Systematic variations of trace element and sulfur isotope compositions in pyrite with stratigraphic depth in the Skouriotissa volcanic-hosted massive sulfide deposit, Troodos ophiolite, Cyprus

Abstract

The Troodos ophiolite represents one of the best-preserved fossil analogs of modern oceanic crust and includes numerous volcanic-hosted massive sulfide deposits. The Skouriotissa deposit can be separated into a stockwork ore zone and an overlying massive sulfide lens that is covered by metalliferous sediments representing the former sulfide–seawater interface. Pyrite is the dominant sulfide mineral within these ores. The trace element composition of pyrite varies systematically with stratigraphic depth (down to ~150mbsf) probably reflecting fluid temperature variations and effects of phase separation (Co, Ni, Se, Te, Bi and Cu). Metal remobilization due to hydrothermal zone refining (Zn, Sb and Pb) and fluid-seawater mixing at the seafloor (Mo) represent further important processes controlling the pyrite chemistry. Massive sulfide-hosted sphalerite and euhedral pyrite probably formed from hot fluids (~400°C), while the occurrence of colloform pyrite indicates lower precipitation temperatures (<400°C). Similar δ18O quartz-fluid equilibration temperatures (~400°C) in the stockwork zone suggest that the Skouriotissa fluids did not cool significantly during the final 150m of fluid ascent to the seafloor. The δ34S composition of deep stockwork pyrite (−1.4‰) suggests that an isotopically light magmatic volatile phase (<0‰) was added to the hydrothermal system of Skouriotissa. During further fluid ascent about 38% of Cretaceous seawater (δ34S=18–19‰) was added leading to the precipitation of stockwork pyrite with positive δ34S values (6.1‰). In addition, the chemical and textural similarities between Skouriotissa and modern seafloor vent systems and massive sulfide deposits suggest that the Skouriotissa hydrothermal system has a modern analog.