Geochemical data for gold-bearing rocks from the Goldfield mining district, Nevada
Geochemical data for gold-bearing rocks from the Goldfield mining district, Nevada by Ashley, Roger P. ashley@usgs.gov, Snow, Geoffrey Greacen (1991). Full…
Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.
UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY GEOCHEMICAL DATA FOR GOLD-BEARING ROCKS FROM THE GOLDFIELD MINING DISTRICT, NEVADA By Roger P. Ashley and Geoffrey G. Snow U.S. Geological Survey Open-File Report 91-614 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
TABLE OF CONTENTS Introduction 1 Methods of investigation 1 Sampling methods 1 Analytical methods 2 Characteristics of the samples 4 Results and discussion 4 References cited 16 Appendix-sample locations 19 LIST OF TABLES Table 1. Geochemical data for Goldfield silicified rocks 7-15 LIST OF ILLUSTRATIONS Figure 1. Map of Goldfield mining district, Nevada, showing locations of analyzed gold-bearing samples 3
INTRODUCTION The Goldfield mining district, Nevada, is located within a 48-km2 area underlain by hydrothermally altered Tertiary volcanic rocks, Mesozoic granitic rocks, and Paleozoic metasedimentary rocks (Ashley, 1974,1975). The bonanza gold ores of the district were found mainly in silicified zones in a 1.3-km2 area immediately northeast of the town of Goldfield, but silicified zones crop out throughout the altered area. Between July, 1966 and September, 1972, the U.S. Geological Survey sampled silicified zones in the entire altered area, excluding only the easternmost 8 km2, which lies within Department of Defense lands not open to mineral entry. This activity was one component of a comprehensive study of the geology and geochemistry of the Goldfield district. The main objective of the sampling program was to provide information on the geochemistry of silicified rocks throughout the altered area, as a guide to possible undiscovered ore bodies. Another objective was to look for lateral changes in the character of hydrothermal alteration within the altered area. The samples were analyzed by semiquantitative emission spectrography, by atomic absorption for gold and mercury, and by a colorimetric method for arsenic. Results for selected elements, including gold, silver, arsenic, bismuth, copper, molybdenum, mercury, and lead, were presented in a series of geochemical maps (Ashley and Keith, 1973a- h), and discussed in a report (Ashley and Keith, 1976). The results were also included in later summaries of geochemical investigations in the Goldfield mining district (Ashley and Keith, 1978a,b). Of the 1337 silicified-rock samples analyzed, 161 showed 100 ppb or more gold. We have reanalyzed 140 of these gold-bearing samples (those with sufficient remaining material) using modern methods for multiple elements (acid leach and analysis by inductively coupled plasma-atomic emission spectrometry) and gold (acid digestion and graphite-furnace atomic absorption spectrometry). These methods provide much lower detection limits for many elements than did our earlier analyses, allowing us to re-evaluate the suite of elements accompanying gold at Goldfield. In addition, we have obtained low-cost whole-rock analyses by a modern technique (borate fusion and acid dissolution, with analysis by inductively coupled plasma-atomic emission spectrometry). These data will allow us to compare minor element and major element data, for additional insights into mineralogical associations of various elements. The purpose of this report is to present analytical data for the 140 goldbearing samples. Data obtained in the 1966-1972 period are also included, because the reports cited give only summary statistics. METHODS OF INVESTIGATION SAMPLING METHODS At each sample locality we collected the most limonite-rich material available. Most samples consisted of composite chips from a 0.1- to 0.5-m2 area on the outcrop surface, but a few were grab samples (one piece). Sample size was 1-2 kg. Selection of sample sites was briefly discussed by Ashley and Keith (1976).
