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Ironstones of the Olary Block, South Australia: the Use of RNAA and INAA to Understand Their Genesis

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Ironstones of the Olary Block, South Australia: the Use of RNAA and INAA to Understand Their Genesis AU9716144 Ironstones of the Olary Block, South Australia: The use of RNAA and INAA to understand their genesis. Ian R. Plimer (School of Earth Sciences, The University of Melbourne, Parkville, Victoria, 3052), Bernd G. Lottermoser, Paul M. Ashley and Dave C. Lawie (Department of Geology and Geophysics, The University of New England, NSW, 2351). INTRODUCTION Broken Hill Block The Early Proterozoic Willyama Supergroup in the Broken Hill Block is host to the huge Broken Hill Pb-Zn-Ag ore deposit and numerous occurrences of other types of mineralisation. The Broken Hill deposit formed during a period of crustal extension at 1700 Ma associated with rapid sedimentation, the influx of mantle-derived mafic melts and lower crustal melting. The circulation of seawater in the fractured rift sediments, volcanics and intrusives was driven by cooling of granite plutons and resulted in leaching of metals from crustal rocks, the leaching of continental evaporites, fluid reduction, fluid heating and focussed fluid exhalation. Sudden changes to fluid P-T-X resulted in rapid submarine sulphide precipitation in the deepest part of the rift {Parr and Plimer (1)}. Facies changes from sulphides in the deepest part of the rift to stratigraphically-equivalent exhalites reflects fluid mixing, oxidation and temperature changes. Such changes are reflected in the geochemistry of the proximal and distal exhalites {Lottermoser (2), (3)}. Olary Block The adjacent Olary Block of South Australia contains Willyama Supergroup rocks stratigraphically equivalent to those in the Broken Hill Block. No massive sulphide deposits are known from the Olary Block despite the presence of numerous ironstones of submarine hydrothermal origin. This study reports RNAA and INAA analyses to document the geochemistry of ironstones as a mechanism of locating the locus of 1700 Ma plumbing conduits and the economic centres of mineralising systems. GEOLOGY Metamorphism and deformation The Olary Block comprises upper greenschist to lower amphibolite low P-high T metamorphic facies Early Proterozoic metasediments and igneous rocks {Clarke et al. (4)} deposited at 1700 Ma {Cook et al. (5)}. Pre-regional metamorphic intrusions of leucocratic A-type granite at 1700 Ma formed andalusite-rich aureoles in metasediments. The Willyama Supergroup was intruded by I-type granodiorites at 1630 Ma, which may have initiated low P-high T metamorphism, and underwent coeval metamorphism and deformation (Ml-Dl and M2-D2) at 1600 Ma (Olarian Orogeny). A number of S-type granitoids and associated rare metal-bearing pegmatites derived from the partial melting of composite 11 gneisses crystallised at 1S90 Ma and a retrograde metamorphic event (D3) has been dated by Ar-Ar methods at 1S30 Ma. Ar-Ar dates of 1100-1200 Ma (Grenvillian Orogeny) and 850 Ma (opening of Adelaide Geosyncline) are associated with brittle deformation. The Willyama Supergroup was unconformably overlain by the Adelaidean sequence of pelite, psammite, carbonate and diamictite and the Delamerian Orogeny (460-500 Ma) resulted in greenschist facies metamorphism, multiphase folding of the Adelaidean, faulting and shearing of the Willyama- Adelaidean unconformity and reactivation of retrograde shear zones. Lithology Five major lithological units are recognised in the Willyama Supergroup. The total thickness of the Supergroup is unknown, basement is not exposed and unconformities have not been recognised. The basal Composite Gneiss of psammopelitic composition grades via migmatite into banded, migmatitic and massive granitoids. The Quartzofeldspathic Suite is dominated by quartzofeldspathic rocks, most of which comprise massive to laminated albite-quartz rocks. The Quartzofeldspathic Suite contains disseminated magnetite and a diversity of iron formations comprising quartz-magnetite +/- hematite, baryte, chalcopyrite. Iron formations are laminated, massive, brecciated and replaced. Some iron formations are enriched in Cu and Au. The quartzofeldspathic rocks grade via calc-albitites into the Calc-silicate Suite. The calc-silicate rocks contain calc-silicate pseudomorphs after carbonate and gypsum and the presence of hypersaline fluid inclusions suggests evaporite mineral formation during deposition and early diagenesis. The influence of evaporite-derived brines and local hot spring exhalation have greatly modified the felsic metasediments. The Bimba Suite is a thin heterogeneous discontinuous unit comprising calc-silicates, carbonates and psammopelitic and pelitic schist. Sulphides are ubiquitous as are surficial gossans and lags enriched in Cu, Pb, Zn, Co, As, Ag, Au, Mn, Ba, W, Mo, Bi and U {Lawie and Ashley (6)} The Bimba Suite formed in a shallow water lacustrine/sabkha/marine environment into which were episodic hot spring exhalations. The Pelite Suite is widely distributed and comprises pelitic and psammopelitic schist, a distinct graphitic facies, calc-silicate ellipsoids and rare tourmalinite, FeMn garnet-quartz rocks and laminated manganiferous banded iron formation. The presence of common graded bedding, sediment composition, lamination and the thickness of the unit suggest deposition of a turbiditic sequence in a deepening marine or lacustrine environment. Mineralisation styles In contrast to the Broken Hill Block, mineral occurrences in the Olary Block are dominated by Cu and U with the only significant deposit being the Radium Hill U(-REE) mine exploited from 1954-1961. Minor feldspar has been produced from pegmatite and baryte production from iron formations. Ashley et al. (7) have recognised stratiform iron formations (with minor to trace Cu and Au) in the Quartzofeldspathic Suite, FeCuZnPbCoAs minerals in the Bimba Suite, and small banded iron formations and Mn silicates in the Calc-silicate and Pelite Suites. Stratabound Fe, Cu, Zn, Co, W, U and F occurrences are common in the Quartzofeldspathic, Calc-silicate and Bimba Suites. U, Th and REE veins, breccias and stockworks derive from 1590 Ma granites as do pegmatite-hosted feldspar, beryl, U, Th, REE, Nb, phosphates and muscovite. Structurally- and lithologically-controlled epigenetic veins and replacements occur in iron formations, calc-silicate rocks and albitites (Fe) and associated with D3 retrogression and Delamerian events (FeCuAu, FeZnCuPb, U-REE, CuAu, Au). Regolith- 12 related gossans, supergene-enriched Cu(CoAu) deposits and surficial ironstones formed during Tertiary to Recent weathering. IRONSTONE GEOCHEMISTRY RNAA, INAA and electron microprobe analyses of ironstones and baryte from ironstones indicates a composition unlike metamorphic, vein-type, intrusive-hydrothermal, sedimentary and karstic barytes. In contrast, the iron formations and baryte from the Olary Block are geochemically similar to modern and ancient submarine hydrothermal precipitates. Geochemical analyses of iron formations and baryte generally show low Mn, Pb, Zn, As and Sb concentrations and elevated Au and Cu values. Major (Al, Si) and trace (REE, Th, U, PGEs) element data show that the ironstones and baryte rocks are of hydrothermal origin and that mixing occurred between oxidised (and possibly hypersaline) surficial waters and a hydrothermal fluid similar to that from modem sites of submarine exhalation. Such geochemical similarities imply rapid burial in the volcano-sedimentary pile or sub-seafloor replacement. The REE patterns in the iron formations demonstrate a range from low temperature reduced hydrothermal precipitates, seawater and oxidised hydrothermal precipitates. Geochemical signatures are due to precipitation from hydrothermal fluids of lower temperature with further influence by prolonged exposure to the overlying water column and subaqueous oxidation. CONCLUSIONS RNAA and INAA analyses can be used to; 1. Delineate the focus of an ore-forming plumbing system, and 2. Assess whether a mineralising system is likely to contain economic metals. REFERENCES 1. Parr, J. and Plimer, I. R., (1995): Models for Broken Hill-type lead-zinc-silver deposits. In: Mineral deposit modelling; eds. Kirkham, R. V., Sinclair, W. D., Thorpe, R. I. and Duke, J. M.; Geol. Assoc. Canada Special Paper 40, 253-288. 2. Lottermoser, B. G. (1989): Rare earth element study of exhalites within the Willyama Supergroup, Broken Hill Block, Australia. Mineralium Deposita 24, 92-99. 3. Lottermoser, B. G. (1991): Trace element composition of exhalites associated with the Broken Hill sulfide deposit, Australia. Economic Geology 86, 870-877. 4. Clarke, G. W., Burg, J. P. and Wilson, C. J. L. (1986): Stratigraphic and structural constraints of the Proterozoic tectonic history of the Olary Block, South Australia. Precambrian Research 34,107-137. 5. Cook, N. D. J., Fanning, C. M. and Ashley, P. M. (1994): New geochronological results from the Willyama Supergroup, Olary Block, South Australia. In: Australian Research on Ore Genesis Symposium, Adelaide. Australian Mineral Foundation. 6. Lawie, D. C. and Ashley, P. M. (1994): Reconnaissance geochemical sampling of ferruginous regolith in the Olary Block, South Australia. In: Australian Research in Ore Genesis Symposium, Adelaide. Australian Mineral Foundation. 7. Ashley, P. M., Cook, N. D. J., Lottermoser, B. G. and Plimer, I. R. (1994): Notes on geology and field guide book to excursion stops in the Olary Block, Geological Survey of South Australia Report Book 94/8, 37pp. 13.
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