The Rochester district encompasses a number of precious metal vein deposits which produced $7 million in silver and gold from 1912 to 1928. The largest deposits were mined on Nenzel Hill where over 100 million tons of low-grade silver-gold mineralization have recently been defined. The veins and low-grade mineralization occur in rhyolites of the Permian-Triassic Koipato Group. The Koipato Group includes mafic to siliceous volcanic and intrusive rocks scattered throughout north central Nevada. Overlying Mesozoic sedimentary rocks, once more than 11,000 ft thick in the vicinity of Rochester, have been severely eroded since the inception of Tertiary basin and range tectonism. Koipato rhyolites were hydrothermally altered on a regional scale to quartz-sericite-pyrite as a result of both the emplacement of comagmatic Koipato intrusives and Late Cretaceous stocks. Precious metal mineralization at Rochester probably accompanied the Late Cretaceous event.The major veins of Nenzel Hill comprise two mineral assemblages which fill and replace faults and strongly fractured rhyolite. Stage 1 veins consist of quartz, K-feldspar, and base and precious metal sulfides. They occur as open space filling and are confined to deeper portions of the Nenzel Hill mineralized zone. Stage 2 veins consist of a similar assemblage but cut stage 1 veins and display strong vertical zoning. With increasing elevation, stage 2 silver minerals are more abundant, Ag/Au increases, and sericite becomes prevalent. In stage 2 veins the FeS content of sphalerite varies from >11 to <1 mole percent over 800 vertical feet, with iron-poor sphalerites occurring nearer to the present surface. The stage 2 assemblage of quartz, sericite, sulfides, and sulfosalts is most common on intermediate and near-surface levels in Nenzel Hill. Low-grade mineralization consists largely of oxidized (80%) and unoxidized (15%) stage 2 assemblages which fill thin, randomly oriented, closely spaced fractures between major veins. Some precious metal grade (5%) is derived from wall-rock pyrite. The low-grade deposit is best developed within several hundred feet of the present surface but appears not to be significantly enriched.Fluid inclusions, oxygen isotopes, mineral compositions, and stabilities and stratigraphic reconstruction indicate that Nenzel Hill mineralization formed at about 300 degrees C and 1 kb. The major components of ore fluid were water, carbon dioxide, and solute approximated by (Na (sub 0.843) Ca (sub 0.075) K (sub 0.006) Mg (sub 2.2) x 10 (super -4) )Cl. Vein sulfides, silicate alteration phases, and sphalerite compositions indicate that f (sub S 2 ) and f (sub O 2 ) of ore fluid in major conduits progressively increased with elevation. The marked correlation of these changes in fluid composition with silver grade suggests that precious metal distribution was governed by hypogene processes. Buffering of the ore solution by a K-feldspar + sericite + quartz wall-rock assemblage permitted dispersion of metal values for great distances from main channel ways, forming a large tonnage of low-grade mineralization.

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