Mineralogical and geochemical controls on the formation of schwertmannite and goethite in the wetland at Dalseong tungsten mine, Korea

Abstract

Schwertmannite and goethite are the most important minerals of ochreous precipitates from the wetland of the Dalseong tungsten mine near southern Daegu. The main factor leading to precipitation of goethite and schwertmannite is the pH of the solution. Schwertmannite forms as a dominant mineral phase below pH 3.5. Formation of goethite takes place at the inlet, whereas schwertmannite predominantly precipitates at both the downstream pond and outlet. Schwertmannite shows bundle of needle morphology with a similar range in size and its fine needles coalesce to form rounded aggregates. Needles are approximately 0.1–0.5 mm long and 0.1 mm wide. Decreasing pH at the down-ward ponds is attributed to formation of schwertmannite from water with high content of iron and sulphate ion. The removal rate of sulphate ion at downstream ponds is unremarkable possibly due to supply of sulphate ion by organic decay in the reduction ponds, although high content of sulphate ion is removed by forming schwertmannite. As precipitation of schwertmannite proceeds, iron is actively consumed and pH decreases.