Monitoring gold precipitation mechanisms using S isotopes: preliminary applications for ore vectoring

Crystal Laflamme, Assistant Professor, Department of Geology and Geological Engineering, University of Laval

Crystal Laflamme is a Canada Research Chair in sulfur isotope geochemistry at Université Laval since 2018. Her research group studies pathways of sulfur through the lithosphere to better understand the secular evolution of geodynamic processes and geochemical reservoirs, responsible for metal accumulation in the Earth’s crust. Prior to 2018, Crystal worked as a research associate at the Centre for Exploration Targeting, University of Western Australia and then as a Consulting Geologist for Terrane Geoscience.

Talk Abstract:

In orogenic gold deposits, the mechanism by which a hydrothermal fluid precipitates gold in laminated quartz veins remains elusive. However, due their small geochemical footprint, understanding the specific mechanism responsible for gold precipitation is critical for targeting for high-grade veins, especially as exploration moves undercover. Reduced, near-neutral fluids ascending along crustal structures in the middle crust, typical to those that form orogenic gold deposits, transport Au as Au(HS)2-. As a result, the precipitation of gold from fluids is best interpreted as the product of the evolving fluid SO42-/H2S ratio, itself a function of processes such as fluid mixing, fluid-wall rock reaction, and/or phase separation (boiling). Recent developments in analytical techniques allow for S isotopes to be measured through a sulfide paragenetic sequence, lending insight into the evolving fluid chemistry leading to destabilization of Au-complexes. In this talk, I will demonstrate new applications of in situ multiple S isotopes combined with trace element composition in sulfides to the understanding of orogenic gold deposits from the Eastern Goldfields, Australia, and the Abitibi subprovince, Canada. A specific example will demonstrate that gold in the Kanowna Belle deposit, Australia precipitates from a single auriferous fluid by phase separation associated with the fault-valve mechanism. This finding demonstrates that fault structures can act not only conduits for auriferous fluids, but also as drivers for gold precipitation, regardless of the host-rock chemical composition. This understanding opens up the search space to the upper Fe-poor lithologies of the greenstone sequence.

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