the vicinity of the Kramer Hill and Preble deposits, the As anomaly bends to the northeast and extends along

the Getchell trend deposits.

North of the Dee, Bootstrap, and Capstone deposits, the As and U/Th anomalies bend to the northeast, then

northward to parallel the Independence group deposits.

It is uncertain whether the northeast bend observed in these anomaly patterns is real, or if they

reflect the extent of Cenozoic volcanic cover of the Banbury formation, or perhaps a sampling

bias. In support of the northeast bend are the points presented in section 8.4.3, and in particular,

eastward lateral offsets in basin-range structure visible (1) north of the northern terminus of the

Battle Mountain–Eureka (Cortez) trend along the Getchell trend deposits (in the Osgood

Mountains), and (2) north of the northern terminus of the Carlin trend, between the Dee deposit

(north Carlin trend) and the Burns Basin and Mill Creek deposits (southern end of the

Independence group deposits; southwestern spur of the Independence Mountains). The

implication is that the Getchell trend represents the northeastward-displaced (sheared?) extension

of the Battle Mountain–Eureka (Cortez) trend, and the Independence group deposits (trend)

represent the northeastward-displaced (sheared?) extension of the Carlin trend. Lateral

displacement of the Getchell and Independence trend deposits away from the Battle

Mountain–Eureka (Cortez) and Carlin trend deposits (respectively) could have been achieved

by right-lateral transcurrent movement within the Humboldt shear(?) zone, parallel to the

northeast–southwest-trending linear features depicted in Figures 8.1a.

Such a scenario requires that certain age and kinematic relationships along the structural zones

be satisfied: (1) the age of mineralization at the “sheared-off” ends of the deposit trends should

be roughly the same—the deposits of the Carlin trend roughly equal in age to those in the

Independence group; the Battle Mountain–Eureka (Cortez) trend deposits roughly equal to those

of the Getchell trend, and (2) mid-Miocene, right-lateral movement along the Humboldt shear(?)

zone should be reconciled with right-lateral movement along the Cortez and Carlin structural

zones and the Walker Lane shear zone.

Hofstra (1997) commented that “evidence continues to mount that Carlin-type deposits formed

in the mid-Tertiary (42 to 30 Ma)” (see references in Hofstra). For the Carlin trend deposits,
123

Christensen (1995) reported two radiometric age date groupings of 95-117 Ma and 36-40 Ma,

with dates as young as 8-35 Ma, and as old as ~120 Ma (Kuehn, 1989; also see Arehart et al.,

1993, 1995). Thorman et al. (1995) proposed that the many of the Carlin trend deposits are

Eocene-Oligocene. Ilchik (1995), based on a reanalysis of data published by Arehart et al.

(1993), suggested that the most reasonable age for the formation of the Post-Betze deposit is

slightly younger than 39 Ma. Most recently, Emsbo et al. (1996) were able to constrain

mineralization at the Meikle and Post-Betze deposit to <38.8 Ma based upon relationships

among igneous dikes on the Goldstrike property. At the southern end of the Independence group

deposits, recent work in the Jerritt Canyon district has produced mineralization age dates of ~40

Ma (Hofstra, 1994; also see Folger et al., 1997; Hofstra, 1997; Wise and Gorman, 1997). Along

the length of the Battle Mountain–Eureka (Cortez) trend, Maher et al. (1993) bracketed the age

of the most economically significant Carlin-style gold producing mines to late-Eocene–earlyOligocene time (radiometric age dates compiled for his study for this time period range from ~40

to ~34 Ma). These ages are comparable with recent Getchell trend mineralization age dates for

the Twin Creeks deposit (Rabbit and Chimney Creek deposits; ~42 Ma; Groff, 1996, Groff et

al., 1997; Hall et al., 1997) and Pinson (42.7±5 Ma; Byron R. Berger, 1998, personal

communication). These age data bridge the timing of mineralization across the Carlin trend and

Independence group deposits and across the Battle Mountain–Eureka (Cortez) and Getchell trend

deposits. This is consistent with the proposal made here that the Getchell trend and

Independence group of deposits represent the sheared-off northern extensions of the Battle

Mountain–Eureka (Cortez) and Carlin mineral trends. Furthermore, if this postulate is correct,

then it may be possible to constrain the most recent movement along the Humboldt shear(?) zone

to between the formation of the Carlin-type deposits (42 to 30 Ma; Hofstra, 1997) and the most

recent igneous activity along the northern Nevada rift zone (~17-14 Ma; Zoback et al., 1994),

which transects the Humboldt shear(?) zone but shows no distinct lateral offsets in its

geomagnetic signature (i.e.—latest appreciable movement occurred between 42-30 to 17-14 Ma).

Questions remain as to the tectonic origin of the Humboldt shear(?) zone, its activity throughout

geologic history, and its relationship to other regional-scale features, such as the Carlin and

Cortez structural zones and the Walker Lane shear zone. As indicated above, there is evidence

as well as a plausible explanation for left-lateral movement along the Carlin structural zone.

Left-lateral motion-sense is kinematically consistent with right-lateral movement along the

Humboldt shear(?) zone. However, given the proposal here that right-lateral movement within

the Humboldt shear(?) zone may have taken place ten’s of millions of years after mineralization

along the Cortez and Carlin structural zones, and presuming that movement along the Humboldt

shear(?) and Carlin structural zones therefore was not concurrent, it is unclear whether or not

motion-sense relationships between these features must or can be resolved. It is perhaps more

important to reconcile motion-sense between the Humboldt shear(?) zone and the Walker Lane,

which has documented activity in the middle and late Tertiary (see Dilles and Gans, 1995;

section 2.6). Research conducted by Hardyman and Oldow (1991) may provide a possible

resolution. Based on field work in the central Walker Lane (Carson sink area, south to Tonopah,

Nevada), Hardyman and Oldow (1991) have outlined a three-stage tectonic history that involves

a 70o

 to 90o rotation in the extension direction since 19-15 Ma: an early 28-17 Ma stage of

extension that was oriented north-northeast–south-southwest, followed by east-west extension

between 17-11 Ma, and, after about 10 Ma, southeast-northwest oriented extension. The result

of this last extensional stage (post-10 Ma through present day) has been dominant right-lateral

movement on northwest-striking faults in the Walker Lane shear zone. To be kinematically
124

correct, this sense of motion would require left-lateral movement along the Humboldt shear(?)

zone, contrary to what is proposed here. However, during the period of early extension (28-17

Ma), Hardyman and Oldow (1991) indicate that extension was oriented north-northeasterly,

resulting in a left-lateral strike-slip regime. This time period of left-lateral strike-slip along the

Walker Lane is in large part concurrent with latest movement within the Humboldt shear(?) zone

proposed here (42-30 to 17-14 Ma), and is kinematically consistent with the right-lateral

movement needed to displace the Getchell and Independence trend deposits northeastward away

from the northern termini of the Battle Mountain–Eureka (Cortez) and Carlin trend deposits

(respectively). Left-lateral motion within the Walker Lane shear zone between 28-17 Ma may

also be related to left-lateral motion along the Carlin structural zone.

8.6 A Mantle Plume 

There has recently been a noticeable increase in the number of regional-scale studies suggesting

that a mantle plume or upwelling asthenosphere could be responsible for the tectonomagmatic

evolution, metallogenesis, and present-day character of the Basin and Range Province and

surrounding environs (see for example, Smith, 1978; Fitton and James, 1987; Mutschler et al.,

1987; Fitton et al., 1988; Leeman and Fitton, 1989; Westaway, 1989; Draper, 1991; Fitton

et al., 1991; Mutschler et al., 1992; Parsons et al., 1994; Saltus and Thompson, 1995; Oppliger

et al., 1997; Murphy et al, 1998; Murphy et al, 1999). Many of the researchers cited above

believe that a mantle plume fits the geographic, geologic, geophysical, and geochemical data

observed in the Great Basin, and that in fact the Great Basin is an example of a plume-related

orogenic process that has been overlooked (Murphy et al., 1998; Murphy et al, 1999). A mantle

plume or diapiric activated/controlled mechanism for magma generation accommodates (1) the

chemistry and isotopic characteristics of mid- and late Cenozoic basalt (Fitton and James, 1987;

Fitton et al., 1988; Glazner and Ussler, 1989; Leeman and Fitton, 1989; Fitton et al., 1991;

Leeman and Harry, 1993), (2) the space-time magmatic and extensional migration patterns across

the region (Armstrong et al., 1969; Stewart 1980; Silberman 1985; White 1985; Seedorff,

1991), and (3) correlates well with the work on the physics and chemistry of mantle plumes

conducted by Courtney and White (1986), Richards et al. (1989), and DePaolo et al. (1991).

A number of datasets compiled for this study, some of which were not directly applied to the

mineral potential modelling performed here (see section 5.3.3), support the presence of a plume

or upwelling asthenosphere. These datasets include (1) igneous rock radiometric age dates

(RAD-B), (2) metallic mineral deposits rock radiometric age dates, (3) Cenozoic igneous rock

time-slices maps, (4) Cenozoic igneous rock composition-slices, (5) 87Sr/86Sr initial values of

Mesozoic and Cenozoic igneous rocks, (6) dip attitude of Tertiary rocks, and (7) mountain peak

elevation. In the Great Basin, magmatism and extension migrated southwestward, spreading

concentrically outward from the northeastern Nevada during the middle Tertiary early rapid

extension through subsequent late Tertiary slow extension stages (see Fig. 6.4 and section 2.5).

(Magmatism in the Great Basin completes only a portion of this ring, forming an arcuate band

in Nevada as shown in Figure 6.4, but the remainder could be concealed under younger strata to

the north or northeast). This outward radiating ring of event inception times follows the timetransgressive effects that a mantle plume could have on magmatism as suggested by a theoretical

model proposed by Courtney and White (1986), where an outward-younging radial and

concentric trend of magmatic inception times is produced above a plume. Maps generated from

the seven aforementioned datasets are all consistent with the time-transgressive effects that a
125

mantle plume could have on regional-scale tectonomagmatism. The space-time migration of

inception times of magmatism is particularly well developed in the igneous rock radiometric age

dates (RAD-B) dataset (Fig. 8.4), while the dip attitude of Tertiary rocks is suggestive of an

overall “doming” or “arching” to the region, as might be expected over a mantle plume or

upwelling asthenosphere. 

A mantle plume, in conjunction with preexisting crustal-scale structures such as the Carlin and

Cortez structural zones and the Walker Lane and Humboldt shear(?) zones, could have had

profound effects on the development of the Great Basin as a metallogenic province. The

southwesterly absolute motion of the North American plate suggests that the Basin and Range

province lies in the “wake” of the plume (Fitton et al., 1991). A mantle plume is believed to

have emerged (made contact with the lower crust?) in northernmost central Nevada, near the

borders of Oregon and Idaho, at the triple-junction intersection of Brothers fault zone, the Snake

River Plain, and the northern Nevada rift zone (see Fig. 2.3) (Parsons et al., 1994; see also

Mutschler et al., 1992). Murphy et al. (1998, 1999) track the plume a little farther south, through

Battle Mountain, Nevada, and along a path which parallels the Humboldt shear(?) zone, as

delineated in this study. However, if the RADB data (Fig. 8.4) and the Stewart and Carlson

(1978) volcanic rock time-slice maps (Fig. 6.4) are any indication of where initial Cenozoic

magmatism was centered in the Great Basin, then slightly north of east-central Nevada is where

the plume may have made contact with the crust. The current location of the plume is thought

to be either under the southern Rocky Mountains, in the region of the Colorado Plateau (Fitton

et al., 1991; Parsons and McCarthy, 1993), or under the region of Yellowstone Park (the

“Yellowstone hot spot”) (Smith, 1978; Westaway, 1989; Draper, 1991; Parsons et al., 1994;

Oppliger et al., 1997; Murphy et al., 1998; Murphy et al, 1999). Oppliger et al. (1997) have

suggested that the ancestral Yellowstone hotspot may have been responsible for sedimentary

rock-hosted mineralization in north-central Nevada, having provided (1) a source for gold (a la

Rock and Groves, 1988), (2) a mechanism for metamorphic devolatization in the lower crust (see

Seedorff, 1991, for metamorphic devolatization sedimentary rock-hosted model), and (3) a

regional-scale heat source that drove fluid convection in the upper crust.

The formation and distribution of the volcanic rock-hosted occurrences could also be related to

plume activity. In context of the arcuate time-transgressive migration pattern of magmatism

across Nevada (Fig. 6.4), the generalized distribution pattern of the sedimentary rock-hosted

occurrences is spatially coincident with the older “core” of the sequence, whereas the volcanic

rock-hosted “epithermal crescent”, which encompasses the sedimentary rock-hosted occurrences,

is coincident with a younger “ring” (see Fig. 6.2 and sections 6.2.2 and 6.2.3). As indicated in

section 8.1 and subsequent sections, the Walker Lane shear zone and the Humboldt shear(?) zone

are believed to have acted as first-order controls over the regional-scale distribution of the

volcanic rock-hosted occurrences. Plume-driven magmatism may have acted as a second-order

control. As Eocene-Oligocene magmatism expanded in a arcuate time-transgressive manner

across the region, it encountered and exploited the Walker Lane shear zone and the Humboldt

shear(?) zone, where together magmatism and preexisting tectonic conditions provided a

favorable environment for ore deposit formation (see, for example, sections 8.2.2, 8.2.1, 3.4, 3.6,

6.4.3, and 6.5.3).
126

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180

Class Type of Relationship Examples 

Field Model

Theoretical Physical/Chemical Principles Groundwater Flow Equations of Motion

Hybrid Semi-Empirical Sediment Transport Transport Equations

Empirical Heuristic or Statistical Mineral Prediction Statistical Regression

Table 1.1. Types of models used in the geosciences (Bonham-Carter, 1994a).
181

Type Model Parameters Examples

Data-driven Calculated From

Training Dataset

Weights of Evidence

Logistic Regression

Neural network

Knowledge-driven Estimated

by an Expert

Fuzzy Logic

Dempster-Shafer Belief Theory

Table 1.2. Types of mineral potential models (Bonham-Carter, 1994a).
182

Population Metallic and Non-Metallic Mineral Occurrences

OccurrenceType

Sample

Primary-Product

Gold-Silver-Bearing

Occurrences (all types)

(2690)

Sedimentary RockHosted Gold-SilverBearing Occurrences

(98)

Volcanic Rock-Hosted

Gold-Silver-Bearing

Occurrences 

(415)

Size Subsample

Lrg.

(59)

Med.

(118)

Sml.

(2269)

Unkn.

(244)

Lrg.

(8)

Med.

(30)

Sml.

(57)

Unkn.

(3)

Lrg.

(33)

Med.

(43)

Sml.

(317)

Unkn.

(22)

Table 1.3. The gold-silver-bearing occurrences examined in this study.
183

Table 3.1. Synoptic mineral deposit model for sedimentary rock-hosted disseminated precious metal deposits. 

This model is derived from the attributes of North American "giant" deposits ( 1,000,000 Troy ounces Au

production and/or reserves) in the Great Basin.

SEDIMENTARY ROCK-HOSTED DEPOSIT-TYPE SYNOPTIC MODEL

SYNONYMS AND DESCRIPTIONS:

Carlin-type; carbonate rock-hosted Au; disseminated replacement-type Au; disseminated Au; invisible Au;

sediment-hosted micron Au (SHMG); erroneously referred to as "epithermal" disseminated as well. Stratiform

deposits of sub-microscopic to microscopic Au coating disseminated sulfide minerals in carbonaceous

calcareous sedimentary rocks. 

COMMODITIES (principal recovered products; by-products in parentheses):

Au Ag (Hg) 

EXAMPLES: (within the Great Basin)

In Nevada: Carlin trend deposits (Bootstrap, Dee, Carlin Gold mine, Genesis, Gold Strike, Pete, Post, Gold

Quarry, Maggie Creek, Rain, etc.), Cortez trend deposits (Gold Acres, Cortez, Horse Canyon, Pipeline, etc.),

Tonkin Springs, Gold Bar, Northumberland, Getchell trend deposits (Twin Creeks (Chimney Creek and Rabbit

Creek), Getchell, Pinson, Preble), Independence group deposits (Big Springs-Sammy Creek, Burns Basin,

Jerritt Canyon, Winters Creek, Wright Window, etc.), Alligator group deposits (Alligator Ridge, Bald

Mountain, Golden Butte, Illipah, White Pine, Yankee, etc.), Standard, Fondaway Canyon, Night Hawk, and

Green Springs. In Utah: Mercur, Barney’s Canyon, Tecoma, and Melco.

TYPICAL GRADE AND TONNAGE (Troy ounces; U.S. short tons):

Gold: 0.015-0.3; up to 0.7-0.8 oz/t

Silver: data sparse; generally much lower than Au (0.02 oz/t Alligator; 0.7 oz/t Dee)

Tonnage: 2-8 to 50 to >500 million tons

ASSOCIATED DEPOSIT TYPES (other genetically related deposit types):

Veins containing stibnite, barite, realgar, and orpiment.

GEOLOGIC SETTING:

Deformed basin and/or platform sediments in mobile belt or stable craton settings. Carbonaceous, calcareous

rocks cut by high-angle normal faults. Many deposits in Nevada aligned along inferred regional-scale

structures, en echelon fault belts, belts of thrust faulting (including fensters in allochthonous plate), and/or

zones of crustal weakness?

MINERALIZED AND HOST ROCKS:

Calcareous sedimentary rocks high in carbon (3-15% C). Rock types usually range from calcareous shale to

silty limestone or dolostone. Commonly thinly bedded. Coarse to fine clastic sedimentary rocks with high

carbon content occur as hosts in some deposits (e.g.—Getchell). Units of high permeability most favorable host

rocks.

ASSOCIATED ROCKS (spatially associated non-mineralized igneous rocks):

Calc-alkaline and/or alkaline plutons and dikes occur at or near most deposits. Intrusives pre-ore or post-ore

at most deposits, although appear to be syn-ore at some. Hydrothermal breccias may be present in ore zones.

Genetic association unclear (may have acted as heat engine in some cases?).
184

Table 3.1, continued.

FORM OF DEPOSIT (ore body and gold mineralization):

Stratabound. Irregular to tabular ore bodies extending out from faults intersecting favorable host rocks. Some

deposits may show vertical continuity of mineralization with no change in mineralogy over vertical intervals

in excess of 700-1000 meters. Mineralization is disseminated, fine grained and decreases away from fault zone

or heat source. Most Au is "invisible" (0.03-10 microns). Au content varies widely within ore zone. Ore may

be partially oxidized. Small veins and stockworks may occur within orebodies. Jasperoid replacement bodies

common and may be ore grade. Au-quartz veins rare. 

ORE CONTROLS:

Permeable carbonaceous calcareous sedimentary rocks. Geochemically favorable horizons. Proximity to high

angle faults, thrust faults, and/or low-angle normal or reverse faults. At some deposits breccia zones/pipes and

impermeable "capping" stratigraphy or structures important (some units or structures may have acted as local

aquitards, blocking upward movement of fluids and redirecting them laterally along permeable units). Regional

antiformal doming and localized (minor) anticlinal folding ("structural domes") important at some deposits

(fracturing and brecciation along crest). Folding is Mesozoic in Nevada. 

MINERALOGY (mode of gold occurrence in ore; other metallic, accessory and gangue minerals):

Native Au: exceedingly fine grained; sub-microscopic to microscopic, usually less than one micron; rarely

visible. Occurs as thin films and fracture fillings on pyrite which may be disseminated, encapsulated in quartz,

or in veins. Free Au also occurs as disseminations in quartz or barite, between quartz and clay grains, on grains

of amorphous carbon or hematite, and as inclusions in other sulfides. Veins contain barite, calcite, pyrite,

realgar, orpiment, other sulfides, sulfosalts, and carbonaceous material. Carbonaceous material may include

hydrocarbons, amorphous carbon, microcrystalline graphite, and humic acids. Minerals in the unoxidized ore

zone include arsenopyrite, pyrrhotite, marcasite, cinnabar, realgar, orpiment, stibnite, lorandite, tennantite,

getchellite, jordanite, galkhaite, gratonite, christite, ellisite, weissbergite, boulangerite, tetrahedrite, carlinite,

galena, sphalerite, chalcopyrite, covellite, chalcocite and molybdenite. Other hydrothermal minerals include

quartz (often jasperoid), calcite, dolomite, barite, fluorite, jarosite, alunite, frankdicksonite, and clay minerals.

Oxidized ore zone may contain hematite, goethite, schuetteite, arsenolite, carminite, scorodite, arseniosiderite,

mimetite, stibiconite, valentinite, cervantite, avicennite, litharge, massicot, anglesite, cerussite, smithsonite,

malachite, azurite, chrysocolla and turquoise. 

ALTERATION:

Decarbonatization, silicification, argillization, pyritization, and oxidation and acid leaching near surface.

Introduction and redistribution of carbon. At the Carlin deposit (Nevada), Au is generally associated with

intermediate alteration zones of carbonate removal. Volume loss related to decarbonatization produces apparent

enrichment in Al, Fe, and K, but there is little change except for major Ca, Mg, CO2 and minor K loss, and SiO2

gain.

 

AGE RANGE:

Host: Cambrian to Tertiary (generally Cambrian through Pennsylvanian for deposits in north-central Nevada)

Ore: Tertiary (42-30 Ma in Nevada)

GENETIC MODEL:

Unresolved. For the Great Basin, three basic models have been proposed: (1) Related to regional-scale crustal

extension, where the southward sweep of Tertiary magmatism and the onset of extensional (and strike-slip?)

tectonism facilitated deep fluid circulation by increasing the geothermal temperature gradient (thinning of the

crust) and crustal permeability via brittle faulting and fracturing in the upper crust. (2) Related to regional

metamorphism, where deep-seated, possibly mantle-derived magmatic activity, preceded crustal extension and

provided fluids (by metamorphic devolatilization reactions at deeper levels) and created sufficient heat (a

regional “geothermal environment”) to circulate fluids. Deeply convecting unevolved meteoric fluids mix at

deep crustal levels with ascending gas-rich metal-bearing fluids that were released at the onset of extension.
185

Table 3.1, continued.

(3) Igneous-related (distal disseminated), where deposits form in the distal portions of magmatic hydrothermal

systems. Magmas provided energy to drive hydrothermal circulation were the source for some fluid

components and metals. Deposits formed within approximately <3 to >5 km of the surface (as compared to

~1km for epithermal mineralization). Main-stage ore depositing fluids were evolved meteoric (mixed) waters,

with elevated CO2 and H2S, low in salinity (<0.2-10 equivalent wt% NaCl; generally <5 wt%, but up to 18%

or higher at boiling), temperature (150-250o

C; most deposits 190 to 225oC), and SO4. Several generations of

hydrothermal brecciation at some deposits suggest multiple mineralizing events. Late-stage ore and/or post-ore

fluids may redistribute Au to some degree. Metals derived at depth from underlying sedimentary pile (Late

Proterozoic miogeoclinal pelitic rocks) or from magmatic sources. Bisulfide, thiosulfide, arsenic, and/or other

poorly documented complexes may transport Au.

COMPILED FROM:

Arehart et al. (1993), Bakken et al. (1987), Bakken and Einaudi (1986), Bakken (1990), Birak et al. (1987),

Birak (1986), Bonham (1985), Cline et al. (1997), Cox and Singer (1986), Cunningham (1988), Dickson et al.

(1979), Hall et al. (1997), Hausen et al. (1967), Henry and Boden (1997), Hitchborn et al. (1996), Hofstra et

al. (1988), Hofstra and Rowe (1987), Hofstra et al. (1987), Hofstra (1997), Hofstra (1994), Holland et al.

(1988), Ilchik and Barton (1997), Ilchik and Barton (1996), Kuehn and Rose (1987), Kuehn (1989), Lamb and

Cline (1997), Leventhal et al. (1987), Lowe et al. (1985), Ludington et al. (1993), Ludington et al. (1996),

Maher et al. (1993), Norman et al. (1996), Northrop et al. (1987), Percival et al. (1988), Radtke (1981), Radtke

(1985), Radtke et al. (1980), Rodriguez (1997), Romberger (1986), Rose and Kuehn (1987), Rye (1985),

Seedorff (1991), Singer (1996), Teal and Jackson (1997), Tooker (1985), White (1985), and Wooden et al.

(1997).
186

Table 3.2. Synoptic mineral deposit model for volcanic rock-hosted gold-silver deposits. This model is derived

from the attributes of North American "giant" deposits ( 1,000,000 Troy ounces Au production and/or

reserves), and is primarily based on, but not limited to, deposits in the Great Basin.

VOLCANIC ROCK-HOSTED DEPOSIT-TYPE SYNOPTIC MODEL

SYNONYMS AND DESCRIPTIONS:

Epithermal veins; volcanic disseminated Au. Epithermal vein and/or disseminated deposits (often co-occurring)

of Au and Au-bearing minerals in or associated with a variety of fault zones, breccia zones, fracture zones,

shear zones, and diatremes in volcanic and spatially associated rock types. Formed at relatively shallow depths

(< 1 km) below paleosurface in geothermal system. May be divided into low-sulfidation systems (LS)

characterized by adularia-sericite alteration or (high-sulfidation systems (HS) characterized by acid-sulfate

alteration. Systems may be Au-dominated (LS or HS) or Ag-dominated (LS).

COMMODITIES (principal recovered products; by-products in parentheses):

LS: Au, Ag, Zn, Pb, (Cu, Sb, As, Hg, Se)

HS: Cu, Au, Ag, As, (Pb, Hg, Sb, Te, Sn, Mo, Bi)

EXAMPLES (including notable deposits outside of the Great Basin):

Vein mineralization generally more common than disseminated:

Aurora (Nevada), Bullfrog (Nevada; Ag-dominated), Comstock (Nevada; Ag-dominated), Goldfield

(Nevada), Rawhide (Nevada), Rochester (Nevada; Ag-dominated), Sleeper (Nevada), Tonopah (Nevada;

Ag-dominated), Oatman (Arizona), Bodie (California), Zaca (California; Ag-dominated), Cripple Creek

(Colorado), Silverton-Telluride (Colorado), Summitville (Colorado), Silver City-Delamar (Idaho; Agdominated), Republic (Washington), Guadalupe y Calvo (Chihuahua), Guanacevi (Durango; Ag-dominated),

Tayoltita (Durango; Ag-dominated), Guanajuato (Guanajuato; Ag-dominated), Pachuca (Hidalgo; Agdominated), El Oro (Michoacan), Abangares (Costa Rica), San Sebsatian (El Salvador), El Rosario

(Honduras; Ag-dominated), Coco mine (Nicaragua), and Pis Pis (Nicaragua).

Disseminated mineralization generally more common than vein:

Hollister-Ivanhoe (Nevada), Paradise Peak (Nevada), Round Mountain (Nevada), Golden Sunlight

(Montana), Los Cacaos (Dominican Republic), Montana Tunnels (Montana), and Pueblo Viejo (Dominican

Republic).

TYPICAL GRADE AND TONNAGE (Troy ounces; U.S. short tons):

LS generally higher than HS

Vein deposits:

Gold: 0.06-0.79 to 1.7; up to 2.24 oz/t 

Silver: 0.047-0.3 to 3-30; up to 73 oz/t 

Tonnage: 0.07-8.8 to 20-90; up to 118 million tons 

Disseminated deposits:

Gold: 0.034-0.14 to 0.55; up to 1.1 oz/t

Silver: 0.08-3.5 to 5 oz/t

Tonnage: 0.12 to 12.3-86; up to 195 million tons 

ASSOCIATED DEPOSIT TYPES (other genetically related deposit types):

Volcanic rock-hosted vein and disseminated mineralization commonly co-occur. Mantos and vein deposits in

sedimentary rocks may occur with vein deposits. Granite molybdenite systems may occur with vein and/or

disseminated deposits hosted by high-silica rhyolite. Sedimentary rock-hosted disseminated Au may be

spatially associated with volcanic rock-hosted disseminated Au mineralization.
187

Table 3.2, continued.

GEOLOGIC SETTING:

LS: Geothermal environment.

HS: Volcanic-hydrothermal environment, closer to volcanic centers.

Volcanic centers in tectonic mobile belts or in anorogenic or extensional tectonic regimes. Commonly, but not

necessarily, associated with volcanic collapse structures.

MINERALIZED AND HOST ROCKS:

Felsic to intermediate calc-alkaline volcanics most common; in some cases alkaline volcanics (e.g.—Cripple

Creek, Colorado). Accompanying mafic volcanics, subvolcanic and epizonal plutons, volcaniclastic, and

sedimentary rocks sometimes mineralized. Includes rhyolite, rhyodacite, latite, dacite, andesite, trachyandesite,

trachyte, phonolite, basalt, granite, shale, sandstone, conglomerate. Acid-altered rock, a silica residue termed

vuggy quartz, is commonly the ore host in HS deposits. High-silica rhyolite source rocks typically produce Agdominated deposits.

ASSOCIATED ROCKS (spatially associated non-mineralized igneous rocks):

Intermediate to felsic calc-alkaline or alkaline pre-ore volcanic, subvolcanic, and plutonic rocks; mafic rocks

may be present. Includes rhyolite, rhyodacite, latite, dacite, andesite, trachyte, phonolite, monchiquite, vogesite,

basalt, olivine basalt, tonalite, granodiorite, diorite, monzonite, and syenite. Post-ore igneous rocks rare.

FORM OF DEPOSIT (ore body and gold mineralization):

LS: Open-space vein fillings dominant, stockwork ore common. Disseminated and replacement ore minor.

HS: Disseminated ore dominant, replacement ore common. Stockwork ore minor, veins commonly

subordinate.

Vein deposits:

Au-bearing quartz-carbonate-sulfide veins in, or spatially associated with various faults (high-angle normal,

listric normal, detachment structures, and strike-slip), shear zones, breccia zones, fracture zones, and

diatremes. Formed at relatively shallow depths (< 1 km) below paleosurface by open-space filling. Includes

stringers, stockworks, sheeted veinlet systems, and single or multiple anastomosing, sygmoidal, and

bifurcating veins; rhythmic banding, crustification, and comb textures typical. Larger veins contain highgrade ore shoots, and involve replacement mineralization. Disseminated mineralization surrounding veins

common. Except in alkaline rock-related systems (e.g.—Cripple Creek, Colorado), vein systems show

distinct vertical zoning; a "barren gap" occurs below bonanza Au and other Au-Ag mineralization, and above

deeper Ag-rich and base-metal mineralization. The productive horizon within a district is typically domal

and delimited by changes in grade and type of mineralization. Some systems Au-dominated near surface,

passing downward into Ag-dominated, grading into base-metal dominated at depth. Zonation may be

telescoped, or more generally expressed as a precious-metal zone overlying a base-metal zone. Zonation may

not be consistent within an individual orebody, within a deposit, or between deposits within a district.

Volcanic rock-hosted disseminated deposits gradational with vein deposits.

Disseminated deposits:

Disseminations of medium- to fine-grained ore minerals in diatremes, breccia zones, surrounding vein and

shear zones, and proximal to changes in attitude of major vein systems; usually within zones of pervasive

silicification. Disseminations may also occur as uniformly dispersed or wispy masses within large vein

systems. Quartz-carbonate veins commonly present. Most disseminated deposits show precious and basemetal zonation as a function of depth; Au-dominated near paleosurface, passing downward into Agdominated, grading into base-metal dominated at depth. Zoning may be telescoped, or more generally

expressed as a precious-metal zone overlying a base-metal zone. Zonation may not be consistent between

deposits in same district. Volcanic rock-hosted vein deposits gradational with disseminated deposits. 
188

Table 3.2, continued.

ORE CONTROLS:

Vein deposits:

Structure; fracture, fault, breccia, and shear zones. Changes in pressure-temperature-composition conditions.

Disseminated deposits:

Usually structure, but may be concealed. Permeable zones; breccia pipes, shear and fracture zones. Areas

surrounding major bends or strike change of vein systems. Changes in pressure-temperature-composition

conditions.

MINERALOGY (mode of gold occurrence in ore; other metallic, accessory and gangue minerals):

Vein deposits:

Native Au: as microscopic to visible grains, plates, wires, clots, fracture fillings, and crystalline masses

interstitial to and replacing gangue; as inclusions in, and as fine grained coating on, pyrite and other sulfides.

Au-bearing tellurides (calaverite, krennerite, petzite, sylvanite, and nagyagite) major ore minerals in alkaline

rock-related systems. Other important ore minerals may include native Ag, argentite, and Ag-bearing As-Sb

sulfosalts and selenides. Cinnabar, realgar, stibnite, and tetrahedrite may be locally important. Principal basemetal sulfides and ferroalloy minerals include pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, and

bornite. Main gangue minerals are quartz and calcite. Fluorite, barite, alunite, and adularia locally may be

main gangue minerals. Cryptocrystalline quartz, various carbonates, and sheet silicates also present in

gangue. Ag-dominated deposits are distinguished from Au-dominated deposits by relative abundance of

sulfosalts (especially ruby silvers) and base-metal sulfides, and by the general absence of As-, Sb-, and Hgbearing phases. Au-dominated deposits are relatively enriched in As- and Sb-bearing minerals and may be

depleted in base-metal sulfides. Ag-dominated deposits may be relatively enriched in Se-bearing phases

whereas Au-dominated deposits may show Te-bearing phases.

Disseminated deposits:

Native Au: as submicroscopic to microscopic grains of Au dispersed in gangue (sometimes encapsulated

in quartz), as inclusions in disseminated pyrite, and less commonly as fine-grained coatings on sulfide grains.

Au-bearing tellurides (calaverite, krennerite, petzite, sylvanite, and nagyagite) locally major ore minerals.

Other important ore minerals include native Ag, argentite, and Ag-bearing As-Sb sulfosalts and selenides.

Cinnabar, realgar, stibnite, and tetrahedrite may be locally important. Principal base-metal sulfides and

ferroalloy minerals include pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, and bornite. Main gangue

minerals are quartz and calcite. Fluorite, barite, alunite and adularia locally may be main gangue minerals.

Cryptocrystalline quartz, various carbonates, and sheet silicates also present in gangue. Ag-dominated

deposits are distinguished from Au-dominated deposits by relative abundance of sulfosalts (especially ruby

silvers) and base-metal sulfides, and by the general absence of As-, Sb-, and Hg-bearing phases. Audominated deposits are relatively enriched in As- and Sb-bearing minerals and depleted in base-metal

sulfides. Ag-dominated deposits may be relatively enriched in Se-bearing phases whereas Au-dominated

deposits may show Te-bearing phases. 

LS: Low-sulfidation state minerals. Distinguished from HS by common occurrence of Zn-Pb minerals

(sphalerite and galena), and electrum. Ore mineralogy generally less diverse than HS, as reflected by

higher Ag/Au ratio.

HS: High-sulfidation state sulfide minerals. Distinguished from LS by common occurrence of Cu-As

minerals (enargite-group minerals, chalcopyrite, tennantite, covellite) and Au-Ag tellurides (usually

present but seldom in major amounts).

ALTERATION:

LS: Quartz-adularia-sericite±carbonate assemblage shows silicification and K-metasomatism in or

immediately adjacent to veins or near disseminated ore bodies, and grades outward to kaolinite and illite

or sericite marginal to ore bodies, to outermost propylitic zone (possibly extensive). Adularia is the

characteristic alteration mineral, which does not occur in HS systems and indicates near neutral pH and

reducing conditions.
189

Table 3.2, continued.

HS: Advanced argillic assemblages (quartz-alunite±sericite±kaolinite±S±barite±

pyrophyllite±zunyite±diaspore) in and adjacent to veins or near disseminated ore bodies, and grades

outward to sericitic-argillic zone, to outermost propylitic zone. Outermost propylitic zone commonly

of regional extent. Alunite is the characteristic alteration mineral, which is not common to LS systems

and indicates relatively acidic and oxidizing conditions.

AGE RANGE:

Host: Tertiary; few Paleozoic and Mesozoic (Tertiary in Nevada)

Ore: Same as host rock; most formed between 27 to 5 Ma in Nevada

GENETIC MODEL:

Au mineralization at shallow depths (within ~1 km) below paleosurface in geothermal or hot springs system

(LS) or volcanic hydrothermal system (HS). Mineralizing system forms during waning stages of magmatic

events, contemporaneous to extension, and during waning extensional activity. Formed in relatively short order

(0.05-0.5 to 3 m.y.) under conditions of unrestricted flow (high-permeability). Deeply circulating fluids leach

metals at depth from volcano-sedimentary pile (underlying rocks) and exit along structural channel ways.

Mineralization is episodic, taking place when system ruptures, rapidly releasing confining pressure and

decreasing temperature. After physiochemical equilibrium is achieved, the system seals for another cycle of

mineralization. Ore minerals precipitate as fluids boil (effervesce) and/or mix (dilution) with cooler oxygenated

ground waters. Bisulfide, thiosulfide, or other complexes that are not well understood (e.g.—cyanide or

arsenide complexes) may transport Au in upper levels of system; chloride complexes in lower levels. Vertical

metal zoning common; base-metal zone often underlies precious metal zone.

LS: Form from near neutral-pH, reduced waters close to equilibrium with host rocks. Fluids are low-salinity

(<1 wt% NaCl equiv.), relatively gas-rich (1-2 wt% CO2, subordinate H2S), at 140 to 325o

C (240oC

average during precious metal deposition), and are meteoric-water dominant; however, there is evidence

for an early magmatic component that may also introduce ore metals to the system. Boiling is the

critical process to deposit high concentrations of Au.

HS: Processes responsible for Au deposition are not understood since the ore fluid has not been

characterized fully, though ore minerals, and hence ore fluid, evolve with time to less acidic and more

reduced conditions. Have a significant magmatic component throughout their history, particularly in

the early, leaching stage when the fluid is most acidic and oxidized. Meteoric-water involvement may

increase as magmatic plume collapses. A continuum may exist between hydrothermal processes related

to mineralization in a porphyry to an epithermal environment (suggesting critical link to late magmatic

fluid input).

COMPILED FROM:

Berger and Eimon (1982), Berger and Henley (1989), Berger and Bonham (1990), Bonham (1988), Buchanan

(1981), Cox and Singer (1986), Cox et al. (1991), Dewelley (1984), Douglas et al. (1982), Durning and

Buchanan (1984), Gott et al. (1969), Hayba et al. (1985), Heald et al. (1987), Hedenquist (1997), Hedenquist

and Lowenstern (1994), Hedenquist et al. (1996), Henry et al. (1997), Henley (1983), John et al. (1991), Henry

et al. (1997), Kesler et al. (1981), Krupp and Seward (1987), Lindgren and Ransome (1906), Ludington et al.

(1993), Mills (1984), Mosier et al. (1986), Nash et al. (1991), O'Neil et al. (1973), Panteleyev (1986), Porter

and Ripley (1985), Randall (1972), Russell et al. (1986), Seedorff (1991), Seward (1989), Silberman (1983),

Sillitoe et al. (1985), Sillitoe (1995), Smith et al. (1982), Stoffregen (1987), Swinden (1984), Taylor (1996),

Thompson et al. (1985), Thorman and Christensen (1991), Tingley and Berger (1985), White (1981), and

White and Heropoulos (1983).
Table 5.1. The geoscience datasets composing the Nevada and Great Basin GIS database. The data are grouped by discipline and chara

data composing a dataset; DATA TYPE%the scale of measurement in which the data are expressed; OBJECT-TYPE%the spatial object-type of 

processing with SPANS GIS; SOURCE DATA%the file format, and/or spatial data model (raster or vector), and/or spatial object-type of the 

before pre-processing and integration into the database; NUMBER OF ENTITIES%the number of sample points or arcs making up the data

size in bytes of the source data file; COVERAGE%the areal extent of the dataset and distribution of points where appropriate; DATASET S

source dataset and/or source of the data composing the dataset.

DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

REGIONAL GEOLOGY

Bedrock and Surficial

Geology Nominal Line, Area ARC INFO, Vectors 84,955 36,778,743 Nevada Turner 

Metamorphic Rocks Nominal Line, Area SPANS, Digitized Lines 158 108,228 Nevada 

Metamorphic Core

Complex Locations Binary Point SPANS, Digitized Points 25 2,145 Great Basin A

Corridor of Metamorphic

Core Complex

Distribution

Binary Line, Area SPANS, Digitized Lines 1 490 Great Basin A

Volcanic Centers Nominal Line, Area SPANS, Digitized Lines 77 46,629 Nevada 

Cinder Cones Binary Point SPANS, Digitized Points 19 757 Nevada 

Mesozoic Plutons

Distribution Percentage Ordinal Line, Area SPANS, Digitized Lines 3 3,456 Nevada 
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

Cordilleran Geosyncline

Facies Boundaries Nominal Line SPANS, Digitized Lines 3 18,052 Nevada Thorman 

Regions of Strong Upper

Crustal Extension Binary Line, Area SPANS, Digitized Lines 24 33,300 Nevada W

Fault Coverage&

Nevada Binary Line ARC INFO, Vectors 13,471 ~ 3,116,375 Nevada Sawatzky (199

from Ste

Fault Coverage&

Historical Binary Line

Generic ASCII

Plotter Output

File

116 39 Nevada Dohrenwend (19

from 

Fault Coverage&

Holocene Binary Line

Generic ASCII

Plotter Output

File

489 118,242 Nevada Dohrenwend (19

from 

Fault Coverage&

Holocene - Late

Pleistocene

Binary Line

Generic ASCII

Plotter Output

File

860 209,004 Nevada Dohrenwend (19

from 

Fault Coverage&

Late Pleistocene Binary Line

Generic ASCII

Plotter Output

File

1,169 319,494 Nevada Dohrenwend (19

from 

Fault Coverage&

Offsetting Volcanics Binary Line

Generic ASCII

Plotter Output

File

2,374 604,221 Nevada Dohrenwend (19

from 

Fault Coverage&

Undifferentiated Binary Line

Generic ASCII

Plotter Output

File

1,979 509,379 Nevada Dohrenwend (19

from 
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

Fault Coverage&

Undated Binary Line

Generic ASCII

Plotter Output

File

2,778 773,232 Nevada Dohrenwend (19

from 

Fault Coverage&

Regional Binary Line ARC INFO, Vectors 18,549 4,291,117 Great Basin Sawatzky (19

Strike-Slip Faults Binary Line SPANS, Digitized Lines 96 9,376 Nevada Putnam I

Modifi

Golconda Thrust Front Binary Line SPANS, Digitized Lines 1 1,568 Great Basin 

Roberts Mountain Thrust

Front Binary Line SPANS, Digitized Lines 1 1,247 Great Basin 

Deep-Seated Basement

Fracture Systems

(Interpreted from

aeromagnetic data)

Binary Line SPANS, Digitized Lines 126 12,834 Nevada 

Furnace Creek Fault Binary Line SPANS, Digitized Lines 1 6,051 Great Basin P

Garlock Fault Binary Line SPANS, Digitized Lines 1 4,917 & P

Las Vegas Fault Binary Line SPANS, Digitized Lines 1 2,867 Nevada P

San Andreas Fault Binary Line SPANS, Digitized Lines 1 19,629 & P
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

Stewart Valley Fault Binary Line SPANS, Digitized Lines 1 3,056 Great Basin P

Snake River Fault Binary Line SPANS, Digitized Lines 1 4,674 & P

Antler Orogenic Belt Binary Line SPANS, Digitized Lines 1 20,388 Great Basin P

Sevier Orogenic Belt Binary Line SPANS, Digitized Lines 8 37,023 Great Basin P

Sonoma Orogenic Belt Binary Line SPANS, Digitized Lines 1 21,630 Great Basin P

Northeastern Snake River

Plain Outline Binary Line SPANS, Digitized Lines 1 17,439 Great Basin Oppenheimer (1

s

Lithotectonic Terranes Nominal Line, Area ARC INFO, Vectors 145 222,475 Great Basin Raines (1995

from Silberling 

Attitude of Tertiary Rocks Interval Point, Surface SPANS, Digitized Points 392 13,695 Nevada, Irregular Distribution 

PHYSICAL

GEOGRAPHY

Elevation / Topography 

(30 Arc-Second Grid)

Interval Surface Raster & 77,000,000

Western US,

Great Basin

 ~ 801 m Grid

U.S. Ge

Elevation / Topography 

(5 Arc-Minute Grid) Interval Point, Surface ASCII Flat File, Points 11,664 315,221 Great Basin ~ 8 km Grid Hitt
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

Mountain Peak Heights Interval Point, Surface SPANS, Digitized Points 417 26,697 Nevada U.S. Ge

Great Basin Physiographic

Province (Generalized) Binary Line, Area SPANS, Digitized Lines 1 4,427 Great Basin 

Great Basin Physiographic

Province (Detailed) Binary Line, Area SPANS, Digitized Lines 1 12,077 Great Basin The

GEOPHYSICS

Gravity Anomaly - 

Observed, Free Air,

Bouguer, Isostatic

Interval Point, Surface ASCII Flat File, Points 79,704 8,156,456 Nevada, Irregular Distribution Hitt

Geomagnetism - Total

Residual Field Anomaly Interval Point, Surface ASCII Flat File, Points 288,983 8,669,771 Nevada, 1 km Grid Buhmann (19

Geothermal Heat Flow Interval Point, Surface ASCII Flat File, Points 1,080 85,933 Great Basin, Irregular Distribution Blackwell (199

Hitt

Geothermal Conductivity

(Gradient) Interval Point, Surface ASCII Flat File, Points 1,059 86,124 Great Basin, Irregular Distribution Blackwell (19

Geothermal Heat

Production Interval Point, Surface ASCII Flat File, Points 1,059 86,124 Great Basin, Irregular Distribution Blackwell (19

Geothermal Wells/Springs

Temperature Interval Point, Surface SPANS, Digitized Points 795 77,609 Nevada Tre

B
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

Paleothermal Anomaly Binary Line, Surface SPANS, Digitized Lines 1 3,846 Nevada Cunningham 

SEISMOLOGY

Crustal Thickness

(Depth to Reflection Moho) Interval Point, Surface SPANS, Digitized Points 103 3,663 Great Basin, Irregular Distribution Allenby and S

Seismicity

(Earthquake density, depth to focus,

and magnitude)

Interval Point, Surface ASCII Flat File, Points 15,045 681,140 Great Basin Hitt

Orientation of Minimum

Principal Crustal Stress

( 0-5 Ma, 5-38 Ma, 38-65 Ma)

Interval Line SPANS, Digitized Lines 118 11,991 Nevada, Irregular Distribution Eaton (19

GEOCHEMISTRYRadiometric Ages&

Igneous Rocks (RAD-B) Ratio Point, Surface ASCII Flat File, Points 4,819 1,041,127 Great Basin, Irregular Distribution Ward (1991

Radiometric Ages&

Metallic Mineralization Ratio Point, Surface ASCII Flat File, Points 157 59,644 Nevada, Irregular Distribution

Bonham (1982); BJones (1989); KuMosier et al. (198al. (1976); Stageand Christensen (al. (1988); White (1983); Wilkins (
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

Strontium Isotope&

87Sr/86Sr Initial Values Ratio Point, Surface ASCII Flat File, Points 511 52,847 Great Basin, Irregular Distribution

Bacon et al (1984)Elison et al (1990)Kistler (1993, persRoss (1990); KistlLeeman (1970); (1987)

87Sr/86Sr ISr = 0.706

Isopleth Nominal Line SPANS, Digitized Lines 3 7,969 Great Basin Farmer 

Kist

87Sr/86Sr ISr = 0.706

Isopleth, Restored Nominal Line SPANS, Digitized Lines 1 1,486 Great Basin Levy an

87Sr/86Sr ISr = 0.708

Isopleth Nominal Line SPANS, Digitized Lines 1 783 Great Basin Farmer 

Kist

NURE

(Major and Minor Element

Geochemistry)

Ratio Point, Surface ASCII Flat File, Points Varies by Element > 10,000 Nevada Hof

PETROS

(Igneous Rock Major

Element Geochemistry)

Ratio Point, Surface ASCII Flat File, Points 37,300 8,196,802 Global Mut

REMOTE SENSING

Linear Features From

LANDSAT Imagery Binary Line ARC INFO, Vectors 6,398 1,410,596 Great Basin Sawatzky (19

Radar Imagery & & (on microfiche) & & Portions of Nevada U.S. Geo
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

AVHRR

Satellite Imagery Nominal Surface Raster & Variable Conterminous US U.S. Geo

ECONOMIC GEOLOGY

MRDS Metallic Mineral

Occurrences

Binary,

Nominal,

Ordinal

Point, Surface ASCII Flat File, Points 6,197 7,295,344 Nevada U. S. Geolo

Mineralization Belts and

Trends Binary Line SPANS, Digitized Lines 22 4,816 Nevada Roberts (1966

MAS Minerals Availability

System

Binary,

Nominal,

Ordinal

(Attributes)

ASCII

Flat File 7,571 6,269,300 Nevada U. S. B

Million Ounce

Gold Camps

(current through 1984)

Binary,

Ratio Point ASCII Flat File, Points 15 9,000 Nevada M

HYDROLOGY

Drainage Divides Nominal Line, Area SPANS, Digitized Lines 46 108,489 Nevada A

Streams Binary Line DLG, Vectors 2,363 2,780,800 Nevada U.S. Geo

Water Bodies Binary Line, Area DLG, Vectors 1,164 1,246,560 Nevada U.S. Geo

HUMAN FEATURES

Major Cities Binary Point SPANS, Digitized Points 153 10,445 Nevada Nicho
DATA SET DATA

TYPE

OBJECTTYPE SOURCE DATA NUMBER OF ENTITIES SIZE (bytes) COVERAGE DATA

Table 5.1, continued. 

County Borders Nominal Line DLG, Vectors 327 386,880 Nevada U.S. Geo

Nevada State Border&

High Resolution Binary Line, Area ARC INFO, Vectors 1 342,000 Nevada Turner 

Nevada State Border&

Low Resolution Binary Line DLG, Vectors 1 54,170 Nevada U.S. Geo

Borders of

Neighbouring States Nominal Line DLG, Vectors 176 57,738 & U.S. Geo

Roads and Highways Binary Line DLG, Vectors 1,008 1,054,880 Nevada U.S. Geo

Railways Binary Line DLG, Vectors 397 449,920 Nevada U.S. Geo
199

Sample Primary

Gold-Silver

Sedimentary

Rock-Host.

Volcanic

Rock-Host.

SubSample

Large 59 8 33

Medium 118 30 43

Small 2269 57 317

Unknown 244 3 22

Grand Totals: 2690 98 415

Table 5.2. Total number of observations in all gold-silver-bearing occurrence-type samples.
Binary

Map

Area of

Pattern

Present

(km2)

Occurrences W+ Stdev (W+) W– Stdev (W–) C Stdev (C) Stud C

C/Stdev(C)

Description of Binary-Class Pred

Primary (Occurrences of all Sizes and Types) Mineral Potential Model Evidence Layers and Associate

BANABIPR 21740 445 0.7790 0.0479 -0.1019 0.0212 0.8809 0.0524 16.8176 Ba/Na geochemical anomaly (mainly soil and sediment samples)

BFFTBIPR 77225 1361 0.6264 0.0273 -0.3898 0.0275 1.0162 0.0388 26.1929 Buffer zones surrounding faults(all types and ages; 0.0, 0.5, 1.0, 1.5BFPLBIPR 21185 810 1.4221 0.0358 -0.2829 0.0231 1.7050 0.0427 39.9717 Buffer zones surrounding plutons (mainly Mesozoic; 0.0, 0.5, 1.0, 1GEDVBIPR 43805 1070 0.9598 0.0310 -0.3418 0.0249 1.3016 0.0397 32.7513 Diversity of lithology (units per 2.5 X 2.5 km neighbourhood cell; geGEOBIHI 2925 342 2.6252 0.0575 -0.1267 0.0207 2.7520 0.0612 44.9958 Lithology—higher W+ (geologic map of Nevada)

GEOBILOW 11473 349 1.1853 0.0544 -0.0985 0.0208 1.2838 0.0582 22.0630 Lithology—lower W+ (geologic map of Nevada)

GEOBIMED 12761 709 1.8140 0.0386 -0.2620 0.0226 2.0760 0.0447 46.3991 Lithology—medium W+ (geologic map of Nevada)

GISOBIPR 152680 1792 0.2139 0.0238 -0.3252 0.0335 0.5391 0.0411 13.1306 Isostatic gravity anomaly

KNABIPR 4398 153 1.3241 0.0823 -0.0433 0.0199 1.3674 0.0847 16.1493 K/Na geochemical anomaly (mainly soil and sediment samples)MAGBIPR 81957 1145 0.3904 0.0298 -0.2147 0.0255 0.6052 0.0392 15.4309 Total residual field geomagnetic anomalyTRRNBIPR 76290 1166 0.4816 0.0295 -0.2565 0.0257 0.7381 0.0391 18.8573 Lithotectonic (allochthonous) terranes

Sedimentary Rock-Hosted Mineral Potential Model Evidence Layers and Associated Weights

BANABISD 10604 34 2.2292 0.1718 -0.3880 0.1250 2.6173 0.2125 12.3195 Ba/Na geochemical anomaly (mainly soil and sediment samples)

BFFTBISD 77225 56 0.7403 0.1337 -0.5291 0.1543 1.2693 0.2042 6.2171 Buffer zones surrounding faults(all types and ages; 0.0, 0.5, 1.0, 1.BFPLBISD 21185 15 0.7164 0.2583 -0.0884 0.1098 0.8048 0.2807 2.8675 Buffer zones surrounding plutons (mainly Mesozoic; 0.0, 0.5, 1.0, 1GEDVBISD 43805 43 1.0433 0.1526 -0.4097 0.1349 1.4531 0.2036 7.1359 Diversity of lithology (units per 2.5 X 2.5 km neighbourhood cell; geGEOBISD 14367 64 2.5593 0.1253 -1.0068 0.1715 3.5661 0.2124 16.7901 Lithology (geologic map of Nevada)GISOBISD 21401 40 1.6882 0.1583 -0.4461 0.1313 2.1343 0.2057 10.3783 Isostatic gravity anomalyKNABISD 923 12 3.6390 0.2906 -0.1274 0.1078 3.7664 0.3099 12.1521 K/Na geochemical anomaly (mainly soil and sediment samples)MAGBISD 119721 71 0.5390 0.1187 -0.7399 0.1925 1.2789 0.2261 5.6556 Total residual field geomagnetic anomalyTRRNBISD 50690 71 1.3993 0.1188 -1.0922 0.1925 2.4914 0.2262 11.0165 Lithotectonic (allochthonous) terranes

Table 7.1. Evidence layers for the primary and sedimentary rock-hosted occurrence-type mineral potential models. Light grey rows represefrom the models in an attempt to mitigate severe conditional dependence (see section 7.4.2 and Appendix C for details). White rowsreprese

in the primary 7-layer (top) and sedimentary rock-hosted 8-layer (bottom) mineral potential models. White and light grey rows represent the primary 11-layer (top) and sedimentary rock-hosted 9-layer (bottom) mineral potential models (i.e.—conditional dependence was not factors used to combine binary-class predictor maps are highlighted in bold.
Binary

Map

Area of

Pattern

Present

(km2)

Occurrences W+ Stdev (W+) W– Stdev (W–) C Stdev (C) Stud C

C/Stdev(C)

Description of Binary-Class Pred

Volcanic Rock-Hosted Mineral Potential Model Evidence Layers and Associated Weights

BANABIVL 10604 17 0.0901 0.2427 -0.0037 0.0502 0.0937 0.2479 0.3782 Ba/Na geochemical anomaly (mainly soil and sediment samples)

BFFTBIVL 77225 205 0.5954 0.0699 -0.3632 0.0690 0.9587 0.0983 9.7550 Buffer zones surrounding faults(all types and ages; 0.0, 0.5, 1.0, 1.BFPLBIVL 21185 81 0.9615 0.1113 -0.1396 0.0548 1.1010 0.1241 8.8751 Buffer zones surrounding plutons (mainly Mesozoic; 0.0, 0.5, 1.0, 1GEDVBIVL 43805 110 0.5397 0.0955 -0.1401 0.0573 0.6799 0.1113 6.1061 Diversity of lithology (units per 2.5 X 2.5 km neighbourhood cell; geGEOBIVL 21955 194 1.8042 0.0721 -0.5500 0.0673 2.3543 0.0986 23.8679 Lithology (geologic map of Nevada)GISOBIVL 154093 260 0.1413 0.0621 -0.1997 0.0804 0.3410 0.1015 3.3578 Isostatic gravity anomalyKNABIVL 3274 9 0.6304 0.3338 -0.0103 0.0497 0.6408 0.3375 1.8987 K/Na geochemical anomaly (mainly soil and sediment samples)MAGBIVL 50725 136 0.6054 0.0859 -0.2000 0.0599 0.8054 0.1047 7.6925 Total residual field geomagnetic anomalyTRRNBIVL 41229 191 1.1543 0.0725 -0.4598 0.0668 1.6141 0.0986 16.3648 Lithotectonic (allochthonous) terranes

Table 7.1, continued. Evidence layers for the volcanic rock-hosted occurrence-type mineral potential model. Light grey rows represent pr

the model in an attempt to mitigate severe conditional dependence (see section 7.4.2 and Appendix C for details). White rows represent 

the volcanic rock-hosted 7-layer mineral potential model. White and light grey rows represent predictor maps included in the volcanic rpotential model (i.e.—conditional dependence was not mitigated). The weighting factors used to combine binary-class predictor maps a
50o

45o

40o

35o

-115o

-120o

-125o

-110o -105o

App ro xima te Limit s of the Southwest ern Region of th

it

e

e Un

e

St a t

d

s

App ro xima te Limit s of the Southwest ern Region

of

th

it

e

e

Un

e

St a

t

d

s

Great Basin

Great Basin Physiographic Province,

Southwestern United States

Figure 1.1. Location map of the Great Basin (highlighted in darker grey). The State of Nevada is lighter grey

and outlined in black. For the purpose of this study, the "southwestern" United States encompasses the region

south and west of the grey dashed line, particularly the Great Basin, Colorado Plateau, and southern Basin and

Range provinces, as well as part of the Pacific Mountain System, and small portions of the Columbia and

Snake River Plateaus and the Rocky Mountain System. The "western" United States is herein defined as

those states composing this base; Washington, Idaho, Montana, Oregon, Wyoming, California, Nevada,

Utah, Colorado, Arizona, and New Mexico. The Great Basin covers ~ 452,340 km (as circumscribed by

Thelin and Pike, 1991). Nevada covers ~ 283,219 km . The Great Basin occupies about 95% of Nevada.

