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**GEOCHEMICAL DATA FOR HISTORIC MINING AREAS,**

### CENTRAL WESTERN SLOPE, COLORADO

By

J. Thomas Nash^1^ , Warren C. Day^2^ and Anna B. Wilson^3^

#### Open-File Report 01-0003

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# 2001

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# 

# 

# 

# 

#  This report can be downloaded from the Central Region website:

[http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-01-0003](http://greenwood.cr.usgs.gov/pub/open-file-reports/OFR-01-003)

# 

# 

# This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey editorial standards or with North American Stratigraphic code. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

# 

## U. S. Department of the Interior

## U. S. Geological Survey

# 

#  ^1^ Box 25046 MS 973, Denver CO 80225; <tnash@usgs.gov> 

#  ^2^ Box 25046 MS 905, Denver CO 80225; <wday@usgs.gov>

^3^ Box 25046, MS 905, Denver, CO 80225; <awilson@usgs.gov>

# **TABLE OF CONTENTS**

Introduction 3

Field Methods 3

#  Water sampling and analysis 4

Sampling and analysis of mineralized rocks 6

Data Files 7

Quality control for water analyses 8

# References cited 11

# 

List of figures and tables

Figure 1: Diagram of analytical error 10

Table 1: Elements determined in water and leachate solutions by ICP-MS 5

#  

# **INTRODUCTION**

This report contains the analytical results for samples collected at 451
sites on the Central Western Slope of Colorado, including the Gunnison
and Uncompahgre National Forests and adjacent public lands administered
by the Bureau of Land Management. Included are results for 190 samples
of surface waters, 185 samples of mined rocks, mill tailings, and
altered rocks, and passive leach analyses of 116 samples of those
mineralized materials, and 65 samples of host rocks. These geochemical
analyses are part of studies designed to improve the understanding of
known ore deposit types and to characterize the geochemical signature of
historic mines and their host rocks and to determine their actual or
potential contamination of surface or ground waters. Seventeen districts
with significant historic mining activity were examined and sampled, but
no active mines were investigated.

One focus of these reconnaissance geochemical studies was on materials
and processes that are actively contributing, or have the potential to
contribute acid or metals to surface waters. Water samples were
collected wherever possible from springs, mine adits, streams, mine pits
or puddles, and where there was no accessible water (ground water was
not sampled) solid materials were collected and tested in the
laboratory, using a passive leach test, for their potential to release
acid and metals. Although the results for many metals of concern are
reported to levels below 1 part per billion, the focus of this study is
on the high concentrations that span four to six orders of magnitude for
toxic metals such as Cu and Zn. This investigation is concerned with
geochemical processes that create those very high metal concentrations,
and the geologic processes that attenuate acidity and high metal
concentrations. The analytical methods used are appropriate for
determination of many metals of potential concern and expected to have a
wide range in concentration, and the ICP-MS method in particular is good
for trace metals in water such as As, Cu, Pb, and Zn. (Crock and others,
1999). A secondary goal of the hydrogeochemical studies is screening of
mining sites of interest to Federal land managers. If water compositions
reported here suggest a problem source of contamination, follow-up work
should be done using trace-metal sampling protocols and more precise
analytical methods appropriate for water quality definition.

*Acknowledgements*. This study has benefited in many ways from helpful
suggestions from staff of the BLM and USFS in Colorado, in particular
Barbara Hite (BLM) and Daryl Gusey (USFS). Our colleagues in the USGS
have offered helpful advice, comments on manuscripts, and chemical
analyses; We thank in particular Paul Briggs, Jim Crock, Bob Eppinger,
David Fey, Harley King, Al Meier, Bill Miller, and Brad van Gosen.

### FIELD METHODS

Field studies were conducted from 1997 to 1999, typically in July and
August when there was sufficient surface water for sampling. Sites
visited and sampled were determined by reference to published
information, U.S. Geological Survey records in the Mineral Resources
Data System (MRDS), information on U.S. Geological Survey 1:24,000 scale
topographic maps, and by accessibility. Sample localities were recorded
on the topographic maps and measured with a conventional GPS (global
positioning system) instrument having an accuracy of about ±100 to 150
feet (horizontal) based on tests at known sites. Sample locations are
reported here in decimal degrees of latitude and longitude.

A six to eight digit identifier was assigned to each site or sample; an
example is NGD201. Nash uses a system in which the first two characters,
NG, designate the sampler (Nash) and study area. The third or fourth
character designates the sample media or site type, most commonly D,
mine waste dump rocks; R, altered (unmined) rocks; W, water (of several
types); L, leachate derived from a lab test. Other types are explained
in the data tables. Field numbers assigned by Day indicate the sampler
(WD, Warren Day; AW by Anna Wilson), the year, and a unique number for
the site and sample.

