Basin dynamics and mineralisation, McArthur Basin, northern Australia

The Broadmere Structure is a concealed complex inversion structure that developed in a well preserved part of the Batten Trough in the southern McArthur Basin of northern Australia. The structure formed in response to a number of major tectonic events over a period of at least 600 million years. Evaluation of more than 300 km of seismic data gathered across the structure has provided significant insight into the evolution of the basin architecture which, in turn, has provided a framework for understanding the spatial and temporal distribution of mineralisation. The Palaeoproterozoic McArthur Basin, a polyphase basin with a long and complex evolutionary history, was initiated some time after 1.8 Ga as a sag basin following mantle activity associated with supercontinent assembly. This shallow sag then began to accumulate a relatively mature clastic sedimentary succession with interspersed basic volcanics. Subsequent extension of the weakened crust, beginning at approximately 1730 Ma, led to the development of a series of normal faults and half grabens within zones (E.G. Batten Trough) defined laterally by major strike slip fault complexes. The half grabens were filled initially by volcaniclastic sediments with associated bimodal volcanics that were later followed by mixed carbonate and clastic sediments exceeding 8 km in thickness. A major change in basinal and regional dynamics between 1575 and 1500 Ma (Isan Orogeny) led to the inversion of the thickened succession overlying the half grabens and extensive erosion of earlier depositional units, followed by deposition of a further 3 km of shallow marine clastic sediments. The analysis of the Broadmere Structure has provided a simple framework in which to evaluate prospectivity. The primary northwest striking half grabens formed localised anoxic, sediment starved depocentres in which organic rich sediments that potentially host mineralisation accumulated. The distribution of subsequent ore bodies is a function of the timing of fluid movement and the nature of fluid pathways, as determined by the evolving basin architecture. Fluids moved laterally through the permeable channels into the Batten Trough in two stages. Spatial data show that Pb and Zn occurrences lie close to the bounding faults, while Cu occurrences are distributed across the width of the Batten Trough, supporting a more or less syngenetic origin for the former and a much later, post inversion, epigenetic origin, for the latter. Mineralisation thus occurred in association with inversion structures formed above the primary half grabens that were generally concealed by later sedimentation. Because the half grabens occur at regularly spaced intervals along the Batten Trough there may well be more mineral occurrences similar to the McArthur River deposit. Clearly, these conclusions must be integrated into any exploration strategy.