Uranium-lead and lead-lead investigations of minerals from the Broken Hill lodes and mine sequence rocks

Mineral separates from the main lodes and mine sequence rocks at Broken Hill have been analysed using the U/Pb and Pb/Pb methods. This investigation had four main objectives and conclusions: to determine 1) the age of crystallization of apatite-the apparent 1595 m.y. ages for monazite and sphene and the 1550-1575 m.y. age range for apatites are most simply explained as cooling ages, the Broken Hill Block remaining above 600oC for 60-70 m.y. after metamorphism; 2) the age of source material of the mine sequence rocks-zircon U/Pb ages were reset during the granulite facies metamorphism at 1663 m.y., thereby obscuring the source age; 3) the possible volcanic or sedimentary parentage of the mine sequence rocks-zircons from the gneisses have undergone a complex history, judging from their morphology (unlike zircons in felsic to intermediate volcanics) and are not simply derived in a single-stage process from a volcanic source; and 4) the isotopic homogeneity of the main sulphide lenses-sphalerites and pyrrhotites show uniform lead isotope ratios throughout the ore-body because of the presence of (inseparable) galena inclusions.-G.J.N.