Great Basin boundary from Thelin and Pike (1991).

2

2

202

200 km
Figure 1.2. Shaded relief of the topography of the Great Basin, illuminated by an artificial light source

located at 290 azimuth and 45 zenith. topography consists of alternating, unusually

even-spaced, parallel mountain ranges and intervening broad, flat, alluviated desert basins. The ranges and

valleys trend roughly north-south ( 10 ) and are typically 15-20 to 50 km across, giving the Great Basin a

pronounced north-northeast south-southwest structural grain. The ranges are large coherent tilted fault

blocks, bound on one or more sides by high-angle normal or reverse faults, and the basins are adjacent

grabens and half-grabens. The tilts of the fault blocks range from a few degrees to about 30 degrees from

horizontal. Basin-range topography was not attained before ~10 Ma (at the earliest), and formed as a result of

block-faulting during late (~17-0 Ma) brittle extension of the upper crust. Text modified from Stewart

(1978), Wilkins (1984), and Zoback et al., (1981). Great Basin outline from Thelin and Pike (1991). Digital

elevation data from U.S. Geological Survey (1996).

o o

o

"Basin and Range"



n

"Basin and Range" Topography

203

200 km
Principal Metallogenic

Features of the Great Basin

Figure 1.3.

1

2

3

Principal precious-metal deposit trends, locations, and metallogenic subdivisions in the Great

Basin. The Great Basin metallogenic province can be divided into an eastern base-metal and a western

precious-metal metallogenic sub-province (dotted black line). The east-west sub-province division is mainly

defined by the preferential distribution of gold-bearing deposits for the western half of the basin. The

main sedimentary rock hosted deposits are aligned along the Carlin and Battle Mountain-Eureka (Cortez)

mineral trends. The main volcanic rock-hosted deposits occur within (or proximal to) the Walker Lane

belt, a broad zone of shear deformation (wavy fill pattern). Nevada in light gray shading. Great Basin in dark

gray shading. Metallogenic sub-provincial boundary adopted from Roberts (1966). Walker Lane belt from

Stewart (1988). Great Basin boundary from Thelin and Pike (1991).

n

n

n

Precious-Metal

Sub-Province

Base-Metal

Sub-Province

Battle Mtn. - Eureka

(Cortez) Trend

Carlin

Trend

Walker

Lane

Belt

204

200 km
Cordilleran Orogenic Belt

North America

Figure 2.1.

central

Generalized extent of the Cordilleran orogen in North America (darkest and lightest gray). The

Great Basin physiographic province (medium gray) lies in the Cordillera, as opposed to regions

north and south. The Cordilleran orogenic belt proper (western hinterland and eastern foreland fold-thrust

belt) in darkest gray. The Cordilleran foreland (lightest gray, typically related to the Laramide phase of the

Cordilleran orogeny) lies northeast of and beneath the eastern foreland fold-thrust belt, and is assumed to be

external to the hinterland and the foreland fold-thrust belt because brittle deformation structures in the

foreland are probably not continuous with those of the orogenic belt proper. Dashed line indicates eastern

limit of foreland (thrust front may or may not be present).

Text and figure modified from Drewes (1991). Great Basin boundary

from Thelin and Pike (1991).

" "

The Cordilleran orogenic belt is one of the two

largest orogenic belts in the world, and extends several thousands of kilometers from northern Alaska

(possibly even from Siberia), down through Canada, the United States, and Mexico, across the Bartlett

transform in Central America, southward along the western margin of South America into Chile, and ends(?)

on the Palmer Peninsula ofAntarctica.

Central

Cordillera

Southern

Cordillera

Northern

Cordillera

205
The Great Basin and Environs

Figure 2.2. Major physiographic divisions (heavy weight lines), provinces (medium weight lines),

subprovinces (dotted lines), and State boundaries (light weight lines) of the western United States. Great

Basin physiographic province in dark gray shading. State of Nevada in light gray shading. Province

boundaries from Thelin and Pike (1991). Province names from Fenneman and Johnson (1946).

R o c k y M o u n t a i

n

S

y s

t

e

m

C

olum

bia

&

Snake River Plateaus

P

a

c i f i c

M

o

u

n

t

a i n

S

y s t e m

Southern Basin and Range

Colorado Plateau

Great Basin

206

200 km
Plate Tectonic Setting

and Selected

Tectonomagmatic Elements

of the

Western United States

Figure 2.3. Present-day tectonic setting of the westernUnited States showing the approximate locations andextents of major Precambrian and Phanerozoic tectonicand magmatic elements. The age of a tectonic feature, inparentheses, represents the approximate time of initialactivity.

The dashed line labelled "0.706" is the initial

Sr/ Sr isopleth = 0.706 for Mesozoic plutons, and

approximates the edge of the Precambrian crust (fromMutschler et al., 1992; modified from Carlson et al.,1991; Fleck and Criss, 1985; Kistler, 1990; Kistler andPeterman, 1973; Farmer and DePaolo, 1983). Thedash-double-dot line labelled "0.708" is the Nd = -7 andinitial Sr/ Sr = 0.708 contour, which is interpreted asthe edge of the Precambrian crust by Farmer andDePaolo (1983). Compiled from Armstrong and Ward(1991); Atwater (1970); Axen et al. (1993); Burchfielet al. (1992); Christiansen and Yeats (1992); Cowanand Bruhn (1992); Dilles and Gans (1995); Eaton(1982); Hoffman (1989); Lipman (1992); Mann andMeyer, (1993); Miller et al. (1992); Mutschler et al.(1987); Reidel and Tolan, (1994); Stewart (1988);Stewart et al. (1975); Stewart and Crowell (1992);Suppe (1985); Thelin and Pike (1991); Thorman andChristensen (1991); Warner (1978); Zoback et al.(1981); and Zoback et al. (1994). Older age forinception of Garlock fault from Craig Jones (1994,personal communication). Trace of Snake River faultfrom Poole et al. (1992).

In the case of trends (i.e. Yellowstone hot spot

trend), the ages span the period of earliest to latestactivity.

n

87 86

87 86



n N.

i

M

n

e x i co)

a i

. cep i .. n

t

~29 M

o

n N.

i M

n e x i co) 

a i

. cep i .. n t ~29 M o

0.706

Northern Rio Grande

Rift Zone (~35-26 Ma)

Jemez Lineament (

)

0.706

0 7. 08

Garlock lt

Fau Zone (16-10 Ma

G

)

arlock lt Fau Zone (16-10 Ma)

0.706

e River 

4 Ma)

Northern Nevada Rift Zone (17-14 Ma)Northern Nevada Rift Zone (17-14 Ma)

Zr oneZr

one

Axis of metamorphic core complex trend...

~60-40 Ma in WA & ID ~36-24 Ma in NV

~20-6 Ma in S. NV ~35-10 Ma in AZ & Mex.

C C

C

Mesozoic Magmatic Arc

(~150-125 to 90-80 Ma)

Mesozoic Magmatic Arc

(~150-125 to 90-80 Ma)

Columbia Plateau basalts feeder

dike swarms (~ 16.5-15.5 Ma)

Yellowst no e Hot spot trend (16-10 to <5 Ma)Yellowst no e Hot spot trend (16-10 to <5 Ma)

E

ne

d

li

g

os

e o

ge

f C

io

ord er M

ill na

ync (Hin

e

g o e r W i a at s hc L ) n

E ne

d

li

g

os e o

ge

f C io

ord er M ill na

ync (Hin e

g o e r W

i a at s hc L ) n

Great Falls Tectonic Zone (~1.8 Ga)

Colorado Mineral Belt / Lineament (~75-17 Ma; Proterozoic inception)

Colorado Mineral Belt / Lineament (~75-17 Ma; Proterozoic inception)

Oligocene or younger;

late or

inception

Oligocene or younger;

late

or

inception

Postulated Snake River Strike-Slip

Fault Zone (earliest Miss. Cenozoic;

latest movement Cenozoic right-lateral)

n

207

200 km
Primary Tectonic Subdivisions

of the Central Cordilleran Orogen

Figure 2.4. First-order tectonic subdivisions of the Cordilleran orogen in the western United States. The

descriptions below are keyed by number to the tectonic subdivisions illustrated on the next page.

#1nPaleozoic and Mesozoic accretionary belt (shaded light grey, slightly darker where subdivisions #1 and

#2 overlap; heavy line delimits approximate western edge of the Precambrian craton, dashed where inferred).

Composed of native and exotic terranes of deep-oceanic, shelf-slope, island-arc, and continental origin that

were accreted and obducted onto the margin of the Precambrian cratonal shelf in a passive-margin setting

(sub-island arc subduction) during the Paleozoic Antler orogeny and the Paleozoic-Mesozoic Sonoma

orogeny. Oceanic and displaced terranes of arc affiliation were again accreted during the Mesozoic Nevadan

orogeny in an active-margin setting (sub-continental subduction). The accretionary belt makes up from 20 to

50 percent of the width of the Cordillera at different latitudes (Burchfiel et al., 1992).

#2n , and as the

of the Cordilleran orogen (shaded dark grey, slightly lighter where subdivisions #1 and #2

overlap). Defined as the region east of the Mesozoic Andean-type arc (stipple pattern, greatly generalized)

and west of the Sevier foreland fold-thrust belt (#3), and includes the Roberts Mountain, Golconda, and

Luning-Fencemaker thrust sheets (see Leventhal et al., 1995; Miller et al., 1992). Affected by: (1) middle

Paleozoic to earliest Mesozoic thin-skinned deformation of latest Proterozoic to Middle Paleozoic

continental shelf clastics and carbonates (miogeoclinal) strata during the Antler and Sonoma passive-margin

orogenies; and (2) Mesozoic thin-skinned thrusting and folding of earlier deformed terrain during the

Cordilleran active-margin orogeny (includes the Nevadan-Elko and Sevier phases, and possibly the

westernmost earliest Laramide phase deformation). This region was most active and initially established

during Mesozoic (Sevier) deformation and is in part underlain by subdivision #1. It is within this region that

the Great Basin would develop in the Cenozoic.

Late Jurassic to Tertiary Sevier hinterland, also known as the central Cordilleran interior

western region

#3nLatest Cretaceous to Tertiary Sevier foreland fold and thrust belt, includes the regions immediately west

and adjacent to the frontal Sevier foreland thrust (series of heavy and subparallel broken lines). Delineates the

eastern limit of the Sevier hinterland and the western limit of Laramide deformation in the Rocky Mountain

foreland region. Coeval and to the west of Sevier deformation, the pacific margin magmatic arc batholiths

were emplaced into accreted Paleozoic and Mesozoic continental crust (northern California), Cordilleran

miogeoclinal-eugeoclinal sequences (south-central California), and Paleozoic craton (southern California

andArizona) (Suppe, 1985).

#4nLatest Cretaceous through middle-late Eocene Rocky Mountain foreland (Laramide deformational belt),

also referred to as the of the Cordilleran orogen. "Classic" Laramide deformational features

are fault-bounded, basement-cored, asymmetric anticlinal uplifts in the central Rocky Mountains (gradientfilled polygons; approximate locations). The uplifts are adjacent to deep basins filled with marine and

continental clastic sediments, largely derived from the Sevier orogenic belt (Hamilton, 1987; Mutschler,

1987). The uplifts developed during the latest Cretaceous to Eocene, and are believed to be related to

basement contraction in response to a decrease in the dip of the subducted slab at depth beneath the southern

Cordillera (Dickinson, 1981).

eastern region

#2 & #3nThe Cordilleran orogenic belt proper, composed of a western hinterland region (#2) and an eastern

foreland fold and thrust belt (#3). In the southwestern United States, the Cordilleran orogenic belt is locally

referred to the (Cowan and Bruhn, 1992). Sevier orogenic belt

#2, #3, & #4nThe " " Cordilleran orogen of the western United States, relative to the "northern"

segment in Montana and British Columbia, and the "southern" segment in southeastern California, southern

Arizona, and southwestern New Mexico (Burchfiel et al., 1992; Cowan and Bruhn, 1992).

central

Figure based on Burchfiel et al. (1992). Accretionary belt from Burchfiel et al. (1992). Sevier thrust front from Pool et al. (1992). Sevier

hinterland (central Cordilleran interior) from Leventhal et al. (1995). Mesozoic magmatic arc from Suppe (1985). Laramide deformation

uplifts from Mutschler et al. (1987). Boundary of Great Basin from Thelin and Pike (1991).

208
Figure 2.4., continued.

1

3

4

2

Accretionary Belt

Hinterland

Fold-Thrust Belt

Foreland

(Approximate width of the subdivisions along the traverse ) 1-2-3-4.

209

200 km
Major Orogenic Belts and Thrusts

of the Central Cordilleran Interior

Figure 2.5.

R

G

LF

S

The central

Cordilleran interior region

(the Sevier hinterland, darker

grey shadings) is the area east of the Mesozoic

magmatic arc (Nevadan orogenic belt) and west of the

Sevier foreland fold-thrust belt (Miller et al., 1992), and was most active during the Mesozoic (Sevier

activity). Great Basin in lighter gray shadings. Phanerozoic thrust fronts: Roberts Mountain (Antler

orogeny, Upper Devonian to Upper Mississippian); Golconda (Sonoma orogeny, mid- to UpperPermian); Luning-Fencemaker (Nevadan-Elko activity, Middle-Late Jurassic to Early Cretaceous;

western counterpart to coeval " " in northeastern Nevada and northwestern Utah, both probably

related to emplacement of the Sierra Nevadan magmatic arc); and Sevier (Sevier activity, eastern limit of

the Sevier foreland fold-thrust belt, western limit of Laramide deformation; thrusts are Sevier and earliest

Laramide in age, Late Jurassic to early Tertiary). The Nevadan orogenic belt is delineated by great batholitic

complexes (i.e. Sierra Nevada and Idaho batholiths), emplaced during the Late Jurassic to Upper

Cretaceous. Laramide deformation (latest Cretaceous through mid- to Late-Eocene) delineates the Rocky

Mountain foreland region. Thrust fronts from Poole et al. (1992) and Hardyman and Oldow (1991). Central

Cordilleran interior (Sevier hinterland) from Leventhal et al. (1995). Boundary of Great Basin from Thelin

and Pike (1991).

n

n

n

n

C

Elko orogeny

Laramide Deformational Belt (basement b ol

ck uplif st and swel sl )

Laramide

Deformational Belt (b

a

se

ment

b

ol

ck

uplif st

and swel sl )

N e v a d

a

n

O r

e

o g e n i c

B

l t

N

e

v

a

d

a

n

O

r

e

o g e n i c

B

l t

S

S

R

R

G

LF

G

Rocky

Mountain

Foreland

F o r e l a n d F o l d a n d T h r u s t F r o n tF o r e l a n d F o l d

a n d

T h

r

u

s t F r o

n t

Hinterland

LF

210

200 km
Figure 2.6. Time chart of orogenic events affecting the Great Basin. Compiled from Burchfiel et al. (1992),

Cowan and Bruhn,(1992), Eaton (1982), Hoffman (1989), Miller et al. (1992), Poole et al. (1992), Speed

(1983), Speed et al. (1988), Stewart (1980), Suppe (1985), Thorman and Christensen (1991), and Thorman et

al. (1991). Inset text modified from Thorman and Christensen (1991). Period bounding dates from Harland

et al. (1990). Lower Proterozoic boundary from Van Eysinga (1975).

EVENT PERIOD ERA

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

MESOZOIC

PALEOZOIC

CENOZOIC

Proterozoic

Tectonic quiescence...

Development of the Cordilleran Geosyncline along the

Precambrian craton margin a northerly trending, westwardthickening miogeoclinal-eugeoclinal depositional regime.

Dominant geologic feature of eastern, central, and southern

Nevada in early and middle Paleozoic time. Continued in

various forms of disruption from Late Proterozoic to middle

Mesozoic time until finally obliterated in CretaceousPaleocene time.



RIFTING (Early Proterozoic, ~ 2.6-2.0 Ga)

ARC ACCRETION (1.8-1.65 Ga)

ANTLER OROGENY

(Roberts Mountain Thrust)

CORDILLERAN...

Laramide Phase

Sevier Phase

Nevadan-Elko

Phase ...OROGENY

SONOMA OROGENY

(Golconda Thrust)

Humboldt Disturbance

Onset of Subcontinental Subduction (~235-215 Ma)

1.46 Ma

65 Ma

145.6 Ma

208 Ma

245 Ma

290 Ma

322.8 Ma

362.5 Ma

408.5 Ma

439 Ma

510 Ma

570 Ma

2.6 Ga

RIFTING (800-500 Ma)

Divergent Passive Margin Formation (~ 600 Ma)

active margin

passive margin

211
ic time and space relationships between Mesozoic and early Cenozoic orogenies in the Great Basin and adjoining areas. Figure from

(1991), with modifications incorporated from Cowan and Bruhn (1992), Eaton (1982), Miller et al. (1992), and Speed et al. (1988).

NEVADA UTAH

Eugeocline Miogeocline Craton

CALIFORNIA

Paleocene

and Eocene

Late Early Late Middle

Laramide

Sevier

Nevadan Elko

Luning

&

Fencemaker

Thrust Belt

?

Phases of the Cordilleran Orogeny

Space-Time Relationships

n

212
mmary of the major tectonomagmatic events affecting the Great Basin in Cenozoic time. The thickness of a time-line indicates the relative

mpiled fromAllmendinger (1992),Armstong et al. (1969),Armstrong and Ward (1991),Atwater (1970, 1989),Axen et al. (1993), Best and

er and Bonham (1990), Christensen and Yeats (1992), Coney (1980), Dickinson (1992), Dickinson and Snyder (1978), Eaton (1982),

Mahood (1987), Hamilton (1987), Harry et al. (1993), Humphreys (1995), Hutchinson and Albers (1992), Livaccari and Perry (1993),

chler et al. (1987), Patino-Douce and Humphreys (1987), Rehrig (1986), Seedorff (1991), Speed et al. (1988), Stewart (1978), Stewart

n (1976), Suppe (1985), Thorman and Christensen (1991), Ward (1991), Wernicke et al. (1987), and Zoback et al. (1981).

eologic Events Affecting the Great Basin

and Environs During the Cenozoic

CenozoicMesozoic

retaceous

Tertiary

Paleogene Neogene

Paleocene Eocene Oligocene Miocene

Q

Plio

metamorphic core complex exhumation

inception

"Great Ignimbrite Flareup"

(proportion of basalt increasing)

magmatism waning...

? brief magmatic lull...

...bimodal rhyolite-basalt volcanism

late slow extension... continuing

climax of block-faulting

early rapid localized extreme extension

& regional uplift

hases of Sevier complete... culmination of

ide overthrusting in Sevier foreland fold-thrust belt

on plate

s & moves inland... ...slab hinge "roll-back" from east to west

rates increase... ...rapid decrease in rates

~30 Ma Pacific & N. A. plates collide in N. Mexico;

~29 Ma inception San Andreas & Mendocino triple junction

subduction to transform margin transition

~20 Ma S. Great

Basin latitudes

~5 Ma N. Great

Basin latitudes

ongoing subduction

to the North

70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

of granitic plutonism (~80 Ma igneous activity shuts down in the Sierra Nevada)

metamorphic core complex migmatitic nuclei emplaced

"Laramide Magmatic Gap" (Paleogene magmatic lull)

t and Carlson (1976) Time Slices: 34 Ma 17 Ma 6 Ma 43 Ma 0 Ma

Mendocino triple

junction migrates north

?

213
Figure 2.9.Latest

Proterozoic through

Devonian depositional framework of the Cordilleran geosyncline, a thick succession of continental

platform (shelf, slope, and rise) strata that accumulated

along the western margin of the craton in a broad northerly-trending two-fold miogeoclinal-eugeoclinal belt.

The miogeoclinal facies ( was deposited on the continental shelf in a shallowwater subtidal to supratidal environment, forming an apron of thin terrigenous siliciclastic detrital material, a

few hundred meters thick in cratonic areas in central Utah, that extended west across the craton margin,

thickening into a prism of siliciclastic carbonate sequences over 15 km thick in central Nevada. The

eugeoclinal facies ( was deposited on the continental slope and rise in a deepwater environment, coeval with miogeoclinal strata, and consisted of siliciclastic-chert-volcanic sequences,

shale, radiolarian chert, quartzite, and mafic pillow lavas. A transitional facies is recognized where the

miogeoclinal shelf facies interfingers with the eugeoclinal slope-rise facies. Facies boundary from Roberts

(1966), Stewart (1980), and Thorman and Christensen (1991). Great Basin boundary from Thelin and Pike

(1991). Great Basin in dark grey. Text from Dickinson (1981), Hamilton (1987), Hutchinson and Albers

(1992), Thorman et al. (1991), and Thorman and Christensen (1991).

eastern carbonate assemblage)

western siliceous assemblage)

Depositional Framework

of the Cordilleran Geosyncline

T r a n s i t i o n a l F a c i e sT r a n

s i t i o n a l F a c i e s

Miogeoclinal Facies

(eastern (eastern carbonate assemblage) carbonate assemblage)

Eugeoclinal Facies

(western (western siliceous assemblage) siliceous assemblage)

214

200 km
The Western Edge of

Precambrian North America

Figure 2.10. The western edge

of the NorthAmerican Precambrian craton,

as delineated by isotopic and paleothermal data.

egional paleothermal

anomaly, as defined by the distribution of Devonian rocks containing conodonts with a conodont color

alteration index over 4.5. The age of the anomaly is uncertain, but a major episode of regional heating took

place in the late Paleozoic and in the early Mesozoic.

Nevada in light gray shading. Boundary

of Great Basin in dark gray shading. Paleothermal anomaly from Cunningham (1988). Boundary of Great

Basin from Thelin and Pike (1991). Text modified from Cunningham (1988).

The

heavy black line labelled " " is the initial Sr/ Sr isopleth = 0.706 for Mesozoic plutons, and is thought

to approximate the edge of the Precambrian crust (Kistler and Peterman, 1973). The dashed line labelled

" " represents the initial Sr/ Sr = 0.708 and the Nd = -7 contours, and is interpreted as the edge of the

Precambrian crust by Farmer and DePaolo (1983). The gridded area outlines a r

The eastern edge of the paleothermal anomaly may be

a sensitive indictor of the position of the edge of the buried craton.

0.706

0.708

87 86

87 86



0.706

0.708

215

200 km
Crustal Formation Provinces

of the Western United States

Figure 2.11. Approximate

boundaries of crustal formation

provinces in the region of the Great Basin

(heavy dashed lines; ages as indicated). Provinces

are distinguished on the basis of regional differences in Nd isotopic evolution paths as determined by

measurements of crustally derived granitoids of various ages. The western edge of the Precambrian craton is

commonly thought to be approximated by

Nevada in light

gray shading. Boundary of Great Basin in dark gray shading. Province boundaries based on Farmer et al.

(1988), with modifications from Bennett and Depaolo (1987) and Livaccari and Perry (1993). Sr/ Sr I

isopleth based on Kistler and Peterman (1973) and Farmer and DePaolo (1983); this trace from Mutschler et

al. (1992), who had incorporated modifications from Carlson et al. (1991), Fleck and Criss (1985), and Kistler

(1990). Boundary of Great Basin from Thelin and Pike (1991). Text modified from Bennett and DePaolo

(1987).

the initial Sr/ Sr I = 0.706 isopleth for Mesozoic plutons (line

labelled ). The dashed line labelled " " represents the initial Sr/ Sr I = 0.708 ( = -7) isopleth,

which is interpreted as the edge of the Precambrian crust by Farmer and DePaolo (1983).

87 86

87 86

Sr

"0.706" 0.708 Sr Nd 

87 86

Sr

0.706

0.708

?

?

> 2.7 Ga (Wyoming Province)

2.3-2.0 Ga

2.3-2.0 Ga

2.0-1.8 Ga

1.8-1.7 Ga

< 1.4 Ga

216

200 km
Variability of Selected Geologic Attributes

Across the Southern Great Basin

Figure 2.12. Simplified profiles at 38 latitude showing the relationships between average elevation, crustal

thickness (Allenby and Schnetzler, 1983), long wavelength regional Bouguer gravity (Hildenbrand et al.,

1982), temperature at depth (Eaton et al., 1978; Eggler et al., 1988), and giant Cenozoic gold camps

(production and/or reserves of 1,000,000 Troy ounces or more Au). Redrawn from Mooney and Mutschler

(1990).

o

Average

Elevation

(km above MSL)

Depth

to Moho

(km below MSL)

Regional

Bouguer Gravity

( 250 km, Mgal) 

Temperature

at 50 km

(degree C)

Number of

Giant Cenozoic

Gold Camps per

2 of Longitude o

1

2

3

20

30

40

50

-100

-140

-180

-220

600

800

1000

1200 Basalt Solidus

2

4

6

8

10

California Nevada Utah

Great Basin

217
ozoic Tectonostratigraphy of The Great Basin

1

2

3

4

3 5

4 6

6 34

3 12

7

gram of regional tectonostratigraphic units at the surface in the western Great Basin and surrounding environs. The stacking section trends

west approximately along 40 N between the Sierra Nevada and Elko, Nevada (see inset lower left). Heavy lines and sutures are tectonic

aries, light lines are depositional. Unit constitutes Precambrian sialic North America plus parautochthonous cover, and probably

ned relatively coherent over Phanerozoic time. Unit represents the eastward thrusting of eugeoclinal rocks (siliceous and volcanic

blages) over coeval miogeoclinal rocks (carbonate shelf-slope strata), during Antler (Roberts Mountains thrust) and Sonoma (Golconda

) orogenies, respectively. Unit is presumed to be a microplate, or collection of microplates, whose subsurface extent is unknown. Unit

d as parautochthonous Mesozoic cover strata, mainly basinal, but locally shelfal, to unit after it was accreted to NorthAmerica. Unit is

med to be the source region for the Jurassic-Triassic basinal sequences of unit . Unit consists of displaced and suture-related terranes of

zoic melange or of arc affiliation. The contact of unit with units and is the Sierra Nevada Foothills suture, a major accretionary surface

parates terranes that attached to the passive margin of North America before Middle Triassic time from those that arrived during the later

margi

n phase of orogenesis(see figure 2.2, #1). The suture separatin

g unit fro

m units an

d is presume

d to exist, an

d is probabl

y blind.

rnary and upper Cretaceous cover (Unit ) not shown (see text). Figure redrawn from Speed et al. (1988). Text modified from Speed et al.

) and Speed (1983).

o

Precambrian Crystalline Basement

Paleozoic-upper (North America)

Precambrian Continental Shelf-Slope Strata

Roberts Mountains Allochthon

Golconda Allochthon

Triassic Shelfal Sequence Jurassic- Triassic Basinal

Sequence

aceous

Continental

Rocks

onomia:

ozoic Arc 

quences

suture

1

2

2

1

3

4

4

5

218
tribution of Sedimentary Rock-Hosted and

lcanic Rock-Hosted Gold-Silver Deposits

anic

osted

Sedimentary

Rock-Hosted

219

200 km

bution of sedimentary rock-hosted and volcanic rock-hosted gold-silver deposits. Volcanic rock-hosted deposits include disseminated,

elated (i.e. breccia zones) epithermal mineralization which is hosted primarily by volcanic rocks, and less commonly by associated

ntary rocks closely associated with proximal subvolcanic intrusions. Solid circles represent quartz-adularia veins; open circles, quartz

t spring gold deposits. Note the crescent-shape distribution (the ), and conspicuous lack of volcanic rock-hosted

eastern regions of Nevada. Sedimentary rock-hosted deposits include disseminated mineralization hosted by clastic as well as carbonate

and large have no obvious association with volcanic centers. The sedimentary rock-hosted deposits lie mainly inward of the epithermal

to the epithermal crescent in central and eastern Nevada, and are older and formed deeper than the volcanic rock-hosted deposits. Figure

3). Text modified from Ludington et al. (1993).

n

"epithermal crescent"
Figure 3.2. The 27 Ma age boundary between sedimentary and volcanic rock-hosted deposits. The

sedimentary rock-hosted deposits are believed to be generally pre-27 Ma in age, whereas the volcanic rockhosted deposits are primarily post-27 Ma in age. The 27 Ma age boundary is based on the spatial coincidence

of: (1) the western extent of sedimentary rock-hosted deposits and the eastern extent of volcanic rock-hosted

deposits (line delimiting eastern extent of volcanic rock-hosted deposits in central Nevada, and shaded area

marking region of known sedimentary rock-hosted deposits); (2) the position of a volcanic time-line that

separates calderas and other volcanic source regions with ages that are pre- or post-27 Ma (dotted line); and

(3) the northeastern limit of Walker Lane shear zone deformation, as delineated by northwest-trending

geomagnetic anomalies (dashed line). Figure and text modified from Ludington et al. (1993) and Cox et al.

(1991).

Sedimentary and Volcanic RockHosted Deposit Age Boundary

Northern Nevada Rift zone, as

interpreted from aeromagnetic

data (northern segment), and

Bouguer and isostatic gravity

data (southern extension).

?

220

200 km
Tuscarora-Spruce Mtn.

Independence

Group

Cherry Creek

Alligator Group

Battle Mt. - Eureka

(Cortez Trend)

Battle Mt. - Eureka

Lynn-Railroad

Lovelock-Austin

Humboldt

"Trend"

Hamilton-Ely

Fallon-Manhattan

Pioche Aurora

Beatty-Search Light

Carlin Trend

Walker Lane

(Virginia (Virginia City-Tonopah) City-Tonopah)

ShoshoneJarbidge

Mineral Trends and Belts of Nevada

Figure 3.3. Principal mineral trends and belts of Nevada. Trends and belts are labelled and illustrated as

dashed white lines (trends) and heavy white lines (belts) superimposed on a shaded relief map of topography.

Mineral belts commonly refer to regional-scale alignments of deposits, which may or may not be of the same

age or type, while mineral trends refer to district-scale deposit alignments (Seedorff, 1991). The topography

effectively shows the north-northeast trending structural grain of Nevada in contrast to most trends and belts,

which are oriented northwest-southeast. Mineral belts from Roberts (1966) and Shawe and Stewart (1976).

Trends from Sweeney (1990), with additions from Thorman and Christensen (1991) and Percival et al.

(1988).

Getchell

Trend

221

200 km
i

ii

Response

Variable

Predictor

Variable

W &W

+ -

(weights of spatial association)

Prior

Probability

Bayesian

Map

Overlay

Modelling

Predictor

Variables

(layers of evidence)

Posterior Probability

Mineral Potential Map

Point

Density

Measurement of

Spatial Association

Area

Analysis

Point-in-Polygon

Analysis

Classification

Scheme

multi- to binary-class reclassification

222

Weights of Evidence

Mineral Potential Modelling Method

Figure 4.1. Flow chart illustrating the weights of evidence mineral potential modelling method.
of gold-silver-bearing occurrences in the Nevada Great Basin. These occurrences represent economic mineralization having gold and/or

modity in production and/or reserve (i.e. unknown size occurrences are not plotted). Large: n=59; Medium n=118; Small n=2269.

nnes. See text for discussion.

n

223
Figure 6.2. Distribution of sedimentary rock-hosted (left) and volcanic rock-hosted (right) gold-silverbearing occurrences in the Nevada Great Basin. These occurrences represent economic mineralization

having gold and/or silver as their primary commodity in production and/or reserve (i.e. unknown size

occurrences are not plotted). Note the "crescent-shaped" distribution of volcanic rock-hosted occurrences in

relation to the ovoid-shaped distribution of the sedimentary rock-hosted occurrences, which lie inward and

"core" the volcanic rock-hosted occurrence crescent distribution (see bottom inset; Ludington et al., 1993).

Sedimentary rock-hosted: Large n=8; Medium n=30; Small n=57. Volcanic rock-hosted: Large n=33;

Medium n=43; Small n=317. Weight units (t) in metric tonnes.

n

Generalized Distribution of

SedimentaryRock-Hosted Occurrences

"Crescent"

"Core"

Generalized Distribution of

Volcanic Rock-Hosted Occurrences

224

200 km
Precious and other

Metallic Mineral Occurrences

ver-Bearing

ccurrences

Generalized

Distribution

of all gold-silver-bearing occurrences (left; n=2690) and all metallic mineral occurrences (right; n=5572). The density map of all metallic

des precious metal as well as base and other metallic mineralization (e.g. Fe, Mo, W, etc.). The gold-silver-bearing occurrence density

ese occurrences are preferentially concentrated in the central and western half of the State (western Great Basin). A weak " " or

ern (see inset under right map) is also present in the gold-silver-bearing occurrence distribution, but is better resolved in the metallic

ty map.

n

U-shaped

Mineral Occurrences

per 10,000 square km

225
aps of Cenozoic igneous rocks in Nevada. Derived from information contained in the legend and correlation chart on the geological map of

son, 1978). The "amagmatic zone" (noted on 6-17 and 17-34 Ma slices) marks the termination of the southwestward sweep of Tertiary

). See textfor discussion. Click on above image for animation.

34-43 Ma17-34 Ma6-17 Ma

amagmatic zone

amagmatic zone

226

200 km
A. B. C. D.

E. F. H.

227

G.

A B

D E F GH

of various geoscientific datasets reflecting the " -shaped" or "horse-shoe-shaped" distribution pattern of the precious- and non-precious

ada (note line on each map highlighting " -shaped" trend). Brighter "warmer" colors denote higher magnitudes of the phenomenon

er, warmer, etc.), whereas darker "cooler" colors represent lower magnitudes (i.e. less dense, lower, cooler, etc.), with the exception of

s), where darker greys indicate thicker crust. precious and non-precious metal occurrence density; density of Mesozoic plutonic

ped faults; depth to reflection Moho; topography; Bouguer gravity; heat flow; temperature of geothermal wells and

U

U

n

n n

n n n nn
WL

BME

WL

HZ

f image of topography in the southwestern United States highlighting regional-scale linear trends and features: Walker Lane shear

Mountain-Eureka (Cortez) mineral trend (yellow); trend of Humboldt gravity zone (blue). See text for discussion. Image from

An examination of Thelin and Pikes's (1991) wall-size map and accompanying information booklet is highly recommended, where these

ominent.

n

n

228

200 km
ap of Nevada (Stewart and Carlson, 1978).

229

Note: Geologic unit abbreviations appearing in legend are used throughout text.

See appendix A for full description of lithologic units.

QaQpQlsQmQToaQTrQtaQtbQTsTriTmiTiTbrTt3TrtTr3Ta3TbaTbTbgTtsTs3ThsTKsuTt2Tr2Ta2TobTs2Tt1Tr1Ta1

Ts1TKsKsTJgrTgrMzgrKgrKJdJgrTrgrKJimJTrsvJdJgbJvJTrsTrcTrkTrlgrJTraTrchTrmtJPuTrPsTrPvsPMhMsvMlTrPdPPaMDmcPpcd

PcdMDsPcPMcPscPPcPcMcPzspDCsvDslDsSeSsOsvOsChCscDtStOtOCtCtCZsZwDcDCcScSOcOcOCcCcCZqCssZqsYgrXmuncls
emblage map, derived by reclassification of the 101 lithologic units making up the geological map of Nevada (Stewart and Carlson, 1978).

he reorganizatio

n of Stewart an

dCarlson's(1978)lithologic units b

y age an

d generalroc

k type (see appendi

x

A).

230
y occurrences (subdivided by size) per lithologic assemblage unit. W+ shown for units (highest in red). 231

Frequency of Primary Occurrences by Lithologic Assemblage:

Subdivided by Occurrence Size

 (no occurrences)

 1.6881

 -0.7636

 1.0436

 2.1781

1.1963

 1.3374

 -1.0317

 0.2213

0.5878

 2.3831

 -0.6318

 1.3153

 1.8975

 0.2865

 0.2865

 -2.0989

 0.9067 

 Continental ClasticsMid MZ 

 Marine & ShelfEarly-Mid MZ 

 Marine & Shelf- Late

PZ 

 Marine & ShelfEarly-Mid PZ 

 Shelf & MarineEarly PZ-PC 

 Silicious Rocks- Late

PZ 

 Silicious Rocks- Mid

PZ-PC 

 Tuff & Sed RocksLate CZ 

 Breccia & TuffsLate CZ 

 Fel-Int Ex- & IntrusLate CZ 

 Fel-Int Extrus- EarlyMid MZ 

 Int-Maf Ex- & IntrusLate CZ 

 Granitic Pluton- Late

MZ-CZ 

 Granitic Pluton- PC 

 Gabbroic PlutonEarly-Mid MZ 

 Ultramafic RocksLate PZ 

 Ign/Meta ComplexEarly-Mid MZ

 Ign/Meta ComplexPC 

Lithologic Assemblage Unit

Large Size OccurrencesMedium Size OccurrencesSmall Size OccurrencesUnknown Size Occurrences
gic units (of 101) host over 90% of the primary occurrences (subdivided by size). See Appendix A for unit

Lithologic Units Hosting Over 90% of Primary Occurrences:

Subdivided by Occurrence Size

JTrs

Ta3

CZq

Tr3

Jgr

Ta2

JTrsv

Kgr

Osv

Mzgr

Oc

Trc

Trk

Zw

Dc

PMh

Tba

CZs

Tt3

Ch

OCt

Os

Tts

Tr2

Tgr

Ts3

Ta1

Qtb

Xm

MDs

TrPvs

Ct

St

Jd

Lithologic Unit

Large Size OccurrencesMedium Size OccurrencesSmall Size OccurrencesUnknown Size Occurrences

Large: Greater than 500 tonnes (~16,075,373 Troy ounces) Au content (production plus reserves). Greater than 10,000 tonnes (~321,507,465 Troy ounces) Ag content (production plus reserves).Medium: 500-25 tonnes (~16,075,373 to ~803,768 Troy ounces) Au content (production plus reserves). 10,000-500 tonnes (~321,507,465 to ~16,075,375 Troy ounces) Ag content (production plus reserves).Small: Less than 25 tonnes (~803,768 Troy ounces) Au content (production plus reserves). Less than 500 tonnes (~16,075,375 Troy ounces) Ag content (production plus reserves).Si l ifi ti d i d f G ild' (1968M t ll i M f N th A i
Figure 6.11. Lithologic units having a strong spatial association with primary occurrences. These units represent those in the 10th and hig

of W+.

Lithologic Units Having a Strong Spatial Association with Primary Occurrences

4.242

4.1052

3.6781

3.0134

2.8557

2.7905

2.7124

2.5979

2.4991

2.2961

2.2598

2.1288

2.1085

2.0187

1.9993

1.999

1.9801

1.9563

1.9282

1.9103

1.8975

1.7838

1.7758

1.6883

1.6742

1.63

1.5603

1.4722

1.3494

1.3064

1.2496

1.1931

1.1147

1.056

0.9628

0.9421

0.9194

0.9067

0.8988

0.8437

08189

 >> 5.0

0.51.01.52.02.53.03.54.04.55.0

Pzsp

Csc

Zw

TrPd

JPu

Ot

Ch

Trk

Jd

Tmi

Se

OCt

CZs

Trlgr

Tgr

CZq

Jgr

JTrsv

MDmc

Trc

St

Ygr

Ct

Ss

Cc

KJd

JTrs

Tri

Zqs

Osv

TrPvs

TJgr

PMh

Ta3

Mzgr

Ti

Qm

Tr2

Xm

Ta2

Kgr

PPa

Lithologic Unit

W+

 Igneous Rocks (mainly volcanic)

 Sedimentary Rocks (carbonates or cla

 Metamorphic Rocks

 Mixed Igneous, Sedimentary, or Metam

 Quaternary Cover (moraine)
entary rock-hosted occurrences (subdivided by size) per lithologic assemblage unit. W+ shown for units (highest in red). 234

Frequency of Sedimentary Rock-Hosted Occurrences by Lithologic Assemblage:

Subdivided by Occurrence Size

 0.8786

 (no occurrences)

 (no occurrences)

 (no occurrences)

 (no occurrences)

 (no occurrences)

 (no occurrences)

 0.9629

1.9675

 -0.0066

 1.6697

 2.6309

 -1.8571

 -1.254

 -2.1021

 -0.9895

 2.5006

 7.2755

 Continental ClasticsMid MZ 

 Marine & ShelfEarly-Mid MZ 

 Marine & Shelf- Late

PZ 

 Marine & ShelfEarly-Mid PZ 

 Shelf & MarineEarly PZ-PC 

 Silicious Rocks- Late

PZ 

 Silicious Rocks- Mid

PZ-PC 

 Tuff & Sed RocksLate CZ 

 Breccia & TuffsLate CZ 

 Fel-Int Ex- & IntrusLate CZ 

 Fel-Int Extrus- EarlyMid MZ 

 Int-Maf Ex- & IntrusLate CZ 

 Granitic Pluton- Late

MZ-CZ 

 Granitic Pluton- PC 

 Gabbroic PlutonEarly-Mid MZ 

 Ultramafic RocksLate PZ 

 Ign/Meta ComplexEarly-Mid MZ

 Ign/Meta ComplexPC 

Lithologic Assemblage Unit

Large OccurrencesMedium Size OccurrencesSmall Size OccurrencesUnknown Size Occurrences
gic units (of 101) host 100% of the sedimentary rock-hosted occurrences (subdivided by size). See Appendix A for unit

Lithologic Units Hosting 100% of Sedimentary Rock-Hosted Occurrences:

Subdivided by Occurrence Size

Qa

Dc

MDs

Cc

Osv

Oc

JTrs

PMh

Tt2

Ct

Pzsp

Ts3

Ta1

Tgr

Jgb

Trc

PMc

Psc

Ch

CZq

Lithologic Unit

Large Size Occurrences Medium Size Occurrences Small Size OccurrencesUnknown Size Occurrences

Large: Greater than 500 tonnes (~16,075,373 Troy ounces) Au content (production plus reserves). Greater than 10,000 tonnes (~321,507,465 Troy ounces) Ag content (production plus reserves).Medium: 500-25 tonnes (~16,075,373 to ~803,768 Troy ounces) Au content (production plus reserves). 10,000-500 tonnes (~321,507,465 to ~16,075,375 Troy ounces) Ag content (production plus reserves).Small: Less than 25 tonnes (~803,768 Troy ounces) Au content (production plus reserves). Less than 500 tonnes (~16,075,375 Troy ounces) Ag content (production plus reserves).Si l ifi ti d i d f G ild' (1968M t ll i M f N th A i
Figure 6.14. Lithologic units having a strong spatial association with sedimentary rock-hosted occurrences. These units represent all of th

value for W+.

Lithologic Units Having a Strong Spatial Association with Sedimentary Rock-Hosted Occurrences

4.8882

3.2779

2.8039

2.5006

2.2321

2.1545

2.0568

2.0219

1.9404

1.9189

1.8365

1.5461

1.3281

1.2976

1.109

 7.2755

0.01.02.03.04.05.06.07.08.0

Pzsp St Os Ct Jgb Ch MDs Osv PMh Tgr PMc Dc Cc JTrs Oc Trc 

Lithologic Unit

W+

 Igneous Rocks (mainly volcanic)

 Sedimentary Rocks (carbonates or clastics)

 Metamorphic Rocks

 Mixed Igneous, Sedimentary, or Metamorphic R

 Quaternary Cover
nic rock-hosted occurrences (subdivided by size) per lithologic assemblage unit. W+ shown for units (highest in red). 237

Frequency of Volcanic Rock-Hosted Occurrences by Lithologic Assemblage:

Subdivided by Occurrence Size

 1.775

 -0.2357

 0.2384

 0.5801

1.5725

 0.7911

 -0.4102

 -0.4688

 -0.8101

 0.6309

 -1.8770

 1.3259

(no occurrences)

(no occurrences)

(no occurrences)

(no occurrences)

(no occurrences)

(no occurrences)

 Continental ClasticsMid MZ 

 Marine & ShelfEarly-Mid MZ 

 Marine & Shelf- Late

PZ 

 Marine & ShelfEarly-Mid PZ 

 Shelf & MarineEarly PZ-PC 

 Silicious Rocks- Late

PZ 

 Silicious Rocks- Mid

PZ-PC 

 Tuff & Sed RocksLate CZ 

 Breccia & Tuffs- Late

CZ 

 Fel-Int Ex- & IntrusLate CZ 

 Fel-Int Extrus- EarlyMid MZ 

 Int-Maf Ex- & IntrusLate CZ 

 Granitic Pluton- Late

MZ-CZ 

 Granitic Pluton- PC 

 Gabbroic PlutonEarly-Mid MZ 

 Ultramafic RocksLate PZ 

 Ign/Meta ComplexEarly-Mid MZ

 Ign/Meta ComplexPC 

Lithologic Assemblage Unit

Large OccurrencesMedium Size OccurrencesSmall Size OccurrencesUnknown Size Occurrences
gic units (of 101) host 100% of the volcanic rock-hosted occurrences (subdivided by size). See Appendix A for unit abbreviations.

Lithologic Units Hosting 100% of Volcanic Rock-Hosted Occurrences:

Subdivided by Occurrence Size

Qa

Ta2

Tba

JTrs

JTrsv

Tts

Tt3

Ta1

Tmi

CZq

Ts3

Mzgr

Tt1

Tr2

Qm

Osv

Qtoa

Tb

Kgr

Trc

PMh

OCt

Ygr

Qls

Trt

Trk

Trlgr

JPu

Os

Oc

Lithologic Unit

Large Size OccurrencesMedium Size OccurrencesSmall Size OccurrencesUnknown Size Occurrences

Large: Greater than 500 tonnes (~16,075,373 Troy ounces) Au content (production plus reserves). Greater than 10,000 tonnes (~321,507,465 Troy ounces) Ag content (production plus reserves).Medium: 500-25 tonnes (~16,075,373 to ~803,768 Troy ounces) Au content (production plus reserves). 10,000-500 tonnes (~321,507,465 to ~16,075,375 Troy ounces) Ag content (production plus reserves).Small: Less than 25 tonnes (~803,768 Troy ounces) Au content (production plus reserves). Less than 500 tonnes (~16,075,375 Troy ounces) Ag content (production plus reserves).Si l ifi ti d i d f G ild' (1968M t ll i M f N th A i
Figure 6.17. Lithologic units having a strong spatial association with volcanic rock-hosted occurrences. These units represent all of those hfor W+.

Lithologic Units Having a Strong Spatial Association with Volcanic Rock-Hosted Occurrences

2.4052

2.4034

2.1298

2.0219

1.9495

1.7844

1.775

1.3924

1.3038

1.1629

1.138

1.1078

1.0817

1.0585

0.9845

0.7839

0.7679

0.5727

 3.7674

0.00.51.01.52.02.53.03.54.04.5

Tmi Qm Ta3 Trlgr JTrsv JPu Ta2 Ygr JTrs Tr3 OCt CZq Tts Tt2 Ta1 Tr2 Tba Tt1 Mzgr 

Lithologic Unit

W+

 Igneous Rocks (mainly volcanic)

 Sedimentary Rocks (carbonates or clastics)

 Metamorphic Rocks

 Mixed Igneous, Sedimentary, or Metamorphic R

 Quaternary Cover (moraine & landslide)
gy ternary-class predictor map for gold-silver-bearing occurrences of all sizes and deposit types. Derived from lithologic units shown in

al association for ternary-class predictor map indicated in legend. See text for discussion.

B.

240

200 km
(A) (B) 

binary-class predictor maps for sedimentary and volcanic rock-hosted gold-silver-bearing occurrences of all sizes. Weights of

ry-class predictor maps indicated in legend. See text for discussion.

B.

241

200 km
(A) (B) (C) 

units reclassified by posterior probability of hosting small, medium, and large primary-commodity gold-silver-bearing

types. Note that maps (A) and (C) form "map-distribution" end-members between those units with potential to host small and large

radational to some degree between the two end-members, although its overall map pattern is more like map (C). See text for discussion.

B. C.

242

200 km
Serpentinite

B. C.

(A) (B) (C) 

units having elevated posterior probabilities for hosting small, medium, and large primary-commodity gold-silver-bearing

types. Note that the large and medium size occurrences, as a group, exhibit similar host-rock spatial associations relative to the small size

posterior probability values are defined as those above a lower threshold delineated by the most pronounced increase in the slope of a

d frequency distribution curve. See text for discussion.

243

200 km
Figure 6.22. (A) (B) Lithologic units having elevated posterior probabilities for hosting small and

primary-commodity gold-silver-bearing occurrences of all deposit types. The large and medium size

occurrences are combined into a single sample primarily because they exhibit similar host-rock

spatial associations, which are different from those of the small occurrences sample, but also to create a larger

statistical sample for the purposes of analysis. "Elevated" posterior probability values are defined as those

above a lower threshold delineated by the most pronounced increase in the slope of a posterior probability

sorted frequency distribution curve. See text for discussion.

"big"

("big")

Serpentinite

A. B.

244

200 km
A. B.

Figure 6.23. (A)

(B)

Comparison between the extent and trend of the Cordilleran geosyncline "transitional

assemblage" and lithologic units having elevated posterior probability for hosting small size gold-silverbearing occurrences. Note the similar distribution of the units in map (B) to the red color units in map (A).

245

200 km
A. B.

Figure 6.24. Lithologic units having elevated posterior probabilities for (A) hosting sedimentary rockhosted and (B) volcanic rock-hosted gold-silver-bearing occurrences. "Elevated" posterior probability

values are defined as those above a lower threshold delineated by the most pronounced increase in the slope of

a posterior probability sorted frequency distribution curve. See text for discussion.

(MDs)

(Osv)

(Os)

246

200 km
Figure 6.25. (A)

(B)

Lithologic units having "substantial" posterior probabilities for hosting volcanic rockhosted gold-silver-bearing occurrences, compared to a map illustrating the space-time distribution of

volcanic rocks in Nevada. Note the spatial coincidence between the units in map (A) to those of the 34-

17 17-6 Ma units in map (B). "Substantial" posterior probability values are defined as those equal to and

greater than the 25th percentile. See text for discussion.

C

A. B.

34 - 17 Ma Rocks

17 - 6 Ma Rocks

6 - 0 Ma Rocks

43 - 34 Ma Rocks

247

200 km
(B) 

y of lithology and a diagrammatic representation of the diversity neighborhood. The diversity map represents the number of lithologic

by 2.585 km neighborhood cell (~6.684 km area), and indicates areas where contacts between various surface lithology units might be

ccurrence of structural, stratigraphic, or intrusive features. See text for discussion.

2

(cell sizes not to scale)

(one quadtree map cell = 83.4 m)

A. B.

31 by 31

Quadtree

Map Cells

~2.5 km

~2.5 km

248

200 km
Figure 6.27. Lithotectonic terranes of Nevada. The terrane map coverage is a generalized representation of

allochthonous terranes, with particular focus on the area of the Great Basin. Most of this coverage is based on

the maps of N. J. Silberling (Silberling et al., 1987; Silberling, 1991), which should be consulted for a better

understanding of the lithotectonic units. In order to provide full map coverage for Nevada, an additional map

unit labelled " " was added. It is approximate in extent and is derived from the

position of Precambrian rock outcrops on the geological map of Nevada (Stewart and Carlson, 1978). The

following information about this coverage was provided by G. L. Raines (1995, personal communication),

the source of which is N. J. Silberling. The area between the Sand Springs and Jungo terranes is an area of

controversial terrane assignment, belonging to either terrane. The lines bounding Cenozoic cover have a

convoluted nature that gives them an aspect of false precision, and should be treated as approximate contacts

only. An understanding of the distribution of the terranes in the third dimension is important to interpreting

the map pattern. For example, lines bounding the eastern sides of the Roberts Mountain and Golconda

allochthonous, unlike other terranes, simply indicate the easternmost exposures of these terranes thus

within the area of the Roberts Mountain allochthon, there are "windows" of the North American

miogeocline, and within the area of the Golconda allochthon, there are windows of both the Roberts

Mountain terrane and the North American miogeocline. The configuration of the eastern-bounding lines for

these terranes reflect the present position of rocks whose exposures are controlled by a variety of

compressional and extensional structures ranging from Paleozoic to Cenozoic in age. Only a minor part (at

most) of the upper crustal thickness is actually composed of rocks characteristic of the named terranes. To

some degree this is also true of the younger, more westerly terranes, particularly when their bounding faults

were originally thin-skinned thrusts. In general, the boundaries of the terranes are discrete faults and, while

even in two dimensions their regional map pattern is not well constrained, the terrane pattern can be regarded

as a surface boundary. The crustal thickness of various terranes is variable and a matter of interpretation.

PC Crystalline Basement (?)

C

249

200 km
ry occurrences per lithologic diversity unit (subdivided by size).

Frequency of Primary Occurrences by Lithologic Diversity Unit:

Subdivided by Occurrence Size

3 small occurrences

0 occurrences

0 occurrences

0 occurrences

2 3 4 5 6 7 8 9 10 11

Less Diverse . . . Number of Lithologic Units per ~2.5 by ~2.5 km Grid Cell . . . More Diverse

0102030405060708090100

Area Percent

Big Size OccurrencesSmall Size OccurrencesUnreported Size OccurrencesCumulative % of Total Area% of Total Area
entary and volcanic rock-hosted occurrences per lithologic diversity unit.

Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences by Lithologic Diversity Unit

1 volcanic rock-hosted occurrenc

0 occurrences

0 occurrences

0 occurrences

0 occurrences

e

2 3 4 5 6 7 8 9 10 11

Less Diverse . . . Number of Lithologic Units per ~2.5 by ~2.5 km Grid Cell . . . More Diverse

013253850637588100

Area Percent

Sedimentary Rock-Hosted OccurrencesVolcanic Rock-Hosted Occurrences% of Total AreaCumulative % of Total Area
al association (C) between lithologic diversity units and primary occurrences (subdivided by type and size). Note that (C) is

Stren

gth of the S

patial Association Between Primar

y Occurrences and Litholo

gic Diversit

y Units 

2 3 4 5 6 7 8 9 10 11

Less Diverse . . . Number of Lithologic Units per ~2.5 by ~2.5 km Grid Cell . . . More Diverse

0510152025303540

Area Percent

Volcanic Rock-Hosted OccurrencesSedimentary Rock-Hosted OccurrencesSmall Size OccurrencesBig Size OccurrencesAll Size Occurrences% of Total Area
ry occurrences per lithotectonic terrane unit (subdivided by size).

Frequency of Primary Occurrences by Lithotectonic Terranes:

Subdivided by Occurrence Size

1 small size occurrence

Cenozoic Cover

Deposits

Golconda Allochthon

Gold Range

Jungo (Fencemaker

allochthon)

Sierra Nevada Batholith

North American

Miogeocline

Paradise

Pine Nut

Roberts Mountains

Allochthon

Sand Springs

PC Crystalline

Basement (?)

Lithotectonic Terranes

Big Size OccurrencesSmall Size OccurrencesUnreported Size Occurrences
entary and volcanic rock-hosted occurrences per lithotectonic terrane unit.

Frequency of Sedimentary and Volcanic Rock-Hosted by Lithotectonic Terrane

Cenozoic Cover

Deposits

Golconda Allochthon

Gold Range

Jungo (Fencemaker

allochthon)

Sierra Nevada

Batholith

North American

Miogeocline

Paradise

Pine Nut

Roberts Mountains

Allochthon

Sand Springs

PC Crystalline

Basement (?)

Lithotectonic Terranes

Sedimentary Rock-Hosted OccurrencesVolcanic Rock-Hosted Occurrences
al association (C) between lithotectonic terrane units and primary occurrences (subdivided by occurrence type and size).

Strength of the Spatial Association Between Primary Occurrences and Lithotectonic Terranes

Cenozoic Cover

Deposits

Golconda Allochthon

Gold Range

Jungo (Fencemaker

allochthon)

Sierra Nevada

Batholith

North American

Miogeocline

Paradise

Pine Nut

Roberts Mountains

Allochthon

Sand Springs

PC Crystalline

Basement (?)

Lithotectonic Terranes

-3.0-2.0-1.00.01.02.03.0

Contrast (C) of W+ and W-

Area

Occurrences

Occurrencestary Rock-Hosted Occurrences

Rock-Hosted Occurrences
Figure 6.34. (A) (B) Diversity of lithology map and diversity binary-class predictor map for gold-silverbearing occurrences of all sizes and types, sedimentary rock-hosted occurrences, and volcanic rock-hosted

occurrences. The diversity map represents the number of lithologic units occurring in a ~2.5 by ~2.5 km

neighborhood cell (~6.684 km area), and indicates areas where contacts between various surface lithology

units might be complicated by a greater occurrence of structural, stratigraphic, or intrusive features. Weights

of spatial association for binary-class predictor map indicated in legend. See text for discussion.

2

A. B.

W+ = 0.9598 W- = -0.3418 All Size Occurrences

W+ = 1.0433 W- = -0.4097 Sed. Rock-Hosted Occurrences

W+ = 0.5397 W- = -0.1401 Vol. Rock-Hosted Occurrences

n

n

n

256

200 km
(A) (B)

rrane binary-class predictor maps for gold-silver-bearing occurrences of all sizes and types, sedimentary rock-hosted occurrences,

ted occurrences. Weights of spatial association for binary-class predictor maps indicated in legend. In descending order of contrast, the

nary predictor maps are as follows: map (A) Paradise, Sand Springs, Golconda Allochthon, Pine Nut, Jungo (Fencemaker allochthon),

hon, and the Gold Range; map (B) Roberts Mountain allochthon and Golconda allochthon; map (C) Sand Springs, Pine Nut, Jungo

and Paradise. See text for discussion.

n

n n

B. C.

65 All Size and Type Occurrences n W+ = 1.3993 W- = -1.0922 Sed. Rock-Hosted Occurrences n W+ = 1.1543 W- = -0.4598 Vol. Rock-Hosted Occurrences n

257

200 km
A. B. C. D.

(B) (C) (D) 

rocks of Nevada, buffered plutons, pluton density, and the density of all gold-silver-bearing occurrences. The plutonic rocks

n age, the remainder are Cenozoic intrusive rocks. Pluton buffers do not extend into the plutonic bodies the 0.0-0.5 km buffer represents

ing the plutonic body to a distance of 0.5 km. The pluton density map illustrates a relative measure of density. Note the broad similarity

nsity distribution pattern and (D) the gold-silver-bearing occurrences density distribution pattern. See text for discussion.

C

258
(B) (C) 

om the geological map of Nevada, fault distance buffers, and density of faults. The faults are of all types (dip-slip, strike-slip, etc.)

concentration for the fault density map is the total length of faults within a grid cell (~ 8.6 by 8.6 km) in geographic data units (latitude and

the relative frequency per unit area of the grid (for further explanation, see Raines, 1978). A 10-times smoothing factor was applied to the

scale trends. For all practical purposes, the fault density map illustrates where there is a high concentration of faults per unit area, and is

urpose of visual comparison. See Sawatsky and Raines (1981) for additional details on the fault density mapping techniques. See text for

>

A. B. C.

259
ry occurrences (subdivided by size) within buffer zones surrounding plutonic and intrusive bodies (predominantly Mesozoic).

Frequency of Primary Occurrences Within Buffer Zones Surrounding Plutonic Bodies:

Subdivided by Occurrence Size

0.5 - 1.0 km 1.0 - 1.5 km 1.5 - 2.5 km 2.5 - 5.0 km 5.0 - 10.0 km > 10.0 km

Buffer Width and Distance from Plutonic Body

0142943577186100

Percent

Big Size OccurrencesSmall Size OccurrencesUnreported Size Occurrences% of All OccurrencesCumulative % of Total Area

y
entary and volcanic rock-hosted occurrences within buffer zones surrounding plutonic and intrusive bodies (predominantly

uenc

y of Sedimentar

y and Volcanic Rock-Hosted Occurrences Within Buffer Zones Surroundin

g Plutonic 

Bodies

0.5 - 1.0 km 1.0 - 1.5 km 1.5 - 2.5 km 2.5 - 5.0 km 5.0 - 10.0 km > 10.0 km

Buffer Width and Distance from Plutonic Body

020406080100120

Area Percent

edimentary Rock-Hosted Occurrencesolcanic Rock-Hosted Occurrences

umulative % of Total Area

y
262

 Figure 6.40. Variation in contrast (C) across distance intervals surrounding plutonic and intrusive rocks for

 primary occurrences (subdivided by type and size).

Variation of Contrast (C) with Successive Area-Cumulative Distance Intervals 

from Plutonic Bodies for Primary Occurrences

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 - 0.5 km 0.5 - 1.0 km 1.0 - 1.5 km 1.5 - 2.5 km 2.5 - 5.0 km 5.0 - 10.0 km

Buffer Width and Distance from Plutonic Body

Contrast (C) of W+ and WVolcanic Rock-Hosted Occurrences

Sedimentary Rock-Hosted Occurrences

Small Size Occurrences

Big Size Occurrences

All Size Occurrences

Big Size Occurrences (non-area-cumulative)

Sedimentary Rock-Hosted Occurrences (non-area-cumulative)

Volcanic Rock-Hosted Occurrences (non-area-cumulative)
ry occurrences (subdivided by size) within buffer zones surrounding faults (predominantly Cenozoic).

Frequency of Primary Occurrences Within Buffer Zones Surrounding Faults:

Subdivided by Occurrence Size

m 0.5 - 1.0 km 1.0 - 1.5 km 1.5 - 2.5 km 2.5 - 5.0 km 5.0 - 10.0 km > 10.0 km

Buffer Width and Distance from Fault

020406080100120

Percent

Big Size OccurrencesSmall Size OccurrencesUnreported Size Occurrences% of All Size OccurrencesCumulative % Area
entary and volcanic rock-hosted occurrences within buffer zones surrounding faults (predominantly Cenozoic).

quency of Sedimentary and Volcanic Rock-Hosted Occurrences Within Buffer Zones Surrounding Faults

m 0.5 - 1.0 km 1.0 - 1.5 km 1.5 - 2.5 km 2.5 - 5.0 km 5.0 - 10.0 km > 10.0 km

Buffer Width and Distance from Fault

013253850637588100113

Area Percent

Sedimentary Rock-Hosted OccurrencesVolcanic Rock-Hosted OccurrencesCumulative % Area
265

 Figure 6.43. Variation in contrast (C) across distance intervals surrounding faults for primary occurrences

 (subdivided by type and size).

Variation of Contrast (C) with Successive Area-Cumulative Distance Intervals 

from Faults for Primary Occurrences

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 - 0.5 km 0.5 - 1.0 km 1.0 - 1.5 km 1.5 - 2.5 km 2.5 - 5.0 km 5.0 - 10.0 km

Buffer Width and Distance from Fault

Contrast (C) of W+ and W0

10

20

30

40

50

60

70

80

90

100

Percent

All Occurrences

All Occurrences (non-cumulative)

Big Size Occurrences

Big Size Occurrences (non-cumulative)

Small Size Occurrences
Figure 6.44.

(A)

(B)

Pluton buffer and fault buffer binary-class predictor maps for primary, sedimentary rockhosted, and volcanic rock-hosted gold-silver-bearing occurrences. Based on distance buffer zone (0-1

km) surrounding Mesozoic plutons and Cenozoic intrusive rocks, and distance buffer zone (0-1 km)

surrounding faults of all types and ages. Weights of spatial association for binary-class predictor maps

indicated in legend. See text for discussion.

A. B.

W+ = 1.4225 W- = -0.2830 All Size Occurrences

W+ = 0.7164 W- = -0.0884 Sed. Rock-Hosted Occurrences

W+ = 0.9615 W- = -0.1396 Vol. Rock-Hosted Occurrences

n

n

n

W+ = 0.6260 W- = -0.3894 All Size Occurrences

W+ = 0.7403 W- = -0.5291 Sed. Rock-Hosted Occurrences

W+ = 0.5954 W- = -0.3632 Vol. Rock-Hosted Occurrences

n

n

n

266

200 km
A. B. C.

1

1a 1a 1a

1

3

3

3

4

4

4

5

5

1

2

2

2

267

(B) (C) 

residual field geomagnetic anomaly, the isostatic residual gravity anomaly, and the Bouguer gravity anomaly of Nevada. The total

d isostatic residual gravity anomalies reflect geologic features in the shallow crust (~ 1 km), whereas the Bouguer gravity anomaly reflects

y basement (as well as density contrasts in the upper- and mid-crust and upper mantle). The main "local-scale" shallow crustal features

de: the northern Nevada rift zone, a narrow magnetic anomaly with a north-northwest trend extending 280 km through north-central

nue south across the entire state as indicated by a in the isostatic gravity anomaly (increasing the total extent to 500-600

flanking the main anomaly to the west is indicated as ); the Walker Lane shear zone belt, expressed by arcuate northwesterly

ies over the broad region of the shear zone, generally trending parallel to the belt and at oblique angles to basin-range structure; the

orth-trending zone in the northeastern quadrant of Nevada characterized by a general lack of high-amplitude magnetic anomalies, which is

western margin of the Precambrian craton and thicker crust; the southern Nevada , a magnetic "low" region of flat

s a transition from low to high gravity values and a Bouguer gravity ; and the Humboldt gravity zone, a generally high

ding northeast-southwest across northern Nevada. The regional-scale geophysical anomaly is dominated by an enormous Bouguer gravity

central part of the state, and correlates with high regional topography and the distribution of thick accumulations of low-density Cenozoic

ly is also believed to reflect density distributions within the pre-Tertiary basement ranging from 5-10 km in depth). The low is flanked to

ional highs, and is characterized by bilateral symmetry. The axis of symmetry trends roughly northwest-southeast and is aligned with the

magnetic and isostatic gravity anomalies. Text modified from Blakely (1988), Blakely and Jachens (1991), Eaton et al. (1978), Maybe et

8).

1—

1a 2—

3—

4—

5—

"gradient ridge"

plays"

"amagmatic zone"

"gradient bowl"

5

00 km
ary occurrences relative to total residual field geomagnetic anomaly strength (subdivided by size).

requency of Primary Occurrences Relative to Strength of Total Residual Field Geomagnetic Anomaly:

Subdivided by Occurrence Size

> 450

100 - 125

125 - 150

150 - 175

175 - 200

200 - 225

225 - 250

250 - 275

275 - 300

300 - 325

325 - 350

350 - 375

375 - 400

400 - 425

425 - 450

25 - 50

50 - 75

75 - 100

-0 - 25

-75 - -50

-50 - -25

-25 - -0

250  225

-225 - -200

-200 - -175

-175 - -150

-150 - -125

-125 - -100

-100 - -75

Nanoteslas (nT)

02468101214

Area Percent

Big Size OccurrencesSmall Size OccurrencesUnreported Size
mentary and volcanic rock-hosted occurrences relative to total residual field geomagnetic anomaly strength.

Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Relative to Strength of Total Residual Field Geomagnetic Anomaly

> 450

100 - 125

125 - 150

150 - 175

175 - 200

200 - 225

225 - 250

250 - 275

275 - 300

300 - 325

325 - 350

350 - 375

375 - 400

400 - 425

425 - 450

25 - 50

50 - 75

75 - 100

-0 - 25

-75 - -50

-50 - -25

-25 - -0

-225 - -200

-200 - -175

-175 - -150

-150 - -125

-125 - -100

-100 - -75

Nanoteslas (nT)

02468101214

Area Percent

Volcanic Rock-Hosted OccurrencesSedimentary Rock-Hosted Occurrences% Total AreaTrend Volcanic Rock-Hosted OccurrencesTrend Sedimentary Rock-Hosted Occurrences
trast (C) across total residual field geomagnetic anomaly intervals for primary occurrences (subdivided by type and size).

on in Contrast (C) with Successive Area-Cumulative Total Residual Field Geoma

gnetic Anomal

y Values For 

Primary Occurrences

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

25

50

75

0

-75

-50

-25

-225

-200

-175

-150

-125

-100

Nanoteslas (nT)

0.0 12.5 25.0 37.5 50.0 62.575.087.5100.0

Area Percent

All Size OccurrencesBig Size OccurrencesSmall Size OccurrencesSedimentary Rock-Hosted Occurrences

asts are area

om higher to

yvalues(right to

art).
271

 Figure 6.49. Number of primary occurrences relative to isostatic residual gravity anomaly strength (subdivided

 by size).

Frequency of Primary Occurrences Relative to

Strength of Isostatic Residual Gravity Anomaly:

Subdivided by Occurrence Size

0

100

200

300

400

500

600

700

800

900

1000

> 100 - 10-10 - -0-20 - -10< -20

Milligals (mG)

Number of Occurrences

0

20

40

60

80

100

120

Percent

Big Size Occurrences

Small Size Occurrences

Unreported Size Occurrences

Cumulative % Area (right to left)

% Total Area
272

 Figure 6.50. Number of sedimentary and volcanic rock-hosted occurrences relative to isostatic residual gravity

 anomaly strength.

Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences

Relative to Strength of Isostatic Residual Gravity Anomaly

0

20

40

60

80

100

120

140

160

> 100 - 10-10 - -0-20 - -10< -20

Milligals (mG)

Number of Occurrences

0

10

20

30

40

50

60

70

80

Area Percent

Sedimentary Rock-Hosted Occurrences

Volcanic Rock-Hosted Occurrences

% Total Area

Trend of Volcanic Rock-Hosted Occurences

Trend of Sedimentary Rock-Hosted Occurrences
273

 Figure 6.51 . Variation in contrast (C) across isostatic residual gravity anomaly intervals for primary occurrences

 (subdivided by type and size)..

Variation in Contrast (C) with Successive Area-Cumulative Isostatic Residual 

Gravity Anomaly Intervals For Primary Occurrences

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

> 100 - 10-10 - -0-20 - -10< -20

Milligals (mG)

Contrast (C) of W+ and W0

10

20

30

40

50

60

Area Percent

All Size Occurrences

Big Size Occurrences

Small Size Occurrences

Sedimentary Rock-Hosted Occurrences

Volcanic Rock-Hosted Occurrences

All Size Occur. (non-area-cumulative)

Big Size Occur. (non-area-cumulative)

Small Size Occur. (non-area-cumulative)

Sedimentary Rock-Hosted Occur. (non-area-cumulative)

Volcanic Rock-Hosted Occur (non-area-cumulativ

Note: Contrasts are area-cumulativefrom higher to

lower anomaly values (right to left on this chart).
Figure 6.52. (A, B, C)

(D, E, F) (A, D) (B, E)

(C, F)

Geophysical binary-class predictor maps derived from total residual field

geomagnetic and isostatic gravity geophysical anomaly data, for primary, sedimentary

rock-hosted, and volcanic rock-hosted occurrences. See text for discussion.

Geomagnetic Anomaly

(top row: A, B, C)

Isostatic Gravity Anomaly

(bottom row: D, E, F)

A.

E. F.

B. C.

D.

W+ = 0.3908 W- = -0.2150

All Size Occurrences

W+ = 0.2137 W- = -0.3248

All Size Occurrences

W+ = 0.5390 W- = -0.7399

Sed. Rock-Hosted Occurrences

W+ = 1.6882 W- = -0.4461

Sed. Rock-Hosted Occurrences

W+ = 0.6054 W- = -0.2000

Vol. Rock-Hosted Occurrences

W+ = 0.1413 W- = -0.1997

Vol. Rock-Hosted Occurrences

274

200 km
(A)

idual field geomagnetic anomaly intervals determined to have a strong spatial association with sedimentary rock-hosted occurrences

sted occurrences. On a regional level, the generally round shape of the broad low-amplitude anomaly in the northeastern quadrant of

ent in extent with the distribution of sedimentary rock-hosted occurrences. This area is part of the , a region of low

stretches along the eastern part of the state and into the of the Laramide magmatic gap. In contrast, the high-amplitude

st widespread in the Walker Lane shear zone (with the exception of the northern Nevada rift anomalies), and along an arcuate distribution

ent-shape distribution of the volcanic rock-hosted occurrences. See text for discussion.

"magnetic quite zone"

"amagmatic zone"

B.

275

200 km
c residual gravity anomaly intervals that are spatially associated with sedimentary rock-hosted occurrences, and volcanic rock- (A) (B)

overall distribution of the isostatic gravity highs in the northeastern quadrant of Nevada (map A) is broadly coincident in extent with the

y rock-hosted gold-silver-bearing occurrences. This region of gravity high is referred to as the , and corresponds to the

he gravity lows (map B) reflect the presence of thick accumulations of low-density Cenozoic volcanic rocks (as well as sediments) within

c depressions, and on a regional level, are broadly correlative to the crescent-shape distribution of volcanic rock-hosted occurrences. See

"Humboldt zone"

B.

276

00 km
n-slice maps of Cenozoic igneous rocks in Nevada. Derived from information contained in the legend and correlation chart on the geologic

n

d Carlson, 1978; see Appendi

x

A). The "amagmatic zone" marks the terminatio

n of the southwestwar

d swee

p of Tertiar

y magmatism

or discussion.

fic Intermediate Felsic

amagmatic

zone

277

200 km
(B) 

ntary rocks and Cenozoic volcanic, subvolcanic, and related intrusive rocks of Nevada. Maps do not include tuffaceous sedimentary

rine units, or Quaternary cover. Prepared from information contained in the legend and correlation chart on the geologic map of Nevada

8; also see Appendix A). An marks the termination of the southwestward sweep of Tertiary magmatism (McKee,

"amagmatic zone"

A. B.

amagmatic zone

278
(C) 

mical sample coverage, K/Na, and Ba/Na element ratio maps derived from NURE (National Uranium Reconnaissance Evaluation)

Na, and Ba were provided in ppb).

(B)

overage K/Na Ba/Na

A. B. C.

279
ary occurrences per K/Na anomaly interval (subdivided by size).

Frequency of Primary Occurrences Relative to K/Na Anomaly:

Subdivided by Occurrence Size

no occurrences

0100200300400500600700800900

1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 6 - 7 7 - 8 8 - 9 9 - 10 10 - 15 15 - 25 25 - 50 50 =>

K/Na Interval

Number of Occurrences

Big Size OccurrencesSmall Size OccurrencesUnreported SizeO
ary occurrences per Ba/Na anomaly interval (subdivided by size).

Frequency of Primary Occurrences Relative to Ba/Na Anomaly:

Subdivided by Occurrence Size

no occurrences

0200400600800100012001400

< 0.10 0.10 - 0.15 0.15 - 0.20 0.20 - 0.25 0.25 - 0.30 0.30 - 0.35 0.35 - 0.40 0.40 - 0.45 0.45 - 0.50 0.50 =>

Ba/Na Interval

Number of Occurrences

Big Size OccurrencesSmall Size OccurrencesUnreported Size OccurrencesCumulative % All Size Occurrences% Total Area
mentary and volcanic rock-hosted occurrences per K/Na anomaly interval.

Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences Relative to K/Na Anomaly

68

05101520253035404550

1 - 2 2 - 3 3 - 4 4 - 5 5 - 6 6 - 7 7 - 8 8 - 9 9 - 10 10 - 15 15 - 25 25 - 50 50 =>

K/Na Interval

Number of Occurrences

Sedimentary Rock-HostedOccurrencesVolcanic Rock-Hosted Occurrences

Note: High end of frequencyscale is truncated in order tobetter illustrate sedimentaryrock-hosted distribution.
mentary and volcanic rock-hosted occurrences per Ba/Na anomaly interval.

Frequency of Sedimentary and Volcanic Rock-Hosted Occurrences Relative to Ba/Na Anomaly

236

no occurrences

05101520253035404550

< 0.10 0.10 - 0.15 0.15 - 0.20 0.20 - 0.25 0.25 - 0.30 0.30 - 0.35 0.35 - 0.40 0.40 - 0.45 0.45 - 0.50 0.50 =>

Ba/Na Interval

Number of Occurrences

Volcanic Rock-Hosted OccurrencesSedimentary Rock-HostedOccurrences

Note: High end of frequencyscale is truncated in order tobetter illustrate sedimentaryrock-hosted distribution.
rast (C) across K/Na anomaly intervals (subdivided by type and size).

Variation of Contrast (C) with Successive Area-Cumulative K/Na Anomaly Intervals for

Primary Occurrences

-2.0-1.00.01.02.03.04.0

2 2 - 3 3 - 4 4 - 5 5 - 6 6 - 7 7 - 8 8 - 9 9 - 10 10 - 15 15 - 25 25 - 50 50 =>

K/Na Interval

Contrast (C) of W+ and W

l Size Occurrences

g Size Occurrences

mall Size Occurrencesedimentary Rock-Hosted Occurrencesolcanic Rock-Hosted Occurrences

umulative % Area

asts are area

m higher to lower

s (right to left on
trast (C) across Ba/Na anomaly intervals (subdivided by type and size).

Variation of Contrast (C) with Successive Cumulative-Area Ba/Na Anomaly Intervals for

Primary Occurrences

-1.0-0.50.00.51.01.52.02.53.0

0.10 - 0.15 0.15 - 0.20 0.20 - 0.25 0.25 - 0.30 0.30 - 0.35 0.35 - 0.40 0.40 - 0.45 0.45 - 0.50 0.50 =>

Ba/Na Interval

Contrast (C) of W+ and W

All Size OccurrencesBig Size OccurrencesSmall Size OccurrencesSedimentary Rock-Hosted OccurrencesVolcanic Rock-Hosted OccurrencesCumulative % Area

sts are area-cumulativefrom higher to

ly values (right to left on this chart).
Figure 6.64. (A, B, C) (D, E, F)

(A, D) (B, E) (C, F)

Geochemical binary-class predictor maps derived from K/Na and Ba/Na

geochemical anomaly data for primary, sedimentary rock-hosted, and volcanic rockhosted occurrences. See text for discussion.

A.

E. F.

B. C.

D.

W+ = 1.3242 W- = -0.0433

All Size Occurrences

W+ = 0.7794 W- = -0.1020

All Size Occurrences

W+ = 3.6379 W- = -0.1274

Sed. Rock-Hosted Occurrences

W+ = 2.2292 W- = -0.3880

Sed. Rock-Hosted Occurrences

W+ = 0.6301 W- = -0.0103

Vol. Rock-Hosted Occurrences

W+ = 0.0901 W- = -0.0037

Vol. Rock-Hosted Occurrences

K/Na Anomaly

(top row: A, B, C)

Ba/Na Anomaly

(bottom row: D, E, F)

286

200 km
(A) (B) 

bution of the K/Na and Ba/Na geochemical anomalies relative to the distribution of the sedimentary rock-hosted occurrences (light

and Battle Mountain-Eureka (Cortez) mineral trends. The distribution of the occurrences and geochemical anomaly highs is coincident,

line. The gray dots represent polymetallic replacement occurrences. See text for discussion. 

aped

287

B.

Carlin Trend 

ttle Mountain

- Eureka (Cortez) Trend

ttle Mountain

-

Eureka (Cortez) Trend

Carlin Trend Battle Mountain

- Eureka (Cortez) Trend

Battle Mountain

-

Eureka (Cortez) Trend

Roberts Mountain

Thrust Front

Roberts Mountain

Thrust Front

50 km
289

predictor maps used in the primary occurrence-type mineral potential models, and the strength of spatial association between

es

Relative Influence of Evidence:

Primary Mineral Potential Models

EOBIMED BFPLBIPR KNABIPR GEDVBIPR GEOBILOW BFFTBIPR BANABIPR TRRNBIPR MAGBIPR GISOBIPR

Binary-Class Predictor Map

-20-100102030405060

Studentized Contrast (C/Std(C))

C (W+ - W-)W+WC/Stdev(C)

Isostatic

Gravity

Anomaly

Magnetic

Anomaly

Lithotectonic

Terranes

Ba/Na

Anomaly

Fault

Buffers

Lithology:

medium

potential

Lithology:

lower

potential

Pluton

Buffers

K/Na

Anomaly

Lithologic

Diversity
predictor maps used in the sed. rock-host. occurrences mineral potential models, and the strength of spatial association between

es

Relative Influence of Evidence:

Sedimentary Rock-Hosted Mineral Potential Models

GEOBISD BANABISD TRRNBISD GISOBISD GEDVBISD MAGBISD BFFTBISD BFPLBISD

Binary-Class Predictor Map

-7.5-5.0-2.50.02.55.07.510.012.515.017.520.0

Studentized Contrast (C/Std(C))

C (W+ - W-)W+WC/Stdev(C)

Lithology Ba/Na

Anomaly

 Litho- tectonic Terranes

Isostatic

Gravity

Anomaly

Lithologic

Diversity

Magnetic

Anomaly

Fault

Buffers

Pluton

Buffers
predictor maps used in the vol. rock-host. occurrences mineral potential models, and the strength of spatial association between

es

Relative Influence of Evidence:

Volcanic Rock-Hosted Mineral Potential Models

TRRNBIVL BFPLBIVL BFFTBIVL MAGBIVL GEDVBIVL KNABIVL GISOBIVL BANABIVL

Binary-Class Predictor Map

-10-5051015202530

Studentized Contrast (C/Std(C))

C (W+ - W-)W+WC/Stdev(C)

Lithotectonic

Terranes

Pluton

Buffers

Fault

Buffers

Magnetic

Anomaly

Lithologic

Diversity

K/Na

Anomaly

Isostatic

Gravity

Anomaly

Ba/Na

Anomaly

Wnear

zero

Wnear

zero
Figure 7.4. Sedimentary rock-hosted occurrence-type posterior probability map, overlain on shaded relief of

topography. The favorability map is based on the weights of evidence sedimentary rock-hosted 8-layer

mineral potential model. Latitude-longitude grid spacing is half a degree.

39.0

-117.0

291

200 km
Figure 7.5. Volcanic rock-hosted occurrence-type posterior probability map, overlain on shaded relief of

topography. The favorability map is based on the weights of evidence volcanic rock-hosted 7-layer mineral

potential model. Latitude-longitude grid spacing is half a degree.

39.0

-117.0

292

200 km
A. B. C.

(A)

(C)

tween mineral potential maps (in absolute magnitude of posterior probability). Primary occurrence-type 11-layer and 7-layer mineral

mentary rock-hosted occurrence-type 9-layer and 8-layer mineral potential models, and volcanic rock-hosted occurrence-type 9-layer

ial models. Maps classified using "quantiles" method.

293

200 km
WOE

B.

8-Layer Model, WOE

C.

9-Layer Model, WLR

(A)

(C)

rock-hosted occurrence-type mineral potential model results. Posterior probability maps for weights of evidence (WOE) 9-layer

er evidence model, and weighted logistic regression (WLR) 9-layer evidence model. The 9-layer WLR-derived map is included for

yer WOE-derived map to determine if the WOE-derived mineral potential pattern was severely effected by conditional independence

WOE- and WLR-derived maps show the same overall map patterns, except that the WOE-derived maps appear to be "warmer" and better

al potential areas. The WLR map combination method generally produces lower posterior probabilities than does the WOE method. The

obability map was generated using the WOE-derived posterior probability classification scheme (to facilitate comparison between the two

n for the overall "less-favorable" appearance ("cooler colors") of the WLR-derived map. If the WLR-derived map was reclassified using a

d on the range of WLR-derived posterior probabilities, the resultant map would be very similar in appearance to the WOE-derived map. 294

200 km
WOE

B.

7-Layer Model, WOE

C.

9-Layer Model, WLR

(A)

(C)

-hosted occurrence-type mineral potential model results. Posterior probability maps for weights of evidence (WOE) 9-layer evidence

ce model, and weighted logistic regression (WLR) 9-layer evidence model. The 9-layer WLR-derived map is included for comparison

rived map to determine if the WOE-derived mineral potential pattern was severely effected by conditional independence violations. In

WLR-derived maps show the same overall map patterns, except that the WOE-derived maps appear to be "warmer" and better highlight

al areas. The WLR map combination method generally produces lower posterior probabilities than does the WOE method. The WLR

lity map was generated using the WOE-derived posterior probability classification scheme (to facilitate comparison between the two

n for the overall "less-favorable" appearance ("cooler colors") of the WLR-derived map. If the WLR-derived map was reclassified using a

d on the range of WLR-derived posterior probabilities, the resultant map would be very similar in appearance to the WOE-derived map. 295

200 km
B.

(A)

(B)

of posterior probabilities used to generate sedimentary rock-hosted occurrence-type 8-layer mineral potential map. Total uncertainty

ces of weights, plus, sum of uncertainty due to missing data) and relative certainty of the posterior probability (posterior probability

iation, in effect, the application of an approximate t-test).

296

200 km
B.

(A)

(B)

of posterior probabilities used to generate volcanic rock-hosted occurrence-type 7-layer mineral potential map. Total uncertainty

ces of weights, plus, sum of uncertainty due to missing data) and relative certainty of the posterior probability (posterior probability

iation, in effect, the application of an approximate t-test).

297

200 km
n of posterior probabilities associated with big (large and medium), small, and unknown size sedimentary rock-hosted occurrences. 298

Posterior Probabilities Appended to Sedimentary Rock-Hosted Gold-Sliver-Bearing Occurrences:

Calculated Using Sedimentary Rock-Hosted 8-Layer Mineral Potential Model

NORTH GENERATOR HILL DEPO.

CANDELARIA MINE

GOLD PICK DEPO.

CORTEZ GOLD MINE

POST DEPO.

SOUTH BULLION DEPO.

RABBIT CREEK MINE

MEIKLE MINE

STONEHOUSE DEPO.

ATLANTA MINE

COVE MINE

WINDFALL MINE

LANTERN DEPO.

DEEP STAR DEPO.

BULLION-MONARCH OPEN PIT MINE

PREBLE MINE

MAC RIDGE DEPO. (BIG SPRINGS)

FELIX CANYON

TRIPLET GULCH PROJECT

BUFFALO VALLEY MINE

GOLD RIDGE DEPO.

NORTH SAMMY CREEK DEPO. (BIG SPRINGS)

NORTHERN LIGHTS DEPO.

ELDER CREEK MINE

HORSE CANYON MINE

CAPSTONE DEPO.

WOOD GULCH MINE

AUSTIN GOLD VENT. AU MINE (QUITO MINE)

REED DEPO. (FONDAWAY CANYON)

NORTH SELOX CLAIMS.

GREEN SPRINGS MINE

MAG DEPO.

MILL CREEK DEPO.

STEIGMEYER PROP. (SHALE PIT MINE)

NIGHTHAWK RIDGE MINE (EASY JUNIOR)

STANDARD MINE (GOLD STANDARD)

SANTA FE GOLD MINE

RELIEF CANYON MINE

STAR POINTER MINE (& ROBINSON MINE)

SOUTH MOUTH DEPO. (FONDAWAY CANYON)

ALLIGATOR RIDGE MINE

LONG LAC DEPO.

Gold-Silver-Bearing Occurrence Site Name or Property Owner (sites are randomly labelled)

Size Unreported Occurrences (n=3)Small Size Occurrences (n=57)Big (Large and Medium) Size Occurrences (n=38)

Prior Probability = 0.0003
ution of posterior probabilities associated with big (large and medium), small, and unknown size volcanic rock-hosted occurrences. 299

Posterior Probabilities Appended to Volcanic Rock-Hosted Gold-Sliver-Bearing Occurrences:

Calculated Using Volcanic Rock-Hosted 7-Layer Mineral Potential Model

ROUND MOUNTAIN GOLD MINE

BUCKEYE AND OHIO

EIRA

NEVADA WONDER MINE

NAVAJO

BLUFF MINE

KINNEY PROS.

LAGUNA GRP.

KNICKERBOCKER MINE

ALTA SHAFT

G U B CLAIMS

HAILSTONE GRP.

AMERICAN FLAT CANYON MINE

PAYMASTER

SOUTH COMSTOCK MINE

DOUGLAS GRP. (CAMP DOUGLAS ?)

GOLDFIELD-BELMONT MINE

BUSTER G.M. CO.

UNAM. PROS. (TRUCKEE DISTRICT)

S.E.D. CLAIMS

LIBERTY MINE (REPRESENTING SAN ANTONE DIST.)

GOLD ZONE DIVIDE MINING CO. PROP.

MAHONEY MINE

ATLANTA MINES CO. CLAIMS

MAZUMA HILLS MINE

KENO CLAIMS

QUARTZITE AND BLACK BUTTE MINES

DEVANEY PROP.

BUCKSKIN NATIONAL MINE

BLACK BOY MINE

RENO MAY

MOHAWK MINE (ARGENTITE MINE)

BEN HUR PROS.

NEVADA HILLS FLORENCE MINE

TRINITY SILVER MINE

PET CLAIM

NBMG SAMPLE SITE 1480

NBMG SAMPLE SITE 788 UNAM. PROS.

SEVEN TROUGHS DISTRICT

KIM BLAIMS, NO.S 1-9

UNAM. PROS. (JESSUP DISTRICT)

WILLARD GRP. (NORTH)

PAN CLAIM

STONE HOUSE CANYON MINE

UNAM. PROS. #5

ARKELL MINE

KING TONOPAH MINE

KOSSUTH MINE

STEINER MINE

BLUE DRAGON NOS. 1-3 PROS.

MIZPAH MINE

PIONEER MINE

PYRAMID MINE

SILVER GLANCE GRP. (SILVER BOW DIST.)

AMALGAMATED MINE

NBMG SAMPLE SITE 3941

WIND MOUNTAIN MINE

TUSCARORA MINE

OKEY DAVIS MINE

REDLITE CLAIMS

MOTHER LODE MINE

MAMMOTH PROS.

BIG FOUR MINE

RUBY-JUNE ROSE GRP.

GOLCONDA-GOLD WEDGE GRP.

BUNCE PROS.

WATER WITCH MINE

WEST SINTER DEPO.

CHARLEY ROSS MINE

BELLE ISLE

BUFFALO HUMP MINE

BOVARD - RAND DISTRICT

SAM JACK GRP.

APRIL FOOL MINE

SILVER CENTER MINE

UTAH SPUR MINE

DEER PROS.

COEUR D'ALENE-JARBIDGE G.M.CO.

DULUTH MINE

GEIGER SHAFT

COYOTE PROS.

ST. PAUL

MAMMOTH MINE

HATCH MINE

WOMENS RIGHT GRP.

NEVADA RAND

DAYTON MINE

CROFOOT PROJECT

AG PROS. (RAGGED TOP DISTRICT)

Gold-Silver-Bearing Occurrence Site Name or Property Owner (sites are randomly labelled)

Size Unreported Occurrences (n=22)Small Size Occurrences (n=317)Big (Large and Medium) Size Occurrences (n=76)

Prior Probability = 0.0015
B.

(A)

(B)

y rock-hosted occurrence-type 8-layer mineral potential model the distribution of elevated favorability mineral potential areas in

ution of known sedimentary rock-hosted occurrences (n=98). In general, the distribution of occurrences with high posterior

oincident with areas that are predicted by the model to have elevated mineral potential favorability (map A). Note, for example, the

hlighted by the dashed lines.

C

300

km
Approximate limits of a corridor region within

which most of the volcanic rock-hosted

occurrences lie. This corridor generally

trends parallel to the Humboldt zone and the

Walker Lane shear zone, and is roughly

coincident to the extent of these zones.

B.

(A)

ock-hosted occurrence-type 7-layer mineral potential model the distribution of elevated favorability mineral potential areas in

ribution of known volcanic rock-hosted occurrences (n=415). In general, the distribution of occurrences with high posterior probabilities

th areas that are predicted by the model to have elevated mineral potential favorability (map A). Note, for example, the coincident

y the dashed lines.

C

301
between sedimentary rock-hosted mineral potential areas delineated in this study (center) and tracts permissible for Carlin-style sediment

n as delineated by teams of experts in the OFR-96-2 (left) and OFR-96-96 (right) mineral assessment surveys (Ludington et al., 1996;

entary rock-hosted mineral potential map (center) is superimposed upon a the permissible tract delineated in the OFR-96-2 survey (grey

-hand map). See text for discussion.

302

6-2 OFR-96-96This Study

Great Basin
n between volcanic rock-hosted mineral potential areas delineated in this study (center) and tracts permissible for epithermal gold-silver

ed by teams of experts in the OFR-96-2 (left) and OFR-96-96 (right) mineral assessment surveys (Ludington et al., 1996; Singer, 1996).

mineral potential map (center) is superimposed upon a the permissible tract delineated in the OFR-96-2 survey (grey underlay; same as

ft-hand map). See text for discussion.

303

6-2 OFR-96-96This Study

Great Basin

Hg,

c Centers
Figure 7.17. Blind test of the weights of evidence sedimentary rock-hosted 8-layer mineral potential model,

conducted with 12 newly discovered sediment-hosted gold occurrences that were not part of the dataset used

to calibrate (train) the model. The posterior probabilities were appended to the occurrence points (labled with

name and value) and color-coded according to the quantile-based classification scheme used to generate the

sedimentary rock-hosted mineral potential map in Figure 7.4. Latitude-longitude grid spacing is half a

degree.

304

39.0

-117.0

Wright Window (0.0008)

Trenton - Valmy (0.0174)

Pinon Range -

Cord Ranch (0.0000)

South Pipeline (0.4921) Pipeline (0.4921)

Gold Canyon (0.0593) Golden Butte (0.0023)

Saddle (Toiyabe Project) (0.0018)

Reona (0.2124)

Pan (0.0003)

Treasure Hill (0.0097)

Winter's Creek (0.0046)

200 km
305

39.0

-117.0

Figure 7.18. Blind test of the weights of evidence volcanic rock-hosted 7-layer mineral potential model,

conducted with 5 newly discovered epithermal gold-silver occurrences that were not part of the dataset used

to calibrate (train) the model. The posterior probabilities were appended to the occurrence points (labled with

name and value) and color-coded according to the quantile-based classification scheme used to generate the

volcanic rock-hostedmineral potentialmap in Figure 7.5. Latitude-longitude grid spacing is half a degree.

Mule Canyon (0.0017)

Hydra - Hercules (0.0044)

Midway (0.0024)

Golden Arrow (0.0480)

Gemfield (0.0220)

Note: The locations of Hydra-Hercules, Midway, Gemfield, and

Golden Arrow are approximated from Township-Range section

coordinates, which at the time, were the only coordinates

available (J.V. Tingley, Personal Communication, 1998).

200 km
Figure 7.19. Regional-scale exploration target areas for sedimentary rock-hosted occurrence-types, as

predicted by the weights of evidence 8-layer mineral potential model. The mineral potential map is overlain

with a mask consisting of gold-silver-bearing occurrences (see legend above). Five regions of interest have

been identified for further investigation (delineated in red, and numbered), which are discussed in the text and

in Figures 7.20, 7.21, and 7.22. Latitude-longitude grid spacing is half a degree.

Sedimentary rock-hosted occurrences,

as classified in the MRDS database.

Volcanic rock-hosted occurrences,

as classified in the MRDS database.

Other primary gold-silver-bearing occurrences.

Includes various occurrence types, as well as

sedimentary and volcanic rock-hosted occurrences

not identified as such in the MRDS database.

10 km radius circular buffers around

known gold-silver-bearing occurrences:

1.

2.

3.

4.

5.

39.0

-117.0

306

200 km
Volcanic rock-hosted occurrences, as classified in the MRDS database.

Sedimentary rock-hosted occurrences, as classified in the MRDS database.

Other primary gold-silver-bearing occurrences. Includes various occurrence

types, as well as sedimentary and volcanic rock-hosted occurrences not

identified as such in the MRDS database.

2 km radius circular buffers around known gold-silver-bearing occurrences:

2.

1.

2.

1.

Carlin Trend deposits

-116.0

41.0

Island Mt. dist. area

Jarbidge

dist. area O'Connell Group

Arizona mine (Au-bearing Cu-skarn)

Pequop dist. area

(Ag prospect)

Independence Group

Tuscarora

dist. area

Hunter

Prospect

Aura &

Edgemont A. dist. areas

B.

(Prior = 0.0003)

Figure 7.20.

A

B

C

Sedimentary rock-hosted occurrence-type regional exploration target areas #1 and #2:

mineral potential map overlain by known gold-silver-bearing occurrences (see legend), with selected

sites labelled; mineral potential map (favorable areas greater than prior probability) superimposed on

shaded relief of topography, overlain by faults (Stewart and Carlson, 1978) and deep-seated basement

fracture systems interpreted from aeromagnetic data (Berger and Bonham, 1990; Blakely, 1988);

lithologic assemblage map overlain by known occurrences (see legend). Latitude-longitude grid spacing

is half a degree (see Fig. 7.20b above). See textfor discussion.

C

C

C

Volcanic rock-hosted occurrences, as classified in the MRDS database.

Sedimentary rock-hosted occurrences, as classified in the MRDS database.

Other primary gold-silver-bearing occurrences. Includes various occurrence

types, as well as sedimentary and volcanic rock-hosted occurrences not

identified as such in the MRDS database.

2 km radius circular buffers around known gold-silver-bearing occurrences:

2.

1.

C.

307
Volcanic rock-hosted occurrences,

as classified in the MRDS database.

Sedimentary rock-hosted occurrences,

as classified in the MRDS database.

Other primary gold-silver-bearing occurrences. Includes various occurrence types,

as well as sedimentary and volcanic rockhosted occurrences not identified as such

in the MRDS database.

2 km radius circular buffers around known goldsilver-bearing occurrences:

3. 4.

3. 4.

A.

B.

Trout Creek & South Bullion deposits (SE end of Carlin Trend)

Mineral Hills mines (polymetallic replacements Ag, Au, Cu, Pb, Zn) C

Alpha dist. area (polymetallic replacements Ag, Pb, Zn, Cu, Au)) C

Rain mine & Emigrant Sprgs. (SE end of Carlin Trend)

Battle Mt. Eureka (Cortez) Trend C

Cove Mine

Cortez gold mine

Tonkin Springs mine

Antelope dist. area

Gold Bar mine

Horse Canyon mine

Elder Creek mine

Gold Acres deposit

Triplet

Gulch

Project

Volcanic rock-hosted occurrences, as classified in the

MRDS database.

Sedimentary rock-hosted occurrences, as classified in the

MRDS database.

Other primary gold-silver-bearing occurrences. Includes various

occurrence types, as well as sedimentary and volcanic rockhosted occurrence not identified as such in the MRDS database.

2 km radius circular buffers around known gold-silver-bearing occurrences:

C.

3. 4.

Figure 7.21.

A

B

C

Sedimentary rock-hosted occurrence-type regional exploration target areas #3 and #4:

mineral potential map overlain by known gold-silver-bearing occurrences (see legend), with selected

sites labelled; mineral potential map (favorable areas greater than prior probability) superimposed on

shaded relief of topography, overlain by faults (Stewart and Carlson, 1978) and deep-seated basement

fracture systems interpreted from aeromagnetic data (Berger and Bonham, 1990; Blakely, 1988);

lithologic assemblage map overlain by known occurrences (see legend). Latitude-longitude grid spacing

is half a degree (see figure 7.21b above). See textfor discussion.

C

C

C

-117.0

40.0

308
Volcanic rock-hosted occurrences, as classified in the

MRDS database.

Sedimentary rock-hosted occurrences, as classified in the

MRDS database.

Other primary gold-silver-bearing occurrences. Includes

various occurrence types, as well as sedimentary and

volcanic rock-hosted occurrences not identified as such

in the MRDS database.

2 km radius circular buffers around known gold-silver-bearing

occurrences:

Volcanic rock-hosted occurrences, as classified in the

MRDS database.

Sedimentary rock-hosted occurrences, as classified in the

MRDS database.

Other primary gold-silver-bearing occurrences. Includes

various occurrence types, as well as sedimentary and

volcanic rock-hosted occurrences not identified as such

in the MRDS database.

2 km radius circular buffers around known gold-silver-bearing

occurrences:

A.

B.

C.

Northumberland

Eureka dist. area

Bellevue

mine

Candelaria & Rock Hill dist. areas

Kingston dist. area

Twin River dist.

area

Jett dist. area

Santa Fe gold mine

Manhattan dist. area

Morey dist.

area

Gilbert dist. area

Tonopah

dist.

area

Figure 7.22. A

B

C

Sedimentary rock-hosted occurrence-type regional exploration target area #5: mineral

potential map overlain by known gold-silver-bearing occurrences (see legend), with selected sites labelled;

mineral potential map (favorable areas greater than prior probability) superimposed on shaded relief of

topography, overlain by faults (Stewart and Carlson, 1978) and deep-seated basement fracture systems

interpreted from aeromagnetic data (Berger and Bonham, 1990; Blakely, 1988); lithologic assemblage

map overlain by known occurrences (see legend). Latitude-longitude grid spacing is half a degree (see Fig.

7.22b above). See textfor discussion.

C

C

C

39.0

-118.5

309
Figure 7.23. Regional-scale exploration target areas for volcanic rock-hosted occurrence-types, as predicted

by the weights of evidence 7-layer mineral potential model. The mineral potential map is overlain with a

mask consisting of gold-silver-bearing occurrences (see legend above). Five regions of interest have been

identified for further investigation (delineated in red, and numbered), which are discussed in the text and in

figures 7.24, 7.25, 7.26, and 7.27. Latitude-longitude grid spacing is half a degree.

Volcanic rock-hosted occurrences,

as classified in the MRDS database.

Sedimentary rock-hosted occurrences,

as classified in the MRDS database.

Other primary gold-silver-bearing occurrences.

Includes various occurrence types and sizes, as well

as sedimentary and volcanic rock-hosted occurrences

not identified as such in the MRDS database.

10 km radius circular buffers around

known gold-silver-bearing occurrences:

1. 2. 3.

4. 5.

39.0

-117.0

310

200 km
C.B.A.

Volcanic rock-hosted occurrences, as classifiedin the MRDS database.Sedimentary rock-hosted occurrences, asclassified in the MRDS database.Other primary gold-silver-bearing occurrences.Includes various occurrence types, as well assedimentary and volcanic rock-hosted occurrences not identified as such in the MRDSdatabase.

2 km radius circular buffers around known goldsilver-bearing occurrences:

c rock-hosted occurrences, as classified

MRDS database.

entary rock-hosted occurrences, as

ed in the MRDS database.

primary gold-silver-bearing occurrences.

s various occurrence types, as well as

ntary and volcanic rock-hosted occurrences

ntified as such in the MRDS database.

circular buffers around known gold

g occurrences:

A

B

ock-hosted occurrence-type regional exploration target area #1: mineral potential map overlain by known gold-silver-bearing

with selected sites labelled; mineral potential map (favorable areas greater than prior probability) superimposed on shaded relief of

aults (Stewart and Carlson, 1978) and deep-seated basement fracture systems interpreted from aeromagnetic data (Berger and Bonham,

lithologic assemblage map overlain by known gold-silver-bearing occurrences (see legend). Latitude-longitude grid spacing is half a

ve). See textfor discussion.

C

C

C

Low-sulfide Au-qtz vein occurr ne cesLow-sulfide Au-qtz vein occurr ne ces

Crofoot / Lewis mine

-119.0

41.5

Virgin Valley dist. area

311
A.

B.

2.

3.

Volcanic rock-hosted occurrences, as classified in the

MRDS database.

Sedimentary rock-hosted occurrences, as classified in

the MRDS database.

Other primary gold-silver-bearing occurrences. Includes

various occurrence types, as well as sedimentary and

volcanic rock-hosted occurrences not identified as such

in the MRDS database.

2 km radius circular buffers around known gold-silver-bearing

occurrences:

Carlin

Trend

Getchell

Trend

Hollister (Ivanhoe)

Tuscarora

dist. area

Jarbidge

dist. area

Arizona mine (Au-bearing Cu-skarn)

Valdez gold prospect

Independence

group

Buckskin National mine

Spring City Paradise Valley district areas C

Gold Circle dist. area

Silver Hills

Cornucopia

C.

Figure 7.25. A

B

C

Volcanic rock-hosted occurrence-type regional exploration target areas #2 and #3: mineral

potential map overlain by known gold-silver-bearing occurrences (see legend), with selected sites labelled;

mineral potential map (favorable areas greater than prior probability) superimposed on shaded relief of

topography, overlain by faults (Stewart and Carlson, 1978) and deep-seated basement fracture systems

interpreted from aeromagnetic data (Berger and Bonham, 1990; Blakely, 1988); lithologic assemblage

map overlain by known gold-silver-bearing occurrences (see legend). Latitude-longitude grid spacing is half

a degree (see Fig. 7.25b above). See textfor discussion.

C

C

C

2.

3.

2.

3.

Volcanic rock-hosted occurrences, as classified

in the MRDS database.

Sedimentary rock-hosted occurrences, as classified

in the MRDS database.

Other primary gold-silver-bearing occurrences.

Includes various occurrence types, as well as

sedimentary and volcanic rock-hosted occurrences

not identified as such in the MRDS database.

2 km radius circular buffers around known gold-silverbearing occurrences:

41.0

-115.0

312
A. B. C.

ck-hosted occurrences, as classified

S database.

ry rock-hosted occurrences, as classified

S database.

ary gold-silver-bearing occurrences.

rious occurrence types, as well as

y and volcanic rock-hosted occurrences

d as such in the MRDS database.

cular buffers around known gold

occurrences:

Volcanic rock-hosted occurrences, as classifiedin the MRDS database.Sedimentary rock-hosted occurrences, as classifiedin the MRDS database.

Other primary gold-silver-bearing occurrences.Includes various occurrence types, as well assedimentary and volcanic rock-hosted occurrencesnot identified as such in the MRDS database.

2 km radius circular buffers around known goldsilver-bearing occurrences:

Rawhide deposit

Duluthmine

38.5

-118.0

Aspingroup

Borealis mine

Gilbert mine

ooseberry 

ines

313

A

B

C

ock-hosted occurrence-type regional exploration target area #4: mineral potential map overlain by known gold-silver-bearing

with selected occurrences and districts labelled; mineral potential map (favorable areas only; those greater than prior probability)

relief of topography, overlain by faults (Stewart and Carlson, 1978) and deep-seated basement fracture systems interpreted from

r and Bonham, 1990; Blakely, 1988); lithologic assemblage map overlain by known gold-silver-bearing occurrences (see legend).

acin

g is half a degree (see Fig. 7.26

b above). See textfor discussion.

C

C

C
nic rock-hosted occurrences, as classified

MRDS database.

mentary rock-hosted occurrences, as classified

MRDS database.

primary gold-silver-bearing occurrences.

es various occurrence types, as well as

entary and volcanic rock-hosted occurrences

entified as such in the MRDS database.

us circular buffers around known gold

aring occurrences:

Volcanic rock-hosted occurrences, asclassified in the MRDS database.Sedimentary rock-hosted occurrences, asclassified in the MRDS database.

Other primary gold-silver-bearing occurrences.Includes various occurrence types, as well assedimentary and volcanic rock-hosted occurrences not identified as such in the MRDSdatabase.

2 km radius circular buffers around known goldsilver-bearing occurrences:

A. B. C.

A

B

C

ock-hosted occurrence-type regional exploration target area #5: mineral potential map overlain by known gold-silver-bearing

with selected occurrences labelled; mineral potential map (favorable areas only; those greater than prior probability) superimposed on

y, overlain by faults (Stewart and Carlson, 1978) and deep-seated basement fracture systems interpreted from aeromagnetic data (Berger

kely, 1988); lithologic assemblage map overlain by known gold-silver-bearing occurrences (see legend). Latitude-longitude grid

e Fig. 7.27b above). See textfor discussion.

C

C

C

mines (e.g. Pioche) C

Utah Spur mine

-115.0

37.0

Delamar

(Magnolia) mine 

Vigo Dist. Area

314
ds highlighting possible regional-scale crustal features that may have been important to the localization of sedimentary rock-hosted occurrences.

ntary rock-hosted occurrences. The trace of all linear trends were fit to fault and fracture patterns visible in shaded relief as offsets in mountain

features, and to the distributio

n patter

n of occurrences(red dots; Fig. 8.1b). The positioning and extent of the linear trendsis approximate. is

ne, as interpreted from geomagnetics. The and structural zones are parallel to the Battle Mountain Eureka (Cortez) and Carlin

allel to the The linear features labelled and are parallel to, and occur within, the proposed Humboldt shear(?) zone. Important

at the intersection of most of these linear features: the Preble and Kramer Hill mines (southern Getchell trend) at and ; the Battle

d mine) at and ; the Bullion district (GoldAcres deposit) at and ; the Cortez andAntelope districts in the vicinity where

and intersect; the Windfall and Ratto Canyon deposits just south of where intersects the ; the

sits at and ; the southern Carlin trend deposits (Rain and Emigrant Springs) at , , and the .

eat Basin boundary in orange (from Thelin and Pike, 1991). Roberts Mountain thrust front in yellow (from Poole et al., 1992). The crosscutting

linear features is not intended to denote relative age.

NNR

Cortez Carlin

NNR. "A", "B", "C"

Cortez "A"

Cortez "B" Cortez "C" NNR,

ront, Cortez Cortez Roberts Mountain thrust front

Carlin "B" Carlin "C" Roberts Mountain thrust front

C

B.

Roberts Mtn.Thrust Front

NNR

tez

Carlin

314

200 km
(A)

. Broad regional-scale crustal structures that may have been important to the localization of volcanic rock-hosted occurrences.

rock-hosted occurrences. The red transparency in (A) envelopes the NW SE-trending Walker Lane shear zone in southwestern Nevada and the

g Humboldt shear(?) zone in northern Nevada. The extents of these two features were fit to fault and fracture patterns visible in shaded relief as

significant changes in the orientation (trend) of mountain ranges, and to the distribution pattern of mineral deposits (red dots; Fig. 8.2b). The

ese boundaries is approximate. The northern boundary of the Humboldt shear(?) zone and the northeastern boundary of the northern Walker Lane

the shaded relief, and is probably greatly complicated by the intersection of these features and concealed beneath late Cenozoic volcanic cover.

y denoted with a dashed red line, may be represented in the shaded relief by any of a myriad of faults or fracture zones, or may be a broad zone that

tectonic elements. A rough boundary might consist of the postulated Snake River fault zone, which underlies the Snake River volcanic plateau,

in Oregon, which has been proposed as a northern extension of northern Nevada rift zone (see Pool et al., 1992; Christiansen and Yeats, 1992).

e volcanic rock-hosted occurrences lie within the Walker Lane and Humboldt(?) shear zones. Nevada border in green. Great Basin boundary in

e, 1991).

l cross-roads"

n

B.

