U.S. GEOLOGICAL SURVEY CIRCULAR 997 

Geochemical Sampling in Arid 

Environments by the 

U.S. Geological Survey 

Geochemical Sampling in 

Arid Environments by the 

U.S. Geological Survey 

By Margaret E. Hinkle 

U.S. GEOLOGICAL SURVEY CIRCULAR 997 

This report discusses geochemical 

sample media and describes case 

histories leading to the selection 

of nonmagnetic heavy-mineral 

concentrates for mineral resource 

potential evaluation studies by the 

U.S. Geological Survey 

1988 
Department of the Interior 

DONALD PAUL HODEL, Secretary 

U.S. Geological Survey 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Hinkle, Margaret E. 

Geochemical sampling in arid environments by the U.S. Geological Survey. 

(U.S. Geological Survey circular; 997) 

Bibliography: p. 

Supt. of Docs. no.: I 19.4/2:997 

1. Geochemical prospecting-United States. 2. Arid regions-United 

States. I. Title. II, Series. 

TN270.H56 1988 622'.13'0973 87-600210 

Free on application to the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 
CONTENTS 

Abstract ·--------------------------------------------

Introduction ------------------------·-- ·--· · • • • • • • • • • 

Previous investigations -----.- •• -------.---------------

Stream sediments ------••• --------.--------------

Heavy-mineral concentrates -------------·---------

Soils -- -- --- -- -- - - - -- -- --- - - - - - - --- -- -• • - -• • • • • • 

Waters ------• • • ·---· • • • ·----·-• · • · • • • • • ·----·-• 

Vegetation ----- --- -- -- --------- · • • -• • • ·- ----- ·-

Other sample media ---.-.--------- •• ------------

Comparison of media ----- ----- ---- - -- -- -- -------

Sample preparation and analysis -----------.------------

Preparation-----------------------------------·-

Standard anal)1ical techniques --------------- ------

Page Page 

1 Sample preparation and analysis-Continued 

1 Partial-ex1raction techniques ·-------------------- 10 

2 Rock analysis storage system -·-·-· · • • ---- ----- ·-- • ·-- - 13 

2 Case histories from the U.S. Geological Survey ---------- 13 

3 Inyo Mountains Wilderness Study Area -·---------- 13 

4 Nopah and Resting Spring Wilderness Study Areas - 18 

5 Indian Pass and Picacho Peak Wilderness Study Areas 18 

5 Kelso Dunes Wilderness Study Area ·------------- 19 

6 South Providence Mountains Wilderness Study Area 19 

7 Riordan's Well Wilderness Study Area ------------ 22 

8 South Egan Range Wilderness Study Area -------- - 22 

8 Conclusions - -• - - - - - - - - -• -• • • - -• - - - - - - - - • - -- - - - - - - - - 23 

9 References -----------------------------·-------·--- 23 

ILLUSTRATIONS 

Page 

FIGURE 1. Average yearly precipitation in western North America -----------------·---------------------------------- 2 

2. Reconnaissance ex-ploration using stream-sediment samples as contrasted to detailed ex-ploration using soil samples 5 

3. Carbon disulfide in soil gases at the North Silver Bell deposit ------------·-----------------·--------------- 6 

4. Distribution of total copper in stream sediment -----------------·-· ·----------- ·-----------------·--- ·--- 9 

5. Distribution of total copper in panned concentrates-----------------·--·------·-------------------·------- 10 

6. Distribution of total copper in vegetation ash -------------------------• ·-·-·------------·---------------- 11 

7. Flow sheet for separation of sediment samples into magnetic and nonmagnetic fractions -- ·-------·------------ 12 

8. Flow chart for the chemical fractionation of sediments and outcrop samples- -- ·-·-------- • ------ -- --·-·------ 15 

9. Location of the Inyo Mountains Wilderness Study Area, Inyo County, California-----·---··--------------·---- 15 

10. Anomalous elements in stream-sediment samples, Inyo Mountains Wilderness Study Area, Inyo County, 

California - - - - -- - - --- -- - - - -- -- -• - - - - - - - --- -- -- - - - - - --- - - - - - --- -- --- -- -- - - - -• -• - - - - --- -- - - - - - - - - -- - 16 

11. Anomalous elements in the nonmagnetic fraction of panned heavy-mineral concentrates, Inyo Mountains 

Wilderness Study Area, Inyo County, California---- •• ----------------·-----·-------------------·------ 17 

12. Location of the Nopah and Resting Spring Wilderness Study Areas, San Bernardino, California----------------- 18 

13. Location of the Indian Pass and Picacho Peak Wilderness Study Areas, Imperial County, California----·-------- 19 

14. Location of the Kelso Dunes Wilderness Study Area, San Bernardino County, California----------------------- 20 

15. Location of South Providence Mountains Wilderness Study Area, San Bernardino County, California------------ 21 

16. · Location map of Riordan's Well and South Egan Range Wilderness Study Areas, White Pine, Lincoln, and Nye 

Counties, Nevada---··-···----------------------------------·------------------------------------- 22 

TABLES 

Page 

TABLE 1. Copper and molybdenum content of soils and plants collected on or near Prospect hill, Pima County, Arizona ------ 8 

2. Limits of determination for the spectrographic analysis of rocks and stream sediments------------- ·---------·- - 13 

3. Chemical methods used by the Branch of Geochemistry ------------------·--------·---·-----------------·-- 14 

III 

Geochemical Sampling in Arid Environments by the 

U.S. Geological Survey 

By Margaret E. Hinkle 

ABSTRACT 

The U.S. Geological Sutvey (USGS) is responsible for the 

geochemical evaluations used for mineral resource assessments of 

large tracts of public lands in the Western United States. Many of 

these lands are administered by the Bureau of Land Management 

(BLM) and are studied to determine their suitability or nonsuitability for wilderness designation. 

Much of the Western United States is arid or semiarid. This 

report discusses various geochemical sample media that have been 

used for evaluating areas in arid environments and describes case 

histories in BLM wilderness study areas in which stream-sediment 

and heavy-mineral-concentrate sample media were compared. As a 

result of these case history studies, the nonmagnetic fraction of 

panned heavy-mineral concentrates was selected as the most 

effective medium for reconnaissance geochemical sampling for 

resources other than gold, in arid areas. Nonmagnetic heavymineral-concentrate samples provide the primary analytical 

information currently used in geochemical interpretations of 

mineral resource potential assessment of BLM lands. 

INTRODUCTION 

The arid region of the United States includes most 

parts of the states of Arizona and Nevada, as well as 

parts of Oregon, Idaho, Utah, Colorado, New Mexico, 

and California. This area is characterized by receiving 

less than 10 inches of annual rainfall (fig. 1 ). It 

encompasses 700,000 square miles, much of which lies 

within the Basin and Range physiographic province. 

The adjacent semiarid lands receive 10-20 inches of 

annual rainfall. Most of this arid and semiarid land is 

managed by the Bureau of Land Management 

(BLM), and is being evaluated for mineral resource 

potential under the Federal Land Policy and 

Management Act of 1976 (Public Law 94-579). 

Under this law, the BLM must review all public lands 

under its jurisdiction and determine their suitability or 

nonsuitability for wilderness designation. Areas 

recommended as suitable for wilderness designation 

1 

by the BLM are also required to have mineral surveys 

conducted by the U.S. Geological Survey (USGS) and 

the Bureau of Mines (Beikman and others, 1983). 

Most exposed mineral deposits in this vast region 

were discovered long ago by prospectors. Additional 

exposed and near-surface deposits have been 

discovered more recently by geologists using geologic 

mapping techniques, geophysical techniques, and 

drilling. Consequently, exploration emphasis has 

shifted from the search for more easily discovered · 

deposits to the search for environments that might be 

favorable for concealed mineral deposits. A variety of 

techniques are used in the search for concealed 

mineral deposits, and for evidence of environments 

favorable for mineral deposits, with increasing 

importance placed on the role of geochemical 

prospecting methods. 

Geochemical exploration for mineral deposits 

includes any method of mineral exploration based on 

the systematic measurement of one or more chemical 

properties of a naturally occurring material, such as 

the trace-element analysis of samples of rocks, soils, 

and stream sediments (Rose and others, 1979). In arid 

environments, the sample media selected, in addition 

to rocks, have generally been stream sediments and 

heavy-mineral concentrates. The same geochemical 

techniques and sample media that are used in 

exploration for favorable mineral environments are 

used in the mineral resource assessment of public 

lands carried out by the U.S. Geological Survey. 

This report discusses sample media that have been 

selected by geochemists for exploration in arid 

environments, and the reasoning behind those 

selections. Sample media used, and the preparation 

and analysis of these media, are also described. The 
KEY TO 

TION 

(INCHES) 

< 10 

10 to 20 

> 20 

------r---

1 

I 

I 

I 

___ I 

500 MILES 

500 KILOMETERS 

FIGURE I.-Average yearly precipitation in western North America. (Modified from E. C. Jaeger, 1957, 

p. 14.) 

effectiveness of stream-sediment and heavy-mineral 

concentrate sample media are compared in case 

histories that resulted from studies in BLM wilderness 

study areas in the Western United States. The report 

discusses the rationale behind the selection of sample 

media for BLM mineral resource assessments. 

2 

PREVIOUS INVESTIGATIONS 

STREAM SEDIMENTS 

Stream sediments generally represent the rock 

material eroded from within a drainage basin. Trace 
elements contained within the lattice of rock-forming 

minerals and within minerals formed . during 

weathering probably make up the greatest proportion 

of the . .element variation in most stream sediments 

(Meyer and others, 1979). Material that is transported 

by moving water has been the most common geochemical sampling medium used in arid regions, 

despite the fact that most stream channels are dry 

most of the time. Chemical weathering of rocks is 

generally not significant in arid areas because of the 

lack of rainfall. Therefore, transport of weathered 

material away from bedrock sources is largely 

mechanical, and only occurs during infrequent 

rainfalls. 

The sediment material most often chosen for geochemical exploration is the minus-SO-mesh fraction 

(material smaller than 0.1S mm), although the minus60-mesh fraction (less than 0.25 mm) and even the 

very fine minus-200-mesh fraction (less than 0.075 

mm) have been used. 

Stream sediments can be transported for long 

distances by flash floods and therefore can indicate the 

presence of mineralized rocks far from the source. 

However, elements of geochemical interest in the 

finer fractions described above may be so diluted by 

quartz, feldspar, and other components of unmineralized rock also present in these fine sediments, that 

the upstream source of mineralization may not be 

detected by chemical analysis of the sediment. 

Wind-blown dust can also dilute fine sediments. 

Griffitts and Cooley (197S) noted that analyses of the 

minus-200-mesh fractions of stream-sediment 

samples from the Sheeprock Mountains in Utah did 

not yield as much evidence for the presence of nearby 

beryllium as did analyses of the plus-200-mesh 

fractions. They attributed the lower beryllium 

concentrations in the minus-200-mesh material to 

dilution of the samples by wind-blown dust. 

Coarser sediment fractions have been used 

successfully in geochemical exploration programs in 

desert areas. In the southeastern desert of Egypt, 

Soliman (19S1, 19S2) successfully used the minus-1-

mm fraction of sediments collected from dry arroyos 

in surveys for copper alone, and for copper, gold, tin, 

and niobium mineralization. The 1S- to 35-mesh (0.5-

to 1.0-mm) size fraction was also selected for streamsediment reconnaissance for uranium in the seven 

western states under the National Uranium Resource 

Evaluation Program (Leach, 1977). Both the minus30 to plus-SO-mesh (0.1S- to 0.60-mm) and minus-10-

to plus-30-mesh (0.60- to 2.0-mm) fractions of stream 

sediments have been successfully used in geochemical 

reconnaissance studies in Saudi Arabia (Overstreet, 

197S; Cheeseman and Thekair, 1979). 

3 

Bugrov (1974) found that different kinds of geochemical sampling techniques could be successfully 

applied to prospecting for ore deposits in the eastern 

desert of Egypt. He found that by removing the 

minus-0.25-mm fraction of stream sediments, which is 

mostly eolian material, the elemental concentrations 

of tin, molybdenum, and copper in the minus-1.0 to 

plus-0.25-mm fraction of stream sediments were 

enriched. Panning to produce heavy-mineral 

concentrates also removed eolian material and 

concentrated minerals that contained tin, tungsten, 

tantalum, niobium, and zirconium. 

Coarse alluvium collected from dry washes and 

sieved to minus-2-mm was used by El Shazly and 

others (1977) to delineate beryl and tourmaline 

occurrences in the southeastern desert of Egypt. The 

elements beryllium, lithium, and boron were detected 

in the minus-2-mm sediment and correlated well with 

the chemistry of the underlying bedrock. However, the 

elements copper, nickel, lead, zinc, and cobalt were 

either not detected or were only detected in very low 

concentrations in alluvium, even though these 

elements were present in higher concentrations in the 

underlying bedrock; the loss of these elements was 

attributed to leaching by waters containing chloride 

and sulfate ions. 

HEAVY-MINERAL CONCENTRATES 

Panned heavy-mineral-concentrate samples 

derived from stream sediments have been used for 

many years in geochemical reconnaissance for ore 

deposits. The panned concentrates may be chemically 

analyzed "as is" after panning, or they may be further 

separated by density (using heavy liquids such as 

bromoform) and (or) separated on the basis of 

magnetic susceptibility into different magnetic and 

nonmagnetic fractions. 

Heavy-mineral concentrates have two advantages 

over stream sediments in arid environments: (1) 

Diluting minerals such as quartz and feldspar have 

been removed in the panning process, and (2) The 

concentrate sample can be separated into different 

fractions, each of which may be studied under the 

microscope to confirm the presence of ore-related 

minerals determined by chemical analysis. 

Examination of the sample under a binocular 

microscope is useful to confirm the presence of 

minerals that are indicated by the chemical 

composition of the sample. Microscopic examination 

is also necessary to determine the presence of 

contaminants, such as lead shot, scrap metal, and 

other metallic artifacts, that will cause false metal 

anomalies in samples. 
Analyses of panned heavy-mineral concentrates "as 

is" and after removal of magnetite have been 

successfully used for geochemical reconnaissance in 

Saudi Arabia (duBray and Doebrich, 1981; Woolf, 

1983). Ayalon and others (1981) separated heavymineral concentrates from stream-sediment samples 

collected near the Gulf of Aqaba into seven magnetic 

fractions. The element content of each magnetic 

fraction correlated with the mineralogical content of 

the fraction, which, in turn, could be related to 

bedrock type. 

In a study of mineralization in northern Sonora, 

Mexico, both a minus-80-mesh stream sediment and 

material for a heavy-mineral-concentrate sample were 

collected at more than 1,200 sites. Heavy minerals 

were separated from the panned concentrate using 

bromoform and were then further separated into 

several magnetic and nonmagnetic fractions using an 

isodynamic separator. The fraction that was nonmagnetic at 0.6 amperes was analyzed and used for 

geochemical interpretation because this fraction 

contained most of the primary and secondary ore 

minerals and was found to be geochemically 

independent of rock type (Turner and others, 1982). 

The USGS began extensive use of heavy-mineralconcentrate samples following the geochemical study 

of the Hillsboro and San Lorenzo quadrangles in 

southwestern New Mexico (Alminas and others, 

1975a, b, c; Alminas and Watts, 1978; Hedlund, 1979; 

Watts and others, 1979). In this study, the concentrate 

samples were separated into fractions that were nonmagnetic and magnetic at 1.0 ampere. The nonmagnetic fraction contained light-colored, high-density 

accessory minerals, such as zircon, sphene, clinozoisite, and apatite, and, in mineralized areas, primary 

and secondary ore minerals, such as fluorite, galena, 

barite, scheelite, cassiterite, pyrite, chalcopyrite, 

molybdenite, and wulfenite. High metal concentrations in the nonmagnetic fraction occurred 

where primary and secondary ore minerals were 

exposed and were being mechanically transported to 

the stream bed. 

The magnetic fraction from this area contained 

secondary hydrous iron and manganese oxides derived 

from fracture fillings and joint coatings. This material 

was mechanically transported to the stream bed. In 

mineralized areas, the magnetic fraction contained 

metal-rich oxides that were believed to result from 

processes associated with mineralization and later 

weathering. 

Under certain geologic conditions, the most 

magnetic fraction of stream sediments (detrital 

magnetite) can be useful as a supplemental sample 

medium. Lovering and Hedal (1987) collected detrital 

magnetite with a hand magnet from dry sediment at 

several localities in southwestern New Mexico. The 

samples contained magnetite derived from the gangue 

of pyrometasomatic or hydrothermal base metal, 

tungsten, and tin deposits in carbonate host rock, and 

accessory magnetite derived from felsic plutonic rocks 

that are genetically associated with these deposits. 

Magnetite from these environments commonly 

contains abnormally high concentrations of ore 

metals. Analysis of the samples revealed geochemical 

anomalies that could be traced downdrainage for 

several miles. 

