U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3084

September 2008

Central Colorado Assessment Project—Application of 

Integrated Geologic, Geochemical, Biologic, and Mineral 

Resource Studies

By T.L. Klein, S.E. Church, J.S. Caine, T.S. Schmidt, and E.H. DeWitt

Printed on recycled paper

Introduction

Central Colorado is one of the fastest-growing 

regions in the Western United States. Population 

along the Front Range increased more than 30 percent 

between 1990 and 2000 (http://www.demographia.

com/db-metro3newworld.htm) with some counties 

within the study area such as Park County 

experiencing greater than 100 percent growth (http://

www.censusscope.org/us/s8/rank_popl_growth.html). 

This growth has caused tremendous demand for 

natural resources and has created challenging landmanagement issues related to the interface between 

wilderness and urban expansion. Management of 

this wilderness/urban interface will benefit from 

current digital geoscience information collected 

by the U.S. Geological Survey (USGS) Central 

Colorado Assessment Project that began in 2003. 

Approximately 20,800 square miles (53,800 km2

) of 

land divided almost equally between the public and 

private sectors were part of the assessment (fig. 1).

The study area includes much of the Colorado 

Mineral Belt, a northeast-trending zone that 

contains 30 economically significant metal deposits. 

Historically, the area provided much of Colorado’s 

metal production. The only active gold and 

molybdenum mines in Colorado lie within the study 

area. Recently, metal and uranium exploration activity 

has increased sharply in response to record prices 

for metals and uranium. This further underscores the 

need for up-to-date geoscience information presented 

in compatible GIS databases in order to facilitate 

rapid land-management decisions. 

Cooperative studies by USDA Forest Service, 

National Park Service supported by the USGS 

Mineral Resources (MRP), and National Cooperative 

Geologic Mapping Programs (NCGMP) contributed 

to the mineral-resource assessment and included 

regional geologic mapping at the scale 1:100,000, 

collection and geochemical studies of stream 

sediments, surface water, and bedrock samples, 

macroinvertebrate and biofilm studies in the 

riparian environment, remote-sensing studies and 

geochronology. Geoscience information available as GIS layers has 

improved understanding of the distribution of metallic, industrial, 

and aggregate resources, location of areas that have potential 

for their discovery or development, helped to understand the 

relationship of tectonics, magmatism, and paleohydrology to the 

genesis of the metal deposits in the region, and provided insight on 

Figure 1. Location of Central Colorado Assessment Project study 

area. The thick black line in the larger figure is the outline of the study 

area. Red lines are major highways. Blue shading is water. White fill 

represents private lands and green fill represents public lands, managed 

mainly by the U.S. Department of Agriculture Forest Service, the National 

Park Service, and the Bureau of Land Management.

Vail

Salida

Dillon

Granby

Walden

Denver

Pueblo

Boulder

Alamosa

Leadville

Kremmling

Walsenburg

Estes

Park

Canon

City

Fort Collins

Colorado 

Springs

0 3 6 12 18 24 MILES

N

Location map
the geochemical and environmental effects that historical mining 

activity and natural, mineralized rock exposures have on surface 

water, ground water, and aquatic life. 

Geologic Mapping

One of the primary objectives of the CCAP is preparation of a 

regional geologic map (1:100,000) from previously published maps 

and new mapping by the USGS and the Colorado Geological Survey. 

The need for this new compilation is illustrated in figure 2 where 

a small part of the new map is compared with the same area of the 

only regional digital geologic map (Tweto, 1979) available before 

this study. This new geologic map provides the underlying geologic 

framework for environmental, mineral-deposit, tectonic, geochemical, 

geochronology, remote-sensing, and geophysical studies.

Colorado Mineral Belt Studies

Metal mining in Colorado began in 1858 and was instrumental in 

the early economic development of the State. Gold, silver, lead, and 

later, tungsten and molybdenum were produced from eight general 

types of hydrothermal deposits identified in figure 3. With the exception of the large gold deposit at Cripple Creek, nearly all the deposits 

in the CCAP study area occur within the Colorado Mineral Belt. 

