Lode-gold deposits of the Yilgarn block: products of Late Archaean crustal-scale overpressured hydrothermal systems

Abstract

Abstract Although the lode-gold deposits of the Yilgarn block are hosted by a variety of rocks, and their structural style, associated alteration and ore mineralogy are also variable, common parameters suggest that they represent a coherent group of epigenetic deposits, most of which formed during a widespread (500 000 km 2 ) and broadly synchronous (2635 ± 10 Ma) hydrothermal event in the closing stages of the Late Archaean tectonothermal evolution of the host granitoid-greenstone terrains. Progressive variations in deposit parameters can be correlated with the metamorphic grade of the enclosing greenstone successions. These systematic variations, combined with evidence for their timing and the P-T conditions of their formation, indicate that the deposits form a continuum in which gold deposition took place from < 5 km depth (1 kbar, 180°C) to > 15 km depth (> 5 kbar, 700°C), marking hydrothermal fluid flow and fluid evolution through the middle and upper crust. The primary ore fluid appears to have been an overpressured, low salinity H 2 O-CO 2 -CH 4 fluid originating from a deep source. Upward fluid advection was strongly channelized along vertically extensive conduits. Although there is a gross regional association between clusters of gold deposits and craton- or greenstone-scale deformation zones, these do not appear to have been the primary fluid conduits, at least during their major phase of structural and magmatic activity. Further, all kinematic types of lower-order structure are mineralized, and the fossil fluid conduits exposed at the mine scale are extremely variable, some with no obvious fault or shear control. A potentially unifying hypothesis that can explain this extreme variability in the nature of the conduits is that fluid flow was focussed into zones of low mean stress in the granitoid-greenstone terrains. This can explain the selective occurrence of gold deposits adjacent to irregular or fault-bounded granitoid contacts in some goldfields, the selective mineralization of competent units (e.g. dolerites) in elongate greenstone belts that contain such units and are oriented sub-perpendicular to the far-field compressive stress, and the lack of mineralization in major, largely planar, shear zones undergoing simple shear. The orientation of the greenstone belts with respect to the far-field compressive stress appears to be a crucial factor in defining the potential for strike-extensive zones of low mean stress. This potentially can explain why granitoid-greenstone belts with a high density of sub-parallel craton- and greenstone-scale deformation zones are most highly mineralized and commonly contain the giant gold deposits which, themselves, are located in geometrically anomalous zones within these greenstone belts.