Overstreet and Day (1985) surveyed the literature 

on the use of magnetic concentrates in geochemical 

exploration. In their review, the term "magnetic 

concentrates" referred to those minerals in panned 

concentrates that have a magnetic permeability 

considerably greater than one, a positive magnetic 

susceptibility, and that are attracted to a permanent 

magnet. These characteristics permit the magnetic 

fraction to be separated simply from the less magnetic 

components of concentrates. Magnetic concentrates 

are recovered readily by hand magnet from alluvial 

sediments or panned concentrates. They have been 

used successfully in exploration as a geochemical 

sample medium for copper, zinc, cobalt, chromium, 

molybdenum, niobium, vanadium, tin, and beryllium, 

particularly in arid environments where alluvial 

sediments may be contaminated by eolian debris. 

However, according to Overstreet and Day, the 

ferromagnetic fraction of raw concentrates has been 

found to be less effective in general regional 

applications than either of the fractions of the raw 

concentrates magnetic at 0.6 ampere or nonmagnetic 

at 0.6 ampere. The ferromagnetic concentrate is a 

special-purpose medium. It may be the most suitable 

geochemical sample medium for the platinum-group 

metals where native alloys of these elements are in 

micron-size particles that defy gravity concentration. 

In the search for titanium and vanadium in deposits of 

titaniferous magnetite, the ferromagnetic concentrate 

appears to be the best geochemical sample medium. 

SOILS 

Desert soils tend to be very thin, with poorly 

developed profiles. The trace element composition 

corresponds generally to the underlying parent 

material (Lovering and McCarthy, 1978). Because 

soils are derived from a more restricted source area 

than are alluvial samples, desert soils are more 

suitable for studies of selected deposits than for 

reconnaissance geochemical sampling (fig. 2). In 

addition, the usefulness of soil samples is limited to 
3 KILOMETERS 

0 Background sediment sample 

• Anomalous sediment sample 

• Background soil sample 

e Anomalous soil sample 

~Ore 

FIGURE 2.-Model of reconnaissance exploration using streamsediment samples as contrasted to detailed exploration using 

soil samples. (Figure modified from Rose and others, 1979, p. 

3.) 

areas of low to moderate topographic relief because of 

problems in interpreting analyses of transported 

material. Another potential problem arises in 

sampling strongly wind-winnowed soils; high-density 

minerals, such as zircon and magnetite, may become 

enriched in these soils due to the removal of lower 

density minerals (Petrov, 1976, p. 271). 

In spite of the potential problems described above, 

Snoep and Zeegers (1979) successfully used soil 

samples for a detailed study of the Socos deposit in 

Peru. Analyses of the soil samples showed that 

molybdenum was the most reliable element to 

indicate the location of primary mineralization in this 

area of 10-25 rom annual rainfall. In addition, multielement analyses of the samples enabled these 

researchers to forecast the size of the deposit. 

WATERS 

The use of surface water for geochemical sampling 

is rarely possible in desert areas. However, wherever 

springs, seeps, or wells are prevalent, water may prove 

to be a good sample medium to use to complement 

other geochemical sampling media. Analyses of 

waters collected from springs were useful for geochemical assessment of mineral resource potential in the 

Riordan's Well and South Egan Range Bureau of 

Land Management Wilderness Study Areas in Nevada 

(Hofstra and others, 1984; Rowan and others, 1984), 

because anomalous element concentrations in the 

water samples correlated well with anomalies in 

stream sediments and panned heavy-mineral 

concentrates. 

5 

Because interpretation of data resulting from multicomponent water analyses may not be 

straightforward due to various hydrologic and lithologic effects on the groundwater, factor analysis is 

often used to relate data from the water analyses to 

the geologic setting. Factor analysis is a statistical 

technique for resolving a large number of elements 

into a smaller number of components. These 

components can be examined for significance in terms 

of geological processes, types of samples, or other 

geological and geochemical information (Rose and 

others, 1979). For example, factor analysis was used 

successfully to show that element concentrations in 

water samples from deep irrigation wells are spatially 

related to a deeply buried porphyry copper deposit 

near Casa Grande, Arizona (Nowlan and others, 

1981). 

VEGETATION 

Deep-rooted trees and plants can be useful sample 

media in desert environments because they can 

assimilate elements dissolved in groundwater flowing 

through concealed ore deposits, as long as the water 

comes into contact with the ore minerals and then the 

roots. 

El Shazly and others (1971) made use of twigs, 

stems, and roots of two species of acacia trees to locate 

sulfide mineralization in the eastern desert of Egypt. 

In this area, analyses of copper, nickel, zinc, lead, and 

cobalt in acacia twigs yielded better geochemical 

anomalies than did analyses of corresponding 

alluvium samples. No differences in element 

concentrations were seen for the two species of acacia. 

Although no elemental differences were seen for 

the two species of tree in the Egyptian example, 

different plant species, as well as different parts of a 

plant (for example, leaves, twigs, or roots) may 

accumulate different concentrations of elements. 

Therefore, the same plant species and the same part of 

the plant should be used for biogeochemical sampling 

in a given area. Selection of a single plant species that 

is uniformly present throughout a reconnaissance area 

may be difficult, and pilot studies will be required to 

select the best two or three species for the area. Plants 

should be sampled within a relatively short period of 

time, because of seasonal variations in the plants. 

Chaffee and Hessin (1971) used soils and the leaves 

and stems of creosote, ironwood, and foothill paloverde to locate the concealed Vekol porphyry copper 

deposit southwest of CasaGrande, Arizona. Although 

anomalous concentrations of copper and molybdenum 

in soils located the buried deposit, the concentrations 

of the~e elements were much higher in the ash of plant 
parts, and, in addition, the plant anomalies covered a 

larger area because the roots of the plants passed 

through barren alluvium and reached the buried 

deposit. 

Chaffee (1976) conducted a biogeochemical survey 

over the Mineral Butte copper deposit, Pinal County, 

Arizona. Species collected were mesquite, catclaw 

acacia, blue paloverde, and ironwood. The first three 

of these species are phreatophytes that have deep and 

extensive root systems that may reach the permanent 

water table. Ironwood is not a true phreatophyte but 

grows larger in this area near stream channels where 

water is commonly available throughout the growing 

season. Results of analyses of ash from the vegetation 

were compared to results of analyses of soil and rock 

samples collected in the same area. Some conclusions 

from this study were as follows: 

1. The relative abundances of copper, zinc, and 

molybdenum in the rocks and soils of the study 

area were not directly reflected by plants growing in 

the same materials. 

2. The concentration of a given element varied with 

the plant species and plant part sampled. 

3. All elements were not necessarily enriched in the 

same part of a given plant species. 

4. Samples that contained anomalous concentrations 

of an element in the ash of both leaves and stems of 

a plant species were generally more reliable 

indicators of significant anomalies than were 

samples containing anomalous concentrations in 

only one part of the plant. 

5. The Mineral Butte copper deposit was best located 

in this biogeochemical survey using the distribution 

of copper anomalies in plant ash. 

6. The copper deposit was best located using analyses 

from mesquite samples. Catclaw acacia analyses 

were almost as effective; blue paloverde analyses 

may also be useful. Analyses from ironwood were 

much less effective in locating the deposit. 

OTHER SAMPLE MEDIA 

The sample media discussed previously are the 

most commonly used for geochemical exploration and 

mineral resource evaluation. However, other media 

are used on an experimental basis and should be 

mentioned. Data from these unconventional media 

are generally used to supplement more conventional 

data. 

Soil gases are the most commonly used nonroutine 

medium. The samples are analyzed for carbon 

dioxide, oxygen, helium, sulfur compounds, and 

hydrocarbons. Anomalous concentrations of these 

6 

32° 

25' 

30" 

0 1h 

I··· ,, I 'I I I 

0.5 

Approximate outline 

of North Silver 

Bell ore body 

I 

1 KILOMETER 

1 MILE 

I 

EXPlANATION 

Sample site 

Carbon disulfide in head-space gas 

Range: not detected at lower limit 

of detection (50 ppb)-1000 ppb 

L .. :··:J ~1oo ppb 

FIGURE 3.-Carbon disulfide in soil gases at the North Silver Bell 

deposit, near Tucson, Arizona (from Hinkle and Dilbert, 

1984, p. 331). 

volatiles rising from oxidizing minerals may help to 

detect concealed deposits (Hinkle, 1986; Hinkle and 

Dilbert, 1984; Lovell and others, 1980, 1983). 

Interstitial gases between soil grains are collected 

by pounding a hollow probe about 0.5-meter into the 

ground, inserting a hypodermic needle through the 

septum of an air-tight fitting attached to the probe, 

and withdrawing the gas with a hypodermic syringe. 

Soil-gas samples are generally analyzed by gas 

chromatography or mass spectrometry. 

Gases adsorbed on soils are another type of sample. 

This sample utilizes the soil as a natural trap to adsorb 

gases rising from below. Adsorbed gases are removed 

from soil by heating samples in closed containers. The 

gas over the soil in the container (the headspace gas) 

is removed by inserting a hypodermic needle through 

a septum in the cap of the container and withdrawing 

the gas with a hypodermic syringe. These gas samples 

are also analyzed by gas chromatography or mass 

spectrometry. 

