Abstract
A variable magmatic volatile influx into subduction-related seafloor hydrothermal systems is hypothesised to explain variations in trace element compositions (de Ronde et al. 2005; Herzig, Hannington, et al. 1998). Sulphur isotopes, Se/S ratios (Layton-Matthews et al. 2013), and trace element signatures serve as a proxy for magmatic influx within these systems. In seafloor hydrothermal systems sulphur may be derived from multiple sources: microbial reduction, leaching from igneous rock, a magmatic volatile phase or thermochemical reduction of seawater (TSR) (McDermott et al. 2015). In active seafloor systems, δ34S values in pyrite vary from −4.9 ‰ at Hine Hina (Herzig, Hannington, et al. 1998) in the Lau back-arc basin to + 6.4 ‰ at TAG on the Mid Atlantic Ridge (Herzig, Humphris, et al. 1998). This demonstrates the presence of variable sources of sulphur in modern Seafloor Massive Sulphides (SMS).
The Troodos ophiolite is the principal on-land locality for the investigation of mafic or Cu-rich VMS deposits. This study presents the first comprehensive δ34S (n = 196) data for VMS ores spanning the entire ophiolite. Sulphur isotope data of pyrite separates range from −3.4 ‰ (n = 6) at Mala to+ 7.2 ‰ (n = 19) at Kalavasos. Within a single deposit, δ34S ranges from −1.6‰ in deep stockwork (Keith et al. 2016) to+ 6.4 ‰ (n = 38) in massive ore reflecting decreased seawater interaction with stratigraphic depth. However, δ34S variations between VMS deposits suggest a variable source for sulphur.
The Mala VMS deposit is located in the SW of the Troodos ophiolite and consists of a massive pyritic lens underlain by a Cu-rich stockwork zone, with massive bedded gypsum bands present within the pyrite mound. In addition to containing the lightest average δ34S composition in pyrite compared to all other sites analysed to date, Mala also shows the highest Se (>9500 ppm) and elevated Au concentrations in pyrite, averaging 1.65 ppm Au (n = 58) compared to 0.81 ppm Au (n = 1481) within all other Troodos VMS deposits. Conversely, the Kalavasos group deposits exhibit the highest average δ34S in pyrite, reflecting increased seawater TSR as a result of proximity to a seafloor transform fault zone.
We interpret the Mala deposit to be a magmatic end-member VMS deposit. Similar light δ34S values are recorded in active SMS systems at Hine Hina and Brothers volcano in the SW Pacific (de Ronde et al. 2005; Herzig, Hannington, et al. 1998). Modern analogues have experienced an increased magmatic volatile influx due to the reduction of magma derived SO2 to H2S (Herzig, Hannington, et al. 1998). This suggests (a) an immature source at Mala VMS, (b) magmatic influx leads to the enrichment of Au, Se and Te and, (c) TAG may not be a comparable analogue to Troodos VMS deposits.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.