Compilations of published geochronology, igneous-rock geochemistry, and lead isotope data, and additional data from new samples have 

been used to understand the geochemical and temporal relationships of 

magmatism to metalliferous hydrothermal systems in the area. These 

studies have been integrated with the structural studies summarized 

below to develop a regional model for the formation of metal deposits 

in the northeastern part of the Colorado Mineral Belt. 

Structural Studies and Controls of Mineralization

New research for the CCAP in the central Colorado Front Range 

has resulted in alternative interpretations that indicate that tectonic 

Figure 2. Geologic map of the Swandyke area in the western part of the Colorado Front Range. Map A (Tweto, 1979) 

was published at a 1:500,000 scale and was the only regional 

geologic map available as a GIS product. Map B is the same 

area from the regional geologic map compiled at a 1:100,000 

scale for the Central Colorado Assessment Project. Note the 

differences in detail and the number of rock units. 

Figure 3. Relationship of the Colorado Mineral Belt to porphyry 

molybdenum (Mo) and copper-molybdenum (Cu-Mo) deposits and 

significant deposits. Mineralized areas are classified by dominant 

hydrothermal mineral deposit type. Significant deposits are those 

with contained metal in the upper 10 percent of their deposit type 

worldwide.

Cripple

Creek

Monarch

Tincup

Alma

Vail

Salida

Walden

Denver

Rosita

Boulder

Alamosa

Leadville

Jamestown

Estes

Park

Canon

City

Breckenridge

Fort Collins

Central

 City

Climax

Gilman

Henderson

Sangre de Cristo 

Mountains

0 5 10 15 20 MILES

Significant

deposit

Porphyry

deposit

Polymetallic

vein

Porphyry 

Mo or Cu-Mo

Polymetallic

replacement

Fluorite vein

Tungsten

vein

Uranium vein

Stratabound

Cu-Zn

Alkaline 

gold telluride

Colorado 

Mineral Belt

boundary

Major 

highway

City

Mineralized area

 type

N

EXPLANATION

Xfh

Xb

Qd

Xb

Qd

Xb

Yg

Kp Qd

TKi

KJdm

KJdm

Xmv

Xv

Xw

Xfv Qtu

Xmv

YXmig

Xfv

Qpt

Xigy

Qtu

Xw

Yikp

Xp

Qls

Kp

Xmv

Xv

Qc

Qpt

Xv

Xigy

Yikp

Qls

Kp

Xfv Qpt

Yigr

Yigr

Qa

Xigy

Xigy

Qtu

Yigr

Xw

Yigr

Xmv

Qc

TKimg Qa

Xigy

Xcs

Yigr

Xmv

Qls

Qls

Xcs

Qls

Kp

Yigr

Qls

Qc Qc Qf

Qc

Xfv

Qc

Xmv

Xmv

Qls

Qls

Yigr

Qc

Qls

Qtu

Qpt

Qbt

Qc

Qa

Yigr

Qc

Qf Yigr

Qls

Yigr

Yigr

Xv

Qf

Qf

Xmv

Qls

Qa

Qls

Qc

Kp

Qls

Qls

Tigd

Tigd

TKixgd Tigr

Yigr

Yigr

Yigr Yigr

Xfv

Xfv Swandyke

Swandyke

0 1/2 1 MILE

0 1/2 1 MILE

B

A

N
processes in the Early Tertiary combine to form shallow, polymetallic 

vein (PMV) deposits, the most common type of deposits in the study 

area. These deposits in the northeastern part of the Colorado Mineral 

Belt were the first to be discovered in Colorado and combined to 

produce more than $850,000,000 (in 2007 dollars) of base and precious 

metals by 1970 (Lovering and Goddard, 1950; United States Bureau of 

Mines Yearbooks 1947 through 1970). 

