Summary:
The Black Pine property is located within the southern structural block of the Black Pine Mountains where exposures consist of the lower plate units of the Jefferson Formation and Manning Canyon Shale, along with middle and upper plate units of the Oquirrh Formation, including weakly metamorphosed limestone and dolomite, silty and sandy limestone, calcareous sandstone and siltstone, quartzite, and shale.
The pre-Cenozoic strata are strongly folded and cut by faults. Virtually all contacts between formations and units are interpreted or observed to be fault contacts (Smith, 1982, 1983; Smith et al., 2020), making construction of a true stratigraphic sequence for the Project area problematic, although fossil data do loosely constrain ages of the units.
As presently understood, the Black Pine property geology is comprised of a lower structural plate that includes the Devonian Jefferson Formation and Mississippian Manning Canyon Shale, a middle plate characterized by Pennsylvanian carbonate rocks of the Oquirrh Group, and an upper plate predominantly consisting of Permian siltstones and sandstones of the Oquirrh Group. The lithologic contact between the lower plate and middle plate is sheared and brecciated, and middle plate units are complexly structurally interleaved. Middle plate strata are considerably more deformed than strata in the upper and lower plates.
The middle plate, which hosts the gold mineralization of interest, has a structural thickness ranging from approximately 656.2 to 1,640 ft (200 to 500 m). At least two major deformational events are evident, manifested by Mesozoic thrust faults and tight to open folds, overprinted by Cenozoic, low- to high-angle normal faults. Gold is distributed throughout the middle structural plate, with higher-grade mineralization occurring within favorable stratigraphic units, such as calcareous siltstones, as well as in and adjacent to breccia bodies and along variously orientated low- to high-angle brittle faults.
Deposit Type
Black Pine mineralization is best described to be in the class of sedimentary rock-hosted, Carlin-type gold deposits (“CTGDs”). While CTGDs are not unique to the eastern Great Basin, they exist in far greater numbers and total resource size in northern Nevada than anywhere else in the world. They are characterized by concentrations of very finely disseminated gold principally in silty, carbonaceous, and calcareous marine sedimentary rocks. The gold is present as micron-size and smaller disseminated grains, often internal to iron-sulfide minerals (arsenical pyrite is most common), or with carbonaceous material in the host rock. Free particulate gold, and particularly visible free gold, is not a common characteristic of these deposits except where strongly oxidized.
CTGDs in the Great Basin have some general characteristics in common, although there is a wide spectrum of variants. Anomalous concentrations of silver, arsenic, antimony, and mercury are typically associated with the gold mineralization. Elevated concentrations of thallium, tungsten, tellurium, and molybdenum can also be present in trace amounts. Alteration of the gold-bearing host rocks is typically manifested by decalcification, often with the addition of silica, fine-grained disseminated pyrite and marcasite, remobilization and/or the addition of carbon, and the deposition of late-stage barite and/or calcite veins. Small amounts of white clay (illite or kaolinite) are generally present. Decalcification of the host produces volume loss, with incipient collapse brecciation that enhances the pathways of the mineralizing fluids. Due to the small size of the gold grains, CTGDs generally do not have coarsegold assay issues common in many other types of gold deposits.
Deposit configurations and shapes are quite variable. CTGDs are typically somewhat stratiform in nature, with mineralization largely confined within specific favorable stratigraphic units. Fault and solution-collapse breccias can also be primary hosts to mineralization.
The Black Pine gold deposits also have characteristics that differ from typical CTGDs. The general location of the Project is outside the major gold deposit trends in Nevada. There are multiple silver-lead-zinc occurrences within the Black Pine property, although the temporal association with the gold mineralization is not clear.
Using the Carlin Type model as a guide, exploration at Black Pine has been focused on targeting zones of favorable stratigraphy primarily silty, sandy carbonates in Pola, Polb, Polc and sections of Ppos units where they intersection high or lower angle fault zones. Exploration has also been driven by surface geochemical anomalies in soils and rock chip sampling where elevated Carlin-type pathfinder elements such as arsenic, antimony and mercury are present.
Mineralization
The Black Pine gold mineralization can be best classified as sedimentary rock-hosted, Carlin-style mineralization.
Gold mineralization, consisting of finely disseminated, micron- and submicron-size gold particles, is hosted in middle plate, calcareous shale and siltstone, as well as fault and dissolution/collapse and tectonic breccias and zones of heavy fracturing developed in more massive limestone and dolomite. Gold mineralization is enhanced where these favorable stratigraphic units intersect, or lie along, large, pre- to syn-mineral, primarily listric normal faults. Gold was likely hosted within the lattice of arsenical pyrite rims on pyrite grains, but the mineralized rocks are now thoroughly oxidized, such that gold is present as “free” gold, associated with goethite, hematite, limonite, barite, calcite, quartz, and rare scorodite. Gold-bearing rocks are typically strongly decalcified and weakly clay altered, with areas of weak to (rarely) moderate silicification. Areas of calcite veining or calcite-cemented breccias are common, probably as a result of decalcification. Lenses of carbonaceous material, either remobilized or concentrated by decalcification, are locally present. The alteration, gold mineralization, host rocks, and geochemical associations are consistent with a Carlin-style deposit model.
Reflected-light microscopy has shown that native gold occurs in quartz and calcite veins, in hematite pseudomorphs after pyrite, and along grain boundaries (Hefner et al., 1991). In (rare) unoxidized material, an electron microscope is required to detect the gold grains, which are commonly less than 0.5 microns in diameter (Brady 1984). Gold is disseminated in clayey or silty matrix of clastic rocks and micrite groundmass of limestone. Carbon is locally present as both graphite and organic matter; gold is associated with organic matter in both clastic and carbonate sedimentary rocks.
Alteration
Strata throughout the Black Pine Mine area is weakly to strongly hydrothermally altered and contain widespread gold mineralization over the entire thickness of the middle plate, and over an area measuring at least 14 km2 . In general, the rock types with higher porosity, permeability, and geochemical reactivity, such as calcareous siltstone and sandstone, and a higher degree of brecciation, are more strongly altered.