Geology and hydrothermal alteration of the Palmetto property, Esmeralda County, Nevada

Abstract

Hydrothermally altered andesite/dacite lavas containing detectable precious metals and largely unaltered Paleozoic sedimentary rocks are exposed at the Palmetto property. Within the volcanic rocks alteration envelopes are centered on prominent E-W-trending silica+alunite ribs in sharp contact with argillically altered rocks that grade outward into propylitized rocks. The ribs vary from quartz+alunite in the north to cristobalite+alunite in the south. Sulfur isotopic determinations suggest coarsegrained alunite (£34S = 23.5 per mil) and pyrite (^34S = 3.7 per mil) are hypogene, whereas fine-grained alunite (£34S=0.7 per mil) is supergene. The hypogene alunite has a K-Ar age of 16.1+0.7 m.y. Gold increases below the zone of oxidation with the appearance of approximately 1% pyrite. Based on the presence of hydrothermal breccias and adularia and the abundance of sulfate minerals, boiling probably occurred. The lack of repeated boiling episodes may explain the absence of ore.