Geology and Mineral 

Resource Assessment of the 

Venezuelan Guayana Shield

U.S. Geological Surve\ 

Corporation Venezolana de
Cover. Rocks of the Early and Middle Proterozoic Roraima Formation, Canaima 

National Park in the Gran Sabana region of Estado Bolfvar, Venezuela. View is southsoutheast toward Cerro Venado (left background). The waterfalls in the center foreground 

are Salto Hacha on Rio Carrao; Canaima lagoon is in the foreground. Photograph by 

Dennis P. Cox, 1987.
Geology and Mineral Resource Assessment 

of the Venezuelan Guayana Shield

By U.S. Geological Survey and Corporacion Venezolana de Guayana, 

Tecnica Minera, C.A.

U.S. GEOLOGICAL SURVEY BULLETIN 2062

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

For sale by

USGS Map Distribution

Box 25286, Building 810

Denver Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Library of Congress Cataloging-in-Publication Data

Geological Survey (U.S.)

Geology and mineral resource assessment of the Venezuelan Guayana Shield / by U.S. Geological 

Survey and Corporaci6n Venezolana de Guayana, T6cnica Minera, C.A. 

p. cm. (U.S. Geological Survey bulletin ; 2062)

Includes bibliographical references.

Supt. of Docs, no.: GPO119.3:2062

1. Geology Venezuela 2. Minerals Venezuela. 3. Guayana Shield. I. Corporaci6n Venezolana de Guayana. Tccnica 

Minera. II. Title. HI. Series. 

QE75.B9 no. 2062 

[QE251] 

557.3 s dc20

[558.7] 9328283

CIP
FOREWORD

The importance of this volume on the mineral resources of Venezuela lies not only 

in its delineation of the vast mineral richness of the Guayana Shield but also in its illustration of the productive cooperation between the Corporaci6n Venezolana de Guayana 

and the U.S. Geological Survey. Geologic research described herein regarding the wise 

use of mineral resources is a model of cooperative scientific effort that can be used to 

assist the making of decisions at the highest levels of the government. This information 

can provide the basis for long-range mineral production and land use policy, for planning 

strategies for mineral exploration and development, for the direction of future research, 

and for expanding the inventory of mineral deposits of Venezuela. Special features of this 

cooperative project included the creation of a Mineral Resources Data System file and the 

use of mineral-deposit models to estimate the mineral resources of mineralized areas, 

both known and suspected, in the Guayana Shield and adjacent areas.

This project was funded by the Corporaci6n Venezolana de Guayana. It incorporated aspects of technology transfer, including specialized training for Venezuelan personnel, the acquisition of analytical equipment, and an inventory of mineral resources of the 

Guayana Shield. Field and laboratory studies by personnel of the U.S. Geological Survey 

and the Corporaci6n Venezolana de Guayana were started immediately after a Memorandum of Understanding, which described the cooperation, and a Project Implementation 

Plan were signed in 1987. These studies were completed in December 1991.

This compilation should be viewed as the necessary first step in the development of 

a long-range policy for mineral resource exploration, mineral production, and land use in 

Venezuela. Personnel of the Coporaci6n Venezolana de Guayana and the U.S. Geological 

Survey are to be complimented for the dedication and the outstanding cooperative spirit 

that they brought to this work and that made this publication possible.

Dallas Peck

Director, U.S. Geological Survey
CONTENTS

Summary............................................................................................................................... 1

Introduction, by Jeffrey C. Wynn, Floyd Gray, and Norman J Page................................... 3

Geology of the Venezuelan Guayana Shield, by Dennis P. Cox, Jeffrey C. Wynn,

Gary B. Sidder, and Norman J Page................................................................................ 9

Geophysics of the Venezuelan Guayana Shield, by Jeffrey C. Wynn.................................. 17

Mines, Prospects, and Occurrences of the Venezuelan Guayana Shield, by G.J. Orris, 

Norman J Page, Karen Sue Bolm, Floyd Gray, William E. Brooks, 

Marguerite M. Carbonaro, and Richard Kibbe................................................................. 29

Mineral Resource Assessment of the Venezuelan Guayana Shield..................................... 55

Introduction, by Floyd Gray, Dennis P. Cox, G.J. Orris, Norman J Page,

Jeffrey C. Wynn, William E. Brooks, and J.D. Bliss........................................... 55

Deposits in Deeply Eroded Terrane............................................................................ 58

Algoma- and Superior-Type Iron Deposits, by Dennis P. Cox,

Norman J Page, and Floyd Gray .................................................................... 58

Sedimentary Manganese Deposits, by Floyd Gray and Norman J Page............... 61

Low-Sulfide Gold-Quartz Veins, by U.S. Geological Survey and

Corporaci6n Venezolana de Guayana, T6cnica Minera, C.A......................... 63

Kuroko-Type Massive Sulfide Deposits, by Dennis P. Cox................................. 69

Synorogenic-Synvolcanic Nickel-Copper Deposits and Related Platinum

Deposits, by Floyd Gray and Norman J Page................................................. 70

Dolomite-Marble Deposits, by G.J. Orris............................................................. 72

Deposits Formed in Supracrustal Rocks and Epizonal Plutons.................................. 73

Carbonatite Deposits, by William E. Brooks, G.J. Orris, and Jeffrey C. Wynn... 73 

Thorite-Rare Earth Element Veins, by Floyd Gray.............................................. 75

Diamond-Bearing Kimberlite Pipes, by Floyd Gray............................................ 75

Tin Greisen Deposits, by Dennis P. Cox and William E. Brooks........................ 77

Rhyolite-Hosted Tin Deposits, by Dennis P. Cox................................................ 78

Porphyry Copper Deposits, by Dennis P. Cox...................................................... 78

Veins of Plutonic and Volcanic Association, by Dennis P. Cox.......................... 79

Volcanic-Hosted Magnetite Deposits, by Dennis P. Cox..................................... 79

Unconformity Uranium-Gold Deposits, by William E. Brooks........................... 80

Quartz-Pebble Conglomerate Gold-Uranium Deposits, by William E. Brooks... 80 

Deposits Formed by Surficial Processes..................................................................... 82

Laterite-Type Bauxite Deposits, by Floyd Gray................................................... 82

Residual Kaolin Deposits, by G.J. Orris and Floyd Gray..................................... 84

Sedimentary Kaolin Deposits, by G.J. Orris......................................................... 85

Placer Gold, by G.J. Orris..................................................................................... 86

Placer Diamond, by Floyd Gray and G.J. Orris.................................................... 86

Tin-(Rare Earth Element, Niobium-Tantalum) Placers, by William E. Brooks

and Floyd Gray............................................................................................... 88

Placer Titanium and Other Heavy Minerals, by Jeffrey C. Wynn........................ 89

Conclusion............................................................................................................................ 91

References Cited................................................................................................................... 92

Appendix Commodity Deposits Index for the Precambrian Shield of Northeastern

South America including Venezuela, Guyana, Suriname, and Northern Brazil.............. 99
VI CONTENTS

PLATES

[Plates are in pocket]

1. Map showing selected geographic features of the Venezuelan Guayana Shield, by Paul G. Schruben, Floyd Gray, and 

Jeffrey C. Wynn.

2. Geologic and tectonic map of the Venezuelan Guayana Shield, compiled by Jeffrey C. Wynn, Dennis P. Cox, 

Floyd Gray, and Paul G. Schruben.

3. Side-looking airborne radar image of the Venezuelan Guayana Shield, by U.S. Geological Survey.

4. Simple Bouguer gravity anomaly map of the Venezuelan Guayana Shield, by Jeffrey C. Wynn and Kevin Bond.

5. Map showing distribution of mines and mineral occurrences in the Venezuelan Guayana Shield, by Norman J Page, 

GJ. Orris, Floyd Gray, and Karen Sue Bolm.

6. Map showing distribution of mineral occurrences in northeastern South America, by Norman J Page, Floyd Gray, and 

Karen Sue Bolm.

7. Map showing permissive domains for Algoma iron deposits, sedimentary manganese deposits, low-sulfide gold-quartz 

veins, tin greisen deposits, porphyry copper deposits, and volcanic-hosted magnetite deposits in the Venezuelan Guayana 

Shield, by Floyd Gray, Norman J Page, Dennis P. Cox, GJ. Orris, Jeffrey C. Wynn, and William E. Brooks.

8. Map showing permissive domains for Kuroko-type massive sulfide deposits, synorogenic-synvolcanic nickel-copper 

deposits and related platinum deposits, carbonatite deposits, diamond-bearing kimberlite pipes, and sedimentary kaolin 

deposits, by GJ. Orris, Floyd Gray, Dennis P. Cox, Norman J Page, William E. Brooks, and Jeffrey C. Wynn.

FIGURES

1-5. Maps showing:

1. Location of Precambrian Guayana Shield..................................................................................................... 4

2. Venezuelan Guayana Shield and quadrangles that cover area at a scale of 1:500,000.................................. 5

3. Gravity coverage of Venezuelan Guayana Shield.......................................................................................... 20

4. Aeromagnetic coverage of Venezuelan Guayana Shield............................................................................... 21

5. Contours of total field aeromagnetic data in area of unusual, deep-source negative magnetic anomaly

south of Puerto Ayacucho, Territorio Federal Amazonas.............................................................................. 23

6-8. Models of:

6. Negative magnetic anomaly south of Puerto Ayacucho, Territorio Federal Amazonas................................ 24

7. Highly magnetic, shallowly dipping peridotite body at Piston de Uroy gold prospect, Rfo Chicanan,

EstadoBolfvar......................................_ 24

8. Moderately magnetic, steeply dipping dike (probably diabase) south of Cerro Autana, Territorio

Federal Amazonas......................................................................................................................................... 24

9. Map showing contours of simple Bouguer gravity anomaly in area of deep high-density body east of

Rfo Orinoco and south of Puerto Ayacucho, Territorio Federal Amazonas........................................................... 26

10. Model of simple Bouguer gravity anomaly in area of deep high-density body east of Rfo Orinoco and

south of Puerto Ayachuco, Territorio Federal Amazonas...................................................................................... 27

11. Tonnage and grade diagrams for Algoma iron and Superior iron deposits............................................................ 59

12. Photograph of steeply plunging recumbent limb of iron ore, El Pao deposit, Estado Bolivar............................... 60

13, 14. Cross sections showing:

13. Iron orebodies of Cerro Bolfvar, Venezuela.................................................................................................. 60

14. Relation between sedimentary facies and sedimentary manganese deposits................................................ 61

15. Tonnage and grade diagrams for sedimentary manganese deposits....................................................................... 62

16. Diagrams showing tonnage and grade model curves for low-sulfide gold-quartz vein deposits ........................... 63

17. Photograph showing Colombia and Las Americas gold-vein systems, El Callao mine, Estado Bolfvar............... 64

18. Sketch map of area of Lo Increfble gold deposit, Estado Bolfvar.......................................................................... 64

19. Photographs showing saprolite-gold system, kaolinite-bearing surficial deposit, Kilometre 88,

Estado Bolfvar........................................................................................................................................................ 67

20. Schematic cross section of kuroko-type massive sulfide deposit........................................................................... 68
CONTENTS VII

21,22. Diagrams showing:

21. Tonnage and grade model curves for kuroko-type massive sulfide deposits................................................. 69

22. Precious-metal grades of kuroko-type massive sulfide deposits................................................................... 70

23. Tonnage and grade for synorogenic-synvolcanic nickel-copper deposits .............................................................. 71

24. Diagrams showing byproduct grades of synorogenic-synvolcanic nickel-copper deposits ................................... 71

25. Tonnage and grade diagrams for carbonatite deposits............................................................................................ 74

26. Idealized model showing relations between thorium-rare earth element veins and alkalic rocks and

carbonatite.............................................................................................................................................................. 75

27. Tonnage and grade diagrams for thorium-rare earth element vein deposits........................................................... 76

28. Cross section of typical kimberlite pipe................................................................................................................. 76

29. Tonnage and grade diagrams for diamond kimberlite pipe deposits...................................................................... 77

30. Diagrams showing tonnage and grade model curves for laterite-type bauxite deposits......................................... 82

31. Photograph of Los Pijiguaos bauxite deposit, Estado Bolfvar................................................................................ 84

TABLES

1. Field areas where cooperative geologic mapping took place, Venezuelan Guayana Shield........................................ 6

2. Geophysical characteristics of geologic map units in the Venezuelan Guayana Shield.............................................. 18

3. Mines, prospects, and mineral occurrences of the Venezuelan Guayana Shield.......................................................... 30

4. Known low-sulfide gold-quartz vein deposits of the Venezuelan Guayana Shield..................................................... 43

5. Known placer gold deposits of the Venezuelan Guayana Shield................................................................................. 45

6. Known diamond deposits of the Venezuelan Guayana Shield..................................................................................... 46

7. Known iron and manganese deposits of the Venezuelan Guayana Shield................................................................... 47

8. Known bauxite deposits of the Venezuelan Guayana Shield....................................................................................... 48

9. Alphabetical listing of mines, prospects, and mineral occurrences, Venezuelan Guayana Shield.............................. 49

10. Topographic surfaces of the Guayana Shield............................................................................................................... 83

11. Estimates of undiscovered deposit types, Venezuelan Guayana Shield....................................................................... 91
GEOLOGY AND MINERAL RESOURCE ASSESSMENT 

OF THE VENEZUELAN GUAYANA SHIELD

By U.S. Geological Survey and Corporaci6n Venezolana de Guayana,

Tecnica Minera, C.A.

SUMMARY

The Venezuelan Guayana Shield comprises the Precambrian terrane of the southern and eastern half of Venezuela and encompasses more than 415,000 km2. This report 

describes the geology and geophysics of the Venezuelan 

Guayana Shield and presents a mineral resource assessment 

of the area. It includes a geographic map, geologic and 

tectonic map, simple Bouguer gravity anomaly map, sidelooking airborne radar image, two mineral occurrences 

maps, and two permissive domain maps. The report is a 

product of a cooperative project on the Venezuelan Guayana Shield between the U.S. Geological Survey and the 

Corporaci6n Venezolana de Guayana, Tecnica Minera, 

C.A., that was begun in 1987 and concluded in 1991.

GEOLOGY

The Guayana Shield is the northern part of the 

Amazonian craton in South America and includes parts of 

Venezuela, Colombia, Guyana, Brazil, Suriname, and 

French Guiana. The Guapore, or Western Central Brazil 

Shield, south of the Amazon River basin, forms the southern part of the Amazonian craton.

The Guayana Shield in Venezuela consists of five 

lithotectonic provinces: (1) Archean amphibolite- to 

granulite-facies gneiss terrane (Imataca Complex); (2) 

Early Proterozoic greenstone-granite terrane(s); (3) Early 

Proterozoic unmetamorphosed volcano-plutonic complex; 

(4) Early to Middle Proterozoic continental sedimentary 

rocks; and (5) Middle Proterozoic anorogenic rapakivi-type 

granite.

The Archean Imataca Complex consists of gneiss and 

granulite and minor dolomitic marble and banded iron 

formation, typically isoclinally folded. The grade of metamorphism in the Imataca Complex ranges from amphibolite facies in the southwestern part of the belt to granulite 

facies in the northeastern part. The Imataca Complex was 

intruded by granitic rocks, and injection gneiss and migmatite were developed during the pre-Trans-Amazonian

tectonomagmatic event between about 2,800 and 2,700 

Ma. South of the Imataca terrane mountainous ridges of 

subaqueous sequences of tholeiitic mafic volcanic rocks 

between large domelike masses of granite of the Supamo 

Complex resemble both Archean and Early Proterozoic 

greenstone belts. Although the ages of these belts are 

poorly constrained, they are considered Early Proterozoic 

in this study. Overlying the Early Proterozoic rocks is a 

conformable, broad area of tholeiitic to calc-alkaline basalt 

to rhyolite and an interval of turbiditic graywacke, volcaniclastic rocks, and chemical sedimentary rocks. Ages 

obtained for the Supamo Complex range from 2,230 to 

2,050 Ma; the best ages for the greenstone belts range 

from 2,250 to 2,100 Ma. The Trans-Amazonian orogeny 

represents a period of continental collision between about 

2,150 and 1,960 Ma; during this orogeny the Imataca and 

the greenstone-granite terranes were deformed and metamorphosed. Unmetamorphosed volcanic and plutonic 

rocks of the Cuchivero Group represent postcollisional, 

post-Trans-Amazonian magmatism between about 1,930 

and 1,790 Ma in the Guayana Shield from Venezuela to 

Suriname. Unmetamorphosed, posttectonic sedimentary 

rocks of the Roraima Group were deposited in fluvial, 

deltaic, shallow-marine, and lacustrine or epicontinental 

environments and are about 1,670 Ma in age. Middle Proterozoic, nontectonized rapakivi granite characteristic of 

the Parguaza Granite is about 1,500 Ma. At least two ages 

of tholeiitic diabase dikes intrude rocks of the Guayana 

Shield.

GEOPHYSICS

Geophysical data available for the southern and eastern parts of Venezuela include aeromagnetic data covering about 60 percent of the Venezuelan part of the 

Guayana Shield, gravity data covering about 50 percent, 

and aeroradiometric data covering less than 40 percent. 

Side-looking airborne radar (SLAR) imagery is available
GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

for the entire Venezuelan Guayana Shield. In areas where 

the geologic coverage is poorest, such as Territorio 

Federal Amazonas, the geophysical data were complete 

enough to permit production of a new geologic and tectonic map that is reasonably even in detail throughout the 

entire Venezuelan part of the shield.

MINERAL OCCURRENCES

More than 450 mines, prospects, and mineral occurrences were documented in the Venezuelan Guayana Shield, 

of which more than 200 are gold. Information was compiled 

for more than 100 diamond mines and occurrences, 14 ironmanganese deposits and prospects, 35 banded iron formation deposits, and 40 bauxite occurrences. Other reported 

mineral occurrences include manganese, alluvial tin, 

titanium, barite, kaolin, dolomite, sand and gravel, molybdenum, uranium, rare earth element minerals, and tungsten.

MINERAL DEPOSITS

For purposes of mineral resource assessment, known 

mineral deposits in the Venezuelan Guayana Shield were 

classified by mineral deposit type. Most of these deposit 

types are defined by tonnage and grade distributions based 

on known deposits. Although deposits can be classified 

using descriptive models, they must also conform to 

tonnage-grade models. Known deposit types include 

Algoma- and Superior-type iron, laterite-type bauxite, sedimentary manganese, low-sulfide gold-quartz veins, placer 

gold, placer diamond, diamond-bearing kimberlite pipes, 

residual and sedimentary kaolin, and carbonatite. The mineral resource assessment also considered types of deposits 

that could be present given the geology of the Venezuelan 

Guayana Shield but which have not yet been found. For 

example, no deposits of kuroko-type massive sulfides have 

been discovered in Venezuela, possibly because of the deep

weathering and forest coverage, but the terrane permissive 

for their occurrence is widespread. Other deposit types for 

which the geology is permissive include dolomitic marble, 

thorite-rare earth element veins, tin greisen, veins of plutonic and volcanic association, volcanic-hosted magnetite, 

unconformity-hosted uranium, synvolcanic-synorogenic 

nickel-copper-platinum, rhyolite-hosted tin, tin placers, 

placer titanium, and porphyry copper.

UNDISCOVERED DEPOSITS

In an attempt to quantify our knowledge about the 

mineral resources of the Venezuelan Guayana Shield, we 

made probabilistic estimates of numbers of undiscovered 

deposits by deposit type. Although there are more than 

125 known low-sulfide gold-quartz vein occurrences in the 

shield forming at least 30 deposits that fit the model, we 

estimate that there is a 50 percent probability of finding 40 

additional deposits having a median tonnage of 30,000 

tonnes (metric tons) of ore. One carbonatite intrusive body 

(Cerro Impacto) is known to be present in the Venezuelan 

Guayana Shield; we estimate that there is at least a 50 

percent probability of finding five or more additional carbonatite bodies having a median tonnage of niobium- and 

rare earth element-bearing zones of about 60 million 

tonnes. We also estimate that there is a 50 percent probability that 26 or more Algoma-Superior iron deposits having a median ore tonnage of 170 million tonnes remain to 

be found in Venezuela. We were unable to make quantitative estimates for other deposit types because of a lack of 

detailed information such as reconnaissance geochemical 

maps showing the distribution of elements including Cu, 

Pb, Zn, Ag, Mo, Be, Nb, Ba, and rare earth elements. 

Lack of geophysical data for the southeastern and northwestern parts of the shield and lack of regional geomorphological maps also hampered our analysis of additional 

deposit types.
INTRODUCTION

By Jeffrey C. Wynn, Hoyd Gray, and Norman J Page

The Guayana Shield is the northernmost ArcheanMiddle Proterozoic craton in South America. It underlies 

most of Guyana, Suriname, and French Guiana, as well as 

parts of neighboring Brazil and Colombia (fig. 1). The 

Venezuelan Guayana Shield underlies all of Estado Bolfvar and Territorio Federal Amazonas, as well as that part 

of Estado Delta Amacuro south of the Rfo Orinoco (pi. 1). 

It encompasses more than 415,000 km2 and is covered primarily by jungle and savannah. The northern part of the 

shield near the Rfo Orinoco has relatively flat topography 

and constitutes the southern margin of the vast plains 

region referred to as Los Llanos; the Gran Sabana makes 

up most of the southeastern part of the shield and includes 

high, flat-topped mountains called tepuis (singular, tepuy). 

Between these two regions are extensive areas of gently 

rolling topography covered with jungle. Territorio Federal 

Amazonas makes up the southwestern half of the Venezuelan Guayana Shield and is a combination of rugged 

mountainous regions and flat, commonly inundated plains 

covered by jungle.

This report describes a mineral resource assessment 

of the Venezuelan Guayana Shield that was done using 

existing information supplemented extensively by new 

geologic, geophysical, and geochemical data generated 

during a cooperative project between the U.S. Geological 

Survey (USGS) and Te"cnica Minera, C.A. (TECMIN), a 

part of the Corporaci6n Venezolana de Guayana (CVG).

Representatives of the CVG first approached the 

USGS in 1985 to discuss a joint project of mineral 

resource assessment and mineral exploration. Ensuing discussions and negotiations resulted in the signing of a 

Memorandum of Understanding in February 1987, and 

both groups began the planning of a cooperative project to 

investigate geoscience aspects of the Venezuelan Guayana 

Shield, with emphasis on mineral deposits. An advisory 

group of two resident USGS scientists (later three) and as 

many as ten visiting scientists per year was established to 

work with CVG-TECMIN. This group was originally 

based in Puerto Ordaz, Venezuela, but for two years maintained a one-man office in Ciudad Bolivar, Venezuela. 

The USGS worked principally with two groups from 

CVG-TECMIN: Grupo de Inventario (Inventory) and 

Grupo de Prospecci6n (Mineral Exploration). The 5-year

project was funded by the CVG and concluded at the end 

of December 1991.

PROJECT OBJECTIVES

The primary objectives of the USGS-TECMIN cooperative project were threefold:

1. To carry out regional geologic mapping in conjunction with the CVG-TECMIN Inventory Group in 

order to produce a new geologic and tectonic framework 

map of the Venezuelan Guayana Shield.

2. To carry out mineral exploration and site evaluations in conjunction with the CVG-TECMIN Mineral 

Exploration Group.

3. To perform a mineral resource inventory of the 

Venezuelan Guayana Shield by identifying domains that 

are permissive for the occurrence of mineral deposit types 

and estimating the number of undiscovered mineral 

deposits.

In addition, the cooperative project included specialized training for Venezuelan personnel and technology 

transfer components.

ACCOMPLISHMENTS

A total of 16 USGS scientists worked with CVGTECMIN geologists at more than 60 mines, concessions, 

and mineral prospects in 23 areas and mining districts in 

Venezuela (table 1).

Previous studies pertinent to the project include geologic, metallogenic, and geophysical investigations. At 

least two countrywide geologic maps of Venezuela have 

been published (Bellizzia and others, 1976; Bellizzia, 

Pimentel de Bellizzia, and Mufioz, 1981). During the 

course of the USGS-TECMIN cooperative project, extensive new mapping from USGS-TECMIN site visits and 

information obtained from geologic compilations made by 

the CVG-TECMIN Inventory Group were acquired. These 

were combined with side-looking airborne radar (SLAR) 

images, only some of which were available to previous 

workers, and with aeromagnetic (Herrero and Navarro, 

1989) and gravity data (Graterol, 1988) to assemble a new
GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

70° 65° 60° 55° 50°

\ _._.___^

L T-1"''1 *^ x_^ ^ _

o° 

Figure 1. Location of the Precambrian Guayana Shield (dashed line) of northern South America.

1:1,000,000-scale geologic and tectonic map of the Venezuelan Guayana Shield (pi. 2).

Twelve l:500,000-scale quadrangle maps (fig. 2, pi. 

2) were compiled for the area of Venezuela south of the 

Rio Orinoco including Estado Bolivar, Territorio Federal 

Amazonas, and Estado Delta Amacuro. These maps comprise the most recent geologic and geophysical data and 

interpretations gathered by CVG-TECMIN and USGS. All 

of the information on these maps, including drainage and 

cultural features (pi. 1), was digitized and reprojected to 

1:1,000,000 scale using GSMAP (Seiner and Taylor, 

1991), a USGS-authored Geographic Information System 

(GIS).

A side-looking airborne radar (SLAR) survey was 

carried out by Goodyear Aeroservices, C.A., in two stages 

in 1971 and 1977. Images were acquired originally as 

l:100,000-scale flight-strips and compiled and released in 

Venezuela at 1:250,000 scale. These images were subsequently compiled by the USGS at 1:1,000,000 scale (pi. 3) 

and printed on a Venezuelan projection called Proyecci6n 

C6nico Secante Compensada, an equidistant conic projection using lat 4° N. and 9° N. as standard parallels and

long 66° W. as the central meridian. This projection is 

used in numerous Venezuelan cartographic products and 

was used as the standard for this study.

Simple Bouguer gravity anomaly data, provided by 

Professor Victor Graterol (Universidad de Sim6n Bolfvar, 

Caracas, Venezuela) and the U.S. Defense Mapping 

Agency, were assembled and edited, then plotted at 

1:1,000,000 scale (pi. 4). Aeromagnetic data for the Venezuelan Guayana Shield are incomplete, but information 

from available maps was incorporated into the assembly 

of the new geologic and tectonic map.

A mineral occurrence map for the Venezuela Guayana Shield (pi. 5) was prepared from a Mineral Resources 

Data System (MRDS) file assembled by USGS and CVGTECMIN. This map reflects more than 450 different mineral occurrence sites reported in the published and unpublished literature or identified in the field by cooperative 

project teams. For reference, a smaller scale map (pi. 6) 

was compiled using the MRDS database that shows the 

location of mineral occurrences in northeastern South 

America; the unedited MRDS data are given in the appendix. Inquiries about information stored in MRDS may be
INTRODUCTION

Puerto Ordaz \DELTAAMACURO

Boundary of Venezuelan 

Guayana Shield

200 KILOMETERS

Figure 2. The Venezuelan Guayana Shield and the quadrangles that cover the area at a scale of 1:500,000.

obtained from the regional MRDS representative at the 

U.S. Geological Survey, Minerals Information Office, 

Corbett Building, 340 North 6th Avenue, Tucson, Arizona 

85705-8325, or at the U.S. Geological Survey, 12201 

Sunrise Valley Drive, Reston, Virginia 22092.

Two metallogenic maps of Venezuela have been previously published (Bellizzia, Pimentel de Bellizzia, and 

Rodriguez, 1981; Rodriguez, 1986). During the course of

the USGS-TECMIN cooperative project, numerous additional field sites were visited and mineral occurrence sites 

identified. For a number of deposit types, sufficient 

geologic and mineral deposit information was available to 

allow a USGS-TECMIN assessment team to make estimates of the number of undiscovered deposits contained 

within the Venezuela Shield. Two maps showing permissive domains were developed (pis. 7, 8). These maps
GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Table 1. Field areas where cooperative geologic mapping 

took place, Venezuelan Guayana Shield.

[See plate 1 and tables 3 and 9]

__________________Field area_________""_________

La Flor-Carapo-La Esmeralda area

Cerro Bolivar-San Isidro iron mining district

El Pao-El Palmar iron-manganese mining district

El Callao-Lo Increible gold mining district

Bochinche area

Las Flores project area

Marwani area

Sierra Verdun-Cerro Piedra de Supamo

Anacoco Sur project area

Anacoco-Los Caribes area

Piston de Uroy-Rio Chivao-Cerro Arrendajo gold mining area

Venamo site

Los Pijiguaos bauxite mine

Kilometro 88-Las Cristinas gold mining district

1989-90 northern Amazonas survey

1988-89 southern Estado Bolivar survey 

Cavanayan-Camarata area of the northern Gran Sabana 

Cerro Roraima-Cerro Cuquenan tepuis 

Santa Elena de Uairen-Icabaru diamond mining district 

Caflo Yagua-Cerro Yapacana 1990 survey

Rio Negro-Brazo Casiquiare 1991 survey 

Rio Negro-Piedra de Cocuy 1991 survey 

Upata-El Silencio-El Manteco 1991 survey

identify regions or areas permissive for the occurrence of 

specific mineral deposit types and can be used to aid in 

the estimation of undiscovered mineral deposits in the 

Venezuelan Guayana Shield.

In addition to this report of assembled maps, texts, and 

tables, additional accomplishments resulting from the cooperative project include more than 70 internal reports, maps, 

and scientific publications released to CVG-TECMIN, 

some of which have been published by the USGS.

To assist in accomplishing the primary objectives of 

the cooperative project, equipment and instrumentation 

were purchased and installed in CVG-TECMIN facilities 

in Puerto Ordaz, Tumeremo, and Ciudad Bolivar. These 

items include computers, plotters, digitizers, and appropriate software, as well as an emission spectrograph and an 

atomic absorption system. An extensive sample preparation system and analytical support equipment were 

installed in the CVG-TECMIN facility in Tumeremo, 

including rock crushers, grinders, centrifuge, furnaces, and 

digital balance.

Also as part of the objectives of the cooperative 

project, seven formal short courses, each 1-4 weeks long, 

were taught in Venezuela (in Spanish), and numerous 

advisory training sessions for individual CVG-TECMIN 

professionals were held. One CVG-TECMIN professional 

spent 11 weeks in the United States training in the operation of the analytical equipment installed in Tumeremo.

Acknowledgments. Initial planning by Vicente 

Mendoza S. of T6cnica Minera, C.A. (CVG-TECMIN), 

and Richard Krushensky, Gary Raines, and Norman J 

Page of the U.S. Geological Survey (USGS) resulted in 

a 5-year project to do mineral resource assessment and 

regional geologic mapping of the Venezuelan Guayana 

Shield and technology transfer to CVG-TECMIN. 

Planning, coordination, and operation of the project were 

conducted by Darrell Herd, Floyd Gray, Norman J Page, 

and Jeffrey C. Wynn (USGS) and by Nestor Angulo, 

Juan Candelaria, Indalecia de Espinoza, Ce"ysar Guti6rrez, Jose" Gutie'rrez, Concepci6n Suarez, and Fernando 

Susach (CVG-TECMIN).

Numerous people from CVG-TECMIN were instrumental to this project by providing information and insight 

into the geology and mineral resources of the shield. They 

include Enrique Acosta, Juan Acosta, Margarita Alberdi, 

Jesus Arusp6n, Angel Baez, Cruz Briceno, Henry Bricefio, 

Antonio Brojanigo, Gloria Contreras, Freddy DaVila, 

Maria-Elena Delgado, Yasmin Estanga, Alf Ferndndez, 

Luis Franco, Andre"s Garcfa, Marta Garcfa, Acenk Guerro, 

Luis Guzmdn, Luisa Heredia, Yolanda L6pez, Glenda 

Lowry, F61ix Martfnez, Miguel Martinez, Giovanny Nava, 

Fernando Nunez, Jorge Penott, Freddy Prieto, Jacobo Quesada, Enot Quintana, Haid6e Rinc6n, Ramon Rincon, Ivan 

Rivero, Nelson Rivero, Ibel Romero, Edixon Sdlazar, 

Henry Sdnchez, Reyes Simoza, and Galo Ydnez. Additional CVG-TECMIN personnel whose work contributed 

to this compilation include Reinaldo Balazar, Virginia 

Behm, Rafael Borges, Henry Brito, Juan Candelaria, Maria 

Chacon, Eric Dumas, Maria Escauriaza, Victor Fernandez, 

Yajaira Fernandez-Gray, Antonio Ferroro, Jose" Freites, 

Silverio Galindo, Ce"sar Guti6rrez, Oliveros Jimenez, 

Victor Lopez, Elis Lugo, Luis Manzanilla, Iris Marcano, 

Hector Marquez, Pedro Mata, Jaime Molina, Abigail 

Morales, Ramon Olivares, Javier Parra, Pedro Petit, Rangel 

Petit, Carlos Pulido, Ine"s Rend6n, Jesus Rodriquez, Yesenia Sierra, Tommaso Tiosiani, Carlos Torrealba, and 

Alberto Torres. Other people in Venezuela who contributed to the completion of this project include Louis 

Gilloux (Bureau de Recherches G6ologiques et Minieres of 

France), Victor Graterol (Universidad de Sim6n Bolivar), 

Emilio Herrera (Ministerio de Energia y Minas), Pedro 

Lira (Ministerio de Energia y Minas), and Alfredo 

Menendez (consultant to CVG-TECMIN).

Colleagues in the USGS who contributed in this 

project include Lindreth Cordell, Warren Day, John 

Dohrenwend, Robert Earhart, Andrew Grosz, Steve Ludington, Sherman Marsh, Anne McCafferty, Barry Moring, 

Stephen Olmore, Robert Oscarson, Jeff Phillips, Gary 

Sidder, Jack Stewart, Richard Tosdal, and Kenneth Watts. 

Paul Schruben prepared the geologic-tectonic map, base 

map, and domain maps using the computer program 

ARC-INFO; Karen Bolm prepared the mines, prospects, 

and occurrences maps. An earlier version of this report
INTRODUCTION 7

(1991) was edited by G.J. Orris and Norman J Page of the carino by Judith Stoeser. Wayne Hawkins prepared the

U.S. Geological Survey's Center for Inter-American Min- maps and illustrations, Marie Melone typeset the book,

eral Resource Investigations (CIMRI) in Tucson, Arizona, Judith Stoeser prepared the tables, and Art Isom designed

and presented to officials of T6cnica Minera as a USGS the cover. Denny Welp and Robert Wells also assisted in

Administrative Report. This bulletin was edited con the preparation of the book.
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD

By Dennis P. Cox, Jeffrey C. Wynn, Gary B. Sidder, and Norman J Page

In this section we provide, as fully as possible, 

descriptions of the rock units shown on the geologic and 

tectonic framework map of the Venezuelan Guayana 

Shield (pi. 2), and we lay the basis for the discussion of 

the geologic environments permissive for mineral deposits 

that follows. We rely heavily on recent reviews of the 

geology of the Venezuelan Guayana Shield and its relation 

to the geology of the rest of the Guayana Shield by Sidder 

and Mendoza (1991, in press). Whereas we emphasize 

lithologic descriptions, Sidder and Mendoza present a 

broader view of the relation of major geologic features 

and the timing of erogenic events.

The rocks of the Venezuelan Guayana Shield can be 

divided generally into two older terranes and two younger 

sequences of sedimentary and igneous rocks. The oldest 

terrane comprises the Imataca Complex, Archean metamorphic rocks of granulite and amphibolite facies. The 

second terrane comprises the Early Proterozoic Supamo 

Complex and Pastora Supergroup and is made up of a 

granite-greenstone belt association and associated younger 

eugeosynclinal rocks. A transitional suprajacent sedimentary and volcanic sequence is composed of continental 

clastic rocks and tuff that may be metamorphosed, isoclinally folded, or, in the case of the Cuchivero Group and 

younger rocks, undeformed. The Cuchivero Group, the 

most important unit in this sequence, consists of voluminous ash-flow tuff and associated granitic plutonic rocks. 

A younger suprajacent sedimentary sequence comprises 

relatively undeformed Early to Middle Proterozoic continental clastic sedimentary rocks of the Roraima Group 

that cover large parts of the shield. Diabasic dikes and 

sills of at least two ages, between 1,850 and 1,650 Ma 

(Early Proterozoic) and between 210 and 200 Ma (Mesozoic), are present in all terranes and sedimentary 

sequences. Cenozoic deposits are mostly restricted to Tertiary delta sediments of the Rio Orinoco and floodplains 

of modern rivers.

IMATACA TERRANE

The Imataca Complex (unit Ai, pi. 2) is composed of 

quartz-feldspar gneiss, commonly garnet bearing, and 

subordinate mafic gneiss of the pyroxene granulite facies

and, locally, amphibolite facies of regional metamorphism 

(Newhouse and Zuloaga, 1929; Kalliokoski 1965a, b, c, d, 

1974). Near Cerro Bolivar plagioclase-quartz-pyroxene 

gneiss makes up 50 percent of the Imataca section, granitic 

gneiss 30 percent, microcline-quartz-plagioclase-pyroxene 

and quartz-feldspar-cordierite-garnet-biotite-sillimanitegraphite gneiss each 5-10 percent, and banded iron formation 2 percent (Chase, 1965). In the Upata area, 80 percent 

of the section is felsic granulite composed of quartz, plagioclase (An30), minor microcline that locally exceeds plagioclase in abundance, and rare pyroxene and biotite; 

15 percent of the section is mafic gneiss containing equal 

proportions of plagioclase (An>40) and pyroxene plus 

amphibole; and 5 percent of the section is iron formation 

(De Ratmiroff, 1965). Mafic gneiss and iron formation are 

closely associated (Kalliokoski, 1965a). The iron formation 

is composed of thin layers of magnetite interbedded with 

quartzite and ferruginous quartzite, and variable amounts 

of manganese oxide lenses are commonly associated with 

the iron formation. Northeast of Upata a layered sequence 

of dolomite marble (locally containing forsterite and tremolite), ferruginous quartzite, and manganiferous schist is in 

a northeast-trending band of outcrops (the Guacuripia Formation of Morrison, 1953; Drovenik and others, 1967).

Dougan (1977, p. 262), on the basis of a geochemical 

study of the Imataca terrane in a small area near Cerro 

Bolivar, described the Imataca as a "metamorphosed igneous association which although it exhibits characteristics 

intermediate between post-Precambrian island-arc calcalkaline and tholeiitic series and continental calc-alkaline 

associations, most resembles the calc-alkaline association 

of active continental margins." The age of the protolith of 

the Imataca terrane may be as old as 3,700-3,400 Ma 

(Montgomery, 1979), and Hurley and others (1976) indicated that the first metamorphic event in the Imataca Complex took place at 2,800-2,700 Ma, followed by a second at 

2,150-2,000 Ma.

The Imataca terrane includes granitic rocks (unit 

XWgr, pi. 2) that contain plagioclase and microcline in 

equal amounts, quartz, biotite, and hornblende. Quartzfeldspar pegmatite forms small bodies. Migmatite (unit 

XWm) composed of granitic rocks intimately layered 

with lineated gneiss is associated with granitic rocks of 

the Imataca terrane. A complex inlier block in the area
10 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

of La Flor contains an assemblage of younger rocks 

(units Xsp, Xa) including arkosic and pelitic metasedimentary rocks, manganiferous chert, and amphibolitized 

metavolcanic rocks (Kalliokoski, 1965d; Olivares, 1991).

The Guri fault, one of the most prominent geologic 

features of the Guayana Shield, forms the southern boundary of the Imataca Complex, but local outliers mapped as 

Imataca Complex are present south of this structure. 

Rocks of the outliers typically are metamorphosed to 

amphibolite grade. A fault inferred from gravity profiles 

forms part of the northern boundary of the terrane.

PASTORA-SUPAMO TERRANE

To the south of the Imataca terrane is a broad area of 

metavolcanic rocks of the Pastora Supergroup (Korol, 

1965; Mene"ndez, 1968) and granitic plutonic rocks of the 

Supamo Complex (Men6ndez, 1968). Some of the metavolcanic rocks are in belts between large domelike masses 

of granite of the Supamo Complex and resemble Archean 

greenstone belts, although both the metavolcanic rocks 

and the Supamo Complex are Early Proterozoic. A broad 

area of younger (?) metavolcanic rocks in the southeastern 

part of the Pastora-Supamo terrane, however, lacks a clear 

relation with the Supamo Complex, and the rocks resemble eugeosynclinal deposits of younger erogenic belts.

Volcanic rocks of the Pastora Supergroup are regionally metamorphosed to greenschist facies and in the northern part of the Pastora-Supamo terrane, within 30 km of 

the Guri fault, to amphibolite facies. Localized zones of 

amphibolite schist are developed in mafic volcanic rocks 

near contacts with the Supamo Complex. Large parts of 

these areas underlain by volcanic rock can be described as 

greenstone belts on the basis of their elongate form, greenschist facies metamorphism, and flanking granitic batholiths. In the El Callao area, the Pastora Supergroup has 

been subdivided into the Carichapo Group, which contains 

a lower pillow basalt and local talc schist lenses (El Callao 

Formation, unit Xec, pi. 2) and an upper mafic to intermediate metatuff (Cicapra Formation, unit Xcc) (Mene"ndez, 

1968). Overlying the Carichapo Group is the Yuruari Formation, an extensive unit of mica schist and phyllite (unit 

Xys) and felsic metatuff (unit Xyf). Similar rock types are 

present in other greenstone belts that are separated from 

the El Callao area by wide areas of granite, but in these 

belts topography is more subdued and jungle cover more 

dense, and stratigraphic relations between rock types are 

difficult to determine. It is likely that cycles of mafic and 

felsic volcanism of several different ages, but all Early 

Proterozoic (2,250-2,100 Ma), have contributed to the 

various greenstone belts in the northeastern part of the 

Venezuelan Guayana Shield. In addition to these volcanic 

rocks, extensive areas of gabbro (unit Xg1) and peridotite 

(unit Xu1) make up most of the greenstone belt extending

south from El Callao. In our compilation, we retain formation names in the El Callao area but in other belts divide 

the rocks on a lithologic basis only into unit Xm1, mafic 

to intermediate rocks containing abundant chlorite and 

actinolite; unit Xg1, gabbro; unit Xu1, peridotite; unit 

Xf 1, felsic metavolcanic rocks containing abundant quartz 

and white mica; and unit Xs1, mica schist and phyllite.

