- © 2015 Society of Economic Geologists.
The Cambrian Lemarchant Zn-Pb-Cu-(Au-Ag) bimodal felsic volcanogenic massive sulfide (VMS) deposit, Tally Pond group, central Newfoundland, Canada, contains metalliferous mudstones that occur either immediately on top or laterally along strike from massive sulfide mineralization, or as interflow mudstones within hanging-wall basaltic rocks. The mudstones are brown to black, graphite rich, and locally have intercalations of siliciclastic, volcaniclastic and/or amorphous chert layers, and, in some cases, fine lamina of organic matter. The main sulfide phases are pyrite (framboidal and euhedral) and pyrrhotite, with minor chalcopyrite, sphalerite, arsenopyrite, and galena; barite is a common sulfate.
The metalliferous mudstones occur at various stratigraphic levels and have variable inputs of hydrothermal (high Fe/Al and base metal values) and detrital components (lower Fe/Al and base metal values) with positive shale normalized Eu anomalies (Eu/Eu* ≥1), negative Ce anomalies (Ce/Ce* ≤1), and an average Y/Ho ratio of ~28.3. These signatures suggest precipitation from reduced, high-temperature hydrothermal vent fluids in an oxygenated water column, coupled with the hydrothermal particles having a short residence time within the hydrothermal plume (i.e., a vent proximal setting). Deposition from an oxygenated water column is also supported by the presence of abundant barite in both the Lemarchant massive sulfides and the metalliferous mudstones, as well as locally preserved marcasite in the mudstones. Redox-sensitive trace elements (As, U, V, Mo, Cr, Ni, and Co) were scavenged as oxyanions onto Fe oxyhydroxides during mudstone formation, and these elevated levels of scavenged redox-sensitive trace elements cause apparent anoxic signatures. Immobile element systematics (La-Th-Zr/10 and Th-La-Sc) indicate that the detrital components of the Lemarchant mudstones were mixtures of both continental and mafic sources, most likely deposited within a pericontinental rifted arc environment. Our research illustrates that the lithogeochemistry of metalliferous mudstones can identify vent proximity and provides insights into the physiochemical conditions of the deposit formation and the ambient environment. These results have implications for similar precious metal-bearing VMS deposits globally.