Geology, Alteration, Geochemistry, and Paragenesis of the Vista Vein Shear Zone Deposit, Humboldt County, Nevada

Abstract

The Vista Vein shear zone Au deposit (VVSZ) is a structurally-controlled, shear zone-hosted deposit located underneath the historic Vista Pit of the Twin Creeks Mine, which is owned and operated by Newmont Mining Corporation. The deposit is located approximately 45 miles northeast of Winnemucca, NV in the Potosi mining district of Humboldt County. This Carlin-type gold deposit is unique in that it is hosted entirely within volcanic rocks of the Ordovician Valmy (Ov) Formation, in contrast with typical Carlin-type deposits, which are typically hosted in shelf-slope sedimentary rock environments. The upper part of the formation is dominated by pillow and massive basalts, with minor siliceous mudstones, volcaniclastic sediments, and flow breccias. Two mineralization events are recognized in this deposit: (1) a brecciated quartz-base metal vein (Galena Vein); and (2) Carlin-type Au mineralization. In the latter, gold is hosted in arsenian pyrite and marcasite disseminated through the VVSZ characterized by decarbonatization, silicification, phyllic, and argillic alteration.The VVSZ has a mineable strike length of 4600 feet, with strikes between N40-45º E, and a dip extent length of 1000 feet with a dip of 65-70º to the NW. There is an ore shoot that has a rake of 35° to the northeast. Average gold grades range from 0.2 – 0.4 ounces per ton. The deposit is named the “Vista Vein” because of the brecciated quartz-base metal vein, known as the Galena Vein, which occurs within the shear zone of the Trench Fault. The “Vista Vein” name is misleading in that it implies a vein deposit, when in fact it is a shear zone deposit. The 1-3 foot wide Galena Vein mineralogy includes (early to late): milky vein quartz, fractured pyrite, massive fractured sphalerite with chalcopyrite disease, subhedral-euhedral pyrite, massive fractured galena, tennantite-tetrahedrite, microcrystalline quartz, sericite, and specular hematite. The VVSZ has been reactivated multiple times and shows strike-slip, normal, and possibly oblique displacement. Rock preparation in the VVSZ for later gold deposition included: decarbonatization, silicification, and phyllic alteration. The shear zone is brecciated and contains clasts of pervasively phyllic altered wallrock. The matrix of the fault breccia is mostly composed of microcrystalline quartz, kaolinite, and carbon. Opaque mineralogy include: pre-ore and ore-stage pyrite, ore-stage marcasite, chalcopyrite, galena, and sphalerite. Several 114 ± 2 Ma porphyritic dacite dikes and sills play a significant role as secondary controls on gold mineralization in the Vista pit as well in the Vista Vein shear zone. These porphyritic dacite dikes are characterized by pervasive phyllic alteration and locally are mineralized.The Carlin pathfinder elements that have strong positive correlations with gold include: arsenic, silver, antimony, mercury, selenium, tellurium, thallium, and tin. Arsenic, thallium, mercury, selenium, and tellurium all increase at depth to the northeast. On the opposite spectrum of the Carlin-type pathfinder elements, the base metal elements have elevated values that increase to the northeast with increasing elevation in the deposit. This suggests that these two mineralization events occurred separately.There is no recognized “ore horizon” with the VVSZ deposit; the only constraint on the extent of gold mineralization is the surface and the 20K fault that truncates the Vista Vein deposit up dip to the northeast. The Vista Vein deposit has potential for expansion down dip and along strike.This research has led to understanding that: (a) the porphyritic dacite dikes act as important secondary controls on mineralization, (b) steeper dips of the Vista Vein shear zone correlate with higher gold values, (c) base metals have an increase in concentrations higher in elevation, and (c) the zonation of Carlin-type pathfinder elements suggest an increase in intensity of mineralization at depth to the northeast.