Economic significance of revised age relations of rocks in the Cornucopia mining district, Elko County, Nevada

United States Department of the Interior

Geological Survey

ILLUSTRATIONS ECONOMIC SIGNIFICANCE OF REVISED AGE RELATIONS OF ROCKS IN THE CORNUCOPIA MINING DISTRICT, ELKO COUNTY, NEVADA

By R. R. COATS

The Cornucopia district is a silver-gold mining district in northwestern Elko County, Nev., (;fig. 1). According to Granger, Bell, Simmons, and Lee (1957, p. 41) it has had a production of about $114 million in gold and silver, ninetenths before 1883. The silver-gold ratio by weight for the latest productior was. about 68:1; earlier recon:ls are not adequate to permit an esUmate of the ratio.

Figure 1.-Index map of Elko County, Nev., showing location of the Cornucopia mining district.

The mines are no more accessible now than they were in 1908. In the course of a brief examination of the surfa~e in 1966, I made the sketch map presented herewith (fig. 2). Fortunately, recent surficial prospecting disclosed critically important information on the rock relations.

The oldest exposed rock is the andesite in which the ore deposits occur. The fresher rocks examined in thin sections are pyroxene andesites. Most of the clinopyroxene is fresh augite; some patches of chlorite and nontronite appear to be pseudomorphs after orthopyr'lxene. More highly altered rocks are bleached and partly replaced by calcite, pyrite, and se""icite. Some of these bleached rocks have nests of secondary quartz that simulate quartz phenocrysts in hand specimen.

The andesite and the mineral der'lsits that it contains are overlain unconformably by a series wur• utur ..... of rhyolitic and rhyodacitic rocks. The lower unit of these rocks, which in some places can be seen to rest on the andesite, is a biotitic rhyolite ignimbrite, the basal part of which is buff wnwelded tuff that grades upward into a reddish-brown welded phase. Because of lack of exposures the maximum thicknrss in the area is not known. The minin1um thickne~s is about 30 feet-measured in an exposure just west of the place where the road passes over the saddle which is near the center of sec.-18, T. 42 N., R. 51 E. The ignimbrite is overlain, possibly unconformably, by a dark, partly glassy, pigeonite rhyodacite containing much xenocrystic material. This rock fm111s the summit of Silver Peak and the surface of the long north-trending ridge immediately south of the mine shafts shown on the map.

Figure 2.-Geologic sketch map of the Cornucopia mining district.

The relations of the volcanics of Silver Peak to the mineralized andesite are critically important to the interpretation of the geology. Because of the prevalence of faulting a:nd the ready disintegration of the tuffaceous lower portion of the ignin1brite, exposures showing the depositional contact are few. The most accessible exposure is in some artificial cqts about 0.4 mile cast and 0.45 mile north of the southwest corner of sec. 18. As the n1ap reveals, f~ulting is prevalent in the Cornucopia area. Some of these faults display characteristics indicating that at least some of the movement on them is premineral; others may also be in part premineral, but either cut the mineral deposits, or displace the contact of the volcanics of Silver Peak and the mineralized andesite.

An example of a fault that is probably in part, at least, premineral is marked "B" in figure 2. At the time of my visit, it was reported by one of the persons engaged irn exploration that the Leopard vein, which has been extensively worked under the n1ine dumps shown on the map, has been traced northward to the point where it is cut off by a silicified fault-vein trending rnorthwest (B in fig. 2). A branch of this fault may be exposed in the portrul of an adit on the south side of the gully that bounds the mined block on the north. The fault at the portal has a strike about N. 10° W. and dips about 70° southwest. It also shows extensive silification. If, as the silicification along it suggests, this fault, or these faults, are premineral in age, th~ likelihood of finding a continuation of the Leopard ore body northeast of them is not great, a.lthough ore is kmown to occur in that area.

The vokanics of Silver Peak may be observed in fault contact with the mineralized andesite in a series of cuts about 0.4 mHe east of the southwest corner of sec. 18. The buff, poorly consolidated, tuffaceous phase of the ignimbrite forms the hanging wall of a fault (A in fig. 2) that trends northwest and dips from 36° to 60° southwest. The fault trace is marked by a brick-red gouge that contains fragments of quartz vein material up to 4 inches in diameter. In the gully to the east of this point, severaL.. cuts expose massive buff rhyolitE. tuff. Obscure bedding in the tuff dips west about 45 °. This steep westerly dip is thought to be caused by drag on a normal fault that must pass just east of the cuts and follows the course of the north-trending guUy.

The geologic evidence that suggests that the rhyolite and rhyodacite are postandesite is confirmed by the petrographic evidence. The buff tuffaceous lower part of the Hotite rhyolite ignimbrite has a substantial content of clear unaltered sanidine phenocrysts, green euhedral biotite tablets, phenocrysts of quartz and plagioclase, and equant fragment~ of vesicular black glass. It is not likely that th~ biotite, sanidine, and glass would have survived unaltered while the adjacent andesite was b~ing intensely propylitized and sericitized ..

The lack of mineralization ir. the rhyolite and rhyodacite can thus be explained by the fact that they are younger thar the andesite and the mineralization. The faults that separate the rhyolite and rhyodacite from the mineralized andesite in the mine area are postmineral; the entire ridge that lies southwest of the area ·of active mining is part of a block, doW!Ilthrown with respect to the blocks lying to the north and east. As the available exposures suggest that the most important ore--bearing veins of the district are cut off by ther~ postmineral normal faults, extensions of these veins may exist beneath the rhyolite and rlcyodacite. Unfortunately, the thicknesses of th~ rhyolite arnd rhyodacite units are not well kr0wn both because an unknown amount of postdepositional erosion has occurred and because original thickness varied g-reatly frmn place to place due to deposition of the rhyolite and rhyodacite units uporn an irregular surface. Hence, it is not possible to give a precise estimate of the throw of the fault on the northeast boundary (A in fig. 2) of the ridge block that lies south of the mine shafts. However, if the tuff of the rhyolite and rhyodacite unit, exposed west of the road in the west half of sec. 18, corresponds to the tuff exposed in the hanging wall of this fault, then the buried upper surface of the mineralized andesite may be only a few scores of feet below the present groUind surface, and concealed veins may offer attractive exploration targets.

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