Evolution of the subseafloor hydrothermal system associated with the Ming VMS deposit, Newfoundland Appalachians, and its controls on base and precious metal distribution

Abstract

The ~ 487 Ma Ming volcanogenic massive sulfide (VMS) deposit consists of four subparallel, elongated, semi-massive to massive sulfides lenses (the 1807, 1806, Ming North, and Ming South zones) hosted in rhyodacite of the Rambler Rhyolite formation, Newfoundland Appalachians. A discordant Cu-rich Lower Footwall zone underlies the semi-massive to massive sulfide lenses. Alteration associated with mineralization can be divided into nine facies that formed in three paragenetic stages: (1) weak quartz–calcite ± spessartine, quartz–sericite, and quartz–sericite–chlorite alteration (stage 1); (2) quartz–chlorite, quartz–chlorite–sulfide, and quartz–chlorite–sericite assemblages (stage 2); and (3) quartz–sericite–sulfide and localized Mn-rich carbonate assemblages (stage 3). A thin syngenetic silica-rich layer immediately overlies part of the VMS deposit and likely formed during the early stages. The volcanic architecture and synvolcanic faults controlled the lateral distribution of extrusive rocks and hydrothermal alteration. Precipitation of the high temperature, discordant to semi-conformable Cu-rich chloritic assemblages (stockwork), was laterally restricted to one of these synvolcanic faults and the transition from coherent- to volcaniclastic-dominated lithofacies. Lower temperature, sericitic assemblages (stages 1 and 3) are controlled by the distribution of volcaniclastic rocks and generally form the immediate footwall to the semi-massive to massive sulfide lenses. Lithogeochemical mass balance calculations illustrate the alteration minerals and mineralization: chlorite-rich assemblages—gains in SiO2, Fe2O3t, MgO, Cr, Ni, and Cu and losses in Na2O, MnO, and CaO and sericite-rich assemblages—gains in K2O, Zn, and Ag and losses in MnO, MgO, CaO, Na2O, and Y. Calcium- and magnesium-rich alteration assemblages are restricted to the northwest fringe of the deposit, distal to the main chloritic and sericitic alteration, and have elemental gains in P2O5, Y, and losses in K2O. The late stage 3 quartz–sericite–sulfide assemblage overprints most assemblages, hosts sphalerite–galena–sulfosalt–Ag–Au-rich veins, and is spatially associated with coherent volcanic rocks. The less permeable nature of these rocks is interpreted to have acted as a physical barrier for ascending metal-rich hydrothermal fluids. Results from the detailed reconstruction of the hydrothermal architecture and paragenetic evolution of the Ming deposit suggest that precious metals were introduced during the waning stage of the hydrothermal system, associated with decreases in temperature and pH of the hydrothermal fluids.