Amphibolite (unit Xa, pi. 2) is a strongly foliated 

hornblende-andesine schist derived from mafic volcanic 

rocks. It is abundant in metavolcanic tuff sequences in the 

El Manteco area and within 30 km of the Guri fault. 

Amphibolite facies metamorphism probably took place 

during the suture event that formed the Guri fault. 

Amphibolite is also present at numerous locations where 

mafic volcanic rocks (unit Xm1) are in contact with the 

Supamo Complex.

Common to the Supamo Complex (unit Xsp, pi. 2) 

are domelike batholiths composed of trondhjemite gneiss 

(Chase, 1965; Dougan, 1976). These biotite-quartzoligoclase gneisses and subordinate hornblende-quartzandesine gneisses lack potassium feldspar. They underlie 

monotonous plains on which outcrops are rare.

A small area of sodic biotite-quartz-oligoclase gneiss 

near Guri mapped by Dougan (1976) was included with 

the Supamo Complex by Martfn (1975) and by the Inventory Group of T6cnica Minera, C.A. (TECMIN), a part of 

the Corporaci6n Venezolana de Guayana (CVG) (unpub. 

map, 1988, 1:500,000 scale). Whole-rock oxide and traceelement analyses for 37 samples (Dougan, 1976) indicate 

that the composition of the gneiss is compatible with a 

derivation from partial melting of oceanic tholeiite or, 

alternatively, deep crustal melting of metagraywacke. It is 

more likely that this gneiss is Imataca and not Supamo 

Complex.

In the El Callao area, rocks of the Pastora Supergroup are apparently overlain by the Caballape Formation 

(unit Xcb, pi. 2), a weakly metamorphosed felsic tuff that 

was deformed only once and is less intensely folded than 

rocks of the Pastora Supergroup (Men6ndez, 1968). Its 

contacts with the Pastora Supergroup and the granite of 

the Supamo Complex are not exposed. The name 

Caballape Formation was extended by Benaim (1972, 

1974) across a 30-km-wide area of Supamo Complex and 

applied to mafic and felsic metavolcanic rocks and phyllite in the basin of the Rio Botanamo. Subsequently, this 

name was applied to metavolcanic rocks in a large area 

extending south of Kilometro 88 and east to the Rfo Marwani by the CVG-TECMIN Inventory Group and other 

CVG-TECMIN geologists. This lithologic correlation is 

difficult to support because of the distance from the type 

area and the differences in rock types between the two 

areas. Contacts between these rocks and the Supamo Complex are rarely exposed, but a post-Supamo age for these 

rocks is suggested by the absence of amphibolite facies 

metamorphism where Supamo Complex granite is adjacent
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD 11

to mafic and intermediate metavolcanic rocks. Amphibolite weathers to form hills that are prominent in the radar 

images even in areas of dense vegetation. The contact 

between rocks referred to as Caballape Formation and the 

Supamo Complex is not marked by any such topographic 

features. Unfortunately, contacts between rocks of the 

Supamo Complex and volcanic rocks of felsic composition 

show only minor metamorphic effects that are easily 

obscured by weathering. Thus, age relations with the 

Supamo Complex are unclear for large areas of volcanic 

rocks, including the Caballape Formation, in the type area. 

In our compilation, we do not use the name Caballape for 

rocks outside of the El Callao area because of their uncertain relation with the Supamo Complex; instead, we divide 

these rocks into units Xm2, Xf2, and Xs2, mafic metavolcanic rocks, felsic metavolcanic rocks, and schist, respectively.

These metavolcanic and metasedimentary rocks (units 

Xm2, Xf2, and Xs2, pi. 2) in the Rio Botanamo, Anacoco, 

Rio Marwani, and Kilometre 88-Las Cristinas areas are 

uniformly metamorphosed to greenschist facies and are 

strongly folded, judging from the steep dips of bedding 

and schistosity shown in maps by Benaim (1974), the 

northeast-trending grain in the radar image (pi. 3), and 

field observations. They may represent a volcanic environment distinctly different from that of the older greenstone 

belts. The area of exposure is broad rather than beltlike, 

and limited petrochemical data from the Anacoco area 

(Day and others, 1989) indicate that the volcanic rocks 

belong to a calc-alkaline suite rather than to a bimodal 

assemblage such as described by Gibbs (1987) for more 

typical greenstone belts in adjacent areas of Guyana. We 

refer to these rocks as the eugeosynclinal metavolcanic 

rocks of northeastern Estado Bolivar.

Gabbro and peridotite intrude volcanic rocks of both 

age groups. Some of these intrusive rocks are penetratively metamorphosed to greenschist facies, but others are 

unmetamorphosed. In the Sierra Verdun area, fresh, zoned, 

calcic plagioclase is in the unaltered interior parts of large 

gabbro, and plagioclase-bearing pyroxenite intrusive bodies have well-preserved cumulus textures (Floyd Gray and 

others, written commun., 1991). Narrow zones of shearing 

and greenschist facies metamorphism along the faulted 

contacts of these intrusive bodies suggest intrusion during 

the waning stages of regional metamorphism. In the Pfston 

de Uroy area, auriferous quartz veins cutting an intrusive 

complex of peridotite and gabbro imply that gold mineralization occurred during the latest stage of regional metamorphism (Wynn, Page, and others, in press). Mafic and 

ultramafic rocks intruding older greenstone-belt rocks 

(including amphibolite inliers in the Imataca terrane) are 

designated units Xg1 and Xu1 on plate 2, respectively, 

and those intruding the eugeosynclinal metavolcanic 

sequence are designated units Xg2 and Xu2.

A younger suite of granitic rocks (unit Xgr, pi. 2) 

intrudes metavolcanic sequences, as well as the Supamo 

Complex, and forms a characteristic rugged radar image in 

many areas across the northern part of the shield (pi. 3). 

These granitic rocks have a wide range of composition 

and are characteristically more potassium rich than those 

of the Supamo Complex.

A basement complex (unit Xbc, pi. 2) of probable 

Early Proterozoic to earliest Middle Proterozoic age 

(1,640126 Ma; Miranda and others, 1991) underlying the 

southern part of Territorio Federal Amazonas is composed 

of well-foliated granite to granodiorite gneiss. The stratigraphic relations between this unit and rock units in the 

Imataca-Supamo-Pastora terranes to the north are 

unknown. On the lower Rfo Pasimoni, the unit is described 

as migmatite (Haydee Rincdn, CVG-TECMIN Inventory 

Group, oral commun., 1991). On the middle Rfo Negro, it 

is well-foliated, chloritized, quartz-bearing, biotite granite 

gneiss. One description (I. Marcano, E. Lugo, and N. Rivero, CVG-TECMIN Inventory Group, unpub. report, 

1991) includes the term "monzodiorite." The distribution 

of this unit is inferred from aeromagnetic data. The rocks 

are moderately magnetic, and the magnetic anomalies 

show no significant directional trends.

Distinguished from the basement complex by aeromagnetic mapping is a metamorphic-plutonic complex of 

probable Early Proterozoic age (unit Xmp, pi. 2) covering 

large parts of the southern Territorio Federal Amazonas. 

These rocks are best exposed at San Carlos de Rfo Negro 

and crop out along most of the Rfo Guainfa and Rfo Negro. 

They include granite, porphyritic granite, granite gneiss, 

and augen gneiss and relatively abundant pegmatite (Marcano, Lugo, and Rivero, CVG-TECMIN, Inventory Group, 

unpub. report, 1991). This complex is characterized by a 

series of strong, sinuous, east- to N. 70° W.-trending, elongate magnetic anomalies.

A group of undivided metamorphic and plutonic 

rocks (unit Xmu, pi. 2) in the upper Rfo Orinoco and its 

headwaters resembles greenstone belts in the northeastern 

part of the shield. This unit includes greenstone, amphibolite, granite gneiss, and granite. Steep magnetic gradients 

were observed along the upper Rfo Orinoco; anomalies 

tend to be sinuous and linear and resemble magnetic 

anomalies in greenstone belts in eastern Estado Bolfvar. 

Part of the area underlain by these rocks is distinguished 

by high-amplitude, moderate- to short-wavelength aeromagnetic anomalies that lack the preferred trend characteristic of greenstone belts, and in some places a clear 

distinction between rocks of unit Xmu and surrounding 

rocks of the Cuchivero group cannot be made. The unit 

shows low to moderate topographic relief in the sidelooking radar imagery (pi. 3).
12 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

TRANSITIONAL SUPRAJACENT 

SEDIMENTARY AND VOLCANIC ROCKS

Metamorphosed and unmetamorphosed volcanic and 

sedimentary rocks transitional in age between the Supamo 

Complex and the Roraima Group are widely distributed 

over parts of the shield south and west of the areas of the 

Imataca-Pastora-Supamo terranes. These transitional formations characteristically contain beds having a red coloration and, locally, beds of air-fall tuff indicating a 

continental origin. They are described below in order of 

decreasing metamorphism and deformation. All are 

assigned an Early Proterozoic age.

The Moriche Formation (unit Xmo, pi. 2) is characterized at its type locality (Cerro Moriche on the middle 

Rio Ventuari) as metasedimentary oligomictic conglomerate, breccia, quartzite, subgraywacke, phyllite, and, locally, 

felsic tuff (Ghosh, 1977; Mendoza and others, 1977). In 

the Cerro Moriche area and along the middle and upper 

Rio Orinoco and Rio Mavaca, these rocks are highly magnetic and generally form long, linear magnetic anomalies 

suggestive of rocks folded during regional metamorphic 

events. A structural grain, parallel with these anomalies, is 

commonly visible in the radar imagery (pi. 3). Map unit 

Xmo includes two other, possibly correlative formations: 

the Cinaruco Formation (McCandless, 1962) consisting of 

micaceous quartzite, phyllite, and conglomerate and the 

Esmeralda Formation (Sellier de Civirieux, 1966) composed of quartz-chlorite-epidote schist and fine-grained 

quartzite. These formations typically are isoclinally folded.

Metasedimentary rocks of Rio Oris (unit Xo, pi. 2) 

(Maracapra Formation of Martin, 1975) are composed of 

hematitic quartzarenite, reddish phyllitic siltstone, feldspathic arenite, conglomeratic arenite, quartz-sericite phyllite, and red laminated fine-grained meta-arenite that has 

graded bedding and erosional channels (Felix Martmez, 

CVG-TECMIN, Inventory Group, oral commun., 1991). 

The unit is folded on an east-west axis.

The Cuchivero Group (McCandless, 1965) represents 

a major postorogenic magmatic episode in the Guayana 

Shield. The group is divided into volcanic and plutonic 

units of calc-alkaline composition (Sidder and Martmez, 

1990) (units Xc and Xcg, respectively; pi. 2). The volcanic unit, the Caicara Formation (unit Xc, pi. 2), includes 

widespread crystal- and lithic-rich rhyodacitic to rhyolitic 

ash-flow tuff and dacitic and andesitic lava flows. The 

more mafic rocks are commonly hydrothermally altered. 

The ash-flow tuff is unaltered to only slightly contact 

metamorphosed and typically shows well-preserved, devitrified glass shards and contorted flow banding. Andesitic 

to basaltic dikes and rhyodacitic granophyre intrude the 

volcanic rocks. Rocks of the Caicara Formation unconformably overlie the Pastora-Supamo terrane and are in 

sharp contact with overlying volcanic rocks in the Ichiin

area. The volcanic rocks in the Ichiin area, named the 

Ichiin Formation by Bricefio and others (1989) to call 

attention to their unmetamorphosed, undeformed character, 

are in a well-stratified sequence of tuffaceous rocks that 

closely resembles the Caicara Formation in composition. 

The age of the Cuchivero Group ranges from about 1,930 

to 1,790 Ma (Sidder and Mendoza, in press). Circular 

structures, as much as 10 km in diameter, visible in radar 

imagery (pi. 3) possibly represent calderas. In some places 

these structures correlate with arc-shaped gradients in the 

aeromagnetic pattern. Gravity data in this area are too 

sparse to show correlation with these structures.

In eastern Estado Bolivar, the Los Caribes Formation 

(Benaim, 1972) (unit Xs3, pi. 2) is composed of weakly to 

unmetamorphosed red arkose and polymictic conglomerate 

and minor intercalated felsic tuff. The unit overlaps the 

Marwani fault, which separates greenstone-belt rocks 

(units Xm1, Xf1, Xs1) from eugeosynclinal metavolcanic 

and sedimentary rocks (units Xm2, Xf2, and Xs2). Clasts 

in the conglomerate are predominantly unmetamorphosed 

felsic volcanic rocks that resemble tuffs of the Caicara 

Formation; fragments of metavolcanic rocks typical of the 

underlying greenstone belts and eugeosynclinal metavolcanic sequence are absent. The Los Caribes Formation probably was derived from sediments eroded from a highland 

composed of Caicara-type rocks lying to the south of the 

Rio Cuyunf and now covered by strata of the Roraima 

Group. Deposition of the Los Caribes was under continental conditions. Although many gold prospects are known 

to be present in the underlying metavolcanic rocks, none 

have been found in the Los Caribes Formation (Juan 

Acosta, CVG-TECMIN Mineral Exploration Group, oral 

commun., 1990). The Orapu conglomerate in French Guiana is equivalent to the type section Los Caribes Formation, and it is gold bearing (Ledru and others, 1991; Mile's* 

and others, 1992).

The term "pre-Roraima Group sedimentary rocks" 

(unit Xpr, pi. 2) was applied by Bricefio (1982) to an 

unmetamorphosed sequence of rocks underlying the basal 

Roraima Group along the Rio Caroni near Canaima. The 

sequence includes fine-grained to very fine grained, clayrich sandstone that locally contains granule-size quartz 

grains and is interbedded with red shale and sandy shale 

(Bricefio, 1982). Rocks of this sequence are commonly cut 

by quartz veins. They overlie, with apparent unconformity, 

the Caicara Formation and are gently folded and mildly 

discordant with the overlying beds of the Roraima Group. 

They weather to low-relief teirane broken by low cuestas 

(Ghosh, 1977; Bricefio, 1982). To the east, in the headwaters of the Rio Chicanan, the Roraima Group is underlain by the Urico Formation of Alberdi and Contreras (in 

press). The Urico is composed of graywacke, shale, mudstone, breccia, and volcaniclastic and quartz-rich sandstone; some of the strata are pyritic, indicating that their 

environment of deposition was chemically different from
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD 13

the more oxidized rocks typical of the transitional suprajacent sedimentary sequence.

The Ichun Formation of Briceno and others (1989) 

(unit Xi, pi. 2) is composed chiefly of interlayered ashflow tuff and clastic sedimentary rocks. The Ichun Formation is concordant with the overlying Roraima Group and 

discordantly overlies volcanic rocks of the Caicara Formation. The Ichun Formation crops out extensively around 

the Ichun and Guanacoco tepuis and from Raudal Los 

Brasileros on the Rfo Paragua to Salto Espuma on the Rio 

Ichun. Three major members comprise the Ichun: a basal 

member consisting of lapilli and ash tuff and volcaniclastic sandstone and andesite flow rocks; a middle member 

consisting primarily of quartzarenite, andesitic flow rocks, 

volcaniclastic sandstone, and ash-flow tuff; and an upper 

member consisting of lithic and crystal-rich tuff interbedded with minor volcaniclastic sandstone (Briceno and others, 1989). The well-stratified tuffs and sedimentary rocks 

of the Ichun Formation suggest deposition in an environment distal to volcanic centers, perhaps during the waning 

stages of Cuchivero magmatism.

GRANITIC ROCKS OF POST-SUPAMO, 

PRE-RORAIMA AGE

Granitic rocks (unit Xcg, pi. 2) of the Cuchivero 

Group of Early Proterozoic age are hypabyssal biotite 

granite and granodiorite that have equigranular to porphyritic, medium- to coarse-grained textures. Hypabyssal 

porphyritic intrusive rocks near Canaracuni that have a 

microaplitic groundmass texture indicate that high-level 

plutons of this group were locally preserved from deep 

erosion. Granitic rocks of the Cuchivero Group represent 

possible batholithic source rocks for the ignimbrites of the 

Caicara Formation.

Granitic rocks (unit Xgu, pi. 2) of probable Early 

Proterozoic age underlie waterflooded plains and jungle in 

southeastern Territorio Federal Amazonas. Where present 

south and east of San Fernando de Atabapo, they are 

described as biotite hornblende granite and granite gneiss 

(Marcano, Lugo, and Rivero, CVG-TECMIN Inventory 

Group, unpub. report, 1991). These rocks are weakly magnetic, and the magnetic pattern shows no apparent trend. 

Contacts between the granitic rocks and other rocks have 

not been observed, but interpretation of the aeromagnetic 

pattern suggests that the granitic rocks intrude the Cuchivero Group.

Granite and other silicic plutonic rocks (unit Xg, pi. 2) 

west and north of Cerro Duida in Territorio Federal Amazon Yas were described by Fe"lix Martfnez (CVG-TECMIN 

Inventory Group, unpub. report, 1991) as massive, coarsegrained, gray, equigranular biotite granite having rapakivi 

textures similar to granite of the Parguaza batholith. These

rocks are weakly to moderately magnetic; they commonly 

show east-west to west-northwest trends in the magnetic 

anomalies and pronounced west-northwest-striking lineaments in the side-looking radar imagery, especially in the 

Atdbapo quadrangle (fig. 2).

A carbonatite intrusive body (unit YXci, pi. 2) at 

Cerro Impacto has an unknown but probable Early to 

Middle Proterozoic age. This feature, originally thought to 

be an impact structure, is an irregularly shaped depression 

as much as 20 km in diameter and includes outcrops of 

tonalite, granodiorite, and granite (unit Xcg) that show 

some signs of fenitization. Most of the area is covered by 

a 200-300-m-deep laterite containing anomalous amounts 

of niobium and rare earth elements that are inferred to be 

derived from weathering of a carbonatite (Aarden and others, 1973). A prominent north-trending ridge in the center 

of the circular feature contains abundant barite float. The 

circular structure is surrounded by a set of radial fractures 

visible in the radar imagery (pi. 3). A distinctive annular 

magnetic anomaly (negatively polarized ring and positively polarized central core) is characteristic of the southern part of the structure. A strong thorium anomaly 

corresponds partly to the north-trending ridge but has a 

broad extension to the west.

RORAIMA GROUP

The Roraima Group of Early and Middle Proterozoic 

age is composed of quartzarenite, arkose, silty arenite, 

conglomeratic arenite, conglomerate, siltstone, shale, and 

minor tuff and has a total thickness of more than 3,200 m 

(Reid and Bisque, 1975). Bedding is cross stratified, laminated, or massive and flat lying or gently inclined. The 

unit weathers to form high tepuis (flat-topped mesas) and 

ledge-and-slope topography. Based on direct and indirect 

isotopic dating, the range of possible ages of the entire 

group is from 1,900 to 1,500 Ma (Sidder and Mendoza, in 

press; see also Ghosh, 1985). In Estado Bolivar, the group 

rests with probable unconformable relation on the Los 

Caribes Formation south of the Rio Cuyuni, on the Pastora 

Supergroup and Supamo Complex, on Cuchivero volcanic 

and plutonic rocks, and on pre-Roraima rocks. It rests conformably on the Ichun Formation.

Strata of the Roraima Group preserved in erosional 

remnants in Territorio Federal Amazonas show a lack of 

correlation with Roraima Group strata in the Gran Sabana 

in Estado Bolivar. The beds in Territorio Federal Amazonas rest on eroded Parguaza Granite (1,545 Ma; Gaudette 

and others, 1978) and are probably younger than the rest 

of the group; these rocks are shown on plate 2 as 

Roraima Group undivided (unit YXr, pi. 2). Ghosh (1985) 

showed that the age of Roraima-type deposition decreases 

from west to east in the shield. He (p. 38) described five 

sedimentary environments represented by the Roraima
14 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

strata: "1) low-sinuosity river channels and flood plains; 

2) delta distributaries above tranquil interdeltaic and 

inter-distributary lakes; 3) coastal lagoons to interdeltaic 

bays; 4) wave-dominant non-barred beach and tidal environment; 5) intertidal mud flat environment, etc." Ghosh 

showed, using paleocurrent data, that the source of the 

Roraima sediment was northeast, east, or south of the 

present outcrops.

The Roraima Group was divided regionally by Y£nez 

(1985) into three formations, the Canaima, the Guaiquinima, and the Auydntepuy; Reid and Bisque (1975) had 

previously subdivided the group into four formations. The 

subdivisions of Ydnez were used in compiling the geologic map (pi. 2) because the distribution of the units was 

presented in map form by Ydnez (1985).

The Canaima Formation (unit YXrc, pi. 2) is composed of quartzarenite and arkose, conglomeratic arenite, 

conglomerate, siltstone, and shale; cross strata are abundant 

in the arenite and arkose units. The formation weathers to 

form cliffs of resistant arenite, arkose, and conglomerate 

and slopes underlain by relatively nonresistant siltstone, 

shale, and silty arenite. It is about 1,100-2,000 m thick and 

is equivalent to the Uaire'n and Cuquen£n Formations and 

the lowermost part of Uaimapue' Formation of Reid and 

Bisque (1975).

The Canaima Formation is conformably overlain by 

the Guaiquinima Formation (unit YXrg, pi. 2). The 

Guaiquinima is made up of fine-grained quartzarenite and 

arkose that are cross stratified, laminated, and massive; 

siltstone and graywacke; and red, green, and greenish-gray 

jasper composed of devitrified and (or) silicified volcanic 

ash and small crystals of quartz and feldspar. The formation weathers to form flat or gently sloping topography, 

and the upper part of the formation is mostly covered by 

debris from the overlying Auydntepuy Formation. The 

Guaiquinima is several hundred meters thick and is equivalent to all but the lowermost part of the Uaimapue' Formation of Reid and Bisque (1975).

The Auyantepuy Formation (unit YXra, pi. 2) is the 

youngest part of the Roraima Group. Composed mainly of 

quartzarenite and minor arkose, it forms steep cliffs 300- 

700 m high and flat-topped mesas. It is equivalent to the 

Matauf Formation of Reid and Bisque (1975).

INTRUSIVE ROCKS OF PROBABLE 

POST-RORAIMA AGE

The Middle Proterozoic Parguaza Granite (unit Yp, 

pi. 2) is a massive, coarsely crystalline, porphyritic granite 

and biotite granite commonly having rapakivi textures. It 

contains xenoliths of fine-grained granite and rhyolite 

(Gaudette and others, 1978) and pegmatite bodies enriched 

in tin and tantalum (Mendoza and others, 1977; Rodriguez

and Pe'rez, 1982). The Parguaza Granite was reported by 

Mendoza and others (1977) and Ghosh (1985) to be 

unconformably overlain by the Roraima Group, although 

isotopic ages for the Parguaza Granite (zircon age, 1,545 

Ma; Rb/Sr age, 1,531±39 Ma; Gaudette and others, 1978) 

are within the possible age span of the Roraima Group. 

These age relations can be explained if the Roraima Group 

varies significantly in age within the Guayana Shield; that 

is, the Roraima Group may be relatively young (less than 

1,545 Ma) in the western part of the shield, where it 

reportedly rests on the Parguaza Granite, but may be older 

in the eastern part of the Guayana Shield (Ghosh, 1977, 

p. 47, 48).

The Parguaza Granite is characterized by lowamplitude intermediate-frequency magnetic anomalies that 

have no preferred trend. In the southwestern part of the 

batholith, a number of narrow, dikelike bodies observed 

in the side-looking radar imagery (pi. 3) form radial patterns. In the same area, several large, generally east 

trending magnetic anomalies were observed that have 

negative polarization. Modeling profiles of these anomalies yield depths of 1-5 km for their presumed source. In 

addition, gravity data in this area indicate a dense body 

5-15 km below the surface. Together, these results suggest a batholith feeder or a later intrusion postdating 

emplacement of the Parguaza batholith.

In southern Territorio Federal Amazonas, Middle 

Proterozoic granitic rocks (unit Ylg, pi. 2) typically penetrate through and dome strata of the Roraima Group. An 

intrusive body in Caflo Yagua was described as coarsely 

equigranular granodiorite having rapakivi textures. In the 

southern part of the Rfo Negro basin, a similar body 

named the Piedra de Cocuy was described as a granodiorite containing 30 percent quartz, 40 percent feldspar, 20 

percent biotite, and 10 percent hornblende (Marcano, 

Lugo, and Rivero, CVG-TECMIN, Inventory Group, 

unpub. report, 1991). This intrusive body is characterized 

by small, subrounded, generally strong magnetic anomalies and produces erosional patterns visible in side-looking 

radar imagery (pi. 3).

Silicic to intermediate intrusive rocks (unit Xgd, 

pi. 2) of probable Early Proterozoic age form small domes 

intruding and folding strata of the Ichun Formation in the 

Rfo Caura quadrangle. One of these small plutons, northwest of Cerro Ichun, was described as a granite (Felix 

Martfnez, CVG-TECMIN Inventory Group, oral commun., 1991). Another, on the east side of Cerro Ichun, was 

described as felsic intrusive rocks (Bricefio and others, 

1989). These silicic to intermediate rocks are generally 

strongly and heterogeneously magnetic; tightly compressed, high-frequency magnetic anomalies are centered 

on a typically elliptical core. Erosion of these rocks produces patterns that are visible in Landsat imagery but not 

clearly identifiable in side-looking radar imagery (pi. 3).
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD 15

Some circular structures as much as 5 km in diameter and visible in side-looking radar imagery (pi. 3) are 

believed to represent alkaline intrusive complexes of probable Middle Proterozoic age (unit YXac, pi. 2). At La 

Churuata, at the base of the eastern flank of Cerro Duida 

southeast of Cerro Marahuaca, one such complex has a 

Rb/Sr age of 1,300 Ma and is described as "saturated 

syenites, quartz-syenites, nepheline syenites, and granites" 

containing aegerine, riebeckite, and monazite or bastnaesite (Soares, 1985). A complex mapped south of the Santa 

Elena quadrangle (fig. 2) and just north of the Brazilian 

border (lat 3°59' N., long 63°01' W.) is a coarsely crystalline gabbro that has flow-aligned phenocrysts of plagioclase 1-2.5 cm long in a fine-grained groundmass. Rocks 

in these alkaline intrusive complexes commonly are commonly only very weakly magnetic.

Dark-gray to greenish-gray, fine- to coarse-grained, 

tholeiitic diabase (unit d, pi. 2), locally containing disseminated pyrite, is present as dikes, laccoliths and sills in the 

Roraima Group and older rocks. Isotopic dating of these 

dikes throughout the Guayana Shield indicates that the 

dikes formed from about 1,850 to about 1,650 Ma and

from about 210 to 200 Ma (Teggin and others, 1985; 

Sidder and Mendoza, in press).

CENOZOIC DEPOSITS

Near Puerto Ordaz and westward, the Imataca Complex is overlain by a sequence of sedimentary rocks correlated with the Pleistocene and Pliocene Mesa Formation of 

Hedberg and Pyre (1944). This unit (unit Tm, pi. 2) is 

composed of massive to laminated, generally reddish siltstone and sandy siltstone and light-tan or pink to gray 

claystone. The unit grades upward into unconsolidated 

gravel and sand. No fossils other than petrified wood have 

been found. The formation may represent a remnant of 

extensive delta deposits of the Rfo Orinoco (Gonzales de 

Juana, 1946).

Deposits of sand, gravel, and silt of Quaternary age 

(unit Qa, pi. 2) cover large areas in the lower reaches of 

Rfo Orinoco and are present sporadically along rivers and 

small streams throughout the shield. This unit includes terrace gravels containing rounded pea-size clasts at locations 

along the Rfo Caronf.

GEOPHYSICS OF THE VENEZUELAN GUAYANA SHIELD

By Jeffrey C. Wynn

Geophysical data, principally side-looking airborne 

radar, gravity, and aeromagnetic data, are primary sources 

of information for geologic mapping, mineral exploration, 

and mineral resource assessment. These types of data are 

particularly useful in the weathered, densely forested geological environment of the equatorial area of Venezuela. 

Side-looking radar imagery of the Earth's surface displays 

fine detail and spectacular relief and thereby illuminates a 

variety of surficial geological features. Gravity data are 

used to distinguish hidden boundaries between lithologic 

units having different densities and to estimate the depth 

to bedrock in sediment-filled basins. Aeromagnetic data 

are used to distinguish differences in magnetic susceptibility, such as between volcanic and sedimentary rocks or 

between fresh and hydrothermally altered rocks. Aeromagnetic data provide a measure of the content of magnetite and other magnetic minerals in rocks and, as such, 

provide information on the distribution of different rock 

types at the surface and at depth.

Geophysical data for the Venezuelan Guayana Shield 

are available but uneven in coverage. Nevertheless, they 

provided crucial information for the compilation of the 

new geologic and tectonic map of the shield (pi. 2) and for 

the mineral resource assessment, especially in areas where 

geologic information is sparse due to remoteness or to 

extensive laterite and jungle cover. A provisional interpretation of these data delineated areas having similar gravimetric, radar reflectance, and magnetic properties that are 

useful in synthesizing the geology and in determining the 

boundaries of areas permissive for specific types of mineral deposits. Fortunately, where the ground-based geologic coverage is poorest, such as Territorio Federal 

Amazonas, geophysical coverage is relatively good. This 

circumstance allowed compilation of a new geologic map 

that is reasonably consistent in the overall quality of its 

detail for the entire shield.

Table 2 describes the geophysical characteristics of 

many of the geologic map units shown on plate 2.

AVAILABLE DATA AND PROCESSING 

TECHNIQUES

SIDE-LOOKING AIRBORNE RADAR

X-band side-looking airborne radar (SLAR) imagery 

was acquired for the entire Venezuelan Guayana Shield by 

Goodyear Aeroservice Corporation in 1971 and 1977. The 

data were acquired using a Goodyear electronic mapping 

system (GEMS) flown at 12,000 m elevation; flightlines 

were generally east-west in Estado Bolivar and northsouth in Territorio Federal Amazonas, and the radar 

antenna illuminated the land toward the west and north, 

respectively. To obtain the precise location of the aircraft 

during the survey an array of some 30 points throughout 

the Amazon Basin was established by the transit satellite 

(Jensen and others, 1977). At each point a transponder 

was set up for a shoran radio-positioning system. By 

simultaneously receiving the broadcast from two such 

points the position of the aircraft was determined to an 

accuracy of about 75 m and was checked every 6 seconds. 

During the survey the plane's position was recorded on 

magnetic tape, along with its altitude and other significant 

data. The acquisition system used a transmission wavelength of 3.12 cm. Image strips were compiled at 

1:100,000 scale and assembled into mosaics at 1:250,000 

and 1:500,000 scales. Resolution of the images is 10 m. 

Registration disagreements of as much as 5 km are apparent between these images and Venezuela's National Cartographic Base. The mosaics formed the base maps for all 

geologic studies.

A l:l,000,000-scale image mosaic of the Venezuelan 

Guayana Shield was assembled by the U.S. Geological 

Survey using density-corrected 1:500,000-scale sheets 

compiled onto a Venezuelan polyconic grid base called 

Proyecci6n C6nico Secante Compensada (pi. 3), an equidistant conic projection using lat 4° N. and 9° N. as standard parallels and long 66° W. as the central meridian. 

Because of the selection of wavelength, the radar imagery 

commonly penetrates vegetation and thus is an important

17
18 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Table 2. Geophysical characteristics of geologic map units in the Venezuelan Guayana Shield.

[Geologic map units are shown on plate 2 and described in text. Side-looking radar imagery (SLAR) is shown on plate 3. Location of quadrangles is shown in figure 2]

Unit Name Regional geophysical signature

Qa

d

Ylg

YXac

Yp

YXr

Xpr 

Xi 

Xgu 

Xgd,Xgr

YXci 

Xg

Alluvial deposits 

Diabase

Late granite 

Alkaline complexes 

Parguaza Granite

Roraima Group (arenite and 

conglomerate)

Pre-Roraima Group sedimentary 

rocks

Ichun Formation (volcanicsedimentary rocks)

Intrusive rocks, undivided

Silicic to intermediate intrusive 

rocks (forming small domes)

Carbonatite intrusion of Cerro 

Impacto

Calc-alkaline granite

None on a regional scale. In the SLAR imagery no pattern is recognized.

Strong, high-frequency, generally northeast trending (Guri, Rio Mavaca, Santa Elena, and Puerto 

Ayacucho quadrangles) or northwest trending (Atabapo, Santa Elena, and Piedra de Cocuy 

quadrangles) linear magnetic anomalies. Some of these bodies are visible in the SLAR imagery as 

narrow linear features.

Small, subrounded, and generally strong magnetic anomalies. Unit is commonly visible in the 

SLAR imagery as small, relatively homogeneous rings in the drainage pattern.

Very weakly magnetic. Unit is commonly visible in the SLAR imagery as small, faint, relatively 

smooth textured, circular or subcircular bodies.

Intermediate-frequency, low-amplitude magnetic anomalies without any preferred trend. Internal 

to the batholith, several large, buried, generally east trending bodies having negative magnetic 

polarization have been observed, as well as narrow, high-frequency linear anomalies (probably 

diabase dikes). At least one dense, disklike body can be seen in gravity data and appears as a 

domelike expression in the southwest margin of the batholith. In the SLAR imagery the batholith 

shows as areas of generally high relief and prominent N. 45°-60° E.-trending faults and shear 

zones (some right- and left-lateral movement).

No magnetic mineral content These units are effectively transparent to aeromagnetic data (units 

beneath them could possibly be mapped using aeromagnetic data). Resistant strata are very 

distinctive and show clearly in the SLAR imagery as long jagged shadows on the distal side of the 

aircraft flight path.

Apparently nonmagnetic, but no direct measurements have been made to verify this. In the SLAR 

imagery rocks appear as relatively smooth textured, low-relief material.

Rock descriptions suggest that these rocks should be highly magnetic, but no direct measurements 

have been made. The unit forms very pronounced bands or laminations in the SLAR imagery.

Weakly magnetic with no apparent orientation to patterns (southwest Territorio Federal 

Amazonas).

Generally strongly and heterogeneously magnetic, showing tightly compressed, high-frequency 

magnetic anomalies centered on a (typically) elliptical core. In Landsat imagery generally 

subrounded structures have slightly different vegetation color, but in SLAR imagery rocks of unit 

are generally difficult to distinguish from surrounding rocks, which have similar characteristics.

Distinctive annular magnetic anomaly (negatively polarized ring having a positively polarized 

central core) overlies southern part of intrusive body, and strong thorium anomaly overlies the 

north-south ridge and extends to the west A prominent set of radial fractures is visible in the 

SLAR imagery, and the surrounding plain shows as an irregularly shaped area of low relief.

Weakly to moderately magnetic; apparent west to west-northwest trends common in magnetic 

anomalies. In the SLAR imagery pronounced west-northwest-striking lineaments are 

characteristic, especially in the Atabapo quadrangle.________________________

geologic mapping tool. Distinct changes in lithology generally produce consistent differences in the pattern of 

radar reflectance. Geological analysis of images can suggest areas for detailed mapping, and preliminary structural 

interpretation of images can provide clues for revising and 

expanding information (Sardi and others, 1988). Sidelooking radar imagery is a useful tool to complement field 

mapping that can help identify the extent of regional structure and gross lithologic units.

On the radar image mosaic (pi. 3) rivers are shown as 

solid dark lines; this is due to the strong absorption of 

radar energy by water. Distinctive platy laminae are visible 

in most places where rocks of the Roraima Group or Ichun

Formation are present; cliffs of the Roraima Group give 

distinctive long, thick black shadows that assist further in 

its identification. Strong topographic relief in the northeastern part of the shield is due to greenstone-belt rocks 

that rise above the relatively flat lying topography of the 

Supamo terrane. Vast areas of Territorio Federal Amazonas are almost featureless in the radar image due to the fact 

that they are mostly flat, water-inundated plains.

GRAVITY DATA

Available gravity coverage in the Venezuelan Guayana Shield was acquired from the U.S. Defense Mapping
GEOPHYSICS OF THE VENEZUELAN GUAYANA SHIELD 

Table 2. Geophysical characteristics of geologic map units in the Venezuelan Guayana Shield Continued.

19

Unit Name Regional geophysical signature

Xcg

Xc

Xmu

Xsp

Xmo

Xml,2, Xfl,2, 

Xgl,2, Xul,2

Xmp 

Xbc 

Ai

Granitic rocks of the Cuchivero 

Group

Caicara Formation (volcanic 

rocks)

Greenstone-belt rocks in Territorio 

Federal Amazonas (volcanicplutonic terrane)

Supamo Complex (intrusive rocks)

Moriche Formation 

(metaconglomerate)

Greenstone-belt rocks

Metamorphic-plutonic terrane of 

San Carlos

Basement complex

Imataca Complex

Moderate magnetic anomalies without preferred trends. Unit forms medium to high topographic 

relief in the SLAR imagery.

Very strong, high-amplitude, high-frequency magnetic anomalies without preferred directional 

orientation. In the SLAR imagery these rocks have low to moderate relief and local directional 

orientation of the drainage patterns; several 5-10-km-diameter circular structures can be identified 

(possibly calderas?).

Almost identical in aeromagnetic expression to undifferentiated volcanic rocks of the Caicara 

Formation discussed above. Extremely limited reconnaissance geologic sampling indicates the 

presence of Caicara volcanic rocks, as well as rapakivi-textured granitic rocks, greenstone-belt 

volcanic rocks (with extensive illegal gold exploitation reported in the national press), and some 

amphibolite. In the SLAR imagery this terrane is very similar to that of Caicara volcanic rocks, 

and some of the granite bodies are clear, circular to subcircular features as much as 13 km in 

diameter.

Nonmagnetic rocks, at least in the eastern Guri quadrangle; some intrusive rocks mapped as 

Supamo in the Sierra Verdun area have magnetic signatures very similar to units Xgd and Xgr and 

are probably related younger granitic intrusive rocks. In the SLAR imagery Supamo rocks 

generally form a terrane of low topographic relief.

High-amplitude magnetic anomalies (linear when the bodies are long and narrow, as in the La 

Esmeralda quadrangle). Anomalies follow structural lows along the middle and upper Rio 

Orinoco and Rio Mavaca; some of the linear bodies show folding by regional metamorphic events. 

These shapes are visible in both aeromagnetic maps and SLAR imagery.

Highly magnetic; high- to medium-frequency anomalies reflect units at different depths. In places 

in northeastern Estado Bolivar rocks are apparently covered by Supamo rocks. Ultramafic (units 

Xu 1, Xu 1) and mafic (unit Xm 1) rocks are generally the most magnetic; the other rocks are 

slightly less magnetic. The granitic rocks (units Xg1, Xg2) cannot be distinguished from the rocks 

that they intrude. Some magnetic anomalies are without preferred orientation, but many are linear 

in one place and bent into broad arcuate structures in another. In the SLAR imagery these rocks 

show as zones of significant topographic relief having pronounced shadows and lineaments 

standing above surrounding Supamo rocks (especially in Bochinche and Marwani areas, at El 

Callao, and south along Sierra Verdun).

Strong and distinctive, sinuous, east- to N. 70° W.-trending, elongate magnetic anomalies stacked 

together. In the SLAR imagery these rocks form an almost featureless, flat plain.

Moderately magnetic terrane, generally without complex significant directional trends. In SLAR 

imagery these rocks form an area of generally low relief and little or no textural pattern.

Generally highly magnetic, reflecting ridges of iron formation and amphibolite; magnetic patterns 

suggest broad regional folds. In SLAR imagery, these rocks appear as areas of moderate relief 

with preferred N. 60° E. trends in the drainage pattern east of the Rio Caroni. The pattern changes 

to broad arcs, as well as generally noncoherent textural trends, west of Rio Caroni._________

Agency (DMA) Library, 3200 South Second Street, St. 

Louis, Missouri 63118-3399, and from the Universidad de 

Simon Bolivar (USB), Caracas, Venezuela, through Professor Victor Graterol. The extent of the gravity coverage 

is shown in figure 3. A Bouguer gravity anomaly map 

was compiled based on the available data (pi. 4). Coverage is almost nonexistent in the southern parts of the 

shield but relatively complete in the northern parts where 

there is substantial road and river access. Data are sufficient in areas around and south of Puerto Ayacucho to 

permit some limited modeling to better understand the 

core of the Parguaza granite batholith (discussed later). 

Station identification, latitude, longitude, elevation, and

observed gravity were compiled for each gravity station 

using the principal facts provided by DMA and USB. 

Simple Bouguer anomalies were calculated using formulas 

published by the International Association of Geodesy 

(1967) and a Bouguer reduction of 2.67 g/cm3 .

Gravity data provide information about the distribution of masses of rock at depth that have different densities. These densities may reflect different degrees of 

metamorphism or simply different mineral assemblages 

(such as clays from hydrothermal alteration, which reduce 

density, or higher contents of ferromagnesian minerals, 

which generally increase density). In areas where gravity 

stations are available in sufficient density, it is possible to
20 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

69° 66° 63°

ESTADO 

Puerto Ordaz \PE"* AMAC^IRO

Boundary of Venezuelan 

Guayana Shield

200 KILOMETERS

Figure 3. Gravity coverage (shaded area) of the Venezuelan Guayana Shield. Names of individual l:500,00-scale quadrangles are 

shown in uppercase letters.

model the buried masses of different densities and learn 

something about the three-dimensional distribution of 

rocks at the surface. Because of the paucity of gravity data 

in the Venezuelan Guayana Shield and because of difficulties in data reduction due to lack of reliable elevation data, 

gravity data were used only rarely in the compilation of 

the geologic and tectonic map (pi. 2).

