A Basin System and Fluid-Flow Analysis of the Zn-Pb-Ag Mount Isa-Type Deposits of Northern Australia: Identifying Metal Source, Basinal Brine Reservoirs, Times of Fluid Expulsion, and Organic Matter Reactions

Abstract

Paleoproterozoic rocks of northern Australia host one of the worlds largest base metal repositories and are the worlds most important zinc repository. The McArthur-Mount Isa-Cloncurry mineral belt contains several world-class Zn-Pb-Ag, U, Cu, and Cu-Au deposits (Ewers and Fergusson, 1980; Williams, 1998; Betts et al., 2003; Large et al., 2005; Fig. 1). The province has the potential to host additional base metal and uranium reserves. Advances in exploration techniques in the 1980s led to the discovery of several major new Zn and Cu-Au deposits, including Cannington, Century, Ernest Henry, and Osborne. However, recent exploration results have been disappointing and new exploration strategies are required if the region is to further its growth potential and if new resources are to be realized beneath shallow cover. Between 1975 to 1995 geoscientists from Geoscience Australia, the Geological Survey of Queensland, and the Northern Territory Geological Survey mapped the Paleoproterozoic outcrop belt of northern Australia at a scale of 1:100,000. Subsequently, researchers at Monash University undertook detailed studies of the deformation history of the Mount Isa block, placing the ore deposits within a tectonic context (e.g.,ODea et al., 1997; Betts et al., 1998, 2003; Betts and Lister, 2002). Researchers at James Cook University carried out structural, metamorphic and mineralization studies across the Mount Isa block, with their principal focus concentrating on the deposits and their immediate environs (Bell et al., 1988; Broadbent et al., 1998; Williams, 1998). Between 1990 and 1998 a multidisciplinary research group based at CODES, University of Tasmania, completed studies aimed at better understanding the origin of the regions zinc deposits and their alteration halos in both the McArthur and Mount Isa regions(Cooke et al., 1998, 2000; Large et al., 1998, 2000, 2005;Garven et al., 2001; Yang et al., 2004). Each of the studies outlined above were based on lithostratigraphic concepts in which rock units were subdivided, mapped, and labeled, but the units defined are diachronous and cannot be used for reconstructions of basin shape and sediment architecture at the times of fluid migration. This requires an event-based chronostratigraphic framework. In 1995 the Australian Geological Survey Organisation commenced an integrated, multidisciplinary study of the Paleoproterozoic rocks of northern Australia with the aim of generating an understanding of the chronostratigraphy and evolution of the basin (Scott et al., 1998; Jackson et al., 2000, 2005; Sami et al., 2000; Southgate et al., 2000; Neumann et al., 2006). This new understanding links basin evolution in prospective stratigraphic packages to the underlying basement. These developments allowed us to (1) better constrain the shape of the basin through time, (2) define the internal stratigraphic architecture and sedimentary composition of the basin fill, and (3) establish the relationships between basin history, basement, basin faults, and fluid flow. This improved understanding of basin shape and sediment architecture at the times of fluid-flow and base metal deposit formation provides predictive capability in the modeling of the ore-forming system. The data and interpretations presented in this thematic issue of Economic Geology summarize the principal results of Australian Minerals Industry and Research Association (AMIRA) International Project P552: Fluid flow modeling in the Mount Isa and McArthur basins. This project aimed to develop an ore-forming, basin system understanding of the Zn-Pb-Ag and U deposits of northern Australia and was a collaborative program between Geoscience Australia, Queens University in Canada, CODES, University of Tasmania, University of Queensland, Commonwealth Industrial Scientific and Research Organization, and the Geological Survey of Queensland. Five fundamental questions were addressed: (1) which parts of the stratigraphy provided the source rocks for the base metals? (2) at what burial depths or temperatures did the basinal brine(s) become enriched in base metals? (3) what was the timing of brine expulsion and sulfide precipitation? (4) where in the basin did the metal-rich brines reside and along which pathways did they migrate? (5) what are the relationships between metal-bearing brine and organic matter at the site(s) of metal precipitation? To address these issues three integrated approaches were adopted: 1. Regional geometric models were constructed to depict sediment architecture and basin shape at the time(s) of metalbearing fluid migration. The models were built using the results of the earlier chronostratigraphic basin analysis studies (Figs. 2, 3; Tables 1, 2). 2. Mineral paragenesis and associated geochemical studies were undertaken at the regional scale to determine the relationships between early and late fluid evolution in siliciclastic, carbonate, and volcanic rocks to establish the links between the deep basinal brines and the deposits (Polito et al., 2006ac)and to characterize the thermal history and organic matter evolution in sediments that host the ore deposits (Glikson et al., 2006; Golding et al., 2006). 3. The geometric basin models were digitized and imported into fluid-flow modeling software packages so that scenario-based simulations could be run to test concepts for the origin of the deposits. The results of the mineral paragenetic and geochemical studies were used to constrain thermal gradients and physical properties used in the models (Yang et al., 2006; Zhang et al., 2006).