OLIGOCENE BATHOLITH IN EAST-CENTRAL NEVADA

Three, large, widely separated exposures of plutonic rocks in the North Egan Range and one in the Schell Creek Range in White Pine County, Nevada, are believed to be part of a batholith-sized igneous body that air magnetic patterns suggest underlies an area at least 50 miles long (N-S) and 20 to 30 miles wide (E-W). The pluton appears to have formed at a time when the sedimentary rocks of the region were more nearly horizontal than they are today. The youngest sedimentary rocks that have been invaded and replaced by plutonic rocks of the batholith are beds of the Early Cambrian Prospect Mountain Quartzite in the Cherry Creek Mining District. Elsewhere the pluton replaced beds no younger than those of the Late Precambrian McCoy Creek Group. The plutonic rocks are largely of quartz monzonite composition but rocks of other compositons are locally present. Porphyritic textured dikes, that are also largely of quartz monzonite composition and probably also of Oligocene age, are younger than the plutonic rocks and are abundant in many areas in and near the plutonic rocks. The dikes strike mostly north-south and dip at variable angles to the east. Some are truncated up-dip by gently east-dipping denudation faults related to early basin-range extension. It is speculated that the dikes may have contributed material to some of the Oligocene volcanic units in the region and were also responsible for setting in motion geothermal cells that created mineral deposits in the Cherry Creek Mining District. The Cherry Creek District has been a modest producer of gold, silver and tungsten.