Opaque mineralogy of the Gochager Lake Ni-Cu deposit (Saskatchewan, Canada)

The Gochager Lake magmatic gabbro-hosted Ni-Cu deposit in northern Saskatchewan, Canada, is situated within the Reindeer Zone of the Paleoproterozoic Trans-Hudson orogenic belt. In order to partially address the paucity of knowledge about this deposit, the present contribution provides new data about its opaque minerals and their chemistry, based on microscopy and electron microprobe analyses. The main sulfide and oxide minerals observed in the massive ore at the Gochager Lake deposit are pyrrhotite (~65 vol.%), pentlandite (~20 vol.%), magnetite (~8 vol.%), and chalcopyrite (~5 vol.%). Minor to trace minerals, each well below 1 vol.%, are euhedral cobaltite, ilmenite, and an unknown Pd-Ni-Bi telluride. The most economically significant mineral is the Co-rich pentlandite (Co ~2.4 wt.%), hosting the vast majority (~97%) of the Ni present at the deposit, but also approximately two-thirds of the Co here. The second most important economic mineral is chalcopyrite, the only Cu host. Minor euhedral cobaltite is important as a major host of PGEs (Pd up to 1.12 wt.%, Pt up to 0.14 wt.%, presence of Rh and Ir) and the second Co host (one-third of the Co present at the deposit). A speculative three-stage formation model of the massive ore at Gochager Lake involves the main sulfidation stage, with pyrrhotite, pentlandite, chalcopyrite, followed by a Co-PGEs stage, with cobaltite and the unknown Ni-Pd-Bi telluride, and a late stage represented by magnetite and the minor ilmenite.