Crustal-scale structures in the Proterozoic Mount Isa Inlier of north Australia: their seismic response and influence on mineralisation

Abstract

The Proterozoic Mount Isa Inlier in northern Australia is prospective for base metals and gold. It contains a Western Fold Belt (including the Leichhardt River Fault Trough) and an Eastern Fold Belt, separated by older basement rocks of the Kalkadoon Leichhardt Belt. Sediments and volcanics in both fold belts were deposited in rifts which were subsequently shortened by up to 50%. Mineralisation appears to be partitioned: large-tonnage lead and zinc deposits are more prevalent in the Leichhardt River Fault Trough, and most gold and copper occurrences are in the Eastern Fold Belt. Cross-sections of the inlier derived from coincident seismic reflection and refraction profiling are dominated by the youngest tectonic events. The refraction data imply a west-dipping lens of high-velocity intermediate-to-mafic rock in the middle to upper crust in the east of the inlier. It is collinear with another lens in the lower crust in the west of the inlier. The lenses form a belt of high-velocity rock cutting the crust from top to bottom and from east to west. The reflection data reveal different styles of compression-related structures in the east and west of the inlier. Thin-skinned tectonics dominate in the Eastern Fold Belt. The sediments and volcanics are thrust to the west along a number of shallowly east-dipping upper-crustal detachments. The detachments in turn are cut by steeply east-dipping reverse faults which link into the zone of high-velocity rocks defined by the refraction data. In contrast, faults in the Western Fold Belt are steep and penetrate to mid-crustal levels and probably also link into the belt of high-velocity material deeper in the crust. The partitioning into different tectonic styles occurs across the Kalkadoon Leichhardt Belt, which appears to have acted as a buttress during the crustal shortening. Published mineralisation models attribute the lead-zinc mineralisation to circulating fluids at shallow crustal levels within the Leichhardt River Fault Trough. High reflectivity of faults with a close spatial association with mineralisation is attributed to alteration along the fault caused by migrating fluids. Copper-gold mineralisation in the Eastern Fold Belt is scattered, but known major deposits lie along-trend from a thrust fault shown in the seismic data to be highly reflective. This fault links via the upper-crustal detachments and the high-velocity lenses into the middle to lower crust. and is seen as a likely control on fluid migration pathways from lower crustal levels into the supracrustal Eastern Fold Belt. The partitioning of the tectonic styles seen in the seismic data and the mode of linking of faults into the middle to lower crust are seen as primary factors in the partitioning of mineralisation in the region.