The petrochemistry of the arcadia pegmatites of the Harare greenstone belt of Zimbabwe: implications on genesis and mineral exploration on pegmatites

Abstract

The aim of this project was to conduct additional petrographic and geochemical analyses on selected samples to ascertain the mineralogical and geochemical makeup of the stacked pegmatites comprising the Main Pegmatite Zone (MP) and the Lower Main Pegmatite Zone (LMP) at Arcadia. The findings were essential in understanding the following; any similarities or variations within these two major units in terms of the distribution of Li-bearing minerals; the mineralisation phases that characterise the pegmatites so as to come up with a detailed paragenetic sequence and to explain the mineral chemistry of the observed deep pink coloured mineral referred to as petalite. Petrography, XRD (Rietveld) and XRF analyses were conducted on 15 representative samples, 7 from the LMP unit and 8 from the MP unit. The results from the fieldwork and petrographic analyses revealed no significant variations between the two units other than, the following slight variations; the MP unit appeared to have a significant concentration of the lithium minerals petalite and spodumene compared to the LMP unit, while the LMP unit had a significant concentration of secondary minerals. Petrographic analysis also indicated the presence of disseminated accessory garnets, within the LMP zone and this observation concurs with the MSA findings in which accessory garnets were identified within the LMP layer. The observed textural difference between the two units was the abundance of the coarse-grained texture within the MP unit, while the LMP unit appeared dominated by the fine-grained aplitic texture as well as the replacement textures from the hydrothermal alteration of feldspars and petalite. The pit mapping revealed patchy irregular concentric zonations on the exposed MP zone, characterised by both a change in textural and mineralogical assemblage. The only visible zones from the pit were the intermediate and the aplitic zones. The coarse-grained texture defined the intermediate zone which hosts the lithium mineralisation and it comprises the following mineral assemblage; blocky or coarse-grained K - feldspar + quartz + spodumene + petalite + minor albite, while the aplitic zone which is characterised by a fine-grained texture comprising fine-grained quartz+ albite+ muscovite (green) +garnet (pink). The core zone rich in coarse-grained quartz was only visible in some sections of the core. The aplitic and the intermediates zones are demarcated by sharp contacts. The petrographic analysis corroborated by the XRD Rietveld technique revealed that the mineralisation at Arcadia occurred in two phases the first phase comprising the early magmatic crystallisation of primary petalite and spodumene and the second mineralisation phase which is characterised by the alteration of the primary mineral phases petalite and spodumene producing secondary minerals, eucryptite, bikitaite and Spodumene+ Quartz intergrown (SQI). The hydrothermal alteration of plagioclase feldspars to smectite, stilbite and calcite constitutes the second mineralisation phase and the mineral phases were documented in a paragenetic sequence in Chapter 5. The whole rock geochemical analysis on major oxides revealed high silica (SiO2-88.64wt%) and alumina (Al2O3-20.69wt%) contents for this suite of pegmatites, and low concentrations for Na2O (-0.03-9.21wt%), K2O (0.05-9.04wt %), CaO (0.08-3.11wt%), Fe2O3 (0.30-1.36wt%), MgO (0.01-0.48wt%), MnO (0.02-0.33wt%), P2O5 (0.02- 0.19wt%), TiO2 (0.01-0.05wt%) and Cr2O3 (0.01wt%). The alumina concentration appeared to be higher than the total alkali of Na2O+ K2O+CaO for all the samples, and this coupled with high silica concentrations > 70% and low MgO, CaO and Fe2O3, implied that the rocks are both highly evolved and peraluminous. The majority of the binary plots of the above mentioned oxides against SiO2, revealed a negative correlation with SiO2, suggesting a non- association with silica, while Fe2O3 and TiO2 revealed a weak positive correlation with silica. The geochemical classification of the samples revealed the majority of the analysed samples from both units as calcic-rich and further classification iv | P a g e revealed the rocks as sub-alkali basalts. The two units did not reveal any significant geochemical variation, other than the significant concentrations of CaO, MgO, P2O5 and TiO2 in the LMP unit as compared to the MP unit. The fact that there are slight variations is an indication that all the samples crystallised from the same magma although the protolith according to geochemical evidence appears to have been from different or mixed sources; igneous/sedimentary source. From the analyses conducted it can therefore be concluded that the MP and the LMP are genetically affiliated seeing they originated from the same magma and they don’t exhibit significant mineralogical and geochemical variations. The petrographic analysis also confirmed the deep pink coloured coarse-grained mineral as secondary petalite and it mostly occurs adjacent to the grey primary petalite in the intermediate zone of the MP unit although it still occurs in minor concentrations in the LMP unit, however without the primary petalite. This version of petalite appears to have distinctive and prominent cleavage planes just like the grey petalite