USING VOLCANOGENIC MASSIVE SULFIDES (VMS) AS PROXIES FOR MID-PALEOZOIC TECTONICS, CRUSTAL COMPOSITION, AND BASIN REDOX ALONG THE ANCIENT PACIFIC MARGIN OF NORTH AMERICA

Volcanogenic massive sulfide (VMS) deposits are magmatically driven hydrothermal systems formed through the circulation of hydrothermal fluids through submarine crust with deposition of metal-rich fluids on the seafloor. Their mode of formation and the compositions of their host volcanic, subvolcanic, and sedimentary rocks, and massive sulfides, provide insight into tectonic regime, composition of crustal substrate, and the basin redox conditions at the time of formation. Correspondingly, VMS deposits that formed along the Ancient Pacific Margin of North America in the peri-cratonic Yukon-Tanana Terrane (YTT) and cratonic rocks of the Selwyn Basin and Pelly-Cassiar Platform are critical proxies for the processes outlined above. In VMS deposits along the Ancient Pacific margin, host volcanic and subvolcanic rocks have geochemical and radiogenic isotopic signatures indicative of varying contributions of recycled North American cratonic crust, coupled with increased mantle contributions in rocks proximal to massive sulfide mineralization (e.g., high HFSE and REE contents and less negative εNd and εHf signatures in host volcanics). Crustal influence is also recorded in Pb isotopic data in massive sulfides, which have signatures that are similar to the Cordilleran shale curve, indicative of recycling of Pb from the Northern Cordilleran cratonic margin. Shales intimately associated with some deposits indicate deposition under local anoxic conditions with evidence for enhanced primary productivity in the upper water column, coupled with deposition in restricted basins under anoxic conditions. Sulfides formed in these basins formed by subseafloor replacement (+/-exhalation) and had a significant contribution of S from H₂S present in both the water column and in their unconsolidated sedimentary host packages. In contrast, other deposits have evidence of deposition under oxygenated conditions with abundant barite (i.e., free SO₄ in the water column) and had both exhalative and subseafloor replacement type mineralization. The fortuitous combination of extensional tectonics, high temperature magmatism, and an efficient trapping mechanism at the site of deposition were critical in the proliferation of VMS deposits along the Cordilleran margin in the late Devonian to early Mississippian (~365-345 Ma).