F2 MINERAL BELT
LOW-SULFIDATION EPITHERMAL OCCURRENCES IN THE CENTRAL AND WESTERN TRINITY RANGE, PERSHING COUNTY, NEVADA:
SINTER, BUNCE, AND VELVET
LOW-SULFIDATION EPITHERMAL GOLD-SILVER DEPOSITS ON THE F2 MINERAL BELT
The Trinity Range, occupies the north and west sides of Interstate 80 for more than 80 kilometers as it tracks eastward and northward through Churchill and Pershing Counties, Nevada.
Renaissance Gold's Sinter, Bunce and Velvet projects (SBV), in the west-central part of the range, lie on a 300-kilometer-long, curvilinear magnetic anomaly interpreted as a typical back-arc rift and known currently as "F2" (Ponce and Glen, 2002).
The anomaly that defines the rift corresponds to basalt/andesite outcrops and numerous mafic dikes that typically align N10E in the SBV area. Epithermal vein deposits along F2 are also dominated by a primary N10E trend, with NW-, NE-, and E-W-striking antithetic veins, and include SBV and other historically mined, low-sulfidation deposits at Jessup, Seven Troughs, Rosebud, Sleeper and current operations at Hycroft. Deposits at Rosebud, in the north, and Seven Troughs, in the central part of F2 are dated at approximately 14.7 and 13.8 Ma, respectively. The host rocks at SBV are dated by RenGold at approximately 14.2 Ma, and crosscutting mineral deposits are necessarily coeval or younger. The age for Jessup is unknown, but presumably similar, and Hycroft was dated at only 3.9 Ma in an earlier study. Hycroft totals 14.1 M oz gold and 485.2 M oz silver in past production and current reserves, and another 13.5 M oz gold and 250.4 M oz silver in resource categories. Past-production and current reserves and resources for the other four F2 deposits listed above total only around 981,000 oz gold and 7.4 M oz silver.
Basement rocks in the Trinity range comprise phyllites and other weakly metamorphosed sedimentary rocks probably belonging to the Triassic-Jurassic Auld Lang Syne Group, and Jurassic and/or Cretaceous granodiorite to quartz monzonite intrusions. These Mesozoic units are unconformably overlain and intruded by a bimodal suite of Miocene rhyolite and basalt/andesite, which are themselves overlain by younger lacustrine sediments, diatomite, and basalt. The range hosts a variety of mineral deposits, including a number of Late Cretaceous tungsten skarns, Late Cretaceous orogenic silver, lead, and/or gold veins, the Oligocene Trinity silver vein-stockwork, several epithermal gold-silver veins hosted in Miocene bimodal-rift (and older?) volcanic rocks, and a few gold placers in Quaternary sediments.
At SBV, hypogene mineralization is confined to the bimodal Miocene volcanic and basement rocks, and is characterized by surficial to very shallow expressions of typical low-sulfidation epithermal systems (terminology from White and Hedenquist, 1990). Features include preserved sinters and shallow-level, chalcedonic to opaline silica (originally precipitated as silica gel) veins with varying textures. The strongest gold-values sampled by Renaissance Gold Corp. (RenGold) geologists are associated with quartz pseudomorphs of platy calcite, and banded quartz-adularia-calcite veins. These presumably deeper, more productive veins crop out at Sinter and Velvet, but are not exposed at Bunce, where recrystallized silica-gel veins are present at surface. All of these mineralized occurrences are associated with moderately to completely argillized host rocks, with or without moderate to strong hematite staining.
A better-studied analog for F2 occurs in north-central Nevada along the 500-kilometer-long Northern Nevada Rift (NNR), which hosts several low-sulfidation deposits including Midas, Hollister, Mule Canyon, Fire Creek, and Buckhorn. The NNR is temporally related to the emergence of the Yellowstone hotspot, the beginning of back-arc extension, and related genesis of the Basin-and-Range physiographic province. NNR deposits are slightly older than those on F2, ranging from 15.1 to 15.8 Ma, and ore is commonly characterized by banded chaldedonic quartz veins with electrum and silver selenides. Past-production and current reserves and resources on the NNR total more than 11.4 M oz gold and 69.3 M oz silver.
Mineralization at SBV shares similarities with Hollister and Fire Creek, where bonanza-grade zones are mostly blind, and expressed by weakly mineralized sinters at a paleosurface that is coincidentally also exposed at present. The ore-grade veins at Hollister occur 225 meters below the sinter environment, and those at Fire Creek are 150 to 200 meters below their sinters. In contrast, ore at Hycroft occurs at less than 50 meters below exposed sinter. There is no way to accurately predict depth to ore below the paleosurface, but these key analogs provide minimum constraints for practical exploration drilling depths, and for considering drill data from earlier campaigns in the SBV area. More mapping is required in any case.
Ponce, D.A., and Glen, J.M.G., 2002, Relationship of epithermal gold deposits to large-scale fractures in Northern Nevada: Economic Geology, v. 97, p. 3-9.
White, N.C., and Hedenquist, J.W., 1990, Epithermal environments and styles of mineralization: variations and their causes, and guidelines for exploration: Journal of Geochemical Exploration, v. 36, p. 445-474.