Fluid inclusion and sulphur isotope evidence for syntectonic mineralisation at the Elura mine, southeastern Australia

Abstract

Abstract. Fluid inclusion and sulphur isotope data for the discordant, metasediment-hosted massive sulphide deposit at Elura are consistent with a syntectonic origin of the orebodies. Thermometric and laser Raman microprobe analyses indicate that two-phase, primary fluid inclusions are low salinity and HzO-CO2-CH 4 types. Inclusion fluids from quartz in ore yield homogenisation temperatures (T0 ranging from 298 ~ to 354~ (mean 320 ~ They are likely to have been trapped close to the solvus of the H20-COz-(CH4-NaC1 ) system and thus should give temperatures of the mineralising fluid. An additional, low T h population of later fluid inclusions is recognised in quartz from ore and syntectonic extension veins in the adjacent wallrock. Th'S for these low CO2-bearing inclusions range from 150 to 231~ (mean 190 c~C), and should be considerably lower than true trapping temperatures. Sulphur isotopic composition (~34S) of pyrite, sphalerite, pyrrhotite and galena ranges from 4.7 to 12.6%o and indicates a sulphur source from underlying Cobar Supergroup metasediments. An average temperature of 275 ~ from the sphalerite-galena sulphur isotopic thermometer suggests isotopic re-equilibration below peak metamorphic temperatures.

Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits

“…We cannot exclude intra-basinal or local basement sources for base metals for this deposit, but a metamorphic source should also be considered. Other examples may include deposits that are structurally controlled and related to deformation/ metamorphism such as the Elura/Endeavor, deposit in the Cobar district in southeastern Australia where metals are thought to be sourced from deep metasediments (Seccombe 1990;David 2008) or (Cu)-Pb-Zn mineralization within the Willyama Supergroup, Olary Block, South Australia, where Pb isotope signatures point towards a deep crustal metal source (Bierlein et al 1996a, b). Early Pb-Zn mineralization in the Coeur d'Alene district, Idaho and Montana, was also proposed to be associated with metamorphism (Leach et al 1998) and syn-metamorphic exhumation of crustal rocks in British Colombia may have contributed metamorphic-derived base metals to extension-related vein systems (Beaudoin et al 1992a, b).…”