Geological Evolution of the Stuart Shelf and Proterozoic Iron Oxide-associated Mineralization: Insights from Regional Geophysical Data

2019-02-15T04:21:03Z (GMT) by Paul A. Gow
Analysis of regional aeromagnetic and gravity datasets shows the Stuart Shelf is host to a major volcano-plutonic complex consisting of large volumes of felsic-dominated volcanic rocks and shallow-crustal level granitic plutons. The complex is interpreted as a major eruptive centre associated with the Gawler Range volcano-plutonic event at approximately 1600 Ma to 1580 Ma. The Mesoproterozoic basement surface in the northern and central areas of the Stuart Shelf represents a section through the lower-middle levels of the igneous complex, and shows evidence of collapsed cauldrons, epizonal granites, and ring faults associated with pluton emplacement. Some of the syn-emplacement structures acted as fluid pathways during an extensive episode of Fe-metasomatism associated with the volcano-plutonic event, and now host significant volumes of iron oxide alteration +/- Cu-U-Au-(Ag-REE) mineralization. The enormous volume of iron oxide alteration allows excellent delineation of alteration zones and fluid conduits in the geophysical data, providing a unique opportunity to document the plumbing systems in these felsic volcano-plutonic systems.
The major period of Fe-metasomatism associated with the volcano-plutonic event produced deposits of hydrothermal iron oxide-associated Cu-U-Au-Ag-REE mineralization. Three common deposit morphologies are recognized. These are I) tectonic breccias, 2) stratabound units, and 3) vein-networks. The tectonic breccias are commonly hematite-dominated, mineralized, and may be hosted by all rock types. The stratabound units are commonly magnetite-dominated and are hosted by the carbonate/chert-bearing Wandearah Formation. Evidence presented in this study supports a model of ore genesis involving two temporally distinct fluids, as previously proposed for the enormous Olympic Dam deposit. The first fluid was of magmatic origin and produced skarn-like magnetite-dominated assemblages. The second fluid was a lower temperature meteoric or evolved meteoric-hydrothermal fluid, and deposited the mineralization in a hematite-dominated assemblage, often overprinting precursor magnetite. Fluid flow was strongly structurally controlled. The magmatic-sourced first phase of fluid flow was localized along a regionally-dominant northwest-trending set of faults, and within arcuate roof-zone faults that formed during emplacement of the Hiltaba Suite granitoids (possibly caldera ring fractures). The second phase of fluid flow was controlled by reactivation of these faults during prolonged multistage intrusion of the Hiltaba Suite granitoids. Overprinting of the magnetite-dominated assemblages by the later hematite-dominated alteration is not a spatial coincidence, but is the result of formation of brecciation-related permeability in the magnetite-rich zones associated with reactivation of the localizing faults. Mineralization is most richly developed where the second stage of fluid flow was associated with prolonged fault activity and/or volcanism as at Olympic Dam.
Whole-rock and mineral separate (quartz - iron oxide) oxygen isotope analyses are consistent with this model and suggest that the initial fluids were magmatic, with a 8180 value of -6.5 - 7.5 %0 and a temperature of 490 - 570°C, whereas the later mineralizing fluids had lower values of <4.6 %0 and a temperature of 380 - 460°C, perhaps indicative of an evolved meteorichydrothermal fluid.
The study suggests that several important controls are necessary to create significant iron oxide-associated mineralization as seen on the Stuart Shelf. These include large volumes of felsic magma, large syn-emplacement brittle structures, and most importantly long-lived volcano-plutonic activity.