AEROMAGNETIC DATA

Available aeromagnetic coverage of the Venezuelan 

Guayana Shield is shown in figure 4 and was mostly 

acquired by Goodyear Aeroservice in 1959 and 1961. 

Handcontoured aeromagnetic data at 1:50,000 scale are 

available from CVG-Tecnica Minera, C.C. Chilemex,
GEOPHYSICS OF THE VENEZUELAN GUAYANA SHIELD 21

Boundary of Venezuelan 

Guayana Shield

TERBITORIO 

O FEDERAL AMAZONAS

200 KILOMETERS

Figure 4. Aeromagnetic coverage (shaded area) of the Venezuelan Guayana Shield. Names of individual 1:500,000-scale quadrangles 

are shown in uppercase letters.

Piso 1, No. 6, Puerto Ordaz, Venezuela. The original 

flightlines were spaced roughly 1 km apart and were 

flown at 152-m mean terrain clearance in a N. 22° W. 

orientation that was chosen to cut across the trend of 

regional foliation. Internal evidence, such as poor correlation between surveys spaced two years apart, suggests

that incorrect registration of fiducial marks may have led 

to errors in horizontal location of as much as 2.5 km.

Aeromagnetic contour maps at 1:50,000, 1:100,000, 

and 1:200,000 scales available from the Venezuelan Ministerio de Energia y Minas were analyzed for information 

on depth-to-source and horizontal boundaries for discrete
22 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

anomalies and also to delineate distinctive magnetic 

domains based on characteristic aeromagnetic signatures 

(Ministerio de Energfa y Minas and Corpoven, 1989). 

These maps were one of the prime sources of information 

on the distribution of rock types in the southern part of 

Territorio Federal Amazonas where geologic mapping has 

not been done except as the most general regional reconnaissance. A preliminary map was assembled using these 

data and previous geologic maps and radar images, then 

field checked by geologists of the U.S. Geological Survey 

(USGS) and T6cnica Minera, C.A. (TECMIN), a part of 

the Corporacidn Venezolana de Guayana (CVG), who 

found good correlation despite the fact that geologic unit 

identifications in the field are not usually based on magnetite content.

A widely spaced (4-6 km) aeromagnetic survey was 

flown in the Gran Sabana region (southeastern Venezuela) 

during the 1970's, but the data were never reduced to contour maps and have never been released (Emilio Herrero, 

Ministerio de Energia y Minas, oral commun., 1987, 

1990).

During the early 1970's, a combined airborne magnetic and radiometric survey was flown in Territorio Federal Amazonas. Although digital tapes were provided to 

the Ministerio de Energfa y Minas by Hunting-UK, the 

contractor, the location data were provided in a nonstandard format, and the flightline sheets were not made available in a format that permits conversion of the data to 

digital form. These data are of sufficient quality, even in 

contoured form, to permit modeling in some cases. Apparently, no large-scale magnetic data acquisition has been 

carried out in the Venezuelan Guayana Shield since the 

mid-1970's, and no regional geophysical data are available 

for the area between the Rfos Caura and Caroni in Estado 

Bolivar.

AERIAL GAMMA-RAY SPECTROMETRY

Aerial gamma-ray spectrometry data were acquired as 

part of the airborne aeromagnetic data acquisition effort in 

Territorio Federal Amazonas. An aerial gamma-ray system 

flown near ground level effectively detects terrestrial 

gamma radiation from an area along the flightline. Aerial 

gamma-ray measurements represent the near-surface 

(<50 cm) distribution of the natural radioelements potassium (K), uranium (eU), and thorium (eTh). (The "e" prefix, for equivalent, denotes the potential for disequilibrium 

in the uranium and thorium decay series.) The only data 

available to the author for the Venezuelan Guayana Shield 

were interpretations in the form of generally, but not 

always, closed domains of radiometric anomalies for uranium and thorium on the 1:200,000-scale compilations of 

aeromagnetic data for Territorio Federal Amazonas.

In some areas, overlaps and inconsistent compilation 

make it impossible to tell which side of a decorated line

the anomaly is on, especially if the anomaly covers a 

large area. Where there are unequivocal anomalies (generally small in size) of uranium and thorium, they were 

incorporated into the geologic map. In some areas, these 

anomalies coincide with a feature on the radar imagery, 

with a magnetic anomaly, or with a discrete feature 

mapped on the ground. Important examples are the 

thorium anomaly associated with the Cerro Impacto 

carbonatite (pi. 2, lat 5°50' N., long 65° 15' W.) and some 

discrete uranium-enriched areas associated with local 

subcircular magnetic anomalies along the border between 

the Puerto Ayacucho and Rfo Caura 1:500,000-scale 

quadrangles (fig. 2).

PROCESSSING OF GEOPHYSICAL DATA

TO COMPILE THE GEOLOGIC AND

TECTONIC FRAMEWORK MAP

The normal procedure for utilization of airborne geophysical data in geologic mapping is to convert the data to 

digital form (if it is not provided digitally) and then to 

develop a series of derivative products such as low pass 

filtered, horizontal gradient of pseudogravity, reduced to 

the pole, and terrace magnetization images. These derivative maps can be used to provide information on structure 

and on contacts between geologic units that have different 

magnetic mineral contents (magnetite, ilmenite, or pyrrhotite). An example of this type of analysis is the study of 

the Bochinche area reported in Wynn, McCafferty, and 

Salazar (in press). A complete geophysical analysis usually concludes with modeling of individual anomalies of 

significance for one reason or another or, rarely, with 

developing a complex three-dimensional model for an 

entire area.

In Venezuela, it proved impossible to obtain digital 

aeromagnetic data except by digitizing hand-contoured 

maps. Digitization of the Bochinche 1:50,000-scale quadrangle alone required a person-week of effort, as well as 

time for editing, and resources were not available to 

devote to digitizing the several hundreds of 1:50,000-scale 

maps in the Venezuelan Guayana Shield. For this reason, 

a different approach was taken in the analysis of the Venezuelan airborne geophysical data.

The analysis, and its incorporation into a new geologic and tectonic map, was done using a multistep procedure. Initially, a number of anomalies were modeled 

for differently shaped two-dimensional bodies at different 

depths. The unusual deep source negative magnetic 

anomaly at lat 5° 15' N., long 67°40' W., south of Puerto 

Ayacucho, for example, can best be modeled as a threedimensional prism having a strong remanent magnetization (figs. 5, 6). A model of the magnetic anomaly of a 

shallowly dipping peridotite body from the Piston de 

Uroy area is shown in figure 7, and a model of the magnetic anomaly of a vertically dipping mafic dike south of
5°30

5° 15' 

GEOPHYSICS OF THE VENEZUELAN GUAYANA SHIELD

67°30'

23

Figure 5. Contours of total field aeromagnetic data in the area of an unusual, deep-source negative magnetic anomaly south of the city 

of Puerto Ayacucho, Tenitorio Federal Amazonas. Contour interval 10 nT. Line of profile of model in figure 6 is also shown. Modified 

from Ministerio de Energfa y Minas and Corpoven (1989).

Cerro Autana in Tenitorio Federal Amazonas is shown 

in figure 8.

This kind of forward modeling was necessary 

because the inclination of the Earth's magnetic field in

southern Venezuela is about 35°-40°. Because of this 

shallow inclination, anomalies are distorted and shifted; 

the normal positive anomaly is diminished and shifted laterally as much as several kilometers in a S. 22° E.
24 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

4,200 N. 10° W. S. 1 0" E. SOUTH

ooooo Fitted 

 Base level

10,000

1 5,000

x 20,000

SI

Q 25,000

1,000 2,000 3,000 

DISTANCE, IN METERS

4,000 5,000

Figure 6. Model of the negative magnetic anomaly south of the 

city of Puerto Ayacucho shown in figure 5. A three-dimensional 

prism was used to model the source; depth to top of source is 12.4 

km, upper surface of cross section is 10.0 km by 11.0 km, and body 

dips 41° S. Thickness of source body is 19.1 km and susceptibility 

is 3.8xlO~3 emu. The best fit is obtained by using a remanent magnetization to susceptibility magnetization ratio (Koenigsburger 

ratio) of 10, a remanence field inclination of -23°, and a remanence field declination of-10°. The Earth's field strength is taken 

to be 35,000 nT, field inclination 40°, and declination -11°.

4,200

w

< 4,100

wUJ

O 4,000

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Q

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UJ

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z

i

t 6,000

Q

8,000

NORTH

o o o o o o Fitted 

 Base level

4,000 8,000 12,000 

DISTANCE, IN METERS

16,000 20,000

Figure 8. Model of a moderately magnetic, steeply dipping dike 

(probably diabase) south of Cerro Autana, Territorio Federal 

Amazonas. The best susceptibility fit uses a long, narrow body that 

is buried 84 m below the surface, dips 88° N., and has a half-width 

of 947 m. The modeled susceptibility is 2.08x10~3 emu. The 

Earth's field strength is taken to be 35,000 nT, field inclination 

40°, and declination-11°.

SOUTH NORTH

44,000 




UJ

O 40,000

z

<

z

a 36,000

_j

UJ

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O

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28,000 




£ o

fc

z 100

I

t 200

EXPLANATION 

 Observed

ooooo Fitted 

 Base level

200 400 600 800

DISTANCE, IN METERS

1,000

Figure 7. Model of a highly magnetic, shallowly dipping peridotite body at the Piston de Uroy gold prospect, Rio Chicanan, 

Estado Bolivar. The best susceptibility fit uses a long, narrow 

body that is buried 24 m, dips 10° N., and has a half-width of 

72 m. The modeled susceptibility is 0.358 emu. The Earth's 

field strength is taken to be 32,700 nT, field inclination 31°, and 

declination-11°.

direction. The normally subtle negative part of the dipolar 

magnetic anomaly is enhanced and is shifted as much as a 

kilometer in the direction N. 22° W. (the direction of the 

Earth's field declination) from the source body. In areas of 

complex anomalies, this shift can lead to mixing and overlap that make interpretation extremely complicated 

(Grauch and Cordell, 1987).

In areas where discrete anomalies could be identified, 

they were interpreted for depth of burial (1) superficial, 

(2) burial by at least 500 m overburden, (3) burial by at 

least 1,500 m of overburden and for horizontal boundaries. These data were incorporated into an overlay that 

was then used in assembling the preliminary geologic map 

through digitizing the interpreted boundaries and reprojecting them at 1:500,000 scale. Where there was extensive overlap of anomalies, boundaries were drawn around 

zones having different magnetic signatures such as different frequency of components, presence or lack of directional trends and their orientations if present, and so forth.

A series of l:500,000-scale geophysical interpretation 

maps were developed, geologic maps were assembled, and 

the information was evaluated and incorporated. This was 

a difficult and mostly subjective process because some of 

these maps were made from photo interpretation with as
GEOPHYSICS OF THE VENEZUELAN GUAYANA SHIELD 25

little as three field sample points in a l°xl° quadrangle 

area. Extensive interviewing was carried out by the author 

and other USGS colleagues of those individual geologists, 

both within and outside of CVG-TECMIN, who had 

worked in the areas of interest to incorporate their observations and to examine the rocks they had collected. Indisputable radiometric anomalies and some gravity data 

modeling results were added at this stage to generate new 

preliminary geologic maps at 1:500,000 scale.

Following assembly of these preliminary geologic 

maps, the side-looking radar imagery was evaluated and 

compared. At this point, serious registration problems 

were discovered of as much as several kilometers between 

the side-looking radar imagery and the l:500,000-scale 

preliminary geologic maps, due mostly to the fact that the 

side-looking radar images were mosaiked on a Cartesian 

base (no projection) with poor geodetic control, as well as 

to scaling problems between adjacent radar image strips. 

For example, it is possible to see cliffs in the Roraima 

Group offset by as much as 2 km between adjacent strips 

(for instance, along the southern margin of Auyan Tepuy). 

These disparities are probably caused by deviations from 

the horizontal in the flightpath of the acquisition aircraft. 

The problem is aggravated by the fact that different generations of geologic maps used bases of differing quality.

The side-looking radar imagery was therefore used 

only to refine the previously developed l:500,000-scale 

preliminary geologic maps, overlaying small parts (areas 

no wider than 15-20 km) where drainages were clearly 

visible on both the geology base and the side-looking radar 

imagery. Side-looking radar imagery was very helpful 

when used in this fashion, particularly in refining mapping 

of the Roraima Group and the Ichun Formation in Estado 

BoHvar. It nevertheless became obvious at this stage that 

side-looking radar imagery is not always reliable as a 

mapping tool, particularly when trying to resolve contacts 

between volcanic and granitic rocks of the Cuchivero 

Group.

As a final step, the 1:500,000-scale preliminary geologic maps, refined using side-looking radar imagery, were 

combined into a single l:l,000,000-scale map. Several 

iterations followed to resolve edge conflicts, usually by 

additional interviews with geologists who had worked in 

the field and by returning repeatedly to all the layers of 

data incorporated to that point.

REPRESENTATION OF GEOPHYSICAL 

DATA IN THE GEOLOGIC MAP

CIRCULAR FEATURES

More than 30 circular or subcircular features identified using geophysical data (not including the Nuria ringdike complex, which is transparent in both magnetic and

gravity data) are shown on the geologic map of the Venezuelan Guayana Shield (pi. 2). A few of these features are 

apparent only in the aeromagnetic data, such as the 70-kmdiameter subcircular structure on the west side of the middle Rio Caura (lat 5° 10' N., long 64°20' W.), which has an 

intensely magnetized core and a superimposed uranium 

anomaly. In two areas of the northwestern part of Territorio Federal Amazonas (lat 4°00' N., long 67°25' W., and 

lat 4°20' N., long 66°20' W.), the magnetic signatures and 

proximity of Cuchivero volcanic rocks suggest that these 

circular features represent hidden volcanic centers. Other 

circular features in southern Estado Bolivar visible in radar 

imagery (for example at lat 4°50' N., long 62°55' W.), for 

which aeromagnetic data are unavailable, may also represent calderas in Cuchivero volcanic rocks. These features 

are all relatively large, on the order of 10 km in diameter, 

and they imply an enhanced potential from the point of 

view of economic geology.

Numerous smaller circular and subcircular features 

are visible in the side-looking radar imagery for which we 

have no ready explanation. Several near Puerto Ayacucho 

have been investigated on the ground without determining 

any apparent cause for their existence (Vicente Mendoza, 

CVG-TECMIN, oral commun., 1988). They may represent buried intrusive bodies, similar to the feature exposed 

at La Churuata, or perhaps hidden carbonatite intrusive 

bodies.

DEEP HIGH-DENSITY BODIES

In parts of the Puerto Ayacucho 1:500,000-scale 

quadrangle (fig. 3), there is sufficiently close spacing of 

gravity data to observe and model two pronounced Bouguer gravity anomalies. One of these anomalies (fig. 9) is 

centered around lat 4°30' N., long 67°30' W., and is represented on the geologic map (pi. 2) by a coarse-mesh square 

hachure pattern. The modeled source body (fig. 10) is a 

dense mass buried beneath about 5 km of granite and a 

thin veneer of alluvium; two circular subanomalies may be 

deeper extensions. Another deep dense body (shown using 

the same map pattern) is about 150 km east of Puerto 

Ayacucho at approximately lat 5°15' N., long 66°00' W. 

Although other deep high-density bodies may be present, 

gravity coverage is insufficient to provide direct identification. The first, and larger, body may have domed up the 

older Parguaza granite, exposing the underlying Santa Rosalia granite.

POSSIBLE RIFTLIKE FEATURE IN TERRITORIO 

FEDERAL AMAZONAS

South of Puerto Ayacucho and crossing the westflowing part of the Rio Orinoco is a large linear feature, 

trending about N. 30° W., that is mostly apparent only in
26 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

67° 66°

50 KILOMETERS

Figure 9. Contours of the simple Bouguer gravity anomaly in the area of a deep high-density body east of Rfo Orinoco and south of 

the city of Puerto Ayacucho, Territorio Federal Amazonas. Contour interval 2 mGal. Solid circles are gravity stations. Line of profile of 

model in figure 10 is also shown.

the geophysical data. It begins in the south at lat 3°00/ N., 

long 67°00/ W., and is manifested south of the westflowing branch of the Rio Orinoco by a 10-20-km-wide 

band of highly magnetic rocks interpreted in the geologic 

map as Cuchivero volcanic rocks (unit Xc, pi. 2). In the 

gravity data (pi. 4), a sharp gradient follows the southern 

half of this axis; the lower density side is to the east and 

possibly represents the downthrown side of a major fault. 

This gravity gradient bends around the east side of the 

high-density body discussed previously.

North of where the structure crosses the Rfo Orinoco, 

this feature extends at least to lat 4°45' N., long 67°45' W., 

and is manifested in both magnetic and gravity anomalies. 

A volcanic center is interpreted to be present at lat 4°05/ N., 

long 67°25/ W. The northern two points anchoring this feature are the possible feeders for the deep high-density body 

referred to previously. We interpret this feature as a possible ancient rift system, with volcanic rocks filling the

edifice. This feature is barely visible in the radar imagery, 

mainly because the area is a plain of low relief. The main 

exception is the long linear course of the Rio Guayapopo, 

which is north of the west-flowing branch of the Rfo 

Orinoco that follows the axis of this string of geophysical 

anomalies.

HIDDEN, HIGHLY MAGNETIC BODIES

A number of magnetic anomalies, generally of high 

frequency (shallow source) and linear in form, are represented in the geologic map (pi. 2) as diagonal hachures 

without a solid border. If the source body was interpreted to be negatively polarized (for instance, the body 

at lat 5°15' N., long 67°40/ W., modeled in fig. 6), it is 

represented by horizontal hachures.

Where there is sufficient aeromagnetic coverage, such 

as in the northeastern reaches of Estado Bolivar, in
GEOPHYSICS OF THE VENEZUELAN GUAYANA SHIELD 27

S. 67° W. N. 67" E.

ooooo Fitted 

, Base level

80 160 

DISTANCE, IN METERS

240

Figure 10. Model of the simple Bouguer gravity anomaly of the 

deep high-density body east of Rfo Orinoco and south of the city 

of Puerto Ayachuco shown in figure 9. Best fit is achieved by a 

prismlike body that is buried 6.84 km below the surface, is approximately 15 km thick, and has an upper surface of 87 km by 42 km 

and a density contrast of 0.064 g/cm3 . The body dips 61° SW.

Territorio Federal Amazonas, and in southwestern Estado 

Bolfvar adjacent to the latter, these generally linear anomalies have been interpreted, in most cases, to be buried mafic 

dikes (unit d, pi. 2). For some of these anomalies, depth-tosource calculations have been done using the steepest tangent method (Vacquier and others, 1951). The results of 

these calculations are indicated on the geologic map as 

integers representing the depth of burial in kilometers.

In Territorio Federal Amazonas and southwestern 

Estado Boivar, results of the aeromagnetic interpretation 

are represented on plate 2 as hachures or dashed lines for 

buried bodies and as geologic units when the magnetic data 

or side-looking radar imagery indicate surficial sources. For 

most of the latter interpretations ground verification is 

available that permitted me to characterize the units geologically in some detail and to describe the stratigraphic relations. In some cases, side-looking radar imagery was used 

to strengthen my interpretation and correlation with similar, 

known features. In the upper Rfo Orinoco drainage, only 

sparse geologic information is available (Vicente Mendoza, 

CVG-TECMIN, oral cornmun., 1991), and airphoto information was used to supplement geologic sampling, aeromagnetic data, and radar imagery (Y£nez, 1991) to 

complete assembly of the geologic map.

MAGNETIC GRADIENTS

In several areas, strong magnetic gradients extend for 

very long distances, sometimes more than 100 km. These 

gradients are represented as strings of heavy, relatively 

widely spaced dots on the geologic map (pi. 2) (not to be

confused with the closely spaced dots in the Imataca Complex that represent ridges of silicified iron formation). 

Several interpretations are possible for these magnetic 

gradients, including sutures or accretions of Precambrian 

terranes or margins of failed rift systems.

In several areas, strings of high-frequency (shallowsource "d" hachures) magnetic anomalies follow the axis 

of the lower frequency (deeper source) magnetic gradients. 

These probably represent dikes intruding a major crustal 

fracture or zone of weakness. Particularly good examples 

are in the northeastern part of the Guri quadrangle (Wynn 

and Cox, unpublished data) and in the Puerto Ayacucho 

quadrangle (the radial dikelike features over the deep 

gravity anomaly previously discussed, fig. 9). In the latter 

area, the strings of high-frequency anomalies are centered 

over the apparent deep intrusive mass and form a radial 

pattern suggestive of dikes radiating from a large buried 

pluton and extending to the surface. The close spatial correlation strongly suggests a genetic relation between the 

narrow radial dikes and the buried pluton.

HIDDEN LATE GRANITE INTRUSIVE BODIES AND

DOMES IN ROCKS OF THE RORAIMA GROUP AND

ICHUN FORMATION

In a number of areas, a magnetic signature distinct 

from the surrounding terrane suggests the presence of a 

hidden intrusive body. In a few examples along the border 

between the Puerto Ayacucho and Rio Caura quadrangles, 

these anomalies also apparently coincide with uranium 

radiometric anomalies and possibly represent extremely 

shallow, if not superficially exposed, granitic intrusive 

bodies. In one example in the upper Orinoco drainage, a 

single outcrop in the river allowed identification and characterization of a granite body visible in the magnetic data 

but not apparent in the Landsat or radar imagery.

CONCLUSIONS

Regional geophysical data, including aeromagnetic, 

radiometric, and gravity, and side-looking radar and airphoto imagery, have proven valuable in helping to map 

remote areas of Territorio Federal Amazonas and southern 

Estado Bolivar in Venezuela, especially where soil and 

vegetation cover is extensive. These data contributed significantly to the compilation of the geologic and tectonic 

framework map of the Venezuelan Guayana Shield 

included herein and were used to guide the delineation of 

geologic contacts and buried tectonic features. Reconnaissance geologic mapping was used wherever possible to 

confirm the geophysically derived contacts and to assign 

geologic names and descriptions to units defined by means 

of these regional data sets.

MINES, PROSPECTS, AND OCCURRENCES OF THE 

VENEZUELAN GUAYANA SHIELD

By G.J. Orris, Norman J Page, Karen Sue Bolm, Floyd Gray, William E. Brooks, 

Marguerite M. Carbonaro, and Richard Kibbe

More than 450 mines, prospects, and occurrences 

within the area underlain by the Venezuelan Guayana 

Shield were compiled for this report. These sites are 

shown on plate 5 and listed by map number in table 3. 

Some of these sites are given in earlier compilations 

including those by the Ministerio de Minas e Hidrocarburos (1958, 1959a, b), Bellizzia, Pimentel de Bellizia, and 

Rodriguez (1981), Rodriguez (1986), and Sidder (1990). 

Additional information was obtained from the literature or 

from field observations made during this project. Although 

the Venezuelan Guayana Shield is rich in a wide variety 

of mineral resources, identification and location of known 

mines and occurrences in the literature is uneven and 

incomplete. Gold, diamonds, iron, and bauxite are the 

major exploited resources of the study area and are better 

documented than other commodities. Even for these commodities, only a few major deposits are discussed with 

any detail, and maps are extremely rare. Other commodities reported in the Venezuelan Guayana Shield include 

manganese, kaolin, dolomite, dimension stone, tin, sand 

and gravel, barite, phosphate, minerals of the rare earth 

elements, titanium, molybdenum, uranium, thorium, and 

tungsten. Information on these commodities is erratic. 

Notably absent in the mining record are occurrences of 

primary deposits of nickel, platinum-group elements, and 

the base metals (copper, lead, and zinc).

Precise location of the known mines and occurrences, 

even at a map scale of 1:1,000,000, was the most difficult 

part of this compilation. Many sites were located solely 

using written descriptions. Other sites were located using 

sketch maps with inconsistent base information. Locations shown on plate 5 represent the best estimate of the 

position of the mine or occurrence given available information and relative to the mapped geology of plate 2 and 

named rivers, roads, and towns. A list of some of the 

mines and occurrences that have not yet been located is 

given at the end of table 3. The symbols for the deposits 

shown on plate 5 represent commodity and deposit type. 

At some locations two commodities are shown. Commodities for each numbered deposit shown on the map are 

given in table 3.

Of the compiled mineral sites, more than 200 are for 

gold. Native gold in quartz veins is reported at 125 sites

(table 4), and placer workings are reported at more than 

80 locations (table 5). At some sites both weathered vein 

and alluvial deposits are present. More than 70 of the vein 

gold mines and occurrences are in the El Callao and Lo 

Increfble districts; they represent only a third of the more 

than 230 gold-quartz veins reported to be present in these 

districts (Rodriguez, 1986). The vast majority of the veingold deposits and prospects can be classified as belonging 

to the low-sulfide gold-quartz vein deposit model (Cox 

and Singer, 1986). The placer operations mine alluvial 

deposits for the most part but locally may exploit residual 

gold from older terrace deposits or weathered veins.

More than 106 diamond mines and occurrences, most 

of which are alluvial, have been identified in the Venezuelan Shield (table 6). Fourteen iron-manganese deposits, 

more than thirty-five banded iron formation, seven sedimentary^) manganese deposits (table 7), and more than 

forty bauxite occurrences (table 8) are known. An alphabetical listing of mines, prospects, and mineral occurrences in the shield, with reference to map number as 

shown on plate 5, is given in table 9.

The mineral occurrence data, as compiled for this 

report, have been entered into the U.S. Geological Survey's Mineral Resources Data System (MRDS). MRDS 

consists of computerized data for more than 92,000 

mineral occurrences (as of December 1992) and deposits 

worldwide and the computer software necessary to access 

that data. During this project, MRDS was installed on personal computers belonging to T6cnica Minera, C.A., a part 

of the Corporaci6n Venezolana de Guayana, in Puerto 

Ordaz and Ciudad Bolfvar. Inquiries about information 

stored in MRDS may be obtained from the regional MRDS 

representative at the U.S. Geological Survey, Minerals 

Information Office, Corbett Building, 340 North 6th Avenue, Tucson, Arizona 85705-8325, or at the U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, Virginia 

22092.

For reference, a smaller scale map (pi. 6) was compiled using the MRDS database that shows the locations 

of mineral occurrences in northeastern South America; 

the unedited MRDS data for this area are given in the 

appendix.

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401 Dolomite quarry

402 Kaolin quarry

403 Upata district

404 Upata bauxite

405 Ancho Caroni

BO 

BO 

BO 

BO 

BO

Upata

Dolomite

Kaolin

Bauxite

Aluminum, manganese

Diamond

Metamorphic (?)

Residual

Laterite

Laterite

Alluvial

Proterozoic

Cenozoic

Cenozoic

Dolomite

High-grade metamorphic rocks

Ferruginous quartzite, gneiss

406 Sand quarry

407 Ganges I

408 Ganges II

409 Ganges III

410 Dimension stone quarry

411 CerroPando

412 Cerro Abanico

413 Delta

414 Gavilan

415 Monte Romero

BO 

BO 

BO 

BO 

BO

BO 

BO 

DE 

BO 

BO

Upata

Sand and gravel 

Manganese, iron 

Manganese, iron 

Manganese, iron 

Dimension stone

Manganese, iron

Manganese

Aluminum

Bauxite

Iron

Alluvial

Residual?

Residual?

Residual?

Dimension stone, metamorphic

Residual?

Residual?

Laterite, residual

Residual

Banded iron formation

Cenozoic

Precambrian-Cenozoic

Precambrian-Cenozoic

Precambrian-Cenozoic

Proterozoic

Precambrian-Cenozoic 

Precambrian-Cenozoic

Archean

Alluvial sediments

Schist, gneiss, dolomite, marble

Schist, quartzite, gneiss, dolomite

Schist

Gneiss

Gneiss, schist, quartzite, dolomite 

Gneiss, quartzite, dolomite 

Gabbroid rocks

3

8

8

416 Cuyubini

417 Las Delicias

418 El Palmar

419 RioAcure

420 Caruachi placers

421 Sand quarry

422 Sand quarry

423 Sand quarry

424 Morocota

425 Unnamed placer workings

426 Wausa

427 PolvodeOro

428 LaLinea

429 Sand quarry

430 San Felix placers

DE

BO Upata

BO

DE

BO Rio Caroni

BO 

BO 

BO 

BO 

BO

DE 

DE 

DE 

BO 

BO Lower Rio Caroni

Iron

Manganese, iron 

Manganese, iron, 

aluminum 

Bauxite, clay

Diamond

Sand and gravel 

Sand and gravel 

Sand and gravel 

Iron, manganese 

Gold

Gold, bauxite

Iron

Iron

Sand and gravel

Diamond

Banded iron formation

Residual

Residual?

Sedimentary (?), alluvial,

secondary

Diamond placers

Alluvial

Alluvial

Alluvial

Sedimentary iron-manganese

Alluvial, placer

Alluvial, residual, laterite

Laterite, banded iron formation

Banded iron formation

Alluvial

Diamond placers, alluvial

Archean

Precambrian-Cenozoic

Precambrian-Cenozoic

Cenozoic

Cenozoic

Cenozoic

Precambrian

Cenozoic

Archean-Cenozoic

Archean

Cenozoic

Gneiss, quartzite, dolomite 

Schist and kaolinized gneiss

Quartz gravel

Alluvial sediments 

Alluvial sediments 

Alluvial sediments

Alluvial sediments

Alluvial sediments 

Quartz gravel

n

C

w

w 

w

N

G W

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431 San Juan de Amacuro DE

432 Kaolin quarry BO

433 Manoa DE ElPao

434 Construction sand quarry BO

435 Amacuro Delta region aluminum DE

436 RioAroi DE

437 Santa Catalina DE

438 Imperial mine DE ElPao

439 Piacoa DE

440 Morocota DE

Iron Banded iron formation Archean

Kaolin Sedimentary Cenozoic

Iron Bedded, banded iron formation Archean 

Sand and gravel Alluvial Cenozoic

Bauxite Laterite bauxite, residual

Bauxite Laterite, residual

Iron Banded iron formation Archean

Iron Banded iron formation Archean

Iron Banded iron formation Archean

Iron Banded iron formation Archean

Alluvial sediments 

Quartzite 

Alluvial sediments

Quartzite

Granitic gneiss and granulite

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CUCUCOCOCO COCOCOHD
MINES, PROSPECTS, AND OCCURRENCES OF THE VENEZUELAN GUAYANA SHIELD 43

Table 4. Known low-sulfide gold-quartz vein deposits of the Venezuelan Guayana Shield.

[Shown by map number on plate 5; leader (-) indicates site is not shown on map. Y indicates production; N indicates no production; U indicates 

production status undetermined. Reserves are shown where known]

Site name

Amparo

Apollo 8

Bochi

Bochinche

Botanamo

Canaima

Cenicero

Cerro Arrendajo

Cerro La Pinto mine

Chile

Cicapra zone

Colombia

Concordia

Cristina-Bizkaitarra

El Callao district

El Carmen

El Carmon

El Choco

El Crucero

El Dorado

ElFoco

ElGrito

ElLeon

El Pauji

El Placer

El Roble

El Tesoro

El Tigre

El Valle

Eureka

Experiencia

Florinda

Hueco Rico

Iguana

Kilometro 88-Las Claritas

La

La Armonica

LaCamorra

La Cubanita

La Culebra

La Divina Pastora

La Espanoa

La Esperanza

La Estrella

La Estrella

La Foraleza

La Foresta

La Introduccion

LaLapa

La Lapa-1

La Lira

LaLoca

La Patricia

La Plata

La Reforma

La Rosa

Map No.

309

-

347

334

211

191

242

108

312

244

331

-

233

123

259

313

198

235

286

184

132

307

-

116

190

274

344

333

282

243

266

311

320

257

124

290

276

186

272

263

294

302

325

177

224

297

303

351

300

284

178

287

-

301

230

219

Production

U

Y

U

N

136,665 t, 32.7 g/t Au (to 1968)

Y

1,324 1, 14.46 g/t Au (to 1968)

Y

N

Y

Y

U

Y

Y

123,962 kg Au (1829-1980)

84.8 15 kg Au (to 1968)

U

Y

U

Y

Y

Y

200 1, 13.0 g/t Au (1968)

Y

l,165t, 16.7 g/t Au (to 1968)

Y

N

Y

Y

2,289 t, 8.5 g/t Au (to 1968)

53,278 t, 10.6 g/t Au (to 1968)

U

U

12,452 t, 8.6 g/t Au (to 1968)

Y

Y

Y

4,218 t, 19.7 g/t Au (to 1968)

Y

27,915 t, 10.2 g/t Au (to 1968)

Y

Y

Y

U

Y

Y

Y

Y

Y

Y

52 1, 20 g/t Au (to 1968)

Y

U

Y

Y

Y

Reserves

2,000 1, 12 g/t Au( 1986)

887,000 1, 19.06 g/t Au (1990)
44 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Table 4. Known low-sulfide gold-quartz vein deposits of the Venezuelan Guayana Shield Continued.

Site name Map No. Production Reserves

La Salvation

Laguna mine-Santa Rita

Las Chichorras

Las Claritas de Uroy

Las Morochas

Las Nieves

Las Tres Rosas-2

358 U

252 1,385,220t, 16.8 g/t Au (1989-1971)

329 Y

126 N

296 Y

367 663t, 7.2 g/t Au (to 1968)

285 Y

Lo Increible

Lo Increible district

Macondo

Malave

Mandingal

Margarita

Marwani-Los Caribes

Marwani I

MarwanilV

Mejico

Mi Venezuela

Mina A

MinaB

MinaC

Mina Cali

Mina Colombia

Mina Corozo

Mina Morocha-Fortaleza

Mina Nueva

MinaOrdaz

Mina Ortega

Mina Plataneal 

Mina Sonia 

Mina Zenovia 

Mocupia 

Nacupal 

Nuevo Callao 

Payapal

Pefla

Peru

Pierina

Piston de Uroy

Potosi

Purgatorio

Quebrada de Oro

Remington

Rio Cuyuni-Rio Uey

Ropeway and Refugio

Salva le Peine

Salvador

San Antonio

San Felipe

San Felipe 

San Miguel 

Santa Ana 

Santa Barbara 

Santa Elena 

Santa Ines 

Sofia

279

256

278

281

336

310

208

226 

221 

234 

283 

299 

_

254

280

253

292

-

19

-

295

20

288

271

246

238

213

192

Y

42

Y

U

Y

Y

Y

Y

Y

Y

31

Y

Y

Y

247

273

137

237

228

337

260

110

248

319

215

249

346

193

304

293

308

291

267

100,364 t, 13.2 g/t Au (to 1968)

Y

Y

Y

Y

N

N 

Y 

Y 

Y 

4,200 t, 80 g/t Au (1989)

429,442t, H.28 g/t Au (to 1968) 1,550,0001, 9.2 g/t Au (1988)

314,869t, 16.5 g/t Au (1928-1946) 4,600,000t, 1.35 g/t Au (1990)

20,568t, 22.96 g/t Au (to 1968)

Y

N

23,898 t, 20.5 g/t Au (to 1968)

Y

Y

50,965 t, 17.7 g/t Au (to 1968)

Y

3,013 t, 10.4 g/t Au (to 1968)

Y

Y

5,2621, 12.2 g/t Au (1866-1968

30,7521, 13.3 g/t Au (to 1968)

1201, 10.5 g/t Au (to 1968)

Y

Y

Y

Y

Y

4,500,0001, 4 g/t Au (1985)
MINES, PROSPECTS, AND OCCURRENCES OF THE VENEZUELAN QUAY ANA SHIELD 45

Table 4. Known low-sulfide gold-quartz vein deposits of the Venezuelan Guayana Shield Continued.

Site name Map No. Production Reserves

Sosa Mendez-Union mine

Sua-Sua

Talisman

Talisman Cova

Te Esperaba

Troya

Union-1

Union-2 

Unnamed

Unnamed gold vein 

Viejo Callao 

Vuelvan Caras 

YiYi

245 27,65017.0 g/t Au (to 1968)

204 Y

270 1,7451, 11.4 g/t Au (to 1968)

269 Y

277 Y

	151, 7.8 g/t Au (to 1968) 

275 Y

298 Y

128 Y

	U

261 761,2721, 68 g/t Au (to 1968)

220 802,533 kg Au (to 1968)

289 Y

2,000,0001, 12 g/t Au (1969)

Table 5. Known placer gold deposits of the Venezuelan Guayana Shield.

[Shown by map number on plate 5; leader (-) indicates site is not shown on map. Y indicates production; N indicates no production; U indicates production status 

undetermined. Where production value is given it is to 1968]

Site name

Agua Negra

Anacoco placers

Aponwao

Avechica

Barrialon placers

Bochinche

Cafio Maraya

Carmen Rosa

Cerro Azul Valley placers1

Cerro Yapacana I

Chiricayen placer

Cicapra zone

Cinco Ranches placers

Corazon de Jesus

Corocoro

Corre Gente

El Callao district

El Casabe placers

El Chivao placers

El Polaco placers

El Tigre

Guaniamo I

Guariche

Guatuaima

Hacha placers

Hueco Rico

Icabani placers

Kamarata

Kamu placers

La Bandera placers

La Carabobo

La Cayena

La Estrella

La Estrella

La Faisca placer

La Hollada placers

La Introduction

La Pena placer

La Playita

La Rosa

Map No.

210

189

176

217

104

334

24

209

339

23

51

331

37

179

442

355

259

163

136

46

333

-

182

185

34

320

32

89

85

33

130

352

177

224

50

42

351

48

167

219

Production

Y

Y

Y

Y

Y

N

Y

1,666 1, 19 g/t Au

N

N

Y

Y

Y

U

3,310kg

U

Y

Y

Y

Y

Y

Y

Y

N

Y

Y

Y

Y

Y

Y

2,041 kg

Y

U

Y

Y

Y

Y

Y

Y

Y

Site name

La Salvacion

Larinal

LasPavas

Macapa

Manaima

Maria Varela

Mina A

Miribisi-Manarevaca

Panama

Pao

Parapapoy

Parupa placers

Paviche

Placers of Bochinche

Polvo de Oro

Pumpiri

Quebrada Cachanaca

Quebrada Mochila placer

Rio Chiguao placers

Rio Cuyuni-Rio Uey

RioMarwani

Rio Suruma

Salto Araguai

San Luis placers

SanRafael

Sesina

Sipao

Unidentified placer workings

Unnamed

Unnamed

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed in Gran Sabana placers

Unnamed in Gran Sabana placers

Unnamed in Gran Sabana placers

Vuelvan Caras placers

Wausa

Yuruan

Map No.

359

102

361

196

117

67

299

-

168

64

169

88

250

335

365

28

133

174

98

110

216

354

118

35

201

205

330

425

119

135

183

188

197

203

58

69

79

225

426

202

Production

Y

Y

U

Y

U

Y

Y

U

U

Y

238.7kg

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

N

Y

Y

Y

Y

Y

Y

N

Y

Y

Y

Y

U

Y

'Reserves 4,800,000 m3, 0.14 g/m3 (1970).
46 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Table 6. Known diamond deposits of the Venezuelan Guayana Shield.

[Shown by map number on plate 5. Y indicates production; N indicates no production; U indicates production status undetermined]

Site name

Agua Colorada

Alto Aponguao

Aho Kukenan

Alto Orinoco

Aho Ventuari

Alto Ventuari

Alto Ventuari

Ancho Caroni

Aparuren

Aponguao

Arabopo

Asa placers

Avequi placers

Avispa

Aza

Bocon placers

Bogarinmine

Buena Esperanza placer

C.O.D.S.A.

Caicara

Campo Grande

Campo Grande

Capaura placers

Carapo

Cardona

Caruachi placers

Camay

Cepillito

Cerbatana

Chicanan

Ciudad Piar

Curao

Don

El Candado

El Caracol

El Caracolito

El Carmen

El Infierno

El Loco placers

El Merey placers

El Milagro

El Pagon placer

El Pao de La Fortuna placers

El Resbalon del Diablo

El Toco

Flora Blanca placers

Gallito

Gran Sabana

Gran Sabana

Gran Sabana

Guacharaca

Guacharaquito

Guaibal-El Mango

Guaniamo district

Guaramoni

Map No.

30

77

56

12

61

63

71

405

73

55

70

159

80

2

195

172

105

229

49

338

139

115

82

92

1

420

68

140

145

173

262

153

170

144

164

161

99

41

74

364

155

236

212

158

141

60

103

57

54

66

86

81

3

207

10

Production

Y

U

U

U

U

U

U

Y

U

U

U

Y

Y

U

U

Y

Y

Y

Y

Y

Y

Y

8,880 carats (1978)

Y

U

Y

Y

Y

Y

Y

U

Y

Y

Y

Y

Y

U

U

Y

6,350 carats (1977-1980)

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

Y

3,654,830 carats (1975-1980)

Y

Site name

Guayaraca

La Bicicleta

La Cu-Ca

LaCuaima

LaCuaimita

LaHoya

La Libertad

La Paragua

La Paragua placers

La Sabanita placers

La Salvacion

Leoncio

Los Caribes placers

Los Frijoles placers

Maijia placers

Matute

Mavaca-Casuquare

Paragua

Paraitepuy placers

Paramichi placers

Pila Blanca

Playa Blanca placers

Quebrada Grande placers

Rio Antavari

Rio Aponguau placers

RioAro

Rio Caroni (no. 18)

Rio Caroni placers

Rio Carrao placers

Rio Claro placers

Rio Los Dos Pozos

Rio Marevari

Rio Merevari

RioOris

Rio Suapure

San Felix placers

San Juan

San Pedro de Las Bocas placers

San Salvador de Paul district

Santa Elena de Uairen placers

Santa Teresa

Solano

Tres Cheques

Unnamed diamond occurrence

Uraday

Uraima-Urutani

Uraima-Urutani

Uraima-Urutani

Unman placers

Veri

Yiguiripin placers

Map No.

