Date of Award
Earth and Environmental Sciences
Iain M. Samson
Arsenopyrite, Gold, LA-ICP-MS, LMCE polymetallic melts, Pyrite
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The Wawa Gold Corridor (WGC), located in the Michipicoten greenstone belt, comprises Au-bearing shear zones that crosscut the 2.75 Ga Jubilee Stock. Shear zones formed during protracted deformation (D1 – D3) and contain Au mineralization produced by two major events. The first comprises syn-deformation gold with arsenopyrite and pyrite in D1 – D2 schists and the second comprises post-deformation gold with Bi-Te minerals, chalcopyrite, and pyrite in carbonate ± riebeckite alteration rims around pyrite in D3 shear veins. Observations of porosity and inclusions in D1 arsenopyrite and D2 pyrite suggest that these sulfides underwent coupled dissolution-precipitation reactions (CDR). Based on trace-element analysis by laser-ablation inductively-coupled plasma mass spectrometry, it is evident that the replacement of Au-rich minerals by Au-poor minerals during CDR generated native gold from Au previously in solid-solution. Textural relationships amongst gold and Bi-Te minerals are consistent with precipitation from a Bi-rich polymetallic melt during the second gold event. The rare presence of riebeckite in the siderite-rich coronae that host Au-Bi-Te assemblages suggests that this event was associated with the emplacement of 1.0 Ga lamprophyre dikes. However, overlap in the δ34S values of pyrite in this event (0.4 – 2.4 ‰) with those of earlier pyrite (0.1 – 4.1 ‰) indicate recycling of pre-existing sulfide sulfur during the 1.0 Ga hydrothermal overprint. By comparing the Au contents of arsenopyrite (10s – 1,000s of ppm) to those of pyrite (<0.01 – 10s of ppm), it is determined that despite the textural and temporal variance with which gold occurs, almost all of the gold in the system originated as solid-solution Au in D1 arsenopyrite. This work therefore highlights the importance of discriminating between the introduction of new Au and the redistribution of existing Au within a deposit, an intricacy that can be resolved using mineral-chemical analysis and mass-balance calculations.
Wehrle, Elliot Andrew, "Gold mineralization in the Archean Wawa Gold Corridor, Wawa, Ontario" (2020). Electronic Theses and Dissertations. 8490.