Summary:
Based on current published explanations for the Mineral Ridge mineralization, it appears that a structurally-controlled orogenic model may be the most applicable to the deposit.
The currently-known mineralized zones occur over an area of approximately 14,000 ft (4,300 m) north south and 15,000 ft (4,600 m) east–west.
The gold mineralization at Mineral Ridge is primarily associated with milky quartz veins and lenses accompanied by local argillization and some sericitization. Prior to mining, the individual zones were as much as 50 ft (15 m) thick, typicallyconsisting of a higher-grade quartz veins from five to 30 ft (9 m) thick, surrounded by a lower-grade envelope of mineralization. Two or more highgrade zones are commonly observed stacked in en echelon patterns.
Gold deposition is structurally-controlled, and some of the highest-grade mineralization is found in shoots that are at an oblique angle to the direction of movement of extension. Gold is present as native gold and electrum that occurs as irregular shaped intergrowths in quartz associated with interstitial space and small fractures. Gold also occurs as irregularly-shaped intergrowths and as fracture fillings associated with goethite, much of which was derived from original pyrite. In partially oxidized mineralization, gold occurs with both pyrite and goethite. Gold particle size varies from 1–2 µm to about 700 µm, but most of the particles are in the 5–50 µm size range. Gold to silver ratios are typically in the 2:1 range. Locally, minor amounts of galena, graphitic to carbonaceous material, sphalerite and anglesite/cerrusite have been observed, with galena and graphite appearing to be associated with higher-grade gold values.
Much of the known mineralization has been mined out in the Drinkwater, Mary LC, Wedge A, B, C, and D, Bluelite, Solberry, Missouri, Brodie, and underground portion of the Oromonte areas. Five areas of remnant mineralization adjacent to the Drinkwater, Mary LC, Bunkhouse Hill, and Brodie open pits are considered to remain prospective for remnant open pit mining, as is the Custer area, which has no open pit mining history, situated to the southeast of Bunkhouse Hill. Four of these areas are on the eastern, Drinkwater, side of the Mineral Ridge antiform (Drinkwater, Mary LC, Bunkhouse Hill, Custer), and two, Oromonte and Brodie, are on the western limb of the antiform.
Drinkwater
An area remains to the northwest of the Drinkwater deposit, and currently forms the highwall of the Drinkwater pit. The mineralized zones in the Drinkwater deposit strike northwest–southeast and dip about 20–25º to the northeast. Mineralized zones are irregular in shape, and have variable thicknesses. In the Drinkwater remnant area, there is a thick sequence of waste material that will require pre-stripping to access the mineralized horizon.
Mary LC ( Mary LC Phase 4 Pit)
The Mary LC deposit is located about 500 ft southeast of the Drinkwater deposit. The stratigraphy and mineralized horizons dip to the northeast at approximately 25º. The fourth proposed pit phase would extract mineralization contained in a zone on the northwest pit highwall.
Bunkhouse Hill
Geologically, the Bunkhouse Hill target is the immediate down-dip extension of the mineralization present in the Mary LC pit. The mineralization at Bunkhouse dips at about 39º NE, which is incontrast to the more typical 25–30º NE dip of the Mary LC mineralization. This dip change is spatially related to a change in geology, whereby the mineralized zone transitions from being hosted in felsic intrusive rocks to being hosted in Wyman Formation sedimentary rocks.
Custer
The Custer zone is situated south of the Mary open pit and the Bunkhouse Hill area. Structurally, Custer is very similar to the Drinkwater deposit, having far less of the post-mineral faulting and folding that was predominant in the Mary and Last Chance deposits.
Brodie (Brodie Phase 2 Pit)
Mineralization at Brodie is more flat-lying than mineralization at Drinkwater and Mary.
Oromonte
Mineralization at Oromonte is located north of the Brodie pit area. Mineralization occurs in a downdropped block located between the mined-out Solberry and Wedge pits.