Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome region3. 3D architecture and permeabilitySimon van der Wielen, Allison Britt, Roger Skirrow3.1 INTRODUCTIONFaults and permeable sedimentary units comprise the fundamental permeability architecture foruranium and other basin-related mineral systems in the region. This architecture is important infocussing fluids at scales ranging from crustal to basin to deposit. The structural and basinarchitecture is a product ultimately of the geodynamic and tectonothermal evolution, aspects ofwhich are described in the first section below. The 3D structural architecture is then documented,based on previous work and on newly compiled 3D datasets such as seismic transects. A series offigures of the architecture are presented as 3D pdf files. Finally, the 3D geometry of Cenozoicunits including the Eyre and Namba formations is presented for a case study area to the northeastof Lake Frome. These units represent some of the key permeability components in uraniummineral systems in the region.The chemical architecture and mapping methodology is presented in Chapter 4.3.2 TECTONIC AND THERMAL EVOLUTIONUranium mineral systems of the Lake Frome region occur within a part of the Australiancontinental crust that appears to be unusually enriched in heat-producing elements includinguranium. The South Australian Heat Flow Anomaly extends over a broad region between the MtPainter Inlier in the east and the eastern Gawler Craton in the west, and includes the FlindersRanges (Neumann et al., 2000). A significant proportion of the heat production is sourced fromupper crustal felsic igneous rocks with elevated uranium, thorium and potassium contents(Neumann et al., 2000). McLaren et al. (2002) contended that a major thermal contribution duringthe Delamerian Orogeny in the Mt Painter region originated from this anomalous high-heatproducingcrust and that intense deformation was guided by zones of thermally weakened crust.The region then experienced the effects of the Alice Springs Orogeny in the period ~310-260 Ma,accompanied by exhumation and cooling (Mitchell et al., 2002). Similarly, Célérier et al. (2005)proposed that the present-day topography of the Flinders Ranges including the Mt Painter Inliercan be explained by Paleogene to Cenozoic inversion and faulting focussed on zones of thermallyweakened crust. Apatite fission track dating indicates significant exhumation (1.5-2.0 km) andcooling commencing as early as the Paleocene (~65 Ma) or late Cretaceous (Mitchell et al., 2002).However, heating above ~100°C appears to have persisted along the Paralana Fault until ~20-25Ma, possibly as a result of hydrothermal fluid circulation that largely switched off after this time(Foster et al., 1994; Mitchell et al., 2002). Other studies suggest that much of the present day reliefin the Mt Painter Inlier could have been generated since ~4 Ma (Quigley et al., 2007). Increasingstrain during the Neogene may be a result of plate boundary forcing as Australian platecommenced its rapid northwards drift (Célérier et al., 2005; Sandiford et al., 2005).The theme of repeated uplift and deformation occurring in parts of the South Australia Heat FlowAnomaly is potentially important in the development of basin-related uranium mineral systems.First, uplifted uranium-rich basement may have provided both a plentiful source of uranium forleaching by surficial waters as well as the topographic driver for fluid flow into adjacent basins.Second, erosion may have provided uranium-rich detritus to basins flanking the uplands, whichare potentially a secondary source of uranium. Third, one of the modes of intraplate compressivedeformation proposed for the Flinders Ranges (long-wavelength elastic deformation) resulted intopographic lows bordering the ranges to the east and west, now occupied by Lake Frome andLake Torrens (Célérier et al., 2005). During wetter climatic periods such lake systems may haveaccumulated abundant organic matter, which is potentially effective as a reductant to precipitateuranium from oxidised waters (see also Chapter 4).Page 19 of 151