Precious metal geochemistry and mineralogy of the ovoid zone of Voisey's Bay magmatic sulfide deposit, Labrador, Canada

摘要

The Voisey’s Bay Ni-Cu-Co deposit, located in northern Labrador, is considered to be a PGE poor, magmatic sulfide deposit, but anomalous regions containing elevated concentrations of Pt, Pd, Au and Ag have been reported during routine assay. Only limited data is available which documents the types of precious metal occurrences that are present in these regions. In this thesis, a thorough investigation of the PGE and precious metal-minerals has been conducted for the Ovoid, Mini-Ovoid and Discovery Hill deposits. The results were compared to previously documented occurrences in a hornblende-gabbro dyke in the Southeast Extension zone. The significance of these occurrences has been evaluated in reference to potential for recovery of Pt, Pd, Au and Ag, and the genesis of the mineralization in the anomalous regions. Because the concentrations of precious metals at Voisey’s Bay deposit are low, unconventional methods of analysis were used to concentrate and analyze the mineral phases present. The dominant mineral phases that are present are sperrylite and froodite. Electrum, stützite and volyksite account for minor to trace proportions of the precious metal minerals. Analysis of sulfide minerals for Pt, Pd, Au and Ag by LA-ICPMS revealed that, in regions that represent very fractionated magma, most of the precious metal mass is present as discrete mineral phases containing Bi, Te, As, Sn and Sb, and only small proportions of Pd are found in solid solution in pentlandite and galena. The presence of semi-metals in the sulfide melt is the key component necessary to crystallize domains rich in PGM. Semi-metals were likely inherited through local crustal interactions of parental magma with Tasiuyak gneisses.udThe major conclusions drawn from this study are 1) that even at low-PGE grade deposits such as Voisey’s Bay, PGM are able to crystallize if sufficient amounts of semi-metals are present; and 2) recovery of the Pt, Pd, Au and Ag at Voisey’s Bay may be possible because the majority of the precious metal mass is associated with minerals that are already recovered in the Ni or Cu concentrates, or they are present as mineral grains large enough to be concentrated alone.