100

157

150

152

146

138

200

181

65

78

147

122

29

72

31

142

8

180

40

26

399

400

162

76

53

231

171

154

134

390

323

38

43

131

127

430

111

206

106

47

45

4

151

109

87

143

148

149

83

194

62

Y

Y

Y

Y

Y

Y

Y

Y

Production

25,590 carats (1975-1980)

Y

Y

Y

Y

Y

Y

Y

U

Y

Y

Y

Y

Y

7,500,000 carats (to 1968)

Y

Y

Y

Y

Y

Y

Y

U

U

U

Y

Y

Y

Y

300 carats (1977-1980)

823,920 carats (1975-1980)

21,020 carats (1978-1980)

U

U

Y

Y

U

Y

Y

Y

13,710 carats (1975-1980)

Y

Y
MINES, PROSPECTS, AND OCCURRENCES OF THE VENEZUELAN GUAYANA SHIELD 47

Table 7. Known iron and manganese deposits of the Venezuelan Guayana Shield.

[Shown by map number on plate 5. Y indicates production; N indicates no production; U indicates production status undetermined]

Site name

Altamira2

Cerro Abanico

Cerro Altamira

Cerro Arimagua

Cerro Bolivar

Cerro de la Esperanza

Cerro Etuna

Cerro Frontera

Cerro Gutierrez

Cerro La Estrella

Cerro Las Adjuntas, Punta del Cerro

Cerro Monte Cristo

Cerro Pando

Cerro Redondo

Cerro San Cristobal district

Cerro Toribio

Cuyubini

El Manganeso

El Manteco

El Padamo

El Palmar

ElPao

El Trueno

Ganges I

Ganges II

Ganges III

Guacuripia

Imperial mine

La Estrella

La Horqueta

LaLinea

Las Delicias

LasGrullas

LasPailas

LasPavas

Los Barrancos I and II

Manoa

Maria Luisa

Monte Bello

Monte Romero

Morocota

Morocota

Piacoa

Placo

Piacoa

Polvo de Oro

Real Corona

Sacupana

San Isidro

San Isidro Iron Quadrilateral

San Joaquin

San Juan de Amacuro

San Lorenzo

Santa Barbara

Santa Catalina

Santa Maria deposit

Santa Rosa

Sierra El Lindero

Tamatama

Terecay

Map No.

327

412

321

348

315

332

Til

316

398

251

264

386

411

324

314

340

416

389

370

18

418

391

223

407

408

409

378

438

322

372

428

417

232

241

362

240

433

357

384

415

424

440

439

-

239

427

265

441

268

305

306

431

379

258

437

377

383

-

13

326

Commodity

Iron, gold

Manganese

Iron

Iron

Iron

Manganese, iron

Iron

Iron

Iron

Iron

Iron

Iron

Manganese, iron

Iron

Mn, Fe, Cu, Pb, Zn

Iron

Iron

Manganese

Manganese

Iron

Mn, iron, aluminum

Iron, manganese

Iron

Manganese, iron

Manganese, iron

Manganese, iron

Manganese

Iron

Iron

Manganese

Iron

Manganese, iron

Iron

Iron

Manganese

Iron

Iron

Iron

Iron

Iron

Iron, manganese

Iron

Iron

Iron, manganese

Iron

Iron

Iron

Iron

Iron

Iron

Iron

Iron

Manganese, iron

Iron

Iron

Manganese, iron

Iron, manganese

Manganese

Iron

Iron

Production

Y

Y

Y

Y

287 Mt (to 1974)

N

U

Y

Y

Y

N

N

N

Y

N

Y

U

U

U

u

N

Y

Y

N

N

N

N

N

N

U

U

Y

Y

Y

U

Y

Y

N

U

U

U

U

N

U

Y

U

U

U

Y

Y

Y

U

Y

N

U

N

N

U

U

N

Reserves

2 Mt, 58 percent Fe (1970)

137 Mt, 63 percent Fe measured (1985)

136 Mt, 63 percent Fe measured (1970)

216 Mt, 64 percent Fe (1981)

1 Mt, 64 percent Fe( 1970)

8Mt(1986)

44 Mt, 62 percent Fe (1986)

90 Mt, 62 percent Fe (1986)

18 Mt, 64 percent Fe (1986)

97 Mt, 61 percent Fe measured (1970)

110 Mt, 62 percent Fe (1986)

0.3 Mt, 20.91 percent Mn, 17.8 percent Fe (1967)

0.05 Mt, 34. 1 1 percent Mn, 10.60 percent Fe (1967)

0.02 Mt, 19.89 percent Mn, 19.25 percent Fe (1967)

6 Mt, 58 percent Fe (1986)

44 Mt, 62 percent Fe (1986)

50 Mt, 50 percent Fe (1970)

80 Mt, 58 percent Fe (1970)

232 Mt, 58 percent Fe measured (1970)

1.4 Mt, 68 percent Fe (1986)

258 Mt (1986)

181.12 Mt, <55 percent Fe (1986)

125 Mt, 45 percent Fe (1970)

5 Mt, 45 percent Fe (1986)

394 Mt, 65 percent Fe (1986)

677 Mt, 57 percent Fe measured (1970)

65 Mt, 62 percent Fe measured (1970)

3.510 Mt, 64 percent Fe (1986)
48 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Table 8. Known bauxite deposits of the Venezuelan Guayana Shield.

[Shown by number on plate 5; leader (-) indicates site is not shown on map. Y indicates production; N indicates no 

production; U indicates production status undetermined]

Site name

Amacuro Delta region aluminum

Casiquiare

Cerro del Hormiguero, A12O3

Cerro Once

Copeyal

Delta

Divina Pastora

El Baiil

El Chorro

El Para

Gavilan

Gran Sabana

Gran Sabana

Gran Sabana

Guaicas

Kamoiran

Kavanayen

Kilometre 88

LaGrulla

Las Nieves

Las Pifias

Los Guamos

Los Pijiguaos

Luepa

Maua

Mesa de La Carata

Moitaco

Nuria Plateau

Platanal II

Platanal I

Rio Acure

Rio Aro

Rio Aroi

Rio Suapure bauxite occurrence

Rio Villacoa bauxite occurrence

Serrania La Cerbutana

Serrania Los Pijiguaos

Serrania Parguaza

Unnamed bauxite occurrence

Upata bauxite

Upata district

Map No.

435

6

343

395

393

413

44

397

392

52

414

97

95

90

120

93

96

113

-

341

_

394

160

94

91

396

366

345

9

11

419

255

436

112

125

107

156

129

175

404

403

Production

N

N

N

N

N

U

N

N

N

U

N

N

N

N

N

N

N

U

U

N

U

N

Y

U

U

N

U

N

U

U

N

U

N

N

N

N

N

N

N

N

N

Reserves

27.5 Mt, 34.49 percent Al (1972)

1.0 Mt (1961)

0.79 Mt (1961)

1.26 Mt (1961)

20 Mt, 35.5 percent A12O3 (1986)

0.5 Mt (1961)

500 Mt, 48 percent A12O3 (1985)

596 Mt( 1961)

8 Mt, 30 percent A12O3 (1986)

4Mt(1973)
MINES, PROSPECTS, AND OCCURRENCES OF THE VENEZUELAN GUAYANA SHIELD 49

Table 9. Alphabetical listing of mines, prospects, and mineral occurrences, Venezuelan Guayana Shield.

[Shown by map number on plate 5; leader (-) indicates site is not shown on map]

Mineral site

Agua Colorada 

Agua Negra 

Aguamena and Boquerons 

Altamira2 

Alto Aponguao

Alto Kukenan

Alto Orinoco

Alto Paragua 

Alto Ventuari 

Alto Ventuari

Alto Ventuari

Amacuro Delta region aluminum 

Amparo 

Anacoco placers 

Ancho Caroni

Aparuren 

Apollo 8 

Aponguao 

Aponwao 

Arabopo

Asa placers 

Avechica

Avequi placers 

Avispa 

Aza

Barrialon placers 

Bochi 

Bochinche

Bocon placers 

Bogarin mine

Botanamo 

Buena Esperanza placer 

C.O.D.S.A. 

Caicara 

Campo Grande

Campo Grande 

Canaima

Cano Aguamena 

Cano Maraya 

Capaura placers

Carapo 

Car dona 

Carmen Rosa

Caruachi placers 

Caruay

Casiquiare 

Caura

Caura placers 

Cenicero

Cepillito

Cerbatana

Cerro Abanico

Cerro Altamira

Cerro Arimagua 

Cerro Arrendajo

Map No.

30 

210 

121 

327 

77

56

12

25 

61 

63

71

435 

309 

189 

405

73

55 

176 

70

159 

217

80 

2 

195

104 

347 

334

172 

105

211 

229 

49 

338 

115

139 

191

24 

82

92 

1 

209

420 

68

6 

166

84 

242

140

145

412

321

348 

108

Mineral site

Cerro Azul Valley placers 

Cerro Bolivar 

Cerro Copeyal 

Cerro de La Esperanza 

Cerro del Hormiguero

Cerro Etuna

Cerro Frontera

Cerro Gutierrez 

Cerro Impacto 

Cerro La Estrella

Cerro La Pinto Mine

Cerro Las Adjuntas, Punta del Cerro 

Cerro Monte Cristo 

Cerro Once 

Cerro Pando

Cerro Redondo 

Cerro San Cristobal district 

Cerro Santa Rosa 

Cerro Toribio 

Cerro Toro

Cerro Yapacana I 

Chicanan

Chile 

Chiricayen placer 

Churuata

Cicapra zone 

Cinco Ranches placers 

Ciudad Piar

Colombia 

Concordia

Conoroto placers 

Construction sand quarry 

Construction sand quarry 

Construction sand quarry 

Copeyal

Corazon de Jesus 

Corococo

Corre Gente 

Cristina-Bizkaitarra 

Cunucunuma

Curao 

Cuyubini 

Delta

Dimension stone quarry 

Divina Pastora

Dolomite quarry 

Dori

El Baul 

El Callao district

El Candado

El Caracol

El Caracolito

El Carmen

El Carmen 

El Carmon

Map No.

339 

315 

388 

332 

343

227

316

398 

101 

251

312

264 

386 

395 

411

324 

314 

385 

340

23 

173

244 

51 

17

331 

37 

262

233

59 

369 

375 

434 

393

179 

442

355 

123 

14

153 

416 

413

410 

44

401 

170

397 

259

144

164

161

99

313 

198
50 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 

Table 9. Alphabetical listing of mines, prospects, and mineral occurrences, Venezuelan Guayana Shield Continued.

Mineral site

El Casabe placers 

El Chivao placers 

El Choco 

El Chorro

El Crucero

El Dorado 

ElFoco

El Grillero 

ElGrito

El Infierno

ElLeon

El Loco placers 

El Manganese 

El Manteco 

El Manteco

El Merey placers 

El Milagro 

ElPadamo 

El Pagon placer 

El Palmar

ElPao

El Pao de la Fortuna placers 

ElParu

El Pauji 

El Placer

El Polaco placers 

El Resbalon del Diablo 

El Roble

El Tesoro

ElTigre

El Toco

El Trueno

El Valle

Eureka 

Experiencia

Flora Blanca placers 

Florinda

Gallito

Ganges I 

Ganges II

Ganges III 

Gavilan

Gran Sabana 

Gran Sabana 

Gran Sabana

Gran Sabana

Gran Sabana 

Gran Sabana 

Guacharaquito 

Guacuripia

Guacuripia area dolomite deposits 

Guaibal-El Mango 

Guaicas 

Guaniamo district 

Guaniamo I

Map No.

163 

136 

235 

392

286

184 

132

317 

307

41

_

74 

389 

318 

370

364 

155 

18 

236 

418

391

212 

52

116 

190

46 

158 

274

344

333

141

223

282

243 

266

60 

311

103

407 

408

409 

414

54 

57 

66

90

95 

97 

81 

378

350 

3 

120 

207

Mineral site

Guaramoni 

Guariche 

Guasipati La Pastora area phosphate 

Guatuaima

Guayaraca

Hacha placers 

Hueco Rico

Icabaru placers 

Iguana

Imperial mine

Kamarata

Kamoiran 

Kamu placers 

Kaolin quarry 

Kaolin quarry

Kavanayen 

Kilometro 88 

Kilometre 88 kaolin deposits 

Kilometro 88-Las Claritas 

La

La Arm6nica

La Bandera placers 

La Bicicleta

La Camorra 

LaCarabobo

La Cayena 

La Churuata uranuim anomaly 

LaCu-Ca

LaCuaima

LaCuaimita

La Cubanita

La Culebra

La Divina Pastora

La Espanoa 

LaEsperanza

La Estrella 

La Estrella

La Estrella

La Faisca placer 

La Foraleza

LaForesta 

LaGrulla

La Hollada placers 

La Horqueta 

LaHoya

La Introduccion

LaLapa 

La Lapa-1 

LaLibertad 

LaLinea

La Lira 

LaLoca 

La Paragua 

La Paragua placers 

La Patricia

Map No.

10 

182 

328 

185

100

34 

320

32 

257

438

89

93 

85 

402 

432

96 

113 

114 

124 

290

276

33

157

186 

130

352 

16 

150

152

146

272

263

294

302 

325

177 

224

322

50 

297

303

42 

372 

138

351

300 

284 

200 

428

178 

287 

181 

65
MINES, PROSPECTS, AND OCCURRENCES OF THE VENEZUELAN GUAYANA SHIELD 51

Table 9. Alphabetical listing of mines, prospects, and mineral occurrences, Venezuelan Guayana Shield Continued.

Mineral site

La Pena placer

La Plata

La Playita

La Reforma

La Rosa

La Sabanita placers

La Salvacion

La Salvacion

La Salvacion

Laguna mine-Santa Rita

Larinal

Las Chichorras

Las Claritas de Uroy

Las Delicias

LasFlores

LasGrullas

Las Margaritas

Las Morochas

Las Nieves

LasNieves

Las Pailas

LasPavas

LasPavas

Las Pittas

Las Tres Rosas-2

Leoncio

Lo Increible

Lo Increible district

Los Barrancos I and II

Los Caribes placers

Los Frijoles placers

Los Guamos

Los Pijiguaos

Luepa

Macapa

Macondo

Maijia placers

Malave

Manaima

Mandingal

Manoa

Margarita

Marhuanta

Maria Luisa

Maria Varela

Marwani-Los Caribes

Marwanil

MarwanilV

Matute

Maua

Mavaca-Casuquare

Mejico

Mercedes

Merevari geochemical anomaly

Mesa de La Carata

Map No.

48

301

167

230

219

78

147

358

359

252

102

329

126

417

214

232

376

296

341

367

241

361

362

-

285

122

279

256

240

29

72

394

160

94

196

278

31

281

117

336

433

310

218

357

67

208

226

221

142

91

8

234

222

27

396

Mineral site

Mi Venezuela

Mina A

MinaB

MinaC

Mina Cali

Mina Colombia

Mina Corozo

Mina Nueva

MinaOrdaz

MinaOrtega

Mina Plataneal

Mina Sonia

Mina Zenovia

Miribisi-Manarevaca

Mocupia

Moitaco

Monte Bello

Monte Cristo

Monte Romero

Morocota

Morocota

Mundo Nuevo

Nacupal

Nuevo Callao

Nuria Plateau

Panama

Pao

Paragua

Paraitepuy placers

Paramichi placers

Parapapoy

Parupa placers

Paviche

Payapal

Pena

Peru

Piacoa

Pierina

Pila Blanca

Piston de Uroy

Placers of Bochinche

Place

Piacoa

Platanal I

Platanal II

Playa Blanca placers

Polvo de Oro

Polvo de Oro

Potosi

Pumpiri

Purgatorio

Quartz mine

Quebrada Cachanaca

Quebrada de Oro

Quebrada Grande placers

Map No.

283

299

-

254

280

253

292

19

-

295

20

288

271

-

246

366

384

-

415

424

440

380

238

213

345

168

64

180

40

26

169

88

250

192

247

439

273

399

137

335

-

239

11

9

400

365

427

237

28

228

342

133

337

162
52 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Table 9. Alphabetical listing of mines, prospects, and mineral occurrences, Venezuelan Guayana Shield Continued.

Mineral site

Quebrada Mochila placer

Rancho Verde

Real Corona

Remington

Rio Acure

Rio Antavari

Rio Aponguau placers

Rio Arc

Rio Arc

Rio Aroi

Rio Caroni (no. 18)

Rio Caroni placers

Rio Carrao placers

Rio Chiguao placers

Rio Claro placers

Rio Cuyuni-Rio Uey

Rio Los Dos Pozos

Rio Marevari

Rio Marwani

Rio Merevari

Rio Nichare

RioOris

Rio Suapure

Rio Suapure bauxite occurrence

Rio Suruma

Rio Villacoa bauxite occurrence

Ropeway and Refugio

Sacupana

Salto Araguai

Salva la Patria

Salva le Peine

Salvador

San Antonio

San Antonio

San Felipe

San Felipe

San Felix placers

San Isidro

San Isidro Iron Quadrilateral

San Joaquin

San Juan

San Juan de Amacuro

San Lorenzo

San Lorenzo

San Luis placers

San Miguel

San Pedro de las Bocas placers

SanRafael

San Rafael

San Salvador de Paul district

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Map No.

174

199

265

260

419

76

53

231

255

436

171

154

134

98

390

110

323

38

216

43

165

131

127

112

354

125

248

441

118

-

319

-

215

-

249

346

430

268

305

306

111

431

379

382

35

193

206

201

-

106

349

356

360

363

368

Mineral site

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Sand quarry

Santa Ana

Santa Barbara

Santa Barbara

Santa Catalina

Santa Elena

Santa Elena de Uairen placers

Santa Elena de Uairen anomalous radioactivity

Santa Ines

Santa Maria deposit

Santa Rita

Santa Rosa

Santa Teresa

Santeli

Serrania La Cerbutana

Serrania Los Pijiguaos

Serrania Parguaza

Sesina

Sierra El Lindero

Sipao

Sofia

Solano

Sosa Mendez-Union mine

Stone quarry

Sua-Sua

Talisman

Talisman Cova

Tamatama

Te Esperaba

Terecay

Tres Cheques

Triunfo

Troya

Uaiparu placers

Union- 1

Union-2

Unnamed

Unnamed

Unnamed

Unnamed bauxite occurrence

Unnamed diamond occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Map No.

371

373

381

387

406

421

422

423

429

304

258

293

437

308

47

75

291

377

374

383

45

_

107

156

129

205

-

330

267

4

245

353

204

270

269

13

111

326

151

187

_

36

275

298

119

128

135

175

109

5

7

15

21

22

183
MINES, PROSPECTS, AND OCCURRENCES OF THE VENEZUELAN GUAYANA SHIELD 53

Table 9. Alphabetical listing of mines, prospects, and mineral occurrences, Venezuelan Guayana Shield Continued.

Mineral site

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold occurrence

Unnamed gold vein

Unnamed in Gran Sabana placers

Unnamed in Gran Sabana placers

Unnamed in Gran Sabana placers

Unnamed placer workings

Uonan placers

Upata bauxite

Upata district

Uraday

Map No.

188

197

203

-

58

69

79

425

39

404

403

87

Mineral site

Uraima-Urutani

Uraima-Urutani

Uraima-Urutani

Uriman placers

Veri

Viejo Callao

Vuelvan Caras

Vuelvan Caras placers

Wausa

YiYi

Yiguiripin placers

Yuruan

Map No.

143

148

149

83

194

261

220

225

426

289

62

202
56 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Any assessment based on deposit models has the 

inherent weakness that resources in unknown or unrecognized models may be overlooked. Given this qualification, we believe that the assessment herein describes, in 

the most explicit manner possible, the mineral potential of 

the Venezuelan Precambrian shield.

For the purpose of this discussion, the term "deposit 

type" is defined as a model of a specific group of deposits 

(Cox and Singer, 1986; Orris and Bliss, 1991; Bliss, 

1992a, b). The term "occurrence" describes a specific 

locality where geologic observations and other data indicate the presence of mineralized rock, and the term 

"deposit" refers to a mineral occurrence of sufficient size 

and grade that might be considered to have economic 

potential. The three-step process of Singer (1975) was 

first used as part of the mineral resource assessment of the 

Nabesna 1:250,000-scale quadrangle, Alaska (Richter and 

others, 1975), and has since been used to assess more than 

170,000 km2 in the United States at a scale of 1:250,000 

and more than 2.2 million km2 in North and South America at a scale of 1:1,000,000.

THREE-STEP ASSESSMENT PROCESS

DELINEATION OF DOMAIN BY DEPOSIT TYPE

Stratigraphic and tectonic information, lithologic 

descriptions, geologic environment, and the deposit type 

or types permissive in that environment (based on deposit 

types in similar geologic environments elsewhere) are factors taken into consideration when domains are delineated. 

The boundaries of the domains are first delineated using 

mapped or inferred geologic characteristics. They may 

then be redefined using the results of geochemical analysis 

of rock samples and (or) exploration history. Geophysical 

techniques, including gravity and aeromagnetic surveys, 

are used to identify buried rock units in areas of poor 

exposure. Deposit types are identified on the basis of host 

and associated rock types, contained metals, tonnage and 

grade, ore minerals, and geologic setting. An inventory of 

known mining activity in the area is also an important step 

in determining deposit type. Incomplete or inaccurate 

descriptions of known occurrences can make it difficult to 

identify the specific deposit type. Projection of deposits 

from adjacent areas is useful if a well-explored area is 

adjacent to a relatively unexplored area and if geologic 

conditions in the two areas appear similar (Menzie and 

Singer, 1990). If two types of deposits share a common 

association, the presence of one suggests the possibility of 

the presence of the other. Later steps in the delineation 

process serve to narrow the list. Once the deposit types are 

identified, limits are placed on what is permissible in a 

domain. The domain boundaries are then adjusted to 

account for areas where it can be demonstrated that a

deposit type could not exist. For some deposit types extensive surface exploration might provide evidence to exclude 

areas from consideration, but for many deposit types only 

close-spaced drilling can be used to exclude areas.

TONNAGE AND GRADE MODELS

Estimated pre-mining tonnages and average grades 

for well-explored, well-documented, and well-characterized deposit types from throughout the world are used to 

construct tonnage and grade models. Tonnage and grade 

models can be used to help selectively classify known 

deposits in the area being assessed and to separate suspected deposits from mineral occurrences or other weak 

manifestations of ore-forming processes. Published tonnage and grade models of deposits in various settings 

worldwide (Cox and Singer, 1986) were used to estimate 

tonnages and grades of undiscovered deposits in similar 

geologic settings in Venezuela. The variation of tonnage 

and grade for deposits belonging to a specific deposit type 

tends to be small in comparison with the variation 

between deposit types.

ESTIMATES OF NUMBER OF 

UNDISCOVERED OCCURRENCES

The number of undiscovered deposits in Venezuela 

was estimated for selected deposit types in a probabilistic 

form in order to show a degree of certainty. A variety of 

factors, including geologic, geochemical (where available), 

and geophysical data and the extent of mineral exploration, 

were integrated to make the estimates. The amount, type, 

and nature of available geologic data variably influence the 

effectiveness with which these data reflect different types 

of deposits and therefore affect the certainty of the assessment exercise. Additionally, the density and spatial distribution of sampling associated with the survey affect the 

estimation of undiscovered deposits. For example, where 

exploration has been extremely thorough, estimates of 

undiscovered deposits may be confidently placed at zero. 

More commonly, however, exploration is less thorough, 

and the effect of previous exploration on estimates of 

undiscovered resource is variable. Singer and Mosier 

(1981) noted, in regard to the effect of previous exploration, that the larger deposits are commonly discovered by 

early efforts and the remaining, undiscovered deposits may 

be, on average, smaller than those already found. Estimates 

of the number of undiscovered deposits for selected 

deposit types in the delineated domains are made at the 

90th, 50th, 10th, 5th, and 1st percentile values. For example, the 90th percentile value is that number of deposits for 

which there is a probability of 0.90 that the actual number 

of deposits equals or exceeds that number. Approximately 

half the estimated undiscovered deposits should have
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 57

tonnages or grades greater than the median tonnage or 

grade for that deposit type. Documented mines and prospects considered to be discovered mineral deposits in 

Venezuela and listed previously (table 3) are not included 

in the estimated number of undiscovered deposits.

The delineation of domains and the identification of 

deposit types facilitate exploration because deposit types 

having similar physical, chemical, and mineralogical features tend to be present in similar geologic environments

throughout the world. Thus, it is important to identify 

deposit types in Venezuela using what is known about geologic environments and deposit types elsewhere. This 

report includes exploration guides and recommendations as 

part of the discussion of selected domains. Future exploration programs in Venezuela should be based on the world 

economic outlook for specific commodities and should 

focus on those domains delineated for specific deposit 

types.
DEPOSITS IN DEEPLY ERODED TERRANES

The Imataca and Pastora-Supamo terranes contain 

mineral deposits that (1) were formed in the Archean and 

Early Proterozoic, infolded into their host rocks, and metamorphosed or (2) were formed during or soon after regional 

metamorphism associated with the Trans-Amazonian orogeny. These deposits were exposed in a later epoch by uplift 

and erosion. They include stratiform deposits of iron and 

manganese and deposits of gold in quartz veins, as well as 

undiscovered deposit types such as massive sulfide deposits 

and deposits of nickel-copper in mafic and ultramafic intrusive rocks.

Algoma- and Superior-Type 

Iron Deposits

By Dennis P. Cox, Norman J Page, and Floyd Gray

THE MODEL

Stratiform deposits of thinly laminated quartz and iron 

oxide minerals intercalated with marine felsic and intermediate volcanic and volcaniclastic rocks have been described 

by Gross (1970) and Goodwin (1973) and termed "Algoma 

iron deposits" by Cannon (1986a). The ores are believed to 

be of submarine volcanic exhalative origin. These deposits 

are mainly Archean in age and are widely distributed in 

Precambrian shields (James, 1983). Commonly, three 

fades of iron formation are present: an oxide facies, a sulfide fades, and a carbonate facies. In terms of tonnage, 

these deposits are similar to the Superior-type stratiform 

iron deposits, which are unrelated to volcanic processes. 

Superior-type iron ores are mainly of Early Proterozoic age 

and are commonly deposited in sequences of rocks that lie 

unconformably on older, stable Proterozoic or Archean 

basement rocks (Cannon, 1986b). Deposits formed distally 

from extrusive centers may be indistinguishable from 

Superior-type deposits. Repetition of ore horizons by faulting and folding, thickening at intersecting folds, and plastic 

flow toward axes may thicken iron formations and increase 

metallurgical quality.

Based on data from 66 deposits in 17 countries, 

Algoma- and Superior-type iron deposits have a median 

ore tonnage of 170 million tonnes (Mosier and Singer, 

1986)(fig. 11A). Ninety percent of the deposits contain at 

least 11 million tonnes of ore, and ten percent contain 

more than 2.4 billion tonnes of ore. The grade of these 

deposits is strongly affected by enrichment of iron during 

weathering. Such enrichment produces residual hematite 

ore containing 60-65 percent Fe, as compared to 30-50

percent Fe for unweathered quartz-magnetite (taconite) 

ore. Both supergene-enriched and primary ores are 

included in the tonnage model resulting in a median grade 

of 53 percent Fe (figs. 11A, B).

EXAMPLES IN VENEZUELA

Iron deposits, believed to be Algoma or Superior type 

(Ruckmick, 1963; Dorr, 1973), are abundant in the 

Imataca terrane, but, because rocks of the Imataca terrane 

are highly deformed and metamorphosed to amphibolite 

and granulite facies, the environment of deposition of the 

iron ore is difficult to determine and assignment to the 

Algoma model is uncertain. A listing of known deposits 

within the study area is given in table 7.

These iron deposits are composed of alternating, fine 

layers of quartzite and iron oxide minerals (hematite and 

magnetite and minor goethite). Only minerals of the oxide 

facies have been recognized. The layers are 0.1-25 mm 

thick, but lenses of almost pure iron ore as thick as 30 m 

are known. Structurally below several of the main orebodies thin layers as thick as 2 m of iron-rich rock intercalated with lithic arkose and felsic gneiss suggest a cyclic 

repetition of depositional conditions prior to main deposition of iron formation (Ferencic, 1969). Zuloaga (1933), 

one of the first to describe deposits in the Imataca Complex, noted that the ore at El Pao is structurally overlain by 

hypersthene granulite, which he termed "norite." Ascanio 

(1985) recognized three types of ore based on grain size: 

(1) coarse grained (>1 mm); (2) medium grained (~1 mm); 

and (3) fine grained (<1 mm). The deposits of El Pao, Las 

Grullas, and Piacoa comprise coarse-grained ore, Maria 

Luisa medium-grained ore, and Cerro Bolivar, San Isidro, 

Los Barrancos I and II, and Altamira 2 fine-grained ore. 

The three groups of deposits are from geographically 

separate areas; the coarse-grained deposits are north and 

east of the El Pao fault, the medium-grained deposits are 

between the El Pao and Guri faults, and the fine-grained 

deposits are in the westernmost part of the Imataca terrane. 

The increase in grade of metamorphism from west to east 

in the Imataca terrane likely affected the grain size of both 

the underlying protore and the enriched ore because both 

decrease toward the west (Gruss, 1973). Mapping by 

Kalliokoski (1965b) and Martm (1975) suggests that the 

major rock types of the Imataca Complex are granitic 

gneiss and interlayered mafic granulite and basaltic 

amphibolite, iron formation, and other metasedimentary 

rocks. Intimate infolding of the iron ore with the metamorphic rocks at many stratigraphic horizons is characteristic 

of the more deformed deposits. Large, extensive blanket

58
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 59

e

O

ut 

O

u. 

O

O

fe

2

o

oc

Q.

1 .U

0.9

0.8

0.7

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0.1

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r

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l;k San Isidro district \

2400^° 0

III I o

1 0.4 1.6 6.3 25 100 4001,6006,300 I 100,000 

A 2 5,000 MILLION TONNES

0.0

0

B

10 20 30 40 50 60 70 

IRON GRADE, IN PERCENT

90 100

0.0

0.01 0.02 0.04 0.0790.16 0.32 0.63 1.3 2.5 

C PHOSPHORUS GRADE, IN PERCENT

10

iron formations common to other cratonic regions are not 

easily observed or interpreted to be present in Venezuela. 

Most of the Venezuelan deposits are aligned east-west following the dominant structural trend of the host complex. 

They form the tops of high-standing ridges comprising 

more resistant iron-rich material and typically 200 m or 

more above surrounding terrain.

The Cuadrilatero Ferrifero de San Isidro (San Isidro 

Iron Quadrilateral) represents the largest known iron 

reserves in Venezuela. It is in the southernmost part of the 

Imataca terrane near the Guri fault zone and consists of five 

orebodies: San Isidro, Los Pailas, San Joaquin, and Los 

Barrancos I and II (which are continuous). The district is 

associated with amphibole-pyroxene gneiss, granitic gneiss, 

and amphibolite. Ferencic (1969) concluded that the primary ore was deposited as a chemical precipitate of volcanic exhalative origin. The orebodies are in range tops 

between 430 and 800 m elevation and formed in the synclinal axes of folds. The average chemical composition of ore 

from the district is Fe, 61-68 percent; SiO2, 0.5-4.05 

percent; A^C^, 0.6 percent; LOI, 3-4 percent; Mn, 3.5 percent; and P2O5 , 0.01-0.45 percent (Ferencic, 1969, p. 293).

The El Pao deposit (fig. 12), described by Zuloaga 

(1933) and Kalliokoski (1965c), is overlain by hypersthene granulite. Three types of ore are present: siliceous 

ore (hematite gneiss), massive high-grade hard ore, and 

canga. The deposit is underlain by quartz-feldspar gneiss, 

and the enclosing rocks are deformed by at least two 

stages of intense folding that produced complexly intersecting, steeply plunging synclines and cup-shaped structural basins (Fernando Lopez, El Pao mine geologist, oral 

commun., 1991). Textural and isotopic evidence indicates 

that the deposit has undergone high-grade metamorphism 

to temperatures as high as 640°C (Kalliokoski, 1965c, 

p. 113). Massive coarse-grained ore and siliceous ore consist of lamellar hematite in which the crystals are oriented 

and strongly deformed. This metamorphism in the orebody 

may be related to the granulite facies metamorphism characteristic of the eastern Imataca terrane.

At the Cerro Bolivar deposit (Ruckmick, 1963), on a 

prominent ridge about 520 m above the surrounding 

savanna, weathered ferruginous laterite and friable ore 

were formed from typically very fine grained iron formation (fig. 13). Outcrops display thin, continuous, parallel 

laminations from 0.05 mm to 1-2 cm in thickness that are 

composed of layers alternately rich and poor in amounts

Figure 11 (facing column). Tonnage and grade diagrams for 

Algoma iron and Superior iron deposits. Tie lines to the curved 

plots represent intercepts for the 90th, 50th, and 10th percentiles. 

Modified from Mosier and Singer (1986, figs. 172-174). A, Tonnages. Tonnages for selected deposits in Venezuela are also shown 

(solid circles). B, Iron grades. Grades for selected deposits in Venezuela are also shown (solid circles). C, Phosphorus grades.
60 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Figure 12. Steeply plunging recumbent limb of iron ore. El Pao deposit, Estado Bolivar. Blocks of hematite ore are in the foreground. 

Photograph by Floyd Gray.

of iron oxide and quartz. Locally, massive ore exhibits 

little or no interlayering or bedding features. Hematite, 

magnetite, and quartz are the main mineral constituents; 

however, Dougan (1977) identified local laminated calcsilicate quartzite associated with laminated quartzmagnetite and orthopyroxene-quartz-magnetite iron formation. Silicate minerals, principally sodic amphibole 

(magnesioriebeckite-crossite) and pyroxene (diopsidic 

acmite), are common phases in the iron formation. The 

Cerro Bolivar deposit represents a stratigraphically thick 

section of iron formation (220 m) repeated by tight folding 

along east-northeast-west-southwest axes and by imbricate 

reverse faulting. At least two generations of folding are 

present. Weathering, an important factor in the enrichment 

of the Cerro Bolivar orebody, has produced an almost 

pure iron oxide cap of ferruginous laterite composed of 

primary grains of hematite in a hard vuggy matrix of secondary goethite. The cap extends downward about 15.6 m, 

below which is the friable ore mainly hematite and 

quartz to a maximum depth of 28 m from the surface. 

Weathering of the deposit is thought to have begun in the

NORTHWEST SOUTHEAST

300 METERS

Iron ore

EXPLANATION

:'v.viv:V-': Banded iron "'' ' ' ' ' " formation Weathered 

gneiss

Figure 13. Cross section of iron orebodies of Cerro Bolivar, 

Venezuela. Modified from Ruckmick (1963); used with permission of Economic Geology.

Oligocene. Mixed ore from the mine yields an average 

grade of approximately 64 percent Fe, 1.3 percent SiO2, 

1.3 percent A12O3, and minor amounts of phosphorous, 

titania, and manganese (Ruckmick, 1963, p. 219).
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 61

Other ore exploited at the present time in the Imataca 

Complex has been enriched by weathering and leaching of 

silica. Unenriched (taconite type) ore is not being 

exploited and has been only partly explored.

PERMISSIVE DOMAIN

The domain permissive for Algoma and Superior iron 

deposits is coincident with the known extent of the 

Archean Imataca Complex and extends north and east 

under covering Cenozoic rocks except where the complex 

is cut off by faulting along the Rfo Orinoco. Domain la 

(pi. 7) contains the many prominent ridges underlain by 

iron formation and all of the known deposits. The west 

end of the Imataca Complex and outliers south of the Guri 

fault are considered to have a low probability of undiscovered deposits because of the absence of prominent ridges 

characteristically developed on iron formation and the lack 

of known deposits. These areas are labeled domain Ib 

(pi. 7). Other marine volcanic areas in the shield are 

excluded because they do not contain the characteristic 

lithologies of the model, they are Proterozoic, and they 

lack known occurrences of this type. The permissive 

domains also exclude granitic plutons within the Imataca 

Complex. Within domain la, there are 22 deposits for 

which a tonnage and grade have been published, of which 

17 are consistent with the tonnage model of Mosier and 

Singer (1986). Approximately 10 deposits are known for 

which no tonnage data are available. Geologic mapping by 

Kalliokoski (1965b) shows approximately 125 ridges of 

iron formation within the complex.

UNDISCOVERED DEPOSITS

Based on these data we made the following estimate 

for domain la. In addition to the 22 deposits for which a 

tonnage and grade are known, there is a 90 percent chance 

of 9 or more undiscovered deposits consistent with the 

tonnage-grade model of Mosier and Singer (1986), a 50 

percent chance of 26 or more deposits, and a 10 percent 

chance of 90 or more deposits. About half of these undiscovered deposits will be of the unenriched (taconite) type 

containing 25 percent Fe or less.

Sedimentary Manganese Deposits

By Floyd Gray and Norman J Page

THE MODEL

Shallow-marine (nonvolcanogenic) sedimentary manganese deposits that formed around rims of anoxic basins 

were termed "sedimentary manganese deposits" by Cannon

Manganese Manganese 

carbonatev oxide \

SEA LEVEL \ \

_0xic \ 

Anoxic

Island, peninsula, or shoal Mainland^

Sandstone and 

conglomerate

KILOMETERS

Figure 14. Schematic cross section showing relation between 

sedimentary facies and sedimentary manganese deposits. Modified from Cannon and Force (1986, fig. 175).

and Force (1986) and Force and Cannon (1988). These 

deposits form at depths of 50-300 m, commonly in 

sheltered sites around paleoislands, and are hosted in and 

associated with shallow-marine sedimentary rocks, commonly carbonates, clays, and glauconitic sand, typically 

with shellbeds, in transgressive sequences associated with 

anoxic basins (fig. 14). These large anoxic basins are in 

interior stable cratonic areas. Deposits formed mainly during anoxic events narrow time periods within the early 

Paleozoic, Jurassic, and mid-Cretaceous but may have 

formed with rocks of any age associated with anoxic 

basins. Manganese is present as oolites, pisolites, laminae, 

and shell replacements. A variety of manganese carbonate 

(mostly basinward) and oxide (mostly landward) minerals 

are present.

Based on data from 39 deposits, the median tonnage 

of this deposit type is 7.3 million tonnes; 90 percent of the 

deposits contain at least 190,000 tonnes and 10 percent as 

much as 280 million tonnes (fig. 15/4). The median manganese grade is 31 percent, and the median phosphorus 

grade is 0.2 percent (figs. 155, C).

EXAMPLES IN VENEZUELA

Secondarily enriched manganese deposits believed to 

be mainly sedimentary in origin are abundant in the 

Imataca terrane. Geologic details, however, do not permit 

us to conclusively assign these deposits to the sedimentary manganese model or to the similar volcanogenicsedimentary deposit model (Roy, 1981; Koski, 1986).

A large cluster of these deposits is in the area of 

Upata-Guacuripia in a stratigraphic sequence approximately 500 m wide and 20 km long (Drovenik and others, 

1967). The strike of the ore-bearing zone is east-northeast, 

parallel with the trend of the Santa Maria fault zone and 

the general trend of structure in the Imataca terrane. 

Manganese-rich beds are generally intercalated between 

overlying quartzite and underlying dolomitic marble
62 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

0.00630.025 0.1 0.4 1.6 6.3 25 100 400 1,6006,300 

A MILLION TONNES

Si 0.6 -

0 10 20 30 40 50 60 70 80 90 100 

B MANGANESE GRADE, IN PERCENT

0.3 

0.2 

0.1

aoo.

P I Q l I

\8

0.2

i i i i

I I I I I 

n=39

i i i i i

01 0.02 0.04 0.079 0.16 0.32 0.63 1.3 2.5 5 

C PHOSPHORUS GRADE, IN PERCENT

-

1

Figure 15. Tonnage and grade diagrams for sedimentary manganese deposits. Modified from Mosier (1986a, figs. 176, 177). A, 

Tonnages. Tonnages for selected deposits in Venezuela are also 

shown (solid circles). Tie lines to the curved plot represent intercepts for the 90th, 50th, and 10th percentiles. B, Manganese 

grades. Grade for Ganges I deposit in Venezuela is also shown 

(solid circle). Tie lines to the curved plot represent intercepts for 

the 90th, 50th, and 10th percentiles. C, Phosphorus grades. Tie line 

to the curved plot represents intercept for the 50th percentile.

horizons. The average thickness of the beds is less than 

10 m. Manganese protore may contain 10-30 percent 

MnC>2 and is hosted in amphibolite schist, quartzite (ferruginous), quartz-biotite schist, spessartine-rhodonite-bearing schist, and forsterite-bearing dolomitic marble. A 

kaolinitic saprolite of unknown protolith is also present. 

Typical associated minerals are spessartine garnet, quartz, 

graphite, cordierite and sillimanite. Sulfide minerals such 

as pyrite, pyrrhotite, and chalcopyrite are present in gondite (Drovenik and others, 1967). The above mentioned 

host rocks intimately associated with manganese occurrences are not described from other locations in the 

Imataca Complex, and they may be part of a younger 

sequence of beds infolded into rocks of the complex. The 

variability in manganese to iron ratios in various prospects and occurrences indicates that the deposition of 

manganese is a product of the. same processes that deposited the iron. Thus, part of the stratified manganesebearing sequence at Upata may be intercalated or gradational to exhalative (volcanogenic) suites. The ore consists 

of manganese and iron oxide and hydroxide minerals. In 

fresh protore minerals such as cryptomelane, psilomelane, 

pyrolusite, goethite, and nsutite are associated with quartz 

and silicate minerals such as spessartine, mica, and clays. 

