Emplacement origins of coarsely-crystalline mafic rocks hosted in greenstone belts: Examples from the 2.7 Ga Yilgarn Craton, Western Australia

Abstract

Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium- to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ∼75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite ± pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (≪1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration.