Finding and identifying platinum group elements in the Eastman layered mafic intrusion

Platinum group elements (PGEs) are commonly hosted by base metal sulfides (BMSs, e.g. chalcopyrite or pentlandite) or platinum group minerals (PGMs). PGE-bearing phases are generally small, ranging from tens of nanometres to a few micrometres. In addition, PGEs can be hosted in PGMs, either intimately associated with BMSs or spatially separated. This poses a difficulty in their detection and analysis, and leads to uncertainty in identification of their mineral hosts. Their erratic occurrence introduces a substantial nugget effect in bulk rock assays, used to evaluate deposit economics on a larger scale. Knowledge of PGE hosts has a direct influence on resource management and extraction efficiency, making the understanding of mineral distribution important for processing. The Eastman Ultramafic Intrusion is a layered mafic–ultramafic intrusive complex in the Eastern Kimberley of Western Australia. Samples from Cu–Co–Ni–PGE exploration drilling at Eastman were bulk-assayed and found to contain up to 2.39 ppm PGEs. The rocks have been strongly altered and no longer represent their primary magmatic mineral assemblage and textures. To better understand the Eastman PGE mineralogy, we applied a multi-method approach of quantitative analytical electron microscopy, 3D scanning and <i>in situ</i> mass spectrometry. We find that Pt is primarily hosted in sperrylite, whereas Pd is mainly associated with kotulskite, pseudomertieite, testibiopalladite, sudburyite and michenerite. Additionally, minor Pt and Pd occur in solid solution within cobaltite, as well as Rh-rich cobaltite (hollingworthite). Spatially, PGMs and cobaltite are mostly clustered on a millimetric scale, and BMSs on a micrometric scale. Three-dimensional computed tomography scanning is a valuable tool for understanding PGM distribution, nugget effect scale and narrowing down interest areas for sample analysis. Guided by 3D scanning, combined quantitative analytical electron microscopy and <i>i</i> mass spectrometry are effective for finding and identifying PGEs. Platinum is overwhelmingly hosted in the arsenide sperrylite, whereas palladium is mostly hosted in various bismuthides and tellurides. Cobaltite contains minor Pt and Pd. The Eastman mineralisation is primarily orthomagmatic, and later reequilibration at low temperature redistributed palladium but not platinum.