Primary alteration and geochemical dispersion haloes of Archaean orogenic gold deposits in the Yilgarn Craton: the pre-weathering scenario

Abstract

Widespread orogenic gold deposits are hosted by practically all rock types over the range of metamorphic grades in the Archaean Yilgarn Craton, Western Australia. They are normally located in second- or third-order structures, most commonly near crustal- to regional-scale deformation zones, in brittle, brittle–ductile and ductile deformational environments. Despite this extreme variability, the alteration and geochemical features of these deposits are strikingly consistent. Each gold deposit is enveloped by an alteration-zoning sequence. In greenschist facies, the alteration zones are, from distal to proximal and according to the diagnostic minerals: calcite–chlorite, calcite–ankerite and sericite. In amphibolite facies, two zones are normally detected, a distal biotite and a proximal calc-silicate zone with no intermediate zone. The ores are enriched in Ag, As, Au, Ba, CO 2 , K, Rb, S, Sb, Si, Te and W, and depleted in Na and Y relative to host-rocks. Base metals, Fe, Mg and Mn are rarely mobile in these settings. From the available, albeit limited, database there is little correlation between rock type or metamorphic grade and the elements enriched in the gold deposit. All enriched components may form primary dispersion haloes around the deposits. Parameters that most consistently define dispersion haloes for those deposits studied in the Yilgarn Craton are As, Au, Sb, Te, W, and the carbonation and sericitization indices. In nearly all cases, at least one of the parameters defines an anomaly significantly more extensive than that defined by Au alone. In some cases, Se or Y anomalies also extend beyond the Au anomaly. Importantly, there is also at least one pathfinder element whose primary dispersion extends beyond the alteration halo based on mineral assemblages, whereas the alteration–index anomalies normally do not extend beyond these limits. The primary geochemical dispersion halo may extend laterally for >200 m, and along strike for a few kilometres, from the gold orebody. A single geochemical parameter rarely defines an extensive and consistent vector towards gold ore. However, useful directional information can be achieved when trends defined by several geochemical parameters are combined.