The Fluorite Deposits at Okorusu, Namibia: Their Origin by the Replacement of Carbonatite Host Rocks

摘要

The fluorite ore deposits at Okorusu, Namibia, occur within a broad area of fenitization of Pre-cambrian Damara metasedimentary rocks along the south margin of the late Cretaceous Okorusu alkaline igneous-carbonatite complex. Although the individual orebodies occur largely within fenite host rock and the fluorite has locally replaced that rock type, it has become increasingly recognized with deeper mining that most of the fluorite ore has selectively replaced newly recognized pyroxene and pegmatitic carbonatites. Evidence for the preferential replacement includes: 1) goethite pseudo-morphs in fluorite ore after pyroxene, magnetite, and pyrrhotite formerly present in the carbonatites, 2) local remnants of carbonatite within the fluorite orebodies which have survived the replacement process, 3) a single near-vertical dike in the A pit and a near-horizontal sill-like intrusion in the B pit, both comprised of carbonatite that shows partial replacement by fluorite, and 4) the presence of about 50-meter thick carbonatite intrusions into fenite along the strikes of the A and B orebodies, in positions where the ore should have projected. The carbonatites were favored for replacement by the fluorine-bearing ore fluids because of the presence of easily soluble carbonatite calcite and its role in providing most of the calcium to form the fluorite.