Minor amounts of hypogene minerals such as magnetite, 

manganese-rich magnetite, spessartine, rhodonite, braunite, and hausmannite are present. Drovenik and others 

(1967) described four types of ore at Upata: earthy, hard, 

pisolitic, and detrital. The first two types form beds containing relict protore minerals and textures.

The deposits at Upata contain from slightly less than 

10,000 to 30,000 tonnes of ore and have a median grade 

of 23 percent MnC>2 (Drovenik and others, 1967). If these 

deposits are of the sedimentary manganese type, they have 

lower tonnages than the 750,000-metric-ton median of the 

tonnage model (Mosier, 1986a).

The Ganges group of deposits (Ganges I-III, table 3) 

in an east-northeast-trending zone between Guacuripia and 

El Palmar forms the largest reserves of secondarily 

enriched manganese deposits presently explored in Venezuela (Drovenik and others, 1967). The deposits are at elevations between 365 and 395 m above sea level. The 

deposits are uniformly deeply weathered but probably are 

associated with black argillaceous schist, underlie ferruginous quartzite, and are intercalated with kaolinitized 

gneiss. Ore is present on weathered surfaces as earthy and 

hard varieties, but the largest reserves are pisolitic ores 

cemented by earthy matrix.

The group of workings along the crest of Cerro 

Pando strikes east-west for a total length of 700 m (Drovenik and others, 1967). The deposits are between 303 and 

430 m above sea level; most workings are at 350-365 m 

elevation. The area is underlain by manganiferous quartzite, black argillaceous schist, and dolomitic marble intercalated with kaolinitized gneiss. The largest occurrence,
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 63

Pando IV, consists of cemented manganese-iron clastic 

material forming a 60-cm-thick cover overlying kaolinized 

gneiss intercalated with black earthy ore and argillaceous 

schist. This sequence is underlain by dolomitic marble. 

The Pando IV deposit is estimated to contain 11,900 

tonnes. The percentage of manganese, iron, and SiO2 in 

the ore is 22.58, 20.74, and 15.18, respectively (Drovenik 

and others, 1967). The combined occurrences at Cerro 

Pando are spatially close enough to be considered one 

deposit and have estimated reserves of as much as 38,000 

tonnes.

PERMISSIVE DOMAIN

We were unable to make a quantitative estimate of 

manganese resources because of the uncertainty in assigning the deposits described above to a particular model or 

type; however, the Imataca terrane (pi. 7, domains la, Ib) 

is permissive for the occurrence of manganese deposits 

similar to those in the Upata area.

Low-Sulfide Gold-Quartz Veins

By U.S. Geological Survey and Corporaci6n Venezolana de 

Guayana, Te"cnica Minera, C.A.

THE MODEL

Auriferous quartz-carbonate veins in faults and shear 

zones in metamorphic rocks and, less commonly, in and 

associated with intrusive rocks are termed "low-sulfide 

gold-quartz veins" (Berger, 1986); they have been 

described as mesothermal gold, shear-zone gold, orogenic 

gold, or Mother Lode gold deposits by other workers 

(Clark, 1970; Hotz, 1971; Barr, 1980). These veins differ 

from other auriferous veins in that they characteristically 

contain only a few percent sulfide minerals, commonly 

pyrite. Base-metal sulfide minerals and scheelite are 

present in small amounts. Most veins are in marine volcanic and volcaniclastic rocks metamorphosed to the greenschist facies of regional metamorphism. Generally, the 

deposits having significant tonnage and grade are in areas 

underlain by greenstone. In addition to volcanic rocks and 

graywacke, favorable host rocks are iron formation and 

intrusive rocks of various compositions, notably granite to 

diorite. The veins may occupy ductile, high-angle shear 

zones, and the largest deposits, such as those in the 

Mother Lode system in California, are along major geologic terrane boundaries of regional extent. Deposits in 

higher level, more brittle, sheared environments form 

stockwork and disseminated systems in and adjacent to 

plutonic bodies. Hydrothermal alteration near veins is 

marked by a lower silica content in wallrocks, the result of

CO

CO

2UJ

0

LL. 

0

z 

o

IE 

2

2Q.

l ,\J

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

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I I l I l I [ l l

\ "=313

-

-

-

0.001

\

\

\

\

\

\

0.03

\

\

VLO Increible

^Botanamo -

\

\jColombia

\J.a Camorra

fXLaguna- Santa Rita 

0.91

I I I I I I I I 0.0001 I 0.0016 I 0.025 0.1 0.4 1.6 6.3 25 100 

0.0004 0.0063 M|LL|ONTONNES

1.0

0.9

0.8

</> 0.7

CO

S 0.6

o

u_

° 0.5

g

1"

0.3 -

0.2

0.1

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Laguna-Sa

0.01 0.0250.063 0.16 0.4 1.0 2.5 6.3 16 40 100 

B GOLD GRADE, IN GRAMS PER TONNE

O

z v> 

o b

si?

Ss

DC

0.

0.1

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0.4 1.0 2.5

1 '

1 1 6.3 16 40

I

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100

I

I

250 630 1,600 4,000

C SILVER GRADE. IN GRAMS PER TONNE

Figure 16. Tonnage and grade model curves for low-sulfide 

gold-quartz vein deposits. Modified from Bliss (1986, figs. 182, 

183). A, Tonnages. Tonnages for selected deposits in Venezuela 

are also shown (solid circles). Tie lines to the curved plots represent intercepts for the 90th, 50th, and 10th percentiles. B, Gold 

grades. Grades for selected deposits in Venezuela are also shown 

(solid circles). Tie lines to the curved plots represent intercepts for 

the 90th, 50th, and 10th percentiles. C, Silver grades. Tie line to 

the curved plots represents intercept for the 10th percentile.
64 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Figure 17. Colombia and Las Americas gold-vein systems. El 

Callao mine, Estado Bolivar. Photograph by Floyd Gray.

reaction with reduced fluids rich in CC>2 and H2S. Mafic 

rocks are converted to pyrite and ferroan dolomite, felsic 

rocks to albite and sericite. Tourmaline is locally abundant. Weathering produces auriferous laterite, gold-bearing 

saprolite, and extensive placer gold deposits.

The tonnage-grade model for this deposit type (Bliss, 

1986) is based on 313 deposits in California, Alaska, 

Nova Scotia, eastern Australia, New Zealand, and other 

countries (fig. 16). In assembling the model, veins within 

1.6 km of each other were considered to be one deposit 

and veins of less than 99 tonnes were not included. The 

median deposit contains 30,000 tonnes of ore; 90 percent 

of the deposits contain at least 1,000 tonnes, and 10 percent contain 910,000 tonnes or more. The median grade is 

16 grams of gold per tonne; 90 percent of the deposits 

have a grade of 6 grams per tonne or more and 10 percent 

a grade of more than 43 grams per tonne. Only 10 percent 

of the deposits have a published silver grade, generally 

about 10 percent of the gold grade.

EXPLANATION

Alluvium 

Tectonic breccia 

Gabbro

Diabase

Pastora Supergroup Divided into:

Caballape Formation 

Yuruari Formation

El Callao Formation of the Carichapo Group Includes 

volcanic breccia (unit Xecb)

Contact Approximately located: dashed where inferred

Fault Dashed where inferred; arrows show direction of

movement. Bar and ball on downthrown side 

Thrust fault Sawteeth on upper plate

Shear zone

Syncline Dashed where inferred

Strike and dip of foliation

 60 Bearing and plunge of lineation

-Q- Pillow lava showing younging direction 

^ Mine or prospect

= Paved road 

= = = = = = Unimproved road

O Town 

Lake

EXAMPLES IN VENEZUELA

Low-sulfide gold-quartz veins are abundant in the 

Precambrian shield of Venezuela in greenstone belts and 

in other areas of greenschist derived from volcanic and 

volcaniclastic rocks. Host rocks are metabasalt of the El 

Callao Formation and correlative units, as well as metaandesite, metatuff of dacitic to rhyolitic composition, 

metagraywacke, gabbro, and ultramafic rocks. Almost all 

of the deposits are in rocks of greenschist facies. A notable exception is Piston de Uroy, where veins cut relatively unmetamorphosed gabbro (Wynn, Page, and others, 

in press). The gabbro was probably intruded shortly after 

the peak of greenschist facies metamorphism, and the 

quartz veins were probably emplaced a short time after 

the gabbro. Some deposits are in amphibolite facies 

metavolcanic rocks. This higher grade metamorphism is 

more widespread near the Guri fault and is probably 

related to post-Supamo movement on the fault that 

occurred considerably later than gold-quartz vein formation. Other distinctive deposits that may be of this type 

are in the Vuelvan Caras and Kilometre 88-Las Cristinas
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 65

3 KILOMETERS

I

-400-

Figure 18 (above and facing page). Geologic map of area of Lo Increfble gold deposit, Estado Bolfvar. Modified from Day and 

others (fig. 2, in press).

areas, where young granitic rocks intrude greenstone-belt 

rocks (Wynn and Sidder, 1991; Floyd Gray, written commun. to CGV-TECMIN, January 1992). The gold-quartz 

veins cut both the granitic stocks and the metavolcanic 

country rock and are in fractures associated with the 

regional high-level intrusive episode. This form of

mineralization is relatively widely known in gold deposits 

of Guyana (Carter and Fernandez, 1969; Bertoni and 

others, 1991).

The Colombia vein in the El Callao district is the 

most completely explored deposit in the region. Estimated 

production to 1968 for this vein and the branching Las
66 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Americas vein is 429,000 tonnes of ore containing 11.28 

grams of gold per tonne (Rodrfguez, 1986). The Colombia 

vein is 2-10 m wide and mined to a depth of 475 m (fig. 

17). It has a zoned alteration envelope comprising an inner 

albite-quartz-carbonate assemblage and an outer zone in 

which the basalt is replaced mainly by ferroan dolomite. 

The vein strikes N. 70° E. and is on a mineralized structure extending at least 10 km toward the southwest that 

includes the Sosa Mendez-Union, Peru, Chile, Mejico, and 

other deposits. A parallel structure 2 km to the north hosts 

the Laguna and Santa Rita veins. The El Callao district 

contains at least 19 named deposits, all in metabasalt of 

the El Callao Formation.

The Lo Increible district (Day and others, in press), 

north of El Callao, is made up of at least 24 closely 

spaced mines that are along a N. 75° E.-striking fault zone 

that is as long as 10 km (fig. 18). This fault separates 

metabasalt of the El Callao Formation on the southeast 

from schistose dacitic metatuff of the Yuruari Formation 

on the northwest. The fault is a zone of intense ductile 

deformation that contains mesoscopic kinematic indicators 

of right-lateral motion. Most of the deposits are in schist 

within or adjacent to the fault zone; a few deposits are in 

the El Callao Formation. This district is believed to have 

produced more than 100,000 tonnes of ore averaging 13 

grams of gold per tonne (Locher, 1974; Rodrfguez, 1986).

Another large deposit is Botanamo, 70 km southsoutheast of El Callao. Mostly mined out by 1940, Botanamo produced more than 136,000 tonnes of oxidized ore 

containing 32.7 grams of gold per tonne (Rodrfguez, 1986). 

The mica schist and chlorite schist that host this deposit 

belong to the calc-alkaline eugeosynclinal sequence of 

metavolcanic rocks that is distributed over a wide area in 

the northeastern part of the shield and that does not have 

the form or structure of a greenstone belt. Low-sulfide 

gold-quartz veins are equally as abundant in these rocks as 

in the greenstone belts.

In the Vuelvan Caras deposit, granitic rocks represent 

high-level intrusive bodies as indicated by the presence of 

mariolitic cavities (Benaim, 1972; Wynn and Sidder, 

1991). Northwest-striking faults localize veins in both 

types of host rocks. Mining in the Vuelvan Caras vein 

produced 802.5 kg of gold between 1850 and 1968 from 

ore that reportedly averaged 63 grams of gold per tonne 

(Locher, 1974). High-grade ore in the oxidized zone 

assayed greater than 400 grams of gold per tonne. The 

grade of ore was noted to decrease with depth in the sulfide zone.

The Kilometro 88 district, 110 km south of Botanamo, is also in calc-alkaline eugeosynclinal rocks (fig. 

19). Auriferous saprolite and placer deposits are mined in 

the district; however, values for tonnage and grade are not 

available. Present-day workings are concentrated in 

saprolite weathering horizons as thick as 23 m (Luis

Guzman, CVG-TECMIN, unpub. data, 1991)). The district forms a northwest-trending elliptical zone approximately 7 km wide by 15 km long that in its central part 

shows a close spatial association between high cupolas of 

probable Late(?) Proterozoic granitic rocks and deformed 

metavolcanic tuff; peripheral mineralized rock is void of 

abundant granitic rocks and is a stockwork controlled by 

north-northwest- to north-northeast-trending shear systems 

(Luis Guzman, CVG-TECMIN, unpub. data, 1991; Floyd 

Gray, written commun. to CVG-TECMIN, January 1992). 

The central part of the district deviates from the typical 

low-sulfide gold-quartz vein model in that it represents 

groups of localized vein systems or stockworks related to 

the intrusive rocks rather than massive persistent veins. 

The newly proposed "plutonic porphyry gold deposit" 

model (Hollister, 1992; see also Rytuba and Cox, 1991) in 

part describes the preliminary findings from this area. 

Similar deposits have been described at Omai, Guyana 

(Bertoni and others, 1991), Dublin Gulch, Canada (Hollister, 1991a), Boddington(?), Australia (Monti, 1987), 

Porgera, Papua New Guinea (Handley and Bradshaw, 

1986), and Fort Knox, Alaska (Hollister, 199Ib).

The central part of the Kilometro 88 area, nominally 

described as Las Cristinas, is underlain predominantly by 

intensely weathered felsic to intermediate metavolcanic 

and metasedimentary rocks (volcanic-derived graywacke) 

that locally form schist. The rocks include strongly foliated massive pyroclastic deposits including debris flows, 

breccia, pumice-rich tuff, thinly bedded tuff, and volcanic 

sandstone. Foliation trends dominantly north-northwest 

and dips steeply 50° N. to 80° SW. The volcanic sequence 

is intruded by quartz diorite, granodiorite, and intermediate porphyry stocks that crosscut foliation. Margins of the 

stocks are commonly altered and brecciated and locally 

contain stockwork veining (mainly iron oxide minerals). 

The stocks occupy a north-northeast-trending zone approximately 850 m wide. Veins in the weathered meta volcanic 

rocks contain the gangue assemblage hematite (with or 

without goethite) and quartz (with or without free gold); 

less weathered impermeable granitic stocks exhibit various 

stages of serialization, tourmalinization, and argillic alteration. Sulfide-bearing veins, mainly pyrite and chalcopyrite, extend below the saprolite zone. Outboard of the 

central zone, mineralized areas are characterized as shearmylonite-controlled, north-northeast- to north-northwesttrending zones that may be exhalative in nature. Mineralized samples of sericite-quartz schist contain the assemblage pyrite, chalcopyrite, and scattered covellite; more 

quartz rich schist contains the assemblage pyrite and 

molybdenite, with or without chalcopyrite, in hand specimen. The lateral extent of this zone, as well as the trend of 

the overall district, is undefined. Unknown but probably 

large annual production from this district and other similar 

districts suggests that this type of deposit is one of the
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 67

Figure 19. Saprolite-gold system, kaolinite-bearing surficial deposit, Kilometre 88, Estado Bolivar. A, Sidewall of 

prospect pit showing relict foliation in saprolitic metavolcanic sequence. Photograph by Floyd Gray. B, Kaolinite zones 

in gold prospect pit; lines are from hydraulic system used in mining saprolite-kaolinite. Photograph by Dennis Cox.
68 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

most important sources of gold in the Precambrian of 

Venezuela.

Table 4 lists 125 low-sulfide gold-quartz vein deposits, 28 of which have a tonnage and grade consistent with 

the model. The more than 45 mines in the Lo Increfble 

district are so closely spaced that they probably should be 

considered as no more than 3 deposits. Of the 19 deposits 

in the El Callao district for which there are tonnage and 

grade information, 14 have a tonnage and grade consistent 

with the model. Probably 9 deposits in this district are 

separated sufficiently from their neighbors to be counted 

as individual deposits. We estimate that there are at least 

30 individual known deposits in the Precambrian shield of 

Venezuela that have a tonnage and grade consistent with 

the model.

PERMISSIVE DOMAIN

The area permissive (pi. 7, domain Ila) for lowsulfide gold-quartz veins is coextensive with greenstone 

belts (units Xm1, Xf1, Xs1, Xg1, and Xu1, pi. 2) and 

eugeosynclinal metavolcanic rocks (units Xm2, Xf2, Xs2, 

Xg2, and Xu2). Areas of amphibolite (unit Xa) are also 

included in the domain because in many areas amphibolite 

facies metamorphism probably postdated gold deposition. 

The total area of domain Ila is 13,175 km2.

The area west and southwest of El Manteco (pi. 7, 

domain lib) has a low probability of undiscovered deposits because of the absence of known gold occurrences 

despite levels of exploration similar to domain Ila. Rocks 

of the Imataca Complex are excluded because of their 

high grade of metamorphism. Areas of Cuchivero volcanic 

rocks, locally contact metamorphosed, are not included 

because of their subaerial origin. Areas of Supamo Complex are excluded mainly because of the absence of known 

deposits and because the complex is, for the most part, 

probably younger than the gold veins. In the southern part 

of Territorio Federal Amazonas, near Rio Ugueto and the 

Sierra Parima, map unit Xmu (pi. 2) is assigned to domain 

Ila. No vein deposits have been identified in this region, 

but the area is permissive on the basis of placer gold 

occurrences and greenstone-belt rocks associated with 

Middle Proterozoic granitic rocks.

UNDISCOVERED DEPOSITS

Although the greatest economic return in the short 

term would be from developing extensions to known 

deposits, particularly mineralized wallrocks of veins, the 

possibility of undiscovered deposits is interesting from a 

long-term point of view.

The density of occurrence of low-sulfide gold-quartz 

vein deposits in California, eastern Australia, and Nova 

Scotia has been analyzed by Bliss and others (1987), who

found that in these areas deposits have a remarkably constant density, between 4.3 and 5.4 deposits per 1,000 km2. 

Assuming that deposits in domain Ha have a similar density, the domain should contain between 57 and 71 deposits. The domain is known to contain 30 deposits having 

known tonnage and grade. Thus, we estimate that 27-51 

undiscovered deposits are present. These numbers are reasonable considering that 50 known occurrences in the 

domain are classified as undiscovered deposits because 

they lack tonnage and grade information. All or some of 

these could be classified as deposits if, based on future 

exploration, they were found to have a tonnage and grade 

consistent with the model. We estimate that domain Ila 

has a 90 percent probability of containing 20 or more 

undiscovered deposits consistent with the tonnage-grade 

model of Bliss (1986), a 50 percent chance of 40 or more, 

and a 10 percent chance of 50 or more.

CHANGES IN

, - ROCK'. ^

CHEMISTRY

+ Mg + Fe -

^ Na - -s '

+ Mg + Fe +

- Na '

Breccia filling and vein

stockwork of quartz,

pyrite, chalcopyrite

V. ~ " - , and gold hlorite +

sericite

Chlorite +

albite

EXPLANATION

Mudstone or tuff

Chert, felsite + pyrite 

± zeolites ± clay 

± carbonates ± hematite

Barite

Black ore: pyrite + sphalerite

± chalcopyrite ± galena

± barite ± pyrrhotite

± quartz 

Yellow ore: pyrite + chalcopyrite

+ quartz ± sphalerite

+ magnetite 

Altered volcanic rocks

Unaltered marine felsic to

intermediate volcanic

rocks 

Subvolcanic felsic

intrusive rocks

Figure 20. Schematic cross section of kuroko-type massive sulfide deposit. Modified from Franklin and others (1981).
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 69

Kuroko-Type Massive 

Sulfide Deposits

By Dennis P. Cox

THE MODEL

Copper- and zinc-bearing massive sulfide deposits in 

felsic to intermediate marine volcanic rocks were called 

"kuroko type" by Singer (1986a) and "Noranda type" by 

Franklin and others (1981)(fig. 20). Kuroko-type deposits 

are in marine flow rocks of andesitic to basaltic composition intercalated with submarine breccia and flow-dome 

complexes of dacitic to rhyolitic composition. The ore is 

commonly massive; that is, it contains more than 60 percent sulfide minerals and is composed of chiefly pyrite, 

chalcopyrite, sphalerite, and barite, subordinate galena, 

magnetite-hematite, and sulfosalt minerals, and traces of 

gold and silver minerals. Orebodies are lens shaped to 

stratiform. Deposits tend to be clustered around centers of 

felsic volcanism. They are syngenetic with respect to the 

surface on which they are deposited and are generally conformable with the bedding of underlying and overlying 

volcanic rocks. Underlying rocks commonly contain a 

mineralized breccia or stockwork that represents the feeder 

zone for the massive ore. This feeder zone is typically surrounded by a halo of magnesian chlorite alteration and an 

outer zone characterized by a potassic alteration assemblage. Transported ore recognized near some deposits consists of sedimentary beds of fine-grained sulfide minerals 

or breccia clasts composed of massive sulfide ore. Deposits are recognized by the appearance of iron-rich gossan 

formed during weathering and by geochemical anomalies 

in base- and precious-metals.

The tonnage-grade model was derived from a worldwide data set containing 432 deposits (Singer and Mosier, 

1986a)(figs. 21, 22). The median deposit contains 1.5 million tonnes of ore; 90 percent of the deposits contain at 

least 120,000 tonnes, and 10 percent contain 18 million 

tonnes or more. The median copper grade is 1.3 percent, 

90 percent of the deposits have a copper grade of at least 

0.45 percent, and 10 percent have a copper grade of more 

than 3.5 percent. The median zinc grade is 2.0 percent, 

and 10 percent have a zinc grade of more than 8.7 percent. 

Only 50 percent of the deposits have a published gold

Figure 21 (facing column). Tonnage and grade model curves 

for kuroko-type massive sulfide deposits. Modified from Singer 

and Mosier (1986a, figs. 146-148). A, Tonnages. Tie lines to the 

curved plots represent intercepts for the 90th, 50th, and 10th percentiles. B, Copper grades. Tie lines to the curved plots represent 

intercepts for the 90th, 50th, and 10th percentiles. C, Lead grades. 

Tie line to the curved plot represents an intercept for the 10th 

percentile. D, Zinc grades. Tie lines to the curved plot represent 

intercepts for the 50th and 10th percentiles.

0.0004 [ 0.0063 0.1 0.4 1.6 6.3 25 100 400 

^0.0016 0.025,MILLION TONNES

1.0 

0.9 

0.8 -

tO

w °'7 

S 0.6-

U_

° 0.5 

g

fe 0.4 

O

I 0.3-

Q.

0.2 -

0.1

0.0

0.45 1.3

0.0320.0560.1 0.180.320.56 1.0 1.8 3.2 5.6 10.0 

B COPPER GRADE, IN PERCENT

0.01 I 0.0630.16 0.4

) ' LEAD GRADE, IN PERCENT

1.0 2.5 6.3 16 40 100

p

-vi

b 0.6

opopp 

i^ k> 1»enu> 

5.01 I 0.0630.16 0.4 1.0 2.5 6.3 16 40 100 

D °'°25 ZINC GRADE, IN PERCENT
70 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

0.6

0.5

0.4

0.3

°'2

0.1

432

0.16

0.0

0.01 0.025 0.063 0.16 0.4 1.0 2.5 6.3 16

A GOLD GRADE, IN GRAMS PER TONNE

40 100

0.7

0.6

0.5

u. 0.4

0.3

O

Q.

O no

DC 0.2

0.1

0.0

/J=432

13

0.4

B

1.0 2.5 6.3 16 40 100 250 630 1,600 4,000 

SILVER GRADE, IN GRAMS PER TONNE

Figure 22. Precious-metal grades of kuroko-type massive sulfide deposits. Tie lines to the curved plots represent intercepts for 

the 90th and 10th percentiles. Modified from Singer and Mosier 

(1986, fig. 149). A, Gold. B, Silver.

grade; 10 percent have a grade of 2.3 grams of gold per 

tonne or more.

PERMISSIVE DOMAIN

No examples of kuroko-type deposits are known in 

the Precambrian shield of Venezuela; however, the geologic environment permissive for the deposits is widespread and their economic attractiveness makes their 

consideration worthwhile. The permissive domain (pi. 8, 

domain III) includes all of the marine volcanic and volcaniclastic rocks of the greenstone belts and other metavolcanic areas. The domain is the same as the permissive 

domain for low-sulfide gold-quartz veins, except that it 

excludes areas of gabbro and peridotite such as Sierra 

Verdun, areas of the Imataca terrane, and those areas considered permissive for Algoma iron deposits. Although 

both Algoma- and kuroko-type deposits are formed by 

submarine exhalative processes, the Imataca Complex contains a higher ratio of felsic to mafic rock than is characteristic of rocks that contain kuroko-type deposits.

UNDISCOVERED DEPOSITS

Massive sulfide deposits could exist in many places, 

undetected because of deep weathering and heavy forest 

cover. Ferruginous crusts derived from weathering of various types of iron-bearing rocks are widespread and easily 

confused with gossans derived from weathering of massive 

sulfide deposits. Mitigating against the existence of these 

deposits is the rarity of copper, lead, and zinc in the whole 

Guayana Shield. The low abundance of base metals may 

be an artifact of inadequate sampling or the result of leaching of near-surface rocks and soils in the tropical environment. Given these circumstances, we estimate that there is 

a 5 percent chance of one or more deposits consistent with 

the tonnage-grade model of Singer and Mosier (1986a).

Synorogenic-Synvolcanic

Nickel-Copper Deposits and

Related Platinum Deposits

By Floyd Gray and Norman J Page

THE MODELS

Lenses of disseminated to massive iron-, nickel-, and 

copper-sulfide mineralization associated with gabbro intrusive rocks in greenstone belts are described as synorogenicsynvolcanic nickel-copper deposits by Page (1986) and as 

gabbroid class deposits by Ross and Travis (1981). They 

form in host rocks that range from gabbro to norite and 

include peridotite and anorthosite that have been intruded 

into technically active terranes. Host rocks are typically 

less magnesian than komatiitic rocks in Archean terranes. 

Chalcopyrite, pentlandite, pyrrhotite, and, less commonly, 

magnetite, ulvospinel, and chromite crystallized from an 

immiscible sulfide liquid and commonly are present as disseminations in spaces between the silicate crystals of rocks 

in the lower to basal parts of intrusive bodies.

A related model, Alaskan platinum-group element 

deposits (Page and Gray, 1986), describes the environment 

in which disseminated platinum-iron and osmium-iridium 

alloys are concentrated in zoned ultramafic and mafic 

rocks that intrude eugeosynclinal erogenic belts. The 

alloys are present with chromite or magnetite in clots or 

schlieren, in magmatic segregation layers, in cumulustextured rocks, or in association with copper and (or) ironnickel sulfide minerals. Placer platinum-group metal 

deposits are typically a result of weathering and are present 

near zoned igneous complexes.
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 71

1 .U

0.9

0.8

co 0.7

CO

§ 0.6

Q

u.

0 nR z 0.5 

g

o °'4

o

£ 0.3

0.2

0.1

0.0

"loll l

\° n=32

-

-

-

-

-

-

0.26

V °

\

\ ^

_

-

-

2

0

' 2.1

"1

V\o _

V

\ -

\ °

\ °

\ o -

\°

1 17 \o\0

I I I I I I I

0.00630.025 0.1 0.4 1.6 6.3 25 100 4001,6006,300

A MILLION TONNES 

1.0 

0.9

O.B

co 0.7

b

o

£ 0.6

Q

U0 0.5

Z

o

cc 0.4

2

0

£ 0.3

0.2

0.1

0.0 

I 

n=32

°\

o \

-

-

-

_

-

-

0.13*

\ °

\°o

\ °

\ °

\ o 

\ °

\ °

\ o 

V,

_

_

-

0.47^ 1.3*^°

i i i

0.0320.056 0.1 0.18 0.32 0.56 1.0 1.8 3.2 5.6 10.0

B COPPER GRADE, IN PERCENT

1 .0 ° 

0.9 

0.8

£ 0.7

Si 0.6

Q

u. 

O

z 0.5 

O

0

oc°- 0.3

0.2

0.1

0.0 

I I ~ I d I I I 

xo n=32 

\o

-

-

_

i 

0.35

\o _ 

\°

\ o -

\o '

\°

\o

~

~

,. °0

l i ill

0.0320.056 0.1 0.18 0.32 0.56 1.0 1.B 3.2 5.6 10.0

C NICKEL GRADE, IN PERCENT

0.1

nn

l i i i l i l i i

n = 32

0.017' ^"^ ° Q

I I I I I I I I I

0.01 0.016 0.025 0.04 0.063 0.1 0.16 0.25 0.4 0.63 1.0 

A COBALT GRADE, IN PERCENT

0.1 

nn

I I I I I

0.035^^ ° Q

I I I I

I I I I 

n = 32

I I I

40 100

PROPORTION Of

0.2 

0.1 

nn

B GOLD GRADE, IN PARTS PER MILLION

I III II 

n = 32

63^ 0

I I I I I I I I I

4 B 16 32 63 130 250 500 1,000 

PALLADIUM GRADE, IN PARTS PER BILLION

0.2

0.1

0.0

D

4 B 16 32 63 130 250 

PLATINUM GRADE, IN PARTS PER BILLION

500 1,000

Figure 24. Byproduct grades of synorogenic-synvolcanic 

nickel-copper deposits. Tie lines to the curved plots represent 

intercepts for the 10th percentile. Modified from Singer, Page, 

and Menzie (1986, fig. 18). A, Cobalt. B, Gold. C, Palladium. D, 

Platinum.

Synorogenic-synvolcanic nickel-copper deposits are 

small as compared to other nickel-copper deposit types 

such as dunitic nickel-copper deposits; median tonnage 

deposits contain 2 million tonnes, and 10 percent of the 

deposits contain 17 million tonnes or more (fig. 23A). 

Median nickel and copper grades are 0.77 and 0.47 percent, respectively (figs. 235, C). Ten percent of the deposits have published grades of cobalt, gold, platinum, and 

palladium (Singer, Page, and Menzie, 1986)(fig. 24). Tonnage-grade data are not available for the Alaskan-type 

deposits.

EXAMPLES IN VENEZUELA

No examples of these deposit types have been discovered in Venezuela, but permissive host environments are

Figure 23 (facing column). Tonnage and grade diagrams for 

synorogenic-synvolcanic nickel-copper deposits. Tie lines to the 

curved plots represent intercepts for the 90th, 50th, and 10th percentiles. Modified from Singer, Page, and Menzie (1986, figs. 

15-17). A, Tonnages. B, Copper grades. C, Nickel grades.
72 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

known and consist of cumulus-textured, clinopyroxenerich, mafic to ultramafic intrusive rocks in some greenstone-belt terranes. These intrusive bodies crosscut 

metavolcanic and metasedimentary rocks and are present as 

multiphase plutons, sills, or dikes. The larger intrusive bodies, such as at Sierra Verdun and Cerro La Esperanza, 

locally contain hornfels in the contact zone, and all the 

intrusive rocks lack the strong penetrative deformation 

exhibited by nearby greenstone host rocks. Sulfide minerals 

are present in these intrusive rocks, most prominently at La 

Paragua, Pfston de Uroy, and Sierra Verdun. The Paragua 

mafic-ultramafic intrusive body, in particular, contains 

abundant sulfide-bearing rock in discrete horizons (Jhonny 

Tapia, oral commun., 1990). Platinum-group-element 

geochemical anomalies have been identified in two areas, 

Pfston de Uroy and La Flor Carapo.

Nickel laterite deposits associated with weathered 

areas of mafic-ultramafic rocks are unlikely in the Venezuelan Precambrian shield because of the lack of residual 

olivine-rich harzburgitic-dunitic rock that commonly 

underlies residual nickel deposits; the pyroxene-rich 

cumulus rocks that are abundant in the shield contain low 

concentrations of nickel.

PERMISSIVE DOMAIN

The domain permissive for this deposit type includes 

areas underlain by mafic and ultramafic intrusive rocks 

(units Xg1, Xg2, Xu1, and Xu2, pi. 2) in the Sierra 

Verdun, Pfston de Uroy, Bochinche, Anacoco, and La 

Paragua areas (domain IV, pi. 8). Areas of intense mineral 

exploration in the El Callao district are excluded from the 

permissive domain.

UNDISCOVERED DEPOSITS

We estimate that there is a 1 percent chance of one or 

more deposits of nickel-copper-platinum group elements 

associated with pyroxenite and gabbroic intrusive rocks in 

the permissive domain.

Dolomite-Marble Deposits

By G.J. Orris

Dolomitic marble of the Guacuripia Formation (Morrison, 1953, 1956) of the Imataca Complex is disconformably overlain by schist and quartzite of the Imataca 

Complex. The Guacuripia Formation crops out in an eastnortheast-trending range of hills that extends from the El 

Palmar area of Rfo Curipiaima to the area of Cerro Peluca. 

The dolomite is reported by Morrison and by Rodrfguez 

(1986) to be uniformly bluish gray, crystalline, and fine to 

medium grained. At Cerro Hacha, near Salto Hacha, the 

formation is 85 m thick. The accessory minerals forsterite, 

tremolite-actinolite, muscovite, and phlogopite form about 

3 percent of the rock. There is a spatial relationship 

between this unit and manganese deposits (see previous 

discussion on manganese). Dolomite mines in the Guacuripia Formation are reported to produce 30,000 tonnes 

of dolomite per year for the steel industry in Puerto Ordaz 

(Rodrfguez, 1986). Other dolomite units within the 

Imataca Complex are not known, and resource potential is 

mostly limited to the identified deposits and their extensions within the known dolomite unit.
DEPOSITS FORMED IN SUPRACRUSTAL ROCKS AND

EPIZONAL PLUTONS

Sedimentary rocks of the Roraima Group, volcanic 

rocks of the Cuchivero Group, and epizonal plutonic rocks 

that intrude rocks of the Cuchivero Group comprise a 

supracrustal terrane that is a potential host for deposits 

that formed near, or within a few kilometers of, the surface during the Early and Middle Proterozoic. These 

deposits include carbonatite and diamond pipes, as well as 

undiscovered deposit types such as tin greisen, porphyry 

copper, volcanic-hosted iron deposits, and polymetallic or 

epithermal veins. It is difficult to assess the potential of 

these undiscovered types primarily because deposits of 

these types are rarely found together in other parts of the 

world. For example, if tin greisen is abundant, then porphyry copper and the other deposit types are unlikely.

Carbonatite Deposits

By William E. Brooks, GJ. Orris, and Jeffrey C. Wynn

THE MODEL

Carbonatite is a carbonate-rich rock of magmatic origin that is present as intrusive bodies, flows, and pyroclastic rocks; it contains 4.0-55.4 weight percent CaO 

(Heinrich, 1966). Many carbonatite bodies are concentrically zoned. Structurally, carbonatite may be associated 

with alkaline intrusive complexes, with or without ring 

dikes, but not all carbonatite is associated with other alkaline rocks. Carbonatite may be of any age but is commonly intrusive into Precambrian shields and stable 

cratonic areas; geologically young carbonatite probably is 

related to rift environments (Guilbert and Park, 1985). 

Because carbonatite is emplaced within a few kilometers 

of the surface and pinches out at depth, it is commonly 

found in supracrustal rocks. Most, though not all, carbonatite bodies are cylindrical or pipelike in shape, and their 

commonly circular surface expression may be identifiable 

on radar images and may be further delineated by radiometric, magnetic, or gravity anomalies, depending on the 

composition of the carbonatite. Carbonatite may contain 

economic grades of phosphate, niobium, rare earth elements, uranium, thorium, titanium, iron, copper, zirconium, vermiculite, fluorite, and barite (Singer, 1986b).

The median tonnage of niobium- and rare earth 

element-bearing zones of carbonatite deposits is 60 million

tonnes; 10 percent of the deposits contain 220 million 

tonnes or more of ore (fig. 25A). The median niobium 

grade is 0.58 percent M^Os; 10 percent of the deposits 

have a grade of 1.9 percent M^Os or greater (Singer, 

1986c) (fig. 255). Ten percent of the deposits contain 

more than 0.35 percent rare earth element oxides (fig. 

25 C). Phosphate-bearing zones of carbonatite deposits 

may be coincident with, and similar in size to, niobiumand rare earth element-bearing zones. Preliminary models 

indicate a median phosphate grade of 18 percent P2Os. 

Ninety percent of reported barite-bearing zones have 

grades exceeding 20 percent barite (Orris, unpub. data, 

1991).

EXAMPLES IN VENEZUELA

Cerro Impacto (6 km long by 2 km wide) is the central of three discontinuous north-trending ridges in Estado 

Bolivar that stand out within an irregularly shaped depression that has a maximum diameter of 20 km. This feature 

was first identified on radar imagery in 1971, and subsequent fieldwork and geochemistry identified the central 

ridge as carbonatite (Aarden and others, 1973). Tonalite, 

granodiorite, and granite showing signs of fenitization are 

adjacent to the carbonatite, and laterite in the area may be 

200-300 m thick (Rodrfguez, 1986; Woolley, 1987). The 

age of Cerro Impacto is unknown; however, it is intrusive 

into plutonic rocks of the Cuchivero Group and is therefore younger than about 1,900-1,800 Ma.

Geochemical analyses of samples from Cerro 

Impacto indicate significant contents (maximum value in 

weight percent oxide) of Ce (6.5), La (2.7), Nb (0.5), Th 

(0.4), Y (0.16), Nd (0.84), and Ba (57.8) (Rodrfguez, 

1986). North of the central carbonatite ridge near Rfo 

Impacto, Aarden and others (1973) sampled a 500 m by 

800 m area having an average cerium content of 0.5 percent. The uranium to thorium ratio of the rocks at Cerro 

Impacto is 2:1 (Premoli and Kroonenberg, 1981); however, the uranium content is too low to be considered an 

economic target, although uranium is recovered as a 

byproduct at Palabora, South Africa (Premoli and 

Kroonenberg, 1981), and Araxa, Brazil (Young, 1984).

La Churuata is a deeply weathered, circular feature in 

Territorio Federal Amazonas that was identified in 1975 

because of an airborne uranium anomaly. This circular feature, composed of several small hills visible on radar imagery, is an alkaline intrusive complex 3-4 km in diameter 

that intrudes rocks as young as the Roraima Group. No

73
74 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

1.0

0.9 

0.8

fj? 0.6

°0.5 

o

§ 0.4

CL

o 

tr

°- 0.3 

0.2 

0.1 

0.0

0.0016 I 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 

0-°063 MILLION TONNES

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

n=20

0.18 0.58

0.0

0.032

B

0.056 0.1 0.18 0.32 0.56 1.0 1.8 3.2 

NIOBIUM OXIDE GRADE, IN PERCENT

5.6 10

0.0025 I 0.016 0.04 0.1 0.25 0.63 1.6 4 10

0.0063 

C RARE EARTH ELEMENT OXIDE GRADE, IN PERCENT

Figure 25. Tonnage and grade diagrams for carbonatite 

deposits. Modified from Singer (1986c, figs. 30, 31). A, Tonnages. Tie lines to the curved plot represent intercepts for the 

90th, 50th, and 10th percentiles. B, Niobium grades. Tie lines to 

the curved plot represent intercepts for the 90th, 50th, and 10th 

percentiles. C, Rare earth element oxide grades. Tie line to the 

curved plot represents intercept for the 10th percentile.

carbonatite has been identified at La Churuata. Rock types 

include syenite, quartz syenite, and nepheline syenite. 

Aegirine, riebeckite, biotite, zircon, monazite, and bastnaesite are present in the syenite. Zirconium and lanthanum are present in altered rock, and Sn, La, Y, As, Pb, Zn, 

W, Zr, and Nb are present in stream sediments (Scares, 

1985). The uranium content of the rocks in the area is low 

(10-160 ppm); however, Scares considered the La Churuata structure as favorable for uranium. Three Rb-Sr 

(whole-rock) and two K-Ar (nepheline, amphibole) ages 

from the intrusive rock give an average age of 1,300 Ma 

(Soares, 1985).

At least two alkaline rock bodies, Morro dos Seis 

Lagos (Brazil) and Kaituma (Guyana), are within 70 km 

of the Venezuelan border. Morro dos Seis Lagos in the 

Pico de Neblina area of the northern Amazonas of Brazil 

consists of three circular structures that represent lateritized carbonatite intruded into Precambrian granite and 

gneiss. The body was identified geochemically; the lateritic soil is characterized by total rare earth element contents of as much as 2.4 percent and by 0.15 percent 

^265, 4.5 percent ThO2, and as much as 25 ppm L^Og. 

The deposit contains an estimated resource of 2,897 

tonnes of niobium. Kaituma is a peralkaline quartz syenite 

intrusive body in a biotite granite. An age of 2,065+ Ma 

was obtained on aegirine-augite (K-Ar) (Snelling and 

McConnell, 1969). The radioactive minerals euxenite, 

samarskite, uranothorite, and titanium betafite have been 

identified.

