Distribution of gold, silver, tellurium, and mercury in the Ely Mining District, White Pine County, Nevada

By Garland B. Gott and J. Howard McCarthy, Jr.

GEOLOGICAL SURVEY CIRCULAR United States Department of the Interior

Geological Survey

The porphyry copper deposits of the Ely district in east-central Nevada have been mined for more than half a century. Spencer (1917), Bauer, Breitrick, Cooper, and Swinderman (1964), and Bauer, Breitrick, Cooper, and Anderson (1966) studied the district and described a pattern of metal zoning having a core of relatively high concentrations of iron and copper surrounded by an envelope containing lead, zinc, gold, and silver in anomalous amounts. We have made additional studies to establish the geochemical setting of the copper deposits in greater detail and to determine the distribution and concentration of valuable metals such as gold, silver, tellurium, and mercury in the bordering areas or envelope. The information obtained may be useful in the search for concealed mineral deposits both in this district and elsewhere.

The eight geochemical m·aps accompanying this report (figs. 2-9) show the broad pattern of sampling and are designed to highlight anomalous concentrations of the principal metals of interest in this study (gold, silver, tellurium, and mercury), as well as of copper.

Gold was determined by a wet chemical method (Lakin and Nakagawa, 1965) using atomic absorption spectrophotometry. Silver was determined by the emission spectrographic method as described by Ward, Lakin, Canney, and others (1963). Tellurium was determined by a sensitive wet chemical method (Lakin and Thompson, 1963). Mercury was determined instrumentally by an atomic-absorption technique (Vaughn and McCarthy, 1964). Limits of detection for these elements were: gold, 0.3 ppm (parts per million); silver, 1 ppm; tellurium, 1 ppm; mercury, 0.1 ppm. The analysts for gold were H. M. Nakagawa, K. W. Leong, and Arthur Hubert; for silver, Uteana Oda, G. C. Curtin, D. J. Grimes, and E. L. Mosier; for tellurium, J. B. McHugh, H. W. Lakin, C. E. Thompson, and Elizabeth Martinez; and for mercury, Henriette McCarthy, W. W. Janes, and H. W. Knight.

The Ely district is an area of Paleozoic sedimE~ntary rocks intruded by quartz monzonite porphyry stocks of Cretaceous age. The sedimentary rocks, about 17,000 feet thick, are predominantly limestone and dolomite but include some shale and quartzite. They are divided into 17 formations (Brokaw and Shawe, 1965; Brokaw and Heidrick, 1966) but in figure 1 are grouped in only two units, pre-Pennsylvanian and Pennsylvanian and Permian.

The main ore deposits of the Ely district are in the central and western parts of a belt of altered and variously mineralized rocks about 1 mile wide that extends 8 miles westward from Ely (fig. 1). Another belt of altered and somewhat mineralized roeks extends about 3 miles northward from Ely.

The strongly mineralized western and central part of the main or west-trending belt coincides with a line of porphyry stocks. Rocks in parts of the stocks are much altered;. and in extensive bordering areas, limestone was silicified or converted to jasperoid. Chalcopyrite and. pyrite are disseminated in the porphyry and the altered or metamorphosed limestone. Ore bodies containing chalcocite formed by supergene enrichment of primary chalcopyrite-pyrite ore were important during the early years of mining, but primary chalcopyrite ore has been the principal source of copper for about 20 years. The central area had produc.~d metals valued at nearly $1 billion by . probably are reasonably representative of the 1964 (Bauer and others, 1964); by far the largest production has been of copper, but appreciable amounts of gold, silver, platinum, palladium, and molybdenum have been obtained as byproducts; and some lead, zinc, and manganese have been mined.'

The altered and somewhat mineralized belt trending north from the town of Ely approximately parallels a set of high-angle faults. It is a narrow belt characterized by partly to completely silicified limestone and dolomite and, as indicated by figures 2-9, by anomalous metal contents. The belt has not as yet been proved to be of economic significance, but it may reflect concealed ore deposits at depth.

