Lead isotope data from the Thalanga, dry river and Mt Chalmers base metal deposits and their bearing on exploration and ore genesis in eastern Australia

The age differences between the Thalanga, Dry River and Mt Chalmers deposits in northeast Queensland are reflected in their lead isotope data. Although all three are classed as volcanic exhalative and exhibit lead-rich (galena-sphalerite) and copper-rich (pyrite-chalcopyrite) zones, the Thalanga and Dry River deposits are located in Cambro-Ordovician metavolcanic-metasedimentary sequences, whereas the Mt Chalmers deposit is thought to be Permian. Lead-rich ores from the Thalanga and Dry River deposits have similar and homogeneous lead isotopic compositions (²⁰⁸Pb/²⁰⁶Pb = 2.111,²⁰⁷Pb/²⁰⁶Pb = 0.865,²⁰⁶Pb/²⁰⁴Pb = 18.10). The pyritic copper ores have a slightly different composition, which is ascribed to the in situ radioactive decay of uranium and thorium to lead since the time of crystallization of the ore. The lead isotope ratios for the younger Mt Chalmers deposit differ significantly from those of the Cambro-Ordovician deposits (²⁰⁸Pb/²⁰⁶Pb = 2.081,²⁰⁷Pb/²⁰⁶Pb = 0.849,²⁰⁶Pb/²⁰⁴Pb = 18.35). At Thalanga and Dry River, the gossans of lead-rich and copper-rich mineralization reflect the lead isotope composition of the primary ore. Hence, lead isotope analysis of weathered surface material may give indications of the nature of the underlying mineralization. The similarity in isotopic composition between the Thalanga and Dry River deposits and other base metal deposits in the Tasman Fold Belt considerably extends the area over which relatively uniform leads occur and points to similar source rocks and/or mineralizing processes for the ores on a large scale. The lead isotope data for the Permian Mt Chalmers deposit are similar to values for the Permian Halls Peak (NSW) deposit and differ from those for the older deposits of the Tasman Fold Belt suggesting an influx of mantle-derived material to the eastern part of the Australian continent during the Permian.