316

200 km
B. D. F.

A. C. E.

(A) (B) (C)

(E) (F)

bution relationships among all gold-silver-bearing occurrences, overlain on occurrence density, K/Na geochemical anomaly,

K/Na anomaly highs overlain on shaded relief of geomagnetic anomaly, isostatic gravity anomaly, and Bouguer gravity anomaly.

urrences an

d K/Na anomal

y highsis coincident, as delineate

d

b

y the "V"-shaped line. Also see Fig. 6.65.

317

50 km
Figure 8.4. Kriged surface map of RADB radiometric age dates for Cenozoic igneous rocks in the Great

Basin region. Note that the approximate center of low age in Nevada is coincident with distribution of

igneous rocks in the 34-43 Ma from Stewart and Carlson (1978) (compare to Fig. 6.4). RADB data from

Ward (1991, personal communication). Great Basin boundary in dashed black (from Thelin and Pike, 1991).

State borders in black. See text for discussion.

318

200 km
A1

APPENDIX A

Stratigraphy And Description

Of Lithologic Units—

Geologic Map of Nevada

Section Contents:

A.1 Correlation Chart of Geologic Map Units with Accompanying Table of Unit Descriptions

A.2 Key to GIS Geological Map Legend and Lithologic Unit Abbreviations Appearing of

Digital Maps

A.3 Key to Lithologic Units Composing Assemblage Map Units
A2

A.1 Correlation of Geologic Units in Nevada

Figure A.1 Correlation of geologic map units. This figure is derived from the “correlation of map

units” shown on Stewart and Carlson’s (1978) geologic map of Nevada, and includes

additional description of the units and assemblages from various sources (see Fig. A.1

caption).

Table A.1 Description of map units for the geologic map of Nevada. This table contains

descriptions of the 101 geologic map units making up Stewart and Carlson’s (1978)

geologic map of Nevada, and is derived from information contained in the legend of

the geologic map.

A.2 Digital Geologic Map Key

Table A.2 Key to lithologic unit acronyms. This table contains the abbreviated unit descriptions

used on the various digital renderings of the geologic map of Nevada and their

correlative unit legend acronyms as they appear on Stewart and Carlson’s (1978)

geologic map.

A.3 Digital Lithologic Assemblage Map Key

Table A.3 Key to the makeup of the lithologic assemblage map units. This table contains the

geologic units, as coded on Stewart and Carlson’s (1978) geologic map of Nevada, that

were combined to create the 20 lithologic assemblage units making up the digital

lithologic assemblage map.
Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Precambrian

~6-0 m.y.

~ 17 - ~ 6 m.y.

~ 34 - ~ 17 m.y.

~ 43 - ~ 34 m.y.

X

Y

Z

Lower

Upper & Middle

Siliceous (Western) Assemblage Transitional Assemblage Carbonate (Eastern) Assemblage & Related

Metamorphic and Granitic Rocks

Ultramafic Rocks

Plutonic Rocks

Quaternary Cover Deposits

Intrusive Rocks

Volcanic and Shallow Intrusive Rocks Sedimentary Rocks

Igneous and

Metamorphic Rocks

Western Nevada

Siliceous and volcanic assemblage

Conglomerate, siltstone, and limestone

sequences within Antler orogenic belt

Carbonate-detrital belt

along eastern margin

of Antler orogenic belt

or western part of

foreland basin

Eastern Nevada

Carbonate rocks deposited in

foreland basin or on shelf

Units of Uncertain

Western and Central Nevada Age or Classification

Eastern Nevada

TJgr

Mzgr

Kgr

Jgr

gr

KJd

Tgr

Qa Qp Qls Qm

Tri Tmi

Ti

KJim

QToa

Tbr

Tt3

Tt2

Tt1

Trt

QTr QTa

Tr3 Ta3

Tr2 Ta2

Tr1 Ta1

Tba

QTb

Tb

Tbg

Tob

Tts

Qts

Ts3

Ts2

Ts1

TKs

Ks

Ths

TKsu

JPu

Ps

J a

ch

mt

Jd Jgb Jv

J s

c

k lgr

J sv

Pvs

PMh

Msv

Ml

Pd

P a

MDmc

P cd

cd

MDs

Pc

Psc

P c

Mc

c

PMc

Pzsp

D sv

Dsl

Se

Osv

h

sc

Ds

Ss

Os

Dt

St

Ot

t

Zs

Zw

O t

Dc

Sc SOc

Oc

Zqs

D c

O c

c

ss

Zq

Ygr

Xm

Stratigraphy

of Nevada:

Correlation of

Geologic Map Units

Figure A1.

Tables A.1 A.2.

Correlation of map units for the

geologic map of Nevada (Stewart and Carlson,

1978). The units are spatially arranged east to west

(right to left). Unit abbreviations and descriptions

are given in and Cenozoic igneous

units are subdivided into a four interval age

classification (43-34Ma; 34-17 Ma; 17-6Ma; 6-0

Ma) and are delineated with dark dotted lines.

These age intervals appear to mark times of

significant change in the type and location of

volcanism, and these changes may reflect

fundamental stages in the tectonic evolution of the

Great Basin (Stewart, 1980). Such relationships

have been recognised by many researchers and have

been used to construct a series of maps to

illustrate the time-transgressive nature of

magmatism in the Great Basin (Armstrong et al.,

1969; Stewart, 1980; Silberman, 1985; White,

1985; and Seedorff, 1991). Three major

depositional provinces, as defined by Stewart

(1980), are highlighted in light gray. The

(upper

Paleozoic and Triassic) are eugeoclinal rocks

originally deposited west of the Antler orogenic

belt. In most places, they are allochthonous and

thrust eastward as the upper plate of the Golconda

thrust. The is

parautochthonous or autochthonous with respect to

theRobertsMountainsthrust. In places,these rocks

are interstratified with the eastern carbonate

assemblages. The

are miogeoclinal

rocks originally deposited east of the Antler

orogenic belt in

They are also

parautochthonous or autochthonous with respect to

the Roberts Mountains thrust. Figure and text

modified from Stewart and Carlson (1978). See

alsoPoole et al.(1992)for correlation chart.

time-slice

Western

Nevada Siliceous and Volcanic Assemblage

Transitional Assemblage

Eastern Nevada Carbonate

Assemblage and Related Rocks

shallow-marine, intertidal, and

supratidal environments.

West East

A3
A4

Unit

Abbreviation

Unit

Name

Unit

Description

Quaternary Cover and Sedimentary, Volcanic, and Shallow Intrusive Rocks

Qa Alluvial Deposits Locally includes lacustrine, shoreline, and

eolian sand dune deposits.

Qp Playa, Marsh, and Alluvial-Flat

Deposits, Locally Eroded

Forms widespread alluvial fan, stream terrace

flood plain, valley flat, and playa deposits.

Grade from coarse gravel near mountians or

ranges to fine silt and clay in the valles and on

the playas.

Qls Landslide Deposits Occur primarily in the mountainous areas of

northern and western Nevada. Qm Morainal Deposits

QToa Older Alluvial Deposits Consist largely of dissected gravel and sand.

QTr Rhyolitic Flows and Shallow

Intrusive Rocks

Rhyolite flows and flow domes and andesite

flows occur in western Mineral and southern

Lyon Counties, and rhyolite domes occur in

QTa Andesite Flows and Breccias Storey and southern Washoe Counties.

QTb Basalt Flows Locally includes maar deposits and cinder

cones.

QTs Sedimentary Rocks Mostly lake deposits.

Tri Rhyolitic Intrusive Rocks Range in age from about 43 to 6 Ma, across the

43-34 Ma, 34-17 Ma, 17-6 Ma time-slice

Tmi intervals of Stewart and Carlson (1976). Intrusive Rocks of Mafic and

Intermediate Composition

Ti Intrusive Rocks

Aphanitic, porphyritic, and coarsely granular

rocks ranging in composition from diorite to

granite. Clark County.

Tbr Breccia Volcanic, thrust, and jasperoid breccia and

landslide mega-breccia.

Tt3 Welded and Non-Welded Silicic

Ash-Flow Tuffs

Locally includes thin units of air-fall and tuff

and sedimentary rock.

Trt

Ash-Flow Tuffs, Rhyolitic

Flows, and Shallow Intrusive

Rocks

Range in age from 17-6 Ma. Coeval with

widespread eruption of mafic lavas and bimodal

assemblages of rhyolite and basalt. Rhyolites

include high-silica subalkaline, as well as

Tr3 peralkaline varieties.

Rhyolitic Flows and Shallow

Intrusive Rocks

Table A.1. Description of map units for the Geologic Map of Nevada (Stewart and Carlson, 1978). See Figure A.1

for the stratigraphic correlation of the map units. See also table 1 in Stewart (1980), Precambrian Z and Lower

Paleozoic Assemblages and Provinces in the Great Basin. Text from Stewart and Carlson (1978), with

supplemental material from Stewart (1980). Refer to Stewart and Carlson (1978) for references herein.
A5

Table A.1, continued. 

Unit

Abbreviation

Unit

Name

Unit

Description

Ta3 Andesite and Related Rocks of

Intermediate Composition

Flows and breccias.

Tba Andesite and Basalt Flows

Mostly in ~17 to ~6 Ma age range. In

Humboldt County, locally includes rocks as old

as 21 Ma May include rocks younger than 6

Ma in places.

Tb Basalt Flows Includes alkali-olivine and tholeiitic basalts.

Tbg Banbury Formation

Basalt, gravel, and tuffaceous sediments locally.

Northeast Humboldt County and northwest Elko

County.

Tts Ash-Flow Tuffs and Tuffaceous

Sedimentary Rocks Range in age from 17-6 Ma.

Ts3 Tuffaceous Sedimentary Rocks Locally includes minor amounts of tuff.

Ths Horse Spring Formation Tuffaceous sedimentary rocks, southern

Nevada.

TKsu Continental Sedimentary Rocks Clark County.

Tt2 Welded and Non-Welded Silicic

Ash-Flow Tuffs

Locally includes thin units of air-fall tuff and

sedimentary rock. In places form thick

sequences that constitute virtually entire

mountain ranges.

Tr2 Rhyolitic Flows and Shallow

Intrusive Rocks

Generally far less voluminous that tuffs (Tt2)

during this time period (34-17 Ma), but

probably most voluminous in northwestern

Nevada.

Ta2 Andesite and Related Rocks of

Intermediate Composition Flows and breccias.

Tob Older Basalt Rocks Basaltic lavas are relatively sparse during this

time interval (34-17 Ma).

Ts2 Tuffaceous Sedimentary Rocks Locally includes minor amounts of tuff.

Tt1 Welded and Non-Welded Silicic

Ash-Flow Tuffs

Locally includes thin units of air-fall tuff and

sedimentary rocks.

Tr1 Rhyolitic Flows and Shallow

Intrusive Rocks Lava flows during this time interval (43-34 Ma)

include moderately to highly potassic andesite,

latite, quartz latite, and rhyolite. Ta1 is

Ta1 composed of flows and breccias.

Andesite and Related Flows

rocks of Intermediate

Composition
A6

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Ts1 Sedimentary Rocks

Includes Sheep Pass Formation (Eocene) and

related units and unnamed tuffaceous

sedimentary rocks.

TKs Continental Sedimentary Rocks

Includes units such as Pansy Lee Conglomerate

in Humboldt County, part of Cretaceous(?) and

Tertiary rocks of Kleinhampl and Ziony (1967)

in northern Nye County, and part of "older

clastic rocks" of Tschanz and Pampeyan (1970)

in Lincoln County.

Ks

Continental Deposits of

Siltstone, Shale, Conglomerate,

and Limestone

Includes units such as King Lear Formation in

Humboldt County, Newark Canyon Formation

in Eureka County, Willow Tank Formation and

Baseline Sandstone in Clark County.

Plutonic Rocks

TJgr Granitic Rocks, Central and

Eastern Nevada

Mostly quartz monzonite and granodiorite.

Inconclusively dated or not dated

radiometrically.

Tgr Granitic Rocks Mostly quartz monzonite and granodiorite.

Mzgr Granitic Rocks, Western Nevada

(Mesozoic)

Mostly quartz monzonite and granodiorite.

Inconclusively dated or not dated

radiometrically.

Kgr Granitic Rocks Mostly quartz monzonite and granodiorite.

KJd Diorite

Jgr Granitic Rocks Mostly quartz monzonite and granodiorite.

Trgr Granitic Rocks Quartz monzonite in northern Esmeralda

County.

Igneous and Metamorphic Rocks

KJim Igneous and Metamorphic

Complex

Pegmatitic granite and other granitic rocks

complexly intermixed with metasedimentary

rocks. Considered to be Mesozoic igneous

complex intruding lower Paleozoic and possibly

Precambrian Z sedimentary rocks. Grades into

units shown on map as lower Paleozoic. Ruby

Mountains and East Humboldt Range, Elko
A7

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Jurassic and Triassic Rocks in Western and Central Nevada

JTrsv

Shale, Sandstone, Volcanogenic

Clastic Rocks, Andesite,

Rhyolite, and Locally Thick

Carbonate Units

Undivided sequence locally containing

recognizable equivalents of the Luning and

Dunlap Formations.

Jd Dunlap Formation (Lower and

Middle Jurassic)

Conglomerate, sandstone, greenstone, felsite,

and tuff. Locally contemporaneous with folding

and thrusting. Mineral County and adjacent

parts of Esmeralda and Nye Counties.

Jgb Gabbroic Complex (Lower and

Middle Jurassic)

Includes gabbro, basalt, and synorogenic quartz

sandstone (Boyer Ranch Formation). Churchill

and Pershing Counties.

Jv

Volcanic Sandstone, Felsic

Ash-Flow Tuffs, Rhyolite, and

Rhyodacite Flows (Upper?

Jurassic)

Pony Trail Group of Cortez Mountains, Eureka

County.

JTrs

Shale, Mudstone, Siltstone,

Sandstone, and Carbonate Rock;

Sparse Volcanic Rock (Upper

Triassic and Lower Jurassic)

Includes Auld Lang Syne Group, Nightingale

sequence on Bonham (1969), and Gabbs and

Sunrise Formations.

Trc

Limestone, Minor Amounts of

Dolomite, Shale, and Sandstone;

Locally Thick Conglomerate

Units (Lower, Middle, and

Upper Triassic)

Includes Tobin, Dixie Valley, Favret, Augusta

Mountain, and Cane Spring Formations and

Star Peak Group in central Nevada and

Grantsville and Luning Formations in westcentral Nevada.

Trk Koipato Group and Related

Rocks (Lower Triassic)

Altered andesitic flows, rhyolitic tuffs and

flows, and clastic rocks. Includes rocks mapped

by Silberling (1959) as Pablo Formation and

originally considered to be Permian in the

Shoshone Mountains, Nye County. Includes

Tallman Fanglomerate (Permian?) in Humboldt

County.

Trlgr Leucogranite and Rhyolite

Porphyry

JTra Aztec Sandstone (Triassic? and

Jurassic)

Friable fine- to medium- grained sandstone with

conspicuous large-scale cross strata; considered

eolian. Age based on correlation with Navajo

Sandstone.
A8

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Trch

Chinle Formation and

Associated Rocks (Upper

Triassic)

Continental deposits of variegated bentonitic

claystone, siltstone, and clayey sandstone;

ledge- forming sandstone; and red siltstone.

Trmt

Moenkopi Formation, Thaynes

Formation, and Related Rocks

(Lower Triassic)

Marine deposits of siltstone, limestone, and

sparse conglomerate.

JPu

Volcanogenic Sedimentary

Rocks, Tuff, Andesitic and

Felsitic Flows, and Carbonate

Rocks

Age uncertain. Mineral, Esmeralda, and

northwest Nye Counties.

TrPs Silty Limestone, Minor Amounts

of Shale, and Some Greenstone

Unnamed sequence in Adobe Range, northern

Elko County.

Western Nevada Siliceous and Volcanic Assemblage

(Upper Paleozoic and Triassic)

TrPvs

Volcanic Flows and Flow

Breccias, Chiefly of Andesitic

Composition, Tuffs, Sparse

Sandstone and Graywacke

Includes Happy Creek Volcanic Series and

related rocks in Humboldt County and similar

rocks in Washoe and Pershing Counties;

includes andesite breccias and volcanogenic

sedimentary rocks in Mineral County.

PMh Havallah Sequence of Silberling

and Roberts (1962)

Chert, argillite, shale, greenstone, and minor

amounts of siltstone, sandstone, conglomerate,

and limestone. Includes Schoonover Formation

of Fagan (1962) and Reservation Hill

Formation in Elko County, Farrel Canyon

Formation in southwestern Humboldt County,

Havallah and Pumpernickel Formations in

Pershing, Lander, and parts of Humboldt

Counties, and rocks originally considered a part

of the Pablo and Excelsior Formations in

northern Nye, northern Esmeralda, and southern

Mineral Counties. Assignment of some rocks to

the Havallah sequence in the East Range,

Pershing County, is highly uncertain. Includes

rocks ranging in age from Late Mississippian to

Early Permian.
A9

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Msv Siliceous and Volcanic Rocks

In Humboldt County, consists of altered pillow

lavas, coarse volcanic breccias, clastic

limestone, and minor amounts of sandstone,

shale, siliceous shale, and chert of the Goughs

Canyon Formation (Lower and Upper

Mississippian). In the East Range, Pershing

County, consists of quartzite, conglomerate,

slate, limestone, chert, and greenstone of the

Inskip Formation (Mississippian?).

Ml Massive Limestone In the San Antonio Mountains, western Nye

County.

Conglomerate, Siltstone, and Limestone Sequences within Antler Orogenic Belt

(each sequence rests unconformably on folded Paleozoic rocks in its respective

area of distribution)

TrPd

Conglomerate, Sandstone, Shale,

and Dolomite of Diablo

Formation Below and Shale,

Sandstone, and Conglomerate of

Candelaria Formation Above

(Lower or Upper Permian to

Lower Triassic)

Mineral, Esmeralda, and northwestern Nye

Counties.

PPa

Antler Sequence of Silberling

and Roberts (1962) (Middle

Pennsylvanian to Early or Late

Permian) (Guadalupian)

Conglomerate, sandy to conglomeratic

limestone, limestone, sandstone, and calcareous

shale. Thin detrital and carbonate sequence

within main part of Antler orogenic belt.

Includes units such as Sunflower Formation of

Bushnell (1967) in Elko County, Battle

Formation, Antler Peak Limestone, and Edna

Mountain Formation in Lander and western

Eureka Counties, and Wildcat Peak Formation

in northern Nye County.

MDmc Conglomerate, Limestone, MetaAndesite, Phyllite, and Shale

Includes Grossman, Banner, Nelson, and

Mountain City Formations. Northern Elko

County.
A10

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Carbonate-Detrital Belt Along Eastern Margin of Antler Orogenic Belt

or Western Part of Foreland Basin

PPcd

Sandy and Silty Limestone,

Conglomerate, and Siltstone

(Upper Pennsylvanian to Upper

Permian)

Includes units such as Strathearn Formation of

Dott (1955) and Buckskin Mountain, Beacon

Flat, and Carlin Canyon Formations of Fails

(1960) in Elko County and Carbon Ridge and

Garden Valley Formations in Eureka County.

Pcd

Limestone, Cherty Limestone,

Sandy Limestone, and

Chert-Pebble Conglomerate

(Lower and Middle

Pennsylvanian)

Includes units such as Moleen and Tomera

Formations of Dott (1955).

MDs

Shale, Siltstone, Sandstone,

Chert-Pebble Conglomerate, and

Limestone

Includes units such as Pilot Shale, Joana

Limestone, Chainman Shale, and Diamond Peak

Formation in northern and eastern Nevada and

Narrow Canyon Limestone, Mercury

Limestone, and Eleana Formation in southern

Nevada.

Carbonate Rocks Deposited in Foreland Basin or on Shelf, Eastern Nevada

Pc

Cherty Limestone and Sparse

Dolomite, Shale, and Sandstone

(Lower and Upper Permian)

Includes units such as Park City Group and

equivalent rocks in northern Nevada and

Toroweap Formation and Kaibab Limestone in

southern Nevada.

PMc Limestone, Dolomite, and Shale

(Upper Paleozoic)

Includes Van Duzer Limestone of Decker

(1962).

Psc

Siltstone, Sandstone, Limestone

and Dolomite (Commonly Silty

or Sandy), and Gypsum (Lower

Permian)

Includes units such as Rib Hill Sandstone and

Pequop Formation of Steele (1959) in Elko

County, Rib Hill Sandstone and Arcturus

Formation in White Pine County, Queantoweap

Sandstone of McNair (1951), Hermit Shale, and

Coconino Sandstone in Clark and southern

Lincoln Counties.
A11

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

PPc

Limestone and Sparse Dolomite,

Siltstone, and Sandstone (Lower

Pennsylvanian to Lower

Permian)

Includes units such as undivided Riepe Spring

Limestone of Steele (1960) and Ely Limestone

or their equivalent in Elko, White Pine, and

northern Lincoln Counties and most of the Bird

Spring Formation and Callville Limestone in

Clark and southern Lincoln Counties. Includes

some stratigraphically higher Permian rocks in

Leppy Peak, easternmost Elko County.

Pc Limestone Includes Ely Limestone (mostly Lower and

Middle Pennsylvanian).

Mc Limestone and Minor Amounts

of Dolomite and Shale

Includes units such as Rogers Spring and Monte

Cristo Limestones.

Ultramafic Rocks

Pzsp Serpentinite (Paleozoic) Mineral, northwestern Nye, and eastern

Humboldt Counties.

Siliceous (Western) Assemblage (Lower Paleozoic)

DCsv Chert, Shale, Argillite, Siltstone,

Quartzite, and Greenstone

Undivided siliceous assemblage. Mostly

Ordovician.

Dsl Slaven Chert Chert and sparse limy sandstone, siltstone, and

limestone. Lander County.

Ds

Shale, Siliceous Siltstone, Chert,

and Minor Amounts of

Limestone

Includes Cockalorum Wash Formation on

northern Nye County and Woodruff Formation

and unnamed rocks in Elko County.

Se Elder Sandstone Feldspathic sandstone, siltstone, shale, and

chert. Lander County.

Ss Shale and Chert

Includes Fourmile Canyon Formation in Eureka

County and Noh Formation of Riva (1970) and

unnamed rocks in Elko County.
A12

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Osv Siliceous and Volcanic Rocks

Chert, shale, quartzite, greenstone, and minor

amounts of limestone. Includes units such as

Valmy Formation of north- central Nevada and

some rocks mapped as Palmetto Formation in

northern part of Esmeralda County and adjacent

parts of Mineral and Nye Counties. Locally

includes rocks of Silurian and Devonian age.

Os

Shale, Chert, and Minor

Amounts of Quartzite,

Greenstone, and Limestone

Includes units such as Vinini Formation of

north- central Nevada, Palmetto Formation in

southern and central parts of Esmeralda County,

and Comus Formation in Humboldt County.

Locally includes rocks of Silurian and Devonian

age.

Ch Harmony Formation (Upper

Cambrian)

Feldspathic and arkosic sandstone and minor

amounts of shale, limestone, and chert.

Csc Scott Canyon Formation (Lower

or Middle Cambrian)

Chert, shale, greenstone, and (Lower sparse

limestone and quartzite. Southeast Humboldt

County and Northwest Lander County.

Transitional Assemblage

Dt Argillaceous Limestone, Chert,

and Shale Elko and Eureka Counties.

St Platy Limestone and Limy

Siltstone, Chert at Base

Includes units such as Roberts Mountains

Formation, and Storff Formation and Chellis

Limestone of Decker (1962). Locally includes

rocks of Early Devonian age at top.

Ot Shale, Chert, and Limestone

Includes Aura Formation of Decker (1962) in

north- west Elko County and Perkins Canyon

Formation of Kay and Crawford (1964) in

northern Nye County.

OCt Phyllite, Shale, and Limestone

Locally includes chert and quartzite. Includes

Tennessee Mountain Formation of Bushnell

(1967) in western Elko County, Broad Canyon

sequence of Means (1962) in Lander County,

and rocks originally mapped as Palmetto

Formation in Toiyabe and Toquima Ranges,

northern Nye County.
A13

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

Ct

Shale and Thin-Bedded or

Laminated Limestone; Also

Thinly Interbedded Limestone

and Chert

Includes units such as Preble and Emigrant

Formations.

CZs

Phyllitic Siltstone, Quartzite, and

Lesser Amounts of Limestone

and Dolomite

Includes Reed Dolomite; Deep Sprint, Campito,

Poleta, Harkless, and Saline Valley Formations;

and Mule Spring Limestone.

Zw Wyman Formation Phyllite and phyllitic siltstone and minor

amounts of limestone, dolomite, and sandstone.

Carbonate (Eastern) Assemblage and Related Rocks

Dc

Dolomite, Limestone, and Minor

Amounts of Sandstone and

Quartzite

Includes units such as Sevy and Simonson

Dolomites, Guilmette and Nevada Formations,

and Devils Gate Limestone.

DCc Dolomite and Limestone (Lower

Paleozoic)

Sc Dolomite

Includes units such as Laketown and Lone

Mountain Dolomites. Locally includes rocks of

Early Devonian age at top.

SOc Dolomite

Includes uppermost part of Ordovician System

(Ely Springs Dolomite and equivalent rocks)

and all of Silurian System.

Oc Limestone, Dolomite, Shale, and

Quartzite

Includes units such as Pogonip Group, Eureka

Quartzite, and Ely Springs Dolomite. Where

Ely Springs Dolomite or equivalent rocks are

included in SOc unit, this unit includes only the

Pogonip Group and Eureka Quartzite or their

equivalents.

OCc Dolomite and Limestone Undivided Cambrian and Ordovician rocks in

part of Clark County; mostly Cambrian.

Cc

Limestone and Dolomite,

Locally Thick Sequences of

Shale and Siltstone

Includes units such as Pioche Shale, Eldorado

Dolomite, Geddes Limestone Secret Canyon

Shale, Hamburg Dolomite, Dunderberg Shale,

and Windfall Formation of northern Nevada and

Carrara, Bonanza King, and Nopah Formations

of southern Nevada.
A14

Unit

Abbreviation

Unit

Name

Unit

Description

Table A.1, continued. 

CZq

Quartzite and Minor Amounts of

Conglomerate, Phyllitic

Siltstone, Limestone, and

Dolomite

Includes Prospect Mountain Quartzite, Osgood

Mountain Quartzite, and Gold Hill Formation in

northern Nevada and Stirling Quartzite, Wood

Canyon Formation, and Zabriskie Quartzite in

southern Nevada..

Css Sandstone and Quartzite Includes Tapeats Sandstone and related rocks.

Rests on Precambrian metamorphic rocks.

Zqs

Quartzite, Phyllitic Siltstone,

Conglomerate, Limestone, and

Dolomite

Includes McCoy Creek Group (excluding Stella

Lake Quartzite) of Misch and Hazzard (1962)

in east- central Nevada and Johnnie Formation

in southern Nevada.

Metamorphic and Granitic Rocks

Ygr Granitic Rocks

Porphyritic rapakivi granite; 1,450 plus or

minus 25 Ma (L. T. Silver, oral

communication., 1973).

Xm Metamorphic Rocks

Gneiss and schist and lesser amounts of gneissic

granite, pyroxenite, hornblendite, migmatite,

pegmatite, and marble. Includes highly folded

granite lenses 1,740 plus or minus 25 m. y. old

(L. T. Silver, oral communication., 1973). In

southern Nye County, may be Precambrian Z

rocks metamorphosed during the Mesozoic.
A15

Table A.2. Key to lithologic units. "#"—map class ID number assigned to lithologic unit in maps;

Legend Name—brief unit description appearing in the map legends; Unit—unit abbreviation. From

Stewart and Carlson (1978).

# Legend Description Unit # Legend Description Unit

1 Alluvium Qa 51 Leucogranite & Ryo. Porphyry Trlgr

2 Playa, Marsh, Alluvial-Flat Qp 52 Aztec sandstone JTra

3 Landslide Qls 53 Chinle Fm.-Clays & Clastics Trch

4 Moraine Qm 54 Moenkopi & Thaynes Fm.-Sed. Trmt

5 Older Alluvium QToa 55 Epiclastic, Tuffs, Flows, Carb. JPu

6 Ryo. Flows, Shallow Intrusives QTr 56 Limestone, Clastic, Greenstone TrPs

7 Andesite Flows, Breccias Qta 57 Andesite Flows, Tuff, Clastics TrPvs

8 Basalt Flows Qtb 58 Havallah Seq.-Clastics & Carb. PMh

9 Lacustrine Sedimentary Rocks QTs 59 Siliceous & Volcanic Rocks Msv

10 Rhyolitic Intrusive Rocks Tri 60 Massive Limestone Ml

11 Mafic-Intermediate Intrusives Tmi 61 Diablo & Candelaria Fm.-Sed TrPd

12 Dioritic-Granitic Intrusives Ti 62 Antler Seq.-Clastics & Carb. PPa

13 Breccia Tbr 63 Clastics, Carb., Meta-Andesite MDmc

14 Silicic Ash-Flow Tuffs Tt3 64 Limestone, Clastics PPcd

15 Tuffs, Ryo. Flows, Intrusives Trt 65 Limestone, Cherty Clastics Pcd

16 Ryo. Flows, Shallow Intrusives Tr3 66 Clastics & Limestone MDs

17 Andesite, Intermediate Rocks Ta3 67 Cherty Carbonates, Clastics Pc

18 Andesite & Basalt Flows Tba 68 Carbonates & Shale PMc

19 Basalt Flows Tb 69 Clastics, Carbonates, Gypsum Psc

20 Banbury Formation Tbg 70 Carbonates, Clastics PPc

21 Ash-Flow Tfs, Tfs Sed. Rocks Tts 71 Limestone Pc

22 Tuffaceous Sedimentary Rocks Ts3 72 Limestone, Dolomite, Shale Mc

23 Horse Spring Formation Ths 73 Serpentinite Pzsp

24 Continental Sedimentary Rocks TKsu 74 Clastics, Metamorphic Rocks DCsv

25 Silicic Ash-Flow Tuffs Tt2 75 Slaven Chert Dsl

26 Ryo. Flows, Shallow Intrusives Tr2 76 Clastics, Chert, Limestone Ds
A16

# Legend Description Unit # Legend Description Unit

Table A.2, continued.

27 Andesite, Intermediate Rocks Ta2 77 Elder Sandstone Se

28 Older Basalt Rocks Tob 78 Shale & Chert Ss

29 Tuffaceous Sedimentary Rocks Ts2 79 Siliceous & Volcanic Rocks Osv

30 Silicic Ash-Flow Tuffs Tt1 80 Clastics, Metamorphics, Carb. Os

31 Ryo. Flows, Shallow Intrusives Tr1 81 Harmony Fm.-Clastics Ch

32 Andesite, Intermediate Rocks Ta1 82 Scott Canyon Fm.-Sed & Meta. Csc

33 Sedimentary Rocks Ts1 83 Argillaceous Carb., Chert, Sed. Dt

34 Continental Sedimentary Rocks TKs 84 Limestone, LimyClastic, Chert St

35 Continent Clastics, Limestone Ks 85 Shale, Chert, Limestone Ot

36 Granitic Rocks, Central & East TJgr 86 Phyllite, Shale, Limestone OCt

37 Monzonite & Granodiorite Tgr 87 Shale, Limestone, Chert Ct

38 Granitic Rocks, West Mzgr 88 Clastics, Meta., Carbonates CZs

39 Qtz-Monzonite & Granodiorite Kgr 89 Wyman Fm.-Phyllite & Sed. Zw

40 Diorite KJd 90 Carbonate, Clastics, Quartzite Dc

41 Qtz-Monzonite & Granodiorite Jgr 91 Dolomite & Limestone DCc

42 Quartz Monzonite Trgr 92 Dolomite Sc

43 Igneous & Metamorphic Complex KJim 93 Dolomite SOc

44 Volcano- & Clastics, Carbonate JTrsv 94 Carbonates, Shale, Quartzite Oc

45 Dunlap Fm.-Clastics & Tuffs Jd 95 Dolomite & Limestone OCc

46 Gabbroic Complex Jgb 96 Dolomite & Limestone, Clastics Cc

47 Volcaniclastic, Tuff, Ryo. Flow Jv 97 Quartzite, Clastic, Carbonates CZq

48 Clastics, Carbonate, Volcanics JTrs 98 Sandstone & Quartzite Css

49 Limestone, Minor Clastics Trc 99 Quartzite, Clastic, Carbonates Zqs

50 Koipato Group - Volc. & Clastic Trk 100 Granitic Rocks Ygr

 101 Metamorphic Rocks Xm
A17

 # Lithologic Assemblage

 Map Unit

 Geologic Map of Nevada Lithologic Units

 Composing the Assemblage Unit

1 Quaternary Cover Qa, Qp, Qls, Qm, QToa, QTs

2 Continental Clastics- CZ-MZ TKsu, TKs, Ks

3 Continental Clastics- Mid MZ JTra, Trch

4 Marine & Shelf- Early-Mid MZ JTrsv, Jd, JTrs, Trc, Trmt

5 Marine & Shelf- Late PZ TrPs, Ml, TrPd, PPa, MDmc, PPcd, Pcd, MDs, Pc, PMc, Psc, PPc, Pc, Mc

6 Marine & Shelf- Early-Mid PZ Dt, St, Ot, OCt, Ct, Dc, DCc, Sc, SOc, Oc, OCc, Cc

7 Shelf & Marine- Early PZ-PC CZs, Zw, CZq, Css, Zqs

8 Silicious Rocks- Late PZ TrPvs, PMh, Msv

9 Silicious Rocks- Mid PZ-PC DCsv, Dsl, Ds, Se, Ss, Osv, Os, Ch, Csc

10 Tuff & Sed Rocks- Late CZ Tts, Ts3, Ths, Ts2, Ts1

11 Breccia & Tuffs- Late CZ Tbr, Tt3, Tt2, Tt1

12 Fel-Int Ex- & Intrus- Late CZ QTr, QTa, Tri, Ti, Trt, Tr3, Ta3, Tr2, Ta2, Tr1, Ta1

13 Fel-Int Extrus- Early-Mid MZ Jv, Trk, JPu

14 Int-Maf Ex- & Intrus- Late CZ QTb, Tmi, Tba, Tb, Tbg, Tob

15 Granitic Pluton- Late MZ-CZ TJgr, Tgr, Mzgr, Kgr, KJd, Jgr, Trgr, Trlgr

16 Granitic Pluton- PC Ygr

17 Gabbroic Pluton- Early-Mid MZ Jgb

18 Ultramafic Rocks- Late PZ Pzsp

19 Ign/Meta Complex- Early-Mid MZ KJim

20 Ign/Meta Complex- PC Xm

Table A.3. Key to the makeup of the lithologic assemblage map units. "#"—map class ID number assigned to

assemblage unit in maps; Lithologic Assemblage Map Unit—brief unit description appearing in the map legends;

Geologic Map of Nevada Lithologic Units Composing the Assemblage Unit—the lithologic units from Stewart and

Carlson’s (1978) geologic map of Nevada that were grouped together to create the lithologic assemblage unit.
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APPENDIX B

Weights of Evidence Mineral Potential

Modelling Theory, Implementation, and

FORTRAN Utilities:

Section Contents:

Listing of FORTRAN Utilities

B.1 GIS Software and Workstation

B.1.1 Introduction

B.1.2 Software Specifications

B.1.3 Hardware Specifications

B.2 Data Analysis and Modelling Methodology

B.2.1 Introduction 

B.2.2 Preliminary Spatial Data Analysis

B.2.3 Weights of Evidence Mineral Potential Modelling—An Overview

B.2.4 Weights of Evidence Mineral Potential Modelling—Theory

B.2.4.1 Procedure i—Calculation of Weights of Spatial Association

B.2.4.2 Procedure ii—Multi-map overlay and weights combination using Bayes' Rule

B.3 Conditional Independence

B.3.1 Introduction

B.3.2 Pairwise Test and the Chi-Square Statistic

B.3.3 Overall Test and the Kolmogorov-Smirnov Statistic

B.4 Posterior Probability Uncertainty

B.4.1 Introduction

B.4.2 Source and Types of Uncertainty

B.5 Practical Implementation of the Modelling Procedures

B.5.1 Introduction

B.5.2 Estimation of the Spatial Weights of Association

B.5.3 Generation of Binary-Class Mineral Predictor Maps

B.5.4 Bayesian Map Overlay, and Generation of Mineral Potential and Posterior Probability

Uncertainty Maps

B.5.5 Conditional Independence Testing
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Listing of FORTRAN Utilities:

CI Conditional Independence Contingency Table Utility.

CONTRAST Individual Case Weights of Spatial Association Estimation.

DLM2VEC Convert Delimited List of Longitude and Latitude Coordinates to Spans

Vec./Veh. Has a bug: longitude must be in column one, latitude in two.

DELIMIT Example Input File for DLM2VEC.

KOLMOG Kolmogorov-Smirnov Calculation Utility.

PREDICT Weights and Various Uncertainty Estimation Utility. Also Does Direct

Bayesian, Logistic Regression, Etc.

THINOUT Utility for Deleting Points Nearer to One Another Than Some User Specified

Threshold.

WEIGHTS Modified Weights Calculation Utility. Cosmetic Modifications to the Output

Tables, and Input Tables Can Be Wider.

WTS Original Weights Calculation Utility.

For additional information about these utilities, consult the comments in the .for source code

files. These programs are written to run with output from SPANS 5.2.
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B.1 GIS Software and Workstation

B.1.1 Introduction

A GIS is a database management system that links data items by geographical position as the

primary means by which to organize spatial information. It provides the functionality for

integrating, analysing, and displaying large volumes of data from many sources. Most

geoscience data can be readily managed and analysed by geographical criteria. Traditionally,

geoscientists have integrated data on paper and stable base maps using manual techniques.

Whereas conventional maps are not quickly or easily modified and manipulated, digital data can

readily be edited, manipulated, transformed, and processed in dynamic environments which

permit exploratory data analysis and modelling.

B.1.2 Software Specifications

SPANS (SPatial ANalysis System) version 5.4 by TYDAC Technologies Inc. is a quadtreebased GIS which uses a raster data structure with a variable pixel size (Fig. B.1). The maximum

raster image resolution is 215 by 215 (32,768 by 32,768) pixels. The quadtree is a hierarchical data

structure which differs from a conventional raster structure in that the pixel size is not fixed, and

that the data items are spatially addressed using the Morton coordinate system rather than the

standard Cartesian coordinate system [x,y]. SPANS quadtrees allow a range of class (intensity)

values of 0-226 as compared to 0-28

 or 0-216 for standard raster images. In addition, the quadtree

equivalent of a raster image requires considerably less storage space. SPANS also employs a

topological vector structure, with vertices defined in precise Morton coordinates, and a variety

of table structures for handling attribute data. Excellent reviews of GIS data structures may be

found in Aronoff (1989), Star and Estes (1990), Maguire et al. (1991), and Bonham-Carter

(1994a).

SPANS is structured to facilitate the integration of data from various sources and accommodates

numerous spatial and non-spatial data structures. Data may be readily exchanged between many

external statistical applications, database managers, spreadsheets, word processors, and drawing

packages using ASCII and other popular file formats. SPANS also provides a comprehensive

and powerful array of analytical and modelling tools which permits the user to perform

customized operations on multiple map layers. Up to 15 quadtree maps can be combined in one

step using a modelling language. A complementary digitizing package, TYDIG, allows analog

data to be directly captured in a vector or point format.

FORTRAN command-line utilities used for implementing some of the mineral potential

modelling procedures carried out in this study are discussed separately in section B.5.

B.1.3 Hardware Specifications

This research was carried out on two IBM-compatible PC workstations. Most of the data

compilation and integration of data into the GIS environment was carried out on an 80486 IBM

PC/AT compatible computer operating at a clock speed of 33 MHZ with ISA-16 bit bus internal

architecture and 16 Mb of total system memory. This system is equipped with a SCSI disk drive
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controller, a 220 Mb hard disk drive, two high density floppy disk drives (51⁄4 and 31⁄2 inch), a

Sony CDU-7101W external CD-ROM drive unit operating on MS-DOS CD-ROM Extensions

version 2.10, a 3.5 by 2.5 foot Gentian Thinline digitizing tablet, a non-interlaced SVGA 17 inch

color monitor powered by an ATI Technologies Inc. Graphics Vantage graphics accelerator video

card which provides 256 colors at a monitor resolution of 1024 by 768 pixels. The workstation

is run under the OS/2 operating system (version 2.1) in Presentation Manager mode with a bus

mouse, networked to a Novell 3.11 file server with 1 Gb storage capacity. Analysis and

modelling of the spatial data were completed on a similar computer operating at a clock speed

of 100 MHZ with ISA-32 bit local-bus internal architecture, 48 Mb of total system memory, and

a 2.7 Gb SCSI hard disk drive operating under OS/2 2.11. SPANS will also operate under DOS

and on a variety of UNIX platforms.

The data listed in Table 5.1 occupy almost 600 Mb of hard disk space. The vector datasets which

represent the geological map of Nevada, for instance, are nearly 38 Mb in size. After these

datasets were processed and converted into quadtree maps, the total storage space required for

vector and raster structures approached 85 Mb (this is for the highest resolution quadtree

structure at a quadlevel of 215 which is equivalent to an ordinary raster of 32,768 by 32,768

cells). A high capacity hard disk is necessary. Computer processing of these data are memory

intensive, particularly for efficient spatial interpolation of large point datasets. Slight

modification of the default OS/2 CONFIG.SYS system file was necessary to successfully process

some of the data. System performance was enhanced by increasing the buffers and disk cache

size.

B.2 Data Analysis and Modelling Methodology

B.2.1 Introduction 

Spatial data analysis can be preliminary in nature, involving non-statistical or summary statistical

approaches (e.g.—visual pattern recognition and simple map reclassification, or, histogram

generation and area analysis, respectively), or it can be more sophisticated, involving statistical

approaches with probabilistic components (e.g.—regression analysis, weights of evidence).

Both preliminary and more sophisticated analysis methods were used to identify and measure the

spatial associations between gold-silver-bearing occurrences and various geoscientific datasets.

Section B.2 reviews these techniques. The error and uncertainty associated with mineral

potential modelling are considered in sections B.3 and B.4. Practical implementation of mineral

potential modelling and error analysis is reviewed in section B.5. For a thorough and detailed

treatment of spatial modelling and GIS in general, see Bonham-Carter (1994a).

B.2.2 Preliminary Spatial Data Analysis

This research was carried out using many spatial data manipulation and analysis tools available

in a standard GIS environment. A comprehensive examination of these tools is beyond the scope

of this study, however, a general overview of the techniques useful for implementing weights of

evidence mineral potential modelling is presented. For detailed reviews of basic spatial data
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manipulation and analysis methods, consult Antenucci et al. (1991), Aronoff (1989), BonhamCarter (1994a), and ESRI (1994).

The preliminary spatial data manipulation and analysis techniques used here include: (1) map

reclassification/generalization; (2) map overlay/combination; (3) spatial and topological

modelling; and (4) visual pattern recognition and spatial query. All of these methods assist in

clarifying and defining trends and patterns.

Map reclassification is one of the most elementary, yet most powerful techniques of analysis in

a GIS. This technique is applicable to nominal, ordinal, interval, or ratio scale data. Map units

may be grouped and displayed to generalize and emphasize spatial relationships and highlight

various attributes. Reclassification may be accomplished using a variety of interactive and semiautomated techniques. One of the most common methods is table reclassification. This method

makes use of polygon or map class attributes that are arranged in a table by rows and columns—a

row (representing a map class) is composed of fields (the columns) which contain spatial or nonspatial attribute information relating to the map class (Table B.1). Each unique class of a map

is keyed to the table by the map class or polygon number. For a geological map, a typical

reclassification table might contain attribute information which categorizes the geologic units

by rock type, lithology, age, or by any characteristic that is desired. In the reclassification

operation, the geological map units that do not satisfy the reclassification conditions are

reclassified to “0”, a null class. Those units that do satisfy these conditions either retain their

original map class values, or are re-coded to new values, and are incorporated into the new map.

In many instances, histograms were produced from the numeric data associated with the map

classes to determine breakpoints for map reclassification schemes.

The process of overlaying or combining two or more maps into a single map is fundamental to

weights of evidence modelling, and to GIS modelling and analysis in general. This is a powerful

technique for examining the spatial patterns caused by the interaction of one or more maps

(Bonham-Carter, 1994a). Maps are typically overlain or combined using a set of rules defined

by various arithmetic and Boolean operators in a variety of combinations (a “map model”). The

main map overlay operations are illustrated in Figure B.2, and include:

C Impose—Portions of one map are cut away based on the boundaries of a second (the second is typically a map

consisting of one class). This technique is useful for creating a “mask” to cover or remove parts of a map (Fig.

B.2a).

C Stamp—A map is overlain (placed on top) of a second. The map placed on top is reclassified by adding the

maximum class value of the bottom map to the class values of the top map. This technique is useful for

constructing complex maps from several simpler (or binary-class) maps (Fig. B.2b).

C Join—The classes of one map are merged with those of a second. This operation is useful for joining two or

more overlapping map sheets together (Fig. B.2c).

C Compare—Two maps are combined so as to highlight where the maps are the same, different, or where map

classes show spatial coincidence. In more sophisticated applications of this technique, the output map class

values can be interactively assigned to each combination of classes of the two input maps (Fig. B.2d)

C Map/Class-Weighted—Two or more maps are combined using a user-specified relative weighting scheme

which ranks the importance of a map and/or map class unit. This technique is useful for assessing the relative

suitability or risk associated with some factor (e.g.—“Given the relative importance of various geological

factors, is this a good place to prospect for gold?”) (Fig. B.2e).

C Unique Conditions—Multiple maps are combined to create a single map composed of unequal-area polygons

that are assigned the same class value if they consist of the same overlap combination or characteristic of input
B6

maps(Fig. B.3). This operation produces an associated (linked) attribute table that contains the unique overlay

combinations of input map classes that compose a particular unique conditions map class (Table B.2). This

is a very powerful map combination method that allows modelling or data analysis to be performed on the table

(“table modelling”), rather than using the input maps directly (“map modelling”). The unique conditions map

can be reclassified according to the modelling or analysis results, which are directly linked to the unique

conditions map via the unique conditions table. This technique plays an especially important role in

implementing weights of evidence mineral potential modelling (for additional details on unique conditions

maps, see Bonham-Carter, 1994a, p. 235-238; Bonham-Carter, 1991, p. 172).

The spatial and topological modelling techniques used for this analysis include geometric,

coincidence, and adjacency analysis. These techniques are applicable to point, line, and polygon

spatial objects. Geometric modelling involves calculating polygon areas and perimeters, the

generation of point and line buffers, and calculating the distance between objects. Coincidence

modelling includes polygon overlay operations and point-in-polygon analysis. Adjacency

modelling deals with spatial interpolation (e.g.—surface contouring or generation) and

neighbourhood operations, as well as network analysis and similar types of operations.

Coincidence and adjacency modelling use spatial and topological information, whereas

geometric analysis does not necessarily require such information (e.g.—map unit area analysis

and vector/point buffering). Most vector and point datasets were transformed to quadtree raster

maps by distance buffering the vector datasets and by surface interpolation of values associated

with the point datasets. Some quadtree maps were further processed using spatial neighbourhood

analysis to determine map complexity (e.g.—diversity of geology) and to examine the “grain”

of geographic/geologic features (e.g.—angle of incidence illumination of topography). Area and

point-in-polygon analyses were performed on most maps to delineate the spatial extent occupied

by each map class, and to determine the number of points, in this case gold-silver-bearing

occurrences, associated with each class. 

In addition to the quantitative analysis techniques described above, each map was also visually

inspected. A number of the datasets were also viewed in shaded-relief and as 3-dimensional

renderings to enhance textural, shape, and directional features. The human eye has an

extraordinary ability to distinguish subtle patterns within otherwise complex spatial

arrangements. Visual examination of the maps facilitated recognition and interpretation of

regional-scale tectonomagmatic trends and patterns which might have otherwise be hidden from

area, point, or polygon-based analysis methods. A technique complementary to visual appraisal

is spatial query. Simple interactive spatial query techniques were used to examine concurrently

the relationships between multiple map layers and an individual or group of mineral occurrences.

In this manner, any number of evidence maps were simultaneously queried so as to determine

their collective or unique properties and characteristics at specific occurrence locations.

B.2.3 Weights of Evidence Mineral Potential Modelling—An Overview

Weights of evidence (WOE) provides a measure of spatial association (a “weight”) between

multi- and/or binary-class map patterns and known point or polygon objects, and uses Bayes’

probability theorem to combine the map patterns to predict the distribution of the point or

polygon objects. As applied to mineral potential modelling, the point objects represent known

mineral deposits. The multi-class map patterns are typically maps of particular geologic

phenomena, such as geology, geochemistry, geophysics, etc., which are likely to be useful as

mineral deposit predictors. These maps are referred to as “evidence maps”, representing geo-
B7

spatial evidence for the occurrence of mineralization. In order to facilitate combination, the

evidence maps are usually reduced to deposit-indicator or mineral “predictor maps” of a few

discrete states (typically binary) where the spatial association between mineralization-favorable

evidence and the occurrences is optimized. The mineral predictor maps collectively constitute

the “layers of evidence” for the mineral potential model.

A layer of evidence need not completely cover the study area, as the WOE modelling method can

easily accommodate missing data (incomplete coverage). This is an important and distinct

advantage of this method. Other advantages include uncomplicated calculations, an objective

procedure for weighting the evidence layers, relatively straightforward interpretation of the

weights, direct user involvement with optimization of the evidence maps, which is an important

inductive process that provides insight into the spatial data relationships, and the ability to model

the uncertainty of the mineral potential map (Bonham-Carter, 1994a, 199b).

The techniques employed by WOE were developed to model spatial data, and depart from

classical (non-spatial) statistical theory. The measure of spatial association and the combining

of spatial data must therefore be performed and evaluated with care because they are sensitive

to choice of the measurement domain, and because their statistical significance is normally not

possible to estimate by conventional means (Bonham-Carter, 1994a). The following paragraphs

describe and illustrate WOE, and are excerpted largely from Agterberg (1989a), Agterberg et al.

(1990), Bonham-Carter (1994a, 1994b), Bonham-Carter (1991), Bonham-Carter et al. (1988),

and Bonham-Carter et al. (1989). 

Weights of evidence (WOE) is a discrete, multivariate statistical method based on a technique

originally developed in a non-spatial context for medical diagnosis (Spiegelhalter and KnillJones, 1984) and has been modified by Bonham-Carter et al. (1989) to deal with spatial

prediction—“diagnosing” mineral deposits using the “symptoms” of various geologic

phenomena (Bonham-Carter, 1994b). WOE evaluates the spatial distribution of known mineral

deposits (the response variable) relative to multi- or binary-class map patterns (the predictor

variables), calculates weights of spatial association (W+ and W–) for each pattern, and produces

a multi-map signature for mineralization (Bonham-Carter et al., 1989). The evidence layers are

combined using Bayes' Rule in a multi-map overlay operation where the prior probability of an

occurrence—the probability of an occurrence given no information (random), equal to the

average density of known occurrences in the study area, and held constant over the whole

area—is updated by the addition of predictor variables and their weights to produce a single

posterior probability map of occurrence (Bonham-Carter et al., 1989; Bonham-Carter, 1991).

In this case, the posterior probability map is a map of mineral potential which reflects the

distribution of known occurrences and predicts the distribution of yet unidentified occurrences

(Bonham-Carter et al., 1988). The whole process is similar to that of an exploration geologist

manually integrating information and combining maps in order to delineate favourable areas of

mineralization (Agterberg et al., 1990).

Weights of evidence is a data-driven method, requiring data about the distribution of known

mineral deposits to estimate weights of spatial association for the mineralization evidence layers.

WOE is similar in some ways to logistic regression in that the distribution of mineral deposits

over the study area must be known, however, in regression modelling, calculation is complex,

the weighting factors are not so readily interpreted, and there is no accommodation for missing
B8

1 As used here, the term “training area” is defined as a sub-region of, or a region outside of, the study area

which is designated an experimental control, where (1) the geologic conditions are similar to the prospecting

region, (2) all the mineral deposits are known, and no more are expected to be discovered, (3) all of the geospatial mineralization evidence is present (i.e.—full evidence map coverage), and (4) the quantitative

relationships (the spatial weights of association) between the mineral deposits and the evidence are

established (Chung and Moon, 1990; Chung, 1995). 

2 Deposit models and modelling will not discussed here, as this topic is the focus of numerous authors,

including Babcock (1984), Barton (1986), Berger and Jefferson (1986), Bliss (1992), Bliss and Menzie

(1990), Cox and Singer (1986), Eckstrand (1984), Kirkham et al. (1993), Singer and Cox (1988), and Singer

(1993).

data (Agterberg et al., 1990; Bonham-Carter, 1994b). Weights of evidence differs from

knowledge-driven methods which do not require deposit distribution data and where the weights

are estimated by an expert, such as in the fuzzy logic method (An et al., 1991; Bonham-Carter,

1994a; Wright and Bonham-Carter, 1996), Dempster-Shafer method (Moon, 1990; Leverington

and Duguay, 1994; Wright and Bonham-Carter, 1996), or in the PROSPECTOR expert system

(see Bonham-Carter, 1994a, chapter 9). 

In WOE modelling, no “training area1” is used to select the geo-spatial mineralization evidence

or to calculate “initial baseline” weights (i.e.—to establish the “initial conditions”). The

selection of mineralization evidence maps is largely guided by a conceptual model (this is the

normal mode of implementation; see Bonham-Carter et al., 1989; Bonham-Carter, 1994a;

Wright, 1996; Wright and Bonham-Carter, 1996). The conceptual model is usually a synopsis

that may contain elements of deposit models, which typically focus on characteristics at the

deposit scale, and/or exploration models, which include deposit model components that apply

to regional-scale exploration, in addition to geophysical, geochemical, remotely-sensed, and

other data useful for delimiting regions that may warrant further detailed investigation2. The

choice of evidence should reflect current understanding on the genesis of the particular deposit

type being modelled as well as the geologic features believed to control its spatial distribution.

An evidence map should ideally provide either universal coverage or coverage over the majority

of the study area (Bonham-Carter et al., 1989). 

The “initial conditions” of the mineral potential model are established using the optimized

mineralization evidence maps (the mineral predictor maps), where the weighting of each

individual layer of evidence is determined automatically using the locations of known mineral

deposits over the entire study region for training (Wright and Bonham-Carter, 1996). A model

may be further calibrated using other factors such as mineral deposit size, where different

schemes for weighting the layers of evidence are calculated for each deposit size subset (Wright

and Bonham-Carter, 1996). For Nevada, numerous maps of various geologic phenomena and

the distribution of known mineral occurrence types and sizes were used to design, train, and

calibrate the models used in this study. The selection of mineralization evidence maps and the

subdivision of the mineral occurrences considered in this study are detailed in Chapter 5. Spatial

optimization of the evidence maps with respect to the mineral occurrences is covered in Chapter

6.