Soil samples were collected over the North Silver 

Bell copper deposit near Tucson, Arizona (Hinkle and 

Dilbert, 1984). Analyses of the volatile constituents 

derived from the soils showed that anomalous 

concentrations of helium, carbon disulfide, and sulfur 

dioxide occurred over the ore body; whereas, 

anomalous concentrations of carbon dioxide and carbonyl sulfide occurred over the alteration zones 

around the ore body. Figure 3 shows the carbon 

disulfide content of gases derived from soils. 
Both soils and soil-gas samples were collected 

along three traverses across the copper-zinc deposit at 

Johnson Camp in southeastern Arizona. The ore 

occurs as isolated sulfide and oxidized sulfide 

replacement bodies in limestone host rock and is 

covered by 10 to 225 meters of alluvium. Samples 

were analyzed for helium by mass spectrometry and 

for carbon dioxide and sulfur compounds by gas 

chromatography. Anomalous concentrations of 

carbon dioxide and helium in soil gases and 

anomalous concentrations of carbon dioxide degassed 

from soils were found over and adjacent to the ore 

bodies. Carbonyl sulfide and carbon disulfide 

degassed from soils were found in the vicinity of ore 

bodies occurring to within 150-meters depth, but not 

to 225-meters depth (Hinkle, 1986). 

Results of several soil surveys in the vicinity of 

concealed gold deposits show that an aerobic sporeforming soil bacterium, Bacillus cereus, often occurs in 

greater numbers or as a greater percentage of the 

Bacillus population in soils overlying mineralized 

bedrock than in adjacent, unmineralized terrain. 

These differences may be of use in locating concealed 

mineral deposits. Other studies of penicillin resistance 

in soil bacteria over two copper deposits indicate that 

B. cereus might thrive over many mineral deposits. 

The ability of B. cereus to resist antibodies produced 

by metal-tolerant fungi in the soil gives B. cereus an 

advantage over other bacterial species in metalliferous 

soils, and appears to be a general phenomenon that 

may have broad applicability in geochemical 

exploration (Parduhn and others, 1985). 

A procedure has been developed (Watterson, 

1985) to estimate the numbers of B. cereus spores in 

soils and stream-sediment samples. The test is 

currently under investigation for its potential in 

mineral exploration, especially for buried deposits. 

COMPARISON OF MEDIA 

Many reconnaissance geochemical sampling 

programs involve the collection of two or more sample 

media and the preparation of subsamples from some 

of the media. More than one kind of sample is 

commonly collected in a pilot program to determine 

the best sample type for the area, while at other times 

whole regions are sampled using different media in 

order to compare the geochemistry of the different 

media to different rock units, or to different types of 

mineral deposits. 

The two most common media collected for 

comparison are stream sediments and panned heavymineral concentrates. Theobald and Allcott (1973) 

compared five size fractions of stream sediments 

7 

(plus-10, minus-10 to plus-30, minus-30 to plus-80, 

minus-80 to plus-200, and minus-200 mesh) in a pilot 

study in Saudi Arabia. Concentrations of boron, 

barium, and nickel in sediments finer than minus-80-

mesh showed no variation, probably because of 

dilution by eolian sands. Analyses of the minus-10 to 

plus-30 size fraction distinguished rock types best. 

This sediment fraction and the nonmagnetic fraction 

of panned concentrates were selected as the two 

media for the study. The nonmagnetic fraction of 

panned concentrates was needed to detect 

molybdenum and tungsten anomalies m this 

environment. 

DuBray (1981) compared five ,media in a granitoid 

terrane in Saudi Arabia. The minus-10 to plus-30 and 

minus-30 to plus-80 fractions of stream sediments, the 

minus-10 to plus-30 and minus-30 to plus-80 fractions 

of the nonmagnetic portions of panned heavy-mineral 

concentrates, and the ferromagnetic portion of the 

panned heavy-mineral concentrates were compared in 

this study. The greatest element enhancements were 

in the minus-30 to plus-80 and minus-10 to plus-30 

fractions of the nonmagnetic portion of heavy-mineral 

concentrates. DuBray concluded that there was no 

need to sieve the nonmagnetic fraction and that the 

unsieved nonmagnetic fraction of stream sediments as 

well as rocks were the best sample media in this 

terrane. 

The minus-0.5-mm (minus-35-mesh) fraction of 

stream sediment and a heavy-mineral concentrate 

were selected by the Bureau of Land Management 

(BLM) for 1,250 sample sites in portions of the 

California Desert Conservation Area. Results of semiquantitative spectrographic analyses of these samples 

by the U.S. Geological Survey were used by the BLM 

to classify these areas for geology, energy, and mineral 

(GEM) resources (Lambie and others, 1983). 

Bugrov and Shalaby (1975) compared several 

sample media in a geochemical survey in the eastern 

desert of Egypt. They determined that either coldextraction analyses of the minus-0.075-mm fraction or 

spectrographic analyses of the minus-1.0-mm fraction 

of stream sediments was superior to panned heavymineral concentrates or bedrock samples for 

reconnaissance surveys in this area. 

Huff (1970) sampled vegetation, soil, and well 

water around alluvium-covered copper deposits in 

Pima County, Arizona. The objective of the work was 

to find sample media that could "see" through about 

50 meters of alluvium to the deposits below. The 

vegetation sampled consisted of shallow-rooted 

xerophytes, such as cacti and grass, and the deeprooted phreatophytes, such as mesquite and paloverde. The roots of mesquite and paloverde may reach 
TABLE 1.-Copper and molybdenum content of soils and plants collected on or near Prospect 

hill Pima County, Arizona 

[Table from Huff, 1970, table 2. All values are in parts per million) 

Samples collected on hill Samples collected on alluvium 20- 200 feet !rom foot of hill 

Sample description Copper 

SoiL ___________________ 40, 40, 40, 

40, 60, 

60, 60, 

80, 100, 

120, 120, 

100, 300, 

400. 

Plants (after ashing) : Mesquite, leaves _____ 300, 450, 

600. 

Mesquite, twigs _____ 120, 120, 

150. 

Palo verde __________ 50, 70, 100. Grass ______________ 50, 100. 

J urn ping cholla ______ 30, 70. 

Prickly pear ________ 15, 30. 

White thorn ________ 20, 70. Ocotillo ____________ 300. 

the permanent ground-water table approximately 80 

meters below the ground surface in this area. Samples 

were collected on and around a small mineralized hill, 

referred to as "Prospect hill" in table 1, to compare 

results from analyses of the various sample media. 

Except when collected directly over mineralized 

bedrock, soil samples did not contain significant 

quantities of copper, or of molybdenum, which is 

derived from molybdenite associated with primary 

copper ore. The ash of mesquite leaves and twigs 

contained higher concentrations of copper and 

molybdenum than did the ash of other vegetation 

collected in the same area. 

Mesquite samples collected up to 12 kilometers 

away from the copper deposits showed the greatest 

molybdenum concentrations in the same areas where 

ground water had high molybdenum concentrations, 

which suggested that most of the molybdenum in 

mesquite was derived from the ground water rather 

than from the alluvium. Huff concluded that plueatophytes, such as mesquite, and ground waters were 

the best sample media for copper exploration in the 

Sonoran Desert environment of southern Arizona. 

Chaffee and others (1976) compared untreated 

panned concentrate samples with minus-60-mesh 

(minus-0.25-mm) stream sediments and with the 

leaves and stems of mesquite and oak trees found in 

arroyos near and downstream from La Caridad 

porphyry copper deposit in Sonora, Mexico. These 

samples were analyzed by various techniques for as 

many as 37 elements. Although the minus-60-mesh 

stream sediment was considered to be the best 

8 

Molybdenum Copper Molybdenum 

1, 1, 1, 1, 10, 10, 10, 1, 1, 1, 1, 1, 

1, 1, 1, 1, 10, 15, 1, 1, 1, 1, 

1, 2, 2, 2, 15, 15, 2, 2, 2, 2, 

2, 5 .. 15, 15, 2, 5. 

15, 15, 

15, 15, 

20, 20. 

30, 30, 45. 60, 80, 120, 14, 15, 20, 

120. 30. 

8, 10, 15. 80, 80, 120, 6, 6, 10, 14. 

120. 

2, 3, 6. 20, 30, 50. 2, 2, 6. 

5, 6. 30, 50. 5, 8. 

2, 2. 30, 30. 1, 3. 

1, 1. 10, 15. 1, 2. 

3, 4. 20, 30. 3, 8. 

8. ------------------------

medium, any of the three media (stream sediments, 

panned concentrates, vegetation) were found to be 

adequate for locating this deposit (figs. 4-6). 

