Petrologic Controls For Fluorspar Deposits at Okorusu, Namibia: Hydrothermal Replacement of Carbonatite, Marble, And Fenite

摘要

The fluorspar ore deposits at Okorusu in north-central Namibia are associated with the late Cretaceous Okorusu alkaline igneous-carbonatite complex. Recent field and laboratory study of the fluorite deposits has shown that the fluorite ores have formed by the replacement of several igneous and metamorphic host rock types. About 68 percent of the fluorite ores have formed by the replacement of coarse-grained calcite-pyroxene-magnetite-ap-atite-pyrrhotite carbonatite bodies that had intruded into ear-lier-formed and brecciated fine-grained greenish-gray fenite. Those ore pods typically exhibit rims of coarsely crystallized massive titaniferous magnetite that originally had formed as rims of the replaced carbonatite intrusions. Locally these ores contain replacement remnants of the carbonatite. The ores are especially characterized by goethite pseudomorphs after pyroxene, magnetite, and pyrrhotite that formerly were present in the replaced carbonatite. Their relatively high phosphorus contents, about 2.5 percent phosporus pentoxide (P_2O_5) or higher, are characteristic and result from the presence of replacement remnants of carbonatite apatite. About 31 percent of the Okorusu fluorite ores have formed by the replacement of late Precambrian Damara marble. Examples include: 1) most of the main ore band, A band, in the B pit, 2) a small satellite ore pod in the hanging wall at the west end of the A pit, 3) most of the C orebody (as shown in drill core), and 4) a new ore prospect, the G deposit, east of the A pit. These fluorite ores are very fine grained, may show banded textures, lack both goethite pseudomorphs and magnetic anomalies that are characteristic of carbonatite replacements, and are characterized by relatively high silica contents owing to quartz in the replaced marble. A small portion, about 1 percent, of the ores at Okorusu formed as veins that cement breccia fragments of fenite. They are exposed in a new pod of ore that extends southeastward from the A band in the B pit.