Because an adequate large-scale base map was not available during the sampling program, we recorded the sample locations on aerial photographs, and transferred these locations by inspection to an orthophoto mosaic. The orthophoto mosaic was fit to the recently-published l:24,000-scale P-series topographic maps (Goldfield, McMahon Ridge, and East of Goldfield quadrangles), and coordinates obtained with a digitizer, using the JKDIGIT program (Kork, 1986). The orthophoto appeared to fit these topographic maps reasonably well, but the locations have not been re-plotted directly on the new maps, so undetermined location errors are possible. The sample locations are shown on figure 1 and listed in the appendix. ANALYTICAL METHODS In the 1966-1972 period the samples were subjected to six-step semiquantitative emission spectrographic analysis (Grimes and Marranzino, 1968). Elements determined by this method (with lower detection limit in parentheses) include iron (0.05 percent), magnesium (0.02 percent), calcium (0.05 percent), titanium (0.002 percent), manganese (10-20 ppm), silver (0.5-1 ppm), arsenic (200- 2000 ppm), gold (10 ppm), boron (10 ppm), barium (20 ppm), beryllium (1 ppm), bismuth (10 ppm), cadmium (20-50 ppm), cobalt (3-5 ppm), chromium (1-10 ppm), copper (5 ppm), lanthanum (20-30 ppm), molybdenum (2-5 ppm), niobium (10-20 ppm), nickel (2-5 ppm), lead (10 ppm), antimony (100-200 ppm), scandium (5 ppm), tin (10 ppm), strontium (100 ppm), vanadium (10 ppm), tungsten (50-100 ppm), yttrium (5-10 ppm), zinc (200 ppm), and zirconium (10 ppm). Detection limits for arsenic, gold, bismuth, cadmium, antimony, tin, and tungsten are too high to yield useful data for most samples in this data set. Consequently gold was also determined by an atomic absorption method, using a cold hydrobromic acid-bromine extraction from 10-gram analytical portions (Thompson and others, 1968), with a reported sensitivity of 0.02 ppm. Arsenic was also determined by the Gutzeit colorimetric method (Ward and others, 1963), with a sensitivity of 10 ppm. Mercury was determined by a vapor atomic absorption method (Vaughn and McCarthy, 1964), with a sensitivity of 0.01 ppm. The recent analyses were performed in August, 1991, by a commercial laboratory, on powders prepared by U.S. Geological Survey. For minor element determination, a 0.5-gram portion was digested with HCl-HNOs solution at 95° C for one hour, diluted and introduced into a plasma (acid leach/ICP). Elements determined by this method (with lower detection limit in parentheses) include silver (0.1 ppm), arsenic (2 ppm), gold (2-3 ppm), boron (2 ppm), barium (2 ppm), bismuth (2 ppm), cadmium (1 ppm), cobalt (1 ppm), chromium (1 ppm), copper (1 ppm), lanthanum (2 ppm), manganese (1 ppm), molybdenum (1 ppm), nickel (1 ppm), lead (2 ppm), antimony (2 ppm), strontium (1 ppm), thorium (2 ppm), uranium (5 ppm), vanadium (2 ppm), tungsten (2 ppm), aluminum (0.01 percent), calcium (0.01 percent), iron (0.01 percent), potassium (0.01 percent), magnesium (0.01 percent), sodium (0.01 percent), phosphorus (0.01 percent), and titanium (0.01 percent). For whole rock determination, a 0.2-gram portion was fused with LiB02, dissolved in HNOs solution, and introduced into a plasma (borate fusion/ICP). Components determined by this method include Si02, Al20s, iron as Fe20s, CaO, MgO, Na20, MnO, Ti02, P205, Cr20s, barium, cobalt (10 ppm), copper (10 ppm), nickel (10 ppm), zinc (10 ppm), strontium (10 ppm), cerium (20 ppm), niobium (20 ppm),
3745' - 3742'30" - KILOMETERS Base from U.S. Geological Survey, Goldfield, McMahon Ridge, Mud Lake South, and East of Goldfield quadrangles, J& provisional editions, 1987 11715' nyiSO" 11710' 1177'30M Fig. 1. Map of Goldfield mining district, Nevada, showing locations of analyzed gold-bearing samples.