## Water sampling and analysis

## 

## The focus of these geochemical studies was the composition of surface waters spatially related to areas where mining has occurred, known mineralized and altered areas, as well as background sites not related to mineralization. Analyses of surface waters provide a quantitative measure of what contaminants are mobile in the vicinity of mines and available to wildlife and humans. Water samples were collected from mines and streams when the water was deemed to be representative of a geologic or mine setting and would yield information on the mobility of metals in that environment. The sampling methods have been used by the senior author since 1994 for reconnaissance characterization of geologic units, rock alteration, mine drainage, and reactions with tailings or dump materials (Nash and others, 1996); these methods are similar to but simplified from those described by Ficklin and Mosier (1999). Flow rate and water characteristics (color, suspended material, bed colors or mineralogy) are recorded, and pH and conductivity are measured using portable instruments. A pocket-sized conductivity meter (Corning CD-55) was used, which has an upper limit of 2,000 μS/cm. This device responded consistently and showed no drift after calibration. The pH meter (Orion 250), with built in temperature electrode, required frequent calibration throughout the day, and at most sites the calibration was checked on a standard solution after the field measurement. The field standards were buffered solutions of pH 4.0, 7.0, and 10.0. The pH measurements are thought to have an uncertainty of about ±0.05 standard units, even though the meter reports to 0.01 units, due to instrument drift, which is possibly larger than normal in the analysis of widely divergent compositions. A representative water sample for analysis was collected with a disposable 60 ml syringe and then filtered using a disposable 0.45-micron cellulose filter. The syringe and the 60 or 120 ml polyethylene bottle were rinsed twice in the sampled water prior to collection. The filtered sample was acidified to a pH of about 2 at the site with 5 drops of ultrapure 1:1 HNO~3~ per 60 ml. At appropriate localities (with pH \>4) an unfiltered sample was collected for determination of alkalinity. Lab and field blank tests using deionized water indicated that contamination introduced by the sampling procedure and equipment is in the low parts per billion level (1-10 ppb), which we consider adequately low in our search for metal concentrations that are orders of magnitude greater than the sampling error. Each group of analyzed water samples included standard solutions and replicate samples (blind to the analyst); quality control is described in other reports (Lamothe and others, 1999; Fey and others, 2000).

Water samples were analyzed by several methods for various constituents
using methods described by Briggs and Meier (1999), Lamothe and others
(1999) and by Crock and others (1999). Concentrations of 40 to 60
elements (cations) were determined by inductively coupled plasma-mass
spectrometry (ICP-MS) by a commercial laboratory (1996 to 1998 samples)
and by the USGS (1999 samples). The elements and the lower limit of
determinations are listed in table 1. Analytical results are in files
GWC.xls and GWC99.xls.

## Table 1: Elements determined in water and leachate solutions by ICP-MS,

## and their lower limits of determination 

\[Elements are listed by atomic weight; lower limit is in parts per
billion; the limits shown are generalized and somewhat higher than
stated by the commercial lab; limits in USGS analyses are similar\].

Li 0.1 Nb 0.01

Be 0.5 Mo 0.04

Na 0.8 Ag 0.01

Mg 0.5 Cd 0.01

Al 0.4 Sn 0.01

Si 1 Sb 0.04

K 1 Te 0.2

Ca 1 Cs 0.01

Sc 0.1 Ba 0.01

Ti 0.04 La 0.01

V 0.04 Ce 0.01

Cr 0.04 Eu 0.01

Mn 0.04 Yb 0.01

Fe 0.3 Ta 0.01

Co 0.01 W 0.01

Ni 0.01 Re 0.01

Cu 0.01 Os 0.01

Zn 0.01 Pt 0.01

Ga 0.01 Au 0.01

As 0.04 Tl 0.01

Se 0.04 Pb 0.02

Br 1 Bi 0.01

Rb 0.01 Th 0.01

Sr 0.01 U 0.01

Y 0.01

Sampling and analysis of mineralized and unmineralized rocks

*Rock chemistry*. Mineralized rock samples were collected from mine
dumps, outcrops, and mill tailings impoundments. In most cases the
intent was to collect a representative sample, but for some sites a
select sample was collected to determine a special property. The most
commonly employed sampling method for dumps and tailings involved the
collection of numerous small portions at 20 to 30 sub-sites, and sieving
the materials through a 2 mm stainless steel sieve to derive a composite
sample weighing about 2 to 4 pounds; this is the standard protocol
developed for USGS-AML investigations (Nash, 1999). Mine dump samples
collected by Day and Wilson (file GOR-ICP.xls) are the best exposed
material available and are thought to be representative of the ore
material of the associated mineral deposit. In most cases, the analyses
represent composite samples of ore samples.

