Summary:
Deposit Types
Gold deposits known and predominantly being explored for on the South Carlin Complex are sedimentary-rock hosted, disseminated, Carlin-type gold deposits, although epithermal- and skarn-type mineralization styles have also been found. When Carlin-type gold deposits were first recognized in Northern Nevada and Utah in the 1960s, the deposits were often informally referred to as “no-see-um” gold deposits or “micron” gold deposits because the gold is rarely visible to the naked eye and cannot be recovered by panning.
Mineralization
The South Railroad property includes Carlin-type gold mineralization in at least six deposit areas: Dark Star, Pinion, Jasperoid Wash, North Bullion, Bowl and Stallion. These deposits are similar in setting and style to that of other deposits in the region, including Rain and Emigrant. Mineralization occurs mainly as finely disseminated, submicroscopic gold in largely stratiform bodies in Devonian, Mississippian, and Pennsylvanian-Permian rocks. The following subsections describe the mineralization for deposits that are located within one of the three parts of the property. The descriptions of the Dark Star, Pinion, Jasperoid Wash, North Bullion, and Pony Creek mineralization are modified from Dufresne and Nicholls.
South Railroad Property
Dark Star Deposit
The Dark Star deposit is hosted primarily within Pennsylvanian-Permian undifferentiated units, with minor amounts of gold mineralization found in the Chainman Formation. The deposit is centered along the north-south-striking Dark Star fault corridor and anticline. As presently defined by drilling, the deposit consists of the Dark Star Main and Dark Star North zones and is approximately 4,900 ft (1,500 m) in length with a maximum width of 3,000 ft (900 m). The deepest known mineralization is 1,900 ft (600 m) below surface.
Pinion Deposit
The Pinion gold deposit is located along the northwest-trending Pinion anticline and proximal to the Bullion fault, which follows the east side of the deposit and down drops the mlbx and mineralization to the east in the North zone and part of the Main zone. The Main zone trends approximately N50°W to N60°W, is approximately 6,300 ft long by 4,000 ft wide (1,900 m by 1,200 m) and vertical thickness between ~50 to 500 ft (~15 to 150 m). Mineralization at the Main zone has been intersected to a depth of 1,500 ft (450 m) below surface where it plunges to the southeast. Mineralization is hosted primarily within a multi-lithic dissolution-collapse breccia at the Devils Gate-Tripon Pass contact. Minor gold mineralization is associated with decalcified limestone and dolostone above and below the breccia.
Jasperoid Wash Deposit
The Jasperoid Wash deposit has an approximate extent of 6,200 ft (1,900 m) in a north direction and a width of about 2,300 ft (700 m). Drilling shows the deposit consists of low-grade (0.003 to 0.012 oz Au/ton (0.1 to 0.4 ppm Au)) bedding-controlled mineralization, and steep east-dipping structurally-controlled mineralization that extends to at least 1,300 ft (400 m) below the surface. Gold is disseminated within altered feldspar porphyry dikes and adjacent conglomeratic rocks, possibly the same sedimentary units that host mineralization at Dark Star. The gold is inferred to be submicroscopic, though no petrographic studies have been completed. Higher-gold grades from 0.03 to 0.44 oz Au/ton (1 – 15 g Au/t) are associated with drusy quartz in fractures, which have a varnish of limonite and/or hematite, and with zones of very fine-grained disseminated sulfide minerals that have a sooty appearance in the argillized feldspar porphyry.
North Railroad Property
The North Bullion deposits in the North Railroad property, which include the North Bullion, POD, Sweet Hollow and South Lodes deposits, are characterized as Carlin-type disseminated-gold mineralization. Only POD and Sweet Hollow are exposed at surface. The bulk of the geological understanding and interpretations of the North Bullion deposits has come from core drilling that was guided by interpretations of gravity and CSAMT data. Gold mineralization is focused on the footwall of the Bullion fault corridor, a north-south-striking zone of normal faults with an overall down-to-the-east displacement. North-south-, northwest- and west-northwest-striking faults appear to be important controls on mineralization. In general, gold-silver mineralization is localized in Webb and Tripon Pass formation rocks, and dissolution-collapse breccia developed above and within silty micrite of the Tripon Pass Formation and calcarenite of the Devils Gate Limestone.
North Bullion Deposit
Gold mineralization in the core of the North Bullion deposit is hosted in mlbx at the upper Devils Gate contact and within or directly below a silty micrite unit interbedded with sandstones and conglomerates of the Chainman Formation. The upper limit of mineralization varies from 250 to 1,300 ft (75 to 400 m) in depth. Auriferous material dips gently to the southeast. Gold is associated with sooty, very fine-grained sulfide minerals, silica, carbon, clay, barite, realgar, and orpiment, and with elevated arsenic, mercury, antimony, and thallium. The North Bullion deposit, as currently defined, is approximately 2,900 ft (900 m) in length, 2,300 ft (700 m) in width and up to 1,650 ft (500 m) in vertical extent.
POD, Sweet Hollow, and South Lodes Deposits
The northeastern-most limit of the Sweet Hollow deposit is situated about 500 ft (150 m) southwest of the North Bullion deposit. Mineralization is potentially traceable along the uppermost mineralization horizon identified at North Bullion, although additional drilling will be required to demonstrate the continuity. As currently defined, the zone is approximately 4,000 ft (1,200 m) in length, 900 ft (275 m) in width, and up to 600 ft (180 m) in vertical extent. Mineralization at the Sweet Hollow deposit is associated with pyrite and hosted primarily by the Webb and Tripon Pass Formations. Alteration is characterized by decalcification and silicification of the sedimentary host rocks.
Mineralization at the POD deposit is restricted to a steep northeast-dipping shear zone located adjacent to and northwest of Sweet Hollow mineralization. As currently defined, the POD deposit is approximately 2,600 ft (800 m) in length, 400 ft (120 m) in width, and up to 750 ft (230 m) in vertical extent. The Chainman, Webb and Tripon Pass Formations are the primary hosts of gold mineralization, with a small amount of gold mineralization hosted in the uppermost Devils Gate Limestone. The center of the mineralized body contains carbon and fine-grained, disseminated pyrite, and accounts for approximately 15% of the deposit. Gold mineralization at POD is associated with various alteration types, including silicification, Jasperoid development, argillization, pyritization, baritization and minor dolomite replacement of calcite.
South Lodes mineralization is also potentially contiguous along stratigraphic horizons that host mineralization at the Sweet Hollow deposit, located to the northeast. Gold grades are generally lower than at North Bullion, Sweet Hollow and POD, and only minor mineralization occurs at the surface.
Pony Creek
The primary zones of gold mineralization at Pony Creek, are the Bowl, Stallion, Appaloosa and Pony Spur Zones. Additional target areas include Stallion-Bowl Trend, Palamino, Willow, Mustang, Elliott Dome, and Robinson.
The gold mineralization discovered to date at Pony Creek is principally hosted within the Tertiary (or Jurassic) rhyolite, or within altered and silicified calcareous clastic rocks of the Pennsylvanian – Permian (Penn-Perm) Moleen Formation. Known stratigraphic controls of mineralization include: the pre-mineral rhyolite intrusion acting as a barrier to focus auriferous fluids along its lower margin and within it at structural intersections.