Metal anomalies in zircon as a record of granite-hosted mineralization

Abstract

Granite-hosted magmatic-hydrothermal mineral deposits are major sources of Cu, Mo, Sn, Li, and W, originating via mineralizing fluids exsolved from volatile-saturated magmas. We show how trace elements in zircon sampled from the granite-hosted Zaaiplaats tin deposit, Bushveld Complex, preserve a record of both the enrichment of incompatible metals during magma fractionation and those arising from magmatic-hydrothermal mineralization processes. The Zaaiplaats granites are subdivided into three groups; mineralized, altered, and unmineralized. Zircon trace element contents define two trends in a plot of Sn against Gd: Sn/Gd ratios in zircons from the unmineralized samples, as well as the majority of altered samples, define a magma fractionation trend with increasing Y at constant Sn/Gd, whereas those from the mineralized samples are displaced to high Sn/Gd ratios at similar Y. Elevated Sn in the Zaaiplaats zircons is attributed to the introduction of a Sn-rich mineralizing fluid during zircon growth, which occurred at an advanced stage of crystallization (>85%) of the host magma. This model is consistent with the preservation of whole-rock Sn zonation in the Zaaiplaats granites modelled by closed-system magma differentiation and the ensuing exsolution of an acidic, saline Sn-rich magmatic-hydrothermal fluid (Groves and McCarthy, 1978). A metal anomaly, Sn/Sn*, is defined which describes the deviation of Sn over that expected through magma fractionation alone (Sn*), and arises from Sn mobilization due to magmatic-hydrothermal mineralization processes. Identification of metal anomalies such as Sn/Sn* and Cu/Cu* in mineral archives or at the whole-rock level, provides an empirical link to the onset of mineralization processes in magmatic-hydrothermal systems, and can be coupled with geochemical proxies to yield a better understanding of the conditions leading up to, and subsequent to, volatile saturation.