Introduction to Archean Rare-Metal Pegmatites
2019
The alkali metal Cesium was first described by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff during the investigation of mineral waters from Durkheim. It is a silvery-gold, soft extremely reactive and pyrophoric metals, has a large ionic radius and a rather low melting point. Cesium belongs to the large ion lithophile elements. The Cesium market is very small. As a result, data on Cs resources and production are not available or very limited. According to the USGS Cs is currently only mined from massive pollucite mineralisation bearing pegmatites in Canada (Tanco) and Zimbabwe (Bikita). Cesium is a typical trace element and normally occurs in the level of a few to some tens of ppm. As Cs is almost incompatible during magmatic crystallisation, it becomes enriched in residual melts. Therefor Cs occurs as trace element in feldspar and mica. However, certain geological processes are capable to enrich Cs to several thousand ppm so that specific conditions can lead to the formation of discrete Cs minerals. At present 31 Cs are known and approved by the International Mineralogical Association. Most of them crystallise in granitic pegmatites or in alkaline complexes at late stage magmatic to hydrothermal processes. However, only the zeolite Cs mineral pollucite as part of the analcime-pollucite series is known to occur in larger and thus economic quantities. Pollucite is classified as tectosilicate and belongs to the zeolite group. It has a general composition of (Cs, Na)2Al2Si4O12xH2O and is isostructural to analcime NaAlSi2O6xH2O. Although it can crystallise in gem stone quality cubic, dodecahedral to trapezohedral crystals with various colours pollucite is commonly developed as glassy, colourless to white polycrystalline masses. Pollucite occurrences are reported from approximately 140 locations worldwide. However, at present only two LCT pegmatite deposits are known to host economic quantities of massive pollucite mineralisations. These are the Bikita LCT pegmatite in Zimbabwe and the Tanco LCT pegmatite deposit in Canada. Both pegmatite deposits have a comparable regional background, as they are hosted within greenstone belts on Archean Cratons and yield Neo-Archean ages of ~2625 Ma. Although several studies focused on the mineralogical and geochemical characteristics of pollucite are done, no systematic investigation focusing on the genesis of these massive pollucite mineralisations was performed so far. Therefore, the massive pollucite mineralisation at Bikita were studied in detail to understand the genetic concepts on their formation. Major Portions of Western Australia consists of Meso- to Neoarchean crustal units (e.g., Yilgarn Craton, Pilbara Craton) that are known to host a large number of LCT pegmatite systems. Among them are the LCT pegmatite deposits Greenbushes (Li, ±Ta) and Wodgina (Ta, Sn ±Li). In addition, small amounts of pollucite were recovered from a single diamond drill core at the Londonderry pegmatite field and more recently from the Sinclair LCT pegmatite, both located within the Yilgarn Craton. Despite that, no systematic investigation and/or exploration were conducted for the mode of occurrence of Cs and especially that of pollucite in Western Australia. Thus, Western Australia represents prospective area that may host one more deposit of a yet merely unique mineralisation type on the planet Earth.
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