PERMISSIVE DOMAIN

The area underlain by the Cuchivero Group is permissive for carbonatite deposits (pi. 8, domain V). Circular features have been noted because of the tendency of 

carbonatites to be associated with elliptical intrusive bodies and alkaline ring complexes as in Jacupiranga, southeastern Brazil (Melcher, 1965). The Imataca Complex, 

Supamo Complex, greenstone belts, and granitic rocks of 

the southern and western parts of Territorio Federal Amazonas are excluded because they probably are eroded too 

deeply for carbonatite deposits of Precambrian age to be 

preserved.

UNDISCOVERED DEPOSITS

Our estimate of undiscovered deposits is based on the 

number of circular features shown on radar imagery and 

inferred from aeromagnetic data, on the tendency of carbonatite deposits to cluster spatially, and on the presence 

of known deposits in the study area and northern Brazil. 

In addition to Cerro Impacto, we estimate that there is a 

90 percent chance of two or more carbonatite deposits, a
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 75

50 percent chance of five or more, and a 10 percent 

chance of twelve or more. Approximately 80 percent of 

carbonatite deposits contain one or more commodities 

having subeconomic to economic potential.

Thorite-Rare Earth Element Veins

By Floyd Gray

THE MODEL

Thorium and rare earth element minerals, as 

described by Staatz (1983), Armbrustmacher (1988), and 

Staatz (1992), are present in quartz-potassium feldspariron oxide gangue in veins that are from about 1 m to 

more than 1 km in length and from less than 1 cm to 

about 16 m in thickness. Veins are generally fine grained 

and commonly stained with iron oxide minerals, with or 

without manganese oxide and hydroxide minerals. Principal ore minerals in most deposits are thorite and monazite. 

Possible associated minerals are brockite, allanite, bastnaesite, and xenotime. The veins are typically associated 

with variable suites of alkaline rocks and (or) carbonatite 

generally in a circular zone outboard of the alkaline rocks 

(fig. 26). Host rocks are mainly Precambrian in age and 

may include quartzite, gneiss, granite, and hornblende 

schist; Late Cretaceous and Tertiary host rocks are also 

known. Veins occupy brittle fractures and joints within 20 

km of source magmas. Hydrothermal alteration near the 

veins is in the form of thin selvages of fenitized rock; 

iron-bearing minerals are altered typically to goethite and 

commonly to lepidocrocite and hematite.

The tonnage-grade model for this deposit type (Bliss, 

1992a) is based on 32 deposits, most of which are in the 

continental United States (fig. 27). The median deposit 

contains 180,000 tonnes of ore; 90 percent of the deposits 

contain at least 7,000 tonnes, and 10 percent contain 4.4 

million tonnes or more. The median grade of thorium 

oxide is 0.39 percent; 90 percent of the deposits contain at 

least 0.13 percent thorium oxide and 10 percent more than 

1.2 percent. The rare earth element grade, where known, is 

as much as 0.5 percent in 10 percent of the deposits.

EXAMPLES IN VENEZUELA

No deposits have yet been identified in Venezuela as 

thorite-rare earth element veins. Placer accumulations of 

thorite, rare earth element minerals, and other heavy minerals in the Cano Aguamena-Cerro Boquerones area may 

have been derived, however, from unidentified sources of 

this deposit type.

Massive carbonatit

Fenite 

Diverse alkalic rocks

Country rock

^Fenite

Diverse alkalic rocks

EXPLANATION

 Thorium vein

Carbonatite dikes

B Country rock

Figure 26. Idealized model showing relations between thoriumrare earth element veins and alkalic rocks and carbonatite. Modified from Staatz (1992, fig. 1). A, Plan view. B, Cross section.

Diamond-Bearing Kimberlite Pipes

By Floyd Gray

THE MODEL

Diamond pipes or kimberlite diatremes and other 

alkaline mafic rocks containing irregularly distributed diamonds are the major source of gem- and industrial-grade 

diamonds (Cox, 1986a; Michalski and Modreski, 1991). 

The rocks are believed to be hybrid ultramafic rocks of
76 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

0.0016 I 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 

.A0.0063 TONNAGE, IN MILLION TONNES

1.0

0.9

0.8

£ 0.7 

Si 06

Q

LL.

° 0.5

g

§ 0.4 h

Q.

o

2i 0.3

0.2 

0.1

n=32

0.13

I

0.39

0.0

0.0320.056 0.1 0.18 0.32 0.56 1.0 1.8 3.2 5.6 10.0

B THORIUM OXIDE GRADE, IN PERCENT

0.4 

0.3 

0.2

0.1

0.0

n=32

0.0320.056 0.1 0.18 0.32 0.56 1.0 1.8 3.2 5.6 10.0

C RARE EARTH ELEMENT OXIDE GRADE, IN PERCENT

Figure 27. Tonnage and grade diagrams for thorium-rare 

earth element vein deposits. Modified from Bliss (1992a, 

figs. 2-4). A, Tonnages. Tie lines to the curved plot represent 

intercepts for the 90th, 50th, and 10th percentiles. B, Thorium oxide grades. Tie lines to the curved plot represent intercepts for the 90th, 50th, and 10th percentiles. C, Rare earth 

element oxide grades. Tie line to the curved plot represent 

intercepts for the 10th percentile.

Archean granite

gneiss and belts

of schist

Figure 28. Cross section of a typical kimberlite pipe. Modified 

from Cox (1978, fig. 3).

upper mantle origin in areas of epeirogenic warping or 

doming and along major basement fracture zones (fig. 

28). It is generally assumed that most kimberlite pipes 

erupted on the Earth's surface, although in most known 

kimberlite provinces surface features are rarely preserved. 

The deposits are hosted in rocks ranging in age from 

2.5 Ga to 30 Ma in crudely outlined clusters of 5-15 

occurrences. The pipes weather rapidly to form topographic depressions. Their geochemical signature consists 

of nickel and niobium and the heavy minerals pyrope garnet, phlogopite, and magnesian ilmenite; trace amounts of 

chromium, titanium, manganese, cobalt, and platinumgroup elements are also present.

Based on data from 20 known deposits in several 

countries these deposits have a median tonnage of 26 million tonnes and a median diamond grade of 0.25 carats per 

tonne (fig. 29). Ninety percent of the deposits contain at 

least 2.5 million tonnes of material, and 10 percent contain 

more than 226 million tonnes. Ninety percent of the 

deposits have a diamond grade of at least 0.072 carats per 

tonne, and 10 percent have a grade greater than 0.83 carats 

per tonne (Bliss, 1992b).

Most pipes consists of several pipe- or dike-like columns of kimberlitic material, each of which has a different grade of diamonds. The economic viability of a given 

pipe is commonly determined not only by diamond grade
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 77

0.00160.00630.025 0.1

A

0.4 1.6 6.3 

MILLION TONNES

25 100 400 1,600

0.032 0.056

B

0.1 0.18 0.32 0.56 1.0 1.8 3.2 

DIAMOND GRADE, IN CARATS PER TONNE

5.6 10.0

Figure 29. Tonnage and grade diagrams for diamond kimberlite 

pipe deposits. Tie lines to the curved plots represent intercepts for 

the 90th, 50th, and 10th percentiles. Modified from Bliss (1992b, 

figs. 2, 3). A, Tonnages. B, Diamond grades.

but also by the ratio of gem- to industrial-grade diamonds 

and by the size, shape, clarity, and color of the gemstones. The ratio of gem- to industrial-grade diamonds in 

most economic deposits is about 1:4 (Michalski and 

Modreski, 1991).

EXAMPLES IN VENEZUELA

Diamond-bearing kimberlite has been identified in the 

Guaniamo district of Estado Bolivar along a stream known

as Quebrada Grande. It is present as dikes and sills discontinuously exposed for a strike length of 7 km (Nixon and 

others, 1989); however, the main tributary and small 

creeks that feed from this area are diamondiferous for a 

distance of at least 40 km. The rocks consist mainly of 

yellow-green smectite and grains of ulvospinel-magnetite. 

Macrocrysts consist of lilac to green garnet that has kelyphitic (reaction) rims, chromite, diamond, and titaniumrich phlogopite. Yimengite is present in Venezuelan 

kimberlitic intrusions associated with spinel (Nixon and 

Condliffe, 1989). Yimengite alteration may be considered 

evidence for potassium-rare earth element metasomatism at 

the base of the craton in Venezuela, during or prior to 

emplacement of the kimberlitic rocks in the Early Proterozoic. Strontium whole-rock geochronology defines a 

poorly constrained Early Proterozoic age of 1,9001200 

Ma. Neodymium model ages yield a similar Early Proterozoic age of 2,06011,950 Ma. Micaceous lamprophyric 

dikes containing pyrope garnet are associated with the 

kimberlite, and mica from the dikes has been dated by the 

40Ar/39Ar method at 850 Ma. The kimberlitic intrusive 

rocks in the Guaniamo area are present in a northnorthwest-trending zone of faults and possible cratonal rifting (Nixon and others, 1989).

PERMISSIVE DOMAIN

The domain permissive for kimberlite intrusive rocks 

is virtually identical to that for carbonatite intrusive rocks. 

Domain V (pi. 8) includes the Cuchivero Group and sedimentary rocks of the Roraima Group. Areas of the 

Imataca Complex, the Supamo Complex and greenstone 

belts, and the granitic rocks of the southern and western 

parts of Territorio Federal Amazonas are excluded 

because they are eroded too deeply for kimberlite to be 

preserved, assuming that the most extensive erosion in the 

shield occurred from late Mesozoic through Tertiary time. 

Two types of areas within the permissive domain have a 

higher probability for undiscovered deposits of kimberlite 

and carbonatite. One type of area consists of crudely circular areas defined by radar imagery, aeromagnetic anomalies, residual gravity anomalies, and the association of 

some diamond prospects not associated with detritus of 

the Roraima Group. The other type of area is defined by a 

riftlike feature described in the section on geophysics. 

Many known kimberlite deposits are clustered in areas of 

crustal weakness or rifting.

Tin Greisen Deposits

By Dennis P. Cox and William E. Brooks

THE MODEL

Tin greisen deposits have been described by Taylor 

(1979) and Reed (1986a) as disseminated cassiterite and
78 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

cassiterite veinlets in stockworks, lenses, pipes, and 

breccias associated with greisenized granite. Greisen alteration consists of a coarse-grained assemblage of muscovite, quartz, tourmaline, and fluorite or topaz that developed in granite during late magmatic or deuteric stages of 

crystallization. Granite that contains tin greisen deposits 

typically is mesozonal and has a high silica content (about 

70 percent SiOa). It contains anomalous amounts of F, Rb, 

Li, Be, W, Mo, Pb, Nb, Ta, Cs, Hf, U, and Th and is 

commonly depleted in Ca, Ti, Mg, Fe, Cu, Ni, V, Sc, Sr, 

La, and Ba. The tin greisen deposits tend to be near the 

apical parts of granite plutons. Placer tin deposits are typically closely associated with tin greisens.

The median tonnage of tin greisen deposits (Menzie 

and Reed, 1986) is 7.2 million tonnes; 10 percent of the 

deposits contain 65 million tonnes or more. The median 

grade is 0.28 percent Sn; 10 percent of the deposits have a 

grade of 0.47 percent Sn or higher.

EXAMPLES IN THE GUAYANA SHIELD

There are no known tin greisen deposits in Venezuela; however, in northern Brazil, less than 400 km south of 

its border with Venezuela, is the Pitinga deposit (lat 0°45' 

S., long 60°00' W.), perhaps the largest tin greisen deposit 

in the world with anticipated 1988 production of 34,000 

tonnes of tin (Jones and others, 1986; Thorman and Drew, 

1988). The Pitinga deposit, in the Mapuera mining district, 

is underlain by a suite of rhyolitic ash-flow and air-fall 

tuff intruded by younger granite. The intrusive rocks at 

Pitinga include an older biotite granite suite (1,700 Ma), a 

biotite-rapakivi granite (1,500 Ma), and a younger series 

of mafic-ultramafic dikes and sills. The Pitinga deposit is 

in the younger granitic rocks. Thorman and Drew (1988) 

indicated that the geologic setting of the Mapuera district 

may extend northward into Venezuela.

PERMISSIVE DOMAIN

The Cuchivero Group and related granitic rocks are 

similar in age and geologic setting to the rocks described 

in the Pitinga area and are considered permissive for tin 

greisen deposits (domain VI, pi. 7). Recently described circular structures, 10-12 km in diameter, in the Cuchivero 

terrane (Garcfa and Lugo, 1991) may indicate tin greisen 

targets that are structurally similar to ring features 

described by Thorman and Drew (1988) in the Rondonia 

mining district in Brazil. The Parguaza Granite terrane is 

also included in the permissive domain because of its 

approximately similar age (1,550 Ma; Gaudette and others, 

1978), the rapakivi texture of the granite, and the presence 

of tin placers derived from pegmatite (Rodriguez and 

Pe"rez, 1982). A period of intrusion of rapakivi granite 

closely preceded intrusion of tin-bearing granite in the

Pitinga, Xingu, and Rondonia tin districts in Brazil (Jones 

and others, 1986).

UNDISCOVERED DEPOSITS

No estimate of undiscovered deposits was made 

because of the lack of geochemical data with which to characterize granitic rocks in the permissive domain. Specifically, fluorine, rubidium, beryllium, and lithium contents 

would be useful in evaluating tin greisen potential.

Rhyolite-Hosted Tin Deposits

By Dennis P. Cox 

THE MODEL

Rhyolite-hosted tin deposits (Reed and others, 1986) 

contain discontinuous veinlets of cassiterite and wood tin 

in rhyolitic flow-dome complexes. Specular hematite, 

fluorite, chalcedony, and adularia are accessory minerals 

in the veinlets. Stream-sediment samples from near these 

deposits typically contain 1,000 ppm Sn and show anomalies of the same elements found in tin granite associated 

with tin greisen deposits. Tin placer deposits are commonly associated with rhyolite-hosted tin.

These deposits are small and of low grade and rarely 

amenable to any but small, nonmechanized mining operations. Median tonnage is 1,000 tonnes, and median grade 

is 0.38 percent Sn. Ten percent of the deposits contain 1.1 

percent Sn or more (Singer and Mosier, 1986b).

PERMISSIVE DOMAIN

Areas underlain by the Cuchivero Group are permissive for rhyolite-hosted tin deposits (see pi. 2 for areas of 

Cuchivero Group). No deposits are known, but their presence is suggested by small amounts of cassiterite in rhyolite, stream sediments, and panned concentrates in streams 

that drain into the upper Rio Paragua (Sidder, 1990). 

Lacking other evidence regarding the distribution of tin in 

the Cuchivero Group, a quantitative estimate for this 

deposit type cannot be made.

Porphyry Copper Deposits

By Dennis P. Cox

THE MODEL

Porphyry copper deposits (Titley, 1982; Cox, 1986b) 

are large irregular masses of intrusive porphyry or country
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 79

rock that contain chalcopyrite in stockworks and disseminations. The deposits form in felsic hypabyssal intrusive 

rocks emplaced at depths of 1-3 km. These intrusive rocks 

have a characteristic texture of small, closely spaced quartz 

and feldspar phenocrysts in a microaplitic groundmass. 

Chalcopyrite and pyrite, with or without molybdenite, are 

associated with pervasive characteristic alteration. Finegrained sericitic or argillic alteration is present in the higher 

parts of the intrusions, and biotite and potassium feldspar 

are present at deeper levels. The deeper alteration produces 

hard, well-crystallized rocks that appear unaltered on first 

inspection. Two important porphyry copper subtypes are 

known; one contains magnetite in place of pyrite and commonly contains as much as 1 gram of gold per tonne and 

another contains pyrite and important byproduct molybdenite. Tropical weathering of the pyrite-rich deposits leaches 

copper and other metals, leaving behind clay-rich zones 

and no geochemical indications of a deposit. Weathering of 

magnetite-rich deposits can result in the deposition of 

copper carbonate minerals in rocks near the surface, as well 

as residual gold anomalies in soils.

Porphyry copper deposits are typically large (2-5 km 

diameter). Their median tonnage is 140 million tonnes of 

ore and their median grade is 0.53 percent Cu (Singer, 

Mosier, and Cox, 1986).

PERMISSIVE DOMAIN

Porphyry copper deposits may be present in two 

domains: areas underlain by felsic volcanic rocks of the 

Cuchivero Group and associated intrusive rocks and areas 

underlain by eugeosynclinal metavolcanic rocks, such as 

Botanamo, Anacoco, and Kilometre 88 (pi. 2). No 

porphyry-style mineralized rocks are documented in these 

areas. Porphyry having characteristic aplitic groundmass 

but lacking mineralization or alteration has been observed 

in granitic rocks of the Cuchivero Group near the village 

of Canaracuni. Pedro Lira (Ministerio de Energia y 

Minas, oral commun., 1991) reported disseminated chalcopyrite mineralization in intrusive rock encountered during drilling in the Kilometre 88 area. There is insufficient 

information to make an estimate of undiscovered porphyry copper deposits.

Veins of Plutonic and 

Volcanic Association

By Dennis P. Cox

THE MODEL

Polymetallic veins are associated with high-level 

plutons and porphyry and skarn base-metal deposits.

Epithermal veins are associated with volcanic centers and, 

in some areas, with resurgent intrusions in calderas. Polymetallic veins form within 1 km of the surface, and epithermal veins form at even shallower levels. Deposits are 

exploited mainly for their gold and silver contents, but 

many contain abundant base-metal sulfide minerals.

PERMISSIVE DOMAIN

No examples of polymetallic or epithermal veins have 

been recognized in the Precambrian shield of Venezuela. 

The area underlain by volcanic rocks of the Cuchivero 

Group is permissive for these deposits (pi. 7, domain VIII), 

and their possible occurrence is supported by low-level Au, 

Ag, Bi, Mo, As, and W anomalies in Cuchivero rocks 

(Sidder and Martinez, 1990), by the presence of porphyrycopper-type intrusive rocks, and by the presence of possible 

caldera structures. The probability of undiscovered vein 

deposits cannot be assessed in this study due to the lack of 

geologic detail in regions potentially permissive for this 

deposit type.

Volcanic-Hosted Magnetite 

Deposits

By Dennis P. Cox

THE MODEL

Lens-shaped bodies of massive magnetite and subordinate apatite in volcanic rocks have been described in a 

poorly constrained descriptive model by Cox (1986c). 

Some deposits making up the model were formed by 

replacement of their host volcanic rocks and are surrounded by alteration zones containing diopside, garnet, 

and scapolite (Anhui Province, China). Other deposits may 

have been emplaced as iron-rich magmas (Kirunavaara, 

Sweden). Deposits may be of any geologic age; Pea 

Ridge, Missouri, is an example of an Middle Proterozoic 

deposit (1.46 Ga; Nuelle and others, 1992).

Median tonnage of these deposits is 40 million 

tonnes, and median grade is 58 percent Fe (Mosier, 

1986b).

Volcanic-hosted magnetite deposits are similar to 

Olympic Dam-type deposits in terms of iron content and 

environment of formation. Olympic Dam in South Australia, the only known deposit of its type, is a 2-billion-tonne 

deposit of hematite, subordinate chalcocite, bornite, and 

uranium, and minor native gold and rare earth element 

minerals in a graben in an Early Proterozoic granitic basement. It is contemporaneous with felsic volcanism of 

about 1.59 Ga and is covered by more than 335 m of Late
80 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Proterozoic and Phanerozoic sedimentary rocks. The possibility of a deposit of this type in rocks of the Cuchivero 

Group is difficult to evaluate because of uncertainties in 

the model.

PERMISSIVE DOMAIN

Areas underlain by Cuchivero Group are considered 

permissive for volcanic-hosted magnetite deposits because 

of the similarity of this environment to that of Pea Ridge, 

Missouri (domain VIII, pi. 7). The Parguaza Granite is 

similar tectonically, chemically, and in age to the Olympic 

Dam deposit (G.B. Sidder, oral commun., 1993). Because 

of uncertainties inherent in the model and the lack of 

known occurrences, an estimate of undiscovered deposits 

was not made.

Unconformity Uranium-Gold 

Deposits

By William E. Brooks

THE MODEL

Unconformity uranium-gold deposits, or veinlike uranium deposits, have been described by Dahlkamp and 

Adams (1981) and Grauch and Mosier (1986) as fractureand breccia-filling associated with Early to Middle Proterozoic unconformities. These deposits are below, above, 

or across unconformities that separate Early and Middle 

Proterozoic rocks. Several subtypes of this deposit type 

exists. In Canada, uranium was originally deposited, along 

with sediments, on Archean basement; these sedimentary 

rocks were metamorphosed about 1,900-1,700 Ma and 

then weathered; the resulting erosion surface was then 

covered by Middle Proterozoic sandstone about 1,600- 

900 Ma (Dahlkamp and Adams, 1981).

Deposits are formed at and along the unconformity 

interface by mixing of reduced fluids from carbonaceous 

metasedimentary rocks and uranium-bearing oxidized 

fluids in overlying sandstone. Locally, Au, Ag, Te, Ni, 

Pd, Re, Mo, Hg, rare earth elements, Y, and Rb may be 

present. According to Mosier (1986c), median tonnage 

for these deposits is 260,000 tonnes, and median grade is 

0.49 percent U3O8.

Deposits are, in some cases, difficult to detect using 

geophysical methods. An airborne radiometric survey at 

Jabiluka, Australia, failed to record any anomaly, and the 

ground radiometric anomaly was only slightly more than 

twice background (Dahlkamp and Adams, 1981).

DISCUSSION

There are no known unconformity uranium-gold 

deposits in Venezuela; however, the geologic setting and 

age of pre-Roraima sedimentary rocks in many parts of 

Venezuela, including the Cinaruco Formation (McCandless, 1962), the Esmeralda Formation (Sellier de Civirieux, 

1966), the Los Caribes Formation (Benaim, 1972), and 

pre-Roraima sediments described by Briceno (1982), indicate the possibility that these types of deposits might be 

present. Pre-Roraima sediments were deposited on Early 

Proterozoic basement complex in Territorio Federal Amazonas and on Cuchivero Group and Early Proterozoic 

schist in the central part of Estado Bolivar (pi. 2); preRoraima rocks include quartzite, schist, and conglomerate. 

The pre-Roraima rocks have been intruded by the Parguaza Granite (McCandless, 1962), deformed into tight 

isoclinal and chevron folds (Ghosh, 1977), and then 

deeply eroded and overlain by the Roraima Group (Ghosh, 

1977). The minimum age of the Roraima Group is 1,600 

Ma (Priem and others, 1973); therefore, pre-Roraima sediments are inferred to be older than 1,600 Ma.

Despite the similarity in age and tectonic environment of the pre-Roraima Group unconformity and unconformities hosting major deposits in Canada and Australia, 

no evidence for the necessary reducing environment or for 

a carbonaceous component has been found in the preRoraima sediments. Therefore, the probability of unconformity uranium-gold deposits in the Venezuelan Guayana 

Shield is considered to be small.

Quartz-Pebble Conglomerate 

Gold-Uranium Deposits

By William E. Brooks

THE MODEL

Gold and uranium deposits associated with ancient 

quartz-pebble conglomerate have been described by Pretorius (1981) and reviewed by Cox (1986d). The world's 

most important source of gold is 2,800-2,200-millionyear-old conglomerate in the Witwatersrand Basin in 

South Africa. Economic occurrences of this type contain 

15 percent of known uranium reserves in the world 

(Young, 1984). These deposits are in mature monolithologic conglomerate within a thick sequence of less mature 

conglomerate and sandstone deposited on Archean granitegreenstone terrane. Basal volcanic rocks are present 

locally. Ore may be concentrated at the base of mature 

conglomerate in a sedimentary environment that includes
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 81

braided stream channels, alluvial fans, trough crossbedding, and current- or wave-winnowed bedding surfaces 

(Minter and others, 1986).

DISCUSSION

The Tarkwa quartz-pebble conglomerate gold deposit 

in Ghana was affected by an orogeny dated at 2,100-2,000 

Ma and is probably younger than the Witwatersrand 

deposits (Mile"si and others, 1991). The Tarkwa conglomerate contains no uranium in the gold mining area. A possible explanation for this lack of uranium is that by 1,645 

Ma, minimum age of the Tarkwa conglomerate (Holmes 

and Cahen, 1955), an oxygen-rich atmosphere had developed that prevented formation of uraninite placers (Vogel, 

1987). Mile's! and others (1991) presented arguments, 

however, that gold mineralization at Tarkwa is more 

closely associated with structures related to deformation 

than with features of detrital origin. Thus, Tarkwa may not 

properly belong to the quartz-pebble conglomerate gold 

model.

Gold deposits are present in French Guiana in the 

Orapu Series, a metasedimentary sequence that includes 

quartzose conglomerate. These deposits are suggestive of 

the quartz-pebble gold type but probably should not be 

included in that model. Unpublished reports by E. Manier, 

and P. Ledru (Mile's! and others, 1991) indicate that gold 

and polymetallic sulfide minerals are present in shear 

zones adjacent to the auriferous metaconglomerate of the 

Orapu Series.

No tonnage-grade model has been prepared for 

quartz-pebble conglomerate gold deposits, but data from 

Witwatersrand presented by Pretorius (1986) show tonnages of 107 and 108 tonnes and average grades of 5-30 

grams of gold per tonne.

The basal formation of the Roraima Group is 850 m 

thick and comprises conglomerate, gravel, and pebbly 

sandstone (Reid and Bisque, 1975). The pebbles (10 cm) in 

the conglomerate beds (30 cm-1 m thick) are well rounded 

and consist of quartz, quartz porphyry, and quartzite 

(Wyant and others, 1953). Unmetamorphosed Cuchivero 

granitic and volcanic terrane is unconformable to conformable beneath the Roraima Group. A radiometric anomaly 

may be present in the basal Roraima (Reid and Bisque, 

1975), and placer gold is commonly associated with the 

basal Roraima near Santa Elena de Uaire"n.

The Roraima Group is Middle Proterozoic (1,600 Ma; 

Priem and others, 1973) in age, younger than the deposits 

used to construct the model. Reconnaissance study of 

sandstone and conglomerate above the Roraima-Cuchivero 

contact near Santa Elena indicates only 1-9 ppm U 

(Brooks and Nuflez, 1991). Of the deposits in the model, 

the Tarkwa deposit in Ghana is a uranium-free end member of the population and younger than most other quartzpebble conglomerate gold-uranium deposits. Vogel (1987) 

hypothesized that by 1,900 Ma (the age of the Tarkwa conglomerates) an oxygenic atmosphere had developed that 

prevented formation of economic uraninite placers but had 

no effect on formation of gold placers. Any deposits in the 

Roraima Group will more likely resemble Tarkwa rather 

than the uranium-enriched deposits of the model.
DEPOSITS FORMED BY SURFICIAL PROCESSES

Erosion and uplift of the Venezuelan Precambrian 

shield, from Mesozoic until late Cenozoic time, has produced many environments for mineral deposits. Deposits 

formed by leaching and residual enrichment of metals 

include bauxite, iron (discussed above), saprolite gold, and 

possibly nickel laterite. Transport and deposition of metals 

and minerals has produced placer deposits of gold, diamonds, titanium, and tin. Some deposit types, such as sedimentary kaolin, are produced by a combination of these 

surficial processes. In the northern part of the shield, most 

placers probably formed during the Miocene, during the 

period of maximum uplift (Olmore and Garcia, 1990; 

Olmore and others, 1990).

Laterite-Type Bauxite Deposits

By Floyd Gray 

THE MODEL

Laterite-type bauxite deposits consist of residual 

material resulting from extreme chemical weathering of 

aluminous silicate rocks (Patterson, 1986). They have 

pisolitic, massive, nodular, or earthy textures and consist 

mainly of gibbsite and mixtures of gibbsite and boehmite, 

together with hematite, goethite, anatase, quartz, and volcanic ash. Deposits formed as a result of surficial weathering on well-drained plateaus in regions having warm to hot 

and wet climates. Locally, deposits may form in poorly 

drained areas and contain only small amounts of iron 

because of its removal by organic complexing. Structural 

and topographic features such as joints, fractures, and 

steepness of slope control the enrichment of A^Os and the 

depletion of SiC>2 (Moreno and Bertani, 1985). Lateritetype bauxite deposits are an end member of a range of 

similarly formed lateritic material found in tropical 

regions. These end members can be described in the following groups: (1) lateritic bauxite, in which the percentages of aluminum and iron are approximately equal or the 

percentage of aluminum is higher; if aluminum is much 

greater than iron, it is called bauxite; (2) lateritic iron, in 

which the percentage of iron is greater than that of aluminum; and (3) nickel laterite, derived from ultramafic rocks.

Based on data from 122 deposits and 32 countries, 

laterite-type bauxite deposits have a median ore tonnage 

of 25 million tonnes (Mosier, 1986d) (fig. 30A). Ninety 

percent of the deposits contain at least 870,000 tonnes of 

ore and 10 percent more than 730 million tonnes. Ninety

0.9 

0.8

°- 6

Q 

U-

°0.5

o

§ 0.4

Q_ 

O

tr ^0.3

0.2 

0.1 

0.0

\

0.9

0.8

0.7

2 0.5- 

g

O 0.4

Q_

o

tr 

a.

0.3 

0.2

0.1

0.0

B

0.87 25

0.025 0.1 0.4 1.6 6.3 25 100 400 1,6006,30025,000 

A MILLION TONNES

35 45

10 20 30 40 50 60 70 80 90 100 

ALUMINA GRADE, IN PERCENT A^Oa

Figure 30. Tonnage and grade model curves for laterite-type 

bauxite deposits. Tie lines to the curved plots represent intercepts 

for the 90th, 50th, and 10th percentiles. Modified from Mosier 

(1986, figs. 191, 192). A, Tonnages. 5, Alumina grades.

percent of the deposits have ore that consists of more than 

35 percent A^C^, and 10 percent of the deposits exceed 

55 percent A^Oa (fig. 305). The median grade of bauxite 

ore in the tonnage-grade model is 45 percent

82
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 83

Table 10. Topographic surfaces of the Guayana Shield.

[Modified from Menendez and Sarmentero (1984)]

Surface

Orinoco alluvial plain..... 

Los Llanos.......................

Intermediate level.... ........

Elevation 

(meters)

1-50 

.. 80-150

.. 200-350

.. 400-500

Age

Recent........................................

Pliocene-Pleistocene... ........ .......

Middle Tertiary. ........................

Middle Tertiarv. ........................

............ Laterite.

Examples of deposits "

............ Ferruginous laterite between Ciudad Piar and La Paraeua. western oart

Nuria........

Kamarata.

600-700 Late Cretaceous-Early Tertiary. 

900-1,000 Jurassic or older.........................

of Los Pijiguaos. 

Nuria Plateau, Los Guaicas Mountains, Los Pijiguaos, £1 Pao, and

Cerro Bolivar iron deposits. 

Roraima (Gran Sabana), southern Los Pijiguaos.

EXAMPLES IN VENEZUELA

Laterite-type bauxite deposits in Venezuela can be 

grouped into two categories, as suggested by Bellizzia, 

Pimentel de Bellizzia, and Rodrfguez (1981): (1) deposits 

derived from mafic rocks such as diabase and gabbrodiorite complexes that intrude the Roraima Group and the 

Supamo Complex, respectively, and (2) deposits derived 

from the weathering of areas underlain by granitic rocks 

such as the Parguaza Granite. The differences between 

deposits in these two categories stems mainly from the 

size of source rock exposure and from the chemical composition of the source rocks. Deposits of lateritic bauxite 

from the first category are generally of smaller tonnage; 

iron is a significant component in the composition of the 

ore and the percentage of iron may exceed that of aluminum. Deposits in the second category, on the other hand, 

are of larger tonnage and have a higher proportion of aluminum with respect to iron.

Several topographic surfaces, interpreted by some 

workers as remnants of ancient erosional surfaces, have 

been identified in the Guayana Shield (table 10). Many of 

these surfaces are covered by laterite and lateritic bauxite 

deposits (Short and Steenken, 1962; Menendez and Sarmentero, 1984, 1985).

The Los Pijiguaos deposit (fig. 31) is by far the most 

important bauxite deposit in the Venezuelan Precambrian 

shield. It formed at between 400 and 1,000 m elevation. 

The deposit is 7-8 m thick and is in the upper part of the 

weathering profile, which is about 50 m thick and developed in situ on the Middle Proterozoic Parguaza rapakivi 

granite (Moreno and Bertani, 1985). The granitic protolith 

contains 65-73 percent SiC>2 and 13.5-15 percent A^Os. 

The area was initially discovered on the basis of geological, geomorphological, and vegetation density criteria, 

guided by correlation of these criteria with other known 

deposits elsewhere in the Guayana Shield. The total area 

of the surface remnants mapped in the Los Pijiguaos 

Mountains and vicinity is 550 km2. The average thickness 

of the bauxitic mantle is 7.6 m and there is almost no 

overburden.

At Los Pijiguaos, the vertical profile of the deposit 

consists of an upper concretionary zone, underlain successively by a mottled zone, a pallid zone, weathered rock, and 

fresh granite. The concretionary zone has a cellular, 

pisolitic, or spongy texture and is composed of 60-90 

percent gibbsite, 5-10 percent quartz, 5-10 percent hematite, less than 5 percent goethite, and traces of kaolinite and 

boehmite. The most accessible part of this bauxite deposit 

has proven mineable reserves of 176.5 million tonnes of 

bauxite having an average grade of 49.5 percent A12O3 and 

an average thickness of 7.6 m. The ore also averages 16.6 

percent Fe2C>3,1.2 percent TiC>2, and 9.3 percent total SiC>2.

In the Upata district at least thirteen deposits of bauxite have been identified, of which five are considered possibly economic (Candiales, 1961). These are El Chorro, 

La Mesa de la Carata, El Baul, Los Guamos, and Cerro 

Once. El Chorro has reserves of 1,259,250 tonnes and the 

other four together a total of 2,904,666 tonnes. The grade 

of bauxite varies from 39 to 67 percent A^C^ and the 

grades of Fe2C»3 and SiC»2 from 3 to 29 percent and from 

less than 1 to 23 percent, respectively. The ore is apparently a weathering product of gabbro or amphibolite.

Nuria is a high plateau that forms an amphitheater 

comprising a flat center and an elevated ringlike perimeter. The hills are 600-700 m in elevation. Granitic 

gneiss comprises the central part of the structure, and diabase (or possibly gabbro) forms the peripheral hills. 

Intense chemical weathering has enriched the mafic rocks 

in alumina and has formed a low-grade ore deposit. Total 

measured, indicated, and inferred reserves are 50 million 

tonnes of 37.5 percent A12O3, 28.9 percent Fe2C»3, and 8.7 

percent SiC»2 (Candiales, 1961; Bellizzia, Pimentel de 

Bellizzia, and Rodrfguez, 1981).

Deposits of aluminum-rich laterite and locally bauxite 

at Los Guaicas and in the Gran Sabana are weathering 

products of Middle Proterozoic and possibly Mesozoic 

diabasic intrusive rocks (Bellizzia, Pimentel de Bellizzia, 

and Rodrfguez, 1981; Rodrfguez, 1986). These deposits 

are relatively low grade (about 35 percent A^Os, 30-40 

percent Fe2C»3, and 3-9 percent SiC>2) and generally small
84 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Figure 31. Los Pijiguaos bauxite deposit, Estado Bolivar. Road cut north of main mine in laterite. Photograph by Jeffrey C. Wynn.

(less than 100 million tonnes). The deposit at Los Guaicas 

contains 3.1 percent TiC>2 (Bellizzia, Pimentel de Bellizzia, and Rodrfguez, 1981). The surface in the Gran Sabana 

area is between 990 and 1,100 m elevation and is called 

the Kamarata Surface. Minor lateritic-type bauxitic occurrences are in Estado Delta Amacuro. Occurrences in the 

Rio Ibaruma region are thin lateritic-saprolitic surfaces in 

or above the Aibe Formation. Exploitable resources are 

small and of poor quality.

PERMISSIVE DOMAIN

The topographic surfaces and geologic characteristics 

of the known bauxite and lateritic deposits are keys to 

determining the permissive domains of laterite-type bauxite. The areas underlain by rocks containing abundant aluminum silicate minerals at elevations between 400 and 

1,000 m are considered permissive for these deposits; however, because detailed topography is unavailable for large 

parts of the shield, no permissive domain was identified.

Residual Kaolin Deposits

By G. J. Orris and Floyd Gray 

THE MODEL

The term "residual kaolin" refers to deposits of 

kaolinite-halloysite that are typically underlain by older 

granite, gneiss, or arkose and overlain by sedimentary 

strata, commonly marine. These deposits, also called 

saprolite or high-alumina kaolin, form in tropical to subtropical climates in areas of well-drained plains and 

plateaus where feldspathic source rocks can be katamorphically altered. Intense weathering leads to hydration of 

the anhydrous aluminous silicate minerals by acidic meteoric water to form kaolinite, potash, and silica. Further 

weathering leads to removal of the silica and alkalies and 

to the formation of laterite and local areas of gibbsitic 

bauxite in the overlying parts of the weathered horizon. 

The ratio of kaolinite to halloysite is determined by the 

amount of mica in the source rock; there is a positive
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 85

correlation between mica content of the source rock and 

the kaolinite to halloysite ratio of the weathered residuum. 

Impurities in the kaolin include quartz, iron oxide minerals, gibbsite, ilmenite, and muscovite.

EXAMPLES IN VENEZUELA

Residual kaolin deposits in Venezuela are generally 

associated with alteration of planation surfaces in 

feldspathic gneiss of the Imataca Complex or with alteration of felsic granitic gneiss; residual deposits also may 

form in acidic to intermediate igneous rocks such as those 

in the Kilometre 88 area (Rodiguez, 1985). Kaolin deposits in granitic gneiss and gneiss complexes are spatially 

associated with bauxite typically underlying lateritic 

bauxite, for example, at Los Pijiguaos; less frequently, 

manganese-enriched zones are associated with these 

deposits.

The Kilometre 88 area may have the largest known 

reserves of kaolin in Estado Bolivar. The deposits, some 

as thick as 30 m, are associated with lateritic weathering 

horizons in granitic or dioritic sequences associated with 

post-Supamo intrusions and are partly covered by young 

sediments of the Rio Cuyunf (Rodriguez, 1986). Estimated 

tonnage in the area may be in excess of 25 million tonnes 

of kaolin. Large deposits of kaolin are associated with 

gold occurrences in the Las Cristinas area; in this area 

normal tropical weathering of the country rocks has been 

enhanced by acidic decomposition of pyrite-enriched 

zones.

The deposits of the Upata area, although small, have 

been extensively mined. The principal deposits of Cerro 

Copeyal and San Lorenzo are in weathered zones in felsic 

granulitic gneiss of the Imataca Complex. Silicification of 

bauxitic material has resulted in the formation of some 

deposits (Rodriguez, 1986).

Minor deposits west of the Rfo Caronf are products 

of alteration of feldspathic gneiss of the Imataca Complex 

(Juan Acosta, CVG, oral commun., 1977). The deposit at 

Las Margaritas is 2.5 km from Rfo Orocopiche. The 

kaolin body trends N. 70° E. and is about 2 km long and 

about 2 m thick. Many small deposits of similar quality 

are being exploited in the area.

PERMISSIVE DOMAIN

Permissive areas for residual kaolin deposits are 

mostly the same as those for bauxite deposits. The permissive domain for these deposit types is more dependent 

on geomorphic and timing constraints than on rock type, 

and it was not possible to delineate the domain.

UNDISCOVERED DEPOSITS

No estimates of undiscovered deposits were made 

because we could not adequately define the permissive 

domains.

Sedimentary Kaolin Deposits

By G.J. Orris

THE MODEL

Sedimentary kaolin, also referred to as high-alumina 

clay or china clay, forms deposits that are typically underlain by much older crystalline rocks and overlain by sedimentary strata, commonly marine. The deposits form in 

tropical to subtropical climates where crystalline source 

rocks can be katamorphically altered. The resultant alteration products are transported and deposited in deltaic or 

tidal-flat environments where they form clay lenses within 

layers that are predominantly sand. Overlying deposits 

locally contain lignite. Postdepositional weathering and 

leaching result in recrystallization of the kaolinite and the 

formation of local areas of gibbsitic bauxite. Impurities in 

the kaolin commonly include quartz, iron oxide minerals, 

gibbsite, ilmenite, and (or) muscovite.

EXAMPLES IN VENEZUELA

Kaolin occurrences of probable sedimentary origin 

are present in the Tertiary Mesa Formation, which is postulated to be of mostly deltaic origin but also has alluvial 

and elluvial components (Hedberg and Pyre, 1944; Royo y 

Gomez, 1956). These occurrences are at predictable elevations in the terraces along the Rio Orinoco. At least one 

mine is operational, and the product is used in the manufacture of ceramic tile. The kaolin likely is a mix of 

kaolinite, quartz, and minor iron oxide minerals. At least 

two kaolin-bearing units are present. The upper unit has a 

quartz sand content that commonly exceeds 40 percent. 

The upper kaolin unit is separated from the lower kaolin 

unit by a layer of bright-orange iron-stained sand. The 

lower kaolin unit is a mixture of kaolinite and small 

amounts of iron oxide minerals without visible quartz sand 

and is the part of the deposit currently being exploited.

PERMISSIVE DOMAIN

Within the study area, the Tertiary Mesa Formation is 

considered permissive for the discovery of additional sedimentary kaolin deposits (domain VII, pi. 8). A report that
86 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

the Mesa Formation extends to the west and to the east of 

its current mapped outcrop could not be confirmed, and 

the delineated domain is therefore limited to the known 

outcrop area.

UNDISCOVERED DEPOSITS

An estimate of numbers of undiscovered deposits 

cannot be made because of a lack of geologic data on the 

Mesa Formation. Although the clay-bearing beds are 

extensive, variations in depositional facies within the formation control the quality (grade) of the kaolin. Data on 

these facies variations are not available.

