Summary:
The Cariboo Gold Project shares many characteristics with an orogenic gold deposit model. This class of deposit is typified by deformed and metamorphosed mid-crustal blocks and major structures, inherent products of orogenesis. Orogenic gold deposits span the entire breadth of the province of British Columbia, occurring predominantly within two main belts. The westerly belt is associated with accreted pericratonic terranes linked to Late Cretaceous to Paleocene movement on crustal-scale dextral strike-slip fault systems along the western margin of the Stikine terrane, and eastern Coast Belt (e.g., Bralorne-Pioneer, Atlin, Cassiar). The easterly belt is crudely cospatial with the Jurassic to Cretaceous accretion of the Intermontane terranes and autochthonous strata of the ancestral North American (e.g., Cariboo, Sheep Creek) (Allan, 2017). Orogenic deposits have significant economic importance, as they are known to host auriferous mineralization as high-grade vein deposits, low-grade bulk-tonnage lode deposits, and are intimately linked with substantial placer accumulations (Goldfarb et al., 2001; 2005).
Gold +/- silver-bearing veins and replacement-style mineral deposits in the Cariboo Gold Project are interrelated but can be subdivided into five principal types:
1. Fault-fill shear veins in fractured early-phase quartz lenses within carbonaceous mud and siltrich, foliation sub-parallel (northwest-southeast trending) shear zones (BC Vein-style);
2. Sub-vertical, foliation-perpendicular (northeast-southwest trending) so-called axial-planar (“AXPL”) veins structurally controlled by late-stage extensional fractures preferentially formed in rheologically brittle sandstone units;
3. Foliation-oblique so-called extensional (“EXT”) veins characterized by greatest mineral potential where in association with AXPL vein systems;
4. Sulphide-replacement bodies structurally controlled by and elongate parallel to the hinges of F2 folds within calcareous sandstones and limestones (Mosquito Creek-style);
5. Fault bound sulphide-replacement bodies within calcareous siltstones (Bonanza Ledge-style).
Vein-related Mineralization
Axial planar quartz veins are the primary source of vein hosted Au +/- Ag within the Barkerville trend, defining the fundamental architecture of the Mosquito Creek, Shaft, Valley, Cow Mountain, Lowhee, and KL deposits. Individual veins range in width from millimeters to several meters. Where density is high, AXPL veins form mineralized corridors extending for up to a few hundred meters along strike and down-dip within rheologically prospective units. Though often advantageous to model and describe AXPL veins as tabular bodies, their morphologies are generally more complex. They are often observed to pinch and swell in thickness with undulatory margins, and commonly network with (and/or refracture and cross-cut) earlier extensional (“EXT”) vein systems. Au-Ag-bearing EXT veins and Au-rich (+/- Ag) sulphide replacement bodies are intimately related to AXPL vein systems, both spatially and presumably in terms of mineralizing fluid dynamics.
The composition of both the AXPL and EXT veins is quartz dominant. Lesser iron carbonate usually occurs as vein-marginal or clustered intergrowths and vein-hosted sericite is also common. Pyrite is the most prevalent sulphide mineral across all deposits, with vein content ranging from trace amounts to tens of percent. Pyrite content appears to have a direct association with gold content within veins. Galena and arsenopyrite are also common vein-hosted sulphides, occurring in individual veins in amounts up to several percent and locally exceeding pyrite content. Additional sulphide minerals generally occurring in trace amounts include pyrrhotite, sphalerite, chalcopyrite and (rarely) argentite. Pb-Ag-Bi sulphosalts including cosalite are found in trace amounts within veins and generally have a close association with elevated Au grades. Scheelite is also locally observed, generally as secondary fill within quartz vein vugs.
Veining can be subdivided into at least two temporally separate events. Both events are characterized by a quartz - iron carbonate ± sericite hydrothermal fluid, but they differ greatly in their Au potential. Early veins may host sulphides (mostly Py, Po, Gal, Sph ± Cpy) but tend to be barren of Au except where mineralized by later fluids. The later, Au bearing veins tend to be more sulphide rich (mostly Py, Aspy, Gal, Sph ± Arg ± Cos) and cross-cut earlier veining.
Replacement-style Mineralization
Replacement mineralization in calcareous siliciclastic and carbonate facies rocks varies from fine to coarsely crystalline pyrite with rare arsenopyrite. This style of mineralization is thought to be the result of a reaction between the slightly acidic, Au bearing hydrothermal fluid and carbonate minerals within the host rocks which results in the simultaneous dissolution of carbonate and precipitation of gold-rich sulphide. Bonanza Ledge-style replacement mineralization is hosted in calcareous siltstone and consists entirely of fine-grained pyrite mineralized material. Sulphide content in replacement mineralized material types is generally high, ranging from 10% (replacing thin calcareous bands) to massive (replacing entire beds). Mosquito Creek-style replacement bodies in limestones and calcareous sands contain the most consistently high Au grades in the Cariboo Gold Project. These replacement bodies are thought to be both spatially and temporally related to mineralized AXPL vein systems.
Vein-related Alteration
Large veins tend to exhibit a strong silica alteration halo with associated vein halo pyrite. Stepping outward, moderate silicification persists, accompanied by moderate sericite, with pyrite present only in trace amounts. A widespread moderate silica envelope with patchy but intense silica closer to the veins is observed within high density vein corridors. Moving further from the fluid source, silicification becomes weak and sericite is present as the dominant alteration mineral. The distal-most alteration halo is characterized by iron carbonate and lesser sericite. Clay minerals (e.g., illite, smectite) and chlorite may be presented as vein forming minerals outside of mineralized corridors.