The genesis and spatial distribution of platinum-group elements in the Coronation Hill Deposit, Northern Territory, Australia, Thesis, 2010

Abstract

The aim of this project is to unravel the fundamental controls on the genesis and distribution of the platinum-group element (PGE) mineralisation at the Coronation Hill deposit in the Northern Territory. Coronation Hill is a low-temperature, hydrothermal Au-Pt-Pd (-Se-SbU) deposit located 80km east of a town called Pine Creek. The deposit is part of the South Alligator Province in the Pine Creek Inlier, a 66,000m2 orogenic belt of Paleoproterozoic age. This deposit is similar to unconformity-related uranium deposits elsewhere in this province, with the precious metals occurring below a major unconformity that separates a deformed and metamorphosed basement (comprised of igneous intrusions, volcaniclastic rocks, and calcareous and carbonaceous sedimentary units) from a cover sequence of little deformed hematitic quartz sandstone and sedimentary breccias. The Coronation Hill deposit does however display some key differences. The PGE mineralisation occurs in very fine alloys (eg. Pt-Pd-Fe, Au-Ag) and platinum-group minerals, such as stibiopalladinite (Pd5Sb2), sudburyite (PdSb), PGE-selenides (Pt,Pd)Se2, and native palladium, broadly hosted in two main settings: 1) U-Au-PGE at or below the major unconformity and/or 2) Au-PGE with very low U confined to stratigraphic units below the unconformity, but with no apparent lithological control. Previous studies have indicated that precious metals were transported and concentrated by hydrothermal fluids from an unidentified source, possibly located outside the immediate deposit. Conversely, in this study we argue that the primary source of the PGE may be more proximal. Geochemical and petrographic data indicate that the least altered diorites contain anomalous concentrations of Au and PGE, mainly associated with finely disseminated magmatic sulphides. We propose that hydrothermal fluids, which circulated in the deposit during multiple metamorphic events, selectively remobilised the precious metals from the sulphides into the surrounding units (breccias and quartz porphyries), which display PGE concentrations and patterns in which the PPGE (Pd, Pt, Rh) are selectively decoupled from the IPGE (Ir, Ru). We interpret this signature as evidence of the hydrothermal transport of the more soluble PPGE from the sulphides into the breccia and porphyries.