Apatite and Biotite in Syenitic Intrusions, Archean Karari Gold Deposit: Evidence for an Oxidized Magma and Oxidized Subsolidus Potassic (Biotite) Alteration

Abstract

Apatite and biotite from syenitic intrusions at the Karari gold deposit, Kurnalpi Terrane, Archean Yilgarn Craton, Western Australia, were targeted for microprobe and LA-ICP-MS analyses in order to determine some parameters (e.g., temperature, oxygen fugacity) during magmatic crystallization and potassic (biotite) hydrothermal alteration. The understanding of magmatic conditions is limited by the almost complete hydrothermal modification of magmatic minerals but made possible by the analysis of refractory magmatic apatite and rare relics of Ba-rich biotite. The SO3 contents of apatite cores (up to 1 wt.%) indicate that the syenitic magmas were strongly oxidized (fO2 > NNO +1) and contained approximately 1360 ppm S. The Cl contents of apatite are very low, suggesting crystallization from a low-Cl magma resulting from early separation of a volatile phase that sequestered Cl from the magma. Trends defined by wt.% SO3, at constant XF/XCl, are consistent with partitioning of oxidized S into the volatile phase during apatite crystallization. Early potassic alteration of the syenitic intrusions produced fine-grained seams of hydrothermal biotite and compositional modifications of igneous biotite phenocrysts. The magnesian composition of the hydrothermal biotite (mg# > 0.5) suggests that oxidized magmatic conditions persisted during potassic alteration. Provinciality of compositional data for different intrusion samples suggests that potassic alteration took place at low fluid:rock ratios. The potassic hydrothermal fluid modified the rims of igneous apatite crystals, depleting them in F, light rare earth elements, Mn, and S. The rare earth elements were rapidly redeposited, as monazite, in the apatite rims or surrounding groundmass. The depletion of S in apatite rims indicates that the potassic (biotite) alteration fluid was under-saturated in S. Application of the biotite-apatite geothermometer suggests potassic (biotite) alteration took place at temperatures of about 660 °C. The results of this study suggest the syenitic magmas at Karari were strongly oxidized, similar to Canadian examples of syenite-associated gold deposits, and therefore potentially fertile for hydrothermal gold.