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

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

Department of the Interior DONALD PAUL HODEL, Secretary

Dallas L. Peck, Director

Free on application to the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225

II

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 minus-60-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 stream-sediment reconnaissance for uranium in the seven western states under the National Uranium Resource Evaluation Program (Leach, 1977). Both the minus-30 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).

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.

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. 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.

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.

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).

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.

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 volatiles rising from oxidizing minerals may help to detect concealed deposits (Hinkle, 1986; Hinkle and Dilbert, 1984; Lovell and others, 1980, 1983).

FIGURE 3.-Carbon disulfide in soil gases at the North Silver Bell deposit, near Tucson, Arizona (from Hinkle and Dilbert, 1984, p. 331).

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.

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 heavy-mineral concentrates. Theobald and Allcott (1973) compared five size fractions of stream sediments (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 heavy-mineral 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 deep-rooted phreatophytes, such as mesquite and paloverde. The roots of mesquite and paloverde may reach

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).

Optical emission spectrography is the most common method of analysis of geochemical samples 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, 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.

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 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 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 stream-sediment 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. 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 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.

FIGURE 7.-Flow sheet for separation of sediment samples into magnetic (M) and nonmagnetic (NM) fractions (from Meyer and others, 1979, p. 427).

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 of their ability to identify mineral deposits will lead to fuller utilization of partial dissolution techniques in geochemical exploration (Chao, 1984).

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).

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).

The Inyo Mountains Wilderness Study Area (CDCA-122, fig. 9) is located in the BLM California

s

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.

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 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).

\ \ J \

ftJS~;;G SILVER

A portion of the nonmagnetic fraction of the panned heavy-mineral concentrate samples was retained for mineralogical identification under the

11aooo·

microscope. Trace-element concentrations in the analyzed portions of the nonmagnetic samples

corresponded well with the mineralogical composition that was identified in the samples. Therefore, the 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 heavy-mineral 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 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 rock-forming 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.

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).

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 generally encompasses terrain lying south of Foshay Pass and north of Interstate Highway 40.

Minus-80-mesh stream sediments, the nonmagnetic (at 1.0 ampere) fraction of heavy-mineral concentrates, and rocks were used in the study area. All samples were analyzed by emission spectrography.

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

gold, arsenic, bismuth, cadmium, antimony, zinc, and In addition, the stream sediment and rock samples mercury. were analyzed by atomic absorption procedures for 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).

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 Highway 318 (fig. 16).

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 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 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). of one

potential or more of the

for epithermal (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).

resource potential in this area was based primarily on samples of minus-80-mesh stream sediment and the nonmagnetic 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.

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. The South Egan Range Wilderness Study Area

Geochemical assessment of the metallic mineral

1.0 ampere) fraction of (at

One region of previously identified mineralization

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.