Placer Gold

By G.J. Orris

THE MODEL

The placer gold model used in this study is that of 

Yeend (1986) and includes elemental gold grains and nuggets mechanically concentrated in sand, silt, and clay of 

alluvial, eolian, and beach deposits. This model also 

includes consolidated equivalents of these units, although 

deposits older than Tertiary are rarely preserved. In placer 

deposits, gold is concentrated at the base of the gravel 

deposits where it was trapped by stream or river riffles, 

fractured bedrock, bedding planes, foliation, or other 

structures perpendicular to the direction of water flow. 

Locally, gold may be concentrated above clay horizons 

that prevent its downward migration. In some cases, 

reworking or longer transport distances may increase gold 

concentration, average nugget size, and (or) fineness. 

There is a strong association between gold and quartz 

clasts in some placer systems, especially those related to 

low-sulfide gold-quartz vein deposits.

Placer gold deposits are commonly spatially associated with known gold-bearing lode. Large placer gold 

systems are also known that have no relation to economic 

gold-bearing lode deposits but instead formed from reworking of sediments originating from black shale, felsic tuff, or 

other rocks of generally low metamorphic grade and having 

low ambient gold contents. Other deposit types spatially 

associated with placer gold include diamond, ilmenitemagnetite, zircon, and (or) monazite placer deposits.

EXAMPLES IN VENEZUELA

Eighty-one placer gold deposits have been reported in 

a large part of the study area (table 5) and are mostly 

worked by small-scale mining techniques. A few of these

deposits may actually be weathered low-sulfide goldquartz vein deposits that are being mined by placer methods. The Carabobo placer deposits produced more than 2 

tonnes of gold prior to 1968. The Corocoro placers produced more than 3.3 tonnes during the same time period. 

Other large placer areas are known, but production 

amounts have not been reported.

PERMISSIVE DOMAIN

Permissive areas for the occurrence of placer gold 

include areas downstream from known placer gold deposits 

and known lode gold deposits. In addition, placer gold 

deposits may be present downstream of areas of greenstone 

rocks and areas of Cuchivero Group rocks permissive for 

epithermal gold deposits. All areas downstream of 

Roraima Group and pre-Roraima rocks are also considered 

to be permissive for gold placers because of reported gold 

in the basal conglomerates. Despite the long history of 

small-scale placer mining within the study area, the mining 

methods used have been relatively inefficient, and deposits 

of very fine sized gold may have been overlooked.

UNDISCOVERED DEPOSITS

Grades and tonnages of placer gold deposits were 

modeled by Bliss and Orris (1986). These deposits have a 

median tonnage of 1.1 million tonnes and a median grade 

of 0.2 grams of gold per tonne. The extensive permissive 

area and the wide range of placer gold grades and tonnages make it difficult to estimate the potential number of 

undiscovered deposits, but their number and the amount of 

probable contained gold are thought to be significant.

Placer Diamond

By Floyd Gray and G.J. Orris

THE MODEL

Diamonds transported from weathered source rocks 

and deposited in alluvial and beach sediments, sandstone, 

and conglomerate are termed "placer diamonds" (Lampietti 

and Sutherland, 1978; Cox, 1986e). Deposits are in coarse, 

clastic-textured sedimentary sequences and are typically 

Tertiary and Quaternary in age. Diamonds are concentrated 

with other heavy minerals in the low-energy parts of 

stream systems and typically decrease in size and increase 

in quality with distance from their source. Streams draining areas of kimberlite pipes or diamond concentrations
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 87

(paleoplacers) in sedimentary or metamorphic rocks form 

the principal environment of deposition; however, alluvial 

diamond deposits may be as far as 1,000 km from their 

source. Where kimberlite or lamproite form the source 

rocks, geochemically anomalous Cr, Ti, Mn, Ni, Co, platinum-group elements, and Ba may be a signature of the 

deposits. Anomalous nickel and niobium, together with the 

heavy minerals pyrope garnet, chrome diopside, magnesian ilmenite, and phlogopite, indicate nearby kimberlite 

pipes.

EXAMPLES IN VENEZUELA

Diamonds are in alluvial and residual deposits within 

the Guayana Shield of Venezuela. Production in 1986 

totalled 212,000 carats of about 20 percent gem quality 

and 80 percent industrial quality and decreased in 1987 to 

about 113,000 carats of about 36 percent and 64 percent 

gem and industrial quality, respectively (Newman, 1988). 

Peak production of 1,248,979 carats was in 1974, and total 

production between 1913 and 1987 was 13,570,917 carats 

and about 25 percent gem quality (Anez, 1985; Baptista 

and Parra, 1985; Newman, 1988).

The source of the diamonds has been variously 

ascribed to the conglomerates of the basal Roraima Group 

(unit YXr, pi. 2) or pre-Roraima rocks (unit Xpr), to kimberlite deposits of West Africa, or to local weathered kimberlite (Quebrada Grande, Guaniamo district, see 

discussion on kimberlite). Although no occurrences of diamonds within the Roraima Group have been documented, 

there is a pronounced spatial association between alluvial 

diamond deposits and outcrops of the Roraima Group. 

With the major exception of the Quebrada Grande area 

workings, most known diamond placer deposits are either 

within areas of mapped Roraima Group or in areas downstream from this unit.

Although the lack of documented diamond finds 

within the Roraima Group has led some authors to postulate that the source of the diamond placers is kimberlite 

deposits of west-central Africa (Reid, 1974; Bricefto, 

1984), most West African kimberlites are younger the 

Roraima. Bricefio (1984) stated that the conglomerates in 

the Uair6n Formation were themselves paleoplacers and 

were the source for diamond-bearing gravels deposited 

about 8,000 years ago at San Salvador de Paul. These 

Holocene paleoplacers probably are the source of diamonds in some deposits in currently active drainages.

Alluvial diamond deposits of the Guaniamo district 

are the most important diamond placers in Venezuela and 

have produced an estimated 12 million carats of diamonds 

during the last 20 years (Baxter-Brown and Baker, 1990). 

The area, discovered in 1968, accounts for approximately 

85 percent of Venezuela's total reported historical diamond production (Fairbairn, 1971; Anez, 1985). The main

area of production is along Quebrada Grande, a tributary 

of Rfo Guaniamo. Rfo Guaniamo flows into Rfo Cuchivero, which in turn enters the Rio Orinoco several kilometers east of Caicara. Quebrada Grande and its tributaries 

are diamondiferous for 40 km or more.

A generalized stratigraphic profile of the Guaniamo 

area consists of a basement of igneous rocks, typically diabase, as at La Salvaci6n, or a weathered granite that is 

essentially ferromagnesian or kimberlitic (Baptista and 

Svisero, 1978; Nixon, 1988; Baxter-Brown and Baker, 

1990). Basal material is typically thoroughly altered and 

forms an irregular depositional surface. A coarse diamondbearing angular to subangular gravel unit directly overlying 

the basal material forms a relatively thin (3 m thick or less) 

horizon consisting of quartz, microcline, chalcedony, chert, 

and lithic fragments of igneous rock in a clay-rich matrix. 

Pebbles 25-50 mm in size in a poorly sorted matrix indicate torrential-like high-energy deposition. The upper part 

of the section consists of fine-grained sedimentary rocks 

such as fine-grained, crossbed-stratified sandstone, mudstone, and organic-rich claystone. Minerals such as pyrope 

garnet, magnesian ilmenite, chrome diopside, olivine, and 

sphene in the coarse gravels also indicate a kimberlitic origin.

Diamond placer deposits associated with rocks of the 

Roraima Group form the second major group of occurrences in the Venezuelan Guayana Shield. These deposits 

are in the basins of Rfos Caronf, Paragua, and Icabaru and 

their tributaries below the high cliffs of the Roraima 

Group in the southeastern part of the country. The most 

prominent locale of this group is the San Salvador de Paul 

area. Although records are poor, overall production in the 

San Salvador area exceeds 2 million carats; production is 

estimated at 2,000 carats per month (Bricefto, 1984). Distribution of stone quality is approximately 49.17 percent 

gem, 36.2 percent industrial, and 14.63 percent bort 

(Maziarek, 1975). The high percentage of gem-grade diamonds and the absence of other minerals of kimberlitic 

association that are less resistant to transportation suggest 

either a distal kimberlitic source or more than one cycle of 

sedimentation or both (Bricefio, 1984). The pattern of sedimentation observed in diamond-bearing unconsolidated 

sediments is fairly systematic (Bricefio, 1984); basal diamond-bearing gravels rest on either iron-rich laterite or 

deeply weathered pre-Roraima rocks. These gravels are 

poorly sorted and clay rich and consist of extremely angular fragments of milky vein quartz and clear quartz crystals showing little wear mixed with coarse to very coarse 

sand that is well rounded. Iron concretions and silicified 

sandstone fragments may be present in the matrix. At least 

one area, Urim£n, contains jasper-rich detrital material that 

fines upward into medium- and fine-grained sands commonly characterized by an organic-rich interval. The 

organic-rich horizon is overlain by either fluvial or windblown sediments extending to the surface.
88 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

The general mechanism of diamond deposition in 

unconsolidated sediment is interpreted as follows. (1) Diamonds were released by weathering, then (2) trapped and 

concentrated in the thick tropical soil profile. (3) With the 

initiation of drying climatic conditions about 9,000-8,000 

years B.P. (representing savannalike conditions), diamonds 

and associated detritus were transported and deposited by 

high-energy streams (Briceno, 1984). (4) Continued dry 

conditions decreased the transporting capacity of streams, 

generating the finer grained upper profile of the sedimentary section. The upper section has been dated by carbon14 methods at 6,470±340 years B.P. (5) Present-day 

conditions are that of a very humid tropical forest 

environment.

PERMISSIVE DOMAIN

The domain permissive for diamond placers is coincident with that outlined for diamond-bearing kimberlite 

pipes and also includes lowlands (about 400 m elevation) 

draining areas of the lower part of the Roraima Group, 

pre-Roraima rocks, and outliers of these formations. 

Extensions of this domain include the total length of 

basin-alluvial deposits of all major rivers and streams 

draining the above mentioned sequences. The large area 

east of Rio Caura and south of the Rio Supamo-Rio 

Yuruani basins and including major drainages such as the 

Caronf and Paragua that drain the Roraima and associated 

formations is considered to have a lower probability for 

undiscovered high-grade deposits because of the absence 

of kimberlitic source rocks (known or suspected) and the 

lack of known deposits. It must be noted, however, that 

the seasonal high-energy load capacity of rivers such as 

the Caronf carries diamond-enriched gravel-size material 

long distances from Roraima sources. The well-rounded 

diamond-bearing pebble conglomerate and modern-day 

river sediments southwest of Puerto Ordaz represent distal 

deposits.

Tin-(Rare Earth Element, 

Niobium-Tantalum) Placers

By William E. Brooks and Floyd Gray

THE MODEL

Placer tin deposits (secondary deposits) form the 

major source of tin for the world's markets. These deposits 

are loosely classified as eluvial (that is, residual, saprolitic), 

colluvial, stream-beach sand, eolian, and so forth (Jenkins, 

1970; Daily, 1973; Taylor, 1979). Precambrian tin provinces are commonly characterized by regions of tin-bearing

pegmatite and closely associated quartz-cassiterite veins. 

Alluvial deposits are usually linked with the unroofing of 

lode-tin-bearing formations and are present in both modern 

and fossil streambeds. Cassiterite and associated heavy 

minerals may be concentrated in streams as silt- to cobblesize nuggets near the source. Ilmenite, zircon, and monazite, as well as columbite-tantalite, may be present if the 

gravels are derived from cassiterite-bearing pegmatite. The 

deposits are commonly within a few kilometers (<8 km) of 

the source, and the minerals may concentrate in riffles and 

other structures transverse to stream flow. Anomalous 

amounts of Sn, As, B, F, W, Be, Cu, Pb, and Zn may be 

present. Deposits may be any age but are usually Tertiary 

or younger (Reed, 1986b).

Placer and residual deposits are significant sources 

for niobium, tantalum, and rare earth elements. These 

deposits are commonly associated with tin- and titaniumbearing placers and share model characteristics with them; 

however, source rocks such as carbonatite, the primary 

host rock for niobium-rare earth element deposits, are 

commonly tin and titanium poor. Deposits in general are 

small but may be sufficiently numerous in a local area to 

constitute an economically important district (Parker and 

Adams, 1973). Local radioactive anomalies caused by 

thorite and monazite commonly characterize these heavymineral accumulations.

EXAMPLES IN VENEZUELA

In the Cano Aguamena-Cerro Boquerones area, 45 

km north of Puerto Ayacucho and east of the town of 

Puerto Paez, cassiterite, tantalite-columbite, and rutile 

have been reported in stream sediments (Rodrfguez and 

P6rez, 1982; Rodriguez, 1986). Heavy-mineral concentrations of from 13 g/m3 to 11 kg/m3 are a few kilometers 

downslope from source pegmatites and associated quartz 

veins in the Parguaza Granite (P6rez and others, 1985).

Areas in which heavy-mineral concentrations are less 

quantifiable, such as the upper Rio Paragua and the region 

of the upper Rio Orinoco near Sierra Parima, suggest the 

occurrence of low-grade (subeconomic) alluvial tin. These 

areas were identified during regional exploration.

There are no known placer niobium-tantalum or rare 

earth element deposits in Venezuela; however, neither 

regional geochemical data nor information from prospects 

and occurrences were available from frontier areas of 

Venezuela at the time of this study.

PERMISSIVE DOMAIN

The domain permissive for alluvial placer tinniobium-tantalum deposits includes drainages underlain by
MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD 89

the Parguaza Granite, drainages of other areas considered 

permissive for tin greisen deposits, and drainages within a 

minimum 8-km-wide band outward from the contact 

between the Parguaza Granite and older rocks. A secondary boundary for tin-niobium-tantalum deposits includes 

areas underlain by other young anorogenic granites including rapakivi-textured calc-alkaline granite (unit Xg, pi. 2), 

late granite (unit Ylg), and small alkaline complexes.

Permissive areas of niobium-tantalum, rare earth element deposits exclusive of tin-bearing areas include areas 

immediately adjacent to suspected carbonatite deposits.

Placer Titanium and Other 

Heavy Minerals

By Jeffrey C. Wynn

THE MODEL

Ilmenite, minor rutile and zircon, and other heavy 

minerals are generally weathered from rocks containing 

primary titanium-rich deposits such as alkalic igneous 

rocks, gabbro, anorthosite, syenitic veins and pegmatites, 

granite, and high-grade metamorphic rocks and are concentrated by fluvial processes in river and stream channels 

and along their margins (see Force, 1986, for a related 

ocean-shoreline model). These heavy-mineral sands are in 

fluvial areas draining continental cratons, along presentday beaches and in deltas and along streams where they 

settle from the slower moving water. Some placer deposits 

are derived from older detrital deposits. These are 

medium- to fine-grained, well-sorted black sands in modern drainages or in river beach deposits; ilmenite is the 

primary titanium-bearing mineral. The black sands, chiefly 

ilmenite and (or) magnetite, are normally interlayered with 

low-feldspar to nonfeldspathic quartz sand. Garnet, zircon, 

rutile, sillimanite, cassiterite, or monazite may also be sufficiently abundant to be economically important. On river 

shorelines the heavy minerals are concentrated in dark layers on beaches and in high-water deposits, and in rivers 

the heavy minerals tend to collect in potholes and deep 

channels that are parallel with the current flow. Other producing titanium mineral deposits may form as a result of 

the winnowing of titanium-bearing saprolite by modern 

streams (Herz and others, 1970)

EXAMPLES IN VENEZUELA

Rodriguez and P6rez (1982) and Pe"rez and others 

(1985) reported titanium (in the form of rutile)-, tin-, niobium-, and tantalum-bearing alluvium in the area of Cerro

Boquerones near the confluence of the Rios Orinoco and 

Meta. These deposits directly overlie the 1,545-Ma 

Parguaza Granite. Samples taken from laterite-derived 

sediments ranging in size from clay to coarse-grained 

sand have concentrations of heavy minerals ranging from 

13 g/m3 to 11 kg/m3 . The heavy fraction of these samples 

contains 0-0.77 percent Sn, 0.01-0.23 percent Nb, 1.8- 

29.1 percent Ti, and 0.5-11.0 percent Zr (Pe"rez and others, 1985). These alluvial deposits apparently were 

derived from the weathering of pegmatite and mineralized 

quartz veins in the Parguaza Granite to the east and 

southeast. The titanium-bearing ore minerals are staniferous rutile and columbiferous rutile (Rodriguez and P6rez, 

1982).

Candiales and Anez (1972) reported heavy-mineral 

concentrates in sediments in the Rfo Chicanan containing 

an average of 71 kg/m3 ilmenite; some concentrates contain as much as 200 kg/m3 ilmenite. Ilmenite makes up 

about 86 percent of the heavy minerals; 5 percent 

chromite and 1.5 percent zircon are present. The percentage of ilmenite increases as the sediments become finer 

grained. Amounts of gold vary from 2 to 5 g/m3 in 

dredged river gravels (Candiales and Anez, 1972).

At least two feasibility studies have been conducted 

in the lower Rfo Caronf drainage (Amelinck, 1974; 

Charles McCallester, oral commun., 1989), as well as 

some limited exploitation of naturally concentrated black 

sands. The sands in this area contain as much as 55 percent ilmenite, as well as minor rutile, magnetite, staurolite, 

and zircon (Andrew Grosz, oral, commun., 1989). Locally, 

the sands contain gold and diamonds. These sands may 

also contain monazite and platinum-group elements. Gold 

miners in the Rfo Caronf use these sands to guide their 

dredging operations. The Rio Aro west of Ciudad Bolivar 

apparently has significant quantities of black sands in 

beach deposits (conversations with local miners, 1990).

PERMISSIVE DOMAIN

The Rios Aro, Caronf, Chicanan, Caura, and Paragua, 

as well as smaller drainages deriving from the Imataca 

Complex, are permissive for the occurrence of black sands 

containing ilmenite and other heavy minerals. Rivers in 

Territorio Federal Amazonas that drain the San Carlos 

metamorphic plutonic terrane (unit Xmp, pi. 2) and the 

igneous-metamorphic basement complex (unit Xbc) may 

also be permissive areas. Rocks of the Supamo Complex, 

the Cuchivero Group, and the Roraima Group can probably be excluded because of their metamorphic grade. The 

Rfo Cuyunf drainage, which contains ilmenite from an 

unknown source, may also be permissive.
90 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Creeks and rivers draining the Parguaza batholith or UNDISCOVERED DEPOSITS 

cutting into alluvial fans produced over the batholith must 

be considered as permissive areas for quartz-vein- and

pegmatite-derived titanium (in the form of rutile), tin, Data are insufficient to allow quantitative estimates 

tantalum, and niobium (Rodrfguez and P6rez, 1982; P6rez of number of undiscovered deposits of placer titanium and 

and others, 1985). heavy minerals (other than gold) in Venezuela.
CONCLUSION

The major objective of this mineral resource assessment was to identify the various permissive domains controlling the distribution of mineral deposits and to define 

and classify the deposit types present or potentially 

present in the Precambrian Venezuela Guayana Shield. 

Thirteen deposit types are present, and nine deposit types 

may be present. Of these twenty-two deposit types, estimates of undiscovered deposits were made for five (table 

11).

We were unable to make quantitative estimates for 

the other deposit types because of a lack of reconnaissance

geochemical maps showing the distribution of Cu, Pb, Zn, 

Ag, Mo, Be, Nb, Ba, Li, rare earth elements, and other 

elements and a lack of regional geophysical data in the 

northwestern part of the shield. A lack of regional topographic maps hampered our analysis of residual deposits.

As more geologic information is developed and interpreted in view of modern mineral deposit models and 

well-documented grade-tonnage characteristics, the geologic community can build on this comprehensive study 

and further evaluate the quality of mineral resources in the 

Venezuelan Guayana Shield.

Table 11. Estimates of undiscovered deposit types, Venezuelan Guayana Shield.

Deposit type

Algoma iron ......................................................

Low-sulfide gold-quartz veins ..........................

Carbonatite.. ......................................................

Kuroko-type massive sulfide ............................

Synorogenic-synvolcanic nickel-copper ...........

90 percent

...... 9 or more

...... 20 or more

...... 2 or more

50 percent

26 or more

5 or more

Chance of occurrence

10 percent

90 or more

50 or more

12 or more

5 percent 1 percent

1 or more

....................................... 1 or more

91
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98 GEOLOGY AND MINERAL RESOURCE ASSESSMENT OF THE VENEZUELAN GUAYANA SHIELD

Thorman, C.H., and Drew, L.J., 1988, A report on site visits to 

some of the largest tin deposits in Brazil, March 11-25,1988: 

U.S. Geological Survey Open-File Report 88-594,19 p.

Titley, S.R., 1982, The style and progress of mineralization and 

alteration in porphyry copper systems, in Titley, S.R., ed., 

Advances in the geology of porphyry copper deposits: Tucson, 

University of Arizona Press, p. 93-116.

Vacquier, V., Steenland, N.C., Henderson, F., and Zietz, I., 1951, 

Interpretation of aeromagnetic maps: Geological Society of 

America Memoir 47, 151 p.

Vogel, W., 1987, The mineralized quartz-pebble conglomerates of 

Ghana, in Uranium deposits in Proterozoic quartz-pebble 

conglomerates: International Atomic Energy Agency 

TECDOC^27, p 235-254.

Woolley, A.R., 1987, Alkaline rocks and carbonatites of the world; 

part 1, North and South America: Austin, University of Texas 

Press, 216 p.

Wyant, D.G., Sharp, W.N., and Ponte R.C., 1953, Radioactive 

source materials in Los Estados Unidos de Venezuela: U.S. 

Geological Survey Trace Elements Investigations Report 222, 

116 p.

Wynn, J.C., McCafferty, A.E., and Salazar, E., in press, Analysis of 

aeromagnetic data used to improve geologic mapping in the 

Bochinche mining district, Estado Bolivar, Venezuela, in Sidder, G.B., Page, N.J, Wynn, J.C., and Garcia, G.A., eds., Geology and mineral resources of the Guayana Shield, Venezuela: 

U.S. Geological Survey Bulletin.

Wynn, J.C., Page, NJ, Contreras, G., Quesada, R.J., Moring, B.C., 

and Oscarson, R.L., in press, Geology, geochemistry, and geophysics of the Piston de Uroy area, Venezuela, in Sidder, G.B., 

Page, N.J, Wynn, J.C., and Garcfa, G.A., eds., Geology and 

mineral resources of the Guayana Shield, Venezuela: U.S. 

Geological Survey Bulletin.

Wynn, J.C., and Sidder, G.B., 1991, Mineral resource potential of 

the NB-20-4 quadrangle, Eastern Guayana Shield, Bolfvar 

State, Venezuela: U.S. Geological Survey Bulletin 1960,16 p.

Yanez, Galo, 1985, Geologia y geomorfologia del Grupo Roraima 

en el sureste de Venezuela: Congreso Geol6gico Venezolano, 

6th, Caracas, Memoria v. 2, p. 1243-1306.

 1991, Aspectos geol6gico-geomorfo!6gicos de la frontera 

con Brasil, en el Territorio Federal Amazonas (Venezuela): 

Las Jornadas de Geologfa de Fronteras, Sociedad Venezolana 

de Geologfa, San Crist6bal, Venezuela, September 10-13, 

1991, 18 p.

Yeend, W.E., 1986, Descriptive model of place Au-PGE, in Cox, 

D.P., and Singer, D.A., eds., Mineral deposit models: U.S. 

Geological Survey Bulletin 1693, p. 261.

Young, R.G., 1984, Uranium deposits of the world, excluding 

Europe, in De Vivo, B., Ippolito, F., Capaldi, G., and Simpson, 

P.R., eds., Uranium geochemistry, mineralogy, geology, 

exploration, and resources: London, Institution of Mining and 

Metallurgy, p. 117-139.

Zuloaga, Guillermo, 1933, The geology of the iron deposits of the 

Sierra de Imataca, Venezuela: American Institute of Mining 

and Metallurgical Engineers Technical Publication 516, 36 p.

Published in the Central Region, Denver, Colorado

Manuscript approved for publication May 18,1993

Edited by Judith Stoeser

Maps and illustrations prepared by Wayne Hawkins

Cover prepared by Art Isom

Photocomposition by Marie Melone

Tables typeset by Judith Stoeser

. GOVERNMENT PRINTING OFFICE' 1993-777-496
APPENDIX COMMODITY DEPOSITS INDEX 

FOR THE PRECAMBRIAN SHIELD OF NORTHEASTERN SOUTH AMERICA INCLUDING 

VENEZUELA, GUYANA, SURINAME, AND 

NORTHERN BRAZIL

A mineral occurrence map for northeastern South America (pi. 6) was prepared from a Mineral Resources Data 

System (MRDS) file. A listing of the unedited MRDS data is given in this appendix. Inquiries about information 

stored in MRDS may be obtained from the regional MRDS representative at the U.S. Geological Survey, Minerals 

Information Office, Corbett Building, 340 North 6th Avenue, Tucson, Arizona 85705-8325, or at the U.S. 

Geological Survey, 12201 Sunrise Valley Drive, Reston, Virginia 22092.
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 TC30209

JTC30208

jTC 30207

JTC30206

JTC30205

' TC30204

JTC30203

JTC30202

jTC 30201

ITC 30200

JTC30199

JTC30198

JTC30197

:TC 30196

JTC30195

JTC30194

JTC30193

JTC30192

JTC30191

JTC30190

JTC30189

JTC30188

JTC30187

[TC30182

JTC30181

JTC30180

JTC30177

JTC30176

[TC30175

JTC30174

JTC30160

JTCOOS82

JTC00527

JW700643

JTC30161

JW700476

JTC30162

JW700054

JW029766

JW026222

 W026202

JW026201

JW0023 1 1

JW002292

JW00213S

JW001413

j WO0 13 90

: WOO 1384

 TC30163

JTC30164

JTC30165

JTC30166

JTC30167

:TC30168

JTC30169

JISM0547

JISM0152

IISM0151

iSite for Sort

iMORROPOTOSI

iSANCARLOS

IABUNA. SAO SEBASTIAN

i ALTO RIO PRETO

i ORIENTE NOVO. PRIMAV

iORIENTEVELHO

i IGARAPE ALVES

iCARITIANAS

: RIO MADEIRA

I BURITI. BOM FUTURO.

j RIO JACI- PARANA

| SAO LOURENCO, MACISA

i QUEIMADA, CACHOEIRIN

:RK> PRETO

FUODASGARCAS

i RIO JIPARANA

i SAO FRANCISCO. TWIR

i RIO con

i SANTA BARBARA

IJACUNOA

i IGARAPE PRETO-TIRIRI

:F»OGUARIBA

j RIO MARMELOS-MAFU

| SERRA DO CACHIMBO

i GARIMPO CENTRAL DO J

JMUHRU

i GARIMPO NOVO PLANETA

: GARIMPO PARANAITA

i GARIMPO NOVO ASTRO

inODOSONO

j CABECBRAS DO RIO NO

i NOVO ASTRO MINE

I SERRA PELADA

ITROMBETAS-SARACA

j NATAL

PITINGA

j BENEnCENTE, COTOVEL

iSERRODONOVtO

i SERRA DO NAVIO. BRAZ

i SERRA DOS CARAJAS AR

iURUCARA-RIOJATAPU

! MAZAGAO DEPOSIT

j FERRADURA-TRES ILHOT

i COLOMA LAURO SCORE

JRIOAMAPARITANTAUT

iAMAPA TIN PLACERS

j AMAZON VALLEY BAUXIT

j K5ARAPE-PRETO TIN DE

JRIO MAMCORE-JATUARA

: GARIMPO MARJI

JRIOTABACAO

JLAJEADO

i RIO TAOUARACU

i RIO AGUA SUJA

JOUATIPURU

I PEDRA PRETA

i AZUL (CARAJAS) DEPOS

iSERRO DO NAVIO MINE

 Country Coda

JBR

i&R

JBR

JBR

JBR

|BR

|BR

iBR

BR

JBR

|BR

JBR

JBR

IBR

JBR

JBR

 ;BR

JBR

:BR

i&R

i&R

JBR

JBR

|BR

JBR

i&R

:&R

|BR

;&R

|BR

JBR

:BR

|BR

JBR

|BR

 BR

JBR

|BR

jBR

|BR

;BR

|BR

|BR

:&R

|BR

i&R

|BR

i&R

|BR

i&R

:&R

|BR

i&R

|BR

i&R

i&R

|BR

i&R

 State Name

JRONOONIA

IRONDONIA

JRONOONIA

JRONDONIA

IRONDONIA

JRONOONIA

IRONDOMA

JRONOONIA

IRONDONIA

JRONOONIA

IRONDONIA

JRONDONIA

IRONDONIA

IRONDONIA

JRONOONIA

IRONDONIA

JMATOGROSSO

IAMAZONAS

JRONOONIA

JRONDONIA

IAMAZONAS

JAMAZONAS

IAMAZONAS

JPARA

JMATOGROSSO

JMATOGROSSO

JMATOGROSSO

JMATOGROSSO

JMATOGROSSO

IGCXAS

JPARA

JPARA

IPARA

JAMAZONAS

i AMAZONIA

JAMAZONAS

: PARA

JPARA

JAMAZONAS

JAMAZONAS

JGOIAS

IGCXAS

JGOIAS

JGCXAS

IPARA

JPARA

|s

i Latitude DMS

09-16-OOS

J09-48-OOS

J09-42-OOS

.09-42-005

109-34-OOS

J09-29-OOS

09-28-OOS

09-24-OOS

;09-09-OOS

J09-23-OOS

J09-22-OOS

 Q9-22-OOS

J09-19-OOS

09-18-OOS

J09-18-OOS

i09-17-OOS

J09-13-OOS

09-13-OOS

J09-12-OOS

J09-09-OOS

J08-37-OOS

: 08-01 -OOS

J08-01-OOS

J09-20-OOS

J09-05-OOS

J08-S3-OOS

J09-31-OOS

 09-30-OOS

J09-04-OOS

J09-07-OOS

J07-18-OOS

i 09-04- S

1 05-54- S

J01-30-OOS

J06-59-OOS

JOO-40-OOS

107-06-OOS

j 00-59- N

01-00- N

06-05-OOS

02-05-OOS

:00-40-OON

06-38-OOS

J01-50- S

01-00- N

j 00-45- N

02- - S

: 09-40- S

I07-20-OOS

J07-53-OOS

09-13-OOS

09-46-OOS

J01-19-OOS

J01-54-OOS

J08-12-OOS

i 07-11- S

'06-06-OOS

JOO-57-OOS

: Longitude DMS

 062-52-OOW

J063-02-OOW

i065-34-OOW

063-14-OOW

082-25-OOW

082-43-OOW

065-1 0-00 W

io63-04-OOW

064-28-OOW

i062-55-OOW

I064-23-OOW

065-00-OOW

J062-17-OOW

062-36-OOW

io64-08-OOW

J061-43-OOW

:061-25-OOW

J065-01-OOW

063-03-OOW

062-54-OOW

J061-13-OOW

|o60-27-OOW

i081-51-OOW

054-50-OOW

J058-34-OOW

J059-10-OOW

|057-07-OOW

056-43-OOW

J058-21-OOW

048-08-OOW

058-34-OOW

i 058-21- W

i 049-42- W

056-28-OOW

060-31 -00 W

J059-55-OOW

J060-44-OOW

j 052-03- W

i 052-00- W

J050-00-OOW

058-00-OOW

J052-00-OOW

052-02-OOW

i 054-43- W

j 052-00- W

|o51-33- W

J058- - W

ioei-15- w

J061-05-OOW

062-09-OOW

048-21-OOW

J048-21-OOW

048-18-OOW

1048-17-OOW

049-22-OOW

1 050-34- W

050-25-OOW

:052-03-OOW

 Commod Present

:SN

|SN

JSN

JSN

JSN

:SN

!SN

JSN

|AU

JSN

JSN

JSN

:SN

JAU

JSN

JDIA

:SN

JSN

JSN

JSN

JSN

JAU

JSN

JAU DIA

JAU

JPB ZN

JAU

|AU

:AU

i DIA

JAU

JAU

JAU PD

JAL1

|MN

JSN

JMN

JMN

JMN

|FE

 fE

jfE

JPB CU ZN

JPB BA

JTA NB

JSN TA NB AU DIA

JAL1

|SN

JSN

JSN AU

JDIA

IDIA

JDIA

JDIA

;NI

iw

JMN

:MN

[Dep Type first

 PLACER. VEINS. DISSE

i PLACER

i PLACER

: PLACER

j PLACER, VEINS

j PLACER. STOCK WORK

i PLACER. VEINS, GREIS

i PLACER. STOCKWORK. G

j PLACER

j PLACER

j PLACERS

i PLACER, VEINS. GREIS

i PLACER, STOCKWORK. G

i PLACER

| PLACER,

i PLACER

I PLACER, VEINS, GREIS

j PLACER GREISEN

: VEINS, PLACER. GREIS

i PLACER, VEINS, GREIS

j PLACER, VEINS, GREIS

I PLACER

j PLACER

I PLACER

i PLACER

iSTRATAFORM

: PLACER

j PLACER

| PLACER

i PLACER

i PLACER

ILCCE

: SEDWENTARY HOSTED G

i LATE RITE

JSTRATAFORM

1 ALLUVIAL AND ELLUVIA

JSTRATAFORM

i SECONDARY ENRICHMENT

i METAMORPHIC STRATABO

i SEDIMENTARY, STRATAF

j SEDIMENTARY, MINETTE

i SEDIMENTARY, LAKE SU

JVEMS

I VEINS AND LENSES

j PLACER

: PLACER

i LATE RITE

I PLACER

I PLACER

! PLACER

j PLACER

j PLACER

j PLACER

i PLACER

JLATERITE RESIDUAL

j VEIN/PLACER

i RESIDUAL ENRICHMENT,

 METAMORPHIC STRATABO

USGSModel first

SN GREISEN

SN GREISEN

SN GREISEN

SN GREISEN

SN GREISEN

SN GREISEN

PLACER AU-PGE

SN GREISEN

SN GREISEN

PLACER AU-PGE

SN GREISEN

SN GREISEN

SN GREISEN

SN GREISEN

SN GREISEN

SN GREISEN

PLACER AU-PGE

PLACER AU-PGE

PLACER AU-PGE

PLACER AU-PGE

PLACER AU-PGE

PLACER AU-PGE

LATERITE-TYPE BAUXITE

LATERITIC NICKEL

i Host Rock Type

i GRAVELS

I GRAVEL

: GRAVEL

| GRAVEL

i GRAVEL

| GRAVEL

| GRAVEL

i GRANITE

i GRAVEL

j GRANITE

| GRANITE DERIVED FROM

i GRANITE

i GRANITE

i GRAVEL

i GRANITE

i GRAVEL

j GRANITE

i GRANITE

i GRANITE

: GRANITE

i GRANITE

i GRAVEL

: GRAVEL

i SANDSTONE

i GRAVEL

! CALCAREOUS SH.TSTONE

i GRAVEL

i GRAVEL

j GRAVEL

| GRAVEL

i GRAVEL

j SCHISTOSE TONAUTE

i GRAPHITIC, MANGANIFE

: SHALES AND SILTSTONE

| SANDSTONE

i GRANITE

i SANDSTONE

GRAPHITIC SCHIST

i MICASCHISTS GARNETIF

i CARAJAS FM., ITABIRI

i ACID AND INTERMEDIAT

j LIMESTONE, (280-345

| PEGMATITE

j METASEDIMENTS

i LATERITIC CLAY AND S

j GRANITE, GRAVELS DER

i GRAVEL

| GRAVEL

i GRAVEL

| GRAVEL

i GRAVEL

i GRAVEL

ILATERITE ON SERPENTI

| METASEDIMENTARY, MET

| SILTSTONE

i MICASCHISTS, GARNETI

i Host Rock Age

\C&t

iCEN

|CEN

JCEN

ICEN

JCEN

;CEN

iLPROT

JPLEISHOLO

ILPROT

ILPROT

ILPROT

JLPROT

JCEN

ILPROT

JCEN

ILPROT

iLPROT

JUPROT

ILPROT

JLPROT

ICEN

JCEN

JPAL

\c&>

iMPROT

\C&t

JCEN

ICEN

JOUAT-HOLO

JCEN

JEPHOT

ITERT

iMPROT

JPREC

IMPROT

; PflEC

; PflEC

JARCH

JPROT

JCARB

; PflEC

i PflEC

JPUO

 PflEC

lc&>

ICEN

|CEN

icei

 CEN

iCEN

iMPROT

EPHOT

; PREC
! Record Number

ITC30170

JTC30171

JTC30172

JD000723

JTC30173

JW002324

:M046527

JTC35034

JTC34901

JTC34902

ITC34903

JTC34904

JTC34905

JTC34906

JTC34651

JTC34907

JTC34652

JTC34908

JTC34909

JTC34653

JTC346S4

JTC34910

JTC346SS

JTC3491 1

JTC34656

JTC34912

JTC34657

JTC34913

JTC34658

JTC34914

JTC346S9

' TC34915

JTC34660

JTC34916

JTC34661

JTC34917

 TC34918

ITC34662

JTC34663

JTC34919

JTC34664

JTC34920

JTC34665

ITC34921

JTC34666

JTC34922

JTC34667

ITC34923

JTC34668

JTC34924

JTC34925

JTC34869

JTC34926

JTC34670

JTC34990

JTC34927

JTC34671

' TC34991

iSite (or Sort

ISERRADOIMAJA

IFAZENDACUMARU

I UNNAMED CHROMITE PRO

i (SUMMARY OF BRAZILI A

FAZENDACOOUERK5

i FRENCH GUIANA BAUXIT

JAICOUPAI

! TIGER HLL

i LOCUST LANDING OUARR

JSEBA QUARRY

i HOPE QUARRY - BLUE M

i LA ROSE CREEK

ioUARTZSTONECREEK

; UPPER WAI AMU

ioMBOPARU CREEK

jSODAM MOUNTAIN

UUDGATEHSJ.

i MIDDLE AREMU TRIBUTA

| NORTHWEST AREMU RTVE

IENACHU RIVER

IEPINGRNER

I FLOOD CREEK

| UPPER EPING RIVER

JWAHRI CREEK

JPERENONGAREA

|ARIAN ISLAND- FLAT

i M.C. CORREIA'S CLAIM

i ST. EDWARD MISSION 0

I WEST OF KUMERAU FALL

IS ANT HOEKPT QUARRY

i RUPUNUNI DIAMOND DIS

| MOCAMOCA QUARRY

| POTARO DISTRCT DIAM

|ARIMAI QUARRY

IIABARATUIK CREEK

j BARACARA QUARRY

i FLAT ROCK AND PALMER

IAKAIWONGRWER

| IANNA CREEK

: BARTICA - POTARO ROA

I UNNAMED PLACER ON BA

i BARTICA - POTARO ROA

MOSCOWCREEK

i KARA ISLAND QUARRY

: UNDO CHEEK

JKURTABU POINT

JROON CREEK

JKALACOON QUARRY

ITABURU CREEK

| SAKARARA BAY QUARRY

i SAXACALU QUARRY

i MARUSSA RIVER

i ROCK POINT QUARRY

JKAMWATTA RIVER

| OLD ENGLAND DEPOSIT

I AGATASH QUARRY

JVENUS CREEK

IDACOURA

 Country Code

|BR

IBR

IBR

|BR

|BR

ire

ire

|GY

IGY

IGY

!GY

|GY

:GY

JGY

JGY

JGY

JGY

JGY

iGY

iGY

JGY

JGY

IGY

lot

JGY

!GY

JGY

JGY

JGY

iGY

|GY

IGY

jGY

IGY

JGY

iGY

iGY

:GY

JGY

,;GY

iGY

iGY

iGY

iGY

JGY

IGY

IGY

|GY

iGY

IGY

|GY

|GY

|GY

|GY

JOY

|GY

IGY

IGY

State Nome i Latitude DMS

PARA jOB-40-OOS

GOIAS i07-48-OOS

PARA :08-58-OOS

10- S

GOIAS |OB-50-OOS

:04- - N

i04-05-13N

03-28-41N

i05-19-56N

|05-40-19N

I06-05-25N

|06-S9-OON

:06-50-OON

J06-45-14N

|06-14-57N

106-46-OBN

06-14-19N

|06-41-14N

i06-43-49N

I06-13-37N

J06-06-27N

06-33-3 1N

i06-04-56N

J06-33-10N

IOG-OB-OON

|o6-23-05N

|06-09-17N

 06-22-38N

I06-08-02N

 06-20-44N

|04-2B-OON

J06-21-46N

i 05- 19-2 IN

06-02-08N

:05-17-16N

06-23-0 1N

:06-23-09N

06-37-54N

07-20-58N

|06-14-OBN

|07-21-59N

|06-23-32N

i04-45-16N

J06-19-25N

|04-4B-57N

J06-23-1 ON

J04-51-05N

06-23-15N

04-51-25N

06-21-51N

J06-34-23N

:04-53-42N

06-32-52N

|04-55-25N

05-53-32N

:06-21-10N

04-46-51N

:05-54-39N

Longitude DMS

: 050- 36-00 W

050-51 -OOW

049-45-OOW

|055- W

 049-2B-OOW

|053- - W

|052-27-47W

|oS9-34-3BW

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058-18-15W

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059-12-38W

059-15-05W

!060-05-59W

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J080-07-06W

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:059-13-50W

060-02-15W

:060-06-35W

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I060-08-03W

I058-56-21W

|060-07-03W

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060-21-40W

058-41 -49W

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:058-41-56W

I059-50-OOW

058-41-02W

059-05-54W

058-40-49W

i 059-23- 16W

058-40-12W

058-40-02W

:059-33-02W

i 059-48-23 W

J05B-3B-44W

060-24-19W

058-37-36W

058-14-27W

|o5B-42-52W

J058-14-40W

:058-41-39W

058-1 5-20 W

058-39-06W

i05B-14-34W

: 058-36-59 W

1058-36-49W

058-15-41W

i05B-37-1BW

058-1 5-08 W

i 058-1 8-04 W

058-36-44W

:058-14-08W

i058-19-S7W

iCommod Present

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JAU

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ISTNI

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PLACER AU-PGE

PLACER AU-PGE

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i Record Number

 TC32108

JTC32215

JTC32107

JTC32106

JTC32214

JTC32249

JTC32105

JTC32213

JTC32248

JTC32212

JTC32104

JTC32211

JTC32247

JTC32139

JTC32103

JTC32246

JTC32210

JTC32138

JTC32102

JTC32137

JTC32245

JTC32101

JTC32244

JTC32136

iTC32100

JTC32279

JTC32135

JTC32243

:T C 3227 8

JTC32242

JTC32134

JTC32277

JTC32241

JTC32169

JTC32133

JTC00809

JTC00808

JTC00876

JTC00875

!TC00874

JTC00873

JI000220

JI000217

JI000221

JI000219

JI000216

JW002322

JW700090

JW700089

JW026887

JW026886

JW02688S

JW026884

 W026883

JW026882

i WO0 1964

JW001961

iSite for Sort

iTUPUHU CHEEK

IBOBB-S WORKINGS

i MARIWA CREEK AND SAR

iWARIRIMINE

: BOURNE, TURPIN. AND

i WILLIAMS HILL

| GROETE CREEK QOLDCO

JN. BANK TERRACES BEL

JMARABUNTA CREEK

I MOUTH OF SUKABICREE

j URLUOWRA AND AMUNGA

JTORRINGTONCAMP

i SAINT ELJABETH CREEK

i TWEETS WORKINGS

i MCNAUGHTEN-S DIAMOND

JNCXTGEDACHT CREEK

iCUMBERBATCWSCAMP

i HENERSOWS WORKINGS

i ARAWAPAI GOLDFIELD

JMUKRU CHEEK

! LOWER KONAWAK FIVER

:IMOTAIQOLDF LD

| MIDDLE KONAWAK RIVER

JKOAMA RIVER

JMINABAHUGOLDFIBJD

: TUMATUMARI

iTRY CHEEK

i KONAWAK RIVER (UPPER

i NORTHFOHK RIVER

! SPECIAL CHEEK

JKAMARU CREEK

iwiLLIPSfWILUS'JL

i QUINTETTE CHEEK

i TIGER CHEEK (SOUTH)