SAMPLE PREPARATION AND ANALYSIS 

PREPARATION 

The two sample media most commonly used for 

mineral resource assessment by the USGS are stream 

sediments and panned heavy-mineral concentrates. By 

convention, the stream sediments are usually sieved to 

minus-80-mesh. 

The panned heavy-mineral samples are first dried, 

then subjected to separation by bromoform (see fig. 7 

for method of separation). Bromoform (specific 

gravity 2.80-2.89) separates the remaining quartz, 

feldspar, micas, and other low-density accessory 

minerals as well as clays and organics from the highdensity heavy minerals which may contain ore 

minerals. The resulting heavy-mineral sample is 

separated into three fractions using a large 

electromagnet (in this case, a modified Frantz Isodynamic Separator1 

). The most magnetic material, 

primarily magnetite, is not analyzed. The second 

fraction, largely ferromagnesian silicates and iron 

oxides, is saved for analysis and archival storage. The 

third fraction (the least magnetic material contains 

zircon, sphene, and nonmagnetic ore minerals such as 

1Use of trade names in this report is for descriptive purposes 

only and does not constitute endorsement by the U.S. Geological 

Sutvey. 
1 09°40' 

30°,-----------------------------------------------------------------~~~---------

22' 

30" 

30° 

STREAM SEDIMENT 

SPECTROGRAPHIC COPPER 

0 ~200 PPM 

• ~150 PPM 

10,000 20,000 FEET 

2500 5000 METERS 

07'~------------------------------------------------------------------_L __________ _ 

30" 

FIGURE 4.-Distribution of total copper in stream sediment, La Caridad porphyry copper deposit, Sonora, Mexico (from Chaffee and 

others, 1976, fig. 6). 

metallic sulfides and their oxidation products) is split 

into two fractions. One half is hand ground for spectrographic analysis; the other half is saved for 

mineralogical analysis. The magnetic separates 

prepared by this procedure are the same separates 

that would be produced by using a Frantz Isodynamic 

Separator set at a slope of 15° and a tilt of 10° with a 

current of 0.1 ampere to remove the magnetite and 

ilmenite, and a current of 1.0 ampere to split the 

remainder of the sample into paramagnetic and nonmagnetic fractions (fig. 7). 

STANDARD ANALYTICAL TECHNIQUES 

Optical emission spectrography is the most 

common method of analysis of geochemical samples 

9 

by the USGS. In the spectrographic method used, a 

10-mg or smaller sample is vaporized in a directcurrent electric arc (Grimes and Marranzino, 1968). 

Spectrographic results are obtained by visual 

comparison of spectra derived from the sample 

against spectra obtained from standards made from 

pure oxides and carbonates. Standard concentrations 

are geometrically spaced over a given order of 

magnitude of concentration as follows: 100, 50, 20, 10, 

and so forth. Samples whose concentrations are 

estimated to fall between those values are assigned 

values of 70, 30, 15, and so forth. The precision of the 

analytical method is approximately plus or minus one 

reporting interval at the 83-percent confidence level 

and plus or minus two reporting intervals at the 

96-percent confidence level (Motooka and Grimes, 
109°40' 109°20' 30°,-------------------------------------------------------------------~-----------

22' 

30" 

PANNED CONCENTRATE 

SPECTROGRAPHIC COPPER 

0 ~300 PPM 

• ~200 PPM 

10,000 20,000 FEET 

2500 5000 METERS 

30° 

07'~-----------------------------------------------------------------L--~----~ 

30" 

FIGURE 5.-Distribution of total copper in panned concentrates, La Carl dad porphyry copper deposit, Sonora, Mexico (from Chaffee and 

others, 1976, fig. 12). 

1976). Table 2 shows the elements analyzed and their 

lower and upper limits of determination. 

Wet chemical analytical methods, especially those 

for atomic absorption analysis, are also frequently 

used to supplement the emission spectrographic 

analyses. Table 3 lists the elements analyzed and 

chemical methods commonly used. Inductively 

Coupled Plasma (ICP) analytical methods are also 

used. 

PARTIAL-EXTRACTION TECHNIQUES 

In many instances, the lower limits of analytical 

determination for elements in rock and 

stream-sediment samples are too high for 

reconnaissance exploration and for exploration for 

10 

blind ore deposits (Aiminas and Mosier, 1975). The 

contrast between anomalous and background 

concentrations of metals can often be improved by 

comparing concentrations of metals that can be 

chemically leached from the surface of the sample, 

rather than by using an analysis of the total sample. 

Higher concentrations of surficially bound 

elements occur more frequently in humid 

environments, where chemical leaching is more 

prevalent, than in arid environments. However, 

partial-extraction analysis can be used to determine 

the concentration of elements adsorbed or loosely 

bound on the surface of sediments, soils, and rocks 

collected in arid environments. 

Copper in stream sediments and panned 

concentrates, collected near La Caridad porphyry 

copper deposit in Sonora, Mexico, was determined by 
1 09°40' 1 09°20' 30°,---------------------------------------------------------------------~-----------. 

22' 

30" 

VEGETATION (ASH) 

SPECTROGRAPHIC COPPER 

@ Leaves and stems anomalous 

0 Leaves or stems anomalous 

• Neither part anomalous 

10,000 20,000 FEET 

2500 5000 METERS 

30° 

07'L---------------------------------------------------------------------~----------~ 

30" 

FIGURE 6.-Distribution of total copper in vegetation ash, La Caridad porphyry copper deposit, Sonora, Mexico (from Chaffee and 

others, 1976, fig. 16). 

a cold-extractable method using a biquinoline 

extraction of copper leached from the sample with 6N 

hydrochloric acid (Ward and others, 1963). Results of 

these partial-extraction analyses gave anomaly 

patterns identical to the patterns for total copper 

analyses of these same media, although the 

magnitudes of the partially extracted copper 

concentrations were lower (Chaffee and others, 1976). 

One of the most popular partial-extraction 

techniques used by the USGS is the oxalic-acid leach 

of Alminas and Mosier (1975). They determined that 

the dried residues remaining after boiling the streamsediment samples with 1.5 N oxalic acid yielded 

analytical data equal in sensitivity and contrast to data 

obtained from heavy-mineral concentrate samples. 

The trace metals are derived from leached iron and 

manganese oxides in the dried residue sample. 

11 

Increasing clay content progressively dilutes the 

quantity of metals leached from a sample; therefore, 

the method may not produce useful data from clayrich samples. Calcium-rich samples are not usable 

because ore-related metals may co-precipitate with 

calcium oxalate and thus be removed from the leachate (Alminas and Mosier, 1975). 

Secondary iron and manganese oxides are 

significant in geochemical exploration. These oxides 

commonly occur as coatings on rocks, as concretions, 

and as colloidal-sized particles in stream sediments 

and soils, and the oxides strongly scavenge many 

ore-related metals in the weathering zone. These 

secondary oxides and their associated metals can be 

partitioned into different mineral phases and chemical 

forms by selective extraction techniques. Knowledge 

of the distribution of metals in various weathering 
products is valuable for selecting sample media and 

extraction procedures, and in helping evaluate the 

significance of analytical data (Chao and Theobald, 

1976). 

Filipek and Theobald (1981) subjected samples of 

minus-80-mesh stream sediments from the North 

Silver Bell copper deposit near Tucson, Arizona, to a 

series of chemical extractions originally designed to 

separate different weathering fractions of samples 

from humid environments. The following fractions are 

effectively separated by this extraction scheme: (1) 

carbonates and exchangeable metals, (2) manganese 

oxides, (3) organic compounds and sulfides, (4) 

hydrous iron oxides, and (5) residual crystalline 

minerals (fig. 8). Jarosite and chrysocolla, two major 

minerals found in the North Silver Bell area, were 

found to dissolve over two or more steps of the 

extraction scheme. Results of these extractions 

showed that caution must be used when applying 

partial solution techniques to investigate geochemical 

partitioning in an arid (or semiarid) environment. The 

attribution of unique phases, such as manganese 

DRAINAGE 

BASIN 

COMMONLY OBSERVED MINERALS 

LIMONITE 

HORNBLENDE 

BIOTITE 

GARNET 

SPHENE 

EPIDOTE 

oxides or organic compounds, to a given extraction can 

lead to incorrect interpretations of weathering 

processes. Copper, lead, and zinc were extracted by 

acetic acid from samples near the mineralized zone, 

suggesting the occurrence of hydromorphic processes 

within the stream sediments in this area. In contrast, 

the residual fractions of the sediment samples gave the 

longest dispersion trains away from the deposit, 

suggesting that mapping total metal concentrations is 

most effective for reconnaissance surveys in arid 

environments for locating metal deposits. 

