RE-OS DATING OF ARCHEAN OROGENIC AU DEPOSITS IN THE YILGARN, WESTERN AUSTRALIA

Orogenic Au deposits constitute a unique and important class of epigenetic deposits that form over a large crustal depth range (2-20 km) in collisional or accretionary terranes of Archean to Phanerozoic age. Orogenic Au deposits, produced by related processes over a continuum of depths, may also span a range of ages within the life-span of an accreting margin, and the array of ages revealed depends on exposure levels across the orogen. The Archean Yilgarn craton of western Australia contains some of the best-studied orogenic Au deposits in the world. Their ages remain controversial, and their origin continues to be debated as deep crustal or mantle.

Direct Re-Os dating of Au is not possible, as it lacks Re and does not permit the isochron age resolution possible with Re-Os dating of Au-associated molybdenite. The U-Pb dating of zircon from spatially associated intrusives may bracket the timing of Au mineralization, but does not always directly address its age. We present Re-Os data and ages for Yilgarn molybdenites that are directly associated with the Au mineralization. These results clearly document cratonic scale, classic orogenic Au deposition in the interval 2640-2620 Ma. However, the Re-Os data also suggest several significantly earlier Au-producing events in the eastern part of the Yilgarn, including ages as old as about 2760 Ma. Also, in one major Au deposit in the southwestern Yilgarn, two periods of Au deposition, both accompanied by molybdenite, are separated by 80 m.y., and reflect the superposition of 2710 Ma porphyry-style, subduction-related Au overprinted by 2630 Ma orogenic Au. The Re-Os data in this study are in agreement with or bracketed by U-Pb ages on zircon for associated rocks. A deep crustal origin for much of the Yilgarn Au mineralization is supported by volumetrically small amounts of molybdenite coupled with relatively low Re concentrations, and in gold by low ppt concentrations of Os with elevated ¹⁸⁷Os/¹⁸⁸Os. Demonstration of timing coupled with a geologic and tectonic understanding of Au deposition in different Yilgarn terranes is vital to building appropriate exploration models.