Epithermal gold-silver-mercury deposits at Paradise Peak, Nevada; ore controls, porphyry gold association, detachment faulting, and supergene oxidation

Abstract

The Paradise Peak gold-silver-mercury deposit in Nye County, Nevada, produced a total of 47 metric tons of gold and 1,255 metric tons of silver. Most of this metal came from four high sulfidation epithermal deposits (Paradise Peak, Ketchup Flat, Ketchup Knob, and County Line), although a small amount came from part of a porphyry gold system, the East Zone deposit.High sulfidation precious metal ore is hosted by strata-bound bodies of pervasively silicified, welded ash-flow tuff. The ore-bearing tuff is bounded on all sides, both stratigraphically and structurally, by aquitards, mainly andesite flows but vitrophyre at the County Line deposit. The highest precious metal values are present in hydrothermal breccia which cuts silicified tuff and, at the Paradise Peak deposit, also overlying andesite flows and felsic tuffs altered to a quartz-alunite assemblage.A lower andesite sequence, beneath the mineralized tuff horizons, is the host for a large zone of low-grade gold mineralization of porphyry type. Gold is present in a quartz veinlet stockwork cutting sericitized andesite flows, which are inferred to be intruded at depth by a porphyry stock.High sulfidation gold-silver-mercury ore contains highly anomalous quantities of Bi, Pb, and Sb, but less As and Cu than in most similar epithermal deposits, such as some of those in the Nansatsu district, Japan. The mercury, as cinnabar, and native sulfur were added to the precious metal ore in the vadose zone during late-stage descent of the paleowater table. Acid leaching, dominated by cristobalite, occurred at this time around the silicified tuff bodies, which themselves were little changed. The porphyry gold mineralization possesses a markedly contrasting lithogeochemical signature, defined by high Mo and very low Ag, which resembles that displayed by the geologically similar porphyry gold deposits in the Maricunga belt, northern Chile.An episode of detachment faulting affected the Paradise Peak district and environs after steep, normal faulting and precious metal mineralization were complete, i.e., between 18 and 10 Ma. All the high sulfidation deposits occur in structural duplexes between low-angle fault strands defined by prominent shearing and gouge. Three of the high sulfidation orebodies are believed to have constituted a single deposit prior to dismemberment during detachment faulting. The fault-localized feeders, which are inferred originally to have underlain the high sulfidation deposits, were detached from them by the low-angle faults.All ore exploited from the high sulfidation deposits underwent supergene sulfide oxidation during weathering, which commenced after detachment faulting had ceased and was ongoing at 10 Ma. The base of oxidation is a districtwide, subhorizontal datum controlled in part by the low permeability of the detachment faults. High sulfidation mineralization in the east lobe of the Paradise Peak deposit and at Ketchup Hill lies beneath the base of oxidation and consists of refractory sulfidic material. Sulfides comprise 10 to 90 vol percent of the unoxidized material and, upon oxidation, produced the friable, powdery ore common in the orebodies. Weathering resulted in only very local redistribution of silver and gold. Hypogene oxidation is not recognized.