Groundwater in the Broken Hill region, Australia: Recognising interaction with bedrock and mineralisation using S, Sr and Pb isotopes

The supergiant Pb-Zn-Ag Broken Hill orebody and numerous other minor mineral deposits occur within the limited outcrop of the Proterozoic Curnamona Province of Australia. The vast majority of this Province is concealed by up to 200 m of transported regolith, hampering conventional exploration strategies. Approximately 300 groundwater samples were collected over the southern Curnamona Province to test whether this medium could be helpful in the search for hidden mineral deposits. Sulphur, Sr and Pb isotope composition of the groundwaters were determined and S excess (S_XS), i.e., the amount of S that can be ascribed neither to evaporation nor to mixing, was calculated. Many samples were recognised to have undergone an addition of ³⁴S-depleted S, which can be attributed to oxidation of sulfides with a Broken Hill type δ³⁴S signature (average ∼0‰ V-CDT). Furthermore, Sr isotopes identify the broad types of bedrock that the groundwater has been interacting with, from the less radiogenic Adelaidean rocks (and minerals) in the west (groundwater ⁸⁷Sr/⁸⁶Sr ratio as low as 0.708) to the highly radiogenic Willyama Supergroup in the east (⁸⁷Sr/ ⁸⁶Sr ratio up to 0.737). The groundwaters have ²⁰⁷Pb/ ²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios comparable to, or intermediate between, various mineralisation types recognised in the area (Broken Hill, Rupee, Thackaringa, etc., types). The few samples taken in the vicinity of known mineralisation yield positive indicators (positive S _XS, low δ³⁴S, ⁸⁷Sr/⁸⁶Sr signature of bedrock type and Pb isotope fingerprinting of mineralisation type). This study also highlights several new locations under sedimentary cover where these indicators suggest interaction with mineralisation.