Summary:
Deposit Types
The Bull River deposit has been described as a Churchill-type vein copper-silver deposit (Lefebure, 1996). The deposit type displays characteristics of relatively low tonnage (typically range from 10Kt to 1Mt) but high-grade (typically range from 1% to 4% Cu). Frequently occurring in Proterozoic-age extensional sedimentary basins, Churchill-type deposits are associated with rifting can comprise single vein to complicated vein systems that vary from centimetres to tens of metres in width and can extend hundreds of metres along strike and down dip. Commonly hosted in clastic metasediments, veins and vein systems are often spatially associated with mafic dykes and sills. The veins are generally associated with major faults related to crustal extension that controls the ascent of hydrothermal fluids to favourable sites for metal deposition. Fluids are believed to be derived from those mafic intrusives that are associated with the vein systems.
Mineralization in Churchill-type deposits is predominantly chalcopyrite, pyrite, and chalcocite with subordinate pyrrhotite, galena, bornite, tetrahedrite, argentite, and covellite and is generally younger than the host lithology. Dilation of veins is commonly caused by cross-structures or folding and results in concentrations of mineralization. Likewise, the intersection of veins is a locus of ore deposition. Mineralization can occur as massive and/or semi-massive sulphides that may be identified as conductors by electromagnetic (EM) surveys. Mafic intrusive bodies and related structures can be defined by magnetic, very low frequency (VLF), or EM surveys.
Alteration usually occurs within host rock in contact with veins and up to tens of metres from the veins with carbonization and silicification as typical alternation types in metasediments (BC MINFILE).
Mineralization
TheBull River Mine (BRM) mineralized zones comprise a vertical to subvertical network of sulphide-bearing quartz carbonate veins striking approximately east-west hosted in sheared and brecciated Aldridge Formation sediments. The vein systems form complex networks within, and adjacent to, the shear zone and often encompasses crushed, deformed, and brecciated host rocks (Baldys, 2001). Host rocks are either partly silicified and chloritized argillites, argillaceous quartzites and quartzites (Masters, 1990). The veins pinch and swell forming stockworks or thick tabular bodies that are often cut by smaller veins and stringers of quartz and quartz-siderite. The main vein structure and associated stringer zones can range from a few centimetres to 30m wide. In 1991, Masters defined five subparallel to en echelon “vein systems” and differentiated them from the Pit Zone that lies within the footwall (Masters, 1991).
Mineralization consists of pyrite, pyrrhotite, and chalcopyrite with minor local galena, sphalerite, arsenopyrite, and cobaltite and traces of tetrahedrite and native gold. Sulphides range from massive, irregular bodies within the vein system to thin discontinuous veins, veinlets, and disseminations in the host rock (Höy et al., 2000). Gangue mineralogy of the veins is variable, with the eastern parts of the deposit consisting of quartz and siderite. The western part of the vein system is dominated by siderite (Baldys, 2001).
A plan view of the modelled mineralized vein system is illustrated in Figure 7-4. The extent of the currently known mineralization is approximately 1,200m along strike, varies in width from 2.5m to 30m and has a down-dip extent of up to 450m based on currently drilling, which remains open at depth. The sub-parallel system is continuous within each of the ten illustrated veins.