A suite of unmineralized rocks was collected by Day and Wilson to better
understand the geologic setting as well as genesis of rock units that
host ore deposits. These geochemical studies were part of a mineral
resource assessment of the central Western Slope of Colorado and are
presented in files GRX-MAJ.xls, GRX-ICP.xls, and GRX-INAA.xls.

Rock samples collected by Nash were pulverized at the USGS lab in Denver
and analyzed by commercial laboratories. In one method the rocks are
dissolved in a mixture of four acids, then the concentrations of 35
major and trace elements are determined using inductively coupled plasma
(ICP); this is considered a total analysis as the strong acids dissolve
all but the most refractory minerals such as zircon. This method is
essentially that of Briggs (1996), although slight differences may exist
between laboratories. Rock samples were also analyzed by a method that
employs weaker acids to dissolve most minerals, then employs an organic
reagent to collect 10 or 15 metals of interest (Ag, As, Au, Bi, Cd, Co,
Cu, Hg, Mo, Pb, Sb, Se, Te, Tl, and Zn); analysis is by ICP; results are
very similar to those of Motooka (1990) but one laboratory includes 5
additional elements. This second analytical method has lower levels of
determination and works well for some elements, such as Sb, Se, Te, and
Tl, that are not effectively determined by the total digestion method.
Quality assurance monitoring shows that the precision and accuracy
(deviation of the reported values from correct ones) is less than 5
percent for most elements. Analytical results of chemical analyses of
solid materials are in a spreadsheet file GMR.xls that is included with
this report.

Rock samples collected by Day and Wilson were analyzed by methods
appropriate for the fresh or mineralized rocks. A set of 35 samples of
mineralized rock samples, many from mine dumps, were analyzed by XRAL
Laboratories, Toronto, Canada by ICP-AES using the method of Briggs
(1996). Sulfur was determined by combustion and infrared analysis
(Curry, 1996). Gold was determined by fire assay. A suite of 65 rock
samples collected to determine the lithogeochemistry of rock units
hosting ore deposits were analyzed by X-ray fluorescence spectroscopy by
J. Taggart of the USGS for major elements (Mee and others, 1996), by
ICP-AES (Briggs, 1996), and by ICP-AES for trace metals (Briggs and
Meier, 199) and fire assay for gold by XRAL Laboratories, Toronto,
Ontario, Canada. The rock samples were also analyzed by instrumental
neutron activation analyses by R. Knight of the USGS using techniques of
Wandless, (1996.)

*Leachate chemistry*. The chemical analyses just described are not
always appropriate for environmental characterization because they
describe total metal rather than mobile (soluble) metal concentrations.
To determine the mobility of metals, as well as the tendency of a
material to generate acid, a passive leach test (Nash, 1999; Fey and
others, 2000) was utilized. Solid materials from dumps, mill tailings,
and outcrops have been processed in the lab by a leach method that
provides a measure of reactions in nature, such as during weathering or
storm events. In this technique, 100 g. of rock sample is sieved to \<
2mm, placed in a beaker with 2,000 ml of deionized water, stirred
slightly, and the initial pH is measured. After about 20 hours, the
upper part of the solution is stirred gently to mix the leachate
solution. At 24 hours the pH and conductivity of the leachate is
measured, and a 60 ml aliquot is taken with a syringe and filtered
through a 0.45 micron filter. The sample is acidified with 5 drops of
ultrapure 1:1 HNO~3~. The leachate is analyzed by the same ICP-MS method
used for water samples. Anions are not determined by ICP-MS and no work
has been done on them in this investigation because the dominant anion
in acidic solutions is expected to be sulfate. The analytical results
(file GLC.xls) resemble those for surface waters degraded by mine waste
(file GWC.xls). However, unlike the water analyses that show the results
of real water-rock interactions, the leachate results show the
*potential* to generate those compositions---actual behavior is more
complex and involves factors such as permeability, kinetics, and
climate, slope and aspect.

For most rock samples no sample preparation was done for these tests.
The majority of dump samples were sieved to pass 2 mm while in the
field. Mill tailings are by their nature ground to fine sand size and
were not sieved. Unmined altered rock samples were crushed but not
sieved. In a few instances, efflorescent crusts on dump or tailing
materials were hand picked in the field or lab to provide material for
leaching that is more than 50 percent crust. The samples were dry and
friable when placed in the reaction vessel.

