Superimposed diagenetic and hydrothermal processes – the key to high grade Zn mineralization in sedimentary basins

Clastic-dominant (CD-type) deposits typically comprise huge base metal and sulfur anomalies (> 106 tons) that are concentrated in a small number of Proterozoic and Phanerozoic basins. Some Phanerozoic deposits also contain barite in the succession hosting mineralization. Traditional models invoke sedimentary exhalative (SEDEX) processes to explain the formation of CD-type deposits, whereby metalliferous hydrothermal fluids were vented into a euxinic (H2S-bearing) water column, resulting in syn-sedimentary sulfide precipitation. Arguments for SEDEX activity are primarily associated with stratiform ore textures and highly positive δ34S values in pyrite. Recently, however, two Late Devonian CD-type deposits at Macmillan Pass (Selwyn Basin, Canada) have been the focus of 3 new approaches aimed at understanding how seawater paleoredox, diagenesis, and hydrothermal processes were interconnected: 1) Fe and Mo based paleoredox proxies provide evidence that anoxic non sulfidic (ferruginous) conditions were dominant during host rock deposition. 2) In situ δ34S values from pyrite and barite can be explained by diagenetic processes associated with the sulfate methane transition zone. 3) Detailed petrographic and fluid inclusion studies have described a comprehensive mineralogical paragenesis, whereby the introduction of hot (300°C) hydrothermal fluids into the shallow subsurface (<1-km depth) resulted in the thermal degradation of organic matter, barite dissolution, and sulfide mineralisation. Importantly, the Macmillan Pass deposits represent subseafloor replacement systems in which there was potential for positive feedbacks between diagenetic and hydrothermal processes, resulting in highly efficient ore-forming systems. These results are also relevant to other Phanerozoic basins in which stratiform barite deposits are spatially associated with sulfide mineralization.