The Hycroft deposit is a low-sulfidation, epithermal, hot springs system that contains Au and Ag mineralization. Radiometric dates of adularia (potassium feldspar) indicate that the main phase of Au and Ag mineralization formed four million years ago (Ebert, 1996) when hydrothermal fluids were fed upward along high angle, normal faults. Low-grade Au and Ag mineralization was co-deposited with silica and potassium feldspar throughout porous rock types.

A subsequent drop in permeability, due to sealing of the system, led to over pressuring and subsequent repeated hydrothermal brecciation. Additional precious metal mineralization was deposited during this event as breccia zones, veins, and sulfide flooding.

Au and Ag mineralization was followed 0.4 to 2.0 million years ago by an intense event of high sulfidation acid leaching of the mineralized volcanic rocks coincident with a regional water table drop. This allowed steam heated sulfur gases to condense into sulfuric acid and leach the upper portion of the mineralized rocks. Oxidation of sulfide mineralization occurs to variable depths over the deposit, depending upon proximity to faults, extent of acid leaching, and depth to water table. Sulfide content through the deposit is variable from 0% to 20%.

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 Hill. The boundaries are typically major faults, namely Break, East, and Ramp.

Brimstone
At Brimstone, the East Fault is a north-northeast striking, west dipping, normal fault with repeated episodes of movement, including approximately 150 ft to 200 ft 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. Only minor mineralization is noted footwall to the fault zone.

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 Au and Ag mineralization. To the south, the Brimstone Zone transitions to the Vortex Zone, with no apparent change in stratigraphy, but changes to alteration zonation. Zones of silicification of limited thickness, oriented parallel to the East Fault, are present in the footwall zone. Alteration extends for 50 ft to 70 ft footwall to the fault, with pervasive silicification and quartz veining dominant.

Au and Ag are spatially associated with fracture and breccia-controlled chalcedony sulfide mineralization. A subsequent acid alteration event produced the current distribution of oxidized and transition sulfide/oxide ore. The lower acid leach material hosts Au and Ag mineralization, as does the underlying silicified and veined volcanics. Mineralization thickness (true width) is 200–1,100 ft thick and remains open to the west toward 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 in Brimstone and is focused primarily along the East Fault. Strong silicification to depths greater than 1,500 ft 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 Au mineralization, high-grade Ag has been encountered at Vortex; with values ranging from 10 to 647 oz/ton. The predominant Ag minerals are pyrargyrite, naumannite and miargyrite, occurring both in veins, disseminated and coarse grains along fractures.

Oxide mineralization is present at a depth of approximately 500 ft below surface, with sulfide mineralization extending to 2,500 ft below surface. Mineralization thickness (true width) is 1,000 to 1,800 ft thick. Banded quartz veins with both high-grade Ag and Au have been noted in core. Drilling to date indicates that the highgrade zones are both high angle banded quartz veins and a more extensive flat lying, massive quartz zone containing visible pyrargyrite and miargyrite.

Bay and Boneyard
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 ft. Mineralization is primarily bedding controlled, with the Range and Central Faults as the main feeders. The Break Fault may also have zoning controls but is poorly drilled in this zone. Mineralized siliceous hot spring sinters have been historically mined indicating that this deposit represents the upper-most levels of a hot spring hydrothermal system.

The predominant alteration type at Bay is silicification. Acid leach alteration in the area is relatively minor and occurs along high angle structures. 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. Au and Ag mineralization is associated with flat lying Camel Conglomerate, above the lacustrine lake sediments. Mineralization thickness (true width) is 20–250 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
Au and Ag 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 ft). 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.

Camel Hill and Cut 5 Zones
Mineralization in the Camel/Cut-5 Zones is hosted by conglomerate rocks and occurs as both disseminated Au and Ag associated with pyrite and marcasite, and higher-grade veins, including Ag bearing pyrargyrite veins. Mineralization thickness (true width) is 200–1,100 ft thick, extends to depths greater than 1,400 ft, and remains open at depth. Oxidation extends to depths greater than 200 ft 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.

OPEN PIT DESIGN
Open Pits were designed by generating Whittle pit shells based on net block values (net smelter returns) and pit slopes recommended by CNI (Call & Nicholas) and Golder Associates. Hycroft designed the final pit with haul ramps, appropriate catch benches, and mining widths. The Hycroft open pit is a large pit, covering an area nearly 3.25 miles long by 1.75 miles wide and reaching a maximum depth of approximately 2,700 feet.