#### **DATA FILES**

Included in this report are 9 data files in spreadsheet format (xls as
from Microsoft Excel) containing descriptive information on sites that
were sampled and analytical results. The files are also provided in
comma delimited format (cvs). The analytical results are mostly as
reported by the chemists, except that some files have been simplified by
deleting elements that are not obviously of high interest and have
concentrations generally below the detection limit (such as ICP-MS
results for more than ten rare earth elements) or for which analytical
results are deemed unreliable or inappropriate (such as Hg by ICP-MS or
Ag by one ICP-MS lab). Note that the spreadsheet files contain more
figures than are significant. The files include:

NSAMD.xls: Description and location of 351 analyzed samples (Nash)

NGMR.xls: Analytical results for 150 samples of mineralized materials

NGLC.xls: Analytical results for 116 leachate solutions from mineralized
materials

NGWC.xls: ICP-MS analyses of 160 surface water samples collected in 1997
and 1998

NGWC99.xls: ICP-MS analyses of 30 surface water samples collected in
1999

GRX-maj.xls: XRF analyses of 65 unmineralized rock samples for major
oxide abundances

GRX-inaa.xls: INAA analyses of 65 unmineralized rock samples for trace
element abundances

GRX-icms.xls: ICP-AES and ICP-MS analyses of 65 unmineralized rock
samples for trace elements and gold

GOR-icp.xls: ICP-AES analyses of 35 mineralized samples, including gold
analyses by fire assay and sulfur by combustion analysis

#### **QUALITY CONTROL FOR WATER ANALYSES**

Standards and replicate samples were routinely submitted with field
samples through the course of these studies. Approximately 5 percent of
the analyzed samples were standards and replicates. A detailed
accounting of quality control is beyond the scope of this report.
Readers desiring such information should consult Lamothe and others
(1999) for the performance of the USGS ICP-MS system. The commercial lab
used for the 1997-1998 analyses reported here are similar. Quality
control information for the ICP-MS analyses in 1997 and 1998 are
presented elsewhere (Fey and others, 2000; Nash, 2000). Analytical
performance differs for various elements, as mentioned below. The
following trends are evident:

1\. Results are reported for four to six orders of magnitude for most
elements of interest in these natural waters and leachates, from less
than 0.1 ppb (part per billion or microgram per liter) to 100,000 or
more than 1,000,000 ppb in some unusual waters. This is a difficult
challenge for any method or chemist. These samples often required
dilution by amounts of 1:10 to 1:100 to keep the analyte within the
calibration range.

2\. Precision and accuracy are complex issues and are not the same for
all elements and all concentration levels. We assume that analytical
error is greater at the very high concentration levels of some of the
unusual mine waters because there are no standards at this level, but
documentation of that error is difficult and is incomplete at this date.
Replicate samples suggest that precision at very high concentration
levels is not much worse than at normal concentrations. Because reliable
standards do not exist for waters with very high "trace" metal
concentrations (\>10,000 ppb), it is not possible to evaluate accuracy
at high levels the way it is done at more normal levels. For reasons
that we can not explain, the precision and accuracy for "major" elements
(Al, Ca, Mg, etc) by this method are much lower than for trace metals.
The user of these results should consider them semi-quantitative with
errors of about 50 percent in some cases (but less in others). Another
method, ICP-AES, provides more reliable results for major elements
(Crock and others, 1999).

3.  Based on standards and replicate samples, the precision for trace
    metals (such as As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Se, V, and
    Zn) appears to be about 10 percent (1 standard deviation) for normal
    water concentrations, and possibly 20 percent at very high
    concentrations. The accuracy of determinations on these trace metals
    seems to be about 10 percent for normal concentrations. The
    precision and accuracy for Fe, which generally is not a trace metal
    in these waters, is not as good as for most metals, and possibly is
    about 20 percent.

4\. Some trace metals pose special problems. Mercury is analyzed, but
the Hg is not stable unless preserved by special methods (Crock, 1996),
thus for water samples the Hg originally present was not present in the
analyte (the analysis is valid, but the sample is not). Silver, a
potentially toxic trace metal, should be high in many of the water
samples, but analytical results for 1996-97 samples are very low and are
not considered reliable. Results for Se, Te, and Tl appear to be
reasonable but no standards contained these elements; precision is about
the same as for other trace elements

5.  These ICP-MS results are deemed satisfactory for the objectives of
    this study, the classification or ranking of water compositions.
    This sampling and analysis protocol has been used for five years in
    Arizona, Nevada, and Colorado AML studies and appears to be
    reproducible from day to day, year to year, at a total error of
    about 20 percent for most trace elements. Replicate analyses show
    that the composition of these acidified water samples change by less
    than a few per cent after a year of storage at room temperature (D.
    Fey, USGS, oral communication, 1999). Many replicate samples have an
    error of about 10 percent, and a few have a higher error.

