Structural controls of the Ernest Henry IOCG deposit: Insights from integrated structural, geophysical and mineralogical analyses. Issue 1 (1st December 2019) - Austin, Jim
Summary: Iron Oxide Copper-Gold deposits (IOCGs) are structurally controlled, and typically display zonation of iron oxides and sulphides, which is potentially related to redox zonation. There are three main factors that determine the location and architecture of an IOCG: 1. fluid pathway(s); 2. trap/host, and; 3. plumbing system (i.e., mechanisms for depressurising the system). Generally, these are loosely referred to as structural controls, but they exercise very different functions within the system. In this study we integrate the results of petrophysical property analyses (including, magnetic susceptibility, remanence, radiometrics and conductivity), structural fabric analyses and TIMA scans that provide information on both mineralogy and texture. The results when placed in an oreproximal- distal-background framework, allow us to understand the footprint of the system. The results show that the initial ductile-brittle metasomatic traps (i.e., the NE-trending shear zones) are highly magnetic due to their relatively reduced (magnetite-albite) mineral assemblage and preserve the pre-existing structural fabric. Conversely, the secondary, brittle, trap (i.e., the breccia) is moderately magnetic and weakly oxidized with a magnetite-hematite-pyrite-chalcopyrite mineralogy, and has randomised magnetic fabric due to brecciation. The least magnetic, and most oxidised zone of the system has a quartz-calcite-chlorite-hematitechalcopyrite mineral assemblage. It overprints the breccia, at the intersection of the trap (NE-trending shear zone(s)) with the fluid pathway (N-S strike-slip fault) and has an NS sub-horizontal AMS lineation. The different metasomatic alteration assemblages present at Ernest Henry (i.e., sodic, potassic and calcic alteration) may be related to redox zonation within one event, or represent overprinting of metasomatic episodes. However, the zonation of the system is initially controlled by the preexisting architecture, a compressive jog within a N-S trending strike-slip fault. When the trap (the jog) is permeable and/or reactive, the redox gradient lies along the trap, between the fluid pathway and the pressure valves. As conditions become more brittle and the trap becomes impermeable and/or non-reactive, and the system becomes over-pressured, leading to brecciation at the intersection of the trap with the fluid pathway.