Manganese mineralization hosted by the Rocky Gap Sandstone in Bland County, Virginia

Abstract

The Stange Mine was one of the largest and most important manganese mines in Virginia . The mineralization is hosted by the Rocky Gap Sandstone of Devonian age. It is a medium to coarse grained sandstone deposited on a shallow marine shelf in a warm water environment. Conodont color indexes indicate the host was buried as deeply as 4 km and heated to 150°C. Romanechite, goethite, and pyrolusite are the three most important oxides, and can coexist at 25°C and 1 bar. In polished section romanechite has three textures: (1) microcrystaline patches, (2) fibers. Pyrolusite occurs as occurs in three textures : botryoidal bands, and (3) interwoven psuedomorphs after manganite. Goethite (1) a mottled and spotted variety characteristic of hand samples with sedimentary layering, (2) a homogeneous goethite , and (3) a banded goethite that occurs as a microbreccia. The paragenesis of the minerals was determined using polished sections, hand samples, and field relationships. Electron microprobe analyses were used to find a nominal chemical formula for romanechite. This formula was used to estimate the Gibbs free energy of formation for romanechite. Manganese deposits in sedimentary rocks are formed by sedimentary processes, submarine volcanic activity, diagenetic activity, residual weathering, and ground water deposition. The stratabound nature, regional extent, burial history, and proximity to fractures on the crest of an anticline indicate the deposit at the Stange Mine was formed by ground water at the interface between deeply-percolating, oxygenated meteoric water and reducing manganese and iron bearing, formation waters where the iron and manganese were precipitated by oxidation .