Geochemical anomalies in the Swales Mountain area, Elko County, Nevada

United States Department of the Interior

ILLUSTRATIONS

GEOCHEMICAL ANOMALIES IN THE SWALES MOUNTAIN AREA ELKO COUNTY, NEVADA

By K. B. KETNER, J. G. EVANS, and T. D. HESSIN

Swales Mountain, about 18 miles northwest of Elko, Nev., is the most southerly peak in the Independence Range (fig. 1). Paleozoic rocks exposed in the area consist of both upper and lower plate rocks of the Roberts Mountains thrust fault. These are cut by two kinds of igneous rock, and small mineral deposits are exposed along the igneous contacts.

The general geologic similarity between formations exposed at Swales Mountain and those of the Carlin mine 13 miles to the west prompted the U.S. Geological Survey in 1967 to map in detail the rocks of the area and to assess its mineral potential as part of the Heavy Metals program. The present report gives the results of surface and subsurface sampling.

Rocks of the upper plate of the Roberts Mountains thrust, which belong to the widespread Vinini Formation of Ordovician age, consist principally of bedded chert and shale. The chert of the Vinini Formation is hard black rock which is composed almost entirely of quartz and which generally occurs in beds a few inches thick. The shale is relatively soft black to dark-gray or tan rock that is finely bedded.

Rocks of the lower plate of the thrust, which consist principally of limestone, are divided into two units. The lower unit, which is probably the Silurian Roberts Mountains Formation, is exposed on the banks of Camp Creek in the NW1,4 sec. 4 (fig. 2). The limestone is dark gray to black, is distinctly bedded, and contains 15 to 40 percent silt. The beds range in thickness from a fraction of an inch to several feet, but the thicker beds are more typical. Drill hole 2 in the NW%, sec. 4 penetrated more than 2,500 feet of nearly flatlying beds of this unit without reaching its base. Throughout this thickness the appearance and composition of the unit are strikingly uniform.

FIGURE 2._Geologic map of part of the Swales Mountain quadrangle, T. 35 N., R. 53 E., Nevada, showing mineralized areas, sample locations, and drill sites. Geology based on mapping by J. G. Evans in 1967.

The upper unit in the lower plate sequence is Devonian and earliest Mississippian in age. Because this unit has not been described or named in previous studies it will be called simply the upper limestone in this report. It is probably the same unit that is called the

Popovich limestone in the Carlin mine area (Hardie, 1966). A 900-foot-thick sequence of the upper limestone is very well exposed on the north bank of Camp Creek in the center of sec. 4. It consists partly of dark-gray limestone resembling that of the underlying Roberts

Mountains Formation and partly of a distinctive sequence of alternating sandy and shaly beds. The sandy beds are brown or tan and consist of sand-size grains of carbonate and quartz. The shaly beds are gray and consist of fine-grained limestone and clay.

Beds of loosely consolidated sandstone and conglomerate of Tertiary and Quaternary age cover the lower slopes east and west of Swales Mountain. Many of the sandy beds consist partly of volcanic ash. Andesite and latite lava flows and ash beds overlie the Vinini Formation north of Swales Mountain.

Intrusive roc.ks in the Swales Mountain area consist mainly of monzonite porphyry and quartz porphyry. A stock of monzonite form.s the south end of Swales Mountain in sec. 18, and a thick dike of monzonite extends from the NW%, sec. 16 northward through sec. 5. Most of the other intrusive rocks shown in figure 2 are qua~tz porphyry. The two types of roc.k are easily distinguished : the monzonite is rather dark and is spotted with black biotite; the quartz porphyry is light colored and is spotted with crystals of clear quartz. Geologic relations and radiometric data indicate that the intrusive rocks are of Tertiary age.

