Hematite geochemistry and geochronology resolve genetic and temporal links among iron-oxide copper gold systems, Olympic Dam district, South Australia

Abstract

The Wirrda Well and Acropolis prospects, ~25 km south from the Olympic Dam deposit, are among several dozen examples of iron-oxide copper gold (IOCG) mineralisation forming a 600 km-long province in the eastern Gawler Craton, South Australia. IOCG systems across the province differ in terms of host lithology, iron-oxide mineral associations, intensity of brecciation, and grade of Cu-(U)-Au mineralisation. Previous efforts to provide a geochronological framework for mineralisation, and correlate this with igneous activity, were inhibited by a lack of clear crosscutting features (particularly in breccia-hosted systems) and a scarcity of reliable hydrothermal mineral geochronometers. The earliest generation of hematite, characterised by enrichment in U-W-Sn-Mo and previously reported from Olympic Dam, provides confident U-Pb dates and insights into hydrothermal fluid signatures. Similar hematite is recognised from Wirrda Well and Acropolis, allowing direct comparison between the three IOCG systems. Relationships between early, silician (Wirrda Well) and titaniferous (Acropolis) magnetite and U-bearing hematite at the two prospects differ. Inferred martitisation of magnetite at Acropolis has generated zoned hematite with U-W-Mo-enrichment at constant Sn concentration, but with a marked loss of REE + Y (REY) and Ti, whereas at Wirrda Well, single-crystal zoned hematite, resembling that from Olympic Dam, retains high REY and Sn concentrations. Although morphologies, textures and compositions of U-bearing hematite at Olympic Dam, Wirrda Well and Acropolis vary significantly, a common U-Pb (laser ablation-inductively coupled plasma-mass spectrometry) age of ~1590 Ma is obtained from the most reliable data (²⁰⁷Pb/²⁰⁶Pb: 1598.9 ± 6.3 Ma at Wirrda Well and 1590.6 ± 6.5 Ma at Acropolis). The geochemical signatures of iron-oxides from the two prospects share common trends for U, W, Sn, Mo, high field strength and siderophile elements, comparable with signatures of Fe-oxides from the ‘outer shell’ of the Olympic Dam deposit. Given that datable iron-oxides would seem to be ubiquitous phases throughout the eastern Gawler Craton, the commonality in geochemistry and age of U-bearing hematite from the Olympic Dam district is significant, as it provides a single, traceable mineral to compare IOCG systems across the province.