Summary:
The Hycroft deposit is a low sulfidation, epithermal, hot springs system that contains gold and silver mineralization.
Regional Geology
The Hycroft Mine is located on the western flank of the Kamma Mountains in the Basin and Range physiographic province of northwestern Nevada. The faults served as conduits of hydrothermal fluids that deposited the Hycroft mineralization.
Local Geology
The deposit is typically broken into six major zones based on geology, mineralization, and alteration. These include Brimstone, Vortex, Central, Bay, Boneyard, and Camel. The boundaries are typically major faults, namely Break, East and Ramp.
Brimstone
The stratigraphy at Brimstone includes up to 100 feet of alluvium, underlain by Camel Conglomerate rocks (0 feet to 400 feet). The Brimstone ore deposit is hosted primarily by Kamma volcanic rocks in the hanging wall of the East Fault.
At Brimstone, the East Fault is a north-northeast striking, west dipping, normal fault with repeated episodes of movement, including approximately 150 feet to 200 feet of alluvial offset. Where exposed in the Brimstone Pit, the fault clearly shows steep normal movement, with slickensides that plunge 80º to 85º. At depth the fault shallows to 45° to 60º and may merge with the Central and Break Faults. Only minor mineralization is noted footwall to the fault zone.
Zones of silicification of limited thickness, oriented parallel to the East Fault, are present in the footwall zone. Alteration extends for 50 feet to 70 feet footwall to the fault, with pervasive silicification and quartz veining dominant.
Drilling has shown that mineralization extends to a depth of over 1,200 ft in the Brimstone Zone. Mineralization thickness (true width) is 200-1,100 ft thick and remains open to the west towards the Break Fault and transitions into Vortex to the south.
Vortex
The upper elevation at Vortex is hydrothermally clay (kaolinite) altered. Acid leaching is less prominent than in Brimstone and is focused primarily along the East Fault. Strong silicification to depths greater than 1,500 feet is due to veining and phreatic hydrothermal brecciation. At least four mineralizing events are present as evidenced by crosscutting vein and breccia relationships.
The mineralization at Vortex is of both vein and disseminated type, with brecciated and altered rhyolite rocks and volcanic clastics acting as favorable hosts.
Oxide mineralization is present at a depth of approximately 500 feet below surface, with sulfide mineralization extending to 2,500 feet below surface. Mineralization thickness (true width) is 1,000 to 1,800 ft thick. Banded quartz veins with both high-grade silver and gold have been noted in core.
Bay and Boneyard
Mineralization in the Bay and Boneyard zones is hosted by gentle, west dipping Camel Conglomerate. Both clastsupported and matrix-supported conglomerate rocks host mineralization. The basal rock type is tuffaceous lake sediments, composed of fine grained clay with minor layers of gravel and conglomerate extending to a depth greater than 1,100 feet. Mineralization is primarily bedding controlled, with the Range and Central Faults as the main feeders.
Gold and silver mineralization is associated with flat lying Camel Conglomerate, above the lacustrine sediments of the Tsg formation. Mineralization thickness (true width) is 20 to 250 ft thick at Bay and 50-300 ft thick at Boneyard. This zone transitions into the upper zone of mineralization at Central. Bay and Boneyard remain open to the north and east.
Central
Alteration along the Central Zone is similar to that of Bay. Acid leach alteration is stronger and more widespread than at Bay and is extensive in the southern portion of the pit. The acid leaching overlies silicified conglomerate rocks, except along the immediate trace of the Central Fault where silicification dominates as the alteration type. Oxidation extends downward approximately 400 feet.
Gold and silver mineralization is associated with favorable stratigraphic horizons in the Camel conglomerate, with an upper and lower zone noted in drilling, separated by a north-south striking, east dipping clay layer. Mineralization remains open to the west, past the Range Fault, and at depth (>1,400 feet). Mineralization thickness (true width) in the upper zone is 50-300 ft thick, while the lower zone ranges from 300-1,200 ft thick, and remains open at depth. The zone mineralization is contiguous to the Vortex and Brimstone Zones to the east, and the Camel/Cut-5 zones to the south.
Camel and Cut-5 Zones
Mineralization in the Camel/Cut-5 Zones is hosted by conglomerate rocks and occurs as both disseminated gold and silver associated with pyrite and marcasite, and higher-grade veins, including silver bearing pyrargyrite veins. Mineralization thickness (true width) is 200-1,100 ft thick, extends to depths greater than 1,400 feet, and remains open at depth. Oxidation extends to depths greater than 200 feet and an area of intense oxidized mordenite alteration is present between the Cut-5 and Camel Zones. Mineralization remains open to the south, west and at depth. To the north, Camel mineralization is contiguous with the lower zone of the Central Zone, while Cut-5 is contiguous with the upper zone. Mineralization is also open to the west of Camel and to the south towards Hades Fault.