SHRIMP U-Pb geochronological constraints on Archean volcanic-hosted massive sulfide and lode gold mineralization at Mount Gibson, Yilgarn Craton, Western Australia

Abstract

The base metal-rich Mount Gibson gold deposits are hosted within the ductile Mount Gibson shear zone in the Retaliation greenstone belt of the Murchison province of the Archcan Yilgarn craton. The deposits are unique in the context of the Yilgarn, as they exhibit two main styles of hydrothermal alteration and sulfide mineralization, consistent with overprinting of synvolcanic, volcanic-hosted massive sulfide-style mineralization by synorogenic, shear-hosted lode gold mineralization. On the basis of model ages based on sulfide Pb isotope ratios, these two events are separated by approximately 300 m.y.Subvolcanic intrusive rocks from within the host sequence to mineralization at the Orion Two and Hornet deposits have SHRIMP II zircon U-Pb ages of crystallization of 2929 +_ 3 and 2934 +_ 6 Ma, respectively. These ages are within error and provide a minimum age for the host sequence, and consequently, the volcanichosted massive sulfide mineralizing event, at Mount Gibson. On the basis of geochemical evidence, the intrusions are interpreted to have been derived from the same parent magma as the rocks they intrude, and consequently ca. 2930 Ma is probably the approximate age of the host rocks to mineralization. The regional granitoid, which intrudes the base of the greenstone belt, has a crystallization age of 2935 +_ 3 Ma. A post-tectonic granite porphyry, which crops out approximately 25 km north-northeast of the Mount Gibson mine site, has a SHRIMP II zircon U-Pb crystallization age of 2623 _+ 7 Ma. This granitoid has identical field relationships, similar whole-rock geochemistry, and similar Pb isotope ratios to those of an internal granitoid which cuts gold mineralization in the Deep South pit and is consequently interpreted to be cogenetic and contemporaneous. The 2623 +_ 7 Ma date is therefore interpreted to be a minimum age for synorogenic gold mineralization at Mount Gibson.A morphologically and chemically distinctive population of zircons extracted from the sample of 2934 Ma altered felsic schist from the Hornet deposit gives a SHRIMP II zircon U-Pb age of 2627 + 13 Ma. These zircons have characteristics which are consistent with a hydrothermal origin and the age is interpreted to represent the timing of synorogenic gold mineralization at Mount Gibson. The interpreted mineralization age is within error of the inferred age of the postmineralization, post-tectonic granitoid. The age is also within error of the majority of absolute ages for gold mineralization from across the Yilgarn craton, which generally fall in the range 2630 + 10 Ma. SHRIMP zircon U-Pb geochronology thus provides strong support for the two-stage model proposed for mineralization at Mount Gibson. Synvolcanic volcanic-hosted massive sulfide-style mineralization occurred at ca. 2930 Ma and was overprinted by lode gold mineralization some 300 m.y. later, at ca. 2630 Ma.

The Composition of Zircon and Igneous and Metamorphic Petrogenesis

“…Hydrothermal zircon has been reported from Archean massive sulfide deposits (Yeats et al 1996), vein-type Archean gold deposits in Canada, China and Australia (Claoué-Long et al 1990, Kerrich and King 1993, Li et al 1997, Yeats et al 1996, and a variety of shear and terrane suture zones (Wyoming Province, Mueller et al 1996;Kaapvaal Craton, Flowers 2000;Newfoundland Appalachians, Ramezani et al 2000). Despite these studies no consistent picture of hydrothermal zircon textures and composition has been forthcoming; anhedral and faceted morphologies and zoned and unzoned crystals have been described as hydrothermal, as have low common-Pb and high common-Pb zircon.…”

“…Despite these studies no consistent picture of hydrothermal zircon textures and composition has been forthcoming; anhedral and faceted morphologies and zoned and unzoned crystals have been described as hydrothermal, as have low common-Pb and high common-Pb zircon. The hydrothermal zircon of Yeats et al (1996) fall into two distinct subpopulations on the basis of REE composition (P.W.O. Hoskin, unpublished data) even though there is no distinction between them on the basis of morphology or internal textures and they form a single U-Pb age population.…”

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

“…Only when initial Nd isotopic data plot Table 1. References for multiterrane features and events include Nelson (1995), Kent & McDougall (1995), Kent & Hagemann (1996), , Yeats et al (1996), (Nelson 1997), Krapez et al (2000), Pidgeon & Hallberg (2000), Witt et al (2001), Chen et al (2001a), Chen et al (2003), Van Kranendonk & Ivanic (2008), Ivanic et al (2010), Pawley et al (2012) and Mole (2012). The geological features/processes active at a given time are shown by different colours, where green represents supracrustal activity/formation; pink represents plutonism (mainly granitic); yellow represents gold-forming events (all types, i.e.…”

Age and significance of voluminous mafic–ultramafic magmatic events in the Murchison Domain, Yilgarn Craton

“…Although *2950 Ma greenstones occur in the southern part of the Murchison Domain (Yeats et al 1996;Wang et al 1998), greenstones in the northern part of the domain were deposited in three groups from 2825 to 2700 Ma (Figure 2). The 2825-2800 Ma Norie Group is intruded at its base by synvolcanic and younger granitic rocks, whereas the younger 2800-2740 Ma Polelle and 2735-2700 Ma Glen Groups (Figure 2) have disconformable to unconformable contacts on the older greenstones.…”