Summary:
Gold mineralization in the Santa Rosa Gold Project has characteristics in common with mesothermal or orogenic and intrusion-hosted gold veins. Mineralization of these types consists commonly of quartz and quartz-carbonate veins located in moderately to steeply dipping, brittle-ductile shear zones and locally in shallow-dipping extension fractures. Veins commonly extend along strike and down dip over very significant distances and occur alone or, typically, in complex vein networks and shear zones. Vein minerals are mostly quartz and carbonates with minor native gold, pyrite, and base metals sulfides. Veins are usually massive or ribbon-textured, but vein breccias and drusy, crystalline quartz can also occur. Wall-rock alteration is zoned and consists of carbonate (often ankerite), sericite, and pyrite.
Hypogene gold mineralization within the Santa Rosa Gold Project is generally associated with the shear zones developed in homogeneous diorite country rock, with higher grades occurring in the associated sulfide-mineralized quartz veins or as steep high-grade quartz-sulfide veins. There are also related saprolitic gold deposits and colluvial gold deposits, both of which have been mined by artisanal miners underground and in hydraulically mined areas known locally as “baticiones.” The shear zones and veins are best exposed in adits and baticiones.
In the San Ramon deposit shear zone, mineralization occurs in fractured, brecciated, and ductily deformed rock. The lowest levels of mineralization (~0.035g Au/t to ~0.1g Au/t) consist of very weak to moderate ductile shearing that locally contains quartz veinlets and/or pyrite. At slightly higher grades (~0.1 to ~0.6 g Au/t), mineralization is characterized by moderate to strong ductile deformation that contains scattered quartz veinlets and sulfides, although the overall quantity of veinlets and sulfides is low. Some sericite, weak to moderate brittle overprinting, and quartz veins greater than 12 mm in thickness that are mostly, if not completely, barren of sulfides, may also be present. The boundary between the two grade ranges is likely gradational, and there are numerous instances where quartz veins or pyrite are not apparent in either. Shear zone intercepts containing these low grades of mineralization may be up to 80 m wide; the maximum true width of the mineralized shear zone is about 60 m.
Higher-grade mineralization, >0.6 g Au/t, within the San Ramon deposit shear zone is characterized by strong shearing, more intense sulfide mineralization, and quartz veins. At relatively lower grades within this higher-grade mineralization (~0.6 to 5.0 g Au/t), strong ductile shearing with abundant sulfide minerals and sericite is almost ubiquitous; quartz veins may or may not be present. Brittle overprinting is moderate to strong and may be genetically related to the high-grade mineralization. At grades in excess of ~5.0 g Au/t, massive and coarse-grained pyrite, pyrite stringers, medium-grained sphalerite, fine-grained galena, and traces of chalcopyrite are present in quartz veins and quartz vein fragments, although some massive pyrite and pyrite stringers occur independently of quartz veins. In the highest-grade intercepts (>50 g Au/t), relatively thick (=2 cm) massive pyrite veins are intermixed with quartz veins that contain coarsegrained and massive pyrite, coarse-grained sphalerite, fine-grained galena, and traces of chalcopyrite.