The fundamental ingredients needed to make PMV deposits include 

heat, fluids, metallic ions, and permeability. Our new research focused 

on permeability, which can be generated in several ways. Previous 

studies in the area suggest that PMV deposits commonly form along 

preexisting geologic structures such as shear zones where the host 

rocks are “weak” and susceptible to fracturing. This in turn makes 

them permeable pathways for magmas and associated metal-rich fluids 

to ascend through the crust to a depositional site. Although this can 

be true, not all shear zones are the same—some are permeable and 

others are not. Some shear zones are “strong” when tectonic forces are 

applied to them, making them resistant to fracturing and thus impermeable. 

One example of a crustal weakness thought to have controlled 

the location of one of Colorado’s most productive regions of PMV 

deposits is the Precambrian Idaho Springs–Ralston Shear Zone (ISRZ). 

However, the analysis of compiled structural and mineral-deposit data 

from existing reports and new data collected from outcrop studies 

results in several key observations shown in figure 4. (1) Locations of 

mineral-deposit-related structures do not correlate well with plutons, 

the ISRZ, or other major brittle faults or fracture zones. (2) Mapped 

structures suggest that myriad directions of potential permeability 

existed. Thus metalliferous, hydrothermal fluids were free to flow in 

many directions at any time during the formation of the metal deposits 

if stress was isotropic. (3) The PMV deposits show either preferential 

east-northeast trends that are well correlated with Laramide orogeny 

stress directions or radial trends proximal to Late Cretaceous and 

Tertiary igneous intrusions and show a weak correlation with structures 

related to the ISRZ. (4) Minor differences in orientation and intensity 

of potential structures existed that may have controlled permeability 

within the ISRZ compared with similar structures outside the ISRZ. 

Our observations suggest that the ISRZ did not host the ascent 

of metal-rich magmas or associated fluids and did not act as the 

primary control of mineral-deposit locations in the Colorado Front 

Range. Rather, responsible processes likely include (1) proximity to 

epizonal plutons; (2) competition between varying magnitudes and 

orientations of shallow regional tectonic stresses, overburden stresses, 

and local stress perturbations related to pluton emplacement; and 

(3) the formation of “self-generated” permeability from magmatic 

heat-generated pressure leading to in-situ fracturing within the various 

stress fields. Our work concludes that all of the processes listed above 

worked together. Thus, tectonic-stress fields, depth range of magma 

formation, and the age, location, and style of potential permeability 

structures should all be considered when determining the potential for 

undiscovered PMV deposits.

Impact of Bedrock Composition, Mineral Deposits, 

and Historical Mining on Water, Sediment, and 

Biology in the Riparian Environment

As part of the CCAP, we conducted an assessment of 198 

watersheds in an area of approximately 54,000 km2

. Water, sediment, 

and aquatic insects were collected from watersheds underlain by 

single lithogeochemical units or from watersheds that contained the 

hydrothermally altered rock. Historical mining for base and precious 

metals was extensive in the Colorado Mineral Belt, which has elevated 

concentrations of base metals (primarily cadmium, copper, lead, and 

zinc) in water and sediment. 

We tested the assumption in the study area that degraded water and 

sediment quality are restricted to watersheds in which historical mining 

occurred and found the four most important parameters that control 

elevated trace-metal concentrations in water and sediment are lithology, 

hydrothermal alteration, the type of contained mineral deposits, and 

presence of historical mining. Base-metal concentrations among 

unaltered rock types showed small geochemical variations that reflect 

host lithology. Base-metal concentrations in sediment from watersheds 

underlain by hydrothermally altered rock are elevated. Classification of 

watersheds (mined and unmined) on the basis of the type of contained 

mineral deposit (fig. 5) discriminates watersheds that have degraded 

water and sediment quality better than the other parameters. Only 

about 5 percent of the watersheds in the study area have been affected 

by historical mining, whereas hydrothermally altered rocks underlie a 

much larger part of the study area and have weathered to produce water 

and sediment that have naturally elevated geochemical baselines.

Ecological risk assessments rarely use models of the natural distribution of metals in the earth. To develop ecologically relevant baselines 

for environmental assessments, we sampled aquatic communities from 

159 streams underlain by single lithogeochemical units (fig. 6) and 

characterized the extent of mining-related disturbance, hydrothermal 

alteration, and deposit type found within each watershed. 

