A review of the geology, mineralisation, and structural controls on the emplacement of the Bikita LCT pegmatites, Masvingo Greenstone Belt, Zimbabwe Craton

Abstract

he world-class Meso-Neoarchaean Bikita pegmatites which host Li, Cs, Ta, Be and Sn mineralisation are enclosed by the elongate shaped Masvingo Greenstone Belt which is situated within the southern part of the Zimbabwe Craton. There are 15 denoted pegmatites within the Bikita mining lease and from these the main pegmatite is the most extensively worked and has been the backbone of the mine since its discovery around 1911 when mining started. These pegmatites have intruded the greenstone-epidiorite country rocks presumed to be metamorphosed basic volcanic rocks with interbedded metamorphosed sediments comprising calcareous and semipelitic schist, siliceous granulite and banded ferruginous quartzite. The Bikita pegmatites are heterogeneous with a complex mineralogical composition containing lithium-bearing minerals (amblygonite, bikitaite, eucryptite, lepidolite, petalite, hectorite, and spodumene). The geology of the Bikita Tinfield was extensively studied prior to its initial mapping by Martin (1964) and most of the interpretations in use have been based on this work. Considering the fact that mining has been ongoing since then, there has been a gap in terms of updating of the geology for these pegmatites which this research work intends to fill. The main purpose of this study is to advance the technical knowledge pertainining to these pegmatites using modern techniques, with particular focus on the geology and mineralisation of the Bikita pegmatites. The possible structural controls on the emplacement of the pegmatites was investigated through satellite imagery and geophysical data. Structural analysis suggests that the NE–SW trending Gono fault and the N-S trending Popoteke fault are the major features on a regional scale understood to have played a vital role in the emplacement mechanism of the pegmatites. These fault zones can provide conduits and with reactivation focus migrating pegmatite melts. On a local structural scale, fractures and shears are the most dominant structures mapped in the study area. Field evidence suggests that most of the structures within the mining lease are on the same strike as that of the pegmatites (NE, N to S, NNE and ENE) therefore cutting across the fabric of the country rock which has an E-W orientation. Some of these structures have been subsequently filled by pegmatitic material proving that they were part of the locus along which the pegmatite intruded. The whole-rock geochemical dataset consists of 1143 samples analysed with AAS for 7 major element oxides in percentages (%) and a dataset of 441 samples analysed by XRF with 6 minor and trace elements (Ta, Nb, Sn, Fe, Rb and Sr) reported in parts per million (ppm). The highest lithium concentration found from the analysed samples is within the Dam Site pit, the highest caesium concentration is at Victor’s pit, the highest tantalum concentration isat Bikita quarry and the highest tin concentration is at the West pegmatite body. The Li2O average abundance across all the pegmatites studied is 2.08 wt %. Results from the Bikita quarry locality have shown the best behaved dataset from which the typical mineral compositions have helped to ascertain mineral deportment and the main definable trends are a high K trend governed by micas i.e. lepidolite and a low K trend which appears to be illustrating the control by petalite. The other Li-Al minerals (spodumene, amblygonite, eucryptite) associate with this low K trend. Essentially all samples show mineralogical control by muscovite and mainly lepidolite (correcting for the build up of radiogenic Sr) exerting a strong influence on Rb contents of extremely differentiated melts containing practically no original Sr. Generally, the Bikita pegmatites are highly fractionated as indicated by their high Rb values (36 700 to 44 283 ppm) from McMurdon pit and Dam Site pit respectively. Spatial data processing, analysis and integration using Remote Sensing, geophysics, geochemistry and GIS applications were useful in understanding the geology, mineralisation and structural controls on emplacement of the Bikita pegmatites. Geochemical work applied in this report work has proved that the Bikita pegmatites are richly endowed in lithium ore minerals with lepidolite and petalite being the most dominant