The Archean lode gold deposits at Wiluna, Western Australia; high-level brittle-style mineralization in a strike-slip regime

Univ. West. Aust., Dep. Geol., Nedlands, West. Aust., Australia

The Wiluna lode-hosted gold deposits comprise a group of individual Archean lode-hosted gold deposits located in the northern part of the Norseman-Wiluna belt, Western Australia. Collectively, they are the fifth largest gold producer in the Yilgarn block. The deposits are hosted in a series of very low grade metamorphosed (prehnite-pumpellyite facies) mafic and ultramafic lava flows with abundant primary igneous textures preserved. Lode-hosted gold mineralization is controlled by the Wiluna strike-slip fault system which consists of north-and northeast-trending and west- and east-dipping sets of principal displacement faults and associated subsidiary structures. The movement on all faults was dextral, with minor oblique-reverse or oblique-normal motion and maximum apparent displacement of 1,350 m.The lode deposits are centered on structural inhomogeneities in the principal displacement faults (e.g., divergent bends and dilational jogs including horsetail splays) and/or subsidiary structures (e.g., overstepping and intersecting faults). The plunge of the ore shoots is generally moderately to steeply to the south and is controlled by the intersections between (1) lithologic layering and fault planes, (2) two or more fault planes, (3) a fault plane and an earlier formed barren or weakly mineralized quartz reef that is semiconcordant to the regional stratigraphy, and (4) a fault plane and a felsic intrusion. The shape and orientation of ore shoots becomes more complex downdip, where they are controlled by additional features such as drag folds and link structures.Fault-hosted ore breccias, including implosion and net-veined breccias and brecciated veins, are significant loci of gold mineralization in all lode deposits; they contain mainly quartz (chalcedony), carbonates, fuchsite and arsenopyrite, pyrite, and/or stibnite. Mineralization occurs also in shear veins and extensional veins and finely disseminated along foliation planes of the mineral fabric formed oblique to the principal displacement faults. Textures of implosion breccias at Wiluna indicate a history of incremental extensional opening across dilational sites and multiple episodes of brecciation and hydrothermal recementation. Quartz textures, such as vugs, cavities, comb textures, and cockade-textured quartz, are ubiquitous, indicating the preservation of open space due to low effective confining pressure.Geologic relations indicate that the mineralized strike-slip fault system was generated late in the tectonic history of the greenstone belt and that high-grade ore zones formed late in the strike-slip deformation event. The very low grade metamorphic facies and the brittle style of deformation in the host rocks, combined with the style of mineralization, indicate that the fault and mineralizing system formed in a shallow, near-surface crustal environment. Textural evidence, combined with the tectonic setting of the lode deposits, favors the suction pump mechanism as a fluid infiltration mechanism for the lode-hosted gold deposits within the Wiluna strike-slip fault system.

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