Geology, Alteration, Paragenesis, and Geochemistry of the Vortex Zone of the Hycroft Gold-Silver Deposit, Humboldt County, Nevada

The Hycroft gold-silver mine is a low sulfidation epithermal hot spring deposit located 55 miles west of Winnemucca, NV near the Blackrock desert. It is located in the historic Sulphur district, which has had mining on and off since the late 1800's. Sulphur was the main commodity initially, with the later discovery and mining of silver, alunite, and mercury through the first half of the 20th century. Gold was discovered in the district in 1974 by the Duvall Corporation. The first gold mining and recovery by heap leach was conducted in 1983 by Standard Slag. Allied Nevada acquired the property in 2008 and discovered the Vortex zone through induced polarization and resistivity surveys.;The oldest rocks in the region are the Permian Happy Creek Volcanic Series. These are overlain by the Auld Lang Syne Group of metamorphosed argillaceous to sandy sedimentary rock. Tertiary volcanic and volcaniclastic rocks overlie the basement rocks. The region underwent folding and regional metamorphism in the Jurassic. In the Late Cenozoic extension was the primary tectonic movement giving rise to the development of normal faults in the basin and range province.;The Auld Lang Syne rocks make up the basement in the Vortex area and are mostly in fault contact with overlying Kamma Mountains volcanic and volcaniclastic rocks. Previously undifferentiated, the Kamma Mountains rocks consists, from bottom to top, of (1) a lower flow-banded rhyolite, (2) ash-fall and lithic-rich tuffs, (3) a massive rhyolite flow, and (4), a clast-to-matrix-supported angular clastic unit. The youngest Tertiary unit is the Sulphur rocks, which consist only of a rounded to subangular clast-to-matrix-supported conglomerate in the Vortex area. The upper parts of the Kamma Mountains and the Sulphur rocks are lithified only where hydrothermally altered. All rocks are cut by a series of north-northeast-striking normal faults, the most important of which is the East fault.;Hydrothermal alteration in the Vortex zone is extensive and focused in layers due to the high permeability of most rock types. There are five types of alteration. An argillic alteration made up of kaolinite + smectite + anhedral quartz + sericite + marcasite + pyrite dominates the deposit. Argillic alteration is distinctly zoned from lower, kaolinite-dominated levels to upper smectite-dominated levels. Argillic alteration has been dated to 4.0 +/- 0.1 Ma. Argillic alteration interfingers with propylitic alteration that consists of chalcedony + chlorite + pyrite + sericite + smectite + marcasite +/- carbonates and occurs in veinlets and flooded into groundmass. Silicic alteration that consists of chalcedony +/- granular quartz + pyrite + marcasite +/- sericite formed above propylitic alteration in the middle part of the Kamma Mountains rocks. Opal or chalcedony + adularia + pyrite alteration locally occurs above the smectite alteration. Adularia from this alteration has been dated to 3.8 +/- 0.09 Ma. Steam-heated acid-sulfate alteration overprints all alteration types at the top of the system. Most elements, except immobile elements were leached and native sulfur added to the upper part of the steam-heated zone. Alunite from this alteration type has been dated to 2.4 +/- 0.1 Ma. The lower part is silica-cemented and has accumulated iron oxides leached from the upper part.;Paragenetic study shows that pervasive hydrothermal alteration occurred early in the system. Pervasive argillic alteration was overprinted with propylitic in the lower parts of the deposit, then silicic in the middle portion, then opal - adularia and acid leach in the upper portions. This was followed by several events of brecciation and veining. Silver mineralization occurs late in brecciation events and locally in veins as pyrargyrite, proustite, tennantite-tetrahedrite, and acanthite.;Geochemistry in the zone shows some typical epithermal zonation. Mercury and antimony show classical volatile zonation, occurring in the upper portions of the system. Arsenic appears to have reverse zoning with higher levels lower in the system, due to inclusion in silver sulfosalts rather than in arsenic sulfides. The base metals occur at very low levels overall and do not show clear zonation, except copper, which has a bi-modal zonation with a horizon of copper occurring in chalcopyrite lower in the system, and one occurring in tennantite-tetrahedrite higher in the system. Correlation of elements shows that gold and silver mineralization are commonly associated with arsenic, selenium, and antimony deposition, though this is variable throughout different levels of the system.;The volcanic rocks in the system were likely deposited between 28 and ∼16 Ma and cut by Basin and Range normal faulting at around 16 Ma. Normal faulting created the necessary conditions to form the volcaniclastic and Camel Conglomerates near the top of the deposit. Hydrothermal alteration began around 4 Ma and lithified these rocks, partially sealing the system. This led to the widespread creation of breccia dikes which roughly coincide with the boundary between the upper rhyolite and volcaniclastic units. Precious metal mineralization occurred in these breccia dikes and later veining. Hydrothermal activity continued after precious metal deposition with late overprints of acid leach.