Overview

Mine Type	Open Pit
Stage	Permitting
Commodities	Gold Silver
Mining Method	Truck & Shovel / Loader
Mine Life	...

Deposit type

• Skarn
• Epithermal

Summary:

Mineralization at Mt. Hamilton consists of skarn hosted tungsten, molybdenum, and copper +/- zinc with later epithermal gold and silver. Gold mineralization is primarily hosted in a 200 to 300 ft thick skarn horizon, bounded by upper (200 ft thick) and lower (450 ft thick) hornfels units. The bounding hornfels had lower permeability and were therefore less receptive to late-stage mineralization. The interbedded skarn in the Centennial area was subject to late-stage, low-angle faulting. These faults were conduits to late mineralizing solutions and oxidation. The result is an oxide-hosted epithermal gold deposit overprinting a retrograde polymetallic skarn. The main Centennial precious metal mineralization is contained within a southeast dipping (15° to 20°) tabular zone that ranges from 20 to 250 ft in thickness. In the NE Seligman area, ore grade mineralization appears to be largely stratiform in shallow-dipping, bedding-parallel, structurally and chemically prepared zones with local high-angle, cross-cutting, possible "feeder" zones (Burgoyne, 1993). At Centennial, the mineralization is controlled by late low-angle structures that are discordant to bedding and oxidized to significant depth. Gold grades of samples within the retrograde alteration range from <0.001 oz/t Au (lower analytical method detection limit) to 0.995 oz/t. The occasional high grades appear to be associated with crosscutting structures and veins within the skarn as described below.

In the Centennial deposit, weathering and oxidation of original sulfide mineralization caused formation of oxide mineralization (with low sulfide mineral residuals) from which gold is recoverable by cyanide heap leaching. In general, the acid generating capacity of the surrounding carbonate rocks is low or nil, and their acid consuming capacity is high. Gold is present as free gold, residing in iron oxide minerals or quartz, and adsorbed on clay minerals. Sulfosalt-bearing veins may be associated locally with the higher grades of gold and particularly silver. These veins cut both skarn and intrusive rocks and are closely associated with zones of retrograde alteration. These veins range in thickness from about 2 to 60 cm. In the Seligman deposit the mineralization is similar to Centennial but is, on average, thinner. Widely spaced drilling between the two deposits indicates that they represent one contiguous mineralized system that could be connected by additional drilling. Locally, more sulfide minerals are preserved at Seligman in comparison to Centennial. As seen in the mine excavations of the NE Seligman deposit, veins seem to exhibit strong continuity along strike.

Mining Methods

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Comminution

Crushers and Mills

Processing

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Production