Ororusu, Namibia Fluorspar Ores



View looking east into the fluorspar open pit mine at Okorusu in north-central Namibia. The orebody is tabular, about 1,000 feet long in an east-west direction, averages 300 feet wide, and dips steeply to the north (left; the sun lies the north in the southern hemisphere). Pods of fluorspar ore extend outward into the footwall (right) and hanging wall (left) that are composed of fine-grained sodic fenite. The original sandstones were fenitized prior to the introduction of the fluorite ore fluids. The early fenitizing fluids and the subsequent ore fluids were derived from a nearby Tertiary carbonatite intrusion. The mine is located on the south side of Okorusu mountain, a topographic feature that has resulted from the resistance to erosion of the hard fenite.



Outcrop showing dramatic partial sodic fenitization of sandstone. Irregularly shaped replacement remnants of sandstone are surrounded by thoroughly fenitized rock. Although original bedding is locally visible in the sandstone, the process of fenitization has completely obliterated the bedding in the sodic fenite. The fenite consists predominately of fine-grained aegirine. The outcrop is located east of the Okorusu fluorspar mine on the hillside shown in the background in the previous photo.



A large float fragment of well crystallized fluorite ore at an outcrop at the top of Okorusu Mountain. The fluorite crystals were deposited in local vugs in the dominantly replacement fluorite. Yellow fluorite represents the last generation of fluorite deposition and it was preceded by green, purple, and grey generations of fluorite as shown in a subsequent photo below.



Cathodoluminescent view of thin section of carbonatite exposed locally in the Okorusu fluorspar mine showing calcite (Cal; yellow) distinctly veined by fluorite (Fl; dark blue). Cathodoluminescence microscopy, low magnification.



Doubly polished thick thin section of Okorusu fluorite ore showing multiple generation of fluorite deposition distinguished by variations in color. The earliest fluorite (designated by the white Fl symbol) to be deposited contains abundant inclusions that cause it to appear opaque in thick thin sections. The inclusions are predominately goethite, quartz, and synchysite (uranium fluorcarbonate). Subsequent generations of green, purple, and grey fluorite were deposited in open space.



Cathodoluminescence microscopic view of a Okorusu fluorite crystal (Fl; deep blue) revealing that banding of the fluorite is even more numerous than shown in thick thin sections. Inclusions of calcite (Cal; yellow) and quartz and goethite (black) are most abundant toward the center of the crystal. CLM, low magnification.



Late deposition of quartz (Qz; deep red) and subsequently calcite (Cal; pinkish yellow) on fluorite (Fl; light to deep blue). Both quartz and calcite show growth zoning that characterized the pulsating nature of the late post-ore fluids that involved the mixing of ground waters with the earlier hydrothemal fluids. Locally the late quartz and calcite form stalactitic growths that indicate the vugs in which they were deposited were no longer filled with fluid. Okorusu fluorite mine, Namibia. CLM, low magnification.