An important assumption made in WOE modelling is that the mineralization evidence layers

included in a model be conditionally independent (CI) of one another with respect to the mineral
B9

deposits (see Bonham-Carter, 1994a, pp. 312-317). In practice, CI is probably always violated

to some degree, and the possibility of the occurrence of CI generally increases with an increase

in the number of evidence layers included in a model (Bonham-Carter, 1991; Bonham-Carter,

1994a). Because of the CI assumption, calculation of the spatial weights of association are

carried out independently between the mineral deposits and each evidence layer, and as a result,

WOE has the opportunity to examine bivariate relationships in some depth (Bonham-Carter,

1994b). The assumption of CI leads to a model that, like most models, does not fit the data

perfectly, but provides a simplification that, when applied carefully, is useful for prediction and

gives insight into the relative contributions of the separate sources of evidence (Bonham-Carter,

1994a). Conditional independence can be checked visually and with statistical tests (pairwise

or overall tests) to isolate the layers that clearly violate the assumption. The offending map

layers can then either be rejected from the analysis altogether or modified in some manner as to

lessen their affect, such as by combining maps using Boolean operators to form a new single

joint map (Bonham-Carter, 1991; Bonham-Carter, 1994b).

As applied in this study, the methodological framework for producing mineral potential maps

using the WOE modelling method consists of four main steps:

1. Determination of the deposit-type characteristics and development of a conceptual model. The

characteristics of the deposit types examined in this study are detailed in Chapter 3 and summarized as

synoptic models. Further geological and statistical evaluation of certain features which may be related to

the regional-scale distribution of the mineral occurrences is reviewed in Chapter 6.

2. Development of the mineral potential spatial database. This step involved the compilation, preparation, and

integration of numerous data sets from a variety of sources, and is reviewed in Chapter 5.

3. Development, design, and training of the model, where information critical to mineral occurrence prediction

was extracted from the mineralization evidence maps. This involved the selection of promising multi-class

evidence maps, the measurement of spatial association between the evidence maps and the mineral

occurrences, determination of the significance of these associations, and, based upon the strength and

importance of the spatial associations, the reclassification of the multi-class evidence maps to binary-class

mineral predictor maps (layers of evidence). This process is the subject of Chapter 6.

4. Application of the model. This step involved the combination of the mineral predictor maps to generate the

mineral potential maps and the examination and evaluation of uncertainty and error. This process is covered

in Chapter 7 and Appendices C and D. Interpretation of the model output with respect to geology and

metallogeny is discussed in Chapter 8.

The theoretical aspects of these steps follows immediately below. Practical implementation is

discussed in section B.5.

B.2.4 Weights of Evidence Mineral Potential Modelling—Theory

The weights of evidence method is illustrated in Figure B.4 and can be subdivided into two main

procedures:

i. Calculation of W+and W– weights of spatial association between the response variable and the predictor

variable(s), in this case point objects representing mineral deposits and multi-class map patterns representing

various geologic phenomena. Generation of the mineral predictor maps (the layers of evidence) by

reclassification of the multi-class predictor variables to binary-class predictor variables, using the weights

of spatial association for each predictor-response variable set as a guide, where: binary pattern present
B10

represents mineralization-favorable areas, and binary pattern absent represents mineralization-unfavorable

areas.

ii. Integration of predictor variables in a multi-map overlay operation where a loglinear version of Bayes' Rule

is used to sum and update the weights associated with each of the map classes that come into coincidence,

producing a compound map which closely exhibits the distribution of known deposits and indicates areas

where more deposits are expected than are observed.

The goal of weights of evidence modelling is to estimate the posterior probability of the

occurrence of a mineral deposit, given the presence or absence of mineralization-favorable

evidence, which, depending on the evidence, may be larger or smaller than the prior probability.

Procedure ii typically involves conditional independence testing. This issue is addressed under

a separate heading below (section B.3).

B.2.4.1 Procedure i—Calculation of Weights of Spatial Association

In the first procedure, Figure B.4i, area analysis is performed on each of the predictor variable

map layers and point-in-polygon analysis is performed on each response/predictor variable set.

The output of the area analysis is a table that contains: (1) the percent of the total area occupied

by each map class, (2) a running total of area by map class, and (3) area in km2

 occupied by each

map class (Table B.3). The point-in-polygon analysis appends the map class of the predictor

variable to the response variable point dataset. The output is a table containing the coordinates

of the response variable points and the predictor variable map classes into which the points fall

(Table B.4).

The area analysis (Table B.3) and point-in-polygon analysis (Table B.4) data characterize the

spatial attributes of the response and predictor variables, and are used to calculate a pair of

weights, W+

 and W–, for each predictor variable map class (see section B.5.2 for implementation

procedure). The value of the weights represent a ratio of area proportions, and is dependent upon

the ratio of the number of deposits that fall within a particular map class to the total number of

deposits, against, the ratio of the particular map class area to the total map area (Bonham-Carter,

1991). In the case of a multi-class predictor variable, each map class is treated individually as

a binary-class map pattern (map class present or absent), and is composed of the area of the

particular map class being evaluated and the combined total area of the remaining map classes.

A mineral deposit is assumed to occupy a small unit area and is likewise regarded as a present

or absent binary pattern. There is no provision for modelling deposit size, grade, or tonnage in

WOE. All area values are expressed in unit cells, where N is the number of unit cells and an area

is denoted as N( ). For the analyses carried out here, a 1 km2 unit cell was used to represent a

mineral deposit.

The coincidence between two binary patterns, a deposit distribution pattern (D) and a mineral

predictor pattern (B), yields four possible spatial overlap relationships:

1. Both patterns are present.

2. Pattern D is present, pattern B is absent.

3. Pattern D is absent, pattern B is present.

4. Both patterns are absent.
B11

P(D|B) ' N(B_D)

N(B) , P(D|B) ' N(B_D)

N(B) , P(D|B) ' N(B_D)

N(B) , and P(D|B) ' N(B_D)

N(B)

B.1

W% ' ln P(B|D)

P(B|D)

, and W& ' ln P(B|D)

P(B|D)

B.2

W% ' W& ' 0 B.3

C ' | W% & W& | B.4

The four overlap states are illustrated in Figure B.5. From these four mutually exclusive

response-predictor variable overlap states, four conditional probabilities can be estimated using

area ratios:

Using the conditional probabilities defined in equation B.1, the weights W+

 and W– for each

predictor variable map class are calculated as log ratios in the following manner:

Detailed discussion and the derivation of these equations are found in Bonham-Carter et al.

(1989) and Bonham-Carter (1994a), as well as Agterberg (1989a), Agterberg et al. (1988),

Bonham-Carter (1994b), Bonham-Carter (1991), Bonham-Carter et al. (1988).

W+

 is a measure of the presence of a map class relative to the total number of deposits, whereas

W-

 is a measure of absence of the map class (as distinct from unknown). W+ and W– are always

opposite in sign, except when the proportion of deposits falling in a map class to the total number

of deposits is the same as the proportion of the map class area to the total study area, then:

and no spatial association between the map class and the points exists. If more deposits are

spatially associated with a map class than would be expected due to chance, W+

 is positive and

W–

 is negative, indicating a positive correlation. Where the presence of a map class “inhibits

deposits”, W+ is negative and W– is positive, indicating a negative or inverse correlation. Where

information is missing (i.e.—incomplete predictor variable coverage), W+ and W– are set to zero

(Bonham-Carter, 1994a). 

The weights can be combined into a single coefficient called the contrast (C), where:

and | | denotes absolute values. The contrast provides a useful measure of the strength of the

spatial association between the mineral deposit pattern (response variable) and the mineral

predictor pattern (predictor variable).
B12

Deposit (D), given presence of binary mineral predictor pattern (B):

P(D|B) ' P(D) P(B|D)

P(B)

Deposit, given two binary mineral predictor patterns:

P(D|B1_ B2) ' P(B1_ B2|D) P(D)

P(B1_ B2|D) P(D) % P(B1_ B2|D) P(D)

The effects of interaction between B1 and B2 can be ignored by making an

assumption of conditional independence, thus, the conditional

probability of a deposit, given two mineral predictor patterns:

P(D|B1_ B2) ' P(D)

P(B1|D)

P(B1)

P(B2|D)

P(B2)

B.5

P(D) ' N(D)/N(A) B.6

B.2.4.2 Procedure ii—Multi-map overlay and weights combination using Bayes' Rule

In the second procedure, Figure B.4ii, Bayes' Rule is used to combine the evidence represented

by the predictor variables (multiple binary- and/or multi-class predictor maps) to determine the

probability that the response variable is present (i.e.—the probability that a mineral deposit is

present at a given location based upon the evidence). One of the main concepts of Bayes'

probability law is that of prior and posterior probabilities. Bayes' Rule gives the probability that

some particular phenomena (B) is “the cause among many causes” of a known end effect (D)

(Mendenhall and Reinmuth, 1974). In the case of mineral potential modelling, the preceding

“cause” phrase might be better replaced with “the most highly correlated geologic phenomena,

among many phenomena, that are spatially associated to known mineral deposits”. 

From Bayes' Rule, the conditional probability that a unit cell contains a deposit, given the

presence of a binary mineral predictor pattern, is given as:

The key to combining binary mineral predictor maps in WOE is the third expression, which

permits multiplication of each predictor map’s effect, independent of one another. The

computed probability P(D|B) is the posterior probability of known end effect (D) given the

information contained in phenomena (B). P(D) is the simple, or prior probability of (D), and

does not take into account the information contained in (B). The value of the prior probability

is assumed to be the average density of known deposits in the region of interest and is constant

throughout. This can be calculated as:

where the area of deposits, D, and total area of the study area, A, are expressed as the count of

unit cells, N(). The posterior probability reflects both the prior and current information, and
B13

ln O(D|B

k(1)

1 B

k(2)

2

...B

k(n)

n

) ' ln O(D) % j

n

j'1

W

k(j)

j B.7

provides a more efficient model for prediction. Bayes' Rule effectively updates or revises the

prior probability by incorporating the observed information contained within (B) into the model

(Mendenhall and Reinmuth, 1974). In this instance, the particular phenomena (B) is some

predictor variable, such as geology or a geochemistry anomaly, and the known end effect (D) is

the response variable (mineral deposits). 

The weights for each predictor variable map class are used to update the prior probability of the

occurrence of a deposit, producing a revised posterior probability with the introduction of each

subsequent layer of evidence (a binary- or multi-class mineral predictor map). As new evidence

is introduced, the prior probability surface is modified, either increasing or decreasing according

to the evidence, to produce a posterior probability surface (Bonham-Carter et al., 1991). Where

the predictor variable map class is present, the value of W+ is added. Where the predictor

variable map class is absent, W– is added. Where a map pattern is unknown or incomplete, W=0,

so that the predictor variable map class neither causes an increase nor a decrease from the prior

probability (Agterberg, 1989a). Thus, given the presence of a favourable predictor variable

(i.e.—a map class with a high W+, such as an associated geochemical anomaly), the posterior

probability of the occurrence is higher than the prior probability (Bonham-Carter, 1994a). The

posterior probability surface is output as a map which represents the incorporation of all layers

of evidence and predicts the distribution of the response variable.

In practice, WOE uses the natural logarithm of odds, also known as log odds or logits,

formulation of Bayes' Rule, which may possibly be more familiar in terms of probabilities and

multiplicative terms. In this form, probabilities are expressed as logits, which make the model

linear and additive, and is expressed as:

where: O are odds, defined as O=P/(1-P); D is the number of unit cells containing a mineral

deposit; j = 1,2,3... n binary-class predictor map patterns; Bj is the area of the j-th predictor

variable map pattern; k(j) is (+) for presence or (-) for absence of the j-th predictor variable map

pattern; Wj is the spatial weight of association for the presence and absence of the j-th predictor

variable map pattern.

The posterior logit of a cell containing a mineral deposit is obtained by the adding appropriate

weights (W+ or W–) for presence or absence of the binary-class predictor maps to the prior logit.

The order of the calculations are as follows:

1. Calculation prior logit from prior probability, using O = P / (1-P).

2. Addition of the prior logit to the sum of the weights to derive the posterior logit.

3. Conversion of the posterior logits to posterior odds.

4. Conversion of posterior odds to posterior probability.

As the final step, a user-defined classification scheme is applied to the calculated posterior

probabilities to produce a thematic map of mineral potential. Each mineral potential map class
B14

represents a unique overlap condition between binary- and/or multi-class predictor maps (which

may consist of a single polygon, or most often, a number of polygons that collectively compose

a mineral potential map class; see section B.2.2). In practice, this procedure can be carried out

on a unique conditions table associated with a unique conditions map. The implementation of

this procedure is described below in section B.5.4.

B.3 Conditional Independence

B.3.1 Introduction

The robustness of a posterior probability map is in part dependent on the conditional

independence (CI) of the predictor variable map patterns to one another (binary-class mineral

predictor map patterns), with respect to the response variable (mineral deposit point

occurrences). The posterior probability map is adversely affected if, at the locations of the

known mineral occurrences, the presence of a mineralization-favorable map pattern in one layer

of evidence is dependent on the presence of a mineralization-favorable map pattern in another

layer of evidence. Conditional independence (CI) is assumed to exist, and is necessary if the

calculated posterior probabilities are to be strictly accurate (Agterberg, 1989b; Bonham-Carter,

1994a). The degree to which this assumption is satisfied can be tested for, and the purpose of

such a test is to determine the magnitude of any binary-class predictor map pattern dependencies

and to identify the map(s) that are responsible for the dependency. If a map is found to be in

serious violation of the assumption of CI, it can then be (1) rejected from the model, (2)

combined with another map in order to minimize the dependency, or (3) modified in some way

as to reduce the problem (for more information, see Agterberg et al., 1990; Bonham-Carter,

1994a). Violation of CI results in either the over-estimation or under-estimation of posterior

probabilities during the Bayesian map overlay procedure, and the expected mineral deposit

frequencies either notably exceed or fall short of the observed deposit frequencies in the most

and least favorable areas of the mineral potential map (Agterberg et al., 1990). In essence, the

most and least favorable binary-class predictor map overlap areas are “artificially” strengthened

or weakened in the posterior probability mineral potential map. In practice, CI is always violated

to some degree (Bonham-Carter, 1994a), but it is important to understand how serious the

violation is so that the appropriate action can be taken to minimize the problem and so that

proper judgments can be made when evaluating areas of high mineral potential. 

Two tests are applied to determine whether the assumption of conditional independence is

satisfied: a pairwise test and an overall goodness-of-fit test, both of which make use of the

observed versus the expected number of observations (mineral deposits). The pairwise method

determines CI between all possible pairings of the binary-class predictor maps by calculating a

x2

 (chi-square) statistic for each map pair, and comparing the calculated value of x2 to the tabled

value of x2 having one degree of freedom (Bonham-Carter, 1994a). The overall method is a

simple procedure involving the relative comparison of predicted versus observed deposits—if

the total predicted number of deposits is much larger than the total observed number (greater

than ~10-15%), it suggests that CI is being violated (Bonham-Carter, 1994a).

The overall goodness-of-fit can be evaluated using the Kolmogorov-Smirnov (K-S) statistic. The

K-S statistic is based on the maximum deviation of the observed number of deposits from the
B15

predicted number. The K-S statistic makes as few assumptions about the distribution of the data

as possible (i.e.—it is non-parametric: it does not depend upon any assumptions regarding the

normality or the distribution of the data), and generally has satisfactory efficiency and robustness

properties (Davis, 1973; Chatfield, 1988). Non-parametric tests of model-fitting are usually used

for ordinal scale data or data from a skewed or otherwise non-normal distribution, which

therefore makes the K-S statistic particularly well suited for this mineral potential modelling

application. The implementation of the pairwise and overall tests is given in section B.5.5.

B.3.2 Pairwise Test and the Chi-Square Statistic

The pairwise goodness-of-fit test measures conditional independence (CI) between all possible

pairings of binary-class predictor maps (with respect to the mineral deposits) by calculating the

x2 (chi-square) statistic for each map pair. CI may be present due to three-way or multi-way

interactions, and testing for these cases is also possible, but for practical purposes, pairwise

testing reveals the most serious CI violations (Bonham-Carter, 1994a).

Normally, the pairwise test is conducted before the binary-class predictor maps are combined in

the Bayesian multi-map overlay procedure (procedure ii, Fig. B.4ii), so that problematic maps

can be eliminated, combined, or modified as necessary. The test is performed using normal 2-

by-2 contingency tables of observed and expected values (i.e.—number of deposits), and tests

the null hypothesis (Ho) that “rows and columns are independent” (the assumption of CI)—that

is, that the probability of an observation falling in any particular column does not depend on

which row the observation is in, and vice versa (Chatfield, 1988; the application of this test to

mineral potential modelling is explained in detail in Bonham-Carter, 1994a, p. 313). Table B.5

shows the algebraic layout of the contingency table, and represents four overlap conditions

between two binary-class map patterns.

B1 Present B1 Absent Totals:

B2 Present N(B1 _ B2 _ D) N(&B1 _ B2 _ D) N(B2 _ D)

B2 Absent N(B1 _&B2_ D) N(&B1 _ &B2 _ D) N(&B2 _ D)

Totals: N(B1 _ D) N(&B1 _ D) N(D)

Table B.5. Contingency table for testing conditional independence, based on cells containing deposits only. N is

the count of unit cells; B refers to the presence or absence of a binary pattern, which is numerically represented in

unit cells (subscript 1 or 2 denoting two different binary patterns); D refers to the presence of a deposit (a point

occurrence), which is numerically represented in unit cells. The four values within the table are either the expected

values assuming independence, calculated from the margin totals, or the observed values measured from the maps.

There is 1 degree of freedom. From Bonham-Carter (1994a).

The x2 statistic is applied only where the mineral deposit binary pattern is present and determines

if two binary-class predictor map patterns are conditionally dependent or independent. To

calculate the x2

 statistic, the observed and expected areas are required (i.e.—the number of

deposits, represented in unit cells, where each deposit equals 1 if a 1 km2

 unit cell was selected

for the analysis). The term N ( B1 _ B2 _ D ) in the contingency table represents the observed

number of deposits occurring in the overlap region where both B1 and B2 are present, and is

counted directly from a map or the deposit dataset (Bonham-Carter, 1994a). The predicted or
B16

2  

N

i
1

(Observedi – Expectedi )

2

Expectedi

B.8

expected number of deposits in the overlap region is calculated by multiplying the number of

deposits on B1 times the number on B2 divided by the total number of deposits, or as expressed

algebraically in terms of the contingency table, N ( B1 D )  N ( B2 D ) N ( D ) (multiplying the

marginal frequencies and dividing by the total).

Once the observed and expected values have been determined, 2 is calculated using the

expression:

which, for this example using the contingency table shown in Table B.5, N=4 and i refers to the

i-th binary-class predictor map pattern.

If conditional dependency exists, the predicted number of deposits in the overlap region of the

two predictor binary map patterns will be higher or lower than the observed number; if

conditional independency exists, the expected number of deposits should be equal to the number

of observed deposits. 

The results of the pairwise test are typically presented in the form of cross-tabulation tables

consisting of a matrix-layout of 2 values for all pairings of binary-class predictor maps. The

value of 2 indicates the degree of independency or dependency between predictor binary map

pairs—the larger the values, the greater the dependency. The values of 2 are compared to a

“tabled” value of 2 with one degree of freedom (DF—number of rows minus one, times, number

of columns minus one, therefore DF = 1) to determine if Ho (the null hypothesis that the binaryclass predictor map patterns are conditionally independent) is to be rejected or accepted, and at

what level of probability Ho should be rejected. For weights of evidence modelling, a probability

level threshold of 98% (2

0.98,1 = 5.41) or 95% (20.95,1 = 3.84) is sufficient (usually one cutoff is

chosen). Test values of 2

 that are greater than either of these two tabled cutoff values indicate

that the null hypothesis (CI) should be rejected.

B.3.3 Overall Test and the Kolmogorov-Smirnov Statistic

The overall goodness-of-fit test is a measure of the conditional independence (CI) between all

of the layers of evidence in a model as a whole. It is normally performed after the binary-class

predictor maps are combined in the Bayesian multi-map overlay procedure, where the posterior

probabilities are calculated for each unique overlap condition between binary-class predictor

maps (which collectively make up the mineral potential map). 

The overall test is very simple, consisting of a comparison of the predicted number of deposits

to the observed number of deposits. In weights of evidence modelling, the prior probability is

assumed to be the average known mineral deposit point density, therefore, the predicted number
B17

N(D)calc ' j

m

k'1

Pk N(A)k B.9

KS ' maximum * O&E * B.10

of deposits occurring within a unique overlap condition between binary-class predictor maps is

determined by summing the areas of the polygons composing each unique overlap condition map

class (i.e.—the total area for a particular unique condition map class), multiplied by the posterior

probability of that map class (see Bonham-Carter, 1994a, p. 317).

The calculation may be expressed as:

where: N(D)calc is the predicted (calculated) number of deposits, which is determined by adding

together the product of the area in unit cells, N(A), times the posterior probability, P, for all

unique condition map classes (k=1,2,...m) derived from the overlapped binary-class predictor

maps that compose the mineral potential map.

A 10% to 15% difference in the number of predicted deposits over the number of observed

deposits indicates that some degree of redundancy exists in the predictor variables (the layers of

evidence), suggesting that CI is being violated, and may warrant a check of the pairwise tests and

some sort of remedial action (Bonham-Carter, 1994a). Unlike the pairwise goodness-of-fit test,

the overall test is not a formal test, and in practice, the predicted number of deposits for the

weights of evidence modelling method is always higher than the observed (Bonham-Carter,

1994a).

The overall goodness-of-fit can also be assessed using the Kolmogorov-Smirnov (K-S) statistic

(non-parametric). Normally, K-S test statistics is used when the expected model can be

completely specified (i.e.—the total number of discrete events, or mineral deposits, is correctly

predicted), and loses its validity when the expected relative frequency is not approximately equal

to 1 at the largest value (Agterberg et al., 1993). For this study, an approximate K-S test is

applied, where the relative cumulative predicted and observed mineral deposit frequencies

(i.e.—the cumulative occurrence proportions) are constrained to reach a maximum of 1.0 (see

Agterberg et al., 1993; Bonham-Carter et al., 1989). Agterberg et al. (1993) pointed out that this

has the advantage of satisfying the assumptions for the K-S test (that the expected roughly equals

the observed number of deposits), but the disadvantage of failing to recognize predicted

frequencies that may be too large. Another advantage is that, because it is not necessary to group

observations into arbitrary categories (binning the data), the K-S test is more sensitive to the tails

of the distribution, as compared with tests that require binning, such as with the pairwise test

which uses the x2

 statistic (Davis, 1986).

The K-S statistic is based on the maximum deviation of the observed frequencies from the

expected frequencies, and is expressed in Davis (1973, p. 276) as:

where O and E are the observed and expected frequency of occurrences and | | denotes absolute
B18

s

2

(W

%

j

) '

1

N(Bj_ D)

%

1

N(Bj_ D)

, and s

2

(W

&

j

) '

1

N(Bj_ D)

%

1

N(Bj_ D)

B.11

values. In this instance, the K-S statistic is being applied to the observed and expected

distributions, not a theoretical Poisson distribution as given in Davis (1973). A more detailed

treatment is given by Davis (1986, p. 99) and Wright (1996, p. 114).

The null hypothesis (Ho) for the K-S statistic states that there is no significant difference between

the observed and predicted distributions. The null hypothesis is rejected if the absolute

difference between the relative cumulative predicted and the relative cumulative observed

frequency distributions (the “K-S statistic”) is larger than the tabled K-S statistic critical value

at a specified level of significance. The null hypothesis is “two-tailed”, because it is rejected if

the observed series is either significantly too high or too low, as compared to the predicted

distribution (Davis, 1973). Where the total number of observations is greater than 40, the critical

K-S value for the 5% (% = 0.05) level of significance can be computed as 1.36'/n, where n is

the total number of observed series of events (Davis, 1973), which in this case is the total

number of unique condition map classes for a given mineral potential model, not the total

number of mineral deposits. If one or more of the binary-class predictor map patterns were

conditionally dependent with respect to the mineral deposits pattern (i.e.—CI is violated), the

expected frequencies would overestimate the observed frequencies where the posterior

probabilities are relatively large and would underestimate the observed frequencies where the

posterior probabilities are small (Agterberg, 1989b).

B.4 Posterior Probability Uncertainty

B.4.1 Introduction

The posterior probability mineral potential map may optionally be augmented with a map of the

posterior probability uncertainty. Agterberg et al. (1990) and Bonham-Carter et al. (1989)

pointed out that if one or more binary-class predictor map patterns is unknown or incomplete in

a given area of the study region, the calculated posterior probability for that area has less

certainty than those based on more, or all, of the binary-class predictor map patterns. A map of

posterior probability uncertainty can be separately compared to a posterior probability map, or

the two can be combined so that the uncertainty map masks out regions of the posterior

probability map that have a high uncertainty factor.

B.4.2 Source and Types of Uncertainty

An important aspect to interpreting a mineral potential map is recognizing and quantifying the

uncertainty inherent to the posterior probabilities. The two primary sources of uncertainty are:

(1) the uncertainty due to variances in weight estimates (W+ and W–); and (2) the uncertainty

due to one or more of the binary-class predictor maps having incomplete coverage (i.e.—missing

data) (Bonham-Carter et al., 1989).

Agterberg et al. (1990) showed that the variance of the weights (W+

 and W–) for each map can

be calculated as:
B19

s

2

(Ppost) ' [

1

N(D)

% j

n

j'1

s

2

(W

k

j

) ] C P

2

post B.12

s

2

j

(Ppost) ' {P(D*Bj)&P(D)}

2 P(Bj

)%{P(D*Bj

)&P(D)}

2 P(Bj

) B.13

s

2

(total) ' s

2

(weights) % j

n

j'1

s

2

j (missing) B.14

where N(Bj _ D) is the number of unit cells where both deposits and the j-th binary-class

predictor map pattern are present, and the remaining terms are defined similarly. In most cases,

the second term in both expressions is very small, and approaches zero as the unit cell becomes

infinitely small. The first term in both expressions, which involve the count of unit cells with

deposits, are unaffected by unit cell size. Bonham-Carter et al. (1989) noted that these

expressions use an asymptotic result which assumes that the number of deposits is large. For

cases with a small number of deposits, the variances of the weights become large (Wright, 1996).

The summed effect of uncertainty due to the variances of the weights for each unique overlap

condition of binary-class predictor maps was shown by Agterberg et al. (1990) to be:

where the superscript k is (+) for presence and (–) for absence of the j-th binary-class predictor

map pattern, and the other terms are defined as before.

Agterberg et al. (1990) and Bonham-Carter et al. (1989) pointed out that if one or more of the

binary-class predictor map patterns is unknown or incomplete in a given region, the estimated

posterior probability for areas with missing data are more uncertain than those based on more,

or all, of the predictor variable map patterns. Bonham-Carter et al. (1989) indicated that, in order

to estimate the uncertainty in posterior probability due to incomplete or missing data in the j-th

binary-class predictor map, the following variance component can be calculated:

where, for any unique overlap condition between binary-class predictor maps, P(D) is the

posterior probability calculated from the non-missing binary-class predictor maps. The terms

P(D*Bj) and P(D*&Bj) are the updated posterior probabilities assuming that the j-th binary-class

predictor map pattern is present and absent, respectively (Bonham-Carter et al., 1989).

The uncertainties due to weights and due to missing data may be examined separately, or

combined to produce a total uncertainty for a given unique overlap combination of binary-class

predictor maps, which is calculated as the variance due to weights, plus, the sum of variances due

to missing data (Bonham-Carter et al., 1989), and is expressed as:

where the terms are defined as before.
B20

The uncertainty due to the weights, which includes the uncertainty of the prior probability, is in

general correlated to the posterior probability, and therefore maps of variance of weights have

the same trends as the posterior probability maps (Bonham-Carter et al., 1989).

In addition to the uncertainties due to weights variances and missing data, a relative certainty

(variance) of the posterior probability can be determined by dividing the posterior probability by

its standard deviation (i.e.—a “studentized” posterior probability), which, in effect, applies a

student t-test (based on a normal distribution) to determine whether the posterior probability is

greater than zero for a given level of statistical significance (i.e.—compared to a tabled t-value)

(Bonham-Carter et al., 1989; Agterberg et al., 1993). The larger the t-value over the critical

tabled t-value cut-off, 1.645 for a significance of 95% for example, the greater the certainty of

the posterior probability. The relative certainty is often more useful than the weights variances

or missing data uncertainties because it indicates the degree of confidence to which the posterior

probabilities are “real”, as opposed to being an artifact of “chance” effects (or due to chance).

As compared to the uncertainty due to the weights variances or missing data, relative certainty

is generally not as highly correlated to the posterior probability.

Ideally, the four uncertainty factors (weight variances, missing data, total, and relative) may be

used to create classified uncertainty maps for comparison to the posterior probability mineral

potential map, or the uncertainty factors may be combined in various ways and reclassified to

a binary-class map which can be used to “mask-out” areas of the mineral potential map that are

deemed to be too uncertain (Bonham-Carter et al., 1989; Bonham-Carter, 1994a). This

procedure is discussed below in section B.5.4.

B.5 Practical Implementation of the Modelling Procedures

B.5.1 Introduction

Weights of evidence modelling may be implemented (1) internal to the GIS, using map and

“unique conditions” modelling techniques, or (2) external to the GIS environment, using unique

conditions modelling in tandem with DOS and OS/2 FORTRAN utilities to expedite the various

statistical procedures described in the preceding sections. The modelling carried out in this study

used a combination of internal and external implementations. The FORTRAN utilities used to

calculate the spatial weights of association (W+ and W–

), the posterior probabilities, and the

uncertainty factors for weights of evidence modelling were provided by G.F. Bonham-Carter

(1996, personal communication) of the Geological Survey of Canada. Wright (1996) contains

detailed procedures for implementing weights of evidence both internally and externally to the

GIS (see Wright's appendix B-2), and gives a detailed history and summary of the FORTRAN

utilities (see Wright's appendix B-2, and p. 138). The FORTRAN utilities are included here in

Appendix B.

B.5.2 Estimation of the Spatial Weights of Association

The information presented under this heading represents weights of evidence procedure i, as

discussed in sections B.2.3 and B.2.4, and illustrated in Figure B.4. 
B21

In order to generate binary-class mineral predictor maps, spatial weights of association (W+

 and

W–) must be calculated between the deposits and each class of each multi-class evidence map

to determine which class(es) of an evidence map has the strongest correlation to the deposit

distribution. This is done in part internally and externally to the GIS. 

The calculation of the weights is done externally using a the FORTRAN utility WTS.EXE (or

any of its various incarnations). WTS.EXE requires two files as input: (1) an area analysis of

the multi-class evidence map to be processed (Table B.3), and (2) a point-in-polygon analysis

between the evidence map and the mineral deposits, where the evidence map class is appended

to the deposit point dataset (Table B.4). These two input files are generated within the GIS first,

then input into WTS.EXE. WTS.EXE outputs a file containing two area-cumulative and one

non-area-cumulative tabulation of the spatial weights of association and contrast (C) between

each multi-class evidence map unit with respect to the deposit distribution (Table B.6).

The non-area cumulative table is normally applied to nominal (or categorical) scale evidence

datasets, such as a geological map, to determine which map units have higher spatial associations

with the deposits (which are more “favorable”). The two area-cumulative tables, one ascending

and the other descending, are normally applied to ordinal (or ranked) or higher-scale evidence

datasets, such as distance from faults or geomagnetic anomaly, and are used to determine

optimum thresholds or cut-offs between successive favorable and not-so-favorable evidence map

units (i.e.—a boundary between “background” and “anomalous”; this is further discussed in the

following section). The weight values (W+

 and W–) should be evaluated in a relative sense, and

augmented with the contrast (C; strength of the spatial association) and with the variances of the

weights factors (standard deviations of W+, W–, and C, and “studentized” C) when determining

the favorability of a particular evidence map unit.

B.5.3 Generation of Binary-Class Mineral Predictor Maps

The multi-class maps analysed above are reclassified to binary maps that serve as layers of

evidence to predict the pattern of mineral deposits. Multi-class maps can be used as layers of

evidence in weights of evidence modelling, but the number of possible overlap conditions

becomes increasingly large and unwieldy (see Bonham-Carter, 1994a, 1994b). The usage of

binary maps also yields more stable and meaningful weights of spatial association, and facilitates

interpretation of the mineral potential map (Bonham-Carter, 1994a). The reclassification of

multi-class maps into binary maps involves both objective and subjective methods. The spatial

weights of association (W+

 and W–) are calculated in order to quantify the spatial relationship

between the mineral deposit pattern and the pattern of a map that is used for prediction. The

weight estimates, the contrast between the weights (C = W+ – W–, which gives a useful measure

of the strength of the spatial association), and the variances of the weights and the contrast, are

fundamental to isolating favorable units from the unfavorable or “background” units, and hence

for determining which map units are optimal for predicting the mineral deposit pattern. In many

cases, however, an optimum threshold between the favorable and not-so-favorable unit(s) is not

always clearly defined, and subjective techniques, tempered with a good amount of geologic and

other practical considerations, are necessary to determine the best cutoff(s).

Nominal (or categorical) scale multi-class maps, such as geological maps, are reclassified to

binary-class maps by combining individual favorable units, constrained only by what seems both
B22

Maximum Area-Cumulative C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3 4 5 6 7 8 9 10

Buffer Zone Distance, or Anomaly Intensity

Cumulative Area Contrast

Figure B.6. Graph of area-cumulative contrast versus buffer zone distances

or anomaly intensity values.

geologically reasonable and numerically advantageous (Bonham-Carter, 1994b). The spatial

weights of association are recalculated to determine new values for W+ and W– for the binaryclass predictor map. Weight estimates may even be recalculated on various groupings of

favorable units to determine which reclassification combination optimises the spatial association

between the deposits and the map pattern.

For maps with an ordinal (or ranked), interval, or ratio level of measurement, such as geophysical

or geochemical anomaly maps, or distance buffer maps, weights are calculated using successive

cumulative map unit areas—weights are calculated for each successive map class using the area

of the map class being evaluated plus the area of the previously evaluated map classes.

Reclassification to a binary-class map respects the map class sequence, where the threshold

cutoff for the optimum number of units to include in the binary-class predictor map is determined

by the number of successively combined units which collectively demonstrate the strongest

spatial association (Bonham-Carter, 1994a, 1994b).

In practice, a graph is constructed which plots area-cumulative C along the y-axis and distance

buffers or anomaly intensities along the x-axis, as shown in Figure B.6. In this example, the

area-cumulative C peaks at the 3rd interval on the x-axis. For buffer zone distances, which are

area cumulative from the closest to the farthest distance interval from the buffered object (0 to

10), the combined map classes 0 to 3 show the highest strength of spatial association. For a

geophysical or geochemical anomaly, area cumulative from the highest anomaly value to lowest

(10 to 0), the combined map classes 10 to 3 show the highest strength of spatial association. 
B23

All binary-class mineral predictor maps were coded as:

C 0 class—Predictor pattern missing (no data).

C 1 class—Predictor pattern absent (pattern not favorable).

C 2 class—Predictor pattern present (pattern favorable).

For additional information, see Bonham-Carter (1994a, p. 319-325), and Wright (1996, appendix

B-1).

B.5.4 Bayesian Map Overlay, and Generation of Mineral Potential and

Posterior Probability Uncertainty Maps

Once the binary-class predictor maps (layers of evidence) have been generated, the Bayesian

multi-map overlay procedure may be performed. Of all the weights of evidence modelling

procedures, the Bayesian map combination procedure is the most flexible in terms of

implementation. It may be carried out purely within the GIS using a map modelling technique,

or external to the GIS using a unique conditions map and table modelling technique in

conjunction with a FORTRAN utility to calculate the posterior probabilities. Both the internal

and external implementation methods of the Bayesian multi-map overlay procedure were

performed in this study.

In the map modelling approach, internal to the GIS, the binary-class predictor maps are combined

using an equation which specifies the weights associated with each layer of evidence and

evaluates the model at each pixel. Using a predetermined classification scheme, the posterior

probability values are grouped into numbered intervals which are assigned to the pixel (or in

other words, assigned as the output map class). A “quantiles” approach to creating a

classification scheme is often used. This is an “area-based” percentiles scheme where the

posterior probability class intervals (the breakpoints) are determined so that each of the posterior

probability classes is roughly equal in area (see TYDAC Technologies Inc., 1993, Reference

Manual, p. 5-56, for more information). An example of a GIS map modelling equation used to

combine the layers of evidence is shown in Table B.7. A detailed review of the implementation

of this technique is given by Wright (1996, appendix B-2(a)).

A more powerful technique is unique conditions map and table modelling, which is implemented

both internal and external to the GIS. The approach involves: (1) the generation of a unique

conditions map and associated (linked) attribute table; (2) the execution of an external

FORTRAN utility (PREDICT.EXE) which calculates probabilities and uncertainty factors for

various map combination methods, including weights of evidence, Bayesian (direct), ordinary

regression, logistic regression, and Dempster-Shafer; and (3) the reclassification of the unique

conditions map using the posterior probabilities and uncertainty factors calculated by

PREDICT.EXE. The main advantage of this method is calculation of the various posterior

probability uncertainty factors by PREDICT.EXE, which for each overlay operation includes

uncertainty due to weights variances, uncertainty due to missing data, the total uncertainty

(weight variances plus missing data), and relative certainty (t-values). PREDICT.EXE also

calculates the total number of expected deposits which are predicted by the model, as well as

generating a table of spatial weights of association (W+

 and W–) similar to WTS.EXE. Other

advantages of this method over the internal map modelling approach is that it minimizes the
B24

number of times the model has to be evaluated, and PREDICT.EXE outputs all the data to an

ASCII table, which can be imported into any number of applications for further analysis (x-y

plots, etc.).

In the first step, a unique conditions overlay map is generated within the GIS from the binaryclass predictor maps which make up the “spatial framework” of mineral potential model. A

unique conditions overlay is a multi-map combination technique that produces: (1) a single map

composed of unequal-area polygons which are assigned the same class value if they consist of

the same overlap combination or characteristic of input maps, and (2) an associated (linked)

attribute table that contains the unique overlay combinations of map classes which compose a

particular unique conditions map class (see Fig. B.3; for additional details, see Bonham-Carter,

1994a, p. 235-238; Bonham-Carter, 1991, p. 172). The objective is to perform the modelling

or data analysis on the table, rather than using the input maps directly, and to reclassify the

unique conditions map according to the results, which are directly linked to the unique conditions

map via the unique conditions table (see Table B.2).

In the second step, two tables are exported from the GIS and input into PREDICT.EXE: (1) the

unique conditions attribute table (Table B.2), generated as a result of the unique conditions

overlay operation between the various binary-class mineral predictor maps (Fig. B.3); and (2)

a table containing the unique conditions map class value appended to the deposit point locations

(i.e.—generated by a point-in-polygon analysis between the deposits and the unique conditions

map; see Table B.4 for an example). PREDICT.EXE performs the overlay operation using the

information contained in the unique conditions and point-in-polygon attribute tables, and for

each unique conditions map class, it calculates a prior probability and the various uncertainty

factors (Table B.8).

In the third step, the output table from PREDICT.EXE (Table B.8) is imported into the GIS and

used to reclassify the unique conditions map by posterior probability or any of the uncertainty

factors. The map reclassification scheme is created as indicated for the map modelling method

above. In this manner, a posterior probability mineral potential map, an uncertainty due to

weights variances map, an uncertainty due to missing data map, a total uncertainty map, and a

relative certainty map may be easily generated for a particular mineral potential model.

B.5.5 Conditional Independence Testing

Conditional independence testing, both the pairwise and overall tests, was implemented external

to the GIS. The FORTRAN utility CI.EXE was used to conduct the pairwise test, whereas a

spreadsheet program was used to carry out a graphical solution (x-y plot) for the overall test.

For the pairwise test, the same tables that were input into PREDICT.EXE (see section B.5.4

above) were input into CI.EXE (a unique conditions attribute table containing the area and a

listing of overlain map class combinations that compose each unique conditions map class, and

a table containing the unique conditions map class associated with every mineral deposit).

Example output is shown in Table B.9.
B25

The overall test was performed using data contained in the output table generated by

PREDICT.EXE (see Table B.8). The data were imported into the spreadsheet program and

processed as follows:

1. The expected number of deposits for each unique conditions map class was calculated by multiplying the map

class area by its respective posterior probability.

2. Relative frequencies for the number of expected and the number of observed deposits were calculated by

normalizing the observed and predicted number of deposits to 1, respectively (i.e.—for the observed deposit

frequency, the number of observed deposits associated with each unique condition map class is divided by

the total number of observed deposits; for the expected deposit frequency, the number of expected deposits

predicted for each unique condition map class is divided by the total number of expected deposits).

3. The observed and predicted relative frequencies were sorted in ascending order by the posterior probability

value. Note that the observed and/or predicted frequencies may also be sorted on a second or third sort key,

but whether sorted or not, the absolute maximum difference between the two frequencies will not be affected

(e.g.—1st sort key: posterior probability; 2nd sort key: observed relative frequency; 3rd sort key:

predicted relative frequency).

4. The observed and predicted relative frequencies were cumulatively summed from lowest posterior probability

to highest (i.e.—in ascending order).

5. The cumulative relative predicted frequency was subtracted from the cumulative relative observed frequency

(observed minus predicted) to determine the absolute maximum difference between the observed and

predicted distributions.

6. The observed and expected cumulative relative frequencies were plotted against their common (shared)

posterior probabilities (the observed and expected cumulative relative frequencies are plotted as two separate

data series on the y-axis; the posterior probabilities are plotted as one data series on the x-axis).

7. The differences between the cumulative relative predicted frequency and the cumulative relative observed

frequency were plotted against their common (shared) posterior probabilities (the differences are plotted as

one data series on the y-axis; the posterior probabilities are plotted as one data series on the x-axis). The

absolute maximum datum point on this curve is the K-S statistic used for comparison to the critical tabled

K-S value.

An example of the graphical output for the overall test is shown in Agterberg (1989b, p. 81;

1993, p. 28), Davis (1986, p. 101), and Bonham-Carter et al. (1989, p. 180), as well as in Chapter

7. Interpretation of the output for both conditional independence tests is discussed in section B.3

above.
B26

 ID geo-rcl

 TITLE GEOLOGY.MAP Reclassification Table

 TABTYPE 4

 FTYPE free

 KEYFIELD 0

 1 3 3.000000 0 geoclass GEOLOGY.MAP map unit class

 2 50 30.000000 0 name Map unit name

 3 25 5.000000 0 code Map unit abbreviation code

 4 1 3.200000 0 area_% Area map unit covers in relative %

 5 3 6.000000 0 area_km Area map unit covers in Km2

 6 3 1.000000 0 0-6ma Igneous units 0-6 Ma in age

 7 3 2.000000 0 6-17ma Igneous units 0-6 Ma in age

 8 3 2.000000 0 17-34ma Igneous units 17-34 Ma in age

 9 3 2.000000 0 34-43ma Igneous units 34-43 Ma in age

 10 3 3.000000 0 felsic Igneous units felsic in composition

 11 3 2.000000 0 inter Igneous units intermediate in composition

 12 3 2.000000 0 mafic Igneous units mafic in composition

 13 3 3.000000 0 igneous Igneous rock units only

 14 3 1.000000 0 rx_type Rock type reclass.--sed, ign, meta.

 15 3 2.000000 0 cmp/age Rock compositions/packages primary, age secondary

 16 3 2.000000 0 age/cmp Age primary, rock compositions/packages secondary 

 17 3 2.000000 0 simple Simplified according to major rock packages

 DATA

 1 "Alluvium " "Qa " 43.22 122395 0 0 0 0 0 0 0 0 1 1 1 1

 2 "Playa, Marsh, Alluvial-Flat " "Qp " 4.61 13064 0 0 0 0 0 0 0 0 1 1 1 1

 3 "Landslide " "Qls " 0.17 487 0 0 0 0 0 0 0 0 1 1 1 1

 4 "Moraine " "Qm " 0.09 250 0 0 0 0 0 0 0 0 1 1 1 1

 5 "Older Alluvium " "Qtoa" 1.03 2907 0 0 0 0 0 0 0 0 1 1 1 1

 6 "Ryo Flows, Shallow Intrusives " "Qtr " 0.00 5 6 0 0 0 6 0 0 6 2 12 5 6

 7 "Andesite Flows, Breccias " "Qta " 0.09 265 7 0 0 0 0 7 0 7 2 12 5 6

 8 "Basalt Flows " "Qtb " 0.58 1648 8 0 0 0 0 0 8 8 2 14 6 7

 9 "Lacustrine Sedimentary Rocks " "Qts " 0.22 623 0 0 0 0 0 0 0 0 1 1 1 1

 10 "Rhyolitic Intrusive Rocks " "Tri " 0.10 275 0 0 0 0 10 0 0 10 2 12 5 6

 11 "Mafic-Intermediate Intrusives " "Tmi " 0.05 134 0 0 0 0 0 0 11 11 2 14 6 7

 12 "Dioritic-Granitic Intrusives " "Ti " 0.17 490 0 12 12 12 0 12 0 12 2 12 5 6

 13 "Breccia " "Tbr " 0.03 89 0 0 0 0 0 0 0 0 1 11 4 5

 14 "Silicic Ash-Flow Tuffs " "Tt3 " 3.42 9675 0 14 0 0 14 0 0 14 2 11 4 5

 15 "Tuffs, Ryo Flows, Intrusives " "Trt " 0.14 387 0 0 0 0 15 0 0 15 2 12 5 6

 16 "Ryo Flows, Shallow Intrusives " "Tr3 " 3.28 9301 0 16 0 0 16 0 0 16 2 12 5 6

 17 "Andesite, Intermediate Rocks " "Ta3 " 1.59 4508 0 17 0 0 0 17 0 17 2 12 5 6

 18 "Andesite & Basalt Flows " "Tba " 2.54 7180 0 18 0 0 0 18 0 18 2 14 6 7

 19 "Basalt Flows " "Tb " 1.74 4925 0 0 0 0 0 0 19 19 2 14 6 7

 20 "Banbury Formation " "Tbg " 1.33 3779 0 0 0 0 0 0 20 20 2 14 6 7

 ...

 ...

 ...

Table B.1. Example of an attribute table used for map reclassification. In SPANS GIS, a table consists of a header

section (above and including the line “DATA”), which describes the contents and format of a table, and a data

section (below the line “DATA”). Polygon or map class attributes are arranged by row, one row for each polygon

or map class. Each row is composed of fields (columns) containing spatial or non-spatial attribute information for

a particular polygon or map class. This example shows part of a table used to reclassify the digital representation

of the Stewart and Carlson’s (1978) geologic map of Nevada (Turner and Bawiec, 1991).
B27

ID uqpr7

TITLE uqpr7

MAPID uqpr7

WINDOW nv 0 0 0 0

TABTYPE 5

FTYPE fixed

KEYFIELD 2

KEYBASE 0

NRECORD 154

 1 0 15.400000 0 area area

 2 4 7.000000 0 uqpr7 uqpr7

 3 4 7.000000 0 bfftbipr bfftbipr

 4 4 7.000000 0 bfplbipr bfplbipr

 5 4 7.000000 0 geobihi geobihi

 6 4 7.000000 0 geobilow geobilow

 7 4 7.000000 0 gisobipr gisobipr

 8 4 7.000000 0 knabipr knabipr

 9 4 7.000000 0 magbipr magbipr

DATA

 35810.4492 1 1 1 1 1 1 1 1

 35414.3555 2 1 1 1 1 2 0 1

 32857.1758 3 1 1 1 1 1 0 1

 28826.8594 4 1 1 1 1 2 1 1

 17017.4883 5 2 1 1 1 2 0 1

 15845.0723 6 1 1 1 1 2 0 2

 12352.0596 7 1 1 1 1 2 1 2

 11994.0527 8 1 1 1 1 1 1 2

 11739.1426 9 2 1 1 1 2 1 1

 11134.8115 10 1 1 1 1 1 0 2

 9268.6045 11 2 1 1 1 1 1 1

 9218.2227 12 2 1 1 1 1 0 1

 5113.7158 13 2 1 1 1 2 0 2

 4804.6558 14 2 1 1 1 2 1 2

 4047.9116 15 2 1 1 1 1 0 2

 3502.2903 16 2 1 1 1 1 1 2

 2172.3958 17 1 2 1 1 2 1 1

 1687.9227 18 1 2 1 1 2 1 2

 1501.7610 19 1 2 1 1 2 0 2

 1417.7291 20 1 2 1 1 2 0 1

 1397.7980 21 1 2 1 1 1 1 1

 1376.5876 22 1 1 1 1 2 2 1

 1198.1527 23 2 2 1 1 2 1 1

 997.4451 24 1 1 1 2 2 1 1

 899.9293 25 2 2 1 1 1 1 1

 876.3833 26 2 2 1 1 2 0 2

 842.8678 27 1 2 1 1 1 0 1

 821.7247 28 1 2 1 1 1 1 2

 ...

 ...

 ...

Table B.2. Example of a unique conditions map attribute table output from SPANS GIS. The table consists of a

header section (above and including the line “DATA”), which describes the contents and format of the table, and

a data section (below the line “DATA”). The data section shows the area of a unique conditions map class (column

1), the unique conditions map class ID number (column 2; the link to the map), and the unique combination of input

map classes that compose the unique conditions map class (columns 3-9; corresponding to the map names listed in

lines 3-9 of the header section). This table was created by a unique conditions overlay of seven binary-class maps,

where 1 and 2 represent a map class and 0 indicates no map classes present.
B28

 SINGLE MAP ANALYSIS

 Map : geology - geology

 Window : nv - Whole of Nevada - Full Screen

 Class Legend Area % Cumm % Area (km2)

 (%) Area (km2)

 ----------------------- ----- ----- -------

 1 Alluvium 43.22 43.22 122395

 2 Playa, Marsh, Alluvial-Flat 4.61 47.83 13064

 3 Landslide 0.17 48.00 487

 4 Moraine 0.09 48.09 250

 5 Older Alluvium 1.03 49.12 2907

 6 Ryo Flows, Shallow Intrusives 0.00 49.12 5

 7 Andesite Flows, Breccias 0.09 49.21 265

 8 Basalt Flows 0.58 49.79 1648

 9 Lacustrine Sedimentary Rocks 0.22 50.01 623

 10 Rhyolitic Intrusive Rocks 0.10 50.11 275

 11 Mafic-Intermediate Intrusives 0.05 50.16 134

 12 Dioritic-Granitic Intrusives 0.17 50.33 490

 13 Breccia 0.03 50.36 89

 14 Silicic Ash-Flow Tuffs 3.42 53.78 9675

 15 Tuffs, Ryo Flows, Intrusives 0.14 53.91 387

 16 Ryo Flows, Shallow Intrusives 3.28 57.20 9301

 17 Andesite, Intermediate Rocks 1.59 58.79 4508

 18 Andesite & Basalt Flows 2.54 61.32 7180

 19 Basalt Flows 1.74 63.06 4925

 20 Banbury Formation 1.33 64.40 3779

 21 Ash-Flow Tfs, Tfs Sed Rocks 0.96 65.36 2713

 22 Tuffaceous Sedimentary Rocks 5.19 70.55 14701

 23 Horse Spring Formation 0.09 70.63 246

 24 Continental Sedimentary Rocks 0.14 70.77 399

 25 Silicic Ash-Flow Tuffs 6.72 77.49 19026

 26 Ryo Flows, Shallow Intrusives 0.45 77.94 1278

 27 Andesite, Intermediate Rocks 1.18 79.12 3347

 28 Older Basalt Rocks 0.03 79.15 85

 29 Tuffaceous Sedimentary Rocks 0.11 79.26 298

 30 Silicic Ash-Flow Tuffs 0.56 79.82 1586

 31 Ryo Flows, Shallow Intrusives 0.58 80.40 1649

 32 Andesite, Intermediate Rocks 0.75 81.16 2137

 33 Sedimentary Rocks 0.19 81.35 547

 34 Continental Sedimentary Rocks 0.09 81.44 254

 35 Continent Clastics, Limestone 0.05 81.49 143

 36 Granitic Rocks, Central & East 0.11 81.60 309

 37 Monzonite & Granodiorite 0.15 81.75 416

 38 Granitic Rocks, West 0.82 82.56 2312

 39 Qtz-Monzonite & Granodiorite 1.13 83.70 3211

 40 Diorite 0.07 83.76 185

 ...

 ...

 ...

 99 Quartzite, Clastic, Carbonates 0.13 99.57 374

 100 Granitic Rocks 0.08 99.65 233

 101 Metamorphic Rocks 0.35 100.00 992

 ----------------------- ----- ----- -------

 Total of 101 classes 100.00 100.00 283219

Table B.3. Map area analysis table output from SPANS GIS. Percent of total area, cumulative percent area, and

area in km2 are given for each map class. This example shows part of the area analysis table for the digital

representation of the Stewart and Carlson’s (1978) geologic map of Nevada (Turner and Bawiec, 1991).
B29

 ID geoarea

 TITLE All MRDS Au and Ag Containing Deposits

 MAPID geology

 TABTYPE 2

 FTYPE free

 NRECORD 2690

 1 5 15.000000 0 morton Morton Coordinate

 2 3 1.000000 0 size Size of Occurrence (0-Unkn, 1-Large, 2-Medium, 3-Small)

 3 4 6.000000 0 geology GEOLOGY.MAP Map Class Number

 DATA

 17ae6b8 1 101

 17b0ec3 1 1

 17b32ef 1 101

 17b8057 1 1

 17b85ae 1 101

 17b88a2 1 101

 17b8ad2 1 101

 17b9391 1 101

 17b9898 1 101

 17ba27a 1 101

 17ba880 1 101

 17bd561 1 17

 17c5a15 1 100

 17c7ed6 1 100

 17cd78b 1 100

 17cd78b 1 100

 17cfc4f 1 101

 17d4873 1 100

 17d48b1 1 17

 17d8928 1 101

 17e20b4 0 1

 17e25f8 0 1

 17e29c7 1 5

 17e2b94 0 5

 17e2bd1 1 5

 17e2c3c 2 17

 17e2c8f 1 17

 17e2e11 1 17

 17e2ec1 3 17

 17e3e1d 1 101

 17e3ec9 1 101

 17e6a8e 1 101

 17e8155 1 5

 17e833d 0 17

 17e8612 0 17

 17e8683 1 17

 17e8c09 1 17

 17e8c5b 1 17

 17e906d 1 12

 ... 

 ... 

 ... 

Table B.4. Point-in-polygon analysis table output from SPANS GIS. The table consists of a header section (above

and including the line “DATA”), which describes the contents and format of a table, and a data section (below the

line “DATA”). The geographic position (column 1), the size (column 2), and the coincident map class (column 3)

are given for each gold-silver occurrence. This example shows part of the point-in-polygon analysis table for the

digital representation of the Stewart and Carlson’s (1978) geologic map of Nevada (Turner and Bawiec, 1991). The

geographic positions of the gold-silver occurrences are given in Morton coordinates (for more information, see

TYDAC Technologies Inc., 1993). Gold-silver occurrence size classification scheme is from the Metallogenic Map

of North America (Guild, 1968).
B30

 Name of file: geolpr.WTS

 Total no of points: 2690

 Area of unit cell: 1.000000 sq. kms

 Total area: 283219.000000 unit cells

 283219.000000 sq. kms.

 The following table is non-cumulative. 

 Areas in unit cells. 

