Venue: Denver, USA
Travel report by Brock Salier
With the support of a JH Lord Travel Grant, I attended the SEG 2002 “Global Exploration: Integrated Methods of Discovery” conference in Denver, USA, from 14-16 April 2002. At the conference I presented a poster on my PhD research in the Laverton area, specifically on the Wallaby gold deposit. The title of my research presentation was “Isotopic Constraints on the Genesis of Alteration at the Wallaby Gold Deposit, Western Australia”. Following the conference, I attended a field trip, run by Richard Goldfarb of the USGS, on which we visited the world-class Carlin, Comstock and Motherlode districts in the western USA.
The Wallaby gold deposit has reserves of over 8 Moz Au, and has a mine life of around eight years, which is expected to take production beyond 2010. The deposit is located near Laverton in a greenschist-facies domain of the Archean Eastern Goldfields Province of the Yilgarn Craton, Western Australia. A 1200m thick, basalt-dominated cobble conglomerate hosts gold mineralisation, and is underlain by volcanoclastic rocks, chert and BIF, with the basal contact defined by a 250m thick ductile shear-zone. Alkaline dykes, which intrude the conglomerate, are associated with a pipe-like zone of actinolite-magnetite-epidote-calcite alteration. The magnetite content of the alteration creates a 600m diameter, circular, aeromagnetic anomaly above the deposit. Gold mineralisation, which is hosted in a series of subhorizontal fracture zones, is associated with silica-albite-pyrite-dolomite alteration, and is largely confined within the magnetite-rich pipe.
My research concentrated on using the sulphur, samarium-neodymium, rubidium-strontium and carbon-oxygen isotope systems to identify if a genetic link exists between gold mineralisation and the magnetite-rich alteration, which aided discovery of this blind deposit.
My isotopic data argue against a proximal magmatic source for auriferous fluids, but are consistent with the formation of the extensive magnetite-actinolite rich alteration pipe through an influx of magmatic fluids related to the Wallaby Syenite. Such an influx is interpreted to have created a brittle iron-enriched zone in an otherwise massive conglomerate. Following and/or during deformation, this created a competency contrast, increased fracture permeability, and geochemically favourable conditions (high Fe/Fe+Mg ratio) for gold mineralisation. As such, the aeromagnetic anomaly associated with the Wallaby deposit only characterises the suitable location for gold mineralisation, and can only be used as a vector to mineralisation where structural and other conditions common to Archean orogenic gold deposits are met.
By attending the conference I was able to share my research with many other workers in the field of Archean gold mineralisation, and received valuable feedback. Furthermore, I was able to meet workers in fields other than Archean gold mineralisation, and several people drew valuable analogies with other younger systems. Such feedback is invaluable, as most of the gold industry in Western Australia, and consequently the economic geology research, is based around Archean gold systems. By broadening my understanding of both magmatic and orogenic gold systems, I feel that my research at the Wallaby gold deposit has benefited tremendously. Once again, I would like to thank Geoconferences for their generous support of my attendance at the conference through the JH Lord Travel Grant scheme.