These error numbers may be better understood by examination of figure 1.
The error bars show that an analysis for a metal with a concentration of
about 10,000 ppb is reliably different from one having 1,000 ppb or 100
ppb. Figure 1 shows the magnitude of 5, 10, 20, and 30 percent error
bars over a wide range of hypothetical metal compositions. Many of the
trace metal determinations (eg. Cu, Pb, Zn) have error bars like those
shown for 10 percent, a few (Cd) may be closer to the 5 percent example,
and some (eg. Fe) are like the 20 percent example. This plot shows that
even with high analytical error (20 or 30 percent) the reported values
for about 10,000 ppb do not overlap those of a sample with 1,000 or
100,000. The ICP-MS results are appropriate for comparison of water
compositions spanning 4 to 6 orders of magnitude (1 to 1,000,000 ppb).

> ![](media/image2.wmf)Figure 1: Diagram of analytical error for
> hypothetical metal concentrations spanning six orders of magnitude.

**REFERENCES CITED**

Briggs, P.H., 1996, Forty elements by inductively coupled plasma-atomic
emission spectrometry, *in* Arbogast, B.F., ed., Analytical methods
manual for the Mineral Resource Surveys Program, U.S. Geological Survey
Open-File Report 96-525, p. 77-94.

Briggs, P.H., and Meier, A.L., 1999, Determination of forty two elements
in geological materials by inductively coupled plasma-mass spectrometry:
U.S. Geological Survey Open-File Report 99-166, 15 p.

Crock, J.G., 1996, Mercury, *in* Methods of Soil Analysis, Part 3,
Chemical Methods: Soil Science Society of America, Book Series 5, p.
769-791.

Crock, J.G., Arbogast, B.F., and Lamothe, P.J., 1999, Laboratory methods
for the analysis of environmental samples, *in* Plumlee, G.S., and
Logsdon, M.J., eds., The Environmental Geochemistry of Mineral Deposits,
Part A.: Society of Economic Geologists, Reviews in Economic Geology,
Vol. 6A, p. 265-287.

Curry, J.K., 1996, Total sulfur by combustion, *in* Arbogast, B.F., ed.,
Analytical methods manual for the Mineral Resource Surveys Program, U.S.
Geological Survey Open-File Report 96-525, p. 106-110.

Fey, D.L., Desborough, G.A., , and Church, S.E., 2000, Comparison of two
leach procedures applied to metal-mining related wastes in Colorado and
Montana and a relative ranking method for mine wastes, Proceedings Fifth
International Conference on Acid Mine Drainage: Denver, Society for
Mining, Metallurgical and Exploration, p. 1477-1488.

Ficklin, W.H., and Mosier, E.L., 1999, Field methods for sampling and
analysis of environmental samples for unstable and selected stable
constituents, *in* Plumlee, G.S., and Logsdon, M.J., eds., The
Environmental Geochemistry of Mineral Deposits, Part A.: Society of
Economic Geologists, Reviews in Economic Geology, Vol. 6A, p. 249-264.

Lamothe, P.J., Meier, A.L., and Wilson, Stephen, 1999, The determination
of forty four elements in aqueous samples by inductively coupled
plasma-mass spectrometry: USGS Open-File Report 99-151, 14 p.

Mee, J. S., Siems, D.F., and Taggart, J. E., 1996, Major element
analysis by wavelength dispersive X-ray fluorescence spectroscopy, *in*
Arbogast, B.F., ed., Analytical methods manual for the Mineral Resource
Surveys Program, U.S. Geological Survey Open-File Report 96-525, p.
135-142.

Motooka, Jerry, 1990, Organometallic halide extraction applied to the
analysis of geologic materials for 10 elements by inductively coupled
plasma-atomic emission spectrometry, in Arbogast, B.F., Quality
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Open-File Report 90-668, p. 92-96.

Nash, J.T., Miller, W.R., McHugh, J.B., and Meier, A.L., 1996,
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Arizona: a preliminary assessment of environmental effects: U.S.
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Nash, J.T., 1999, Geochemical investigations and interim recommendations
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Nash, J. T., 2000, Hydrogeochemical data for historic mining areas,
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Wandless, G.A., 1996, Instrumental neutron activation analysis by
abbreviated count, *in* Arbogast, B.F., ed., Analytical methods manual
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