Sulfur isotope distribution in Late Silurian volcanic‐hosted massive sulfide deposits of the Hill End Trough, eastern Lachlan Fold Belt, New South Wales

Abstract

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 Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough.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 δ34S values range from −7.4‰ to 38.3‰, 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 δ34S 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 δ34S 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 34S‐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.

Sulfur isotope characteristics of the Permian VHMS deposits in Tasik Chini district, Central Belt of Peninsular Malaysia

“…In comparison, the significantly narrow ranges of sulfides with a cluster toward positive δ 34 S values in both deposits are similar to those of several ancient VHMS deposits, including the Osborne Lake deposit in the Snow Lake area, Canada (-1.1‰ to +6.0‰; Sangameshwar, 1972), the El Cobre deposit, Cuba (-1.4‰ to +7.3‰; Cazañas et al, 2003), the Mount Morgan deposit, Australia (-1.6‰ to +5.3‰; Ulrich et al, 2002), the Lewis Ponds, Mount Bulga, Belara and Accost deposits in the Lachlan Fold Belt, New South Wales (range of -1.7‰ to +5.9‰; Downes and Seccombe, 2004). However, the abundance of significant low δ 34 S values in sulfides at the Bukit Ketaya deposit is also probably comparable with a δ 34 S signature exhibited by the Mount Lyell deposits, Tasmania (-10‰ to +10‰; Huston et al, 2011).…”

“…Sangster (1968) was the first researcher to recognize that the trend of δ 34 S variation in Proterozoic and Phanerozoic VHMS deposits closely parallels the ancient seawater curve, but is offset to lighter δ 34 S values by about 18‰ or ~16‰ (Huston, 1999;Huston et al, 2010). Subsequent stud ies have confirmed the general trend that seawater sulfate provides a source of reduced sulfur for many VHMS deposits (e.g., Large, 1992;Downes and Seccombe, 2004;Scotney et al, 2005;Inverno et al, 2008). More recent works on modern seafloor hydrothermal sulfide systems also indicate a consistent role of reduced sulfur in addition to seawater δ 34 S source (e.g., Shanks, 2001;Rouxel et al, 2004).…”

Pb‐isotope compositions of the Tasik Chini volcanic‐hosted massive sulfide deposit, Central Belt of Peninsular Malaysia: Implication for source region and tectonic setting

“…Lead isotope signatures can be used to define the source of Pb (Doe & Zartman, ), and to fingerprint mineral systems with model age information for an individual deposit (Downes & Seccombe, ). Therefore, the application of lead isotope analysis can provide important data to be used in constraining the process of ore formation (Harkins, Appold, Nelson, Brewer, & Groves, ; Zartman & Smith, ; Bozkaya, ; Huston, Pehrson, Eglington, & Zaw, ; Huston, Relvas, Gemmell, & Drieberg, ).…”

“…Lead isotope signatures can be used to define the source of Pb (Doe & Zartman, 1979), and to fingerprint mineral systems with model age information for an individual deposit (Downes & Seccombe, 2004).…”

Geochemistry and geotechnical models – a case study from the proposed Kempfield Silver Project, Bathurst, New South Wales

“…This province is characterised by thick turbidite successions and mafic to felsic lower Ordovician to early Silurian volcanic rocks and considered to be a convergent terrane (Foster et al, 1999). Prolonged crustal extension during the mid-Silurian to Middle Devonian (Collins, 2002) resulted in development of back-arc rift basins including the Hill End Trough with deposition of quartz-rich turbidites, felsic and mafic volcanic rocks, volcaniclastic rocks and black shales, later subjected to greenschist facies metamorphism during the middle to late Devonian (Downes and Seccombe, 2004). VHMS mineralisation occurred in the late Silurian concentrated within the Campbell's (which includes the units observed at Kempfield), Mumbil, Chesleigh and the Tanabutta Groups (Downes and Seccombe, 2004).…”