 Class Area Points W+ Sdt (W+) W- Sdt (W-) C Sdt (C) Stud (C)

 1 1 122395 198 -1.7776 .0711 .4955 .0202 -2.2731 .0739 -30.7446

 2 2 13064 4 -3.4435 .5001 .0462 .0194 -3.4897 .5005 -6.9731

 3 3 487 2 -.8435 .7086 .0010 .0194 -.8445 .7088 -1.1914

 4 4 250 6 .9421 .4132 -.0014 .0194 .9435 .4137 2.2806

 5 5 2907 16 -.5493 .2507 .0044 .0194 -.5537 .2514 -2.2019

 6 6 5 0

 7 7 265 0

 8 8 1648 26 .5142 .1977 -.0039 .0195 .5181 .1986 2.6083

 9 9 623 0 

 10 10 275 12 1.5603 .2952 -.0035 .0194 1.5638 .2958 5.2862

 11 11 134 14 2.4991 .2824 -.0048 .0194 2.5039 .2831 8.8448

 12 12 490 12 .9628 .2923 -.0028 .0194 .9656 .2929 3.2964

 13 13 89 0

 14 14 9675 44 -.7410 .1511 .0184 .0195 -.7595 .1524 -4.9849

 15 15 387 2 -.6126 .7089 .0006 .0194 -.6132 .7092 -.8647

 16 16 9301 113 .2492 .0946 -.0096 .0198 .2589 .0967 2.6772

 17 17 4508 128 1.1147 .0897 -.0330 .0199 1.1478 .0918 12.4972

 18 18 7180 47 -.3748 .1463 .0081 .0195 -.3830 .1476 -2.5938

 19 19 4925 3 -2.7554 .5775 .0166 .0194 -2.7719 .5779 -4.7970

 20 20 3779 0

 21 21 2713 30 .1540 .1836 -.0016 .0195 .1556 .1846 .8429

 22 22 14701 28 -1.6141 .1892 .0433 .0195 -1.6573 .1902 -8.7152

 23 23 246 0

 24 24 399 0

 25 25 19026 300 .5136 .0582 -.0492 .0206 .5628 .0617 9.1194

 26 26 1278 30 .9194 .1848 -.0068 .0195 .9262 .1858 4.9853

 27 27 3347 77 .8988 .1153 -.0173 .0197 .9161 .1170 7.8327

 28 28 85 0

 29 29 298 1 -1.0462 1.0017 .0007 .0194 -1.0469 1.0019 -1.0450

 30 30 1586 15 -.0039 .2594 .0000 .0194 -.0039 .2602 -.0151

 31 31 1649 1 -2.7598 1.0003 .0055 .0194 -2.7653 1.0005 -2.7640

 32 32 2137 26 .2507 .1973 -.0022 .0195 .2528 .1983 1.2752

 33 33 547 4 -.2633 .5018 .0004 .0194 -.2638 .5022 -.5252

 34 34 254 1 -.8859 1.0020 .0005 .0194 -.8864 1.0022 -.8845

 35 35 143 0

 ...

 ...

 ...

Table B.6. Output from the FORTRAN utility WTS.EXE for the weights of spatial association estimation

component of weights of evidence mineral potential modelling. The table consists of a header, which summarizes

the map and point dataset analysed, and a data section. The data section contains the following for each map class:

the map class number (columns 1 and 2); area in unit cells (column 3); total number of coincident points (mineral

deposits, column 4); positive weight of association (W+, column 5); standard deviation of W+ (column 6); negative

weight of association (W–, column 7); standard deviation of W– (column 8); contrast or W+ and W– (C, column 9);

standard deviation of C (column 10); and studentized C (column 11). This example shows part of the weights of

spatial association measurements between the gold-silver-bearing occurrences and the digital representation of the

Stewart and Carlson’s (1978) geologic map of Nevada (Turner and Bawiec, 1991). See text for discussion of

spatial association coefficients.
B31

 :Weights of evidence modelling Bayesian multi-map overlay procedure.

 :Response variable (point dataset) is the Nevada MRDS gold-silver occurrences.

 :Builds a posterior probability map based upon 11 layers of evidence.

 :

 :Begin calculations...

 :

 :Define the prior probability...

 :

 :Prior probability = total # of points / total area of study region

 :

 : (if a unit cell is defined as 1km2)

 : ...or, also considered as...

 :

 : # of unit cells with occurrences / total area in unit cells

 :

 :Number of occurrences = 2,690 (all gold-silver occurrences)

 :Total area of analysis = 283,219 km2

 :Prior probability = 2690/283219 = 0.009498

 :

 priorp = 0.009498

 :

 :Calculation of log of prior odds from prior probability...

 :

 lprioro = log(priorp / (1.0 - priorp)) ;

 :

 :Assign the weights to binary-class mineral prediction maps m1 to m11.

 :The maps (names in quotes) are coded as:

 : 0 class - predictor pattern missing (no data)

 : 1 class - predictor pattern absent (pattern not favorable)

 : 2 class - predictor pattern present (pattern favorable)

 :The weights were determined using WTS.EXE and are hard-wired into the equation.

 :

 : W+ (predictor pattern present) W- (predictor pattern absent

 :

 m1 = { 0.4560 if class('banabipr') == 2, -0.1110 if class('banabipr') == 1, 0 };

 m2 = { 0.6264 if class('bfftbipr') == 2, -0.3898 if class('bfftbipr') == 1, 0 };

 m3 = { 1.4221 if class('bfplbipr') == 2, -0.2829 if class('bfplbipr') == 1, 0 };

 m4 = { 2.6252 if class('geobihi') == 2, -0.1267 if class('geobihi') == 1, 0 };

 m5 = { 1.1853 if class('geobilow') == 2, -0.0985 if class('geobilow') == 1, 0 };

 m6 = { 1.8140 if class('geobimed') == 2, -0.2620 if class('geobimed') == 1, 0 };

 m7 = { 0.9598 if class('gedvbipr') == 2, -0.3418 if class('gedvbipr') == 1, 0 };

 m8 = { 0.2139 if class('gisobipr') == 2, -0.3252 if class('gisobipr') == 1, 0 };

 m9 = { 1.0010 if class('knabipr') == 2, -0.0560 if class('knabipr') == 1, 0 };

 m10 = { 0.3904 if class('magbipr') == 2, -0.2147 if class('magbipr') == 1, 0 };

 m11 = { 0.4816 if class('trrnbipr') == 2, -0.2565 if class('trrnbipr') == 1, 0 };

 :

 :Total the weights...

 :

 wtot = m1 + m2 + m3 + m4 + m5 + m6 + m7 + m9 + m10 + m11;

 :

 :Add the log prior odds to the totalled weights to get log of posterior odds...

 :

 lposto = lprioro + wtot;

 :

 :Convert log odds to odds, but make 0 if all the weights are zero...

 :

 posto = {exp(lposto) if wtot <> 0 , 0};

 :

 :Convert posterior odds to posterior probability...

 :

 postp = posto / (1.0 + posto);

 :

 :Build the posterior probability map...

 :

 postp;

Table B.7. Example of weights of evidence Bayesian map overlay modelling equation implemented in SPANS

GIS. Colons ":" indicate comment lines.
B32

 ID uqpr11

 TITLE Weights of Evidence

 MAPID uqpr11 (unique conditions overlay map of 11 binary-class mineral predictor maps)

 TABTYPE 5

 FTYPE free

 KEYFIELD 2

 NRECORD 1144

 1 0 15.400000 0 area area

 2 4 7.000000 0 uniq unique conditions map class

 3 4 7.000000 0 depos number of deposits

 4 0 10.400000 0 postp posterior probability

 5 0 10.400000 0 tpost Studentized post prob

 6 0 10.400000 0 sdevw Uncertainty (weights)

 7 0 10.400000 0 sdevm Uncertainty (missing)

 8 0 10.400000 0 stot Total uncertainty

 DATA

 28205.3300 1 32 .0010 3.1488 .0001 .0003 .0003

 25433.8100 2 36 .0016 3.1630 .0001 .0005 .0005

 18652.4200 3 41 .0008 11.6187 .0001 .0000 .0001

 13665.1100 4 34 .0014 12.0878 .0001 .0000 .0001

 12665.0000 5 14 .0030 3.1705 .0002 .0009 .0010

 11266.3000 6 27 .0017 11.4680 .0001 .0000 .0001

 9648.6680 7 33 .0018 3.1516 .0001 .0005 .0006

 8824.1040 8 23 .0045 3.1868 .0003 .0014 .0014

 7443.1330 9 23 .0015 11.4479 .0001 .0000 .0001

 7103.5460 10 59 .0026 11.9010 .0002 .0000 .0002

 6235.7220 11 18 .0029 11.9252 .0002 .0000 .0002

 5949.4050 12 8 .0027 3.1626 .0002 .0008 .0008

 5257.2710 13 45 .0165 3.2798 .0012 .0049 .0050

 4657.6000 14 39 .0022 11.6428 .0002 .0000 .0002

 4400.3140 15 29 .0038 12.1258 .0003 .0000 .0003

 4067.2670 16 19 .0060 3.1934 .0005 .0018 .0019

 3665.3160 17 16 .0034 3.1743 .0003 .0011 .0011

 3191.0030 18 8 .0031 11.3124 .0003 .0000 .0003

 2965.6220 19 8 .0048 3.1767 .0004 .0015 .0015

 2938.4030 20 6 .0083 3.2137 .0006 .0025 .0026

 2816.4610 21 3 .0030 10.4588 .0003 .0000 .0003

 2541.2050 22 4 .0051 10.8136 .0005 .0000 .0005

 2317.4540 23 8 .0020 3.1539 .0002 .0006 .0006

 2086.0970 24 5 .0047 11.5095 .0004 .0000 .0004

 2043.0330 25 2 .0080 11.9945 .0007 .0000 .0007

 2023.4960 26 50 .0070 11.9625 .0006 .0000 .0006

 1953.3160 27 13 .0041 11.4850 .0004 .0000 .0004

 ...

 ...

 ...

Table B.8. Output from the FORTRAN utility PREDICT.EXE for the Bayesian multi-map overlay component of

weights of evidence mineral potential modelling. The output table is formatted for direct import into SPANS GIS,

and consists of a header section (above and including the line “DATA”), which describes the contents and format

of a table, and a data section (below the line “DATA”). For each unique conditions map class, the following is

given: area (column 1); unique conditions map class ID number (column 2); total number of coincident points

(mineral deposits, column 3); posterior probability (column 4); studentized posterior probability (relative

uncertainty, column 5); uncertainty due to variances in the weights (column 6); uncertainty due to missing data

(column 7); and total uncertainty (column 8). Any of these attributes may be used to reclassify the unique conditions

map generated for this particular model (in this instance, all 2690 gold-silver-bearing occurrences and 11 binaryclass mineral predictor maps). See text for discussion.
B33

 map number 1 banabipr.map (Ba/Na geochemical anomaly)

 map number 2 bfftbipr.map (buffer zones around faults)

 map number 3 bfplbipr.map (buffer zones around plutons)

 map number 4 geobihi.map (lithology)

 map number 5 geobilow.map (lithology)

 map number 6 geobimed.map (lithology)

 map number 7 gedvbipr.map (diversity of lithology)

 map number 8 gisobipr.map (isostatic gravity anomaly)

 map number 9 knabipr.map (K/Na geochemical anomaly)

 map number 10 magbipr.map (geomagnetic anomaly)

 map number 11 trrnbipr.map (lithotectonic terranes)

 Chi-squared summary table:

 Upper triangle -- conditional independence (deposits only)

 Lower triangle -- conditional independence (non-deposits only)

 1 2 3 4 5 6 7 8 9 10 11

 1 0.00 35.73 32.57 15.05 0.03 0.85 93.47 95.73 486.34 6.60 78.37

 2 22.18 0.00 21.71 0.66 0.39 0.64 124.92 8.46 31.21 14.80 2.93

 3 931.92 554.43 0.00 31.13 44.13 1.19 39.81 45.46 29.91 4.50 29.93

 4 214.73 259.65 3158.10 0.00 57.30 139.23 12.67 8.04 0.78 1.25 95.93

 5 659.93 807.42 12217.88 106.58 0.00 142.54 0.00 5.70 12.39 21.01 49.60

 6 1.44 1117.33 8963.74 115.95 518.89 0.00 4.32 18.25 0.11 5.82 0.58

 7 442.02 13507.94 6342.08 1055.66 947.80 1490.22 0.00 23.44 43.94 1.56 0.19

 8 2002.18 1482.81 530.30 417.44 71.46 3060.30 4819.86 0.00 5.71 20.37 1.81

 9 20876.66 67.08 200.80 241.06 180.59 24.07 85.13 625.62 0.00 17.21 6.14

 10 812.76 21.20 2757.50 94.09 481.81 0.80 9.26 406.94 386.42 0.00 4.51

 11 11541.06 43.93 4319.61 1396.21 10189.16 226.09 276.28 54.55 2342.36 86.13 0.00

Table B.9. Output from FORTRAN utility CI.EXE for the error analysis component of weight of evidence mineral potential modelling. Pairwise conditional

independence test x2

 scores for 11 binary-class predictor map pair combinations. The predictor maps are denoted as numbers 1-11 along the left-hand and top margin

of the lower table, and are keyed to map names shown in a legend which appears over the table of x2

 scores. Note that only the upper right-hand triangular block

of x2

 scores (bold italic numbers) shows the conditional independence between the 11 binary-class predictor map pairs with respect to the deposits. See text for

discussion.
Standard and Quadtree Raster Data Structures

Figure B.1. Afull expanded uniform-sized pixelrasterstructure (A) and a hierarchical variable sized pixel quadtree data structure (B). The vector outline of Nevada

issuperimposed on a grid which representsthe pixel configuration of the data structures. The gray highlightindicatesthe raster equivalent ofthe vector data structure

(cellsizes are exaggerated).

A B
Two- and Multi-Map Overlay

and Combination Methods

Figure B.2. Two-map and multi-map overlay and combination methods. Figure adopted from BonhamCarter(1994a, p. 223, 225, 226, 229, and 230). See textfor discussion.

Map A Map B

Combined

MapsA&B

Impose

Stamp

Join

Compare

Map/ClassWeighted

(map B is weighted

heavier than A)

0 0

0

0

0

4

0

0

0

0

0

0

0

0

0

0

1

1 2

3 4

1

1

1

2

2

2

3

3

3

4

4

1 2

3 4

1

1

1

1

1

1

8

10

7

7

5

3

4

2

2

5

6 5

3

2

2

2

2

2

3 3 3 4

4

1 2

3 4

5

6

1

2

1

2

B35
Entity and Unique Conditions Map

Overlays and Linked Attribute Tables

Figure B.3. The generation of a single map from overlaying multiple maps. There are two basic types of

map overlay operations: (1) an entities overlay, where each new polygon created by the intersection of

polygons composing the input mapsis assigned a unique class value, or entity number (leftside of diagram);

and (2) a unique conditions overlay, where the new polygons are assigned the same class value (grouped) if

they consist of the same overlap combination or characteristic of input maps (a "unique condition"

intersection of input map classes; right side of diagram). The map overlay operation also produces an

associated (linked) attribute table that containsinformation detailing the overlay combinations of input maps

which constitute each overlaymap class. Figure and textmodified fromBonham-Carter(1994a, p. 236). See

text for discussion.

1 1 1

2 1 2

3 1 2

1

2

2

2

2 1

4

3

4

4 3

5

6

7

8

1

2

3

4

5

6

1

1

1

2

2

2

1

2

3

3

4

1

1

2

1

2

2

3

4

4

1

2

3

4

5 7

6

8

1

2

2

3

4 5

5

6

Map A Map B

Unique

Polygons

(Entity)

Map

A•B

Unique

Polygons

(Entity)

Attribute

Table

A•B

Unique

Conditions

Attribute

Table

A•B

Unique

Conditions

Map

A•B

Map

Overlay

Operation

Attribute Tables (below) Linked to Respective Maps (above) Via Map Class

Class

Map A

Map

Class

Map

Class

Class

Map B

Class

Map A

Class

Map B

B36
Weights of Evidence Multi-Map

Overlay Modelling Method

Figure B.4.

.

.

The weights of evidence (WOE) mineral potential modelling

method is illustrated by the flow chart on the opposite page. WOE may be

subdivided into twomain procedures:

Calculation of W and W weights of spatial association

between the response variable and the predictor variable(s),

which in this instance are point objects representing goldsilver-bearing occurrences, and multi-class map patterns

representing various geologic phenomena.

Combination of predictor variables (the )

in a multi-map overlay operation where a loglinear version of

is used to sum and update the weights associated

with each of the map classes that come into coincidence,

producing a compound posterior probability mineral potential

map which closely exhibits the distribution of known

occurrences and indicates areas where more occurrences are

expected than are observed.

See the textfor amore detailed discussion.

i

i i

+ -

W and W are

calculated for each of the predictor variable map classes. The

weights are then used as a guide to reclassify a multi-class

predictor variable to a binary-class predictor variable (a binaryclass deposit predictor map), where binary-class map pattern

present represents mineralization-favorable conditions, and

binary-class map pattern absent represents mineralizationunfavorable conditions.

+ -

"layers of evidence"

Bayes' Rule

B37
Response

Variable

Predictor

Variable

W & W + -

(weights of spatial association)

Prior

Probability

Predictor

Variables

(layers of evidence)

Posterior Probability Mineral Potential Map

Figure B.4, continued.

i

ii

B38
Spatial Overlap Relationships

Figure B.5. Diagrams illustrating the spatial overlap relationships on which the weights of spatial

association (W andW) are measured. The value of the weights depend on the ratio of deposits that fall on

the binary mineral predictor pattern to the total number of deposits, against, the ratio of the binary mineral

predictor pattern area to the totalstudy region area. See text for discussion. Modified from Bonham-Carter

(1994a, p. 305, 311, and 316).

+ -

Distribution of Mineral Deposits

Relative to a Binary Mineral

Predictor Pattern Being Evaluated

Venn Diagram Showing the Spatial

Overlap Relationships Between the

Binary Mineral Predictor Pattern and

the Deposit Distribution Pattern

Study Area

Mineral Deposits

Binary Mineral Predictor Pattern

D B

D B1 D B1

D B1

A

Four Spatial Overlap Combinations:

D B regions where both deposit and predictor patterns present.

D B regions where deposit pattern absent, predictor pattern present.

D B regions where neither deposit nor predictor patterns present.

1

D B1 regions where deposit pattern present, predictor pattern absent.

1

1

C

C

C

C

A

D

B

= Study Area

= Mineral Deposit Distribution

Binary Pattern (either "present”

or "absent")

= Mineral Predictor Binary Pattern

(either "present" or "absent")

B39
C1

APPENDIX C

Mineral Potential Map Generation, 

Conditional Independence,

and Uncertainty:

Section Contents:

C.1 General Introduction (organization of materials presented)

C.2 Mineral Potential Maps Generated

C.3 Conditional Independence

C.3.1 Introduction

C.3.2 Results of the Pairwise Test

C.3.3 Results of the Overall Test

C.3.4 Mitigation of Conditional Dependence

C.3.5 Interpretation of Conditional Dependence

C.3.6 Comparison Between CI-Mitigated and Unmitigated Mineral Potential Maps

C.3.7 Comparison Between Mineral Potential Maps Generated by the Weights of Evidence and

Weighted Logistic Regression Methods

C.4 Uncertainty of the Mineral Potential Maps

C.4.1 Introduction

C.4.2 Maps of Mineral Potential Uncertainty
C2

C.1 General Introduction

Much of the material covered in here is organized in a 2-fold manner: for each of the three

occurrence-types (primary, sedimentary and volcanic rock-hosted), two mineral potential models,

composed of a differing number of evidence layers, are examined (making a total of six models).

The first model is based on all of the binary-class mineral predictor maps, while the second

model is based on a fewer number of predictor maps, where one or more maps have been rejected

in order to make the model more statistically sound and/or geologically robust. As such, a pair

of tables, graphs, figures, or maps is presented for each of the three occurrence-types (primary,

sedimentary and volcanic rock-hosted), resulting in a total number of six tables, graphs, figures,

or maps necessary to characterize or illustrate any one point.

C.2 Mineral Potential Maps Generated

A total of six weights of evidence (WOE) mineral potential maps were produced—two for each

of the primary, sedimentary and volcanic rock-hosted occurrence-types. The first map is based

on all evidence layers (Figs. C.1a, C.2a, and C.3a). The second map is based on a fewer number

of evidence layers, where predictor maps were removed from the model in an attempt to mitigate

conditional dependency violations (Figs. C.1b, C.2b, and C.3b). Conditional independence and

its mitigation are discussed below in section C.3.

In addition, three more mineral potential maps, generated using the weighted logistic regression

(WLR) method of predictor map combination, are presented for comparison with the “allevidence-layers” WOE-derived mineral potential maps for each occurrence-types (Figs. C.1c,

C.2c, and C.3c). Weighted logistic regression is covered below in section C.3.7.

All binary-class mineral predictor maps were coded as:

C 0 class—Predictor pattern missing (no data).

C 1 class—Predictor pattern absent (pattern not favorable).

C 2 class—Predictor pattern present (pattern favorable).

Where data coverage was not complete for an predictor map (e.g.—K/Na and Ba/Na

geochemical anomaly maps), the area was assigned “pattern unknown” and weighted as “0” for

the WOE-derived posterior probability mineral potential maps. For the WLR-derived maps,

where no way for representing missing data is possible, areas of incomplete data coverage were

reclassified to “pattern not present”, and was done so in order to facilitate the comparison

between the WOE and WLR map combination methods.

The WOE-derived mineral potential maps were classified using a “quantiles” approach—an

“area-based” percentiles scheme where the posterior probability class intervals (the break points)

are determined so that each of the posterior probability classes is roughly equal in area (see

section B.5.4; also see TYDAC Technologies Inc., 1993, Reference Manual, p. 5-56, for more

information). The WLR-derived mineral potential maps were also classified using the same

WOE-derived map classification schemes, so that an absolute-scale comparison could be made

between the WOE-derived and WLR-derived maps.
C3

C.3 Conditional Independence

C.3.1 Introduction

Conditional independence (CI) between the binary-class mineral predictor map patterns, with

respect to the gold-silver-bearing occurrences, is assumed to exist and is necessary when adding

the weights of various types of evidence. For a mineral potential model and its resultant

posterior probability map to be robust, the presence of a mineralization-favorable map pattern

in one layer of evidence (a predictor map) should not be dependent on the presence of a

mineralization-favorable map pattern in another layer of evidence. Conditional independence

is tested for using pairwise and overall goodness-of-fit methods. If a predictor map is found to

be in serious violation of the assumption of CI, it can then be (1) rejected from the model, (2)

combined with another map in order to minimize the dependency, or (3) modified in some way

to reduce the problem (for more information, see Agterberg et al., 1990; Bonham-Carter,

1994a).

Both the pairwise and overall CI tests make use of the observed versus the expected number of

observations (gold-silver-bearing occurrences). The expected number of occurrences in this

instance is calculated by multiplying the total area of the predictor map pattern (in km2

) by its

associated posterior probability (this assumes that the total region of the study area is represented

in 1 km2 size unit cells, where an occurrence is represented by one unit cell, or is 1 km2 in size)

(see Bonham-Carter, 1994a, p. 317). The theoretical aspects and the implementation of these

tests are described in the sections B.3 and B.5.

The principal points made in this section are: (1) conditional dependency between the layers of

evidence for the mineral potential models does exist, however, the “predictive” quality of the

mineral potential maps is not seriously affected; and (2) the posterior probability maps generated

from the mineral potential models should be considered as “favorability” rather than

“probabilistic” mineral potential maps. The results of the pairwise and overall goodness-of-fit

tests are presented below in sections C.3.2 and C.3.3. Corrections made to the mineral potential

models to mitigate CI problems are reviewed in section C.3.4. Interpretation of the pairwise and

overall goodness-of-fit test results is reviewed in section C.3.5 and an evaluation of results is

presented in sections C.3.6 and C.3.7.

C.3.2 Results of the Pairwise Test

The pairwise goodness-of-fit test measures conditional independence (CI) between all possible

pairings of the binary-class predictor maps by calculating the x2

 (chi-square) statistic for each

map pair, and comparing the calculated value of x2

 to the tabled value of x2 having one degree

of freedom (Bonham-Carter, 1994a). The results of the test are typically presented in the form

of pairwise cross-tabulation tables (see sections B.3.2 and B.5.5).

The results of the pairwise x2

 test for the primary, sedimentary and volcanic rock-hosted

occurrence-type mineral potential models are presented in summary form in Tables C.1, C.2, and

C.3. A probability level threshold of 98% (x2

0.98,1 = 5.41) was chosen as the null hypothesis (that

CI exists) rejection-acceptance cutoff. Two tables of x2 test results are given for each of the three
C4

occurrence-types. Each table represents a mineral potential model that is identified with a label

consisting of the occurrence-type (primary, sed-hosted, or vol-hosted) and the number of layers

of evidence composing the particular model (11, 9, etc.). Tables C.1a, C.2a, and C.3a show x2

scores for mineral potential models that combine all of the predictor maps for a given mineral

potential model. Tables C.1b, C.2b, and C.3b show x2 scores for mineral potential models that

combine a fewer number of predictor maps, where some maps have been rejected from the

mineral potential model in an attempt to mitigate serious CI violations that appear in Tables

C.1a,C.2a, and C.3a. The respective mineral potential models represented by Tables C.1b, C.2b,

andC.3b are not fully optimized with respect to the x2 test—all map combinations that yield high

rejection-level x2

 values (> 5.41) have not been eliminated. Instead, these tables represent

mineral potential models that are a geologically-sensible compromise between two end-member

models. One end member mineral potential model is the combination of all predictor maps,

which yields many high x2

 scores. The other end member model is a combination of fewer maps,

which yields x2 scores all below 5.41, but consists of relatively few predictor maps (e.g.—to

eliminate all conditional dependency for the primary 11-layer mineral potential model, 8 of the

11 maps would have to be rejected from the model).

C.3.3 Results of the Overall Test

The overall goodness-of-fit test is a measure of the conditional independence (CI) among all of

the layers of evidence in a model as a whole. The overall method is a simple procedure

involving the relative comparison of predicted versus observed occurrences—if the total

predicted number of occurrences is much larger than the total observed number (greater than

~10-15%), it suggests that CI is being violated, and may warrant a check of the pairwise tests and

some sort of remedial action (Bonham-Carter, 1994a). The overall goodness-of-fit can also be

evaluated using the Kolmogorov-Smirnov (K-S) statistic. The K-S test is based on the maximum

deviation of the observed number of occurrences from the predicted number, and the results are

typically presented in graphic form (see sections B.3.3 and B.5.5).

The total expected versus total observed number of occurrences for the various occurrence-type

mineral potential models and other overall CI test result data are given in Table C.4. The

graphical results of the K-S test are shown in Figures C.4, C.5, and C.6. For each of the three

occurrence-types, two mineral potential models were tested (as was performed with the pairwise

x2

 test). Figures C.4a, C.5a, and C.6a show the K-S test results for the various mineral potential

models using all of the available layers of evidence. Figures C.4b, C.5b, and C.6b show the

results for mineral potential models that are composed of fewer layers of evidence, where an

attempt has been made to mitigate serious CI violations (the “a” and “b” series K-S test figures

are complementary to the “a” and “b” series x2

 test tables). The figures also show the critical

K-S statistic cut-off values and the K-S values calculated for each of the models.

C.3.4 Mitigation of Conditional Dependence

Both the x2 pairwise tests and the overall tests indicate that a high degree of conditional

dependency exists between the various layers of evidence, with respect to the occurrences, for

all of the occurrence-type mineral potential models. The lack of CI occurs most notably in the

mineral potential models where all of the available binary-class predictor maps were combined

(the “a” series of Tables C.1, C.2, and C.3, and Figs. C.4, C.5, and C.6).
C5

The primary occurrences mineral potential models showed the greatest violations of pairwise CI

of any of the gold-silver-bearing occurrence-types. The highest x2 scores are found in the

primary occurrences 11-layer model (Table C.1a), between the lithologic diversity and fault

buffers predictor maps, the medium-potential lithology with the high- and low-potential lithology

predictor maps, and most notably, between the K/Na and Ba/Na geochemical anomaly predictor

maps (x2 = 486.34). Many of these x2 scores are an order of magnitude or two higher than most

of the other scores for any of the other sedimentary or volcanic rock-hosted mineral potential

models. The most severe instances of CI violation in the primary 11-layer model were rectified

by removing the Ba/Na, lithology (medium W+), lithologic diversity, and lithotectonic terranes

predictor maps (Table C.1b). This modification, while clearly not solving all of the pairwise CI

violations, did result in a marked decrease in the number of gold-silver-bearing occurrences of

all types and sizes predicted in the overall test (Fig. C.4b; also see Table C.4, the “% expected

over observed”, “K-S critical value”, and “K-S statistic for model” values).

The sedimentary rock-hosted occurrence mineral potential models have less pairwise CI

violations than the primary occurrence models, and relative to the primary 11-layer model, have

substantially lower x2 scores (Tables C.2a and C.2b). The overall test, however, indicates the

highest discrepancy between the number of observed and predicted occurrences for any of the

occurrence-types (Table C.4). In order to correct all of the CI violations observed in the

pairwise test, the pluton buffers, lithologic diversity, and isostatic gravity predictor maps would

have had to been removed from the mineral potential model (leaving only 6 predictor maps).

Even with such a modification, the overall test still predicts 452.57 occurrences to the observed

98—from a relative standpoint, not much of an improvement (it is also interesting to note that

the number of unique conditions from the 9-layer to the 6-layer mineral potential model

decreases from 598 to only 96). Therefore, only the pluton buffers map was removed from the

model (Table C.2b and Fig. C.5b), as the geologic factors represented in the other predictor maps

were deemed to be more important to the formation of sedimentary rock-hosted occurrences.

The removal of the pluton buffers predictor map from the mineral potential model, while perhaps

being logical from a geologic perspective, resulted in a slight increase in the number of

sedimentary rock-hosted occurrences predicted in the overall test (Table C.4). Irrespective of

combination or number of predictor maps included, there is a large degree of conditional

dependence in the sedimentary rock-hosted mineral potential models.

The volcanic rock-hosted occurrences mineral potential models, in comparison to the other goldsilver-bearing occurrence-types, showed the least (yet still noticeable) amount of pairwise and

overall CI violations, when considered together (Table C.4 and Fig. C.6). The Ba/Na

geochemical anomaly and lithotectonic terranes predictor maps were the source of the greatest

number and most severe pairwise CI violations. Removal of these two maps from the mineral

potential model (Table C.3b and Fig. C.6b), while not remedying all of the pairwise CI

violations, resulted in significantly closer agreement between observed and predicted occurrences

(see Table C.4).

C.3.5 Interpretation of Conditional Dependence

The violation of conditional independence in the mineral potential models is not unexpected.

Conditional independence in WOE modelling is always violated to some degree (BonhamCarter, 1994a; see section B.3). The greater the number of evidence maps used in a model,
C6

especially if those maps have similar distribution patterns and represent similar types of

evidence, the greater the possibility is that conditional dependency will exist (that CI will be

violated). For example, the Ka/Na and Ba/Na geochemical anomaly maps are nearly coincident

in their spatial distribution, are derived from the same dataset, represent broadly similar

information, and are highly correlated spatially with gold-silver-bearing occurrences

(sedimentary rock-hosted occurrences in particular). It is not surprising, therefore, that these two

layers of evidence show very high x2

 scores in the pairwise goodness-of-fit tests. In addition,

because these predictor maps have high W+

 values that are essentially compounded during the

Bayesian overlay procedure, the dependence between their predictor map patterns results in

inflated posterior probabilities, and the overestimation of predicted occurrences. The same is

probably true for some of the other dataset pairs, such as the geomagnetic anomaly and lithology

datasets, and the fault buffers and lithologic diversity datasets. The failure of the overall

goodness-of-fit test also indicates a large degree of interdependency that is probably due to the

multiple interactions of many similar and highly mineral-occurrence-correlated predictor maps.

The high degree of CI violation exhibited by these mineral potential models may also be in part

related to:

1. Modelling the distribution of a relatively large number of mineral occurrences (response variables).

2. Compounding effects of mineral occurrence point clustering, where more than one point may fall within the

1 km2

 unit cell size assigned to represent a mineral occurrence.

To investigate these possibilities, two simple and non-rigorous empirical tests were performed

using the primary occurrences 11-layer mineral potential model datasets. These datasets were

chosen because they contain the greatest number of occurrence points (2690), the largest number

of predictor maps (11), and therefore, the highest number of mineral potential map unique

condition classes.

In the case of possibility #1, six pairwise CI test trials were performed on the primary 11-layer

model using six different “dummy” mineral occurrence datasets—consisting of various numbers

(139, 277, 565, 1127, 1565, and 2856) of gridded points—in place of the primary occurrences

point dataset. The results of the trials show a roughly proportional increase in the magnitude of

the highest x2

 scores with an increase in the number of dummy occurrence points (smaller x2

scores also showed a general increase in magnitude, but not necessarily in proportion to the

number of dummy occurrence points). Given the increases in the magnitude of the CI violations,

increases in the number of violations also occur (the increase in the number of violations,

however, does not appear to be proportional to the number of dummy occurrence points). These

findings indicate that increasingly large response variable datasets (i.e.—greater numbers of

mineral occurrences, or points) have increasingly larger magnitude and a greater number of CI

violations (this is intuitively what would be expected).

With regard to possibility #2, pairwise and overall CI tests were performed on the primary 11-

layer model using a version of the primary occurrences point dataset that was “weeded” so as to

remove any points that lay less than 1 km from a neighbouring point. In this manner, the primary

occurrences point dataset was reduced from 2690 to 1625 points (it is interesting to note that

nearly 40% of gold-silver-bearing occurrences lie within 1 km of another). The results of the

pairwise CI test indicate that the magnitudes of the higher x2

 scores for the weeded occurrence
C7

point dataset are proportionally just as large as the x2

 scores for the non-weeded occurrence point

dataset (proportional relative to the total number of occurrence points, similar to the results

demonstrated above with the gridded dummy occurrence datasets). The number of total CI

violations also decreased slightly, but this was directly attributable to the decrease in x2 score

magnitudes “across the board”, where borderline scores dropped below the tabled critical x2

value. The overall test results also indicate negligible differences between the weeded and nonweeded datasets. Using the weeded occurrence point dataset, the overall test predicts

proportionatelythe same number of occurrences as is predicted using the non-weeded occurrence

point dataset—for the non-weeded occurrence point dataset, 2690 occurrences observed and

4525.45 occurrences expected (predicted over observed by ~68%, see Table C.4); for the

weeded occurrence point dataset, 1625 occurrences observed and 2770.50 expected (predicted

over observed by ~70%). 

In summary, the results of this investigation into the influence of large response variable datasets

on conditional independence in the WOE modelling method suggest: (1) mineral occurrence

point clustering (in this case where occurrence points are assigned a unit cell size of 1 km2

) has

no causal effect on whether or not CI between predictor maps, with respect to mineral

occurrences, exists; and (2) large occurrence point datasets influence the calculated x2 scores in

the pairwise goodness-of-fit test, producing larger magnitude x2

 scores with greater numbers of

occurrence points.

The fundamental point to be made here is that the layers of evidence composing these mineral

potential models are not conditionally independent with respect to the mineral occurrences, and

that this lack of CI is real, in both the statistical and the geological sense. Bonham-Carter

(1994a) pointed out that the assumption of CI in weights of evidence results in a model that, like

most models, does not perfectly represent the data, but does provide a working simplification,

which, when applied with care, can be useful for prediction and provides insight into the relative

contributions of the separate sources of evidence. It is therefore important to be aware of, as well

as understand, the degree and causes for the lack of CI in the models. Given the CI violations

discussed in the preceding paragraphs, it is best to consider the mineral potential maps derived

from these models as relative “favorability” maps, rather than as “probabilistic” maps in the

strict sense.

C.3.6 Comparison Between CI-Mitigated and Unmitigated Mineral Potential

Maps

The differences between the first and second mineral potential maps—the “all-evidence-layers”

WOE-derived mineral potential maps and the maps based on fewer evidence layers—are shown

as maps of absolute difference in posterior probability in Figure C.7. These maps indicate that

the differences in the posterior probabilities are generally small (relative to the maximum

posterior probability for the two models compared for a given occurrence-type group). Where

the difference is larger, the corresponding map areas are negligible in size (e.g.—less than 0.5%

of the total area for the primary and sedimentary rock-hosted occurrence mineral potential maps,

and less than 2.3% for volcanic rock-hosted, accounts for the top two red-and-yellow mapdifference classes shown in Fig. C.7).
C8

C.3.7 Comparison Between Mineral Potential Maps Generated by the

Weights of Evidence and Weighted Logistic Regression Methods

In order to help evaluate and measure the effects of conditional dependence in the mineral

potential models, a comparison was made between the Bayesian statistical method for combining

layers of evidence used in weights of evidence (WOE) modelling and a weighted logistic

regression (WLR) method. This comparison was chiefly made because the WLR method does

not require statistical independence between the predictor variables (the layers of evidence) with

respect to the response variable (the mineral occurrences). Logistic regression analysis

techniques for estimating mineral resource potential have been applied by Agterberg (1974,

1992a, 1992b, 1993), Chung and Agterberg (1980), Reddy et al. (1991), and Wright (1996). A

detailed review of the WLR method and its theoretical aspects is given in Agterberg (1992a) and

an overview given in Wright (1996). The WLR map combination procedure was carried out in

a similar manner to WOE (see section B.5.4).

The WLR method usually produces posterior probabilities that are smaller in magnitude than

those generated by the WOE method, which results in a fewer number of predicted occurrences.

The simple overall goodness-of-fit test, when conducted with the WLR posterior probabilities,

results in a predicted number of occurrences that is very close to the observed. Table C.5 shows

that WLR expected frequencies generally fall a few percentage points or less short of the

observed frequencies, whereas the WOE expected frequencies over-shoot the expected

frequencies by 10's to 100's of percent. An examination of the posterior probabilities generated

by each method (Figs. C.8, C.9, and C.10) suggests that the large overestimation of predicted

occurrences based on the WOE method may, to a certain degree, be the result of anomalously

high posterior probability values associated with a small number of unique overlap conditions

between the predictor maps for a particular mineral potential model. This is especially apparent

for the volcanic rock-hosted 9-layer and sedimentary rock-hosted 9-layer mineral potential

models (Figs. C.9a and C.10a, respectively).

In addition, side-by-side visual comparisons were made between mineral potential maps

generated from WLR-derived and WOE-derived posterior probabilities. This was done in order

to determine if spatial distribution of the WOE-derived mineral potential map patterns were

significantly affected by CI violation. These comparisons show only minor differences in the

distribution of favorable mineral potential areas at the local-scale, and at the regional scale, no

significant differences are visible in the overall map patterns, although the WOE mineral

potential maps appear to be somewhat “warmer”(Figs. C.1c, C.2c, and C.3c). In general, the

results of the WLR method appear to be consistent with those of the WOE method, and suggest

that, for this study, the results of the WOE method are within acceptable and expected bounds.

C.4 Uncertainty of the Mineral Potential Maps

C.4.1 Introduction

It is important to recognize and understand the uncertainty inherent to the posterior probabilities

when examining a mineral potential map. Uncertainty may be due to the variances in weight

estimates (W+

 and W–) and/or to one or more of the predictor maps having incomplete coverage
C9

(i.e.—missing data) (Bonham-Carter et al., 1989). As an aid for interpretation, a posterior

probability uncertainty map may be compared separately or combined with a mineral potential

map to “mask-out” regions of high posterior probability uncertainty (see section B.4 for

additional information).

C.4.2 Maps of Mineral Potential Uncertainty

The mineral potential uncertainty maps are presented in Figures C.11, C.12, C.13, and C.14. The

first series of uncertainty maps (Fig. C.11) illustrates all four types of uncertainties (weights

variance, missing data, total, and relative), and was generated using the primary occurrence-type

11-layer mineral potential model. The three remaining series of uncertainty maps (Figs. C.12,

C.13, and C.14) show only the total and relative uncertainties for the “CI-optimized” primary,

sedimentary and volcanic rock-hosted occurrence-type mineral potential models. The maps were

classified using a quantiles technique, as described in section B.5.4, and edited so as to

emphasize the upper portion of the uncertainty values.

The maps of uncertainty due to variance in the weights estimates include the uncertainty of the

prior probability, and in general, are correlated to the posterior probabilities. Hence, the weights

variance uncertainty maps tend to have a pattern similar to those of the posterior probability

maps. The maps of uncertainty due to missing data are (as is expected) strongly controlled by

the incomplete evidence map coverage, which in this case is clearly related to the K/Na and

Ba/Na geochemical anomaly evidence (note the “blocky” uncertainty map patterns, particularly

in Fig. C11b). The relative certainty maps indicate the degree of confidence to which the

posterior probability mineral potential map patterns are “real”, as opposed to being an artifact

of “chance” effects (or due to chance). The larger the t-value over the critical tabled t-value cutoff (e.g.—1.645 for a significance of 95%) the greater the certainty of the mineral potential map

pattern, although the high t-values in some of these relative certainty maps may be related to

violations of CI. In general, the maps of uncertainty due to variance in weights estimates and due

to missing data reflect the large overall variation in the data (i.e.—”noise in the system”).

However, the relative certainty maps suggest that, despite this high degree of variability, the

mineral potential map favorability patterns are reliable.

The uncertainties discussed above are presented in the form of single-theme maps (only

uncertainty), however, the uncertainties may alternatively be presented in the form of 3-

dimensional illustrations that more discernably relate the degree of certainty to areas of high

mineral potential (i.e.—to areas having high posterior probabilities). For example, Fig. C.15

shows a 3-dimensional rendering of relative certainty draped over a pseudo-DEM wire-frame of

posterior probabilities for the sedimentary rock-hosted 8-layer mineral potential model. In this

illustration, height represents posterior probability values that are greater than the prior, and the

color represents the degree of relative certainty of the values, as indicated by t-value. Note that

the regions of higher certainty (yellow and red) are spatially coincident to the “V”-shaped relief

of high mineral potential, suggesting that this favorability pattern has a high degree of certainty,

and is therefore probably not due to chance.
C10

A. Primary (Occurrences of all Sizes and Types) 11-Layer Mineral Potential Model

Map Ba/Na fault

buffers

pluton

buffers

lithology

(high W+)

lithology

 (low W+)

lithology

(med. W+)

lithologic

diversity

isostatic

gravity K/Na geomagnetics

lithotectonic

Ba/Na 0.00 35.73 32.57 15.05 0.03 0.85 93.47 95.73 486.34 6.60 78.37

fault buffers 0.00 21.71 0.66 0.39 0.64 124.92 8.46 31.21 14.80 2.93

pluton buffers 0.00 31.13 44.13 1.19 39.81 45.46 29.91 4.50 29.93

lithology (high W+) 0.00 57.30 139.23 12.67 8.04 0.78 1.25 95.93

lithology (low W+) 0.00 142.54 0.00 5.70 12.39 21.01 49.60

lithology (med. W+) 0.00 4.32 18.25 0.11 5.82 0.58

lithologic diversity 0.00 23.44 43.94 1.56 0.19

isostatic gravity 0.00 5.71 20.37 1.81

K/Na 0.00 17.21 6.14

geomagnetics 0.00 4.51

lithotectonic terranes 0.00

B. Primary (Occurrences of all Sizes and Types) 7-Layer Mineral Potential Model

Map fault

buffers

pluton

buffers

lithology

(high W+)

lithology

(low W+)

isostatic

gravity K/Na geomagnetics

fault buffers 0.00 21.71 0.66 0.39 8.46 31.21 14.80

pluton buffers 0.00 31.13 44.13 45.46 29.91 4.50

lithology (high W+) 0.00 57.30 8.04 0.78 1.25

lithology (low W+) 0.00 5.70 12.39 21.01

isostatic gravity 0.00 5.71 20.37

K/Na 0.00 17.21

magnetics 0.00

Table C.1. Pairwise conditional independence (CI) test x2 scores for primary occurrence-type model. Table A. (upper) shows scores for the combination of all

(11) binary-class predictor maps. Table B. (lower) shows scores for the combination of those maps that have ~10th percentile and lower x2

 values relative to

the scores shown in the upper table. With 1 degree of freedom and a probability level of 98% (x2

0.98,1 = 5.41), many x2

 values in table A exceed the acceptance

cutoff of the null hypothesis (that CI exists) by one or two orders of magnitude, therefore indicating a severe violation of conditional independence in those cases.

In table B, the most severe cases of conditional dependence have been mitigated by removing the Ba/Na, lithology (medium W+

), lithologic diversity, and

lithotectonic terranes maps from the model.
C11

A. Sedimentary Rock-Hosted 9-Layer Mineral Potential Model

Map Ba/Na fault

buffers

pluton

buffers

lithologic

diversity lithology isostatic

gravity K/Na geomagnetics

lithotectonic

Ba/Na 0.00 0.84 0.53 1.08 0.82 0.80 6.11 1.99 0.14

fault buffers 0.00 0.28 0.03 1.58 1.20 0.07 0.24 0.04

pluton buffers 0.00 15.30 0.58 0.13 0.54 4.47 0.05

lithologic diversity 0.00 3.84 1.60 2.90 15.54 2.77

lithology 0.00 10.15 2.79 2.21 2.21

isostatic gravity 0.00 6.98 0.46 11.97

K/Na 0.00 0.92 1.33

geomagnetics 0.00 1.09

lithotectonic terranes 0.00

B. Sedimentary Rock-Hosted 8-Layer Mineral Potential Model

Map Ba/Na fault

buffers

lithologic

diversity lithology isostatic

gravity K/Na geomagnetics

lithotectonic

Ba/Na 0.00 0.84 1.08 0.82 0.80 6.11 1.99 0.14

fault buffers 0.00 0.03 1.58 1.20 0.07 0.24 0.04

lithologic diversity 0.00 3.84 1.60 2.90 15.54 2.77

lithology 0.00 10.15 2.79 2.21 2.21

isostatic gravity 0.00 6.98 0.46 11.97

K/Na 0.00 0.92 1.33

geomagnetics 0.00 1.09

lithotectonic terranes 0.00

Table C.2. Pairwise conditional independence (CI) test x2 scores for the sedimentary rock-hosted occurrence-type model. Table A. (upper) shows scores for

the combination of all (9) binary-class predictor maps. Table B. (lower) shows scores for the combination of eight maps after removal of the pluton buffers map

from the model. With 1 degree of freedom and a probability level of 99% (x2

0.99,1 = 6.63), over 85% of the map pattern pairs in table B approximately satisfy

the assumption of conditional independence. Only the pluton buffer map was removed from the model because of its high degree of spatial correlation with the

distribution pattern of volcanic rock-hosted occurrences. The isostatic gravity, geomagnetics, and lithotectonic terranes maps, although responsible for a

significant amount of conditional dependency, were not eliminated from the model because of their spatial coincidence to the distribution pattern of the

sedimentary rock-hosted occurrences.
C12

A. Volcanic Rock-Hosted 9-Layer Mineral Potential Model

Map Ba/Na fault

buffers

pluton

buffers

lithologic

diversity lithology isostatic

gravity K/Na geomagnetics

lithotectonic

Ba/Na 0.00 0.18 0.17 12.34 10.12 7.08 123.09 7.78 15.94

fault buffers 0.00 2.58 6.17 9.51 2.35 3.96 1.24 0.00

pluton buffers 0.00 13.37 5.72 6.47 0.78 13.56 1.10

lithologic diversity 0.00 0.83 6.21 12.50 3.20 0.85

lithology 0.00 1.70 2.16 11.14 9.02

isostatic gravity 0.00 3.05 0.02 15.40

K/Na 0.00 3.32 7.23

geomagnetics 0.00 0.05

lithotectonic terranes 0.00

B. Volcanic Rock-Hosted 7-Layer Mineral Potential Model

Map fault

buffers

pluton

buffers

lithologic

diversity lithology isostatic

gravity K/Na geomagnetics

fault buffers 0.00 2.58 6.17 9.51 2.35 3.96 1.24

pluton buffers 0.00 13.37 5.72 6.47 0.78 13.56

lithologic diversity 0.00 0.83 6.21 12.50 3.20

lithology 0.00 1.70 2.16 11.14

isostatic gravity 0.00 3.05 0.02

K/Na 0.00 3.32

geomagnetics 0.00

Table C.3. Pairwise conditional independence (CI) test x2 scores for the volcanic rock-hosted occurrence-type model. Table A. (upper) shows scores for the

combination of all (9) binary-class predictor maps. Table B. (lower) shows scores for the combination of seven maps after removal of the Ba/Na and lithotectonic

terranes maps from the model. With 1 degree of freedom and a probability level of 99% (x2

0.99,1 = 6.63), over 76% of the map pattern pairs in table B

approximately satisfy the assumption of conditional independence. The Ba/Na geochemical anomaly map was removed from the model because of the high

degree of association of Ba-bearing mineralization (primarily barite) with sedimentary rock-hosted occurrences. The lithotectonic terranes map was removed

from the model to eliminate the greatest number of conditional dependencies while rejecting the least number of maps from the model. Other map pair

combinations responsible for a significant degree of conditional dependency were not removed from the model because of the relative importance of these features

to mineralization (e.g.—fault buffers).
C13

Occurrence-Type Model

Primary

(all sizes and types)

Sedimentary

Rock-Hosted

Volcanic

Rock-Hosted

Mineral Potential

Model 11-Layer 7-Layer 9-Layer 8-Layer 9-Layer 7-Layer

Observed

Occurrences 2690 2690 98 98 415 415

Expected

Occurrences 4525.45 3261.11 721.99 733.36 654.31 507.21

% Expected

> Observed 68.23 21.23 636.72 648.33 57.67 22.22

Number of

Unique Conditions 1144 154 598 382 658 198

K-S Critical

Value (% = 0.05) 0.0402 0.1096 0.0556 0.0696 0.0530 0.0966

K-S Statistic for

the Model 0.2737 0.1725 0.4254 0.4316 0.2547 0.1599

Table C.4. Summary information of the overall conditional independence test for the various mineral potential

models.
C14

Occurrence-Type Mineral Potential Model

Primary

(all sizes and types)

Sedimentary

Rock-Hosted

Volcanic

Rock-Hosted

Mineral Potential

Model 11-Layer 7-Layer 9-Layer 8-Layer 9-Layer 7-Layer

Observed

Occurrences 2690 2690 98 98 415 415

WLR Expected

Occurrences 2572.94 2606.54 96.42 96.46 411.35 412.07

% WLR Expected

Over Observed -4.35 -3.10 -1.61 -1.57 -0.88 -0.71

WOE Expected

Occurrences 4525.45 3261.11 721.99 733.36 654.31 507.21

% WOE Expected

Over Observed 68.23 21.23 636.72 648.33 57.67 22.22

Table C.5. Comparison of expected versus observed occurrences for the weights of evidence (WOE) and weighted

logistic regression (WLR) methods of evidence combination. In general, the number of occurrences predicted using

the WLR method is nearly equal to the number of occurrences observed. Minus percent values indicate the amount

that the predicted number of occurrences fall below (short of) the observed (the observed equalling 100%).
A.

11-Layer Model, WOE

B.

7-Layer Model, WOE

C.

11-Layer Model, WLR

Figure C.1. (A) (B)

(C)

Primary occurrence-type mineral potential model results. Posterior probability mapsfor weights of evidence (WOE) 11-layer evidence model,

7-layer evidencemodel, and weighted logistic regression (WLR) 11-layer evidence model. The 11-layerWLR-derived map isincluded for comparison with the

11-layerWOE-derived map to help evaluate the effects of conditional dependence in the WOE-based model. Note the regions of favorable mineral potential along

theNW-SEWalker Lane shear zone in southwestern Nevada,theBattleMountain heat high in north-central Nevada, and the weak pattern along theBattleMountainEureka (Cortez) mineraltrend. In general, theWOE- andWLR-derived mapsshow the same overall map patterns, except thattheWOE-derived maps appear to be

"warmer" and better highlight favorablemineral potential areas. TheWLRmapcombinationmethod generally produceslower posterior probabilitiesthan doesthe

WOE method. The WLR-derived posterior probability map was generated using the WOE-derived posterior probability classification scheme (to facilitate

comparison between the two methods), and isthe reason forthe overall "less-favorable" appearance ("cooler colors") oftheWLR-derived map. If the WLR-derived

map wasreclassified using a classification scheme based on the range ofWLR-derived posterior probabilities,the resultant map would be very similar in appearance

to theWOE-derived map.
A.

9-Layer Model, WOE

B.

8-Layer Model, WOE

C.

9-Layer Model, WLR

Figure C.2. (A)

(B) (C)

Sedimentary rock-hosted occurrence-type mineral potential model results. Posterior probability maps for weights of evidence (WOE) 9-layer

evidence mode, 8-layer evidence model, and weighted logistic regression (WLR) 9-layer evidence model. The 9-layer WLR-derived map is included for

comparison with the 9-layer WOE-derived map to help evaluate the effects of conditional dependence in theWOE-based model. Note the prominent " -shaped

mineral potentialtrendsin north-centralNevada that parallelthe BattleMountain-Eureka (Cortez)(western leg ofthe " ") andCarlin (northern portion of eastern leg

ofthe " ")mineraltrends.

V"

V

V
A.

9-Layer Model, WOE

B.

7-Layer Model, WOE

C.

9-Layer Model, WLR

Figure C.3. (A)

(B) (C)

Volcanic rock-hosted occurrence-typemineral potentialmodelresults. Posterior probability mapsfor weights of evidence (WOE) 9-layer evidence

model, 7-layer evidencemodel, and weighted logistic regression 9-layer evidencemodel. The 9-layerWLR-derived map isincluded for comparison with the

9-layerWOE-derived map to help evaluate the effects of conditional dependence in theWOE-based model. Note the favorable mineral potentialtrendsthat parallel

theNWn n SE-trendingWalkerLane shear zone in southwestern Nevada andthe broad NE SW-trending belt across northern Nevada.
C18 Overall Conditional Independence Using Kolmogorov-Smirnov Statistic

Primary Occurrences 11-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Posterior Probability

Cumulative Relative Frequency

Observed Occurrences

Predicted Occurrences

Difference (Observed - Predicted)

A.

Critical K-S Value for 5% Significance Level = 0.0402

K-S Statistic for this mineral potential model = 0.2737

Overall Conditional Independence Using Kolmogorov-Smirnov Statistic

Primary Occurrences 7-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Posterior Probability

Cumulative Relative Frequency

Observed Occurrences

Predicted Occurrences

Difference (Observed - Predicted)

B.

Critical K-S Value for 5% Significance Level = 0.1096

K-S Statistic for this mineral potential model = 0.1725

 Figure C.4. Overall goodness-of-fit test for all size and type occurrences, using the K-S statistic: (A) primary

 11-layer model; (B) primary 7-layer model.
C19 Overall Conditional Independence Using Kolmogorov-Smirnov Statistic 

Sedimentary Rock-Hosted Occurrences 9-Layer Mineral Potential 

Model

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Posterior Probability

Cumulative Relative Frequency

Observed Occurrences

Predicted Occurrences

Difference (Observed - Predicted)

A.

Critical K-S Value for 5% Significance Level = 0.0556

K-S Statistic for this mineral potential model = 0.4254

Overall Conditional Independence Using Kolmogorov-Smirnov Statistic

Sedimentary Rock-Hosted Occurrences 8-Layer Mineral Potential 

Model

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Posterior Probability

Cumulative Relative Frequency

Observed Occurrences

Predicted Occurrences

Difference (Observed - Predicted)

B.

Critical K-S Value for 5% Significance Level = 0.0696

K-S Statistic for this mineral potential model = 0.4316

 Figure C.5. Overall goodness-of-fit test for sed. rock-hosted occurrences, using the K-S statistic: (A) sed-host.

 9-layer model; (B) sed-host. 8-layer model.
C20 Overall Conditional Independence Using Kolmogorov-Smirnov Statistic

Volcanic Rock-Hosted Occurrences 9-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Posterior Probability

Cumulative Relative Frequency

Observed Occurrences

Predicted Occurrences

Difference (Observed - Predicted)

A.

Critical K-S Value for 5% Significance Level = 0.0530

K-S Statistic for this mineral potential model = 0.2547

Overall Conditional Independence Using Kolmogorov-Smirnov Statistic

Volcanic Rock-Hosted Occurrences 7-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Posterior Probability

Cumulative Relative Frequency

Observed Occurrences

Predicted Occurrences

Difference (Observed - Predicted)

B.

Critical K-S Value for 5% Significance Level = 0.0966

K-S Statistic for this mineral potential model = 0.1599

 Figure C.6. Overall goodness-of-fit test for vol. rock-hosted occurrences, using the K-S statistic: (A) vol-host.

