EVIDENCE FOR MULTIPLE PALAEOPROTEROZOIC THERMAL EVENTS AND MAGMATISM ADJACENT TO THE BROKEN-HILL PB-ZN-AG OREBODY, AUSTRALIA

Thermal events at 1690-1680, 1660-1640 and 1600-1570 Ma have been reso lved by SHRIMP U-Pb geochronological study on zircons and monazites fr om seven localities near to the Broken Hill Pb-Zn-Ag orebody, Australi a. The earliest-recognized thermal event included intrusion of now def ormed granites such as Rasp Ridge Gneiss and Alma Gneiss and intrusion of gabbro at Round Hill. Previously these have been interpreted as vo lcanic in origin, and have been assigned to different stratigraphic un its of the Palaeoproterozoic Willyama Supergroup. Because these rocks are intrusions, they should be removed from the Supergroup stratigraph ic sequence. The 1640-1660 Ma thermal event reached upper amphibolite to granulite conditions and produced melt segregations in parts of the Rasp Ridge Gneiss. Granites of this age are the Purnamoota Road Gneis s, previously correlated with 1690-1680 Ma rocks assigned to the Heres Gneiss stratigraphic unit, and granitic veins within Sundown Group me tapelites. The 1600-1570 Ma thermal event also reached upper amphiboli te to granulite conditions. The only possible 1600-1570 Ma intrusive r ock reported in this study is 'Lf-leucogneiss' (granite) at the Purnam oota Road locality. Melt segregations of this age have been found in t he Round Hill gabbro and metamorphic segregations have been found in t he Purnamoota Road Gneiss. The granite intrusions and segregations are absolute time markers for fabric development and therefore can be use d to reevaluate tectonothermal evolution of rocks close to the Broken Hill Pb-Zn orebody. Within the studied rocks several discrete high gra de deformation phases have been observed. The earliest detected deform ation is older than 1640-1660 Ma, but syn- or post 1690 Ma. A later de formation phase can be constrained to be pre-or syn 1640-1660 Ma and a yet later deformation phase to be syn- or post- 1600-1570 Ma. The cur rent consensus classifies the Broken Hill Pb-Zn-Ag orebody as the meta morphosed equivalent of classic SEDEX (sedimentary-exhalative) deposit s, deposited at ca 1690 Ma. This interpretation heavily relies on the Heres Gneiss being a volcanic marker horizon, because the orebody is s ituated, apparently conformably, within the Heres Gneiss. However, res ults of this study show that rocks assigned to the Heres Gneiss are of different age, thus do not form a reliable marker horizon. The presen t results suggest that in the Thackaringa and Broken Hill Groups in th e vicinity of Broken Hill, true supracrustal rocks are greater than or equal to 1690 Ma, rather than ca 1690 Ma as previously suggested. Lar ge parts of rocks surrounding the orebody are intrusions and together with their host supracrustal rocks were metamorphosed and locally reme lted at 1660-1640 and 1600-1570 Ma. (C) 1998 Elsevier Science B.V.