Authors / CoAuthors
Gibson, G.M. | Crooks, A. | Szpunar, M.
Abstract
Exploration targets in Palaeoproterozoic rocks (Willyama Supergroup) of the Outalpa Inlier, South Australia include the regionally extensive Bimba Formation, a marble and calc-silicate dominated unit known for its high base-metal content (Pb, Zn, Mn, Cu, Co). This unit is a correlative of the Ettlewood Calc-silicate in the Broken Hill Group (NSW) and is spatially associated with an important redox boundary that elsewhere in the Olary area served as the locus for several recently discovered mineral deposits (Kalkaroo, White Dam, Portia). This boundary is visible in aeromagnetic images of the Olary region and because of its obvious potential as both a marker horizon and guide to mineralisation, it was selected, along with the adjacent units, for detailed structural investigations as part of the Broken Hill Exploration Initiative. These investigations focussed on Ameroo Hill with the goal of better understanding the three dimensional geometry of the area as well as clarifying the origin of this important boundary and the extent to which it is structurally and/or stratigraphically controlled. Stratigraphy in the Ameroo Hill area incorporates one or more unconformities and has been deformed by at least four phases of pre-Adelaidean deformation (D1-D4). The Bimba formation immediately overlies one such unconformity and typically coincides with the transition of a variably oxidised succession of migmatised psammopelitic gneisses, quartz-albitites, calc-albitite, and minor calc-silicate rock (Curnamona Group) into an overlying sequence of psammopelitic schist that is increasingly graphitic towards its base (Strathearn Group) (Conor, 2000). Tight to isoclinal macroscopic folds identified in both sequences are of D2 and D3 age. They produced widespread structural repetition of lithological units and fold interference patterns that conform to the modified arrow-head type (non-coaxial deformation). D2 was also associated with northeast-directed thrust faulting that locally emplaced high-grade metamorphic rocks over lower-grade metamorphic facies. Microfabrics and metamorphic mineral assemblages in the Ameroo Hill area further indicate that D2 was accompanied by crustal thickening and deforms an even earlier high-grade layer-parallel fabric. The origin of this fabric remains unclear although formation in an extensional tectonic environment cannot be ruled out. A later episode of more upright W-E folding and associated shearing (D4) further complicates the regional structure, making for a complex outcrop pattern in which lateral and vertical continuity of the Bimba Formation is neither assured nor predictable without a thorough understanding of the three-dimensional structural geometry. Pb-Zn-Cu mineralisation in the Outalpa Inlier is not confined to a single horizon. The most obvious gossans in some areas are associated with a 5-20 m thick calc-silicate unit lying some tens of metres below the Bimba Formation and separated from it by a sequence of thin-bedded psammitic schist. This calc-silicate unit forms part of the underlying Curnamona Group (Ethiudna subgroup; Conor, 2000) and constitutes a secondary target for mineral exploration. It is manganiferous as well as sulphide rich, and is distinguished from the overlying Bimba Formation by its occurrence within a sequence of composite biotite gneisses (meta-sandstone) that are also host to a thin but regionally persistent quartzite layer. The most effective drilling programs and exploration strategies are likely to be those that target mineralisation at more than one stratigraphic level, including sub-Bimba depths, and which acknowledge that the original stratigraphy may have been substantially modified by deformation. Conor, C.H.H. (2000): Definition of major sedimentary and igneous units of the Olary Domain, Curnamona Province. Mesa Journal 19: 51-56.
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39193
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- External PublicationAbstract
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- AU-SA
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- Earth Sciences
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2002-01-01T00:00:00
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