Pit phasing has been designed internal to the final pit limit. Phasing was based on lower revenue whittle shells, access, and minimum mining widths. The purpose of the phasing is to improve over-all economics by mining higher economic margin phases first. In total, 22 individual phases have been designed.

Haul ramps are design to be 120 ft wide, including the safety berm for double lane traffic accommodating 320-t class trucks. A 10% maximum grade has been considered in the final design. Some internal pit phase designs considered single-lane travel and 12.5% maximum grade for a very limited number of benches near the bottom of the phase. All pits are designed to be mined on 40- t high benches with catch benches every bench. Catch bench widths varied from a maximum of 46.20 ft to a minimum of 22.85 ft.

OPEN PIT OPERATIONS
Hycroft mining operations are currently planned for typical truck and shovel open pit mining methods.

The following ore routing is available to each block:
- Oxide Ore - ROM heap leach & ¾” crushed heap leach
- Transitional Ore - ROM heap leach, ¾” crushed heap leach, ½” crushed heap leach
- Sulfide Ore - ½” crushed heap leach.

GEOMECHANICS
Slopes in Argilic and Propylitic alterations have been designed at a 38° inter-ramp slope angle. Acid Leach alteration is completely mined out, not having any significant final pit slopes. Any pit slopes which may remain in acid leach alteration is designed at 41° inter-ramp slope angle. Slopes in Silicic alteration are designed at 50° inter-ramp slope angle.

West of the Break Fault, slope stability is controlled by the strength of the unconsolidated Camel Formation. Overall slope angles in the upper portion of the west wall sectors of the Vortex domain vary from 30° to 38°. Below the 3600 elevation, the inter-ramp slope angle is 50°, as competent silicified rock is present in the wall. There are four slope sectors that define pit slopes west of the Break Fault: WBF-U, WBF-M, WBF-L, WBF-2U.

To the east of the Break Fault, the rock strength improves significantly due to the pervasive silica alteration of the Camel and Kamma Mountain Formations. Pit slopes for these sectors have been designed to an inter-ramp angle of 38 - 50°. There are three slope sectors that define the pit slopes east of the Break Fault: Vortex-1U, Vortex-1L and Vortex-3.

Thinly bedded rhyolite of the ALS is located in the footwall of the East Fault at depth. A 38° inter-ramp slope angle has been utilized for this sector: Vortex-2. A minimum 220-foot buttress (which is mined as one of the final mining phases) in the silicified rock in the hanging wall has been designed to manage the weak ALS unit that is present at the toe of the slope.

The hanging wall of the East Fault is composed primarily of silicified rock of the Camel and Kamma Mountain Formations and is defined by 2 slope sectors: FWEF-1, FWEF-2. Pit slopes for these sectors have been designed to an inter-ramp angle of 45°.

North of the deeper Vortex portion of the final pit are sectors associated with the Brimstone Pit. The highest slope in this area of the pit is the 1,200-foot-high east wall. The slope has been designed at an inter-ramp angle of 45° for this sector, identified as WBRIM. The East Fault will be mined out in this sector, thereby avoiding potential failure associated with the weak fault zone.

The west wall of Brimstone is less than 1,000 feet high. The upper 150 to 200 feet of the wall will be excavated in mine waste dumps. This stacked waste rock has been laid back to 35° degrees, roughly the angle of repose. Alluvium is present below the dumped waste. Inter-ramp angles of 42° have been utilized for this material. Silicified and argillically altered rocks of the Camel Conglomerate occur at depth. North Ramp Fault slopes will be constructed mainly in argillically altered Camel Conglomerate. An inter-ramp angle of 42° has been used for slopes in this sector. Both these wall sections are defined by the slope sector NRF.

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The heap leach operation is designed to treat three categories of ore:

- Ore Category 1 (ROM ore) – lower grade ore with high cyanide soluble gold is routed directly to the leach pad and cyanide leached to extract gold and silver. This accounts for 4% of the ore over the life of mine. The gold contents are highly soluble and the remaining refractory gold contents are not projected to justify the time and expense of a pre-oxidation step; therefore it will be stacked as ‘ROM’. The ore in this category is typically defined as ‘ROM oxide’ or ‘ROM transition’.

- Ore Category 2 (3/4” Crushed ore) – higher grade ore with high cyanide soluble gold is crushed to a P80 of ¾” and cyanide leached to extract gold and silver. This accounts for 2% of the ore over the life of mine. The gold contents are highly soluble, but additional size reduction is expected to increase gold and silver recovery enough to justify the additional expense. The remaining refractory gold contents ........