Hydrothermal evolution of volcanogenic Zn-Cu-Pb deposits, Taylors Brook showing, Stirling Belt, southeastern Cape Breton Island, Nova Scotia

Abstract

The Taylors Brook showing is a suspected volcanogenic massive sulfide (VMS) occurrence (Zn-Pb-Cu-Au-Ag) that occurs within the ~680 Ma Stirling volcanosedimentary-plutonic belt, southeastern Cape Breton Island, Nova Scotia, Canada. In addition to mineralized mudstones, there are 7 associated hydrothermal vein types, characterized using three criteria: vein-filling mineral composition and alteration style, cross cutting relationships and mineralization. Mineralized (Zn-Cu-Pb-Sc-Y-Ag-Te) veins show propylitic and potassic alteration assemblages. Compositional zoning in epidote and a pyrrhotite-smythite replacement reaction indicate that the hydrothermal fluid was experiencing fluctuations in /O2 and cooling, respectively, rendering the metals less soluble causing their precipitation. Bulk rock geochemistry indicates mineralization anomalies are associated with host rock Zn-Pb anomalies in pyritic mudstone units and Cu anomalies in type IA (epidote-actinolite-calcite-K-feldspar-quartz-pyrite) veins. Fluid inclusion data from two generations of calcite in later type V veins suggest a CaCl2-rich hydrothermal brine responsible for metal transport and deposition with bulk salinities ranging between 43-54 wt% total salt (absolute: 33-50 wt% CaCl2 and up to 20 wt % NaCl). The homogenization temperatures for calcite A-hosted inclusions range from 106-306[degree]C (n=21) and in calcite B-hosted inclusions from 212-331[degree]C (n=91). Isochore ranges were plotted using microthermometric data, paired with a po-py equilibrium boundary constraint limits trapping conditions to a maximum of 400[degree]C and 5kbar. The distinct Ca-Cl-rich brine composition can only be formed naturally under specific conditions. There are four conceivable explanations for this Ca rich fluid: boiling, cation exchange between Na and Ca, evaporation of Ca-rich paleoseawater and dissolution of Ca-evaporites. However, the combination of high salinity and high Ca is only possible via the latter two processes. The Taylors Brook showing has comparable mineralization to the Stirling deposit, and the Duck Pond-Lemarchant deposits of Newfoundland but has an inconsistent fluid chemistry with VMS deposits.