Late-Archean granitoid-hosted lode-gold deposits, Yilgarn Craton, Western Australia: Deposit characteristics, crustal architecture and implications for ore genesis

Abstract

Granitoid-hosted lode gold deposits constitute a sub-group of late-Archean lode-gold deposits in the Yilgarn Craton of Western Australia. They share a number of common characteristics, including: (1) a strong structural control on a variety of scales; (2) in most instances, the presence of mineralisation in adjacent supracrustal sequence; (3) wallrock alteration and vein assemblages consistent with the addition of SiO 2, K 2O, CO 2, S±Na 2O, but dependent on the P– T conditions of the host environment and ore fluid; (4) a metal association comprising Au, Ag, As, Bi, Te, W±Mo and low concentrations of Cu, Pb and Zn; and (5) deposition from a low-moderate salinity, near-neutral H 2O–CO 2±CH 4 fluid over a temperature and pressure range from 250 to ≈600°C and ≈1 to 4 kbar, which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies. These characteristics suggest that the granitoid-hosted lode-gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone-hosted lode-gold deposits of the craton. In combination, lode-gold deposits irrespective of their host rocks and host terrane, formed over a range of crustal levels and developed late in the tectonic sequence of the craton. Granitoids which host lode-gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and, in combination with thermodynamic and isotope studies, indicate that the host granitoids are not the dominant source of the ore fluid. However, the host granitoids do provide a structural and, to a lesser extent, chemical trap for gold-bearing fluids. In combination, trace-element and isotope studies of greenstone- and granitoid-hosted deposits suggest that the ore fluid originated from a deep (mid to lower crustal) source and, at least, interacted with granitic rocks at some stage. The ore fluid composition is quite constant over the range of crustal levels from deep to shallow, implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial.