DIVERSITY WITHIN A UNIFIED MODEL FOR ARCHEAN GOLD MINERALIZATION IN THE YILGARN CRATON OF WESTERN-AUSTRALIA - AN OVERVIEW OF THE LATE-OROGENIC, STRUCTURALLY-CONTROLLED GOLD DEPOSITS

The Archaean Yilgarn Craton has produced > 3000 tonnes of gold, mainly from structurally-controlled deposits that formed during the latest s tages of an orogenic event that affected the entire craton and culmina ted in the period 2.66-2.63 Ga. As a group, these late-orogenic deposi ts encompass a wide range of host rocks, structural settings and struc tural styles and alteration types. However, several consistent feature s justify grouping the deposits into a single class of mineralization. These features include timing relative to orogenesis, high gold to ba se metal ratios, an association with potassium metasomatism and carbon ation of host rocks, low sulfide contents, and a low-salinity H2O-CO2( -CH4) ore fluid. Most deposits formed in domains of low mean stress du ring east-west regional compression and preserve evidence, in the form of quartz vein arrays and hydraulic breccias, for high fluid pressure s and rock dilation. The range of ore depositional conditions implied by alteration assemblages at various deposits makes the term 'mesother mal' an inappropriate description for this class of mineralization. Th e term late-orogenic, structurally-controlled gold deposits is propose d as one which more completely encompasses the class of mineralization described in this overview. Late-orogenic, structurally-controlled go ld 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. Th ese relationships probably reflect diachronous metamorphism and a sing le gold mineralization event which was contemporaneous with the later, high-grade metamorphism. Geochronological data indicate gold depositi on 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 crat on (Eastern Goldfields Superterrane) but this timing is difficult to r econcile with field relationships that suggest broad contemporaneity b etween regional metamorphism and gold mineralization at ca. 2.66 Ga. T he main, competing genetic models for late-orogenic, structurally-cont rolled gold mineralization in the Yilgarn Craton propose an orthomagma tic, 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. (C) 1998 Elsevier Science B.V. All rights reserved.