Lead isotope study on hydrothermal sulfide mineralisation in the Willyama Supergroup, Olary Block, South Australia

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Abstract
Lead isotope analysis of galena separates and low‐lead sulfides from metamorphosed stratiform to stratabound and epigenetic (i.e. post‐dating peak metamorphism) sulfide occurrences in the Proterozoic Olary Block has been carried out. The results suggest that lead has been extracted from a relatively homogeneous crustal source and that mobilisation occurred intermittently at various times since the Mesoproterozoic. Integrated petrographic, structural and geochemical evidence do not substantiate a mixing model or extraction of lead from an inhomogeneous source during one mineralisation event in the Mesoproterozoic. Multiple‐stage vein‐type deposits were derived by leaching of components from Willyama Supergroup metasediments. The least radiogenic lead from the Olary Block has isotopic characteristics very similar to the lead isotope signature of the giant stratiform Pb‐Zn‐Ag orebody at Broken Hill and formed at ca 1700 Ma. Unlike the extremely homogeneous signature of the Broken Hill orebody, Mesoproterozoic sulfides from the Olary Block are characterised by a wider scatter of isotopic compositions, suggesting that diagenetic fluid movement was governed by local processes involving a single‐pass fluid and/or convective cells of rather limited extension. The lead isotopic signature of possible equivalents to Thackaringa‐type vein deposits in the Olary Block point to their emplacement at ca 500 Ma and imply widespread fault‐controlled fluid circulation during the Delamerian Orogeny.

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