Zircon mineral chemistry and implications for magmatic-hydrothermal evolution of the granite-hosted Zaaiplaats Sn deposit, Bushveld Large Igneous Province, South Africa

Abstract

This study presents trace element compositions for zircons from a series of co-genetic granitic rocks of the Lebowa Granite Suite (LGS) that host the Zaaiplaats Sn mine in the northern limb of the Bushveld Large Igneous Province (LIP). In this area, the LGS comprises the coarse-grained Nebo and Bobbejaankop granites, and the fine-grained Lease granite, of which the two latter are variably altered and host Sn mineralisation. Under cathodoluminescence, zircons of the Nebo granite display long prismatic crystals and well-developed internal oscillatory zoning, whereas those of the Bobbejaankop and Lease granites are porous with faint zoning and vermicular to spongy internal textures. Trace element contents of zircons in the Bobbejaankop and Lease granites show significant enrichments in all the trace elements (e.g., Hf, Nb, Ta, U, Th, Ti, and REE) suggesting a hydrothermal origin for the zircons of the Bobbejaankop and Lease granites, that are different to those for the magmatic zircons of the Nebo granite. Moreover, the inclusion-free hydrothermal zircons of the Bobbejaankop and Lease granites point to lower Ti-in-zircon derived crystallisation temperatures (450-600 °C) and oxygen fugacities [as log (ƒO2) values in the range − 20 to −26] than those of the Nebo granite [where T = 570-625 °C and log (ƒO2) values = −19 to −21]. Collectively, the data record an apparent magmatic differentiation from a single magma, showing derivation of the hydrothermal zircons of the Bobbejaankop and Lease granites from a highly-fractionated late-magmatic-hydrothermal fluid enriched in F, Cl and B, as indicated by non-chondritic Y/Ho and Zr/Hf ratios under reducing conditions, as required typically for the formation of the granite-hosted Sn deposits.