Petrography and Mineralogy

Abstract

For the petrography of Neo-Archean pegmatites at Bikita, Londonderry, Mount Deans, Cattlin Creek and Wodgina, optical microscopy of polished thin sections was precised by backscattered electron imaging (BSE) and energy dispersive X-ray spectroscopy (EDS) with a scanning electron microscope (SEM). Automated mineral liberation analysis (SEM-MLA) of unpowdered aliquots for WR geochemistry allowed to quantify mineral modes of the coarse-grained pegmatites. This modal analysis encloses the accessory minerals at <1 wt% of bulk mode, as cassiterite, monazite, zircon and Nb-Ta oxides, and allows to demonstrate the presence of Cs-enriched lepidolite, nanpingite, sokolovaite, pezzottaite, and even small grains of dispersed pollucite. Inspection of Al-Si ratios in the EDS-spectra in combination with elemental analysis and optical microscopy allowed to recognise Li- and Be-bearing minerals as petalite, spodumene and beryl. The samples were classified into 21 distinct mineral associations or pegmatite mineral zones. At Bikita, pollucite forms lens-like massive mineralisation with a crosscutting vein network of purple lepidolite, petalite, quartz and feldspar. Pollucite exhibits rather inhomogeneous Cs compositions, ranging from <20 up to 37 wt% Cs₂O, with higher Cs concentrations along the lepidolite-quartz-petalite veins and later generations of cracks. Beryl, partly in several generations with up to 10–15 wt% Cs₂O, was observed at Cattlin Creek, Londonderry and Wodgina. Most common are tantalite, columbite and cassiterite. Other Ta-, Nb- and Sn-oxide minerals ascertained during the study are microlite, wodginite, ixiolite and pyrochlor. The early crystallisation stage in the pegmatites is characterised by euhedral plagioclase, K-feldspar, petalite, spodumene, cassiterite, columbite and tantalite which crystallised directly from the hydrous melt under subsolvus conditions of >5 kbar. The subsequent main stage is characterised by the second generation of plagioclase, and the growth of muscovite and quartz. The position of the massive pollucite mineralisation within the crystallisation sequence at Bikita is difficult to ascertain. Inclusions of petalite and feldspar in pollucite signal its crystallisation during the early stage. Replacement along the lepidolite vein network provides arguments for the late phase of the main stage crystallisation. The late stage crystallisation is characterised by numerous replacement reactions which are attributed to circulation of deuteric hydrothermal fluids in consolidated but still not fully cooled pegmatites.