Mark Kendrick, The University of Melbourne (Australia)
Nick H.S. Oliver, James Cook University (Australia)
Masahiko Honda, ANU (Australia)
David Phillips, The University of Melbourne (Australia)
Geordie Mark, Shaw (Canada)
Dave Gillen, James Cook University (Australia)
Combined noble gas/halogen analyses have been used as a new way to investigate fluid origins in contrasting IOCG deposits. This is advantageous because the isotopic composition of He, Ne and Ar varies by orders of magnitude between the atmosphere, crust and mantle; and they are volatile elements that are not easily reset during wall-rock alteration. The halogens (Cl, Br and I) are strongly fractionated by fluid interaction with halite and meta-evaporitic scapolite might have a similar effect.
Two contrasting (1.6-1.5 Ga) IOCG terrane have been investigated. 1) Most IOCG deposits in the Cloncurry District of the Mt Isa Inlier, Australia, are temporally associated with intrusion of regionally extensive Williams-Naraku Batholiths. In contrast, syn-mineralisation granites are unknown in the Wernecke Mountains of Canada.
Both districts yielded similar Ar and halogen results but some key differences are apparent for He and Ne data. High salinity aqueous fluid inclusions have 40Ar/36Ar values of between <1000 and ∼40,000 in the Ernest Henry deposit, some occurrences of barren albitisation in the Cloncurry District, and in the Wernecke prospects. In contrast Mt Isa's Osborne deposit, and most albitisation, is characterised by fluid inclusions with uniformly low 40Ar/36Ar values of <2500. The lowest 40Ar/36Ar values are similar to sedimentary formation waters (or locally-derived metamorphic fluids), consistent with Osborne's pre-intrusive timing, whereas the highest values are most easily explained as a deeply-derived magmatic component, consistent with Ernest Henry's syn-magmatic timing. CO2 fluid inclusions are present throughout the Mt Isa District and dominate in some albitisation samples from the Mary Kathleen Fold Belt. They have a maximum 20Ne/22Ne value of 10.1 (air = 9.8) and 40Ar/36Ar of 14,000, compatible with a mantle CO2 component of 85%. However, further Ne isotope analyses are required to constrain mantle involvement elsewhere in the Cloncurry District. Calc-silicate rocks are the most likely CO2 source at Osborne (40Ar/36Ar 2200). Helium plus Ne isotopic data do not favour mantle involvement in the CO2-poor Wernecke prospects. Fluid inclusions in all of the deposits studied in the Mt Isa Inlier (Ernest Henry, Osborne, Eloise, Starra) and the Wernecke prospects (Igor, Olympic, Slab, Slats, Hoover) have heterogenous halogen signatures including low Br/Cl and I/Cl values that are most easily explained by fluid interaction with halite. Preliminary analyses of Mt Isa scapolite indicate that it could only have been an important source of Cl, if its Br/Cl partition coefficient is >1.
The Mt Isa and Wernecke data suggest mineralisation is related to advection of diverse fluids that enhanced their salinity by dissolution of (meta)evaporites. We propose fluid advection is driven by magmatism in both districts, but that the magmatic component is not essential for mineralisation. Fluid interaction with wall-rocks is more important.