Summary:
Copper-cobalt-silver-zinc-germanium mineralization at the Bornite Property forms disseminations, veins, and massive sulphides in stacked, semi-stratabound bodies closely associated with secondary hydrothermal dolomitization. The cross-cutting nature of the mineralization along with the presence of early pyrite and sphalerite in sedimentary breccia clasts suggest an epigenetic origin that was temporally very close after the deposition of host strata. Re-Os dating supports this interpretation (Selby et al., 2009).
Data are limited regarding the sources and nature of the copper-rich fluids that formed the Bornite deposit, but they suggest that mineralizing fluids may have formed from the interaction of saline basin fluids with mafic volcanic rocks in the area.
Given these constraints, Bornite has characteristics similar to other districts and deposits including: the Mount Isa and McArthur River districts in Australia, the Tynagh deposit in Ireland, the Kipushi deposit in the Democratic Republic of the Congo, and the Tsumeb deposit in Namibia. All of these deposits show early epigenetic characteristics, emplacement in carbonate stratigraphy, and early pyrite-dolomite alteration followed by sulphide mineralization.
These comparable deposits occur in intra-continental to continental margin settings undergoing extensional tectonics and bimodal volcanism similar to Bornite. Basin-margin faults seem to have been important in localizing mineralization (Hitzman, 1983) although basin margin structures at Bornite have not been directly identified.
An early epigenetic carbonate-hosted Cu-Co model is applicable for exploration targeting in the project area.
Property Geology
The geology of the Bornite resource area is composed of alternating intervals of carbonate rocks (limestone and dolostone) and calcareous phyllite. Limestone transitions laterally into dolostone near zones of mineralization and is considered hydrothermally altered. Spatial relationships and petrographic work suggest that dolomitization is genetically related to early stages of the copper mineralizing system (Hitzman, 1986).
Mineral Deposits
Mineralization at Bornite forms tabular mineralized zones that coalesce into crudely stratabound bodies hosted in dolostone conglomerate/breccia. Two significant dolomitic horizons that host mineralization have been identified by drilling and include: 1) the Lower Reef, a substantial 100 m to 300 m thick dolomitized zone lying immediately above the basal quartz phyllite unit of the Anirak schist and 2) the Upper Reef, a 100 m to 150 m thick dolomite horizon that sits roughly 300 m higher in the section. The Lower Reef is separated from the Upper Reef by a zone of ductilely sheared phyllites up to 60 m thick.
The Lower Reef dolostone outcrops along the southern margin of the Ruby Zone and is spatially extensive throughout the deposit area. It hosts a significant portion of the shallow mineral resources in the Ruby Zone as well as higher grade mineral resources down-dip and to the northeast in the South Reef area. The Upper Reef hosts relatively high-grade mineral resources to the north in the Ruby Zone. The Upper Reef zone appears to lie at an important north-east-trending facies transition to the northwest of the main drilled area and appears to be at least partially thrust over the Lower Reef stratigraphy to the southeast.
Mineralization
Copper mineralization at Bornite comprises chalcopyrite, bornite, and chalcocite distributed in stacked, stratabound zones exploiting favourable lithologies (conglomerate/breccia) within the Bornite sequence. Mineralization occurs, in order of increasing grade, as disseminations, irregular and discontinuous stringer-style veining, breccia matrix replacement, and stratabound massive sulphides. The typical mineralization of the Bornite deposit characterized by chalcocite, bornite, chalcopyrite and pyrite.
The distribution of copper minerals is zoned around the bottom-centre of each zone of mineralization, with bornite-chalcocite-chalcopyrite at the core progressing outward to a fringe of chalcopyrite-pyrite. Additional volumetrically minor copper minerals include carrollite, digenite, tennantite-tetrahedrite, and covellite (Bernstein and Cox, 1986). Stringer pyrite and locally significant sphalerite occur above and around the copper zones and locally massive pyrite and sparse pyrrhotite are associated with siderite alteration below copper mineralization in the Lower Reef.
Significant cobalt mineralization is found accompanying bornite-chalcocite mineralization). Cobalt often occurs with high-grade copper as carrollite (Co2CuS4), cobaltite (CoAsS) and as cobaltiferous rims on recrystallized pyrite grains (Bernstein and Cox, 1986). Preliminary geometallurgical work by Trilogy Metals showed that cobalt occurs primarily as cobaltiferous pyrite (approximately 80% of the contained cobalt) and within other cobalt minerals, such as carrollite and cobaltite (CoAsS).
Germanium is also seen to be associated with copper mineralization (Runnels, 1963; Bernstein and Cox, 1986). In 2011, 50 mostly continuous core samples selected from four drill holes were found to have germanium values ranging from <1 to 83 ppm and averaging 10.7 ppm using sample preparation methods specifically for germanium.