Base metal exploration of the Mount Read Volcanics, western Tasmania; Pt. III, Application of lead isotopes at Elliott Bay

Abstract

Lead isotope studies were carried out in the southern part of the Mount Read Volcanics on rock chip, drill core, and bed-rock geochemical samples in order to improve the genetic model of mineralization and to assign exploration priorities. Samples analyzed from 20 separate mineralized occurrences fall into three major isotopic groups which correspond to three different styles and episodes of mineralization.Samples of the Cambrian stratiform massive sulfide Pb-Zn-Ag mineralization at the stratigraphic top of the rhyolite volcanic pile constitute the least radiogenic group ( 206 Pb/ 204 Pb = 18.06 to 18.14), and samples from the Devonian vein-style Pb-Zn-As mineralization emplaced along major faults in the western epiclastic and sedimentary rocks form the most radiogenic group ( 206 Pb/ 204 Pb = 18.44 to 18.53). A third group consists of samples with isotopic ratios mostly intermediate between the other two; the majority of the 206 Pb/ 204 Pb ratios range from 18.18 to 18.30. The samples come from disseminated and vein-type Pb-Zn mineralization associated with bodies of quartz porphyry which intrude the pyroclastic sequence and are considered to be later than the episode of massive sulfide formation. The samples also contain strata-bound gold mineralization which is typically associated with carbonate alteration in the host volcanics. A plot of lead contents versus isotopic ratios of these samples gives an inverse correlation and indicates that the spread in ratios results from radioactive decay since mineralization. The samples as a whole indicate one source of lead emplaced in the Cambrian rather than a mixing of two sources, one Cambrian and the other Devonian.Lead isotope data for all samples of mineralization and volcanics studies can be interpreted in a multistage model to imply an ultimate source for the lead from the Precambrian basement. The ratios of massive sulfide mineralization at Elliott Bay are distinctly less radiogenic than for massive sulfide mineralization farther north in the Mount Read Volcanics at Rosebery and Que River and this implies the possibility of different ages for the separate mineralizing systems or a variation in the U-Th-Pb characteristics of source regions along the volcanic belt.