Broken Hill Pb-Zn-Ag deposit — a product of mantle metasomatism |
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| Authors: | I R Plimer |
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| Institution: | (1) The University of New England, 2351 Armidale, N.S.W., Australia;(2) Present address: University of Newcastle, 2308, N.S.W., Australia |
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| Abstract: | Broken Hill ore deposition occurred during the highest geothermal gradient coeval with an event of bimodal basic-rhydacitic volcanism. The depositional environment is interpreted as an ensialic rift on the basis of the sedimentary facies, ferro- and low-K tholeiitic basalts, and bimodal basic-calc alkaline (rhyolite, rhyodacite) volcanism. The orebody is of unusual composition characterised by abundant carbonate, fluorite and fluorapatite, abundant LIL-, K- and Rb-rich premetamorphic alteration assemblages, primordial S and Pb isotopic values, Sr isotopes possibly indicating a heterogeneous source, S : Se in the magmatic range, and zoning suggestive of cooling of the ore fluid.It is suggested that in the mature stage of rifting, propagation of deep fractures suddenly devolatilized the mantle, released CO2 and other fluids which, together with basalts, ascended and caused crustal melting to form acid magmas. The resultant ore fluid exhalation and basaltic and rhyodacitic volcanism were therefore coeval. Rapid deposition of ore from a fluid of unusual composition in a basin or graben within the rift formed a deposit which is not too dissimilar in composition from a carbonatite.The composition of the associated basic rocks and younger alkaline rocks, the premetamorphic alteration assemblage, and the orebody chemistry all suggest that the Lower Proterozoic source area for the ore fluids was metasomatized mantle. |
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