The Montague gold deposit, located in the Gum Creek greenstone belt within the Youanmi Terrane of the Southern Cross Province, Western Australia, is hosted in greenschist facies metamorphosed basalts and granodiorites. The deposits are spatially associated with the D3 NW striking Caledonian shear zone, which is a subsidiary shear zone off the regional-scale Victory Well shear zone. Despite over 100 years of exploration for gold (and other metals) in the Gum Creek greenstone belt, detailed geological studies only started in the 1990’s. Beeson et al. (1993) provide the only regional scale investigation of the lithostratigraphic and structural framework of the Gum Creek greenstone belt. Hence, the overall economic potential and characteristics of gold mineralization in the Gum Creek greenstone belt remain poorly investigated. New whole rock geochemistry of both pristine and hydrothermally altered rocks from the meta-basalts and the granodiorites that form the host rocks to the Montague gold deposit indicate that hydrothermally altered rocks experienced loss in silica content whith varying K2O, compared to least altered samples. Supported by petrological observations, chloritization, sericitization and K-metasomatism affected the rocks during hydrothermal alteration. Indeed, mass balance calculations (Gresens, 1967) reveal that significant gains in CaO, total C, S and H2O resulted from hydrothermal alteration. Furthermore, chlorite geothermometry from distal and proximal alteration zones in both the meta-basalt and granodiorite host rocks yield temperatures of 301 ± 12°C (distal) and 336 ± 6°C (proximal), respectively. Our new chemical-petrological data suggest that the Montague deposit of the Gum Creek greenstone belt is best classified as an Archean orogenic (mesozonal) gold system. The characteristic features include: (1) structural control on the localization of the ore bodies in second-order brittle-ductile shear zones adjacent to first order fault zones, (2) distinct hydrothermal alteration zoning surrounding the controlling structures, and (3) estimated hydrothermal alteration temperatures of about 301 ± 50°C indicating mesozonal crustal level emplacement and development of the ore-bearing system.

Reference(s)

Beeson, J, Groves, D, and Ridley, JR, 1993, Controls on mineralization and tectonic development of the central part of the northern Yilgarn Craton. Minerals and Energy Research Institute of Western Australia, Report 109. Gresens, RL, 1967, Composition-volume relationships of metasomatism. Chemical Geology, v. 2, p. 47-65.