tantalum (20 ppm), yttrium (20 ppm), and zirconium (20 ppm). In addition, loss on ignition (LOI) was determined. The ICP methods are similar to those described by Lichte and others (1987). For gold, a 10-gram sample was ignited at 600° C, digested with hot aqua regia, extracted with methyl iso-butyl ketone (MIBK), and analyzed by graphite furnace atomic absorption spectrometry (GFAAS). The detection limit was 1 ppb. This method is described by Hall and Bonham-Carter (1988). CHARACTERISTICS OF THE SAMPLES The silicified zones at Goldfield are replacement bodies that formed along faults, fractures, and permeable beds that conducted hydrothermal fluids. They form prominent outcrops, termed "ledges" by Ransome (1909). The remainder of the hydrothermally altered area consists of argillized rocks. Additional zones, which symmetrically envelop the silicified zones, can be defined on the basis of mineral assemblages. The main constituent of most silicified rocks is fine-grained quartz. Alunite and kaolinite are common, preferentially replacing feldspar phenocrysts. Other less common hypogene replacement minerals include pyrophyllite, diaspore, and potassium mica. All silicified rocks contain minor amounts of anatase or rutile, and varying amounts of limonite (any combination of goethite, hematite, and sometimes jarosite. Limonite replaces former pyrite and coats fractures. It is a product of weathering-related oxidation, which generally extends to a depth of at least 10 meters throughout the altered area. Only a few samples contain minor relict pyrite. The fracture coatings represent oxidized pyrite-bearing veinlets, as well as iron that was leached, transported some distance, and reprecipitated during oxidation. About 25 percent of the samples contain gypsum, calcite, or smectite. These minerals were introduced after the main-stage alteration that produced quartz, alunite, kaolinite, pyrophyllite, diaspore, and pyrite, but they may partly or entirely precede oxidation. Most of the silicified samples contain alunite, pyrophyllite, or diaspore, so they are advanced argillic rocks, as defined by Meyer and Hemley (1967; see also Rose and Burt, 1979). Some rocks from the silicified zones, however, do not show advanced argillic mineral assemblages. These include intensely leached rocks consisting of vuggy quartz with minor rutile and limonite (usually closely associated with less-intensely leached advanced argillic rocks), argillic rocks consisting of quartz and kaolinite with limonite and minor anatase, and potassic rocks consisting of quartz, adularia, limonite, and minor anatase, with or without potassium mica. Locally the silicified zones are brecciated and cemented with relatively coarsegrained alunite and jarosite. This jarosite may be hypogene (Keith and others, 1979). The data set includes several samples of this material. RESULTS AND DISCUSSION Table 1 shows all geochemical data available for the 140-sample set of Goldfield silicified rocks, grouped by component. The emission spectrographic and borate fusion/ICP results should reflect total amounts of the elements determined, although analytical precision is much better for the ICP method than for the spectrographic method. For the components
determined by both these methods, results are generally similar, although for many elements, relatively high spectrographic detection limits prohibit a rigorous comparison. Exceptions occur (chromium, niobium) where amounts present in many samples are near detection limits for one or both methods. The acid leach/ICP method yields partial analyses, because it does not dissolve refractory minerals such as zircon, and only partly dissolves silicates. It is biased toward constituents of sulfides and limonite, which are mostly dissolved (Chao, 1984). In most samples similar values were obtained for iron by acid leach/ICP, borate fusion/ICP, and emission spectrography, indicating that iron is located mainly in goethite and hematite, which are effectively dissolved by the acid leach. In the minority of cases where acid leach/ICP showed notably less iron, jarosite is relatively abundant; apparently this sulfate is only partly dissolved. Other elements likely associated with iron in limonite and effectively leached include manganese, boron, silver, arsenic, bismuth, copper, nickel, molybdenum, antimony, and zinc, based on comparisons between acid leach/ICP