Figure 4. Potential permeable structures of the Colorado Front Range 

and Cretaceous and Tertiary plutons (red areas). Also shown are the 

Idaho Springs–Ralston Shear Zone (IRSZ) as irregular thick brown lines, 

brittle faults as thick gray lines with yellow fill indicating zones of extensive hydrothermal alteration, and polymetallic veins (PMV) as bright red 

lines. Note that the PMV cut all major brittle faults, the Cretaceous and 

Tertiary plutons are only coincident with the IRSZ in one location, and 

only a small proportion of the PMV is coincident with the ductile IRSZ.
Results show that the number of different types of aquatic insects 

(richness) and the density of metal-sensitive insects are negatively 

affected by mining within a watershed (fig. 7). However, the large and 

overlapping statistical ranges in these parameters make it difficult to 

use them to differentiate between mined and unmined watersheds. 

The type of hydrothermal alteration also influences aquatic 

communities. Propylitic mineral alteration can cause higher 

concentrations of metals in water and sediment in both mined 

and unmined watersheds, but in some cases these metals are not 

biologically available due to minerals found in the deposit that 

ameliorate metal toxicity. In phyllic-mineral-altered watersheds, pyritebearing minerals release metals and acids into the stream, reducing the 

number of different types of insects and the number of metal-sensitive 

insects found per square meter of streambed. Different deposit types 

also influence aquatic-community responses to the effects of mining.

Figure 5. Concentration data in sediment from watersheds classified by 

mineral-deposit type. (Cd, cadmium; Cu, copper; Pb, lead; Zn, zinc; SO4, sulfate).

Figure 6. U.S. Geological Survey field party sampling aquatic 

invertebrates in central Colorado. 

Figure 7. Box plots illustrating the number of different types 

of aquatic insects (richness) and the density of metal-sensitive 

insects in sampled catchments by alteration and metal deposit 

type. Propylitic and phyllic classes are both mined and unmined. 

Polymetallic vein and polymetallic vein replacement classes are 

mined only. Porphyry Cu-Mo class is prospected. (Cu, copper; 

Mo, molybdenum).

These results show that integrative science linking geoscience 

and ecological data improved our ability to predict the ecological 

risks associated with mining specific rock and deposit types. Future 

ecologically relevant assessments of the effects of geology on the 

environment should include the assessment of hydrothermal alteration, 

deposit type, and the extent of mining disturbance within watersheds 

underlain by single lithogeochemical units. 

References Cited

Lovering, T.S., and Goddard, E.N., 1950, Geology and ore deposits 

of the Front Range Colorado: U.S. Geological Survey Professional 

Paper 223, 319 p.

Tweto, O.L., 1979, Geologic map of Colorado: U.S. Geological Survey 

Special Map, 1:500,000. 

United States Bureau of Mines, 1947–1970, Minerals Yearbook: 

United States Bureau of Mines. 

0.01

0.1

1.0

10

100

1,000

10,000

100,000

Uranium veins

Fluorite veins

Polymetallic replacement 

and skarn

 Porphyry Mo and Cu-Mo

 Polymetallic veins

 Background (unmined)

 catchments

Cd Cu Pb Zn SO4

CONCENTRATION, IN PARTS PER MILLION

EXPLANATION

DENSITY OF MAYFLIES + STONEFLIES +

CADDISFLIES PER 0.1 METER SQUARED NUMBER OF DIFFERENT INSECTS

0

5

10

15

20

25

30

0

100

200

300

400

500

N = 35

N = 27

N = 55

N = 36 N = 6 N = 4

N = 3

mined and unmined

mined

Unmined

Mined

Propylitic

Phyllic

Polymetallic vein

Polymetallic 

Porphyry Cu-Mo

vein replacement

Maximum

90th percentile

75th percentile

Median

25th percentile

10th percentile

Minimum

EXPLANATION

prospected