Summary:
The deposits that comprise the Nevada Operations are considered to be examples of Carlin-style carbonate-hosted disseminated gold–silver deposits and intrusion-related gold–copper–silver skarn deposits.
Phoenix is a skarn-hosted polymetallic massive sulfide replacement deposit.
Au-Cu-Ag-Pb-Zn skarn mineralization at Phoenix is centered about the Eocene Copper Canyon granodiorite porphyry stock. Paleozoic sedimentary rocks within two miles of the stock have undergone strong thermal metamorphism (hornfels) and metasomatism (skarn). Metal zonation is manifest by a CuMo core close to the stock grading outward into proximal Cu-Au-Ag, Au-Ag and distal Pb-Zn-Ag mineralization.
The Phoenix Project site lies within the Basin and Range physiographic province and is composed of two relatively flat valleys and steep-sided ranges with approximately 3,700 feet of relief. Elevations range from approximately 4,520 feet along the Reese River to 8,232 feet at Antler Peak. The mountains serve as hydrologic divides that separate drainage basins. The mountain flanks are deeply incised in places, and the resulting canyons collect and discharge runoff to creeks and alluvial fans. The valley floors grade toward the Reese River and Buffalo Valley Playa.
Quaternary alluvial deposits cover most of the valley floors and lower drainage basins. In general, the alluvium fan away from the mountain fronts, with the coarsest material having accumulated in upper alluvial fan deposits and the finest material being deposited in the center of the valleys away from the mountain front. The valley fill in many basin and range valleys in Nevada can be more than 3,000 feet thick.
The Quaternary volcanic rocks within the Phoenix Project site principally consist of basalt flows southeast of Copper Canyon referred to as the Caetano Tuff. The Caetano Tuff has a maximum thickness of approximately 300 feet and caps ridges near Rocky canyon and Elephant’s Head, which lies to the northeast of the Phoenix Project site.
There are numerous small igneous intrusions of Mesozoic- and Tertiary-age exposed in the Battle Mountain Range. The most hydrologically significant intrusive is the Tertiary Copper Canyon granodiorite, which roughly divides the Phoenix/Fortitude pit from the down-gradient Reona and Bonanza pits.
The Golconda allochthon (or Havallah sequence) is comprised of the Pumpernickel and Havallah Formations. The Havallah sequence is a tectonically interleaved assemblage of chert, argillite, shale, siltstone, sandstone, conglomerate, limestone, and greenstone. The Pumpernickel Formation consists predominantly of argillite and chert that forms ridges and ledges where exposed.
The Havallah Formation consists of a lower basalt and gabbro unit, as well as chert, siltstone, sandstone, and limestone units. The Havallah sequence rocks cover most of the surface of the Battle Mountain Range west of the Phoenix Project site.
The Antler Sequence is an autochthonous, overlap sequence that lies unconformably on the Roberts Mountain allochthon within the Phoenix Project site. It is made up of three formations from oldest to youngest: the Battle Formation, the Antler Peak Limestone, and the Edna Formation. These formations were deposited in a shallow marine environment. The Battle Formation consists of thick-bedded conglomerate, sandstone, shale, and limestone. The overlying Antler Peak Limestone is a limestone that is locally shaley, sandy, and pebbly. The Edna Mountain Formation is mostly sandstone and chert pebble conglomerate, with cherty limestone.
The Harmony Formation is the oldest rock exposed within the Phoenix Project site. Much of the exposed rock of the Battle Mountain Range east of copper canyon is comprised of the Harmony Formation. The formation is mainly medium- to coarsegrained sandstone with lesser shale cemented by calcite approximately 3,000 feet thick.
Base and precious metal mineralization occurs as disseminated and massive sulfide replacements of calcium-silicate rock units. Mineralization also occurs as sulfide veins and fissure fillings within and outside areas of calcium-silicate alteration.
Gold mineralization occurs freely at gangue–gangue or at sulfide–gangue grain boundaries, and only rarely as inclusions within gangue minerals. Some inclusions were noted in quartz, pyroxene, epidote, and orthoclase. The remaining gold occurred as inclusions totally encapsulated by sulfide minerals including pyrite, pyrrhotite, and to a lesser extent arsenopyrite, chalcopyrite, and sphalerite. Silver minerals are dominantly electrum, hessite, and lesser argentite.
Copper oxide mineralization locally contains minor amounts chalcanthite, malachite, chrysocolla, azurite, and lesser cuprite. Enriched copper mineralization typically has chalcopyrite ± covellite present. Covellite locally rims chalcocite grains where the effects of oxidation are more advanced. In hypogene mineralization, chalcopyrite occurs as disseminations and bedded replacements with skarn and silicate minerals, and in conjunction with pyrite.
Phoenix Complex deposits include the following deposits: Fortitude, Bonanza and Greater Phoenix, combined deposit lengths are 16,000 ft long, 3,900 ft wide, and the deposits have a thickness range from 20–550 ft.