Theuseofpartialdissolutiontechniquesingeochemical exploration is expanding in the USGS. Through 

understanding and application of appropriate selective 

and specific extractions, the effects on metal 

distribution caused by mineralization and those 

resulting from lithological and environmental factors 

can be differentiated. Knowledge gained from the use 

of partial dissolution techniques aids in the 

interpretation of chemical analytical data in relation to 

the geochemistry of the area sampled. Further 

development of these techniques and demonstration 

COMMONLY OBSERVED MINERALS 

UNMINERALIZED AREAS 

ZIRCON 

BARITE 

SPHENE 

APATITE 

IN MINERALIZED AREAS 

FLUORITE 

CASSITERITE 

GALENA 

PYRITE 

WULFENITE 

VANADINITE 

SCHEELITE 

OTHER OXIDIZED ORE 

MINERALS 

FIGURE 7.-Flow sheet for separation of sediment samples into magnetic (M) and nonmagnetic (NM) 

fractions (from Meyer and others, 1979, p. 427). 
TABLE 2.-Limits of determination for the spectrographic analysis of rocks and stream sediments, 

based on a 10-mg sample 

[The spectrographic limits of determination for heary-mineral-concentrate samples are two reporting units higher than the limits 

given for rocks and stream sediments] 

Elements Lower determination limit Upper determination limit 

Percent 

Iron (Fe) 

Magnesium (Mg) 

Calcium (Ca) 

Ti tani urn (Ti) 

0.05 

.02 

.05 

.002 

20 

10 

20 

1 

Parts per million 

Manganese (Mn) 

Silver (Ag) 

Arsenic (As) 

Gold (Au) 

Boron (B) 

Barium (Ba) 

Beryll i urn (Be) 

Bismuth (Bi) 

Cadmium (Cd) 

Cobalt (Co) 

Chromium (Cr) 

Copper (Cu) 

Lanthanum (La) 

Molybdenum (Mo) 

Niobium (Nb) 

Nickel ( Ni) 

Lead (Pb) 

Antimony (Sb) 

Scandium (Sc) 

Tin (Sn) 

Strontium (Sr) 

Vanadium (V) 

Tungsten (W) 

Yttrium (Y) 

Zinc (Zn) 

Zirconium (Zr) 

Thori urn (Th) 

10 

0.5 

200 

10 

10 

20 

1 

10 

20 

5 

10_ 

5 

20 

5 

20 

5 

10 

100 

5 

10 

100 

10 

50 

10 

200 

10 

100 

of their ability to identify mineral deposits will lead to 

fuller utilization of partial dissolution techniques in 

geochemical exploration (Chao, 1984). 

ROCK ANALYSIS STORAGE SYSTEM 

Upon completion of all analytical work, the 

analytical results are entered into a computer-based 

file called Rock Analysis Storage System (RASS). 

This data base contains both descriptive geological 

information and analytical data. Any or all of this 

information may be retrieved and converted to a 

binary form for computerized statistical analysis or 

publication (VanTrump and Miesch, 1977). 

13 

5,000 

5,000 

10,000 

500 

2,000 

5,000 

1,000 

1,000 

500 

2,000 

5,000 

20,000 

1,000 

2,000 

2,000 

5,000 

20,000 

10,000 

100 

1,000 

5,000 

10,000 

10,000 

2,000 

10,000 

1,000 

2,000 

CASE HISTORIES FROM THE U.S. 

GEOLOGICAL SURVEY 

Results of sample media studies in arid 

environments by the USGS confirm the results 

described under "Previous Investigations." The case 

histories described here are results of mineral 

resource potential assessment studies of public lands 

proposed for wilderness designation by the Bureau of 

Land Management (BLM), especially in the 

California Desert Conservation Area (CDCA). 

INYO MOUNTAINS WILDERNESS STUDY AREA 

The Inyo Mountains Wilderness Study Area 

(CDCA-122, fig. 9) is located in the BLM California 
TABLE 3.-Chemical methods commonly used by the U.S. Geological Sutvey Branch of Geochemistry 

[AA = atomic absorption; I = instrumental; SI = specific ion; S = spectrophotomeuy; and F = fluorometry] 

Element or Determination limit 

constituent Sample Method (micrograms/ Analyst Reference 

determined Type gram or ppm) 

Gold (Au) AA 

Mercury (Hg) 

Arsenic (As) AA 

Antimony (Sb) AA 

Zinc (Zn) AA 

Bismuth ( Bi) AA 

Cadmium (Cd) AA 

Cobalt (Co) AA 

Copper (Cu) AA 

Lead (Pb) AA 

Nickel (Ni) AA 

Zinc (Zn) AA 

Fluorine (F) SI 

Indium (In) AA 

Tha 11 i urn ( T1 ) 

Molybdenum (Mo) AA 

Tell uri urn (Te) AA 

Tin ( Sn) AA 

Tungsten (W) s 

Uranium (U) F 

Desert Conservation Area, on the eastern flank of the 

Inyo Mountains between Owens and Saline Valleys, 

west of Death Valley National Park in Inyo County, 

California. Stream sediments, sieved to minus-80-

mesh, and the nonmagnetic (at 1.0 ampere) fraction of 

the panned heavy-mineral concentrate were used as 

sample media. These samples were analyzed by 

emission spectrography. 

0.05 Thompson 

and others, 

1968 

0.02 Modification 

of McNerney 

and others' 

1972, and 

Vaughn-;-Fnd 

McCarthy, 

1964. 

5 or 10 Modification of 

V1 ets, 1978 

2 

5 

1 

0.1 

5 Ward, and 

others, 1969. 

5 

5 

5 

5 

100 Hopkins, 1977. 

14 

0.2 Hubert and 

Lakin, 1973. 

Modification of 

Rao, 1971. 

0.1 Chao and 

others, 1978 

Welsch and 

Chao, 1976. 

0.5 or 1 Welsch, 1983. 

0.05 or 1 Modification of 

Centann1 and 

others, 1956. 

For the most part, stream sediments collected in 

the Inyo Mountains Wilderness Study Area (WSA) 

and contiguous areas did not contain trace elements 

known to be associated with mineralized rocks, in high 

enough concentrations to be detected by the analytical 

methods used, even when the samples were collected 

below mines and in known mineralized areas. These 

disappointing results probably occurred for several 
HOAc 

LEACH WITH 

C-Hxl 0.1 M NH20H: HCI 

IN O.Ql M HN03 

CENTRIFUGE 

HzOz 

H-Hxl 

Residual 

25°C, 

112 hr. 

CARBONATES 

EXCHANGEABLE METALS 

HYDROUS MN OXIDES 

(EASILY REDUCIBLE) 

ORGANIC COMPOUNDS, 

SULFIDES 

HYDROUS FE OXIDES 

(MODERATELY REDUCIBLE) 

RESIDUAL CRYSTALLINE 

FRACTION 

FIGURE 8.-Flow chart for the chemical fractionation of 

sediments and outcrop samples (from Filipek and 

Theobald, 1981, p. 160). 

reasons, among them the loss of some resistant 

refractory ore minerals that might have been retained 

in the discarded plus-80-mesh stream sediments, and 

excessive dilution of the stream sediments by windblown sediment to the point where chemical detection 

of ore and ore-related elements was virtually 

impossible. The nonmagnetic fraction of the panned 

heavy-mineral concentrates, however, was very 

effective for detecting trace elements and thus 

delineating mineralized areas. Figures 10 and 11 

compare analytical results for the two sample media 

(from J. E. Kilburn and D. E. Detra, written commun., 

1985). After removal of the rock-forming silicates, 

clay minerals, and organic material, the content of ore 

and ore-related elements was enhanced to the point 

where it could be measured by the spectrographic 

method used. Furthermore, the variation in dilution 

by sedimentary processes was lessened. 

In addition to the above studies, a set of samples 

from 40 sites in the Inyo Mountains WSA and nearby 

areas was selected for further analyses. The samples 

consisted of 40 minus-80-mesh stream sediments and 

40 samples each of three fractions of heavy-mineral 

concentrates: (1) the most magnetic fraction (largely 

15 

118°15' 118°00' 117°45' 

INYO 

NATIONAL 

FOREST 

APPROXIMATE 

BOUNDARY OF 

STUDY AREA 

SALINE 36° VALLEY 45' 

10 15 KILOMETERS 

FIGURE 9.-Location of the Inyo Mountains Wilderness Study 

Area, Inyo County, California. 

magnetite), (2) the fraction containing mostly ferromagnesian minerals and iron oxides, and (3) the 

fraction nonmagnetic at 1.0 ampere. Emission spectrographic analyses were performed on all the 

samples. In addition, ICP analyses were performed on 

the stream sediments, the magnetic fraction, and the 

ferromagnesian mineral fraction of the concentrates. 

Atomic absorption analyses were also performed on 

the stream sediments. 

