Summary:
Silver mineralization at the Bayhorse Property is associated with veins, stringers and disseminations in rhyolite along and in close proximity to the Sunshine fault which is hosted in Late Triassic to Jurassic volcanics and sediments of the Weatherby and Huntington formations.
The Bayhorse Silver Deposit, being at least spatially related to the intrusive rhyolite, is either synor post Weatherby in age and may have formed in an epithermal or sub-epithermal environment. The deposit has a strong structural control along the prominent Sunshine Thrust Fault. Mineralization is dominated by various sulfosalts, including tetrahedrite-tennantite, together with abundant pale colored, low Fe-bearing sphalerite and possibly acanthite (Leitch, 2013). In addition, there is chalcopyrite and covellite with minor galena and pyrite mineralization. Gangue minerals include quartz-silica and carbonate with minor sericite, potassium feldspar, chlorite and clay. Fracture-controlled copper (Cu) oxide mineralization as either malachite, azurite or black Cu-Manganese (Mn) wad is common in the vicinity of the Intermediate and Upper adits. Surface sulfosalt-sulphide mineralization is seen either in massive, 0.4 to 1.6 inch (1 to 4 cm) thick veins along rhyolite-andesite faulted contacts or as fracture-controlled replacements, veins and stockworks hosted by rhyolite.
Mineralization
The primary style of mineralization on the Bayhorse Property comprises fracture-controlled veins, pods and replacements of sulphide and sulfosalt mineralization (mostly tetrahedrite-tennantite with other Ag sulfosalt minerals) that are commonly hosted by the Sunshine Thrust Fault, late rhyolite dykes and sills, or occupy rhyolite-andesite faulted contacts. This Ag-rich ± Cu ± Zn mineralization has been historically mined at the Property and continues to be the main focus of Bayhorse Silver’s ongoing exploration and development work.
Fracture-controlled Cu oxide mineralization as either malachite, azurite or black Cu-Mn wad is common on surface in the vicinity of the Intermediate and Upper adits, where it is hosted by both andesite and rhyolite. The Cu oxide is believed to be the weathering product of Cu-bearing sulfosalt (tetrahedrite-tennantite series) mineralization. Sulfosalt mineralization, with lesser sphalerite, chalcopyrite, covellite and trace galena, is common in stockpiled materials on surface near the Upper and Intermediate Level portals. Also, at the Intermediate Level portal, several mineralized outcrops are present along or close to the Sunshine Thrust, which separates rhyolite from underlying andesitic Huntington rocks. Apart from some rare Cu oxides, no surface mineralization was observed in the vicinity of the Lower Level portal.
As noted by Conway (2018), the main sulphide minerals in the Bayhorse Mine are the tetrahedrite-tennantite series ((Cu,Fe)12(Sb,As)4S13), together with sphalerite (ZnS), and galena (PbS). Geochemical data indicates that antimony (Sb) is much more abundant than arsenic (As) within the Bayhorse mineralization, indicating a predominance of tetrahedrite. Freibergite ((Ag,Cu,Fe) 12 (Sb,As) 4S13) is a tetrahedrite group mineral in which Ag substitutes in the Cu-Fe position. Since Ag is closely associated with sulfosalt mineralization, the dominant sulfosalt mineral present in Bayhorse mineralization may actually be freibergite.
Surface and underground exposures suggest that there is a strong spatial relationship between the Bayhorse sulfosalt-sphalerite mineralization and rhyolite. On surface, two modes of sulfosalt-sulphide mineralization are observed; (i) massive veins and pods hosted by the Sunshine Thrust where it separates rhyolite from andesite; and, (ii) as narrow (<1.2 inch [3 cm]) fracture-controlled veins and stockworks that are entirely hosted by, or replace rhyolite or Huntington rock as seen in the Legend Zone. Type (i) mineralization is observed at the face of the Intermediate Adit where bonanza silver grades of up to 150 kg per tonne occur (Ray, 2015). The sulfosalts are mostly dark grey to black, fine-grained and massive although some coarsely crystalline tetrahedrite-tennantite occurs locally. The sphalerite is low in Fe (Leitch, 2013), which makes it difficult to identify with the naked eye, even though it can make up 20% of the veins. Pyrite is generally uncommon but does occur in small amounts as fine grained disseminations and veinlets. The sulfosalt mineralization is associated with silicification, thin quartz stringers, calcite-dolomite carbonate, chlorite, and minor sericite and K-spar (Leitch, 2013).
Surface and underground mapping (Conway et al., 2014) indicates that, in part, the mineralized body is essentially flat lying and elongated east-west along the Sunshine Thrust. Conway indicates that the highest grade material lies within the Sunshine structure and perhaps within rhyolite fault slices in the 20 to 40 ft (~6 to 12 m) thick fault zone. The mineralization observed so far beneath the Sunshine Thrust is in footwall andesite beneath the Sunshine and Big Dog stopes. It is generally of lower grade, as typified by the newly discovered Legend Zone. However, the footwall argillic to silicic alteration with local chlorite may be quite extensive and occurs mainly as white to pale green veins and patches within the red-brown hematite-rich andesite. Central to the pale alteration veins are gray to black veins and veinlets of very fine-grained mineralization that is presumed to be mostly tetrahedrite. It is possible that these extensive areas of lower grade material represent feeder zones for the overlying and higher grade mineralization. It is also possible that sulphide mineralization at the Bayhorse Mine occurs in both the flat lying Sunshine Thrust, as demonstrated by Conway et al., (2014) and as a steeply dipping body.
A geological structural review by Dr. Clay Conway in April 2022 of the mineralized system deep inside the historic mine at the Big Dog and Goldilocks Zones indicated strongly altered andesite and locally richly mineralized rhyolite are interlensed as 'fault slices' in the complex Sunshine thrust fault zone. Historically mined Ag-rich mineralization has been taken from rich pockets in the fault zone which may be considered to have been a controlling factor for the mineralization. The Sunshine thrust zone is more than 63 feet thick in the Goldilocks area; the bottom is seen but not the top; an inaccessible raise extends another 25 feet higher. In comparison, the mineralized fault zone at the Sunshine stope near the historical upper adit, is about 20 feet thick. The large proportion of rhyolite (greater than 60%) in the thick fault zone of the Goldilocks area, at the far western end of the mine, indicates there is strong potential for further westward extension of silver mineralization. In new sampling from the Goldilocks zone, silver is as high as 145 ounces/tonne (4,500 grams/tonne). An 88 foot high historic raise in the Goldilocks zone is currently being timbered for safety, for further access, and to enable future mining development.
Mineralization here is similar to, and continuous with, the historically extensively stoped Big Dog zone. Grab samples taken during the review at the Big Dog zone returned 218.2 oz/t (6,786.76 g/t) and 30.17 oz/t (938.4 g/t) silver respectively, while a Goldilocks sample returned 145.05 oz/t (4,511.5 g/t) silver. The reader is cautioned that chip, channel and panel samples are considered select samples that may not be reflective of average or mined grades.
The Bayhorse exploration model holds that the silver/copper rich mineralized rhyolite at the Bayhorse Silver Mine could have its source in underlying shallow granites that may be conductive porphyry copper bodies as reflected by the low-resistivity anomalies (Conway, 2024).