Geology, alteration, and geochemistry of the McGinness Hills area, Lander County, Nevada

Abstract

Two distinct hydrothermal systems have been active in McGinness Hills: an older (35 m.y.) system characterized by quartz + kaolinite i alunite alteration of andesite, and a younger (2.7 m.y.) hot-spring system that produced quartz + adularia alteration of rhyolite ash-flow tuff and andesite. The older system is characterized by elevated F and scattered Mo and W anomalies, suggesting that this terrane may overlie a porphyry Mo system. The younger system formed sinter terraces and deposited Au, Ag, Hg, and Sb within 300 feet of the surface with low values of associated base metals. Au enrichment occurs above Ag, As and Hg enrichment; Sb is concentrated within the sinter. Three distinct silica stages are recognized. The highest precious-metal values are associated with late banded quartz/chalcedony + adularia veins. The patterns of geochemical anomalies, alteration, and deformation may be related to the loss of CO2 and H2S from the fluid during boiling.