A U [sbnd]Th [sbnd]Pb lead isotope study of rocks and ores from Broken Hill, Australia

Abstract

A lead “double-spike” calibrated against an NBS Standard was used to obtain the isotopic abundances of totalrock samples of high-grade gneiss from the Mine Sequence, Broken Hill. New lead isotope data for a number of Thackaringa Type ore leads are also reported as are the lead, uranium and thorium concentrations of the rock samples. An age of 1690 ± 35 my is calculated from the apparent linear relationship of the rock leads on a 207Pb/ 204Pb− 206Pb/ 204Pb diagram. This agrees with previously reported Rb Sr results and is similarly interpreted as the time of the regional high-grade metamorphism. Recent mobility of uranium and thorium may account for the considerable scatter of the rock data when plotted on conventional U Th Pb isochron diagrams. The slope of the least squares line through the galena data on a 207Pb/ 204Pb− 206Pb/ 204Pb plot is substantially higher than values previously reported. It is therefore suggested that Thackaringa mineralization occurred approximately 800 my ago, prior to the retrograde metamorphism dated at 500 my. The common lead component in both Thackaringa ores and high-grade gneisses may be isotopically identical to lead from the main lode orebody (that is, Broken Hill Type lead). Hence, formation of this orebody with lead derived from local source rocks at the time of the high-grade metamorphism may be indicated. Alternatively, as suggested by Stanton, metamorphism of ore and host rocks soon after syngenetic deposition would be likely to give the observed isotopic results.