FLUID SOURCES AND FLUID-WALLROCK INTERACTION IN REGIONAL ALTERATION AND IRON OXIDE-CU-AU MINERALISATION, CLONCURRY DISTRICT, AUSTRALIA

A database of 656 d¹⁸O and d¹³C ratios from Cloncurry District ore assemblages, host rocks and alteration systems is presented and employed to clarify metasomatic fluid sources and the role of fluid-wallrock interaction in this important metallogenic district. Isotopic ratios for calcite from the cores of Na-(Ca) alteration systems strongly cluster around 11‰ d¹⁸O and -7‰ d¹³C, with shifts towards higher d¹⁸O values and higher and lower d¹³C values, reflecting interaction with different host rocks. Marine metacarbonates (ca. 20.5‰ d¹⁸O and 0.5‰ d¹³C, calcite) and graphitic metasedimentary rocks (ca. 14‰ d¹⁸O and –18‰ d¹³C, calcite) are the only significant carbon reservoirs in the district, and neither of these are in isotopic equilibrium with the cores of Na-(Ca) alteration systems. Crystalline plutonic rocks in the Cloncurry District contain negligible carbon, and as such are incapable of homogenizing the d¹³C signature of CO₂-rich metasomatic fluids of other postulated sources. In contrast, crystallizing plutons have been documented to exsolve CO₂-NaCl-rich fluids, and represent a likely source of fluid for Na-(Ca) alteration in the Cloncurry District. Carbonates from Cu-Au ore assemblages record a wide range in d¹⁸O and d¹³C values, reflecting complex fluid flow paths, and precluding the use of these isotopic ratios as vectors towards mineralisation. For intrusion proximal, skarn-like ore bodies that lack significant K- and Fe- enrichment (e.g. Mt Elliott), isotopic ratios cluster around values of 11‰ d¹⁸O and –7‰ d¹³C, indicating an isotopically similar fluid source as for Na-(Ca) alteration, and indicating that significant fluid-wallrock interaction was not required in the genesis of these deposits. In contrast, intrusion-distal deposits (e.g. Ernest Henry), record K- and Fe-rich ore assemblages, and significant shifts in d¹⁸O and d¹³C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid-wallrock equilibration prior to mineralisation. Low temperature, low salinity, low d¹⁸O (<10‰), CO₂-poor fluids have been documented in retrograde metasomatic assemblages, but these fluids are paragenetically late, and have not contributed significantly to the mass budgets of Cu-Au mineralisation.