Sulfur isotope distribution in Late Silurian volcanic-hosted massive sulfide deposits of the Hill End Trough, eastern Lachlan fold belt, New South Wales
Downes Peter, M.; Seccombe, P.K.
Australian Journal of Earth Sciences 51(1): 123-139
Volcanic-hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat-Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured delta (super 34) S values range from -7.4ppm to 38.3ppm, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from -1.7% to 5.9%. Ore-forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average delta (super 34) S values ranging from 5.4% to 8.1%. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The delta (super 34) S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9-14.5%, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6% to 38.3%. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6%, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by (super 34) S-enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature.