Geochemistry of the Cambrian manganese deposits of eastern Newfoundland

Abstract

Manganese-rich sedimentary rocks are exposed intermittently throughout an area of 40,000 km² in the Avalon Zone in southeastern Newfoundland. Paleontological data indicate that the manganese-rich rocks form a stratiform deposit of basal Middle Cambrian age and that the manganese horizon is related to a disconformity whose duration increases from south to north. Stratigraphic measurements and chemical analyses show that the manganese horizon becomes thinner (from 25 to 1 m) and progressively enriched in Mn relative to Fe (Mn/Fe from <l to ≃3.0) towards the north as the magnitude of the disconformity increases. The manganese horizon is enriched in Mn, Fe, Mg, Ba, and P relative to enclosing red and green mudrocks. -- Petrographic study of manganese horizon samples shows that the horizon is fossiliferous, containing a fauna which includes trilobites and algal structures. Most of the manganese is present in carbonate minerals (rhodochrosite and manganiferous calcite), although secondary manganese oxide minerals also occur. In part, the manganese horizon contains anomalous concentrations of Fe-rich chlorite (chamosite). Phosphatic nodules and euhedral crystals of barite and pyrite are also present in some manganese horizon samples. Carbonate nodules are abundant and these are believed to have been formed during diagenesis. -- The available evidence suggests that the manganese accumulated slowly in shallow marine waters in a partially restricted basin during a break in normal sedimentation characterized by reduced rates of clastic sedimentation. The manganese was originally precipitated in oxide form, with subsequent alteration during early diagenesis producing the manganese carbonate which now characterizes the horizon. The Mn was derived from the weathering of Late Precambrian rock assemblages exposed around the Middle Cambrian basin. Mn was separated from Fe during transportation of the Mn. towards the north within the partially restricted basin. In this regard, the restricted rate of clastic sedimentation is critical, as it permitted the gradual transportation and concentration of manganese at a given site over long periods of time with no dilution by clastic minerals. -- The shallow marine origin, the stable tectonic environment, the manganese carbonate mineralogy, and the pronounced influence of diagenesis upon the rocks suggest similarities between the Newfoundland manganese horizon and modern oceanic ferromanganese deposits of the diagenetic type (Rona et al., 1978; Bonatti et al., 1972) and with the Nicopol Formation type of ancient deposit (Varentzov, 1964). Manganese-rich rocks of basal Middle Cambrian age present in the Harlech Dome region in North Wales are stratigraphically, tectonically, and chemically similar to the Newfoundland manganese horizon (Mohr, 1966) and may be correlative with the latter.