Geochemistry and origin of the Bald Knob manganese deposit, North Carolina

Geochemical and textural evidence for the submarine hydrothermal origin of the Bald Knob Mn deposit, North Carolina, is preserved in a mineralogically diverse suite of rocks that have experienced amphibolite facies metamorphism. Manganese- and Mn-Fe-rich lithologies have high Mn/Fe ratios and low concentrations of Ni, Cu, Zn, U, Th, and rare earth elements that are similar to concentrations reported from other ancient manganiferous deposits and from crusts and sediments which are forming in hydrothermally active regions of the present sea floor. Manganese- and Ca-rich carbonate minerals, a variety of Mn-rich silicates (including tephroite, the Mn humite-group minerals, spessartine, rhodonite, and pyroxmangite), and the oxides galaxite and jacobsite form fine laminations that represent different bulk compositions. No chemical or textural evidence for replacement of Mn-poor by Mn-rich minerals was found and, in some samples, the same lithologic units are repeated on the scale of a thin section, consistent with a primary sedimentary origin for the layering. Chert, which is one of the country rocks at Bald Knob, and Mn- and Ca-rich carbonate rocks represent original sedimentary protoliths. More than one detrital component may be present in the Mn-rich layers. Biogenic detritus was the probable source of at least some of the siliceous material that allowed for the formation of the Mn-rich silicates. An aluminous detrital component, which probably was derived from a continental or andesitic source, is reflected in spessartine-rich layers. Minor Mn and Co sulfide mineralization (alabandite, cattierite, cobaltite) was contemporaneous with deposition of the Mn-rich sediments. The association of the manganiferous lithologies with chert and meta-igneous rocks (amphibolite) is consistent with deposition of the sediments in a rifting environment, such as a back-arc basin, or possibly, a midocean ridge.