Role of metalliferous mudstones and detrital shales in the localization, genesis, and paleoenvironment of volcanogenic massive sulphide deposits of the Tally Pond volcanic belt, central Newfoundland, Canada

The Cambrian Tally Pond volcanic belt in central Newfoundland contains numerous volcanogenic massive sulphide (VMS) deposits and prospects associated with exhalative metalliferous mudstones. Deposits in the belt are bimodal felsic VMS deposits that are both base metal bearing (e.g., Duck Pond – Boundary), and base metal and precious metal bearing (Lemarchant). At the Lemarchant deposit, metalliferous mudstones are stratigraphically and genetically associated with mineralization. In the remainder of the Tally Pond belt, detrital shales occur predominantly in the northeastern part of the belt (mostly as unrelated mid-Ordovician structural blocks) in the upper sections of the Cambrian volcanic stratigraphy, but locally also are intercalated with metalliferous mudstones. Their relationships to massive sulphides are less obvious, with many spatially, but not necessarily genetically, related to mineralization. Upper Cambrian to Lower Ordovician black shales from Bell Island, which represent pelagic sedimentation not associated with hydrothermal activity and volcanism, are compared with the Tally Pond belt mudstones and shales. Exhalative mudstones, like those at Lemarchant, have elevated Fe/Al and base-metal values, and have shale-normalized negative Ce and positive Eu anomalies, indicative of deposition from high-temperature (>250 °C) hydrothermal fluids within an oxygenated water column. Mudstones and shales sampled from other Tally Pond prospects have more variable signatures, ranging from hydrothermal to nonhydrothermal black shales (no positive Eu anomalies, flat rare earth element patterns, low Fe/Al and base-metal contents), to those that have mixed signatures. Accordingly, mudstones from areas with a Lemarchant-like hydrothermal and vent-proximal character are more attractive exploration targets than mudstones and shales with predominantly detrital signatures.