Abstract

Two key issues related to the origin of the massive Broken Hill Pb-Zn-Ag orebody are the genesis of its host rocks and their depositional age. These granulite facies layered gneisses have been ascribed alternately to volcanic and nonvolcanic regimes. Determination of the depositional age of the sequence has never been seriously addressed.North of Broken Hill, amphibolite facies precursors to the granulite facies gneisses show compelling evidence of a volcanic origin in their texture, chemistry, and zircon morphology. These felsic metavolcanic rocks are in the upper part of the Broken Hill Group, stratigraphically equivalent to the Broken Hill orebody. The present work traces textural, zircon morphological and isotopic changes across the amphibolite to granulite facies transition, in order to determine the primary depositional age of this supracrustal sequence and the timing of highgrade regional metamorphism.Ion microprobe (SHRIMP) U-Pb data on zircons from the lower grade (amphibolite facies) felsic metavolcanic rocks define a major magmatic population at 1690 + or - 5 Ma. This is considered to be the age of zircon crystallization and eruption of felsic volcanics in this part of the Broken Hill Group. These SHRIMP data clarify inheritance and discordance problems that had plagued conventional, multigrain U-Pb analyses.Zircon results from granulite facies stratigraphic equivalents of these rocks, near Southern Cross and at Broken Hill, have patterns similar to those of the amphibolite facies rocks, including igneous relics that provide the same primary age. Additionally, zircon in the highest grade granulite facies rocks ( approximately 800 degrees C) forms metamorphic overgrowths surrounding the magmatic relics. These younger zircon selvedges have a pooled age of 1600 + or - 8 Ma, which determines the timing of this granulite facies event. Consistent U-Pb monazite results yield this same age (1599 + or - 9 Ma), suggesting that the monazite blocking temperature is close to that of zircon or that cooling to <725 degrees C was achieved relatively rapidly. This is some 60 m.y. younger than the previously accepted age for high-grade metamorphism at Broken Hill. Irrespective of metamorphic grade in the gneisses, all biotite Rb-Sr ages have been reset to approximately 480 Ma, a reflection of early Paleozoic (Delamerian) orogenesis.Inherited zircon grains, interpreted as xenocrysts and in some cases mantied by 1690 Ma igneous rims, have older Proterozoic to late Archcan ages (1780-2700 Ma). The presence of xenocrystic zircon components points to an ensialic tectonic setting for the Broken Hill block and reflects accidental incorporation in the felsic volcanic protoliths of lithic fragments and/or complex igneous sources, such as found in crust exposed in the Gawler craton to the west.The 1690 + or - 5 Ma depositional age determined here provides a firm chronologic tie between the early Proterozoic sequences enclosing major stratiform Pb-Zn-Ag orebodies at Broken Hill, Mount Isa (1670 + or - 19 Ma), and McArthur River (1690 + or - 27 Ma) in northern Australia. In light of the uncertainties over timing and mode of origin of these deposits, the similar ages of their host sequences have important implications for early Proterozoic tectonics and metallogeny.

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