Diversity within a unified model for Archaean gold mineralization in the Yilgarn Craton of Western Australia: An overview of the late-orogenic, structurally-controlled gold deposits

Abstract

The Archaean Yilgarn Craton has produced >3000 tonnes of gold, mainly from structurally-controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2.66–2.63 Ga. As a group, these late-orogenic deposits encompass a wide range of host rocks, structural settings and structural styles and alteration types. However, several consistent features justify grouping the deposits into a single class of mineralization. These features include timing relative to orogenesis, high gold to base metal ratios, an association with potassium metasomatism and carbonation of host rocks, low sulfide contents, and a low-salinity H 2O–CO 2(–CH 4) ore fluid. Most deposits formed in domains of low mean stress during east–west regional compression and preserve evidence, in the form of quartz vein arrays and hydraulic breccias, for high fluid pressures and rock dilation. The range of ore depositional conditions implied by alteration assemblages at various deposits makes the term `mesothermal' an inappropriate description for this class of mineralization. The term late-orogenic, structurally-controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview. Late-orogenic, structurally-controlled gold deposits formed in thermal equilibrium with high-grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower-grade host rocks in the cores of greenstone belts. These relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later, high-grade metamorphism. Geochronological data indicate gold deposition at ca. 2.63 Ga in the western part of the Yilgarn Craton. Isotopic data suggest similar depositional ages in the eastern part of the craton (Eastern Goldfields Superterrane) but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca. 2.66 Ga. The main, competing genetic models for late-orogenic, structurally-controlled gold mineralization in the Yilgarn Craton propose an orthomagmatic, deeply-sourced or metamorphogenic ore fluid. None of these models can account satisfactorily for all the features of the late-orogenic, structurally-controlled gold deposits.