PETROLOGICAL AND STRUCTURAL SETTING OF PLATINUM GROUP ELEMENT AND CHROME MINERALIZATION OF THE THETFORD MINES OPHIOLITIC COMPLEX, SOUTHERN QUÉBEC APPALACHIANS, CANADA

Pre-obduction syn-magmatic normal faults dissect the Thetford Mines Ophiolitic Complex (TMOC) crust into tilted blocks composed (base to top) of cumulate Dunitic, Pyroxenitic and Gabbroic Zones, sheeted dykes or subvolcanic breccias, and an extrusive/sedimentary sequence deposited upon an erosional surface that locally penetrated to the Dunitic Zone, and which removed the sheeted dyke complex from horsts. The lower Dunitic Zone hosts stratiform chromitites with moderate PGE (Platinum Group Elements) enrichment (up to 581 ppb Pt+Pd). The Pyroxenitic Zone has rhythmic harzburgite-orthopyroxenite-websterite layering near its base, grading up to massive websterite. All facies are cut by undeformed dunitic, lherzolitic and websteritic sills (1-20m) and tabular intrusions oriented parallel to major syn-oceanic faults. PGE-enriched chromitites form anastomozing vein systems within the Pyroxenitic Zone. The Hall deposit is hosted by a ~100m wide metasomatic dunite, and contains (average) 2230 ppb PGE, and 1438 ppb Pt+Pd (max. 20.4 gr/ton). Star Chrome (max.20.7 gr/ton Pt+Pd) contains complex Cu-Ni-Fe-As-S-Pt-Pd-Rh alloys and potarite [PdHg], located in intergranular fractures and veins, suggesting late-stage enrichment in Pt-Pd by low-temperature fluids. Enrichment in La and Ce in associated pyroxenites and peridotites could represent an exploration tool. Economic values also occur in pyroxenites showing no visible mineralization (768 ppb Pt+Pd, Lac Bisby), however, and may represent a major unsuspected resource. Os isotopic ratios from chromitites (¹⁸⁷Os/¹⁸⁸Os at 480 Ma: 0.1243 to 0.1263) resemble those of other supra-subduction zone ophiolites, implying that Os is principally derived from the mantle wedge. Star Chrome chromitites are the most radiogenic, and may include subduction zone or crustal components. Since the sub-volcanic erosion surface penetrates to the Dunitic Zone, this raises the possibility of paleo-placer PGE mineralization. Finally, our stratigraphic and structural correlation of the TMOC with the Mt Ham and Asbestos ophiolites to the South, suggests that these massifs also represent exploration targets.