The Roberts Mountains Formation and the upper limestone unit are separated from the overlying Vinini Formation by a thrust fault. A thrust fault such as this one, which separates younger limestones from overlying older siliceous rocks, in this region is generaUy called the Roberts Mountains thrust fault (Roberts and others, 1958). Originally the thrust fault probably lay almost horizontally in the Swales Mountain area, but now both lower plate and upper plate rocks and the thrust fault that separates them are folded into an arch or anticline that trends north. The association of intrusive rocks with the anticline suggests that forcible e·mplacement of igneous rocks may have caused the anticlinal structure. The Roberts Mountains Formation and the upper limestone unit are exposed along the axis of this anticline, and they underlie the Vinini Formation at unknown depths on the flanks of the anticline.

In the course of mapping the Swales Mountain area some doubt arose as to whether limestone exposed there was actually in the lower plate of the Roberts Mountains thrust or whether it was an unusually thick body of limestone in the upper plate. Three hoi& drilled to clarify the structural picture indicate the limestone to be in the lower plate. These holes were designated as holes 2, 3, and 4 (figs. 1, 2, 3). Hole 2 was drilled to a depth of 2,547 feet, entirely within the Roberts Mountains Formation. Hole 3 penetrated limestone and skarn to a depth of 861 feet. Hole 4, located near Swales Creek (SE%, sec. 28, T. 36 N., R. 53 E.), started in Vinini Formation in the upper plate of the Roberts Mountains thrust fault, penetrated the thrust fault at a depth of 348 feet, and was extended 200 feet into limestone of the lower plate.

Chert and limestone have been somewhat altered close to the borders of igneous intrusive rocks. Some ,chert of the Vinini Formation has been recrystallized and bleached to a quartzitelike material. Limestone along intrusive contacts has been partly converted to a mixture of garnet and pyroxene minerals called skarn or tactite. This rock is extremely dense, hard, and difficult to drill. Limestone alteration at Swales Mountain thus is different from that at the Carlin mine, where carbonate, in the Roberts Mountains Formation has evidently been extensively leached, leaving a bleached, porous, soft residue of silica.

Exposed mineralized rock is concentrated mainly at the contact of povphyry with upper plate chert on the northeast and southeast slopes of Swales Mountain (SE%, sec. 6; Elh sec. 18) and along contacts between monzonite and limestone (SE%, sec. 5; Elh sec. 8; NE%, sec. 17). A few samples from these areas. contain as much as several tens of parts per million of gold, several hundred parts per million of silver, more than 100 parts per million of molybdenum, and thousands of parts per million of lead, zinc, and copper, but most samples are leaner. The samples whose metallic contents are shown in figures 4 to 10 were randomly selected from visibly mineralized rock Gold, in parts per million

• More than 1

Silver, in parts per million

• More than 70

Molybdenum, in parts per million 2-5

• S-10
• More than 10

Mercury, in parts per million

• More than 0.400

Copper, in parts per million

• More than 2,000

Lead, in parts per million

• More than 1,000

Zinc, in parts per million

• More than 10,000

at each site. The tonnage of mineralized rock cannot be estimated, but the narrow extent of surface exposures indicates that it is small.

Disseminated gold deposits should be sought at Swales Mountain in the kinds. of rocks that carry disseminated gold elsewhere. In the Carlin mine (Hardie, 1966) and the Cortez district (Erickson and others, 1966) gold is disseminated in the Roberts Mountains Formation. At the Gold Acres mine (Ketner, 1965) gold is disseminated in broken rock along the Roberts Mountains thrust. The presence of small gold deposits in the upper plate rocks at Swales Mountain suggests the possibility of more extensive deposits in the Roberts Mountains Formation where it lies concealed by upper plate rocks or in the broken rock along subsurface parts of the Roberts Mountains thrust. Exploratory efforts therefore should include drilling through the upper plate in areas immediately surrounding Swales. Mountain. Unfortunately there is no way of knowing how deep the thrust zone lies or how far below the thrust the Roberts Mountains Formation lies. Both could be very close to the surface without revealing their presence in any way.

Lower plate rocks intruded by igneous rocks are concealed beneath alluvium on the east flank of the mountain (sees. 4, 9) . Exploration of these rocks would require drilling through the alluvium east of the area where lower plate rocks are exposed.

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