Most of the sampling was .done in the central and eastern parts of the district because of the abundance of natural exposures and shallow prospect pits in those areas. Few samples were collected in the western part of the district, where copper has been mined, because most of the original outcrops have been concealed or destroyed as a result of mining activity. Samples collected were largely jasperoids with different colors and textures, gossans with different degrees of leaching, silica boxworks, fracture fillings, and limestones and dolomites with various amounts of silica boxwork or massive silica; some limestone and dolomite also were sampled.

The number of samples collected at each locality depended on the variations within the rock sampled. As many as 6 or 8 samples were collected at many localities but on the average 2 or 3 were collected; a total of 1,400 samples was collected within the district.

Nearly 400 samples were collected from the nearly continuous masses of jasperoid and gossan within the iron- and silica-rich zone of alteration in the central part of the district. Although the concentrations and proportions of metals within these materials are erratic, the samples collected

jasperoid.

Nearly 200 samples were collected within the north-trending eastern area. These samples were p~incipally from partly silicified limestone and dolomite whose compositions range from low silica-high lime to high silica-low lime. The composition of these samples averages about 50 percent sili~a ~nd 50 percent limestone and dolomite.

Many samples were collected outside these two alteration areas. Many of these samples were from apparently barren unmineralized rock, and some were iron- and manganese-rich fracture fillings or sm~ll pods of j asperoid and gossan, all representing small volumes of rock.

Two geochemical maps were made for each element (figs. 2-9), one map showing the average value of analyses of all samples at each locality and the other the highest value obtained at each locality. These maps also show, for the central and western areas, the limits of the iron- and silica-rich zone and the boundary of the area within which mineralized samples contain 0.5 percent or more copper. In the eastern area they show the limits of the zone within which limestone and dolomite are partly to completely silicified.

The analytical results show that gold, silver, tellurium, and mercury are generally enriched in the district as a whole, as shown in the following table and in figures 2-9. In some parts of the central and eastern mineralized areas, these metals are mostly concentrated in pods and discontinuous masses of rock. Their concentration ranges from below the limit of detection to 10,000 ppm of tellurium, 1,600 ppm of silver, 33 ppm of gold, and 70 ppm of mercury.

In the central part of the district, the metals are generally most concentrated in a belt between the zone of high copper and the outer limit of the principal zone of alteration. The average concentrations of tellurium, silver, gold, and mercury in 372jasperoid and gossan samples collected from this bert are given in the following table. For

each metal, about half of these samples contain values below the limit of detection. The higher ·analyses, with the exception of those for mercury, tend to be clustered near the qqartz monzonite stocks. Outlines of four such clusters of high analyses are shown in figure 10. The concentration of the individual metals varies greatly from one cluster to another, as is illustrated by the table at top which showsthe average concentration, in parts per million,of seven metals in each cluster. ' In the eastern mineralized area silver, tellurium, and mercury are most concentrated within a narrow silicified zone. Most of the samples collected from this area, are of box work silica or massive gray silica in incompletely to completely silicified limestone and dolomite. Samples of fracture fillings were largely iron- and m_anganese-rich oxides. All samples collected in this area are grouped and compared in the table at· bottom (values given in parts per million).

Tellurium, silver, and mercury are higher in the eastern area than in any other part of the district, whereas gold is much lower. The ratios among the different metals are also different from those found elsewhere within the district. As compared to the central part of the district, the silver in the high-silica samples is higher by a factor of 2, tellurium by 2.5, and mercury by 200, whereas gold is lower by a factor of 5.

All the metals are more concentrated in the silica-rich material than in the lime-rich material although, surprisingly, silver is nearly as high in the average for all the samples as it is in the high-silica samples. However, the concentration of metals within the rocks in which they now occur does not seem to have been in proportion to the degree of silicification of the rocks. The table on page 4 (values in parts per million except as indicated) shows the random relation between the metals and calcium carbonate in 10 samples.

Gold is concentrated mainly in four local areas of high metal concentration as shown in figure 10;

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.