Summary:
Deposit Types: Based on the common mineralogical, alteration, formational, and geologic characteristics of Mt. Hamilton, it is reasonable to apply the gold-skarn deposit model to guide future exploration of the Property. In addition to the skarn mineralization observed at Mt. Hamilton, the Seligman and Centennial areas both display typical features of a potential Carlin-type overprint.
Mineralization
Mineralization at Mt. Hamilton is characterized by an early polymetallic molybdenum (Mo) copper (Cu) tungsten (W) + Au-Ag skarn-related phase and a late Au-Ag epithermal overprint. Gold-silver mineralization at Mt. Hamilton occurs within a broad north-trending zone of anomalous gold and is hosted in three contiguous deposits known as Seligman, Seligman Stock and Centennial, Combined mineralization spans an area approximately 5,800 ft long and 2,000 ft wide, and ranges from exposed at surface to 730 ft below surface.
High- and low-angle faults along with skarn assemblages developed along lithologic contacts are the main controls to mineralization. Gold and silver are predominantly hosted within garnet-pyroxene, pyroxene-tremolite-quartz-potassic feldspar-calcite assemblages and quartz veins (Meyers et al., 1991). Dominant sulfide minerals include pyrite, arsenopyrite, molybdenite, chalcopyrite, scheelite, and galena.
The Seligman, Stock, and Centennial deposits demonstrate strong geochemical zonation of downhole and soils data. The Seligman deposit transitions from a more proximal zonation of Cu, lead (Pb), zinc (Zn), bismuth (Bi), antimony (Sb), Mo to a more distal arsenic (As), Au, and Ag zonation. Similarly, the Stock is concentrically enriched in Pb, Bi, Sb, Mo, Cu, As, Au, and Ag. The Centennial deposit displays a concentric zonation of elevated W, Bi, Zn, Pb, Sb, As, Mo, Cu, Au, and Ag, with smaller satellite zones of enriched Ag and Au to the south.
Gold occurs as free gold, in association with sulfide minerals (pyrite and arsenopyrite), in association with oxide minerals (hematite and goethite), disseminated with clay, and encapsulated within quartz (Paster, 1988, 1989, and 1990).
Myers et al. (1991) observed that sulfide-sulfosalt bearing quartz veins cut both the skarn and stock and are closely associated with retrograde skarn zones. The veins vary in thickness from <1 to 30 ft and are continuous over an area measuring 2,000 by 4,500 ft. These quartz veins may be Au-Ag bearing and contain sphalerite, galena, pyrite, covellite, bornite, stibnite, chalcopyrite, iron oxides, and minor tetrahedrite, bournonite and jamesonite.
Seligman Deposit
The mineralization at Seligman is laterally continuous and spans an area approximately 3,400 ft long, 2,000 ft wide and extends to a depth of 530 ft, though it is more commonly <100 ft below surface. The deposit has an overall shallow plunge (15°) to the north.
Mineralization is interpreted to be controlled by skarn developed along the contact between the Hamburg Dolomite and Dunderberg Shale and by high-angle faults. Mineralized skarn thicknesses delineated by drilling within mineralized zones vary from approximately 3 to 500 ft, with the thickest skarn intervals occurring at intersections between the Dunderberg-Hamburg contact and northeast- and west-northwest-striking faults.
Centennial Deposit
Gold-silver mineralization at Centennial is laterally continuous and spans an area of 2,400 ft long, 1,600 ft wide, and extends to a depth of 730 ft below surface. Mineralization is hosted by skarn and hornfels units within the Secret Canyon and Dunderberg Shale. Gold-silver mineralization is typically associated with a sub-horizontal (10° to 20°), laterally continuous, highly oxidized, and variably silica altered and brecciated zones along low-angle structures. The zone has a shallow dip to the south-southeast and has been interpreted to be controlled by a low-angle structure by previous workers (Myers et al., 1991; Humbree, 1996). These zones are dominated by goethite-quartz assemblages and represent “Type 2” retrograde alteration of Myers et al. (1991).
The favorable host for gold-silver mineralization is skarn developed within the Dunderberg Shale, Hamburg Dolomite and the Secret Canyon Shale. Intense mineralization typically occurs at the contact between the different units.
Myers et al. (1991) notes that Au mineralization at Centennial consists of two types. The first is predominantly low-grade gold (<0.03 oz/ton Au) associated with garnet-pyroxene zones containing up to 5% fine grained pyrite (or iron oxides) with trace arsenopyrite. The second type ranges from low (<0.03 oz/ton Au) to high grade (>1.0 oz/ton Au) and is associated with retrograde assemblages. This style hosts the majority of high-grade material at Centennial. Polished thin section studies of mineralized material conducted by Westmont concluded that gold is generally <2 µm in diameter (MRDI, 1997). Widespread but variable silver values are also present in quartz-sulfide-sulfosalt veins within the faults (MRDI, 1997).
Seligman Stock (Igneous)
Gold-silver mineralization within the Seligman Stock is laterally continuous over an area approximately 4,200 ft long, 1,400 ft wide, and on average extends to a depth of 450 ft below surface. Mineralization is hosted within the endoskarn, along structures, veins, and breccias within the main stock. The mineralized zone transitions from sub-horizontal in the northern portion of the stock, to shallowly (25°) dipping east in the central portion, to shallowly dipping west (10° to 15°) in the southern portion. The deposit has an overall shallow plunge (15°) to the north.