Hayman, Patrick1, Campbell, Ian2, Cas, Ray3, Squire, Rick3, Doutch, David3, Outhwaite, Michael4
1Queensland University of Technology, Brisbane, Australia, 2Australian National University, Canberra, Australia, 3Monash University, Clayton, Australia, 4Lithify Pty Ltd, East Victoria Park, Australia
Granophyre and quartz dolerite are the evolved fractions of differentiated dolerite (diabase) sills and are an important host to Archean gold deposits, in part because accessory magnetite acts as a chemical reactant for orogenic fluids. Despite their economic importance, the understanding of processes leading to enhanced formation of these favorable rock types is poor. Drill core logging, whole rock geochemistry, magnetic susceptibility, gold assay and thermodynamic modelling data from eleven mineralized and unmineralized ca. 2.7 Ga differentiated dolerites in the Eastern Goldfields Superterrane (Yilgarn Craton, Western Australia) are used to better understand the igneous and emplacement processes that increase the volume of host rock favorable for gold precipitation during orogenesis. Orogenic gold favours differentiated dolerites, derived from iron-rich parental magmas, that crystallize large volumes of coarse quartz (magnetite) dolerite (>25% total thickness). Mineralized sills are commonly >150 m thick and hosted by thick sedimentary sequences. Sill thickness is likely the most important factor as it largely controls cooling rate and hence fractionation. The parental melts must have fractionated large amounts of clinopyroxene and plagioclase (possibly up to 50%) before emplacement in the shallow crust. A second fractionation event at shallow levels (<3 km) operated both vertically and laterally, resulting in an antithetic relationship between quartz (magnetite) dolerite and cumulates (pyroxenites and peridotites). By comparison with younger mafic sills emplaced in syn-sedimentary basins, we argue that the geometry of these high-level sills was more irregular than the often-assumed tabular form. Any irregularities in the lower sill margin act as traps for early formed (dense) ferromagnesian minerals, now represented by pyroxene and peridotite cumulates, while irregularities in the upper sill margin trap the buoyant fractionated liquids when the sill is mostly crystalline, through magma flow on the scale of <1 km. Late formation of magnetite (F>50%) is critical to produce disseminated texture and increase the volume of magnetite-bearing quartz dolerite; thus dry magmas are more prospective. Less Fe-enriched melts are known to host orogenic gold, but these are less common, and probably only become good hosts for economic gold when sufficiently thick to fractionate large volumes of magnetite. We summarize the characteristics of the most prospective hosts relevant for exploration of differentiated dolerites hosting orogenic gold.
Dr Hayman’s principal research focuses on using field data and geochemical techniques to resolve volcanic, tectonic and ore forming processes of the Earth at a range of scales, from outcrops to terranes.