The highs and lows of Archean dolerites and the controls on gold mineralisation: examples from the Eastern Goldfields, Western Australia

Abstract:

Coarsely crystalline mafic rocks, commonly referred to as dolerite, are hosts to major orogenic gold deposits. Although the importance of structures for focusing gold-bearing fluids is well understood, the lithological and mineralogical variety of rock types associated with coarse mafic rocks, their emplacement origins and mineralization remain poorly understood. To address these points, we examined two mineralised (Golden Mile and Cave Rocks Dolerites) and three unmineralised packages from the Eastern Goldfields Superterrane (Yilgarn Craton, Western Australia). The coarsely crystalline mafic rocks are generally 400 m thick, extend for tens of kilometers, are primarily hosted in thinly bedded turbidites, are concordant to stratigraphy and have sharp and finely irregular contacts. The mafic packages consist of up to eight different lithologies, which we characterize by mineralogy, texture, composition, and magnetic susceptibility. The balance of evidence supports origins as high-level intrusive sills. A negative correlation between the thickness of ultramafic (pyroxenite and peridotite) compared to quartz- and magnetite-bearing lithologies is interpreted as evidence for undulations in the sills, with the denser minerals accumulating at low points and the less dense minerals occurring at high points. Gold data positively correlates with the quartz-bearing lithologies, which are thickest in the two mineralised examples. These observations together provide strong support for a primary mineralogical control on gold precipitation and present a tantalizing target for exploration for orogenic gold deposits: locate paleo-highs in the upper surface of sills.

Biography:

Patrick Hayman is a Lecturer at Queensland University of Technology. Upon completion of BSc ENG and MSc degrees in Canada, Pat spent time working for several diamond exploration and mining companies. He then moved to Australia to obtain his PhD in geology (volcanology) from Monash University and spent three years on a Research Fellowship working on regional stratigraphic and structural problems in the Eastern Goldfields, before moving to sunny Queensland.

Dr Hayman’s principal research focusses on physical and chemical reconstructions of volcanoes associated with ore deposits, ranging from deposit to terrane scale. Dr Hayman has particular expertise working with altered and deformed successions from the Archean and Paleoproterozoic, researching volcanogenic massive sulphide (Cu, Pb, Zn, Au), epithermal (Au, Ag), komatiite-hosted (Ni) and orogenic gold systems. Most of Dr Hayman’s research is underpinned by field mapping (outcrop and drill core) and utilises a variety of lab-based techniques (especially geochemistry, geochronology and textural studies) to resolve questions about depositional environments, tectonic processes and exploration targeting. 

 

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