: KURASHI DIAMOND AREA

iMAHDIA

;OMAI PROPERTY

j EAST MONTGOMERY MINE

jPETERSMWE

i FIVE STAR

I AROAIMA MINING COM PA

i TEPURU CREEK QUARRY

i LINDEN KAOLIN

j ST. MARTS QUARRY

ilTABU

jTOPIRA

i GUYANA BAUXITE DEPOS

i MACKENZIE DISTRICT

i KWAKWANI DEPOSIT

i BLUE MOUNTAINS

| TIGER HILL

i WAMARA MOUNTAINS

ipUTARENG

 POMEHOONHEAD

i IRON MOUNTAIN

: BAUXITE -GUYANA

i BAUXITE -GUYANA

: Country Code

:GY

 GY

JGY

:GY

JGY

iGY

IGY

JGY

 GY

JGY

:GY

IGY

|GY

JGY

IGY

iGY

JGY

JGY

JGY

JGY

JGY

JGY

iGY

JGY

JGY

JGY

:GY

JGY

;GY

IGY

JOY

|GY

 GY

JGY

!GY

 GY

|GY

;GY

iGY

JGY

iGY

IGY

|GY

 GY

iGY

:GY

JGY

:GY

JGY

JGY

JGY

JGY

 GY

|GY

 GY

iGY

:GY

State Name | Latitude DMS

I06-37-03N

JOS-23-22N

I06-36-22N

i06-33-18N

JOS-24-20N

|05-16-43N

i06-38-36N

J05-27-28N

|05-18-14N

JOS-24-02N

06-41-OON

iOS-23-28N

:05-18-47N

i06-20-05N

06-51-39N

05-21-24N

JOS-22-28N

:06-20-t2N

J07-06-59N

:06-50-49N

J05-19-47N

J07-14-51N

05-18-02N

J06-47-32N

J07-13-28N

J05-22-04N

:06-51-25N

J05-17-08N

J05-08-48N

05-16-54N

!06-51-43N

J05-14-31N

05-16-54N

06-06-21N

J06-55-37N

05-20-49N

: 05-28- N

06-10- N

J06-15-12N

J07-36-05N

;05-15- N

MA2JJRUNI-POTARO j 06-25- N

EASTDEMERARA 05-10- N

MAZARUNI-POTARO 106-15- N

MAZARUNI-POTARO i 06-25- N

EASTDEMERARA i 05-25- N

|05- - N

i 06-00- N

;05-16-06N

i 06-33- N

! 05-40- N

i 05-09- N

:06-16-OON

J06-52-OON

 05-00-OON

I05-00-OON

:05- - N

i Longitude DMS

;058-53-20W

;059-08-40W

io58-5t-54W

;05fl-56-14W

J059-08-40W

i059-04-43W

I058-41-57W

059-09-43W

059-08-01W

:059-10-02W

060-32-58W

J059-11-22W

J059-07-42W

J060-12-52W

 060-32-05W

J059-07-44W

059-10-41W

J060-12-26W

J059-20-55W

060-22-26W

J059-07-09W

J059-25-08W

059-06-49W

J060-24-41W

059-29-24W

J059-00-05W

J060-17-06W

:059-06-52W

 059-08-38W

059-06-36W

:060-15-09W

i059-01-27W

 059-06-36W

;059-21-46W

J060-19-50W

;059-07-16W

i 058-45- W

i 058-1 5- W

:059-20-53W

J060-22-27W

j 058-00- W

1 058-17- W

i 058-00- W

: 058-1 8- W

i 058-1 7- W

i 058-10- W

J059- - W

j 058-10- W

058-01-09W

j 058-49- W

; 058-25- W

: 058-35- W

J060-10-OOW

I059-12-OOW

iosa-so-oow

J058-00-OOW

1 058- - W

) Commod Present

:AU

|AU

JAU

JAU

;AU

JAU

JAU CU

JAU

:'AU

JAU

JAU DIA

JAU

JAU

JAU

JDIA

JAU

JAU

:AU

JAU

JAU DIA

iAU

:AU

JAU

j AU DIA

JAU

JAU

JDIA

JAU DIA

JAU

JAU

JDIA

JAU

IAU

;AU

: DIA

IAU

:AU

JAL1

;AU

JAU

:AL1

JSTN1

i CLY3 AL1

ISTNI

JSTN1

JCLY3AL1

JAL1 AL2

:AL1

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JFE AL Tl

JFE AL Tl

JFE AL Tl

jfE

JFE Tl

;FE AL Tl

JAL1

:AL1 AL2

iDep Type first

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j PLACER

i PLACER

j VEIN AND PLACER

I PLACER

JVEIN

JVEIN

1 PLACER

j PLACER

I PLACER

j PLACER

j PLACER

i PLACER

i PLACER

 PLACER

j PLACER

i PLACER

j PLACER

: PLACER

i PLACER

j PLACER

I PLACER

j PLACER

i PLACER

j PLACER

i PLACER

i PLACER

i PLACER

: PLACER

i PLACER

j PLACER

j PLACER

i PLACER

i PLACER

j PLACER

I PLACER

JVEIN. SAPROLITE-LATE

JLATERITIC BAUXITE

JVBN.LODE

i LODE AND ELUVIAL

j LATERITIC BAUXITE

j GRANITE AGGREGATE

i RESIDUAL KAOLIN

i AGGREGATE

i GRANTE AGGREGATE

i RESIDUAL KAOLIN

I BRECCIA AND LATERITE

j LATERITIC BAUXITE

JLATERITIC BAUXITE

i RESIDUAL LATERITIC;

i RESIDUAL LATERITIC;

 RESIDUAL LATERITIC;

i MASSIVE- NOT CLASSI

i MASSIVE - NOT CLASSI

j RESIDUAL LATERITIC;

i LATERITE, BRECCIA

:USGSModel first

; PLACER AU-PGE

: PLACER AU-PGE

j PLACER AU-PGE

i LOW-SULFIDE AU-OUARTZ VEIN

 PLACER AU-PGE

i LOW-SULFIDE AU-OUARTZ VEIN

: LOW-SULFIDE AU-OUARTZ VEIN

i PLACER AU-PGE

j PLACER AU-PGE

 PLACER AU-PGE

: PLACER AU-PGE

i PLACER AU-PGE

i PLACER AU-PGE

i PLACER AU-PGE

i PLACER AU-PGE

: PLACER AU-PGE

i PLACER AU-PGE

: PLACER AU-PGE

i PLACER AU-PGE

: PLACER AU-PGE

 PLACER AU-PGE

j PLACER AU-PGE

: PLACER AU-PGE

i PLACER AU-PGE

! PLACER AU-PGE

i PLACER AU-PGE

j PLACER AU-PGE

i PLACER AU-PGE

i PLACER AU-PGE

i PLACER AU-PGE

i PLACER AU-PGE

: PLACER AU-PGE

i LOW-SULFIDE, AU-OUARTZ

:VEM

: LOW SULFIDE-AU QUARTZ VEIN

: LATERITE-TYPE BAUXITE

LATERITE-TYPE BAUXIT

i LATERITE-TYPE BAUXITE

Host Rock Type

ALLUVIAL GRAVELS

TERRACE SANDS

ALLUVIAL AND ELLUVIA

MEDIUM GRAINED AMPH

TERRACE SANDS

BASIC METAVOLCANICS

MICACEOUS SCHISTS CU

TERRACE SANDS

ALLUVIAL GRAVELS

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GRAVELS

TERRACE SANDS

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ALLUVIAL GRAVELS

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GRAVEL OF ANGULAR OU

ALLUVIAL GRAVELS

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ALLUVIAL GRAVELS

ALLUVIAL GRAVELS

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JTC50111

JTC50114

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JTCS0116

 TC50117

ITCSOIIS

JTC50119

JTCS0120

JTCS0121

.JTC50122

JTC50123

JTC50124

JTC50125

JTCS0126

JTC50127

JTC50128

JTCS0129

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JTC50131

JTC50132

JTC50133

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JTC50135

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ITC50138

JTC50139

ITC50140

JTC50141

ITC50142

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JTCS014S

JTCS0146

JTC50147

JTCS0148

JTCS0149

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JTC50151

 TC50152

JTCS01S3

JTC50154

JTC50155

|TC50156

JTC50157

JTC501S8

 TC50159

 Site lor Sort

: UMDENTFIED GOLDOC

: SOFIA

I SANTA ELENA

ILACUBANITA

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I UNION-1

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1 LA LAPA-1

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i UMDENTFIED GOLD OC

I UMDENTFIED GOLDOC

i UMDENTFIED GOLDOC

1 UMDENTFIED GOLDOC

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 Latitude DMS

1 06-43-1 5N

|o7-23-26N

|07-2S-09N

|07-23-49N

|07-23-S5N

|07-23-SON

|07-24-31N

J07-24-08N

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J07-24-37N

|07-24-06N

|07-24-40N

|07-24-43N

|07-24-54N

J07-25-03N

|o7-24-OSN

1 07-24- 1SN

I07-24-18N

I07-24-24N

|06-17-09N

|06-12-S9N

|06-16-23N

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1 07-34-0 IN

1 08-05-1 3N

|08-06-57N

i06-29-39N

|06-16-36N

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|06-49-5SN

|06-29-17N

|07-16-49N

|07-28-06N

|06-51-18N

|06-45-50N

|06-44-06N

|04-51-26N

|OS-29-20N

i03-43-27N

02-18-28N

02-12-01N

I03-22-26N

: 03-44-1 9N

i06-46-38N

1 06- 50-3 IN

: Longitude DMS

 061-54-38W

|061-49-53W

j 06 1 -47 -47 W

061-49-30W

:061-48-57W

I061-49-30W

061-46-24W

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061-51-56W

061-33-55W

J063-28-46W

1 066-08-30 W

i 062-57-26 W

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U.S DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

BULLETIN 2062 

PLATE 1

ATLANTIC OCEAN

EST\DO DELTA AMAC0RO / r i

200 400 BOO 000 KILOMETERS

0 100 200 300 400 MILES 

INDEX MAP

=) Sierra/ \ 

8( "5 Verdun I

t Las MargaritapjZone in dispute between 

Venezuela and Guyana; 

currently in mediation by 

United Nations

. 11 /'"^ ~~~ w

. Las Cristinas Q I! ,,-,

' * Vb Kilometre

ESTADO BOLIVAR

/ vs

jj /

"^

"I ,->

v ^P^JJairen/ \ F

~~* ^-V7

FEDERAL 

AMAZONAS

r--

I Cerro La

N '"-' Nehlina

0°

60°

Compiled using Republica de Venezuela Ministerio del Ambiente y de 

Los Recursos Naturales Renovables, 1979, "Atlas de Venezuela," 

Direction de Cartograf fa Nacional. 331 p.

EXPLANATION

River and major tributary 

=^ Road 

O Town

 - - International boundary

 - State boundary

50

SCALE 1:1 000 000 

50 100 150 KILOMETERS

25 50 75 MILES

SELECTED GEOGRAPHIC FEATURES OF THE 

VENEZUELAN GUAYANA SHIELD

By 

Paul G. Schruben, Floyd Gray, and Jeffrey C. Wynn

1993

68° 67° 66° 65° 64° 63°
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

68°

BULLETIN 2062 

PLATE 2

67'

ATLANTIC OCEAN

Caribbean Sea

-0

DELTA AMAC

100 200 300 400 MILES 

INDEX MAP

Prr°J 

rbani

/XWgr

VENEZUELA

Zone in disoute between Venezuela and Guyana; currently in mediation by 

United Nations

ESFADO BOLIVAR

anomaly

Thorium . Laterite-covered fenitiz

COLOMBIA

YXra 

Cerro (/uaiquinrma Positive magnetic .rare

^Negatively polarized intrusive body 

(10 km to top'of the body) Hidtterrtat intrusion ma'gn'ettc-data-' 

anomaly

Uranium 

Tepuyv

anoooco 

vl **

Xmo J

ApSrBuried intrusive . ' >.. "A complex f-'.. /anomalv^Uramum

;\^;j f { C^). < ^an° ly( 

Ridden 

f 

igneous

'

< m/'+fr- r

t/ rtn^struct^ft^ 

'"'withi urani'urii assoc. sarinarna

Cerrol 

jjvUranium anomal

nomalous 

vegetation

SOURCES OF GEOLOGIC DATA

Boundary of Venezuelan 

Guayana Shield

TERRITQRl

CIUDAD PUR

BOLIVAR

ESTADO 

FEDERAL 

AMAZON COLOMBIA

TEPRITORIO 

FEDERAL AMAZONAS

Names and locations of 2°x 3° 1:500,000-scale quadrangles are shown

CORRELATION OF MAP UNITS

4° - 3° - 2° - 1. 4. Stewart, J.H., and Martinez, Felix, U.S. Geological Survey and Corporation Venezolana de Guayana, Tecnica Minera, C.A., unpublished mapping. 3. Stewart, J.H., U.S. Geological Survey and Corporation Venezolana de Guayana, Tecnica Minera, C.A., unpublished mapping. 2. 5. Cox, D.P., Acosta, Juan, Stewart, J.H., Gray, Floyd, Arusp6n, Jesus, Brooks, W.E., Franco, Luis, Salazar, Edixon, and Lopez, Yolanda, U.S. Geological Survey and Corporation Venezolana de Guayana, Tecnica Minera, C.A., unpublished mapping. 6. Wynn, J.C., Olmore, Stephen, Mendoza, Vicente, Rendon, Ines, Estanga, Yasmin, Rinc6n, Haydee, Martinez, Felix, Garcia, Andres, Lugo, Elis, Rivero, Nelson, and Schruben, Paul, U.S. Geological Survey and Corporation Venezolana de 

Guayana, Tecnica Minera, 

C.A., unpublished mapping.

7. Brooks, W.E., Wynn, J.C., 

Garcia, Andres, Rinc6n, Haydee, Martinez, Miguel, 

Rivero, Nelson, Martinez, Felix, Estanga, Yasmin, Lugo, Elis, Rivero, Ivan, 

Rendon, Ines, Briceno, 

Henry, Mendoza, Vicente, Ubencio, Urbano, and Schruben, Paul, U.S. 

Geological Survey and Corporation Venezolana de Guayana, Tecnica Minera, C.A., unpublished mapping.

8. Gray, Floyd, Wynn, J.C., 

Olmore, Stephen, Lowry, 

Glenda, Dohrenwend, J.C.,

Page, N.J, Salazar, Edixon, 

and Martinez, Felix, U.S. Geological Survey and 

Corporation Venezolana de 

Guayana, Tecrica Minera, C.A., unpublished mapping.

9. Wynn, J.C., Olmore, Stephen, 

Martinez, Miguel, Rincon, 

Haydee, Garcia, Andres, 

Rivero, Nelson. Rendon, 

Ines, Marcano, Iris, Lugo, 

Elis, Mendoza, Vicente, and Schruben, Paul, U.S. Geological Survey and Corporation Venezolana de Guayana, Tecnica Minera, C.A., unpublished mapping.

10. Wynn, J.C., Olmore, Stephen, 

Martinez, Miguel, Garcia, 

Andres, Rincon, Haydee, 

Martinez, Felix, Tovar, 

Frank, Rendon, Ines, Lugo, Elis, Mendoza, Vicente, and Schruben, Paul, U.S. Geological Survey and Corporaci6n Venezolana de Guayana, Tecnica Minera, C.A., unpublished mapping.

11. Wynn, J.C., Olmore, Stephen, 

Rivero, Nelson, Rincon, 

Haydee, Marcano, Iris, Mendoza, Vicente, Rendon, Ines, Lugo, Elis, Gray, Floyd, and Schruben, Paul, U.S. 

Geological Survey and Corporation Venezolana de 

Guayana, Tecnica Minera, C.A., unpublished mapping.

12. Wynn, J.C., Olmore, Stephen, 

Garcia, Andres, Martinez, 

Miguel, Rendon, Ines, Mendoza, Vicente, and 

Schruben, Paul, U.S. 

Geological Survey and 

Corporation Venezolana de Guayana, Tecnica Minera, 

C.A., unpublished mapping.

QUATERNARY

r - SCALE 1:1 000 000 

50

Unconformity

M

Unconformity

1

Unnamed orogeny (about 1,860 to 1,730 Ma)

Xa I Xm1 Xf1 Xsl .

j- TERTIARY 

} MESOZOIC

Trans-Amazonian orogeny (about 2.150 to 1,960 Ma)

Pr*-Trans-Amazonian tectonic event 

(about 2,800 to 2,700 Ma)

MIDDLE AND (OR) 

EARLY PROTEROZOIC

> EARLY PROTEROZOIC

ARCHEAN

DESCRIPTION OF MAP UNITS

SEDIMENTARY, METASEDIMENTARY, 

AND VOLCANIC ROCKS

Alluvial deposits (Quaternary) Sand, gravel, and siltMesa Formation (Pleistocene and Pliocene) Siltstone, sandy siltstone, and clavstone. Unconsolidated gravel and sand in upper 

part

Roraima Group (Middle and Early Proterozoic)

Roraima Group, undivided

Auyantepuy Formation of Yanez (1985) Mainly quartzarenite 

and minor arkose. Forms steep cliffs and flat-topped mesas. Equivalent to Mataui Formation of Reid and Bisque (1975)

Guaiauinima Formation of Yanez (1985) Fine-grained quartzarenite and arkose that are cross stratified, laminated, and 

massive; siltstone and graywacke; red, green, and greenish-gray 

jasper composed of devitrified and (or) silicified ash and small 

crystals of quartz and feldspar. Weathers to form flat or gently 

sloping topography. Upper part is mostly covered with debris from overlying Auyantepuy Formation. Several hundreds of 

meters thick. Equivalent to all but the lowermost part of the 

Uaimapue Formation of Reid and Bisque (1975)

Canaima Formation of Yanez (1985)- Quartzarenite and arkose, 

conglomeratic arenite, conglomerate, siltstone, and shale. 

Abundant cross-strata in arenite and arkose units. Weathers to 

forms cliffs on resistant arenite, arkose, and conglomerate units 

and slopes on relatively nonresistant siltstone, shale, and silty 

arenite units. About 1,100-2,000 m thick (Yanez, 1985). Equivalent to Uairen and Coquenan Formations and lowermost part of Uaimapue Formation of Reid and Bisque (1975)

Ichun Formation of Briceno and others (1989) (Early Proter- 

^^ ozoic) Well-stratified ignimbrite, sandstone, and andesite flowsHxpr H Pre-Roraima Group sedimentary rocks (Early Proterozoic) Finegrained to very fine grained, clay-rich sandstone, locally 

containing granule-size quartz grains, interbedded with red shale and sandy shale (Briceno, 1982). Includes Urico Formation. Probably is unconformabe below Roraima Group

Los Caribes Formation (Early Proterozoic) Weakly metamorphosed to unmetamorphosed sequences of reddish arkose and 

polymictic conglomerate and minor felsic tuff

Caicara Formation of the Cuchivero Group (Early Proterozoic) 

Rhyodatitic to rhyolitic tuff, some of which is crystal and lithic 

rich; rhyodacite porphyry; andesite and dacite lava flows that 

commonly are hydrothermally altered; andesite to basalt dike; 

rhyodacite granophyre; local mylonite. Only slightly metamorphosed

Metasedimentary rocks of Ri'o Oris (Early Proterozoic) Hematitic 

quartzarenite; reddish phyllitic siltstone; feldspathic arenite; 

conglomeratic arenite; quartz-sericite phyllite; red, laminated, fine-grained meta-arenite showing graded bedding and erosional channels

Moriche Formation and correlative rocks (Early Proterozoic) 

Mainly metaconglomerate, quartzite, and mica schist. Includes Cinaruco and Esmeralda Formations

DEEP-SEATED METAMORPHIC AND PLUTONIC ROCKS

AND BASEMENT ROCKS IN TERRITORIO FEDERAL AMAZONAS

(EARLY PROTEROZOIC)

Greenstone-belt rocks

Metamorphic-plutonic terrane of Snn Carlos Granite, granite 

gneiss, and augen gneiss and relatively abundant pegmatite

I Basement complex Well-foliated granite to granodiorite gneiss

EUGEOSYNCLINAL METAMORPHIC ROCKS OF

NORTHEASTERN ESTADO BOLIVAR

(EARLY PROTEROZOIC)

Xg2 Met?qabbro

u2 ^ Ultramafic rocks Mainly metaperidotite, metapyroxenite, serpentinite, and talc schist. Cumulus texture is locally preserved

Mica schist and phyllite Quartz + muscovite ± chlorite ± chloritoid 

± ankerite schist and phyllite and subordinate quartzite or 

metachert derived from sedimentary and felsic volcanic rocks. 

Ankeritic rocks weather to ferruginous schist and phyllite

Felsic metatuff and flows Quartz + muscovite ± chloritoid schist 

containing relict phenocrysts of partially resorbed quartz and broken plagioclase replaced by albite. Groundmass is mainly devitrified glass. Traces of relict lapilli are locally abundant

Xm2 I Mafic to intermediate metalava and metatuff Chlorite + epidote ± 

actinolite schist and phyllite and greenstone, commonly containing relict pyroxene phenocrysts. Relict textures suggest amygdaloidal flows and lithic- and crystal-rich tuff

GREENSTONE-BELT ROCKS OF EL CALLAO AREA 

(EARLY PROTEROZOIC)

Caballape Formation Mainly felsic metatuff and phyllite derived 

from laminated volcaniclastic siltstone and graywacke. Metatuff contains plaqioclase phenocrysts, resorbed phenocrysts of quartz, 

wisps of pumice, and minor lithic clasts

Pastora Supergroup 

Yuruari Formation

Felsic metatuff Quartz ± muscovite ± calcite schist containing relict quartz and feldspar phenocrysts and traces of lapilli and 

breccia clasts. Minor greenstone

Schist and phyllite Finely laminated quartz + muscovite ± 

chloritoid ± biotite ± andalusite ± sillimanite schist locally 

containing volcaniclastic metasandstone and felsic metatuff and breccia 

Carichapo Group

Cicapra Formation Mafic to intermediate metatuff. Mainly albite ± epidote ± biotite amphibolite. Relict textures suggest interlayering of tuff and volcaniclastic sandstone

El Gallic Formation Greenstone, greenschist, and minor talcschist and amphibolite. Relict pillow structures are common. 

Flow rocks are commonly intercalated with flow breccia. Finegrained quartz-hematite rocks are present in uppermost part. 

Minor chert

ROCKS OF OTHER GREENSTONE BELTS 

(EARLY PROTEROZOIC)

Metagabbro Saussuritized and locally amphibolitized. Cumulus texture is locally presentUltramafic rocks Mainly metaperidotite, metapyroxenite, serpen- 

____ tinite, and talc schist. Relict cumulus texture is locally present|Xs1 J Mica schist and phyllite Quartz + muscovite ± chlorite ± ankerite 

schist and phyllite and subordinate quartzite or metachert derived 

from sedimentary and felsic volcanic rocks. Ankeritic rocks 

weather to ferruginous schist and phyllite

Felsic metatuff and flows Quartz + muscovite ± chlorite schist 

containing relict phenocrysts of partially resorbed quartz and broken feldspar

Mafic to intermediate metalava and metatuff Chlorite + albite + 

epidote ± actinolite schist, phyllite, and greenstone that are locally amygdaloidal. Subordinate albite-epidote amphibolite and minor amphibolite. Minor chert

Amphibolite Mainly highly deformed hornblende schist containing 

plagioclase. Locally shows outlines of original phenocrysts replaced by hornblende

Xf1

ROCKS OF IMATACA TERRANE

Imataca Complex (Archean) Amphibolite- to granulite-facies 

quartz-feldspar orthogneiss and paragneiss, commonly garnet bearing, and felsic granulite. Subordinate pyroxene granulite and charnockite. Dashes indicate metamorphosed iron formation and 

ferruginous quartzite. Protolith may be as old as 3,700-3,400 

Ma (Montgomery, 1979) and age of metamorphism is 2,150- 2,000 Ma (Onstott and others, 1989)

Migmatite and gneiss (Early Proterozoic and (or) Archean) 

Granitic migmatite and lineated gneiss

100 150 KILOMETERS

25 50 75 MILES

GEOLOGIC AND TECTONIC MAP OF THE VENEZUELAN GUAYANA SHIELD

Compiled by 

Jeffrey C. Wynn, Dennis P. Cox, Floyd Gray, and Paul G. Schruben

1993

^ P I

INTRUSIVE ROCKS

Diabase (Mesozoic to Early Proterozoic) Dark-gray to greenishgray, fine- to coarse-grained diabase of tholeiitic composition. 

Present as dikes, sills, and laccoliths

Late granite (Middle Proterozoic) Typically penetrating through, 

and doming, Roraima sediments

Alkaline complexes (Middle to Early Proterozoic) Riebeckite- and 

aegerine-bearing granite, gabbro

Carbonatitp. intrusion of Cerro Impacto (probably Middle to Early 

Proterozoic)

Yp | Parquaza Granite (Middle Proterozoic) Rapakivi-textured granite

Silicic to intermediate intrusive rocks (Early Proterozoic) Forms 

____ small domes intruding and folding Ichun Formation rocks

Xgu I Intrusive rocks, undivided (Early Proterozoic) Underlies waterflooded plains and jungle in southeastern Territorio Federal 

AmaTOnas

Calc-alkaline qranite (Early Proterozoic) Massive, coarse-grained, 

gray, equigranular biotite granite that has rapakivi texture similar 

to rocks of the Parguaza Granite

Granitic rocks of the Cuchivero Group (Early Proterozoic) 

Includes Santa Rosalia granite

Granitic rocks of post-Supamo age (Early Proterozoic) Mostly 

monzogranite and granite. Porphyritic, medium to coarse 

grained. Includes some granodiorite and tourmaline-rich rocks

Supamo Complex (Early Proterozoic) Sodic granodiorite and 

trondhjemite, paragneiss, and migmatite commonly in domeshaped intrusions. Rare pegmatite. About 2,230-2,050 Ma (Sidder and Mendoza, in press)

Granitic rocks of Imataca Complex (Early Proterozoic and (or) 

Archean) Granite and granodiorite

-Geologic contact Approximately located. Dashed where concealed

} Area underlain by intrusive magnetic rocks 

 Area of iron formation

* !S ** Major deep-penetrating shear zone inferred from geologic 

mapping and radar imagery

i i i i i Graben-bounding fault Dashed where inferred; ticks point toward 

graben

-^ .7 Fracture or fault determined from geologic mapping, magnetic 

data, or airborne radar imagery Direction of movement, 

where shown, is based on ground mapping. Dashed where 

inferred; dotted where implied

| Anticline I Syncline 

.............. Structural trend

Magnetic source Number, where shown, indicates depth (in kilometers) to top of source; s, shallow; d, deep

Positively polarizedNegatively polarized Deeo dense mass

^ '\ 2< Circular feature of unknown origin identified using geophysical or

side-lookmg radar imagery data In some cases may represent caldera

Axis of strong magnetic gradient May represent major hidden fault 

or suture between different geologic terranes

DISCUSSION

The geologic map of the Venezuelan Guayana Shield was produced digitally 

from 12 1:500,000-scale geologic maps that were compiled by geologists of the U.S. Geological Survey and Corporacion Venezolana de Guayana, Tecnica Minera, C.A., between 1990 and 1992 using geologic (Sidder and Martinez, 

1990; Sidder and Mendoza, 1991, in press) and geophysical (Graterol, 1988; 

Herrero and Navarro, 1989; this report, pis. 3, 4) data and interpretations and numerous published and unpublished larger scale maps. The information on the 

1:500,000-scale maps was digitized and reprojected to 1:1,000,000 scale using 

GSMAP (Seiner and Taylor, 1991). The projection used is Proyeccion Conico 

Secante Compensada, an equidistant conic projection using lat 4° and 9° N. as 

standard parallels and long 66° W. as the central meridian. The 1:1,000,000- scale map was then transformed into ARC-INFO files for further refinement and compilation and for preparation of the materials from which this map was created.

REFERENCES CITED

Briceno, H.O., Tapia, Jhonny, and Estanga, Jazmin, 1989, Formation n Ichun, 

volcanismo acido del Grupo Roraima: Congreso Geologico Venezolano, 

7th, Barquisimeto, Venezuela, 1989, Memoria, v. 1, p. 58-81.

Briceno, M., H.O., 1982, Application of remote sensing to diamond placer

exploration in a tropical jungle environment, Caroni River, Venezuela:Golden, Colorado School of Mines, Ph.D. dissertation, 176 p. 

Graterol, Victor, 1988, Mapa de anomalia de Bouguer de la Republica de

Venezuela: Caracas, Universidad de Sim6n Bolivar, scale 1:20,000,000. 

Herrero, E., and Navarro, J., 1989, Mapa de anomalias magneticas de

Venezuela: Venezuela, Ministerio de Energia y Minas, folio of 1:500,000- 

scale sheets. 

Montgomery, C.W., 1979, Uranium-lead geochronology of the Archean

Imataca Series: Contributions to Mineralogy and Petrology, v. 69, no. 2, p.

167-176. 

Onstott, T.C., Hall, C.M., and York, Derek, 1989. 40Ar/39Ar

thermochronometry of the Imataca Complex, Venezuela: Precambrian

Research, v. 42, p. 255-291. 

Reid, A.R., and Bisque, R.E., 1975, Stratigraphy of the diamond-bearing

Roraima Group, Estado Bolivar, Venezuela: Quarterly Bulletin of theColorado School of Mines, v. 70, no. 1, p. 61-82. 

Seiner, G.I., and Taylor, R.B., 1991, GSMAP system version 7.0; graphics

programs and related utility programs for the IBM PC and compatiblemicrocomputers to assist compilation and publication of geologic maps andillustrations using geodetic or Cartesian coordinates: U.S. GeologicalSurvey Open-File Report 91-0001, 151 p., diskette. 

Sidder, G.B., and Martinez, Felix, 1990, Geology, geochemistry, and mineral

resources of the upper Caura River area, Bolivar State, Venezuela: U.S.Geological Survey Open-File Report 90-231, 29 p. 

Sidder, G. B., and Mendoza, Vicente S., 1991, Geology of the Venezuelan

Guayana Shield and its relation to the entire Guayana Shield: U.S.Geological Survey Open-File Report 91-141. 

 in press, Geology of the Venezuelan Guayana Shield and its relation to the

entire Guayana Shield, in Sidder, G.B., Page, N.J, Wynn, J.C., and Garcia,

G.A., eds., Geology and mineral deposits of the Venezuelan GuayanaShield: U.S. Geological Survey Bulletin. 

Yanez, Galo, 1985, Geologia y geomofologia del Grupo Roraima en el sureste

de Venezuela: Congreso Geologico Venezolan, 6th, Caracas, Ministerio de

Energia y Minas, v. 2, p. 1243-1306.

CONTRIBUTORS

Enrique Acosta, Juan Acosta, Nestor Angulo, Jesus Aruspon, Cruz Briceno, 

Henry Bricefto. William E. Brooks, Juan Candelaria, Gloria Contreras, Warren Day, John C. Dohrenwend, Jacques Edilibi, Yasmin Estanga, Luis Franco, Jose 

Jesus Freites, Andres Garcia, Floyd Gray, Acenk Guerra, Yolanda Lopez, Glenda 

Lowry, Elis Lugo, Iris Marcano, Freddy Malave, Sherman Marsh, Felix Martinez, Miguel Martinez, Vicente Mendoza, Alfredo Menendez, Fernando Nufiez, 

Stephen Olmore, Norman Page, Ines Rendon, Haydee Rincon, Ivan Rivero, 

Nelson Rivero, Edixon Salazar, Henry Sanchez, Gustava Sardi, Gary Sidder, 

John H. Stewart, Frank Tovar, Urbano Ubencio, Manuel Uso, and Galo Yanez.
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

68°

BULLETIN 2062 

PLATE 4

ATLANTIC OCEAN

NURIA 

trHPLANQ

Zone in dispute between 

Venezuela and Guyana; 

currently in mediation by 

United Nations

O BOLIVAR

Cerro Guaiqumima

iUITO /1YAGUCHO

C^fro 

rahi/aca\O

Simple Bouguer gravity data for stations were provided by Professor Victor Graterol (Universidad 

de Simon Bolivar, Caracas, Venezuela) and the U.S. Defense Mapping Agency.

TERRITORIO 

FEDERAL

AMAZONAS

200 400 600 800 KILOMETERS

I I I I 

0 1GD ZOO 300 400 MILES

Gravity station

Piedra de Cocuu

Cerro La 

Neblina

EXPLANATION

 80 Simple Bouguer anomaly contour Contour interval 2 mGal

SCALE 1:1 000 000

50 100 150 KILOMETERS

25 50 7'5 MILES

SIMPLE BOUGUER GRAVITY ANOMALY MAP OF THE

VENEZUELAN GUAYANA SHIELD

By 

Jeffrey C. Wynn and Kevin Bond

1993
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

BULLETIN 2062 

PLATES

60°

ATLANTIC OCEAN

X_-r" 358, 359 '

- i ^c y 266,267,269-278,

x / ; / 280-285,287-304,

7 307-309x

2" 351

-* ""'' 344

Serrania de Im^taca

0 100 200 300 400 MILES 

INDEX MAP

323

Crerro Bfivar

Zone in dispute between 

Venezuela and Guyana; 

currently in mediation by 

United Nations

-OLas Claritas

0 Pi-aiornctro 8S 

114

Auyar/Tepuy 

\ /

o80 o8

^ ^°y ' f

> 74<>/ _ W1 Cerro

\ Cuquenan

 6^

O

ChitQta 

Tepuy

%_>c*t iiti i-.tti m,-., I "^ w

-jj°!&/ \ r

x. **i 

 - F*-JX

Cerrol 

Sarisarina>rna

_ _ ^4*. ^ ^

J

/^ Cerro

A Maveti 

'\^L

TERRITORIO

FEDERAL 

AMAZONAS A

60°

EXPLANATION

[See text table 3 for description of numbered locations]

	Gold (low-sulfide gold-quartz veins) 

 Gold (placers) 

O Diamond 

A Manganese 

V Iron

D Aluminum (bauxite)

O Other

SCALE 1:1000000 

50 100 150 KILOMETERS

26

r i b

25 50

3=

75 MILES '

=1

DISTRIBUTION OF MINES AND MINERAL OCCURRENCES IN THE

VENEZUELAN GUAYANA SHIELD

By 

Norman J Page, G.J. Orris, Floyd Gray, and Karen Sue Bolm

1993

68° 67° 65° 64° 63°
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

68°

BULLETIN 2062 

PLATE 6

0°

ATLANTIC OCEAN

VENEZUELA

GUYANA

SURINAME

FRENCH GUIANA /

^_^' ' -^ U

10°S

o 0 o

BRAZIL

EXPLANATION 

[See text appendix \ for description of mineral occurrences]

x Gold

 Placer gold

o Diamond

A Manganese

V Iron

n Aluminum

o Other

a

D

o 

o

^A

v

O

V

v

o 

O V

A

A 

O

O

V

0

O 

O °

O

O

BOLIVIA t 10°S

6B° 48°

100

SCALE 1:5000000 

200 300 400

I

500 KILOMETERS 

_I

100 200

\ 

300 MILES

DISTRIBUTION OF MINERAL OCCURRENCES IN NORTHEASTERN SOUTH AMERICA

By 

Norman J Page, Floyd Gray, and Karen Sue Bolm

1993
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

68°

BULLETIN 2062 

PLATE?

60°

8C -

7° -

ATLANTIC OCEAN

VENEZUELA

Zone in dispute between 

Venezuela and Guyana; 

currently in mediation by 

United Nations

San Salvador de Paul r

COLOMBIA

Auyan/Tepuy

fcerro Guaiqutnima,

San Juan de > 

Manapiare

Q

erro 

aveti

/--

TERRITORIO

FEDERAL 

AMAZONAS

EXPLANATION

Algoma iron deposits and sedimentary manganese deposits 

Domain la contains prominent ridges underlain by iron formation 

and all of the known deposits and is considered permissive for 

Algoma iron deposits. Domain Ib has a low probability of 

undiscovered deposits because of the absence of prominent ridges 

characteristically developed on iron formation and the lack of 

known deposits. Both domains are considered permissive for 

sedimentary manganese deposits

COLOMBIA

: Low-sulfide gold-quartz vein deposits Domain Ha is coextensive 

witn greenstone belts, eugeosynclinal metavolcanic rocks, and areas 

of amphibolite and is considered permissive for low-sulfide goldquartz vein deposits. Domain lib has a low probability of 

undiscovered deposits because of the absence of known gold 

occurrences despite levels of exploration similar to domain Ha

Tin greisen deposits

Porphyry copper deposits and volcanic-hosted magnetite deposits

-)San Carlos de Ri6 Negro

SCALE 1:1 000 000

50

Piedra de Cocuy

150 KILOMETERS

PERMISSIVE DOMAINS FOR ALGOMA IRON DEPOSITS, SEDIMENTARY

MANGANESE DEPOSITS, LOW-SULFIDE GOLD-QUARTZ VEINS, TIN

GREISEN DEPOSITS, PORPHYRY COPPER DEPOSITS, AND

VOLCANIC-HOSTED MAGNETITE DEPOSITS IN THE

VENEZUELAN GUAYANA SHIELD

By 

Floyd Gray, Norman J Page, Dennis P. Cox, G.J. Orris, Jeffrey C. Wynn, and William E. Brooks

1993

68°

0°

67° 66° 64° 63°
U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

68°

67

BULLETIN 2062 

PLATES

60°

ATLANTIC OCEAN

0 100 200 300 400 MILES 

INDEX MAP

VENEZUELA

Zone in dispute between 

Venezuela and Guyana; 

currently in mediation by 

United Nations

ESTADO BOLIVAR

Auyan/Tepuy

X /

/"VS

Cerro] 

Sarisariharna

Probable ,

v/\LC->^ VV'*^ Los Canbesj 

. Icabaru 

-^-0 

s^ L i 

X _/1y

b;d^-0 ^ .. ^-/

^Magnetic belt 

(rift)

EXPLANATION 

TERRITORIO Kuroko-type massive sulfide deposits

FEDERAL 

AMAZONAS

Synorogenic-synvolcanic nickel-copper deposits and related 

platinum deposits

COLOMBIA

Carbonatite deposits and diamond-bearing kimberlite pipes

Sedimentary kaolin deposits

Seis Lagos and 

Marie Marin 

carbonatites

O

60°

I Circular feature of unknown origin identified using geophysical or 

side-looking radar imagery data

1 Area underlain by intrusive magnetic rocks

I Area of uranium (U) or thorium (Th) anomaly as delineated by 

aeroradiometric survey

SCALE 1:1 000 000 

50 100 150 KILOMETERS

25 50 75 MILES

PERMISSIVE DOMAINS FOR KUROKO-TYPE MASSIVE SULFIDE DEPOSITS,

SYNOROGENIC-SYNVOLCANIC NICKEL-COPPER DEPOSITS AND

RELATED PLATINUM DEPOSITS, CARBONATITE DEPOSITS,

DIAMOND-BEARING KIMBERLITE PIPES, AND

SEDIMENTARY KAOLIN DEPOSITS

By 

G.J. Orris, Floyd Gray, Dennis P. Cox, Norman J Page, William E. Brooks, and Jeffrey C. Wynn

1993

o°

67° 66° 65° 64° 63°