 9-layer model; (B) vol-host. 7-layer model.
A. B. C.

Figure C.7. (A)

(B) (C)

Difference between mineral potential favorability maps(in absolute magnitude of posterior probability). Primary occurrence-type 11-layer and 7-

layer mineral potential models, sedimentary rock-hosted occurrence-type 9-layer and 8-layer mineral potential models, and volcanic rock-hosted

occurrence-type 9-layer and 7-layermineral potentialmodels. Maps classified using "quantiles"method.
C22

WOE Poster Probability Plotted Against WLR Posterier Probability

Primary Occurrences 11-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

WOE-Derived Posterior Probability

WLR-Derived Posterior Probability

B.

X = Y

Comparison of WOE versus WLR Derived Posterior Probabilities

Primary Occurrences 11-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 200 400 600 800 1000 1200 1400

Mineral Potential Observation

Larger Areas . . . (Unique Conditions Map Class) . . . Smaller Areas

Posterior Probability

0

3000

6000

9000

12000

15000

18000

21000

24000

27000

30000

Square Km

WOE Posterior Probability

WLR Posterior Probability

Uniq. Cond. Map Class Area

A.

 Figure C.8 Posterior probability estimates derived from WOE and WLR methods for the primary 11-layer mineral

 potential model. WOE estimates tend to be higher than WLR.
C23 Comparison of WOE versus WLR Derived Posterior Probabilities

Sedimentary Rock-Hosted 9-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 100 200 300 400 500 600 700

Mineral Potential Observation

Larger Areas . . . (Unique Conditions Map Class) . . . Smaller Areas

Posterior Probability

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Square Km

WOE Posterior Probability

WLR Posterior Probability

Unique Conditions Map Class Area

A.

WOE Poster Probability Plotted Against WLR Posterier Probability

Sedimentary Rock-Hosted 9-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

WOE-Derived Posterior Probability

WLR-Derived Posterior Probability

B.

X = Y

 Figure C.9 Posterior probability estimates derived from WOE and WLR methods for sed. host. 9-layer mineral

 potential model. WOE estimates tend to be higher than WLR estimates.
C24

WOE Poster Probability Plotted Against WLR Posterier Probability

Volcanic Rock-Hosted 9-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0.0 0.1 0.2 0.3 0.4

WOE-Derived Posterior Probability

WLR-Derived Posterior Probability

B.

X = Y

Comparison of WOE versus WLR Derived Posterior Probabilities

Volcanic Rock-Hosted 9-Layer Mineral Potential Model

0.0

0.1

0.2

0.3

0.4

0 100 200 300 400 500 600 700

Mineral Potential Observation

Larger Areas . . . (Unique Conditions Map Class) . . . Smaller Areas

Posterior Probability

0

10000

20000

30000

40000

Square Km

WOE Posterior Probability

WLR Posterior Probability

Unique Conditions Map Class Area

A.

 Figure C.10 Posterior probability estimates derived from WOE and WLR methods for vol. host. 9-layer mineral

 potential model. WOE estimates tend to be higher than WLR estimates.
A.

Uncertainty Due

to Variances of

Weights Estimates

B.

Uncertainty Due to

Incomplete Data

Coverage

C.

Uncertainty Due to

Weights Variance and

Sum of Missing Data

D.

Relative Certainty

of the Posterior

Probability

Figure C.11.

(A) (B)

(C)

(D)

Examples of various sources and types of uncertainty in the posterior probability estimates used to generate the primary occurrence-type 11-layer

mineral potentialmap. Uncertainty due to variancesin the estimate ofspatial weights of association, uncertainty due to missing data (primarily incomplete

coverage of K/Na and Ba/Na geochemical anomaly maps), total uncertainty (uncertainty due to variances of weights, plus, sum of uncertainty due to missing

data), and relative certainty of the posterior probability (posterior probability divided by its standard deviation; in effect, the application of an approximate ttest). Allmapswereclassified using a quantiles approach. With the exception of the relative certaintymap,themaps have been classified so asto accentuate the 50th

percentile of area associated with the higher magnitude uncertainty values. Maps A, B, and C have dimensionless units. Map D units represent t-values (at a

significance level of 95%,t-values greatthan 1.645 indicate thatthe posterior probabilities are greaterthan 0). InmapD,themineralpotential favorability estimates

are relativelymore certain in regionsthat appear yellow-orange-red.
A. B.

Figure C.12. (A)

(B)

Uncertainty of posterior probabilities used to generate primary occurrence-type 7-layer mineral potential map. Total uncertainty (uncertainty due

to variances of weights, plus, sum of uncertainty due to missing data) and relative certainty of the posterior probability (posterior probability divided by its

standard deviation, in effect, the application of an approximate t-test). SeeFigureC.11 caption for additional information.
A. B.

Figure C.13. (A)

(B)

Uncertainty of posterior probabilities used to generate sedimentary rock-hosted occurrence-type 8-layer mineral potential map. Total uncertainty

(uncertainty due to variances of weights, plus, sum of uncertainty due to missing data) and relative certainty of the posterior probability (posterior probability

divided by itsstandard deviation, in effect, the application of an approximate t-test). See FigureC.11captionfor additional information.
A. B.

Figure C.14. (A)

(B)

Uncertainty of posterior probabilities used to generate volcanic rock-hosted occurrence-type 7-layer mineral potential map. Total uncertainty

(uncertainty due to variances of weights, plus, sum of uncertainty due to missing data) and relative certainty of the posterior probability (posterior probability

divided by itsstandard deviation, in effect, the application of an approximate t-test). See FigureC.11 caption for additional information.
t-value

Figure C.15. Three-dimensional rendering of relative certainty of posterior probabilities used to generate the sedimentary rock-hosted occurrence-type 8-layer

mineral potential map. Height represents posterior probabilities greater than the prior. Color represents the degree of relative certainty, as indicated by t-value.

Relative certainties (t-values) are draped over mineral potential "relief" wire-frame created with posterior probabilitiesthat are greater than the prior probability(>

0.0003). T-values greater than 1.645 indicate that, at a significance level of 95%,the corresponding posterior probability is greater than zero. Note thatthe highest

relative certainties are coincident to the posterior probability "relief peaks" that delineate the -shaped distribution of the sedimentary rock-hosted occurrences,

suggesting thatthe posterior probabilitymappatternisrelatively certain (or "real"),ratherthan a result of chance.

"V"

Viewed from southeast

at 45 degrees zenith.
D1

APPENDIX D

Mineral Potential at Known

Mineral Occurrence Areas:

Section Contents:

D.1 Summary Statement

D.2 Primary Gold-Silver-Bearing Occurrences

D.3 Sedimentary Rock-Hosted Gold-Silver-Bearing Occurrences

D.4 Volcanic Rock-Hosted Gold-Silver-Bearing Occurrences
D2

D.1 Summary Statement

Approximately b to 3⁄4 of all occurrences, for any given occurrence-type sample, have posterior

probabilities greater than the prior probability.

D.2 Primary Gold-Silver-Bearing Occurrences

Figure D.1 Approximately 76% of the big (large and medium size) and ~69% of the small

size occurrences have posterior probabilities larger than the prior probability,

with a total of ~69% of all of the primary occurrences having a higher posterior

probability.

Figure D.2 The spatial distribution of the primary gold-silver-bearing occurrences with

elevated posterior probability values is coincident with areas of elevated mineral

potential favorability, as predicted by the primary occurrence posterior

probability map. In this figure, a conspicuous pattern of occurrences is

highlighted with a dashed line (map “B”), and is shown to have a spatially

associated and corresponding pattern of areas of elevated posterior probability

(map “A”). The dashed line merely delineates the spatial distribution of the

occurrences, and does not necessarily reflect any geologic, structural, or other

genetic-related trends (particularly in the case of the primary occurrences).

Table D.1 Primary gold-silver-bearing occurrences of all sizes and types having posterior

probabilities $ 0.1000, as determined with the primary 7-layer mineral potential

model (see table C.1b).

D.3 Sedimentary Rock-Hosted Gold-Silver-Bearing

Occurrences

Figure D.3 Approximately 84% of the big and ~82% of the small size occurrences have

posterior probabilities larger than the prior probability, with a total of ~83% of

all of the sedimentary rock-hosted occurrences having a higher posterior

probability.

Figure D.4 The spatial distribution of the sedimentary rock-hosted gold-silver-bearing

occurrences with elevated posterior probability values is coincident with areas of

elevated mineral potential favorability, as predicted by the sedimentary rockhosted occurrence posterior probability map. In this figure, a conspicuous pattern

of occurrences is highlighted with a dashed line (map “B”), and is shown to have

a spatially associated and corresponding pattern of areas of elevated posterior

probability (map “A”). The dashed line merely delineates the spatial distribution

of the occurrences, and does not necessarily reflect any geologic, structural, or

other genetic-related trends, although one may exist..
D3

Table D.2 Posterior probabilities associated with gold-silver-bearing sedimentary rockhosted occurrences of all sizes, as determined with the sedimentary rock-hosted

8-layer mineral potential model (see table C.2b).

D.4 Volcanic Rock-Hosted Gold-Silver-Bearing

Occurrences

Figure D.5 Approximately 72% of the big and ~60% of the small size occurrences have

posterior probabilities larger than the prior probability, with a total of ~60% of

all of the volcanic rock-hosted occurrences having a higher posterior probability.

Figure D.6 The spatial distribution of the volcanic rock-hosted gold-silver-bearing

occurrences with elevated posterior probability values is coincident with areas of

elevated mineral potential favorability, as predicted by the volcanic rock-hosted

occurrence posterior probability map. In this figure, a conspicuous pattern of

occurrences is highlighted with a dashed lines (map “B”), and is shown to have

a spatially associated and corresponding pattern of areas of elevated posterior

probability (map “A”). The dashed lines merely delineate the spatial distribution

of the occurrences, and does not necessarily reflect any geologic, structural, or

other genetic-related trends, although one may exist.

Table D.3 Posterior probabilities associated with gold-silver-bearing volcanic rock-hosted

occurrences of all sizes, as determined with the volcanic rock-hosted 7-layer

mineral potential model (see table C.3b).
D4

Table D1. Primary gold-silver-bearing occurrences of all sizes and types having posterior probabilities $

0.1000, as determined with the primary 7-layer mineral potential model (practical space limitations restrict the

listing of all 2960 occurrences; sorted in order of descending posterior probability). All data, with the

exception of posterior probabilities, from the MRDS mineral occurrences database. Occurrences size from

Guild (1968), and deposit types from Cox and Singer (1986). Prior probability = 0.0095.

County Site Name Size Post.

Prob. Commodities Deposit Type

ELKO HUNTER PROSPECT S 0.6300 AG PB SB -- ---

ELKO PROSPECT NO. 4 S 0.6199 AU -- -- -- ---

ESMERALDA DUTCHESS PROSPECT S 0.6199 AU CU AG F ---

ESMERALDA ENTERPRISE PROSPECT S 0.6199 AU AG CU PB COPPER SKARN

LANDER GOLD TOP MINE ? 0.6199 AU AG CU ZN ---

LANDER BENTLEY MINE S 0.6199 AG PB CU AU POLYMETALLIC VEINS

LANDER BIG FOUR MINE S 0.6199 AG AU PB -- ---

LANDER BUZZARD MINE S 0.6199 AU CU PB AG POLYMETALLIC VEINS

LANDER FAIRVIEW PROPERTY S 0.6199 AU AG PB CU ---

LANDER FULLER GROUP S 0.6199 AU AG FE SB ---

LANDER INDEPENDENCE MINE S 0.6199 AG AU CU ZN ---

LANDER LITTLE GIANT MINE S 0.6199 AU AG CU PB POLYMETALLIC VEINS

LANDER MONITOR CLAIM S 0.6199 AU AG ZN FE ---

LANDER PEGGY GROUP MINE S 0.6199 AG CU AU -- POLYMETALLIC VEINS

LANDER SILVER KING MINE S 0.6199 AU AG -- -- GOLD SKARN

NYE NBMG SAMPLE SITE. 3174 S 0.6199 AU -- -- -- ---

NYE PHILLIPS MINE S 0.6199 AG -- -- -- ---

NYE SHAMROCK DIGGINGS S 0.6199 AG -- -- -- ---

NYE SHAMROCK MINE S 0.6199 AG AU -- -- ---

NYE UNNAMED PROSPECT (ELLSWORTH DISTRICT) S 0.6199 AU AG -- -- ---

PERSHING AGAMEMNON MINE ? 0.5378 AU AG SB -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING BLACK WARRIOR MINE S 0.5378 SB AG CU ZN LOW-SULFIDE AU-QUARTZ VEINS

PERSHING ENTERPRISE MINE S 0.5378 AU CU -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING PACIFIC MATCHLESS MINE S 0.5378 AG PB -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING WHEELER MINE S 0.5378 AG AU CU ZN LOW-SULFIDE AU-QUARTZ VEINS

CHURCHILL BLUFF MINE S 0.4983 AG AU -- -- COMSTOCK EPITHERMAL VEINS

CHURCHILL BUFF CLAIMS S 0.4983 AU AG -- -- COMSTOCK EPITHERMAL VEINS

ESMERALDA MAUD CLAIMS S 0.4875 AU -- -- -- ---

ESMERALDA SILVER KING CLAIMS (TOKOP DISTRICT) S 0.4875 AU AG W AS ---

NYE BAXTER SPRINGS PROSPECT ? 0.4875 AU -- -- -- ---

CHURCHILL DROMEDARY HUMP MINE S 0.4818 AG AU -- -- COMSTOCK EPITHERMAL VEINS

ELKO HUMBOLDT MINE S 0.4818 AU PB CU -- POLYMETALLIC VEINS

ELKO JACKPOT S 0.4818 AU AG -- -- ---

NYE LOCKE MINE S 0.4818 AU AG -- -- ---

NYE SHOE - SHOE MINE S 0.4818 AU AG PB ZN ---

ELKO EDGAR TURQUOISE MINE ? 0.4710 GEM AU -- -- ---

ESMERALDA APRIL MINE S 0.4710 AU -- -- -- ---

ESMERALDA BLACK BEAUTY GROUP S 0.4710 AU AG PB ZN ---

ESMERALDA GOLD LEDGE CLAIM GROUP S 0.4710 AG AU PB -- ---

ESMERALDA LAST CHANCE MINE S 0.4710 AU AG PB -- ---

ESMERALDA MARY MINE S 0.4710 AU AG PB -- LOW-SULFIDE AU-QUARTZ VEINS

ESMERALDA MISSOURI PROSPECT S 0.4710 AU -- -- -- ---

ESMERALDA NEW YORK AND SILVER PEAK MINING CO. MINES S 0.4710 AG AU -- -- ---

ESMERALDA POCATELLO MINE S 0.4710 AG AU CU -- LOW-SULFIDE AU-QUARTZ VEINS

ESMERALDA SHIPPER GROUP S 0.4710 AG AU CU -- ---

ESMERALDA TREASURE CLAIM S 0.4710 AU ? -- -- ---

ESMERALDA VANDERBILT MINE S 0.4710 AG AU CU -- ---

LANDER CARISSA MINE S 0.4710 AU CU AG W GOLD SKARN
D5

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

LANDER EAGLE MINE S 0.4710 AU AG -- -- ---

LANDER GOLD DYKE MINE S 0.4710 AU AG -- -- ---

LANDER GOLD ROD S 0.4710 AU AG -- -- ---

LANDER LABRADOR DEPOSIT S 0.4710 AU AG -- -- COPPER SKARN

LANDER NORTHERN LIGHTS DEPOSIT S 0.4710 AU AG CU -- SEDIMENT-HOSTED AU-AG

NYE GRIZZLY CLAIM S 0.4710 AU -- -- -- ---

NYE KORF PROPERTY S 0.4710 AU -- -- -- ---

NYE WHITE HORSE CLAIM S 0.4710 AU W -- -- ---

WHITE PINE ESSEX TUNNEL ? 0.4710 AU FE -- -- ---

WHITE PINE GOLD KING GROUP ? 0.4710 AU SB -- -- ---

NYE HALLER PROPERTY S 0.3813 AG PB ZN -- ---

ELKO MOHAWK S 0.3712 AU AG FE -- ---

ELKO PITTSBURG SILVER S 0.3712 AU AG FE CU ---

ESMERALDA JACKSON/ELECTRIC CLAIMS S 0.3712 AU CU AG -- ---

ESMERALDA NBMG SAMPLE SITE 1377 S 0.3712 AU AG CU -- ---

ESMERALDA NORTHERN ALPINE PROSPECT S 0.3712 AG AU PB -- ---

ESMERALDA SILVER TOP #2 CLAIM S 0.3712 AG ? -- -- ---

ESMERALDA TREASURE HILL CLAIMS S 0.3712 AU ? -- -- ---

ESMERALDA WEE CLAIM S 0.3712 AU -- -- -- ---

ESMERALDA WEEPAH MINE S 0.3712 AU AG PB CU LOW-SULFIDE AU-QUARTZ VEINS

LANDER BLUE BIRD MINE S 0.3712 AU AG PB CU POLYMETALLIC VEINS

NYE KRAMER-OSGOOD PROPERTY S 0.3712 AG AU CU PB ---

NYE RAY RICKETTS GROUP S 0.3712 AU -- -- -- ---

NYE RAY RICKETTS MINE S 0.3712 AU W -- -- ---

NYE SERGEANT PROPERTY S 0.3712 AG AU PB ZN ---

NYE VALLEY GROUP S 0.3712 AG PB ZN CU POLYMETALLIC VEINS

PERSHING IMPERIAL GROUP S 0.3712 AU AG CU PB POLYMETALLIC VEINS

PERSHING KENNEDY CANYON PROSPECT S 0.3712 AU AG CU -- PORPHYRY CU

PERSHING RUSTY PICK PROSPECT S 0.3712 AU AG -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING SUNNYSIDE MINE S 0.3712 AU AG -- -- POLYMETALLIC VEINS

CHURCHILL FAIRVIEW EAGLE L 0.3516 AG AU MN HG COMSTOCK EPITHERMAL VEINS

CHURCHILL NEVADA HILLS MINE L 0.3516 AU AG CU PB COMSTOCK EPITHERMAL VEINS

CHURCHILL HAILSTONE GROUP S 0.3516 AG AU -- -- COMSTOCK EPITHERMAL VEINS

CHURCHILL LENA GROUP S 0.3516 AG AU -- -- ---

CHURCHILL NBMG SAMPLE SITE 3847 S 0.3516 AG PB AU ? ---

CHURCHILL NBMG SAMPLE SITE 3849 S 0.3516 AG -- -- -- ---

CHURCHILL NBMG SAMPLE SITE 3850 S 0.3516 AG CU PB -- ---

CHURCHILL SEYMOUR FRACTION S 0.3516 AG AU -- -- ---

ESMERALDA GRAND CENTRAL MINE S 0.3516 AU AG -- -- ---

ESMERALDA GREAT WESTERN MINE S 0.3516 AU AG -- -- ---

ESMERALDA JABONRIA CLAIMS S 0.3516 AU -- -- -- ---

ESMERALDA OHIO MINES CORP. DUNFEE GROUP S 0.3516 AU AG -- -- ---

ESMERALDA BIG WEDGE CLAIMS S 0.3418 AG PB CU -- POLYMETALLIC VEINS

ESMERALDA GYPSY CLAIMS S 0.3418 AG ? AU ? ---

ESMERALDA MABLE MINE S 0.3418 AU U -- -- ---

ESMERALDA OLD INGALLS MINE S 0.3418 AG PB AU U ---

NYE BARCELONA MINE S 0.3418 AG AU CU PB POLYMETALLIC VEINS

NYE BEL CLAIMS S 0.3418 AG PB SB CU ---

NYE ED WELCH PROPERTY S 0.3418 AU -- -- -- ---

NYE NBMG SAMPLE SITES 2076, 2077, 3183. S 0.3418 AG AU PB CU ---

PERSHING ROCHESTER L 0.2964 AG AU PB ZN LOW-SULFIDE AU-QUARTZ VEINS

PERSHING LIMERICK GOLD MINE S 0.2964 AU AG -- -- LOW-SULFIDE AU-QUARTZ VEINS
D6

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

PERSHING NENZEL CROWN POINT MINES S 0.2964 AU AG ZN -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING PLAINVIEW GROUP MINES S 0.2964 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

ELKO LAST CHANCE ? 0.2900 AG AU -- -- POLYMETALLIC REPLACEMENT

ELKO HELEN M GROUP S 0.2900 AG CU PB -- ---

ELKO LEE S 0.2900 AG AU CU PB ---

ELKO SYLVANIA MINE S 0.2900 AU AG CU PB ---

ELKO WEB FOOT S 0.2900 AG CU PB AU COPPER SKARN

CLARK WALL STREET MINE M 0.2818 AU AG -- -- GOLD ON FLAT FAULTS

CLARK ELDORADO EMPIRE MINING CO. S 0.2818 AU -- -- -- ---

ESMERALDA NBMG SAMPLE SITE 1952 S 0.2818 AU ? AG ? ---

HUMBOLDT HALL MINE S 0.2818 AU AG FE CU LOW-SULFIDE AU-QUARTZ VEINS

HUMBOLDT NBMG SAMPLE SITE 2966, 2967 S 0.2818 AU AG PB CU LOW-SULFIDE AU-QUARTZ VEINS

MINERAL LUNING GOLD CONSOLIDATED ? 0.2818 AU AG PB CU ---

MINERAL SILVER CHIEF WORKINGS ? 0.2818 AG -- -- -- ---

MINERAL DISPOZITCH MINE S 0.2818 AG PB CU AU ---

NYE SYLVANITE GROUP (HORN SILVER MINE) S 0.2818 AG AU -- -- ---

WHITE PINE MARY ANNE ? 0.2818 AU AG PB CU POLYMETALLIC VEINS

ELKO MEMPHIS S 0.2731 AU AG CU -- ---

HUMBOLDT GOLD RUN NO. 23 CLAIM S 0.2731 AG CU MO W POLYMETALLIC VEINS

HUMBOLDT RED BUTTE MINE S 0.2731 AU CU AG PB POLYMETALLIC VEINS

LANDER BATTLE MOUNTAIN DISTRICT L 0.2731 AG AU CU PB POLY MET VEIN;PORP CU;SKARN

LANDER FORTITUDE MINE M 0.2731 AU AG AS CU COPPER SKARN

LANDER DEAD HORSE MINE S 0.2731 AU AG -- -- ---

LANDER JULIE SHAFT S 0.2731 AU AG ZN PB ZINC-LEAD SKARN

LANDER TRENTON MINE S 0.2731 AG CU PB ZN PORPHYRY CU-MO

LANDER WILSON-INDEPENDENCE MINE S 0.2731 AU AG AS MO ---

LYON SECTION 30 PROSPECTS ? 0.2731 AU AG -- -- ---

LYON CAMBRIDGE MINE S 0.2731 AU AG U -- ---

NYE LIBERTY MINE (REPRESENTING SAN ANTONE DIST.) S 0.2731 AG AU CU -- COMSTOCK EPITHERMAL VEINS

NYE MILLETT MINE S 0.2731 AU -- -- -- ---

NYE VIKEN CLAIM S 0.2731 AU AG CU -- ---

NYE YELLOW GOLD CLAIM AND HOLE IN THE WALL GRP. S 0.2731 AU AG -- -- ---

PERSHING EXCALIBER MINE ? 0.2731 AU -- -- -- POLYMETALLIC VEINS

STOREY BEST & BELCHER MINE ? 0.2731 AG AU -- -- COMSTOCK EPITHERMAL VEINS

STOREY C & C SHAFT L 0.2731 AU AG CU PB COMSTOCK EPITHERMAL VEINS

STOREY CONSOLIDATED VIRGINIA MINE L 0.2731 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY SAVAGE MINE L 0.2731 AG AU MN FE COMSTOCK EPITHERMAL VEINS

STOREY CALIFORNIA MINE M 0.2731 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY HALE & NORCROSS MINE M 0.2731 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY MONTE CRISTO MINE M 0.2731 AU AG CU PB COMSTOCK EPITHERMAL VEINS

STOREY ANDES MINE S 0.2731 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

LANDER KINGSTON MINE ? 0.2704 AU AG -- -- ---

CHURCHILL NEVADA FAIRVIEW MINE M 0.2645 AU AG -- -- COMSTOCK EPITHERMAL VEINS

ESMERALDA BIG BLOSSOM MINE S 0.2561 AU W -- -- ---

NYE BIG SPRINGS MINE S 0.2561 AU -- -- -- ---

NYE BLAKER-SLOANE PROPERTY S 0.2561 AG AU -- -- ---

NYE JUMBO MINE. S 0.2561 AU -- -- -- ---

NYE KEYSTONE MINE S 0.2561 AU F AG -- COMSTOCK EPITHERMAL VEINS

MINERAL WAMSLEY MINE S 0.2518 AG -- -- -- ---

NYE VANDERHOEF CLAIMS S 0.2518 AG PB ZN -- POLYMETALLIC VEINS

CHURCHILL MOTTINI MINE S 0.2437 AU AG PB CU COPPER SKARN

ELKO GOLDEN EAGLE MINE S 0.2437 AU PB -- -- ---
D7

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

ESMERALDA MINERAL RIDGE DISTRICT L 0.2437 AU AG -- -- ---

ESMERALDA CROWNING GLORY MINE S 0.2437 AU AG -- -- ---

ESMERALDA DRINKWATER MINE S 0.2437 AU AG PB -- ---

ESMERALDA ESMERALDA PROSPECT S 0.2437 AU AG PB -- ---

ESMERALDA GOLDEN EAGLE MINE S 0.2437 AU AG PB -- LOW-SULFIDE AU-QUARTZ VEINS

ESMERALDA HOMESTAKE MINE S 0.2437 AU -- -- -- ---

ESMERALDA SODA MINE S 0.2437 AU -- -- -- ---

ESMERALDA W.S. NO. 97 CLAIMS S 0.2437 AU ? AG ? ---

ESMERALDA WESTERN SOLDIER MINE S 0.2437 AU AG PB -- ---

LANDER GREY EAGLE MINE M 0.2437 AG AU CU PB POLYMETALLIC VEINS

LANDER MUD SPRINGS MINE AND ADJACENTPROSPECTS. S 0.2437 AG PB CU AU ---

LANDER PHOENIX S 0.2437 AG PB AU CU POLYMETALLIC VEINS

LANDER TRIPLET GULCH PROJECT S 0.2437 AU -- -- -- SEDIMENT-HOSTED AU-AG

LANDER TWO WIDOWS CLAIM S 0.2437 AG AU -- -- ---

NYE GREEN TOP CLAIM S 0.2437 AG CU U -- ---

NYE LAST CHANCE GROUP S 0.2437 AU AG -- -- ---

WHITE PINE ANNA TUNNEL S 0.2437 AU -- -- -- ---

ESMERALDA THREE METALS PROSPECT S 0.2287 AG PB CU -- ---

LANDER ACE OF DIAMONDS CLAIM S 0.2287 AG AU PB -- ---

LANDER ANNIE BLAINE CLAIM S 0.2287 AG PB FE -- ---

LANDER AVALANCHE PROSPECT S 0.2287 AG PB AU -- POLYMETALLIC REPLACEMENT

LANDER BRYAN MINE S 0.2287 AU AG CU -- ---

LANDER CLEVELAND CLAIM S 0.2287 AU AG -- -- ---

LANDER DRISCOL PROPERTY S 0.2287 AG PB AU CU POLYMETALLIC VEINS

LANDER EAGLE MINE (BLUE EAGLE, BLACK EAGLE...) S 0.2287 AU AG PB CU ---

LANDER GALENA MINE S 0.2287 AG PB AU CU ---

LANDER GOLD CASH MINE S 0.2287 AU CU AG PB ---

LANDER HARD TIMES CLAIM S 0.2287 AU AG CU PB ---

LANDER HUMBUG-LUCKY CHANCE MINE S 0.2287 AU AG CU PB ---

LANDER IRISH ROSE MINE S 0.2287 AG CU AU PB POLYMETALLIC VEINS

LANDER MERCURY CLAIM S 0.2287 AU AG FE -- ---

LANDER MINNIE MINE S 0.2287 AU AG CU PB COPPER SKARN

LANDER MOONLIGHT MINE S 0.2287 AG PB AU CU ---

LANDER NEPTUNE CLAIM S 0.2287 AU AG -- -- ---

LANDER PLUMAS MINE S 0.2287 AG PB CU AU ---

LANDER SPANISH VEIN PROSPECT S 0.2287 AG PB AU ZN ---

LANDER TOMBOY MINE S 0.2287 AU AG CU PB COPPER SKARN

LANDER TRINITY-ARMOR MINE S 0.2287 AG CU PB AU POLYMETALLIC VEINS

MINERAL HECLA MINE ? 0.2287 AG -- -- -- ---

MINERAL UNKNOWN ADITS AND OPEN STOPES ? 0.2287 AG -- -- -- ---

MINERAL UNKNOWN SHAFTS AND PROSPECTS ? 0.2287 AG -- -- -- ---

MINERAL RIP VAN WINKLE MINE S 0.2287 AG PB -- -- ---

MINERAL SILVER GULCH MINE S 0.2287 AG PB -- -- ---

NYE LIME DYKE CLAIMS S 0.2287 AG PB ZN AU ---

NYE THELMA MINE S 0.2287 AG F MO -- ---

NYE UNNAMED GOLD PROPERTY S 0.2287 AU -- -- -- ---

NYE UNNAMED PROSPECT B. S 0.2287 AU -- -- -- ---

PERSHING ALLIED TUNGSTEN PROSPECT ? 0.2287 AG W PB ? LOW-SULFIDE AU-QUARTZ VEINS

PERSHING NEVADA HUMBOLDT GROUP ? 0.2287 AU AG PB ZN LOW-SULFIDE AU-QUARTZ VEINS

PERSHING SHEBA MINE M 0.2287 AG PB SB AU LOW-SULFIDE AU-QUARTZ VEINS

PERSHING AMERICAN CANYON DIVIDE PROSPECT S 0.2287 AG PB -- -- LOW-SULFIDE AU-QUARTZ VEINS
D8

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

PERSHING BLOODY CANYON MINE S 0.2287 SB AG FE PB LOW-SULFIDE AU-QUARTZ VEINS

PERSHING FOUR SISTERS MINE S 0.2287 AU AG CU PB LOW-SULFIDE AU-QUARTZ VEINS

PERSHING HUMBOLDT QUEEN S 0.2287 AG PB AU CU LOW-SULFIDE AU-QUARTZ VEINS

PERSHING LUCKY DOG MINE S 0.2287 AG PB ZN CU LOW-SULFIDE AU-QUARTZ VEINS

PERSHING MARIGOLD MINE S 0.2287 AU AG ZN PB LOW-SULFIDE AU-QUARTZ VEINS

PERSHING MCGEE PROPERTY S 0.2287 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING NBMG SAMPLE SITE 2516 S 0.2287 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING NBMG SAMPLE SITE 2517 S 0.2287 AU ? -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING PQX CLAIMS S 0.2287 AG AU PB ZN LOW-SULFIDE AU-QUARTZ VEINS

PERSHING TEHAMA MINE S 0.2287 AU AG CU -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING TROY CANYON PROSPECT S 0.2287 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING WABASH MINE S 0.2287 AG PB AU CU LOW-SULFIDE AU-QUARTZ VEINS

ESMERALDA BROUGHER-DIVIDE MINING CO. PROPERTY S 0.1924 AG ? AU -- ---

ESMERALDA DIVIDE EXTENSION MINE S 0.1924 AG AU -- -- HOT-SPRING AU-AG (?)

ESMERALDA GOLD ZONE DIVIDE MINING CO. PROPERTY S 0.1924 AU AG CU PB COMSTOCK EPITHERMAL VEINS

ESMERALDA HARMILL DIVIDE MINE S 0.1924 AG ? AU ? ---

ESMERALDA KNOX DIVIDE MINE S 0.1924 AG -- -- -- ---

ESMERALDA NEW ALTO DIVIDE MINING COMPANY PROPERTY. S 0.1924 AG AU -- -- ---

ESMERALDA TONOPAH DIVIDE MINE S 0.1924 AU AG MO CU COMSTOCK EPITHERMAL VEINS

ESMERALDA TONOPAH-DIVIDEND MINING COMPANY PROPERTY S 0.1924 AG ? AU -- ---

PERSHING ABE LINCOLN MINE S 0.1870 AG AU -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING BUCK AND CHARLEY MINE S 0.1870 AG PB ZN AU LOW-SULFIDE AU-QUARTZ VEINS

PERSHING FORVILLY MINE S 0.1870 AU AG -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING LINCOLN HILL MINE S 0.1870 AU AG U ZN LOW-SULFIDE AU-QUARTZ VEINS

PERSHING LOONEY MINE S 0.1870 AU AG PB ZN LOW-SULFIDE AU-QUARTZ VEINS

PERSHING NEVADA PACKARD MINES S 0.1870 AG AU CU SB LOW-SULFIDE AU-QUARTZ VEINS

PERSHING OCTOPUS MINE S 0.1870 AG AU PB -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING RAVEN PROSPECT S 0.1870 AG AU -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING SUMMIT MINE S 0.1870 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

PERSHING WEAVER CANYON MINE S 0.1870 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

CLARK SEARCHLIGHT DISTRICT L 0.1862 AU AG CU W ---

MINERAL JAIME'S RIDGE S 0.1862 AU AG HG -- HOT-SPRING AU-AG

ESMERALDA ORIENTAL MINE S 0.1797 AU -- -- -- ---

LYON BUCKEYE MINE S 0.1797 AG AU -- -- COMSTOCK EPITHERMAL VEINS

STOREY KENTUCK MINE ? 0.1797 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY MERI'S CON VIRGINIA #1 MINE ? 0.1797 AU AG -- -- ---

STOREY CHOLLAR MINE L 0.1797 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY CROWN POINT MINE L 0.1797 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY POTOSI MINE L 0.1797 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY YELLOW JACKET MINE L 0.1797 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY CHALLENGE AND CONFIDENCE M 0.1797 AU AG -- -- COMSTOCK EPITHERMAL VEINS

STOREY ALPHA CLAIM S 0.1797 AG AU -- -- COMSTOCK EPITHERMAL VEINS

STOREY EXCHEQUER CLAIM S 0.1797 AG AU -- -- COMSTOCK EPITHERMAL VEINS

CLARK SOUTHERN NEVADA MINE ? 0.1764 AU AG -- -- ---

CLARK BLOSSOM MINE S 0.1764 AU AG -- -- ---

CLARK J. E. T. S 0.1764 AU -- -- -- ---

CLARK POMPEII MINE S 0.1764 AU AG -- -- ---

WASHOE BLACK WARRIOR PEAK S 0.1764 AU AG -- -- COMSTOCK EPITHERMAL VEINS

DOUGLAS WINTERS MINE S 0.1702 AG AU FE CU POLYMETALLIC VEINS

ESMERALDA RED LIGHT MINE S 0.1702 AU CU -- -- ---

ESMERALDA SOLBERRY MINE S 0.1702 AU AG PB -- ---

ESMERALDA VEGA MINE S 0.1702 AU AG PB -- ---
D9

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

LANDER GOLD QUARTZ M 0.1702 AU AG PB ZN ---

LANDER PAYMASTER & CHRISTOPER S 0.1702 AU AG CU PB ---

MINERAL GOLDEN MILE GROUP S 0.1702 AU AG CU -- ---

CHURCHILL GOLD COIN #2 MINE S 0.1641 AG AU -- -- COMSTOCK EPITHERMAL VEINS

ESMERALDA ALBERT MINE S 0.1641 AG AU PB -- ---

ESMERALDA HORNSILVER MAY CO. SHAFT S 0.1641 AG AU -- -- ---

ESMERALDA NBMG SAMPLE SITE S 0.1641 AG PB AU CU ---

MINERAL ROCK CABIN MINE ? 0.1641 AU BA -- -- ---

ESMERALDA BUCK BOARD CLAIM GROUP S 0.1582 AG AU -- -- ---

ESMERALDA CALLISON PROPERTY S 0.1582 AU AG PB -- ---

ESMERALDA ENTERPRISE MINE S 0.1582 AU -- -- -- ---

ESMERALDA FESLER PROSPECT S 0.1582 AG AU CU PB ---

ESMERALDA GOLD CREST CLAIMS S 0.1582 AU AG PB CU ---

ESMERALDA GOOD HOPE MINE S 0.1582 AG PB CU -- ---

ESMERALDA HART SILVER MINE S 0.1582 AG PB AU ? ---

ESMERALDA NORTH STAR CLAIMS S 0.1582 AU AG ? -- ---

ESMERALDA RUBY D CLAIMS S 0.1582 AU AG -- -- ---

ESMERALDA SNAKE PIT S 0.1582 AU ? -- -- ---

ESMERALDA SUNRISE CLAIMS S 0.1582 AU AG CU -- ---

ESMERALDA VICTORIA PROSPECT S 0.1582 AU AG PB CU ---

NYE ARIZONA CLAIM S 0.1582 AG CU SB AU ---

NYE COMBINATION MINE S 0.1582 AG SB -- -- POLYMETALLIC VEINS

NYE EL DORADO SOUTH MINE S 0.1582 AG CU PB SB POLYMETALLIC VEINS

NYE HIGHBRIDGE MINE S 0.1582 AG CU -- -- ---

NYE HOOPER MINE S 0.1582 AG -- -- -- ---

NYE MOORE AND MARTIN PROPERTY S 0.1582 AG -- -- -- ---

NYE NBMG SAMPLE SITE 2039 S 0.1582 AG PB CU MO ---

NYE NBMG SAMPLE SITE 2040 S 0.1582 AG PB CU SB ---

NYE NBMG SAMPLE SITE 2079 S 0.1582 AG PB CU ZN ---

NYE SAN PEDRO MINE S 0.1582 AG HG ZN PB ---

NYE WAR EAGLE MINE (BARCELONA DISTRICT) S 0.1582 AU AG PB CU POLYMETALLIC VEINS

MINERAL UNKNOWN PROSPECTS ? 0.1529 AG ? -- -- ---

MINERAL UNNAMED PROSPECT (CANDELARIA DISTRICT) ? 0.1474 AG ? -- -- ---

MINERAL BIRDSONG MINE S 0.1474 AU AG PB -- ---

MINERAL RIP VAN WINKLE PROSPECT S 0.1474 AG PB -- -- ---

MINERAL GRASSI MINE S 0.1446 AU AG -- -- ---

ESMERALDA B.D. CLAIMS S 0.1393 AU AG PB -- ---

ESMERALDA COYOTE MINE S 0.1393 AG PB ? -- ---

ESMERALDA CRESCENT MINE S 0.1393 AU AG PB -- ---

ESMERALDA GOLD COIN MINE (TOKOP DIST.) S 0.1393 AU ? -- -- ---

ESMERALDA GOOD BUDDY CLAIMS S 0.1393 AG PB CU -- ---

ESMERALDA GREAT GULCH MINE S 0.1393 AU -- -- -- ---

ESMERALDA MONTEZUMA CAMP S 0.1393 AG AU CU PB POLYMETALLIC REPLACEMENT

ESMERALDA MR LODE CLAIMS S 0.1393 AU AG PB -- ---

ESMERALDA NBMG SAMPLE SITES 1287, 1288 S 0.1393 AG PB CU -- ---

ESMERALDA OLDT CLAIM S 0.1393 AG CU PB -- ---

ESMERALDA RADIO TOWER SHAFT S 0.1393 AU AG ? -- ---

ESMERALDA RED MONSTER MINE S 0.1393 AU AG -- -- ---

ESMERALDA SILVER MOON MINE S 0.1393 AU ? AG ? ---

ESMERALDA TOMMY KNODAN CLAIMS S 0.1393 AU -- -- -- ---

LANDER BULL DOG JACK MINE ? 0.1393 AG AU -- -- ---

LANDER BLUE DICK ANTIMONY MINE S 0.1393 SB AU AG SE SIMPLE ANTIMONY
D10

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

LANDER LIMELIGHT MINE S 0.1393 AG AU -- -- ---

LANDER PITTSBURG MINE S 0.1393 AU AG CU PB POLYMETALLIC VEINS

MINERAL LUCKY HILL MINE ? 0.1393 AG AU PB FE DISTAL DISSEMINATED AG-AU

MINERAL MT. DIABLO MINE ? 0.1393 AG AU PB FE ---

MINERAL PRINCESS SHAFT ? 0.1393 AG -- -- -- ---

MINERAL UNKNOWN PROSPECTS ? 0.1393 AG -- -- -- ---

MINERAL UNKNOWN PROSPECTS ? 0.1393 AG ? -- -- ---

MINERAL CANDELARIA MINE M 0.1393 AG AU PB SB DISTAL DISSEMINATED AG-AU

MINERAL MOUNT DIABLO MINE M 0.1393 AG -- -- -- ---

MINERAL 1905 MINE S 0.1393 AU AG -- -- ---

MINERAL ACKERMAN MINE S 0.1393 AU AG -- -- ---

MINERAL BLUE DUMP MINE S 0.1393 AU AG -- -- ---

MINERAL BOUNCE MINE S 0.1393 AU AG -- -- ---

MINERAL BROWN MINE S 0.1393 AU AG -- -- ---

MINERAL CHIEF MINE S 0.1393 AU AG -- -- ---

MINERAL FORTUNA MINE S 0.1393 AU AG -- -- ---

MINERAL FOTTLER MINE S 0.1393 AU AG -- -- ---

MINERAL LUCKY HILLMINE S 0.1393 AG -- -- -- ---

MINERAL MARY MINE S 0.1393 AU AG -- -- ---

MINERAL NEW PARTY MINE S 0.1393 AU AG -- -- ---

MINERAL ORPHAN BOY MINE S 0.1393 AU AG -- -- ---

MINERAL SNOWBALL MINE S 0.1393 AU AG -- -- ---

MINERAL SWASTIKA MINE S 0.1393 AG -- -- -- ---

NYE BERLIN MINE S 0.1393 AG AU CU PB POLYMETALLIC VEINS

NYE DOONAN PROPERTY S 0.1393 AU -- -- -- ---

NYE LUCKY STRIKE MINE S 0.1393 AU -- -- -- ---

NYE MURPHY MINE S 0.1393 AG AU CU SB POLYMETALLIC VEINS

NYE SULLIVAN MINE S 0.1393 AU -- -- -- ---

PERSHING ALDRICH MINE S 0.1393 AG PB AU CU POLYMETALLIC VEINS

NYE MONTANA - TONOPAH MINING CO. L 0.1352 AG AU -- -- COMSTOCK EPITHERMAL VEINS

NYE TONOPAH MINING CO. L 0.1352 AG AU CU PB COMSTOCK EPITHERMAL VEINS

NYE RESCUE EULA MINING CO. M 0.1352 AG -- -- -- ---

HUMBOLDT QUICK GOLD CLAIMS S 0.1277 AG ? AU ? POLYMETALLIC VEINS

HUMBOLDT HUNTINGTON-WHITMAN GROUP S 0.1243 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS

HUMBOLDT PICK HANDLE MINE S 0.1243 AU CU PB -- LOW-SULFIDE AU-QUARTZ VEINS

HUMBOLDT SILVER STATE MINE S 0.1243 AU AG CU -- LOW-SULFIDE AU-QUARTZ VEINS

MINERAL TODD MINE S 0.1243 AU AG PB -- ---

WHITE PINE VICTORIUE CLAIMS S 0.1243 AU AG -- -- ---

CHURCHILL NBMG SAMPLE SITE 3944 S 0.1197 AG CU MO PB ---

CHURCHILL NBMG SAMPLE SITE 3945 S 0.1197 AG CU BI SB ---

ESMERALDA BLUE DAISY CLAIM S 0.1197 AU ? -- -- ---

HUMBOLDT NBMG SAMPLE SITE 2423 S 0.1197 AU CU SB AG LOW-SULFIDE AU-QUARTZ VEINS

HUMBOLDT SNOWDRIFT MINE S 0.1197 SB AG AS FE SIMPLE ANTIMONY

MINERAL BELLEVUE MINE S 0.1197 AU HG ZN -- ---

NYE GRUSS MINE S 0.1197 AG PB CU -- ---

NYE OPHIR CLAIMS (BLOCK) S 0.1197 AU AG -- -- ---

NYE SUMMIT GROUP S 0.1197 AU -- -- -- ---

PERSHING JERSEY VALLEY-REX GROUP S 0.1197 AG PB AU CU ---

PERSHING NBMG SAMPLE SITE 1483 S 0.1197 AG AU -- -- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 2356 S 0.1197 AU ? AG PB LOW-SULFIDE AU-QUARTZ VEINS

PERSHING STONE HOUSE CANYON MINE S 0.1197 AU AG -- -- COMSTOCK EPITHERMAL VEINS

PERSHING UNNAMED GOLD MINES S 0.1197 AU -- -- -- LOW-SULFIDE AU-QUARTZ VEINS
D11

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D1, continued.

NYE NORTHUMBERLAND MINE M 0.1151 AU AG AS FE SEDIMENT-HOSTED AU-AG

NYE GOODING MINE S 0.1151 AG AU AS SB ---

ESMERALDA TIP TOP CLAIMS S 0.1111 AU AG HG -- COMSTOCK EPITHERMAL VEINS

STOREY GOOSEBERRY MINE S 0.1111 AU AG CU -- COMSTOCK EPITHERMAL VEINS

MINERAL NORTH SECTION 31 PROSPECTS ? 0.1069 AU ? -- -- ---
D12

Table D2. Posterior probabilities associated with gold-silver-bearing sedimentary rock-hosted occurrences of

all sizes, as determined with the sedimentary rock-hosted 8-layer mineral potential model (sorted in order of

descending posterior probability). All data, with the exception of posterior probabilities, from the MRDS

mineral occurrences database. Occurrences size from Guild (1968), and deposit types from Cox and Singer

(1986). Prior probability = 0.0003.

County Site Name Size Post.

Prob. Commodities Deposit Type

EUREKA LANTERN DEPOSIT S 0.9972 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA BAZZA DEPOSIT ? 0.9881 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA BLUE STAR MINE M 0.9881 AU AG GEM CU SEDIMENT-HOSTED AU-AG

EUREKA BOBCAT DEPOSIT S 0.9881 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA LONG LAC DEPOSIT ? 0.9881 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA DEEP STAR DEPOSIT S 0.9588 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA CARLIN GOLD MINE M 0.9073 AU HG AG ZN SEDIMENT-HOSTED AU-AG

EUREKA GENESIS MINE M 0.9073 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA NORTH STAR DEPOSIT S 0.8662 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA BULLION-MONARCH OPEN PIT MINE S 0.7376 AU AG --- --- SEDIMENT-HOSTED AU-AG

EUREKA PETE DEPOSIT S 0.7335 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA TUSC DEPOSIT M 0.695 AU AG AS SB SEDIMENT-HOSTED AU-AG

HUMBOLDT PREBLE MINE S 0.6293 AU BA CU --- SEDIMENT-HOSTED AU-AG

LANDER GOLD ACRES DEPOSIT M 0.4921 AU W CU SB SEDIMENT-HOSTED AU-AG

EUREKA GOLD QUARRY MINE L 0.3904 AU AG AS SB SEDIMENT-HOSTED AU-AG

ELKO MAC RIDGE DEPOSIT (BIG SPRINGS) S 0.3741 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO SOUTH SAMMY CREEK DEPOSIT (BIG SPRINGS) S 0.3741 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA GOLDSTRIKE MINE L 0.3498 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA SCREAMER DEPOSIT S 0.3498 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA MAGGIE CREEK OPEN PIT GOLD MINE M 0.3477 AU AG BA AS SEDIMENT-HOSTED AU-AG

HUMBOLDT FELIX CANYON S 0.2179 AU AG AS HG SEDIMENT-HOSTED AU-AG

HUMBOLDT GETCHELL MINE M 0.2179 AU AG AS W SEDIMENT-HOSTED AU-AG

HUMBOLDT PINSON MINE S 0.2179 AU W --- --- SEDIMENT-HOSTED AU-AG

HUMBOLDT MARIGOLD MINE (BATTLE MT. DIST.) M 0.214 AU AG AS SB SEDIMENT-HOSTED AU-AG

LANDER TRIPLET GULCH PROJECT S 0.214 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO ALCHEM DEPOSIT M 0.1897 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO GENERATOR HILL DEPOSIT M 0.1897 AU AG HG SB SEDIMENT-HOSTED AU-AG

ELKO MARLBORO CANYON DEPOSIT M 0.1897 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO NEW DEEP DEPOSIT M 0.1897 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO NORTH GENERATOR HILL DEPOSIT M 0.1897 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO SAVAL CANYON DEPOSIT (JERRITT CANYON) S 0.1897 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO WEST GENERATOR HILL DEPOSIT M 0.1897 AU --- --- --- SEDIMENT-HOSTED AU-AG

LANDER BUFFALO VALLEY MINE S 0.1847 AU AG CU AS SEDIMENT-HOSTED AU-AG (?)

ELKO EMIGRANT SPRINGS S 0.1368 AU AS SB BA SEDIMENT-HOSTED AU-AG

ELKO NORTH SAMMY CREEK DEPOSIT (BIG SPRINGS) S 0.1226 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA GOLD RIDGE DEPOSIT S 0.1226 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA GOLDSTONE DEPOSIT S 0.1226 AU --- --- --- SEDIMENT-HOSTED AU-AG

HUMBOLDT C-ZONE S 0.0726 AU AG AS HG SEDIMENT-HOSTED AU-AG

LANDER NORTHERN LIGHTS DEPOSIT S 0.0661 AU AG CU --- SEDIMENT-HOSTED AU-AG

MINERAL CANDELARIA MINE M 0.0661 AG AU PB SB DISTAL DISSEMINATED AG-AU

MINERAL LUCKY HILL MINE ? 0.0661 AG AU PB FE DISTAL DISSEMINATED AG-AU

ELKO PATTANI SPRINGS DEPOSIT S 0.0617 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO DEE GOLD MINE S 0.0599 AU AG SB AS SEDIMENT-HOSTED AU-AG

LANDER ELDER CREEK MINE S 0.0599 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA HORSE CANYON MINE S 0.0447 AU BA --- --- SEDIMENT-HOSTED AU-AG

ELKO RAIN MINE M 0.0426 AU BA AS HG SEDIMENT-HOSTED AU-AG

EUREKA GOLD PICK DEPOSIT M 0.0375 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO STEER CANYON DEPOSIT S 0.0269 AU SB --- --- SEDIMENT-HOSTED AU-AG
D13

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D2, continued.

ELKO BOOTSTRAP MINE M 0.0267 AU AG CU PB SEDIMENT-HOSTED AU-AG

ELKO CAPSTONE DEPOSIT S 0.0267 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO TROUT CREEK DEPOSIT S 0.022 AU AG AS SB SEDIMENT-HOSTED AU-AG

ELKO WOOD GULCH MINE S 0.0163 AU AG --- --- SEDIMENT-HOSTED AU-AG

LANDER CORTEZ GOLD MINE L 0.0163 AU HG AS SB SEDIMENT-HOSTED AU-AG

HUMBOLDT CX DEPOSIT S 0.0078 AU AS HG SB SEDIMENT-HOSTED AU-AG

ELKO BURNS BASIN GOLD DEPOSIT M 0.0077 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA POST DEPOSIT L 0.0076 AU AG AS --- SEDIMENT-HOSTED AU-AG

NYE NORTHUMBERLAND MINE M 0.0076 AU AG AS FE SEDIMENT-HOSTED AU-AG

LANDER AUSTIN GOLD VENTURES GOLD MINE (QUITO MINE) S 0.0055 AU AS SB --- SEDIMENT-HOSTED AU-AG

ELKO SOUTH BULLION DEPOSIT M 0.0052 AU AG AS SB SEDIMENT-HOSTED AU-AG

LANDER HILLTOP MINE S 0.0046 AU AG CU PB DISTAL DISSEMINATED AG-AU

CHURCHILL REED DEPOSIT (FONDAWAY CANYON MINE) S 0.0027 AU --- --- --- SEDIMENT-HOSTED AU-AG

WHITE PINE BULL TERRIER PATENTED MINES(HAMILTON MINE?) S 0.0027 AU PB --- --- SEDIMENT-HOSTED AU-AG

WHITE PINE NORTH SELOX CLAIMS. S 0.0027 AU BA --- --- SEDIMENT-HOSTED AU-AG

WHITE PINE TAYLOR MINE (ARGUS PIT, BISHOP PIT) M 0.0027 AG AU SB CU DISTAL DISSEMINATED AG-AU

WHITE PINE WINROCK MINE S 0.0027 AU --- --- --- SEDIMENT-HOSTED AU-AG

WHITE PINE GREEN SPRINGS MINE S 0.0023 AU AG --- --- SEDIMENT-HOSTED AU-AG

WHITE PINE ILLIPAH MINE S 0.0023 AU AG --- --- SEDIMENT-HOSTED AU-AG

HUMBOLDT MAG DEPOSIT S 0.0022 AU HG AS SB SEDIMENT-HOSTED AU-AG

HUMBOLDT CHIMNEY CREEK MINE M 0.0019 AU AG SDG AS SEDIMENT-HOSTED AU-AG

HUMBOLDT RABBIT CREEK MINE L 0.0019 AU AG PB ZN SEDIMENT-HOSTED AU-AG

ELKO MEIKLE MINE L 0.0018 AU AG HG ZN SEDIMENT-HOSTED AU-AG

EUREKA GOLD BAR MINE M 0.0018 AU AG SB AS SEDIMENT-HOSTED AU-AG

HUMBOLDT LONE TREE MINE M 0.0018 AU AG AS SB SEDIMENT-HOSTED AU-AG

HUMBOLDT STONEHOUSE DEPOSIT M 0.0018 AU AG AS SB SEDIMENT-HOSTED AU-AG

WHITE PINE CASINO MINE S 0.0014 AU --- --- --- SEDIMENT-HOSTED AU-AG

ELKO MILL CREEK DEPOSIT S 0.0008 AU --- --- --- SEDIMENT-HOSTED AU-AG

CHURCHILL COLORADO DEPOSIT (FONDAWAY CANYON MINE) S 0.0007 AU AG AS SB SEDIMENT-HOSTED AU-AG

EUREKA TONKIN SPRINGS MINE S 0.0006 AU AG AS HG SEDIMENT-HOSTED AU-AG

NYE STEIGMEYER PROPERTY (SHALE PIT MINE) S 0.0006 AU AG AS SB SEDIMENT-HOSTED AU-AG

WHITE PINE NIGHTHAWK RIDGE MINE (EASY JUNIOR) S 0.0006 AU AG HG TL SEDIMENT-HOSTED AU-AG

WHITE PINE YANKEE MINE S 0.0006 AU --- --- --- SEDIMENT-HOSTED AU-AG

PERSHING STANDARD MINE (GOLD STANDARD) S 0.0005 AU AG AS BA SEDIMENT-HOSTED AU-AG

MINERAL SANTA FE GOLD MINE S 0.0003 AU AG AS SB SEDIMENT-HOSTED AU-AG

NYE STERLING MINE S 0.0003 AU AG F --- SEDIMENT-HOSTED AU-AG

WHITE PINE LITTLE BALD MOUNTAIN MINE (LBM) S 0.0003 AU HG --- --- SEDIMENT-HOSTED AU-AG

PERSHING RELIEF CANYON MINE S 0.0002 AU AG F AS SEDIMENT-HOSTED AU-AG

CHURCHILL S. MOUTH DEPOSIT (FONDAWAY CANYON MINE) S 0.0001 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA NEW WINDFALL SHAFT L 0.0001 AU AG PB CU DISTAL DISSEMINATED AG-AU

EUREKA RATTO CANYON DEPOSIT S 0.0001 AU --- --- --- SEDIMENT-HOSTED AU-AG

EUREKA WINDFALL MINE M 0.0001 AU AG PB ZN DISTAL DISSEMINATED AG-AU

HUMBOLDT KRAMER HILL MINE S 0.0001 AU AG --- --- DISTAL DISSEMINATED AG-AU

LANDER COVE MINE M 0.0001 AU AG MN AS DISTAL DISSEMINATED AG-AU

LINCOLN ATLANTA MINE L 0.0001 AU AG U --- SEDIMENT-HOSTED AU-AG

PERSHING FLORIDA CANYON MINE M 0.0001 AU AG HG CLY SEDIMENT-HOSTED AU-AG

WHITE PINE ALLIGATOR RIDGE MINE S 0.0001 AU AG SB HG SEDIMENT-HOSTED AU-AG

WHITE PINE BALD MOUNTAIN MINE M 0.0001 AU AG CU ZN DISTAL DISSEMINATED AG-AU

WHITE PINE DECKER FLATS DEPOSIT S 0.0001 AU AG --- --- SEDIMENT-HOSTED AU-AG

WHITE PINE STAR POINTER MINE (INCLUDES ROBINSON MINE

CU PORPHYRY) S 0.0001 AU AG --- --- SEDIMENT-HOSTED AU-AG (?)
D14

Table D3. Posterior probabilities associated with gold-silver-bearing volcanic rock-hosted occurrences of all

sizes, as determined with the volcanic rock-hosted 7-layer mineral potential model (sorted in order of

descending posterior probability). All data, with the exception of posterior probabilities, from the MRDS

mineral occurrences database. Occurrences size from Guild (1968), and deposit types from Cox and Singer

(1986). Prior probability = 0.0015.

