~1760 Ma magnetite-bearing protoliths in the Olympic Dam deposit, South Australia: implications for ore genesis and regional metallogeny

Abstract

Spatial associations between banded iron formation and iron-oxide Cu-Au (IOCG) style mineralization are well documented in the Gawler Craton (South Australia), but the possible genetic relationships between these two distinct types of mineralization are hitherto unclear. A texturally conspicuous generation of coarse-grained silician magnetite, intergrown with carbonates and quartz, is observed in drillholes intersecting the ‘outer shell’ of the Olympic Dam IOCG-type deposit. This magnetite is characterised by high U-content (~50 ppm), siliceous chemistry, and unusual zonal textures with respect to Si-Fe-nanoprecipitates. Direct dating of this magnetite by laser ablation inductively coupled plasma mass spectrometry yields reproducible 207Pb/206Pb dates (1761 ± 16 Ma) that are significantly older than the granite hosting the deposit (1593 Ma), or the mineralized breccias constituting the Cu-U-Au-Ag resource (~1592–1589 Ma). The older, Fe-rich crustal material can be correlated with the ~1.76–1.74 Ga (meta)sedimentary Wallaroo Group, host to Fe-rich horizons across the Gawler Craton, including locations ~15 km NW of Olympic Dam. A generation of granitic rocks, which intruded bedrock at ~1.75 Ga are present ~30 km NE of Olympic Dam, and likely exsolved hydrothermal fluids that enriched pre-existing magnetite-bearing protoliths in both U and REE. Such material was physically, and likely chemically, incorporated into the ‘outer shell’ at Olympic Dam some ~150 Ma later, during granite uplift along faults. The coincidence between Fe-rich horizons/BIF and ~1750 Ma granitoids may have provided IOCG systems with an additional source of both Fe and U that predates the ~1.59 Ga craton-scale metallogenic event. The uranium concentrations in some South Australian IOCG systems represent major global anomalies in the element. A combination of the fortuitous geological circumstances outlined here, may help explain the highly anomalous accumulation of uranium found at Olympic Dam.