results and borate fusion/ICP or emission spectrographic results, or both. Amounts of the major elements aluminum, sodium, and potassium determined by borate fusion/ICP are much larger than amounts determined by acid leach/ICP, indicating that the minerals containing these elements are not dissolved by the acid leach. Much of the aluminum resides in kaolinite, pyrophyllite, or diaspore, all of which are relatively refractory. In many samples significant aluminum is also present in alunite, which accounts for most of the potassium and sodium, indicating that alunite is not appreciably dissolved by the acid leach. Titanium, present in anatase or rutile, is untouched by the acid leach. Amounts of phosphorus, barium, and strontium determined by borate fusion/ICP are much larger than amounts determined by acid leach/ICP, indicating that their host minerals are relatively insoluble in the acid leach. Phosphorus may reside in minor aluminum phosphate-sulfate minerals that replace apatite or as a minor component in alunite (Stoffregen and Alpers, 1987). Barite accounts for much of the barium, and barium may also be a minor component of alunite. Celestite is a possible site for strontium, as well as aluminum phosphate-sulfate minerals and alunite. Similarly, emission spectrographic values for lead are large relative to acid leach/ICP values. Although some lead may reside in limonite, much of it substitutes for potassium in alunite, and some probably also substitutes for potassium in jarosite. Lanthanum and chromium are apparently in minor unidentified refractory phases not dissolved by the acid leach. Vanadium and cobalt values determined by emission spectrography and borate fusion/ICP, respectively, are somewhat higher than acid leach/ICP values, suggesting that some but not all vanadium and cobalt reside in limonite. Samples with high borate fusion/ICP values for cobalt often have low acid leach/ICP values; many but not all such samples are jarosite-bearing. Rocks with relatively large amounts of calcium contain calcite or gypsum, both of which are apparently dissolved by the acid leach. Where calcium levels are low, calcium-bearing minerals have not been identified, but calcium is still effectively dissolved by acid, suggesting that refractory sulfates are not important sites for calcium. The hydrated calcium silicate tobermorite has been reported from one silicified-rock locality at Goldfield (Harvey and Vitaliano, 1964).
Hydrothermal alteration reduced magnesium to relatively low levels in all the silicified rocks. Magnesium also appears to be mostly dissolved by the acid leach. Some is probably located in calcite, where calcite is present. Magnesium sulfate, if present, would be readily soluble. Cyanide leach/GFAAS values for gold average about 35 percent higher than values obtained by HBr-Br2/AAS, but there is much sample-to-sample variation in the size of the disparity between the two determinations. Ashley and Albers (1975), reporting on gold analyses for replicate samples, show that the nugget effect can be important in Goldfield silicified rocks, so the differences here may be due in part to sample inhomogeneity. Acid dissolution/ICP values show reasonably good agreement with those obtained by the AAS methods, but the data are not very useful owing to the high (generally 3 ppm) detection threshold. The emission spectrographic data for gold, with relatively low precision and a 10 ppm detection threshold, are of little use. Colorimetric values for arsenic are similar to acid leach/ICP values in many cases, but in some cases are markedly lower, especially for samples relatively rich in arsenic.
Table 1. Geochemical data for Goldfield silicified rocks. ppm=parts per million; ppb=parts per billion. Methods include: ICPf, borate fusion and inductively coupled plasma atomic emission spectrometry; ICPa, acid dissolution and inductively coupled plasma-atomic emission spectrometry; ES, emission spectrography; GFAAS, cyanide leach and graphite furnace atomic absorption spectrometry; HBr/AA, hydrobromic acid-bromine leach and atomic absorption spectrometry, C, Gutzeit colorimetry; I/AA, instrumental atomic absorption spectrometry. N=not detected at the detection threshold shown in parentheses; L=less than the detection threshold shown in parentheses; G=greater than the upper detection limit shown in parentheses; n.d.=not determined.