County Site Name Size Post.

Prob. Commodities Deposit Type

LYON BUCKEYE MINE S 0.1316 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY KENTUCK MINE ? 0.1316 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY YELLOW JACKET MINE L 0.1316 AG AU CU PB COMSTOCK EPITHERMAL VEINS

CHURCHILL BLUFF MINE S 0.0720 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL BUFF CLAIMS S 0.0720 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY ALPHA CLAIM S 0.0713 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY ANDES MINE S 0.0713 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

STOREY CHALLENGE AND CONFIDENCE M 0.0713 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY CHOLLAR MINE L 0.0713 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY CROWN POINT MINE L 0.0713 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY EXCHEQUER CLAIM S 0.0713 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY HALE & NORCROSS MINE M 0.0713 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY MONTE CRISTO MINE M 0.0713 AU AG CU PB COMSTOCK EPITHERMAL VEINS

STOREY POTOSI MINE L 0.0713 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY SAVAGE MINE L 0.0713 AG AU MN FE COMSTOCK EPITHERMAL VEINS

CHURCHILL DROMEDARY HUMP MINE S 0.0640 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL KINNEY PROSPECT S 0.0640 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA GOLDFIELD DEEP MINES CO. S 0.0634 AU AG CU SN EPITHERMAL QTZ-ALUNITE AU

ESMERALDA JUMBO EXTENSION MINING CO. (JUMBO GROUP) S 0.0634 AU AG CU SN EPITHERMAL QTZ-ALUNITE AU

ESMERALDA LAGUNA GROUP S 0.0634 AU AG ZN PB EPITHERMAL QTZ-ALUNITE AU

ESMERALDA RED TOP MINE S 0.0634 AU AG PB ZN EPITHERMAL QTZ-ALUNITE AU

HUMBOLDT RASER AND REEDER MINES S 0.0549 AU AG --- --- HOT-SPRING AU-AG

CLARK WALL STREET MINE M 0.0523 AU AG --- --- GOLD ON FLAT FAULTS

MINERAL JAIME'S RIDGE S 0.0523 AU AG HG --- HOT-SPRING AU-AG

STOREY BEST & BELCHER MINE ? 0.0518 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY C & C SHAFT L 0.0518 AU AG CU PB COMSTOCK EPITHERMAL VEINS

STOREY CALIFORNIA MINE M 0.0518 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY CONSOLIDATED VIRGINIA MINE L 0.0518 AG AU CU PB COMSTOCK EPITHERMAL VEINS

PERSHING WOMENS RIGHT GROUP ? 0.0480 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY ALTA SHAFT S 0.0480 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

STOREY JUSTICE SHAFT L 0.0480 AG AU PB CU COMSTOCK EPITHERMAL VEINS

STOREY KEYSTONE SHAFT ? 0.0480 AU --- --- --- COMSTOCK EPITHERMAL VEINS

STOREY KNICKERBOCKER MINE S 0.0480 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY LADY WASHINGTON MINE. S 0.0480 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY NEW YORK MINE S 0.0480 AU --- --- --- COMSTOCK EPITHERMAL VEINS

STOREY OVERMAN 2 MINE S 0.0480 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA TIP TOP CLAIMS S 0.0464 AU AG HG --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT G U B CLAIMS S 0.0464 AG AS CU --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT SILVER BUTTE MINE S 0.0464 AG AU ZN FE COMSTOCK EPITHERMAL VEINS

HUMBOLDT WILD GOOSE VEIN S 0.0464 AG AS --- --- COMSTOCK EPITHERMAL VEINS

NYE JIM BUTLER TONOPAH MINING CO. L 0.0410 AG --- --- --- COMSTOCK EPITHERMAL VEINS

NYE MAC NAMARA MINING CO. L 0.0410 AG --- --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT HERMIT GROUP S 0.0397 AU AG --- --- HOT-SPRING AU-AG

WASHOE LEADVILLE MINE S 0.0346 AG PB ZN CU COMSTOCK EPITHERMAL VEINS

CHURCHILL FAIRVIEW EAGLE L 0.0335 AG AU MN HG COMSTOCK EPITHERMAL VEINS

CHURCHILL HAILSTONE GROUP S 0.0335 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL NEVADA HILLS MINE L 0.0335 AU AG CU PB COMSTOCK EPITHERMAL VEINS

STOREY GOOSEBERRY MINE S 0.0335 AU AG CU --- COMSTOCK EPITHERMAL VEINS
D15

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

CHURCHILL NEVADA FAIRVIEW MINE M 0.0289 AU AG --- --- COMSTOCK EPITHERMAL VEINS

MINERAL SUNNYSIDE CLAIMS S 0.0289 AU AG CU --- COMSTOCK EPITHERMAL VEINS

WASHOE PEAVINE PEAK GOLD MINE S 0.0289 AU --- --- ---

COMSTOCK OR HOT-SPRING

EPITHERMAL (?)

PERSHING AMERICAN FLAT CANYON MINE S 0.0286 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING WILDCAT MINE S 0.0286 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY COMSTOCK LODE DISTRICT, (SILVER CITY DIST.) M 0.0286 AG AU CU --- COMSTOCK EPITHERMAL VEINS

STOREY KEYES MINE S 0.0286 AG AU --- --- COMSTOCK EPITHERMAL VEINS

PERSHING UNNAMED SILVER PROSPECT (GOLD BUTTE DIST.) S 0.0253 AU AG W --- COMSTOCK EPITHERMAL VEINS

WASHOE GOLDEN FLEECE S 0.0251 AU AG CU PB EPITHERMAL QTZ-ALUNITE AU

WASHOE PAYMASTER S 0.0251 AU AG CU PB EPITHERMAL QTZ-ALUNITE AU

STOREY BELCHER CLAIM M 0.0249 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

STOREY BULLION MINE S 0.0249 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY CALEDONIA MINE S 0.0249 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY IMPERIAL MINE M 0.0249 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY OVERLAND MINE M 0.0249 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY OVERMAN CLAIM M 0.0249 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY SCARPION SHAFT S 0.0249 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL SUMMIT KING-DAN TUCKER MINE L 0.0222 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA COMBINATION MINE M 0.0220 AU AG TE CU EPITHERMAL QTZ-ALUNITE AU

ESMERALDA FLORENCE MINE M 0.0220 AU AG CU BI EPITHERMAL QTZ-ALUNITE AU

ESMERALDA JANUARY MINE (JANUARY & FEBRUARY CLAIMS) S 0.0220 AU AG CU BI EPITHERMAL QTZ-ALUNITE AU

ESMERALDA MOHAWK MINE M 0.0220 AU AG CU SB EPITHERMAL QTZ-ALUNITE AU

ESMERALDA SANGER MINE S 0.0220 AG AU CU PB COMSTOCK EPITHERMAL VEINS

LYON SOUTH COMSTOCK MINE S 0.0220 AU AG --- --- COMSTOCK EPITHERMAL VEINS

MINERAL DOUGLAS GROUP (CAMP DOUGLAS ?) S 0.0220 AU AG PB --- COMSTOCK EPITHERMAL VEINS

STOREY BILLY THE KID PIT M 0.0220 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY DRYSDALE MINE S 0.0220 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY LUCERNE PIT M 0.0220 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING ROSE-DALE MINE S 0.0205 AU --- --- --- HOT-SPRING AU-AG

ESMERALDA GOLDFIELD-BELMONT MINE S 0.0190 AU ZN CU TE EPITHERMAL QTZ-ALUNITE AU

ESMERALDA GREAT BEND MINE S 0.0190 AU AG CU ZN EPITHERMAL QTZ-ALUNITE AU

ESMERALDA LONE STAR GROUP (PATRICK CLAIM) S 0.0190 AU --- --- --- EPITHERMAL QTZ-ALUNITE AU

CHURCHILL NBMG SAMPLE SITE 2882 (TRUCKEE, FIREBALL) S 0.0180 AU ? AG ? COMSTOCK EPITHERMAL VEINS

CLARK HOMESTAKE GROUP S 0.0180 AU AG --- --- GOLD ON FLAT FAULTS

ELKO ALPHA MINE S 0.0178 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO BOURNE MINE S 0.0178 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO BUSTER G.M. CO. S 0.0178 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO LONG HIKE S 0.0178 AU AG SE --- COMSTOCK EPITHERMAL VEINS

ELKO RIDDLE LEASE S 0.0178 AU --- --- --- COMSTOCK EPITHERMAL VEINS

STOREY OPHIR CLAIM L 0.0178 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

STOREY SIERRA NEVADA MINE L 0.0178 AG AU CU PB COMSTOCK EPITHERMAL VEINS

STOREY UNION MINE M 0.0178 AG AU PB CU COMSTOCK EPITHERMAL VEINS

CHURCHILL NEZELDA MINE S 0.0159 AU AG PB --- COMSTOCK EPITHERMAL VEINS

CHURCHILL UNNAMED PROSPECT (TRUCKEE DISTRICT) S 0.0159 AU --- --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT NBMG SAMPLE SITE 2605 S 0.0159 AG CU AU PB COMSTOCK EPITHERMAL VEINS

PERSHING FENCEMAKER MINE S 0.0158 SB AG HG AU HOT-SPRING AU-AG

WASHOE S.E.D. CLAIMS S 0.0158 AU ? --- --- HOT-SPRING HG

CLARK QUO VADIS S 0.0143 AU AG PB --- EPITHERMAL QTZ-ALUNITE AU

ESMERALDA SILVER KING CLAIMS (SPECULATOR, SILVER

KNIGHT...) S 0.0143 AG PB --- --- COMSTOCK EPITHERMAL VEINS

NYE HORN SILVER MINE S 0.0143 AG AU HG --- COMSTOCK EPITHERMAL VEINS

NYE LIBERTY MINE (REPRESENTING SAN ANTONE DIST.) S 0.0142 AG AU CU --- COMSTOCK EPITHERMAL VEINS
D16

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

HUMBOLDT LUCKY JIM NOS. 1-8 AND APEX CLAIMS S 0.0137 AU AG U --- HOT-SPRING AU-AG

CHURCHILL GOLD ORE CLAIMS S 0.0135 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL GOLD COIN #2 MINE S 0.0131 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA DIVIDE EXTENSION MINE S 0.0121 AG AU --- --- HOT-SPRING AU-AG (?)

ESMERALDA GOLD ZONE DIVIDE MINING CO. PROPERTY S 0.0121 AU AG CU PB COMSTOCK EPITHERMAL VEINS

ESMERALDA TONOPAH DIVIDE MINE S 0.0121 AU AG MO CU COMSTOCK EPITHERMAL VEINS

LYON TALAPOOSA DEPOSIT S 0.0114 AU AG HG CU COMSTOCK EPITHERMAL VEINS

ELKO ALTITUDE MINE S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO NEW HOPE GROUP S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA ATLANTA MINES CO. CLAIMS S 0.0113 AU AG CU SN EPITHERMAL QTZ-ALUNITE AU

LYON SILVER HILL MINE L 0.0113 AG AU FE MN COMSTOCK EPITHERMAL VEINS

LYON VOLCANO MINE S 0.0113 AG AU --- --- COMSTOCK EPITHERMAL VEINS

PERSHING FAIRVIEW MINE S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING J AND B MINE S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING KINDERGARTEN AND WIHUJA MINES S 0.0113 AU AG PB CU COMSTOCK EPITHERMAL VEINS

PERSHING MAZUMA HILLS MINE S 0.0113 AU AG CU PB COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 2820 S 0.0113 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY DONOUAN PROPERTY M 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY SUCCOR MINE M 0.0113 AG AU --- --- COMSTOCK EPITHERMAL VEINS

STOREY UTAH SHAFT S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY WOODVILLE SHAFT M 0.0113 AG AU CU PB COMSTOCK EPITHERMAL VEINS

WASHOE MAHONEY MINE S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE PANDORA S 0.0113 AU AG --- --- COMSTOCK EPITHERMAL VEINS

LINCOLN HOMESTAKE MINE S 0.0098 AU AG HG CU COMSTOCK EPITHERMAL VEINS

LINCOLN IRIS84 09 84 09 S 0.0098 AU AG CU --- COMSTOCK EPITHERMAL VEINS

LINCOLN KENO CLAIMS S 0.0098 AU ? AG BA COMSTOCK EPITHERMAL VEINS

LYON COMO-EUREKA M 0.0098 AG AU CU --- COMSTOCK EPITHERMAL VEINS

ESMERALDA JMP CLAIMS S 0.0097 AU AG --- --- EPITHERMAL QTZ-ALUNITE AU

ESMERALDA QUARTZITE AND BLACK BUTTE MINES S 0.0097 AU CU SB TE EPITHERMAL QTZ-ALUNITE AU

PERSHING DEVANEY PROPERTY S 0.0097 AU AG F --- COMSTOCK EPITHERMAL VEINS

PERSHING GOLD CROWN GROUP S 0.0097 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 1491 S 0.0097 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 2815 S 0.0097 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING SANTA MARIA PROSPECT S 0.0097 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING SNOW SQUALL MINE S 0.0097 AU AG F --- COMSTOCK EPITHERMAL VEINS

PERSHING STONE CLAIM SHAFT S 0.0097 AU ? --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT BUCKSKIN NATIONAL MINE S 0.0094 AU AG SB FE COMSTOCK EPITHERMAL VEINS

ESMERALDA BLUE BULL MINE S 0.0086 AU AG CU --- EPITHERMAL QTZ-ALUNITE AU

NYE LAND MARK MINE S 0.0086 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING ANTIMONY IKE MINE S 0.0086 SB AU AG AS HOT-SPRING AU-AG

PERSHING BLACK BOY MINE S 0.0086 AG PB SB ZN COMSTOCK EPITHERMAL VEINS

PERSHING VELVET GOLD MINES S 0.0086 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CLARK JETCO CLAIMS S 0.0081 AU CU --- --- GOLD ON FLAT FAULTS

ELKO CORNUCOPIA S 0.0081 AU AG CU ZN COMSTOCK EPITHERMAL VEINS

ESMERALDA TIP TOP MINE S 0.0081 AU AG --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT LUCKY HORSESHOE GROUP S 0.0081 AU AG U --- HOT-SPRING AU-AG

WASHOE MAZY MINE S 0.0081 AU AG PB ZN COMSTOCK EPITHERMAL VEINS

WASHOE RENO MAY S 0.0081 AU AG ZN PB COMSTOCK EPITHERMAL VEINS

ELKO AJAX PROSPECT S 0.0080 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO BEN HUR PROSPECT S 0.0080 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO PAVLAK MINE JARBIDGE-PAVLAK

G.M.CO.PROPERTIES S 0.0080 AU AG --- --- COMSTOCK EPITHERMAL VEINS
D17

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

ELKO STARLIGHT GROUP S 0.0080 AU AG CU --- COMSTOCK EPITHERMAL VEINS

ESMERALDA MOHAWK MINE (ARGENTITE MINE) S 0.0080 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

MINERAL AURORA DISTRICT (AURORA MINE AREA) L 0.0080 AU AG PB SE COMSTOCK EPITHERMAL VEINS

PERSHING PERSHING MINE ? 0.0080 AU --- --- --- HOT-SPRING AU-AG

PERSHING WONDER METAL MINE S 0.0080 SB AG FE TI HOT-SPRING AU-AG

LINCOLN HORSESHOE MINE MINE S 0.0070 AU AG CU --- COMSTOCK EPITHERMAL VEINS

PERSHING HOLLYWOOD MINE M 0.0069 SB AG AU --- HOT-SPRING AU-AG

WASHOE WESTERN HOG RANCH MINE M 0.0069 AU AG HG U HOT-SPRING AU-AG

CHURCHILL CYCLONE GROUP S 0.0065 AG --- --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL NEVADA HILLS FLORENCE MINE S 0.0065 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL OHIO GROUP S 0.0065 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE BLACK WARRIOR PEAK S 0.0065 AU AG --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT SLEEPER MINE M 0.0062 AU AG K S COMSTOCK EPITHERMAL VEINS

CHURCHILL UNNAMED PROPERTY S 0.0061 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING TRINITY SILVER MINE S 0.0061 AG PB SB AU COMSTOCK EPITHERMAL VEINS

ELKO GOOD HOPE ? 0.0048 AG AU SB --- COMSTOCK EPITHERMAL VEINS

STOREY BALTIMORE SHAFT S 0.0048 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA C.O.D. CONSOLIDATED MINING CO CLAIMS S 0.0044 AU AG CU SB EPITHERMAL QTZ-ALUNITE AU

HUMBOLDT KIM BLAIMS, NO.S 1-9 S 0.0044 AU AG U --- HOT-SPRING AU-AG

HUMBOLDT LUCKY STRIKE PROSPECT S 0.0044 AU AG --- --- HOT-SPRING AU-AG

LINCOLN FORTUNA AND HELEN CLAIMS S 0.0044 AU AG PB ? COMSTOCK EPITHERMAL VEINS

LINCOLN NBMG SAMPLE SITE 788 UNNAMED PROSPECT S 0.0044 AU ? BA ? EPITHERMAL MANGANESE

LYON HULLEY-LOGAN S 0.0044 AG AU CU --- COMSTOCK EPITHERMAL VEINS

LYON RAMSEY COMSTOCK MINE S 0.0044 AU AG --- --- COMSTOCK EPITHERMAL VEINS

LYON RAPIDAN M 0.0044 AG AU --- --- COMSTOCK EPITHERMAL VEINS

LYON THE STONE CABIN CLAIMS S 0.0044 AG AU --- --- COMSTOCK EPITHERMAL VEINS

MINERAL NEVADA RAND ? 0.0044 AU AG MN ZN COMSTOCK EPITHERMAL VEINS

MINERAL RANDALL PROPERTY S 0.0044 AU AG CU --- COMSTOCK EPITHERMAL VEINS

NYE KEY FLOWER MINE S 0.0044 AU AG CU PB COMSTOCK EPITHERMAL VEINS

NYE PARADISE PEAK MINE M 0.0044 AU AG HG BA EPITHERMAL QTZ-ALUNITE AU

PERSHING NBMG SAMPLE SITE 2809 S 0.0044 AU ? --- --- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 1480 S 0.0044 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 1481 S 0.0044 AG AU --- --- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 2836 S 0.0044 AG PB AU CU COMSTOCK EPITHERMAL VEINS

PERSHING PORTLAND MINE S 0.0044 AU AG PB ZN COMSTOCK EPITHERMAL VEINS

PERSHING SEVEN TROUGHS DISTRICT S 0.0044 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING TRINITY SILVER PROSPECT M 0.0044 AG --- --- --- COMSTOCK EPITHERMAL VEINS

PERSHING TRINITY JOINT VENTURE S 0.0044 AG PB CU W COMSTOCK EPITHERMAL VEINS

STOREY FLOWERY MINE M 0.0044 AU AG PB ZN COMSTOCK EPITHERMAL VEINS

STOREY LODY BRYAN MINE ? 0.0044 AU AG --- --- COMSTOCK EPITHERMAL VEINS

STOREY NORTH BONANZA MINE S 0.0044 AU AG PB --- COMSTOCK EPITHERMAL VEINS

STOREY PET CLAIM S 0.0044 AG --- --- --- COMSTOCK EPITHERMAL VEINS

NYE MIZPAH EXTENSION MINING CO. M 0.0040 AG AU --- --- COMSTOCK EPITHERMAL VEINS

NYE MONTANA - TONOPAH MINING CO. L 0.0040 AG AU --- --- COMSTOCK EPITHERMAL VEINS

NYE TONOPAH MINING CO. L 0.0040 AG AU CU PB COMSTOCK EPITHERMAL VEINS

LANDER ASPEN GROUP S 0.0039 AU MN --- --- COMSTOCK EPITHERMAL VEINS

NYE MONTGOMERY-SHOSHONE MINE S 0.0034 AG AU --- --- COMSTOCK EPITHERMAL VEINS

WASHOE GREEN HILL MINES S 0.0033 AU AG CU --- COMSTOCK EPITHERMAL VEINS

CHURCHILL GOLD KING CLAIM AND VALLEY KING CLAIMS S 0.0031 AU AG W --- COMSTOCK EPITHERMAL VEINS

CHURCHILL UNNAMED PROSPECT (JESSUP DISTRICT) S 0.0031 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO BLUSTER MINE S 0.0031 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO FLAXIE MINE S 0.0031 AU AG --- --- COMSTOCK EPITHERMAL VEINS
D18

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

ELKO JARBIDGE DISTRICT L 0.0031 AU AG --- --- HOT-SPRING AU-AG (?)

ELKO NEW STAR GROUP S 0.0031 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO PAN CLAIM S 0.0031 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO PICK AND SHOVEL MINE S 0.0031 AU AG --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT GOLDEN CASH; JACKPOT, BURIED TREASURE,

AND GOLDEN ? 0.0031 AU AG U --- HOT-SPRING AU-AG

HUMBOLDT NBMG SAMPLE SITE 2609 S 0.0031 AG AS --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT SISKIYOU MINE S 0.0031 AU --- --- --- COMSTOCK EPITHERMAL VEINS

PERSHING BROWN PALACE GROUP ? 0.0031 AU AG CU PB COMSTOCK EPITHERMAL VEINS

PERSHING DREAMLAND MINE S 0.0031 AU AG CU PB COMSTOCK EPITHERMAL VEINS

PERSHING ROSEBUD MINE S 0.0031 AU AG CU PB COMSTOCK EPITHERMAL VEINS

PERSHING WILLARD GROUP (NORTH) S 0.0031 AU AG CU PB COMSTOCK EPITHERMAL VEINS

LINCOLN DELAMAR MINE (MAGNOLIA MINE) L 0.0028 AU AG CU FE COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 1483 S 0.0028 AG AU --- --- COMSTOCK EPITHERMAL VEINS

PERSHING STONE HOUSE CANYON MINE S 0.0028 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE FAIRVIEW MINE S 0.0025 AU --- --- --- COMSTOCK EPITHERMAL VEINS

NYE FRANZ HAMMEL MINE S 0.0025 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE TROY MINE S 0.0025 AG CU MN --- EPITHERMAL MANGANESE

NYE UNNAMED PROSPECTS #5 S 0.0025 AG AU CU MN COMSTOCK EPITHERMAL VEINS

PERSHING BELLE ETC. CLAIMS ? 0.0025 AU --- --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL NEVADA CROWN MINE (GOLD CROWN; PORT AND

SAMPSON) S 0.0024 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

ELKO ARGENTA S 0.0024 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO DEFREES M 0.0024 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA HASBROUCK MOUNTAIN S 0.0024 AG AU AS HG HOT-SPRING AU-AG

LINCOLN CULVERWELL MINE S 0.0024 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE KING TONOPAH MINE S 0.0024 AG AU --- --- COMSTOCK EPITHERMAL VEINS

WASHOE ARKELL MINE S 0.0024 AG AU PB ZN EPITHERMAL QTZ-ALUNITE AU

WASHOE BIG MOUTH S 0.0024 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE DESERT KING MINE S 0.0024 AG AU PB ZN EPITHERMAL QTZ-ALUNITE AU

WASHOE SECRET CANYON PROSPECT S 0.0024 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE WEDEKIND MINE S 0.0024 AG AU PB ZN EPITHERMAL QTZ-ALUNITE AU

WASHOE BUSTER MINES M 0.0022 AU AG CU PB COMSTOCK EPITHERMAL VEINS

ESMERALDA GOLDFIELD DISTRICT L 0.0021 AU AG BI K EPITHERMAL QTZ-ALUNITE AU

ESMERALDA NIVLOC MINE S 0.0021 AG AU PB --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT SILVER HILLS MINE ? 0.0021 AU AG --- --- HOT-SPRING AU-AG

LYON DAYTON MINE ? 0.0021 AU AG --- --- COMSTOCK EPITHERMAL VEINS

LYON KOSSUTH MINE S 0.0021 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE WARRIOR MINE S 0.0021 AU AG CU PB COMSTOCK EPITHERMAL VEINS

CHURCHILL FRED BRANCH PROPERTY S 0.0020 AU AG CU MN COMSTOCK EPITHERMAL VEINS

PERSHING STEINER MINE S 0.0020 AU AG PB SB COMSTOCK EPITHERMAL VEINS

CLARK CAMP DUPONT GROUP S 0.0018 AU --- --- --- GOLD ON FLAT FAULTS

PERSHING BLACK STAR CLAIM GROUP S 0.0018 AU AG SB CU COMSTOCK EPITHERMAL VEINS

ELKO COMMON WEALTH M 0.0017 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO NORTH COMMONWEALTH M 0.0017 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO OZARK GROUP ? 0.0017 AU AG --- --- COMSTOCK EPITHERMAL VEINS

EUREKA BUCKHORN S 0.0017 AU AG AS HG COMSTOCK EPITHERMAL VEINS

HUMBOLDT BIGHORN GROUP S 0.0017 AG U --- --- HOT-SPRING AU-AG

HUMBOLDT BLUE DRAGON NOS. 1-3 PROSPECT S 0.0017 AG AU --- --- HOT-SPRING AU-AG

CHURCHILL M. H. CLAIMS ? 0.0015 AU CU --- --- COMSTOCK EPITHERMAL VEINS

ELKO GOLD CIRCLE DISTRICT (MIDAS) L 0.0015 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL BIG LEDGE MINE S 0.0013 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL JELINEK MINE S 0.0013 AG AU --- --- COMSTOCK EPITHERMAL VEINS
D19

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

CHURCHILL MIZPAH MINE S 0.0013 AG AU --- --- COMSTOCK EPITHERMAL VEINS

PERSHING YELLOWSTONE MINE S 0.0013 AG AU ? --- HOT-SPRING AU-AG

WASHOE TIGER GROUP S 0.0013 AU AG CU --- COMSTOCK EPITHERMAL VEINS

NYE KEYSTONE MINE S 0.0012 AU F AG --- COMSTOCK EPITHERMAL VEINS

NYE ROUND MOUNTAIN GOLD MINE M 0.0012 AU AG AS SB HOT-SPRING AU-AG

CHURCHILL BLACK BUTTE MINE S 0.0011 AU AG PB FE COMSTOCK EPITHERMAL VEINS

CHURCHILL CENTURION PROSPECT S 0.0011 AG AU CU --- COMSTOCK EPITHERMAL VEINS

CHURCHILL CIRAN PROSPECT S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL CRIPPLE QUEEN MINE S 0.0011 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL LANSING PROSPECT S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL NBMG SAMPLE SITE 3941 S 0.0011 AG CU PB --- COMSTOCK EPITHERMAL VEINS

CHURCHILL PYRAMID MINE S 0.0011 AG PB MN --- COMSTOCK EPITHERMAL VEINS

CHURCHILL REX CLAIM #6 S 0.0011 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL SCOTIA MINE S 0.0011 AG --- --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WATER SHAFT MINE S 0.0011 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WOLVERTON PROSPECT S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO APRIL FOOL GROUP S 0.0011 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO GRAND PRIZE L 0.0011 AG --- --- --- COMSTOCK EPITHERMAL VEINS

ELKO TUSCARORA MINE S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT ECHO GROUP S 0.0011 AG U --- --- HOT-SPRING AU-AG

LYON PONY MEADOWS CLAIMS S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

MINERAL OMCO MINE (OLYMPIC MINE; ROYAL GEORGE GRP.) M 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE AMALGAMATED MINE S 0.0011 AU F --- --- COMSTOCK EPITHERMAL VEINS

NYE ARROWHEAD MINE (ARROWHEAD SYNDICATE

MINES, INC.) S 0.0011 AG AU SB --- COMSTOCK EPITHERMAL VEINS

NYE BULLFROG DISTRICT L 0.0011 AU AG CU PB COMSTOCK EPITHERMAL VEINS

NYE ELLENDALE MINE S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE GOLD BAR GROUP (GOLDEN ARROW DISTRICT) S 0.0011 AU AG CU --- COMSTOCK EPITHERMAL VEINS

NYE MANHATTAN CONSOLIDATED MINE S 0.0011 AG AU SB AS COMSTOCK EPITHERMAL VEINS

NYE PIONEER MINE S 0.0011 AU --- --- ---

COMSTOCK OR HOT-SPRING

EPITHERMAL (?)

NYE SILVER GLANCE GROUP (SILVER BOW DIST.) S 0.0011 AG AU --- --- COMSTOCK EPITHERMAL VEINS

PERSHING CROFOOT PROJECT ? 0.0011 AU --- --- --- HOT-SPRING AU-AG

STOREY GLOBE CONSOLIDATED MINE S 0.0011 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE TMB ASSOCIATES CLAIMS S 0.0011 AU ?

CL

Y

--- HOT-SPRING AU-AG

WASHOE WIND MOUNTAIN MINE S 0.0011 AU AG CL

Y

AS HOT-SPRING AU-AG

CHURCHILL GRAND CENTRAL MINE S 0.0009 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL LAST HOPE MINE S 0.0009 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

CHURCHILL NBMG SAMPLE SITE 3935 S 0.0009 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL TERRELL MINE S 0.0009 AG PB --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WINGFIELD MINE S 0.0009 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO BUCKEYE AND OHIO L 0.0009 AG SB AS --- COMSTOCK EPITHERMAL VEINS

LINCOLN POPE MINE S 0.0009 AU AG CU ? COMSTOCK EPITHERMAL VEINS

LINCOLN REDLITE CLAIMS S 0.0009 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE GOLDEN CROWN GROUP ? 0.0009 AG --- --- --- COMSTOCK EPITHERMAL VEINS

NYE OKEY DAVIS MINE S 0.0009 AU --- --- --- COMSTOCK EPITHERMAL VEINS

PERSHING NBMG SAMPLE SITE 2927 S 0.0009 AU AG CU --- COMSTOCK EPITHERMAL VEINS

WASHOE WILD ROSE PROSPECT S 0.0009 AU U --- --- HOT-SPRING AU-AG

CHURCHILL DIXIE COMSTOCK MINE S 0.0008 AU AG CU PB COMSTOCK EPITHERMAL VEINS

CHURCHILL GOLCONDA-GOLD WEDGE GROUP S 0.0008 AG AU CU MO COMSTOCK EPITHERMAL VEINS

CHURCHILL LAST CHANCE AND TONY PAN PATENTED CLAIMS S 0.0008 AG AU --- --- COMSTOCK EPITHERMAL VEINS
D20

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

CHURCHILL RUBY-JUNE ROSE GROUP S 0.0008 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL SPIDER AND WASP PROSPECT S 0.0008 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL TREASURE HILL CLAIM #1417 S 0.0008 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WEST JOB CANYON-NORTH ? 0.0008 AU AG CU PB COMSTOCK EPITHERMAL VEINS

CHURCHILL WINDLASS MINE S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO BANNER S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO DEXTER L 0.0008 AU AG TI --- COMSTOCK EPITHERMAL VEINS

ELKO EIRA M 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO ELKO PRINCE S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO HOLLISTER (IVANHOE) M 0.0008 AU AG HG CU HOT-SPRING AU-AG

ELKO WATER WITCH MINE S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ESMERALDA GILBERT MINE S 0.0008 AU --- --- ---

COMSTOCK OR HOT-SPRING

EPITHERMAL (?)

ESMERALDA MAMMOTH PROSPECT S 0.0008 AG AU SB --- COMSTOCK EPITHERMAL VEINS

ESMERALDA RED CLOUD CLAIMS S 0.0008 AU AG ? --- COMSTOCK EPITHERMAL VEINS

NYE AMERICAN EAGLE AND CEDAR VEIN S 0.0008 AG AU CU --- EPITHERMAL MANGANESE

NYE BIG PINE MINE S 0.0008 AU --- --- --- COMSTOCK EPITHERMAL VEINS

NYE BIG FOUR MINE S 0.0008 AU --- --- --- COMSTOCK EPITHERMAL VEINS

NYE BOND GOLD BULLFROG MINE M 0.0008 AU AG CU PB COMSTOCK EPITHERMAL VEINS

NYE BRUNER MINE S 0.0008 AU --- --- --- SADO EPITHERMAL VEINS

NYE LONGSTREET MINE S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE MAMMOTH 1-5 CLAIMS S 0.0008 AU AG HG --- HOT-SPRING HG

NYE MOTHER LODE MINE S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE MUSTANG MINE S 0.0008 AU --- --- --- COMSTOCK EPITHERMAL VEINS

NYE NATIONAL BANK MINE S 0.0008 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE UNION NO. 9 MINE S 0.0008 AU --- --- --- COMSTOCK EPITHERMAL VEINS

PERSHING BUNCE PROSPECT S 0.0008 AU --- --- --- COMSTOCK EPITHERMAL VEINS

PERSHING GOLDBANKS MERGER MINES S 0.0008 AU AG PB HG HOT-SPRING AU-AG

PERSHING QUEENSTAKE'S NORTH STRUCTURE S 0.0008 AG PB ZN --- HOT-SPRING AU-AG

PERSHING TRINITY MINE ? 0.0008 AG --- --- --- COMSTOCK EPITHERMAL VEINS

STOREY OCCIDENTAL MINE S 0.0008 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WEST JOB CANYON-SOUTH ? 0.0007 AU AG CU PB COMSTOCK EPITHERMAL VEINS

ELKO BELLE ISLE S 0.0007 AG AU CU ZN COMSTOCK EPITHERMAL VEINS

ELKO INDEPENDENCE M 0.0007 AG CU PB --- COMSTOCK EPITHERMAL VEINS

EUREKA WEST SINTER DEPOSIT S 0.0007 AU AG --- --- COMSTOCK EPITHERMAL VEINS

LANDER DMB MINING PROPERTIES. S 0.0007 AG AU --- --- HOT-SPRING AU-AG

LANDER FIRE CREEK MINE S 0.0007 AU --- --- ---

COMSTOCK OR HOT-SPRING

EPITHERMAL (?)

LINCOLN CHARLEY ROSS MINE S 0.0007 AU AG --- --- COMSTOCK EPITHERMAL VEINS

LINCOLN CONFIDENCE MINE S 0.0007 AU AG --- --- COMSTOCK EPITHERMAL VEINS

LINCOLN JENNIE MINE S 0.0007 AU AG PB CU COMSTOCK EPITHERMAL VEINS

LINCOLN SNOWFLAKE MINE S 0.0007 AG AU CU FE COMSTOCK EPITHERMAL VEINS

LINCOLN THOR MINE S 0.0007 AU AG --- --- COMSTOCK EPITHERMAL VEINS

MINERAL BOREALIS MINE M 0.0007 AU AG ZN CU HOT-SPRING AU-AG

MINERAL FREEDOM FLATS DEPOSIT S 0.0007 AU AG AS SB EPITHERMAL QTZ-ALUNITE AU

PERSHING SILVER PROSPECT (RAGGED TOP DISTRICT) ? 0.0007 AG --- --- --- COMSTOCK EPITHERMAL VEINS

PERSHING WILDHORSE GROUP S 0.0007 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL BUFFALO HUMP MINE S 0.0006 AG AU CU PB COMSTOCK EPITHERMAL VEINS

ESMERALDA 16 TO 1 MINE (OLD SILVER PEAK DIST.) S 0.0006 AG AU PB ZN COMSTOCK EPITHERMAL VEINS

ESMERALDA B.C.H. MINERALS CO. PROPERTY S 0.0006 AU AG PT ? COMSTOCK EPITHERMAL VEINS

HUMBOLDT CROFOOT/LEWIS MINE S 0.0006 AU AG HG S COMSTOCK EPITHERMAL VEINS

HUMBOLDT LEWIS MINE (AND CROWFOOT?) S 0.0006 AU AG SB --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT YELLOW ROCK GROUP S 0.0006 AU AG U --- HOT-SPRING AU-AG
D21

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

NYE ORIGINAL BULLFROG MINE S 0.0006 AG AU CU --- COMSTOCK EPITHERMAL VEINS

CHURCHILL GOLD BASIN MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL GOLD BUG MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL JACK POT MINE M 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL NEVADA WONDER MINE L 0.0004 AU AG MO CU COMSTOCK EPITHERMAL VEINS

CHURCHILL SILVER CENTER MINE S 0.0004 AG AU ? BA COMSTOCK EPITHERMAL VEINS

CHURCHILL VULTURE MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WONDER MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO BULLION PROSPECT S 0.0004 AU AG CU --- COMSTOCK EPITHERMAL VEINS

ELKO COEUR D'ALENE-JARBIDGE G.M.CO. S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO EASTERN STAR MINE S 0.0004 AU AG CU --- COMSTOCK EPITHERMAL VEINS

ELKO MODOC S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO RED DIKE PROSPECT S 0.0004 AU --- --- --- COMSTOCK EPITHERMAL VEINS

LANDER GOLD BASIN GOLD MINING CO. S 0.0004 AU AG CU PB COMSTOCK EPITHERMAL VEINS

LINCOLN SOLO JOKER CLAIM CLAIM S 0.0004 AG AU BA --- COMSTOCK EPITHERMAL VEINS

LINCOLN UTAH SPUR MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

MINERAL BOVARD - RAND DISTRICT S 0.0004 AU AG CU PB COMSTOCK OR HOT-SPRING

EPITHERMAL (?)

MINERAL BROKEN HILLS MINE S 0.0004 AG PB ZN AU COMSTOCK EPITHERMAL VEINS

MINERAL RAWHIDE DEPOSIT L 0.0004 AU AG CU PB COMSTOCK EPITHERMAL VEINS

NYE AJAX MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

NYE APRIL FOOL MINE S 0.0004 AU F --- --- COMSTOCK EPITHERMAL VEINS

NYE CLIFFORD MINE S 0.0004 AG AU --- --- COMSTOCK EPITHERMAL VEINS

NYE GOLD HILL MINE M 0.0004 AU --- --- --- COMSTOCK EPITHERMAL VEINS

NYE GOLDEN KING MINE S 0.0004 AU AS BE --- COMSTOCK EPITHERMAL VEINS

NYE HANNAPAH MINE S 0.0004 AG --- --- --- COMSTOCK EPITHERMAL VEINS

NYE JEEP GROUP S 0.0004 AG AU --- --- COMSTOCK EPITHERMAL VEINS

NYE SAM JACK GROUP S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

PERSHING AMONETT-FRANK CLAIMS S 0.0004 AU AG --- --- HOT-SPRING AU-AG

PERSHING DEER PROSPECT S 0.0004 AU AS SB HG HOT-SPRING AU-AG

PERSHING VENT CLAIM PROSPECT S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE KEYSTONE NEVADA MINE S 0.0004 AU AG --- --- COMSTOCK EPITHERMAL VEINS

WASHOE PRETTY ROCK GROUP S 0.0004 AU AG U --- HOT-SPRING AU-AG

CHURCHILL BELL MOUNTAIN MINE S 0.0003 AU AG --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL GEIGER SHAFT S 0.0003 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL GOLD LEDGE MINE S 0.0003 AU AG PB CU COMSTOCK EPITHERMAL VEINS

CHURCHILL NBMG SAMPLE SITES 3927, 3928 S 0.0003 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL NBMG SAMPLE SITES 3890, 3926 S 0.0003 AG AU PB SB COMSTOCK EPITHERMAL VEINS

CHURCHILL NBMG SAMPLE SITE 3925 S 0.0003 AG AU --- --- COMSTOCK EPITHERMAL VEINS

CHURCHILL WILLIAMS MINE S 0.0003 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO NAVAJO L 0.0003 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO NORTH BELLE ISLE L 0.0003 AG AU --- --- COMSTOCK EPITHERMAL VEINS

ELKO REX MINE (GOLD CIRCLE DIST.) M 0.0003 AU AG --- --- COMSTOCK EPITHERMAL VEINS

ELKO ST. PAUL S 0.0003 AU AG --- --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT AUTO HILL PROSPECTS S 0.0003 AG AU SB CU COMSTOCK EPITHERMAL VEINS

HUMBOLDT BIRTHDAY MINE S 0.0003 AU AG PB CU COMSTOCK EPITHERMAL VEINS

HUMBOLDT BLUM SHAFT S 0.0003 AG AU HG PB COMSTOCK EPITHERMAL VEINS

HUMBOLDT CAUSTIN MINE S 0.0003 AG FE SB AU HOT-SPRING AU-AG

HUMBOLDT CHEFOO TUNNEL S 0.0003 AU AG CU ZN COMSTOCK EPITHERMAL VEINS

HUMBOLDT CRAWFORD MINE S 0.0003 AU ? AG ? COMSTOCK EPITHERMAL VEINS

HUMBOLDT HATCH MINE S 0.0003 AU AG --- --- HOT-SPRING AU-AG

HUMBOLDT INDIAN VALLEY MINE S 0.0003 SB AU AG --- HOT-SPRING AU-AG
D22

County Site Name Size Post.

Prob. Commodities Deposit Type

Table D3, continued.

HUMBOLDT MAMMOTH MINE S 0.0003 AU AG --- --- HOT-SPRING HG

HUMBOLDT NATIONAL MINE L 0.0003 AU AG CU PB COMSTOCK EPITHERMAL VEINS

HUMBOLDT RADIATOR HILL S 0.0003 AU AG SB --- COMSTOCK EPITHERMAL VEINS

HUMBOLDT SULPHUR DISTRICT S 0.0003 S AG HG AL3 HOT-SPRING HG

NYE BALD MOUNTAIN PROSPECT S 0.0003 AU --- --- --- COMSTOCK EPITHERMAL VEINS

NYE DULUTH MINE S 0.0003 AU AG --- --- SADO EPITHERMAL VEINS

NYE MANHATTAN DISTRICT L 0.0003 AU AG SB AS COMSTOCK EPITHERMAL VEINS

PERSHING COYOTE PROSPECT S 0.0003 AG AU --- --- HOT-SPRING AU-AG

PERSHING D.J. CLAIMS S 0.0003 AG AU --- --- HOT-SPRING AU-AG

WASHOE OBSIDIAN GROUP S 0.0003 AU AG U --- HOT-SPRING AU-AG

WASHOE UNNAMED PROSPECT #2 ? 0.0003 AU AG U --- HOT-SPRING AU-AG
on of posterior probabilities associated with big (large and medium), small, and unknown size primary occurrences. D23

Posterior Probabilities Appended to Primary Gold-Sliver-Bearing Occurrences:

Calculated Using Primary 7-Layer Mineral Potential Model

SILVER GULCH MINE PQX CLAIMS ACE OF DIAMONDS CLAIM WEAVER CANYON MINE J. E. T. BUCK BOARD CLAIM GRP. SAN PEDRO MINE RED MONSTER MINE FORTUNA MINE PICK HANDLE MINE

GOOSEBERRY MINE

DAISY CLAIM GRP.

MAYFLOWER CLAIM ALPHA CLAIMS BIG TWENTY MINE BARTLETT MINE GOOD HOPE MINE UNAM. ADIT NBMG SAMP. SITE 2968 WABASH TUNNEL

PRICE AND DAVIS INSKIP MINE UNKN. (1) MIKE PHELPS AND DICK JOHNSON UNAM. AU PROS.5 GR CLAIMS CLIPPER GRP. DIS. REPORT (MONTEZUMA DIS.) WHEELER MINE FOSS MONARCH PITTSBURG MINE TIGER CLAIMS LADY WASHINGTON MINE. SILVER KING MINE UNAM. PROS. (LEE DIS.) FARRIS PROPERTY CT. LAWRENCE PROPERTY NBMG SAMP. SITE 2252

NBMG SAMP. SITE 2959 BUNKER HILL MINE NBMG SAMP. SITE 3839 JOSHUA CLAIMS

GOLD TRAIL GRP.

STINGER CLAIMS FLORENCE TALAPOOSA DEPOSIT NBMG SAMP. LOCATION 1753. SANGER MINE MC CULLEY PROS.S RETURN MINE

GOLD STRIKE SILVER SULPHIDE GRP. MOBILE & EQUALITY PATENTED MINE JIM CLAIMS STARLIGHT CLAIMS COSTA PROS. TEICHART MINE MACINTOSH TUNNEL NBMG SAMP. SITE 2666 RIDDLE LEASE

SHADOW GRP. HOLLY EXTENSION MINE HULLEY-LOGAN

PORTLAND MINE FRANZ HAMMEL MINE SECTION 10 WASHOE CLAIM MAGPIE PROS. TROY MINE PROVIDENTIAL MINE SOUTH AMERICAN MINE MIDAS MINE

NEV. CROWN DESERT KING MINE RUBY-JUNE ROSE GRP. SILVER ACE MINE

OCCIDENTAL MINE JUNIPER MINE GOLD FEVER CLAIM

SILVER BELL CLAIMS BRADSHAW MINE SAVAL CANYON DEPOSIT (JERRITT SECRET PASS ZONE ELLENDALE MINE ARROWHEAD CONSOLIDATED MINES HIGHLAND QUEEN MINE

MEADOW VALLEY NO. 5 BERNICE MINE

S.E.D. CLAIMS NEVADA CONSOLIDATED MINE PET CLAIM WHITMORE PROPERTY NEVADA CLAMS

LAKEVIEW MINE GREAT BEND MINE NBMG SAMP. SITE 2764 KINGLAND PROS.S GREEN MINE

BLUE CLAIMS HAM-AND OR CURLEY LEASE PROS. PIT LUCKY-BOY DIVIDE MINING CO. CLAIMS LAST CHANCE MINE BIG HORN EXTENSION CLAIMS KINSEY CLAIMS

LANSING PROS.

WINDLASS MINE J AND B MINE UVADA TUNNEL LUCKY JIM NOS. 1-8 AND APEX CLAIMS SILVER REEF PROS. NBMG SAMP. SITE 2815 KEYSTONE NEVADA MINE YANKEE MINE NBMG SAMP. SITE 2972

NBMG SAMP. SITE 2623 CHEFOO TUNNEL FOLEY'S & PALMETTO CO. PRSPTS MARYANN MINE

RELIANCE MINE

YUBA EAST MINE SILVER RANGE MINES CO DEPOSITS ANTELOPE MINE NBMG SAMP. SITE 2942 KEYSTONE MINES

PEACOCK-ECKMAN

CUMBERLAND GRP.

GOLD CRATER MINE BUMBLEBEE-GREY HORSE PROS. LUCKY STRIKE CLAIM CATLIN GRP.

GIANT CLAIMS

IRON HAT MINE

WILLIAMS PROS.

UNKN. ADITS AND OPEN STOPES BULL DOG JACK MINE

SILVER CHIEF PROS.S ESMERALDA VEIN WORKINGS PROS. MOUNTAIN KING CLAIM

VANDERBILT TUNNEL UNKN. PROS.

UNKN. PROS.S CROFOOT PROJECT TRUE FISSURE G.M. CO. 16-TO-1 MINE & MILL JOHNNY FOUR GENERATIONS MINE NE SECTION 23 PROS.S

Gold-Silver-Bearing Occurrence Site Name or Property Owner (sites are randomly labelled)

Size Unreported Occurrences (n=244)Small Size Occurrences (n=2269)Big (Large and Medium) Size Occurrences (n=177)

Prior Probability = 0.0095
Figure D.2. Primary occurrence-type 7-layer mineral potential model the distribution of (A) elevated favorability mineral potential areasin comparison to (B)the

distribution of known primary gold-silver-bearing occurrences(n=2690). The occurrences are classified (quantile-based) by their associated posterior probabilities,

which were calculated using theWOE7-layer mineral potentialmodel and appended to the points. The top four posterior probability intervals of the point map (red,

oranges, and yellow) approximately correspond in value to the categories represented in the elevated favorability mineral potential map. The distribution of

occurrences with high posterior probabilities (map B) is generally coincident with areas that are predicted by the model to have elevated mineral potential

favorability (mapA). Note,for example,the coincident distribution highlighted by the dashed lines.

C

A. B.
ion of posterior probabilities associated with big (large, medium), small, and unknown size sedimentary rock-hosted occurrences. D25

Posterior Probabilities Appended to Sedimentary Rock-Hosted Gold-Sliver-Bearing Occurrences:

Calculated Using Sedimentary Rock-Hosted 8-Layer Mineral Potential Model

GENERATOR HILL DEPO.

NORTH GENERATOR HILL DEPO.

CANDELARIA MINE

GOLD PICK DEPO.

CORTEZ GOLD MINE

POST DEPO.

SOUTH BULLION DEPO.

RABBIT CREEK MINE

MEIKLE MINE

STONEHOUSE DEPO.

ATLANTA MINE

COVE MINE

WINDFALL MINE

LANTERN DEPO.

DEEP STAR DEPO.

BULLION-MONARCH OPEN PIT MINE

PREBLE MINE

MAC RIDGE DEPO. (BIG SPRINGS)

FELIX CANYON

TRIPLET GULCH PROJECT

BUFFALO VALLEY MINE

GOLD RIDGE DEPO.

NORTH SAMMY CREEK DEPO. (BIG SPRINGS)

NORTHERN LIGHTS DEPO.

ELDER CREEK MINE

HORSE CANYON MINE

CAPSTONE DEPO.

WOOD GULCH MINE

AUSTIN GOLD VENT. AU MINE (QUITO MINE)

REED DEPO. (FONDAWAY CANYON)

NORTH SELOX CLAIMS.

GREEN SPRINGS MINE

MAG DEPO.

MILL CREEK DEPO.

STEIGMEYER PROP. (SHALE PIT MINE)

NIGHTHAWK RIDGE MINE (EASY JUNIOR)

STANDARD MINE (GOLD STANDARD)

SANTA FE GOLD MINE

RELIEF CANYON MINE

STAR POINTER MINE (& ROBINSON MINE)

SOUTH MOUTH DEPO. (FONDAWAY CANYON)

ALLIGATOR RIDGE MINE

LONG LAC DEPO.

Gold-Silver-Bearing Occurrence Site Name or Property Owner (sites are randomly labelled)

Size Unreported Occurrences (n=3)Small Size Occurrences (n=57)Big (Large and Medium) Size Occurrences (n=38)

Prior Probability = 0.0003
A. B.

Figure D.4. (A)

(B)

Sedimentary rock-hosted occurrence-type 8-layer mineral potential model the distribution of elevated favorability mineral potential areas in

comparison to the distribution of known sedimentary rock-hosted occurrences (n=98). In general, the distribution of occurrences with high posterior

probabilities (map B) is coincident with areas that are predicted by the model to have elevated mineral potential favorability (map A). Note, for example, the

coincident distribution highlighted by the dashed lines. See FigureD.2 caption for additional information.

C
ution of posterior probabilities associated with big (large and medium), small, and unknown size volcanic rock-hosted occurrences. D27

Posterior Probabilities Appended to Volcanic Rock-Hosted Gold-Sliver-Bearing Occurrences:

Calculated Using Volcanic Rock-Hosted 7-Layer Mineral Potential Model

ROUND MOUNTAIN GOLD MINE

BUCKEYE AND OHIO

EIRA

NEVADA WONDER MINE

NAVAJO

BLUFF MINE

KINNEY PROS.

LAGUNA GRP.

KNICKERBOCKER MINE

ALTA SHAFT

G U B CLAIMS

HAILSTONE GRP.

AMERICAN FLAT CANYON MINE

PAYMASTER

SOUTH COMSTOCK MINE

DOUGLAS GRP. (CAMP DOUGLAS ?)

GOLDFIELD-BELMONT MINE

BUSTER G.M. CO.

UNAM. PROS. (TRUCKEE DISTRICT)

S.E.D. CLAIMS

LIBERTY MINE (REPRESENTING SAN ANTONE DIST.)

GOLD ZONE DIVIDE MINING CO. PROP.

MAHONEY MINE

ATLANTA MINES CO. CLAIMS

MAZUMA HILLS MINE

KENO CLAIMS

QUARTZITE AND BLACK BUTTE MINES

DEVANEY PROP.

BUCKSKIN NATIONAL MINE

BLACK BOY MINE

RENO MAY

MOHAWK MINE (ARGENTITE MINE)

BEN HUR PROS.

NEVADA HILLS FLORENCE MINE

TRINITY SILVER MINE

PET CLAIM

NBMG SAMPLE SITE 1480

NBMG SAMPLE SITE 788 UNAM. PROS.

SEVEN TROUGHS DISTRICT

KIM BLAIMS, NO.S 1-9

UNAM. PROS. (JESSUP DISTRICT)

WILLARD GRP. (NORTH)

PAN CLAIM

STONE HOUSE CANYON MINE

UNAM. PROS. #5

ARKELL MINE

KING TONOPAH MINE

KOSSUTH MINE

STEINER MINE

BLUE DRAGON NOS. 1-3 PROS.

MIZPAH MINE

PIONEER MINE

PYRAMID MINE

SILVER GLANCE GRP. (SILVER BOW DIST.)

AMALGAMATED MINE

NBMG SAMPLE SITE 3941

WIND MOUNTAIN MINE

TUSCARORA MINE

OKEY DAVIS MINE

REDLITE CLAIMS

MOTHER LODE MINE

MAMMOTH PROS.

BIG FOUR MINE

RUBY-JUNE ROSE GRP.

GOLCONDA-GOLD WEDGE GRP.

BUNCE PROS.

WATER WITCH MINE

WEST SINTER DEPO.

CHARLEY ROSS MINE

BELLE ISLE

BUFFALO HUMP MINE

BOVARD - RAND DISTRICT

SAM JACK GRP.

APRIL FOOL MINE

SILVER CENTER MINE

UTAH SPUR MINE

DEER PROS.

COEUR D'ALENE-JARBIDGE G.M.CO.

DULUTH MINE

GEIGER SHAFT

COYOTE PROS.

ST. PAUL

MAMMOTH MINE

HATCH MINE

WOMENS RIGHT GRP.

NEVADA RAND

DAYTON MINE

CROFOOT PROJECT

AG PROS. (RAGGED TOP DISTRICT)

Gold-Silver-Bearing Occurrence Site Name or Property Owner (sites are randomly labelled)

Size Unreported Occurrences (n=22)Small Size Occurrences (n=317)Big (Large and Medium) Size Occurrences (n=76)

Prior Probability = 0.0015
Approximate limits of a corridor region within

which most of the volcanic rock-hosted

occurrences lie. This corridor generally trends

parallel to the Humboldt gravity zone and the

Walker Lane shear zone, and is roughly

coincident to the extent of these zones.

A. B.

Figure D.6. (A)

(B)

Volcanic rock-hosted occurrence-type 7-layer mineral potential model the distribution of elevated favorability mineral potential areas in

comparison to the distribution of known volcanic rock-hosted occurrences(n=415). In general, the distribution of occurrences with high posterior probabilities

(map B) is coincident with areas that are predicted by the model to have elevated mineral potential favorability (map A). Note, for example, the coincident

distribution highlighted by the dashed lines. See FigureD.2captionforadditionalinformation.

C