Apatite and phlogopite

from the Palabora carbonatite in South Africa

apatite/mica [273 kb]

"Rock of the Month # 107, posted May 2010" --- Displaying a spray of coarse, pale blue phosphate prisms up to 2 cm in length, in a friable matrix dominated by lustrous brown phlogopite mica flakes, this rock is an unusual ore from a unique deposit. The Palabora mine is located in eastern South Africa, near the Kruger National Park. Sample 1748, collected in 1995, loose on dumps overlooking the tailings impoundment.

Apatite and phlogopite --- phosphate and mica --- are both industrial minerals long mined together or as separate commodities. The rocks at this mine are composed of a range of minerals, including much magnetite, silicates such as diopside, olivine, serpentine, vermiculite and mica, copper sulphides such as chalcopyrite and bornite, and the carbonates calcite and dolomite. The host structure is the Palabora (Phalaborwa) carbonatite complex, formed in part from a magma dominated by carbonate minerals, such as calcite, rather than the familiar silicates (quartz, feldspars, etc) of typical igneous rocks such as granite and gabbro. The intrusive complex also contains grains of minerals such as baddeleyite and zirconolite, and has been dated at 2060 Ma (million years). Some of the local rocks are known by unusual names, such as the mica-rich glimmerite and a magnetite-dominated, phosphate-rich rock known as phoscorite (foskorite). Some of these rocks are mafic pegmatoids found within the alkaline igneous complex, which is 6.5x2.5 km in area.

The economic exploitation of the local rocks for copper has a history dating back more than 1,000 years, with initial mining of the colourful carbonates malachite and azurite as a tractable source of copper metal. Famed South African explorer Hans Merensky was involved in 20th-century appraisal of the local mineral potential. The modern mining era dates back to the 1950s, with an open-pit mine commencing operation in 1965. Copper was and remains the main product. However, the complexity of the host rocks presents a wide range of byproducts: these include iron (magnetite), apatite, and the sheet silicates vermiculite and phlogopite. Minor elements enriched in the local carbonatite, pyroxenite and syenite include uranium and thorium, zirconium and the REE (rare earth elements).


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Graham Wilson, 11 May 2010

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