Deposit Type
- Epithermal
- Vein / narrow vein
Summary:
The Hycroft deposit is a large, epithermal, low sulfidation hot springs deposit. Gold and silver mineralization is noted as both disseminated and vein controlled, with gold values ranging from detection to 8.8 opt, and silver ranging from detection to 647.5 opt.
Mineralization at Hycroft has been deposited through multiple phases. An early silica sulfide flooding event deposited relatively low-grade gold and silver mineralization, generally along bedding. This mineralization is cross cut by later, steeply dipping quartz alunite veins. Late stage silver bearing veins are found in the Vortex zone and at depth in the Central area. Late to present supergene oxidation along faults has liberated precious metals from sulfides and further enriched gold and silver mineralization, along water table levels.
The deposit is typically broken into six major zones based on geology, mineralization, and alteration. These include Brimstone, Vortex, Central, Bay, Boneyard, and Camel Hill. The boundaries are typically major faults, namely Break, East and Ramp.
The stratigraphy at Brimstone includes up to 100 feet of alluvium, underlain by Camel Conglomerate rocks (0 feet to 400 feet), and Kamma volcanic rocks, as shown in Figure 7-3 and Figure 7-4 respectively. The volcanic rocks are principally eruption breccias, tuffs, rhyolites, and volcanic rocks proximal to vents, and overlie deformed and metamorphosed shale, sandstone, and siltstone of the Auld Lang Syne (ALS) group.
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 80° to 85°. At depth the fault shallows to 45° to 60° and may merge with the Central and Break Faults. The fault may have originally served as a conduit to hydrothermal fluids.
North of the Brimstone deposit, the east-west trending Ramp and Prill faults appear to down drop favorable stratigraphy. Condemnation drilling of the leach pad to the north has shown only local zones of weak gold and silver mineralization. To the south, the Brimstone Zone transitions to the Vortex Zone, with no apparent change in stratigraphy, but changes to alteration zonation.
Acid leach alteration extends to depths of 700 feet in some areas of the Brimstone deposit as seen in Figure 7-3, indicating that the water table was present below the base of the acid leached zone. A siliceous layer (basal acid leach), up to tens of feet thick, occurs at the base of the acid leached material. Underlying the acid leaching is a layer of hydrothermal clay alteration, followed by silica potassium feldspar alteration. Pervasive silicification, veining and hydrothermal brecciation are generally found in the rhyolites and breccias.
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.
The upper elevation at Vortex is hydrothermally clay (kaolinite) altered. Acid leaching is less prominent 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 hydrothermal venting may have contributed to the eruption breccias overlying the Brimstone Zone. Propylitic and/or clay alteration extends outboard of the silicification.
The mineralization at Vortex is of both vein and disseminated type, with brecciated and altered rhyolite rocks and volcanic clastics acting as favorable hosts. In addition to gold mineralization, high grade silver has been encountered at Vortex; with values ranging from 10 to 647 opt. The predominant silver minerals are pyrargyrite, naumanite and miargyrite, occurring both in veins, disseminated and coarse grains along fractures.
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.
Mineralization in the Bay and Boneyard zones is hosted by gentle, west dipping Camel Conglomerate. Both clast supported 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 as shown in Figure 7-6. Mineralization is primarily bedding controlled, with the Range and Central Faults as the main feeders.
The predominant alteration type at Bay is silicification. Acid leach alteration in the area is relatively minor, and occurs along high angle structures (Figure 7-7). Clay alteration of the underlying lacustrine sediments is also noted in limited drillholes, and is illite smectite dominated. Strong oxidation is present in the upper portion of the silicified zone.
Gold and silver mineralization is associated with flat lying Camel Conglomerate, above the lacustrine lake sediments. 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.
The Central Zone geology is similar in nature to that of Bay, with mineralization and alteration fed by high angle faults and fractures, with dominant lateral fluid flow through the porous conglomerate rocks of the Sulphur Group as seen in (Figure 7-8). Camel Conglomerate units are underlain by lacustrine sediments. However, the lacustrine units thin dramatically to the south, with less than 50 feet of the material noted south of Cut-4.
The Central Zone is bounded to the east by the Central and Break Faults. Fault movement is unknown, but extends at least 2,000 feet, with recent reactivation in the quaternary (50 feet to 150 feet), as demonstrated by offset in the alluvium.
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. Underlying the silicification and acid leaching are illite smectite clay altered and clay dominant lacustrine sediments. Hot spring sinter deposits have not been observed.
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 Hill/Cut-5 zones to the south.
Mineralization in the Camel Hill/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.