Chemical analyses of 183 rocks from the Modoc mine area, Lander County, Nevada

Abstract

This report presents analytical data for 183 rock samples from the Modoc mine area and a statistical treatment of the data. The Modoc mine area is located in the southwestern part of the Antler Peak quadrangle, between Willow Creek and Rocky Canyon, Lander County, Nev. (fig. 1). The area we studied is centered on a small middle Tertiary pluton, cropping out across an area of less than 0.1 km 2 , which intrudes sandstone, quartzite, shale, limestone, and chert of the Pennsylvanian and Lower Permian Havallah Formation (fig. 2). Detailed descriptions of the geology and geochemistry of the porphyry-copper environment of the general area are given by Roberts (1964), Roberts and Arnold (1965), Nash and Theodore (1971), Theodore, Silberman, and Blake (1973), and Theodore and Blake (1975). Theodore, Silverman, and Blake (1973) determined ages of 41 and 39 m.y. by the K-Ar method on mineral separates of magmatic hornblende and biotite, respectively, from the pluton of the Modoc mine area. Gold mineralization in the immediate area was apparently first discovered in 1925, and the property has been worked intermittently since that time; mineral rights are presently held by Charles L. Caldwell of Battle Mountain, Nev. The major underground workings explore two steeply dipping and parallel shear zones that strike approx-imately N. 25° W. These zones are up to 1 m wide and contain steeply plunging shoots of high concentrations of precious metals. Little is known about the production from the area, but it appears that only a few thousand dollars in gold and small amounts of silver may have been produced. Gold was found in narrow quartz veinlets near the surface of the pluton, and sulfides were encountered at a depth of about 10 m. Assays indicate that the sulfide-rich rock contains gold and silver. Trace amounts of secondary copper minerals occur in both the underground workings and on the surface. Samples collected away from the two major veins also yielded anomalous gold concentrations.