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 (SXS), 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 SXS, 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.