Results of the analyses showed that the emission 

spectrographic, atomic absorption, and ICP analyses 

of the stream sediments yielded data that were 

essentially equivalent for the purpose of geochemical 

interpretations. However, neither the stream 

sediments nor the ferromagnesian minerals, 

regardless of analytical treatment, indicated the 

presence of elemental dispersion trains away from a 

metallic source. 

The most useful data for geochemical 

interpretation came from the spectrographic analyses 

of the nonmagnetic material. These data indicated 

elemental dispersion trains away from metallic 

sources. 

The most magnetic, or the magnetite fraction, of 

the concentrate samples did not yield data useful for 

geochemical interpretations in this area. Gold was not 

detected by any analytical procedure in any of the 

sample media, although some of the sample sites were 
-------~ 

I 

I 

I 

I 

I 

---.....1 

\ 

\ 

~Ba \ . \lSl 

\ KEYNOTx. 

\ MINE 

\ 

\ 

\ 

\ 

\ 

\ 

.. 

\ 

\ 

\ 

J ........ ---""'\ --· ~-- \ 

\ 

\ 

\ 

" ' '\· 

\ 

\-

\ 

\ 

\ 

BEVERIDGE MINING 

DISTRICT 

\. 

\ 

I 

\ 

\ 

'-... 

' \ . \. 

~§JAg 

ftJS~\;;G SILVER 

MINE 

" ............. __ ............ 

............ . .......... 

" " I 

-........ I " : ( 

"-........ \ 

.......... .................... ' ~ .......... \ ). '-.. I( ............ __ ........... .) 

"' , ___ _...... // 

EXPLANATION 

Anomalous element not represented in partitioned 

hexagon 0 Cerro Gordo Element represented by blackened segment of 

partitioned hexagon 

Sample 

6 KILOMETERS 

6 MILES 

36° 

30'~------------------------------------------------------------~------~ 

FIGURE 10.-Anomalous elements in stream-sediment samples, Inyo Mountains Wilderness Study Area, California (from J. 

E.Kilburn and D. E. Detra, written commun., 1986). 

located in areas where a possibility existed that gold 

might be detected in the samples (J. E. Kilburn and D. 

E. Detra, written commun., 1985). 

16 

A portion of the nonmagnetic fraction of the 

panned heavy-mineral concentrate samples was 

retained for mineralogical identification under the 
11aooo· 11 r45' ,-------------------------------------------------------------------,---------, 

\ 

\ 

\ 

\\ KEYNOTx 

\ MINE 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

.. BEVERIDGE MINING 

DISTRICT 

' 

\ 

\ 

\ 

\ 

\ 

\ 

........ 

EXPlANATION 

Anomalous element not represented in partitioned 

hexagon 

Element represented by blackened segment of 

partitioned hexagon 

Sample 

6 KILOMETERS 

0 Cerro Gordo 

6 MILES 

36°L-----------------------------------------------------------~------~ 30' 

FIGURE H.-Anomalous elements in the nonmagnetic fraction of panned heavy-mineral concentrates, Inyo Mountains 

Wilderness Study Area, California (from J. E. Kilburn and D.E. Detra, written commun., 1986). 

microscope. Trace-element concentrations in the 

analyzed portions of the nonmagnetic samples 

17 

corresponded well with the mineralogical composition 

that was identified in the samples. Therefore, the 
50 100 MILES 

50 100 KILOMETERS 

FIGURE 12.-Location of the Nopah and Resting Spring Wilderness Study Areas, Inyo County, California. 

nonmagnetic heavy-mineral concentrates were used as 

the sample medium for the geochemical appraisal. 

NOPAH AND RESTING SPRING WILDERNESS STUDY 

AREAS 

The Nopah and Resting Spring Wilderness Study 

Areas (CDCA-145, CDCA-150) consist of the 

central part of the Nopah Range, which is located east 

of Death Vaiiey National Park, California, between 

California State Highway 127 and the CaliforniaNevada boundary (fig. 12). Minus-80-mesh stream 

sediments and the nonmagnetic (at 1.0 ampere) 

fraction of panned heavy-mineral concentrates were 

collected for the geochemical reconnaissance of the 

area. In this study, the minus-80-mesh sediments were 

res am pled later in the same year. All samples were 

analyzed by emission spectrography. Strong element 

anomalies in the nonmagnetic heavy-mineral 

concentrate samples delineated an area of mineralized 

rocks near the site of an old mine. Element anomalies 

in the minus-80-mesh stream sediments collected in 

the spring of the year were weak, but they also 

18 

delineated the same area of mineralized rocks. 

However, the minus-80-mesh stream sediments 

collected in the fall of the same year did not delineate 

the area of mineralized rocks. Replicate analyses of 

selected samples from both sets of sediments 

confirmed previous results. The discrepancy in 

analytical results between minus-80-mesh 

stream-sediment samples collected in spring and fall 

of the same year may be due to differences in 

composition of the sediments between spring and fall 

field seasons; hydromorphic enhancement of the 

metal content during winter runoff could have 

increased metal contents of the spring samples; 

however, wind-blown dust could have diluted the fall 

samples (C. L. Smith, written commun., 1983). 

INDIAN PASS AND PICACHO PEAK WILDERNESS STUDY 

AREAS 

The Indian Pass (CDCA-355) and the adjacent 

Picacho Peak (CDCA-355A) Wilderness Study Areas 

are located in easternmost Imperial County, 

California, about 25 miles ( 40 km) north-northwest of 

Yuma, Arizona (fig. 13). 
CALIFORNIA 

Study 

area 

20 

20 

ARIZONA 

40 MILES 

40 KILOMETERS 

FIGURE 13.-Location of the Indian Pass and Picacho Peak 

Wilderness Study Areas, Imperial County, California. 

Sample media used in these two areas were the 

minus-80-mesh stream sediments, the nonmagnetic 

(at 1.0 ampere) fraction of heavy-mineral 

concentrates, and rocks. All the sample media were 

analyzed by emission spectrography. In addition, the 

stream sediments and the rocks were analyzed by wet 

chemical procedures. 

Analysis of the nonmagnetic fraction of heavymineral concentrates derived from stream sediments 

proved to be the most useful in delineating areas of 

possible mineralization within the WSA's. A plot of 

anomalies for barium, strontium, tungsten, bismuth, 

gold, silver, and arsenic in heavy-mineral concentrates 

showed two areas within the Indian Pass WSA with 

distinctive multielement geochemical signatures. High 

concentrations of tungsten and bismuth occur in the 

northwestern quadrant of the WSA, where alteration 

and mineralization seem to be localized near the 

contact of the Orocopia Schist and an intruding 

granite. The northeastern quadrant of the WSA was 

characterized by high concentrations of arsenic, 

barium, antimony, strontium, and boron in nonmagnetic heavy-mineral concentrates; mineralization 

here is thought to be related to hot-spring activity 

localized near faults. 

Analysis of minus-80-mesh stream sediments 

proved to be of minimal value for this study. Stream 

19 

sediments in the northeastern quadrant of the Indian 

Pass WSA showed anomalous arsenic. Gold was not 

detected in the sediment samples. Apparently, 

dilution by quartz, feldspar, and other common rockforming minerals was so great that elements related to 

mineralization were not anomalous in this sample 

medium (Smith and others, 1987). 

Many rock samples contained anomalous elements. 

Spectrographic analyses showed high concentrations 

of arsenic, copper, and zinc; however, atomic 

absorption analyses indicated high concentrations of 

arsenic, gold, silver, antimony, zinc, and tungsten in 

rocks. 

KELSO DUNES WILDERNESS STUDY AREA 

The Kelso Dunes Wilderness Study Area 

(CDCA-250, fig. 14) lies in central San Bernardino 

County, California, about 60 miles west of the 

Colorado River. 

Yeend and others (1984) collected samples from 

sites on the dunes and from sites in dry washes around 

the dunes. A raw sand sample and a sand sample for 

heavy-mineral concentration were collected at each 

site. Four sites were sampled at each dune: the 

windward face, dune crest, slip face on the leeward 

side, and trough between dunes. All samples were 

analyzed by emission spectrography. In addition; 

selected samples were analyzed for gold by atomic 

absorption procedures. 

The geochemical populations for all elements 

within the concentrate samples showed little 

variability. Analyses of the raw sands showed 

somewhat greater variability than the concentrate 

analyses; however, statistical analyses of these data did 

not identify any geochemical associations 

characteristic of economic mineralization. 

Sand samples from dune crests generally contained 

higher concentrations of iron, titanium, manganese, 

cobalt, chromium, niobium, vanadium, yttrium, and 

zinc than did samples collected on other parts of the 

dunes. These elements were determined to be 

associated with magnetite, which was observed to be 

enriched on many dune crests, and to be unrelated to 

any mineral resource (Yeend and others, 1984). 

