HIGHLY MANGANIFEROUS RESIDUAL SOILS OVERLYING INTERLAYERED CARBONATE AND VOLCANIC BEDROCK IN MARYLAND'S WESTERN PIEDMONT

The Maryland Geological Survey undertook an EarthMRI study of seafloor volcanogenic mineralization to identify and map copper-related critical minerals in the western Piedmont, Westminster terrane. Bedrock in the region consists of greenshist-grade metamorphosed basalts, andesites, carbonates and volcaniclastic rocks. As a value-added finding, highly manganiferous soils, 1 to 4 meters thick, were identified overlying locations where carbonate rocks were interlayered with the volcanogenic suite. The black soils contain Mn fragments, nodules, and glossy black films, and overlie locations of dolomitic marble in which black, 1-3 cm thick, Mn-rich weathering rinds are formed. This rind exhibits a porous, friable, Mn- and Fe- rich framework devoid of carbonate minerals. Initial testing with portable XRF indicates the black wad ranges from 8-14% manganese. The exact geologic context and origin of the manganiferous deposits remains unclear. Due to thick soil cover, observations are thus far limited to manually dug soil pits, utility excavation, and quarry exposures. All observations to date indicate that these soils overlie dolomitic marble and occur near volcanic bedrock contacts. The marble in these settings varies from pink, brecciated and mineralized, to white with randomly oriented meter-size clasts of dolomite.

Earlier reports suggest that the manganiferous soils occur near bedrock contacts and were indicative of copper mineralization zones. In the early 2000’s NRCS soil scientists named and mapped this soil, and subsequent studies provided evidence that they were the result of in-situ weathering of the underlying carbonate bedrock. The Westminster terrane manganiferous soils are considered analogous to South African supergene deposits interpreted to have resulted from intense weathering and dissolution of dolostone. Manganese content in the South African dolostone is attributed to deposition in marine waters containing hydrothermal solutions from volcanic deposits.