Characterising mineralising fluids in carbonate-hosted Zn-Pb sulphide deposits of eastern Canada

Abstract

Nova Scotia and Newfoundland are home to Carboniferous- (e.g., Scotia Mine and Walton) and Ordovician- (e.g., Daniel’s Harbour) hosted carbonate-replacement Zn-Pb sulphide deposits. The processes controlling Mississippi Valley-type (MVT), sedimentary exhalative (SEDEX), and Irish-type deposits remain disputed, with significant overlap among the three models. The Scotia Mine, Walton, and Daniel’s Harbour deposits served as case studies to test and refine models of Irish-type, SEDEX, and MVT deposits, respectively. The research approach consisted of detailed petrography followed by a suite of microanalytical techniques (SEM-CL, LA-ICP-MS, and SIMS) whose mutually informing and constraining results will produce valuable insights into the mineralising fluid(s) responsible for each deposit. The analytical techniques indicated that two-fluid mixing was crucial in the formation of all the deposits, but that sulphate reduction took place either on-site (Scotia Mine) or off-site (Daniel’s Harbour and Walton). As a result, disequilibrium textures (e.g., colloform) were more common at Daniel’s Harbour and Walton due to fluid mixing occurring after sulphate reduction. Thermochemical sulphate reduction was most important at Daniel’s Harbour and Scotia Mine, whereas bacterial sulphate reduction was most important at Walton. Hydrocarbons are of varying importance in regard to mineralisation; however, for all deposits hydrocarbons were a likely reductant and additionally could have been a sulphur and/or metal source at Walton. Higher homogenisation temperatures and the significance of elements such as Ag and In could indicate the influence of magmatism during ore precipitation at Walton. Trace element mapping via LA-ICP-MS shows that zone refining was an important process at Walton but not at Daniel’s Harbour and Scotia Mine and might be a distinguishing feature of at least some SEDEX deposits versus MVT and Irish-type deposits.