REFERENCES CITED Ashley, R. P., 1974, Goldfield mining district, in Guidebook to the geology of four Tertiary volcanic centers in central Nevada: Nevada Bureau of Mines and Geology Report 19, p. 49-66. Ashley, R. P., 1975, Preliminary geologic map of the Goldfield mining district, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-681. Ashley, R. P., and Albers, J. P., 1975, Distribution of gold and other ore-related elements near ore bodies in the oxidized zone at Goldfield Nevada: U.S. Geological Survey Professional Paper 843-A, 48 p. Ashley, R. P., and Keith, W. J.,1973a, Geochemical map showing distribution and abundance of arsenic in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-472. Ashley, R. P., and Keith, W. J.,1973b, Geochemical map showing distribution and abundance of bismuth in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-473. Ashley, R. P., and Keith, W. J.,1973c, Geochemical map showing distribution and abundance of copper in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-474. Ashley, R. P., and Keith, W. J., 1973d, Geochemical map showing distribution and abundance of gold in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-475. Ashley, R. P., and Keith, W. J.,1973e, Geochemical map showing distribution and abundance of lead in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-476. Ashley, R. P., and Keith, W. J.,1973f, Geochemical map showing distribution and abundance of mercury in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-477. Ashley, R. P., and Keith, W. J.,1973g, Geochemical map showing distribution and abundance of molybdenum in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-478.
Ashley, R. P., and Keith, W. J.,1973h, Geochemical map showing distribution and abundance of silver in silicified rocks, Goldfield mining district, Esmeralda and Nye Counties, Nevada: U.S. Geological Survey Miscellaneous Field Studies Map MF-479. Ashley, R. P., and Keith, W. J., 1976, Distribution of gold and other metals in silicified rocks of the Goldfield mining district, Nevada: U.S. Geological Survey Prof. Paper 843-B, 17 p. Ashley, R. P., and Keith, W. J., 1978a, Goldfield mining district, Esmeralda County, Nevada, in Levering, T. G., and McCarthy, J. H., Jr., eds., Conceptual models in exploration geochemistry~The Basin and Range province of the western United States and northern Mexico: Journal of Geochemical Exploration, v. 9, p. 204- Ashley, R. P., and Keith, W. J., 1978b, Geochemical exploration for gold deposits at Goldfield, Nevada: U.S. Geological Survey Open-File Report 78-228, 8 p. Chao, T. T., 1984, Use of partial dissolution techniques in geochemical exploration: Journal of Geochemical Exploration, v. 20, p. 101-135. Grimes, D. J., and Marranzino, A. P., 1968, Direct-current arc and alternatingcurrent spark emission spectrographic field methods for the semiquantitative analysis of geologic materials: U.S. Geological Survey Circ. 591, 6 p. Hall, G. E. M., and Bonham-Carter, G. F., 1988, Review of methods to determine gold, platinum and palladium in production-oriented geochemical laboratories, with application of a statistical procedure to test for bias: Journal of Geochemical Exploration, v. 30, p. 255-286. Harvey, R. D., and Vitaliano, C. J., 1964, Wall-rock alteration in the Goldfield district, Nevada: Journal of Geology, v. 72, p. 564-579. Keith, W. J., Calk, Lewis, and Ashley, R. P., 1979, Crystals of coexisting alunite and jarosite, Goldfield, Nevada, in shorter contributions to Mineralogy and Petrology, 1979: U.S. Geological Survey Prof. Paper 1124-C, p. C1-C5. Kork, J. 0., 1986, JKDIGIT, a program to control a digitizing board for geologists, written in BASIC for the IBM personal computer: U.S. Geological Survey Bull. 1616, 69 p. Lichte, F. E., Golightly, D. W., and Lamothe, P. J., 1987, Inductively coupled plasma-atomic emission spectrometry, in Baedecker, P. A., ed., Methods for geochemical analysis: U.S. Geological Survey Bull. 1770, p. B1-B10. Meyer, Charles, and Hemley, J. J., 1967, Wall rock alteration, in Barnes, H. L., ed., Geochemistry of hydrothermal ore deposits: New York, Holt, Rinehart, and Winston, p. 166-235.
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