NEW INSIGHTS INTO THE FORMATION OF MANGANESE ORE DEPOSITS IN THE PIEDMONT PHYSIOGRAPHIC PROVINCE OF VIRGINIA

The James River-Roanoke River Manganese District is a belt of manganese ores extending through Appomattox, Campbell, and Pittsylvania Counties. In the Roanoke River watershed, Mn concentrations in both groundwater and the Roanoke River can exceed the secondary drinking standard of 50 ppb, suggesting a possible connection between Mn ores and elevated Mn concentrations in water. This project seeks to examine if there are connections between geomorphic, geochemical and hydrologic processes that formed the Mn ores to those that release Mn to groundwater.

The model of Mn ore genesis in this region proposed by Espenshade (1954) suggests these are supergene deposits formed when groundwater dissolved soluble materials containing Mn. Mn in solution was transported downward with infiltrating groundwater, forming Mn-oxides at depth (~90m maximum depth). In this study, mineralogical analysis using X-ray diffraction and scanning electron microscopic analysis of ore morphologies support Espenshade’s model of ore formation. The redox chemistry of Espenshade’s model, however, is problematic. As groundwater flows downward oxygen should become depleted with depth, making Mn oxidation and subsequent precipitation with depth unlikely.

We propose a revised model for the formation of these ore deposits. A more feasible mechanism of Mn-oxide precipitation is from the upward movement of reduced groundwater toward discharge zones, resulting in mixing with oxidized waters, promoting Mn oxidation. To test this model, we examined soils maps for the region. Overlays of the soil maps with those of the ore locations reveal that Mn mining localities occur in close proximity to both ancient alluvial deposits at higher elevations in the Piedmont Uplands and modern alluvial deposits at lower elevations in stream valleys. This suggests streams were present in the geologic past in the vicinity of the present-day ore locations, and that ore deposits could have formed near or in discharge zones. Deposits located near modern alluvial soils at low elevations may suggest that the same processes that formed the ore deposits at higher elevations continue today, resulting in elevated Mn concentrations in groundwater near discharge areas and potentially allowing Mn-oxides to form in modern stream valleys throughout the Piedmont landscape.