SOUTH PROVIDENCE MOUNTAINS WILDERNESS STUDY 

AREA 

The South Providence Mountains Wilderness 

Study Area (CDCA-262, fig. 15) is located in San 

Bernardino County, southeastern California, in the 

southern part of the Providence Mountains. The area 
KELSO MOUNTAINS 

3(}(XJ 

10 15 MILES 

10 15 KILOMETERS 

FIGURE 14.-Location of the Kelso Dunes Wilderness Study Area, San Bernardino County, California. 

generally encompasses terrain lying south of Foshay 

Pass and north of Interstate Highway 40. 

Minus-80-mesh stream sediments, the nonmag20 

netic (at 1.0 ampere) fraction of heavy-mineral 

concentrates, and rocks were used in the study area. 

All samples were analyzed by emission spectrography. 
MAP LOCATION 

Kelso Dunes 

south== Pr-~~id:~;e 

Mitchell Caverns 

State Park 

Fountain Peak X 

Mountains 

Wilderness Study Area 

(CDCA-262) 

GRANITE 

6 KILOMETERS 

EXPLANATION 

Boundary of wilderness study area 

Boundary of geologic mapping 

6 MILES 

" ~ 

~ 

~ 

~ 

==,!) 

Paved road 

Dirt road 

Colton Hills 

Clipper 

Valley 

CLIPPER 

MOUNTAINS 

FIGURE 15.-Location of South Providence Mountains Wilderness Study Area, San Bernardino County, California. 

In addition, the stream sediment and rock samples 

were analyzed by atomic absorption procedures for 

gold, arsenic, bismuth, cadmium, antimony, zinc, and 

mercury. 
In some parts of the area, both stream sediments 

and concentrates contained anomalous concentrations 

of ore-related elements. For the stream sediments, 

areas interpreted as having highest resource potential 

were defined by a silver-arsenic-gold-bismuth-coppermercury suite of elements. Anomalous concentrate 

samples had a distinctive element suite of silver-goldcopper-lead-strontium (with or without barium, 

bismuth, and molybdenum). Some of these anomalies 

could be traced to old mines in the area. Other 

anomalies indicated potential for undiscovered 

mineral resources (Miller and others, 1984). 

RIORDAN'S WELL WILDERNESS STUDY AREA 

The Riordan's Well Wilderness Study Area 

(NV-040-166) is located in east-central Nevada 

approximately 12 miles southeast of the town of 

Currant, east of U.S. Highway 6 and west of Nevada 

I 

I 

I 

------; 

I 

I 

COUNTY 

Highway 318 (fig. 16). 38° 

Geochemical assessment of the metallic mineral 

resource potential in this area was based on samples 

of minus-80-mesh stream sediment, the nonmagnetic 

(at 1.0 ampere) fraction of the heavy-mineral 

concentrates, and spring and well waters. All the 

analytical data from these media were subjected to 

factor analysis. Samples of rock that showed evidence 

of mineralization were also collected and analyzed but 

were not subjected to factor analysis. 

All the sample media contained anomalous 

concentrations of metallic elements. Two areas in the 

wilderness study area were determined to have 

moderate potential for gold-tungsten mineralization 

such as occurs in the nearby Troy mining district. 

These areas are located along the southwestern 

margin and at the northwest boundary of the study 

area. The areas were outlined by anomalous 

concentrations of one or more of the 

elements-tungsten, tin, molybdenum, bismuth, lead, 

and zinc-in the nonmagnetic heavy-mineral 

concentrates; gold and lead in rocks; and arsenic in 

stream sediments. 

Two other areas, located in the south-central and 

north-central parts of the WSA, were determined to 

have moderate potential for epithermal 

precious-metal mineralization. These areas were 

outlined by anomalous concentrations of one or more 

of the following elements or constituents: zinc in 

heavy-mineral concentrates; gold, silver, arsenic, 

antimony, boron, zinc, copper, and molybdenum in 

rocks; arsenic, zinc, boron, and silver in stream 

sediments; and arsenic and sulfate ion in spring waters 

(Hofstra and others, 1984). 

22 

10 20 30 40 MILES 

I I I II I I I 

0 10 20 30 40 KILOMETERS 

FIGURE 16.-Location map of Riordan's Well and South EganRange Wilderness Study Areas, Lincoln, Nye, and White PineCounties, Nevada. 

SOUTH EGAN RANGE WILDERNESS STUDY AREA 

The South Egan Range Wilderness Study Area 

(NV-040-168) is located in east-central Nevada at the 

junction of White Pine, Lincoln, and Nye Counties, 

about 25 miles south of the town of Ely, and lying 

between Nevada Highway 318 on the west and U.S. 

Highway 93 on the east (fig. 16). 

Geochemical assessment of the metallic mineral 

resource potential in this area was based primarily on 

samples of minus-80-mesh stream sediment and the 

nonmagnetic (at 1.0 ampere) fraction of 

heavy-mineral concentrates. Supplemental information was obtained from analyses of spring and well 

waters. All data resulting from analyses of these media 

were subjected to factor analysis. Rock samples were 

also collected, mainly from outcrops showing evidence 

of mineralization; rocks were analyzed but not 

subjected to factor analysis. Geochemical anomalies 

in the South Egan Range WSA tended to be broadly 

distributed throughout the area, rather than clustered 

as in the nearby Riordan's Well WSA. 

One region of previously identified mineralization 

is located at the northern end of the WSA and includes 

the southern portion of the Ellison mining district 

where veins were mined for precious and base metals. 
A porphyritic intrusion with potential for 

molybdenum mineralization lies below the 

near-surface vein system. This region was defined by 

anomalous concentrations of one or more of the 

elements lead, copper, zinc, tin, molybdenum, and 

bismuth in concentrates. Only a few stream-sediment 

samples contained anomalous concentrations of 

elements in this region; these elements were silver and 

arsenic. Rock samples generally had anomalous 

concentrations of a large number of metallic elements, 

including silver, bismuth, cadmium, copper, 

molybdenum, tin, tungsten, lead, zinc, arsenic, and 

antimony. Spring water samples collected east of the 

study area contained anomalous lead and copper 

concentrations. 

A second area of known mineralization also occurs 

in the northern part of the WSA. This area has 

subeconomic disseminated gold and silver 

mineralization and was delineated by anomalous 

concentrations of copper, lead, zinc, and cobalt in 

concentrates and copper, arsenic, and nickel in the 

stream sediments (Rowan and others, 1984). 

CONCLUSIONS 

From this review of previous investigations it can be 

seen that the study of a nonmagnetic fraction of 

heavy-mineral-concentrate sample medium is the 

most effective method for locating metals, such as 

copper, lead, zinc, tin, and tungsten, in arid 

environments. The use of this medium has consistently 

enabled researchers to delineate areas of known and 

potentially mineralized rocks. Such identification of 

mineralized areas can be expected, provided that the 

mineralized rocks are exposed to erosion. 

Concentration of the heavy minerals by panning and 

then separation of the minerals by the use of bromoform and magnetic susceptibility isolates those 

minerals, such as metallic sulfides and their oxidation 

products, which can be related to mineralization. 

Stream sediments, in comparison, may or may not 

delineate even known and exposed mineralized areas. 

The dilution of sediment by other rock-forming 

minerals, such as quartz and feldspar, or by eolian 

dust makes stream sediments less effective than the 

nonmagnetic fraction of panned heavy-mineral 

concentrates for locating mineralization in arid 

environments. 

However, the relative success in locating basemetal mineralization with nonmagnetic concentrates 

does not imply that this is the only sample medium 

that should be used for exploration or mineral 

resource assessment. For example, gold may not be 

23 

detected in concentrate samples because the 

distribution of gold in nature is often heterogeneous. 

Therefore, in the search for gold deposits, all sample 

media collected should be analyzed for elements, such 

as arsenic, antimony, or mercury, which may be 

associated with gold. 

Partial-solution chemical techniques have been 

successfully used in various studies to extract orerelated metallic elements from stream sediments 

collected in arid areas, and to obtain data that are 

comparable to data obtained from nonmagnetic 

heavy-mineral concentrates. In some situations, 

partial-solution techniques might replace nonmagnetic heavy-mineral concentrates as a preferred 

sample medium for geochemical exploration in arid 

environments. 

For current BLM mineral resource assessment 

work, the U.S. Geological Survey relies heavily on data 

that are obtained from analyses of the nonmagnetic 

fraction of panned heavy-mineral concentrates. 

Supplemental information may also be obtained from 

analyses of other sample media, such as stream 

sediments, rocks, waters, plants, soils, soil gases, or 

from bacteria surveys. Use of the most effective 

sample media for the environment enhances the value 

of the mineral resource assessment. 

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