Mineralogy and trace element geochemistry of hydrothermal sulfides from the Ari vent field, Central Indian Ridge

Abstract

The Ari vent field (AVF) is an ultramafic-hosted seafloor massive sulfide (SMS) deposit in the middle part of the Central Indian Ridge. In this paper, we describe the detailed mineralogy and geochemistry of hydrothermal sulfide samples from the AVF, which can be classified into Fe–Cu- and Cu-rich types based on the major sulfide minerals. Sulfide mineralisation of the former type comprises: (1) stage I, early deposition of magnetite, pyrrhotite, isocubanite, chalcopyrite, and subhedral–euhedral pyrite under high-temperature fluid conditions (> 335 °C); (2) stage II, deposition of colloform pyrite, sphalerite, galena, and electrum from low-temperature fluids (< 200 °C) during the later mineralisation stage; and (3) stage III, seawater alteration that caused the precipitation of uraninite and chalcocite. This indicates that the fluids in the AVF had decreasing temperature and ƒS2 and increasing ƒO2 as mineralisation proceeded. The Cu-rich sulfide samples have mineral assemblages and a paragenesis similar to those of the Fe–Cu-rich sulfide samples, but the higher proportion of isocubanite is indicative of relatively high-temperatures and reducing conditions during mineralisation. Bulk chemical compositions of the AVF sulfides are characterised by high U contents (up to 51.9 ppm) and a distinct Sn distribution (2.1–86.4 ppm) between the two different types of hydrothermal samples, which differ from those of other ultramafic-hosted sulfide deposits. The U content is controlled mainly by the precipitation of discrete uraninite grains (< 1 μm in size) on altered surfaces of pyrite and hematite. The oxidative alteration of Fe-bearing minerals caused the fixation of seawater-derived U. Laser ablation–inductively coupled plasma–mass spectrometry analysis showed that most trace elements occur in solid solution in the sulfide minerals, mainly controlled by the physicochemical conditions of the hydrothermal fluids (e.g. temperature, ƒS2, and ƒO2). In particular, a comparative analysis of other mid-ocean ridge systems shows that the ultramafic-hosted sphalerite and pyrite are more enriched in Sn as compared with those hosted by basaltic rocks. However, the Fe–Cu-rich sulfide samples of the AVF are Sn-poor (< 10.2 ppm), because pyrite is substantially depleted in Sn (mostly < 1 ppm) as compared with sphalerite, regardless of the effect of the ultramafic-hosted mineralisation. This indicates that in situ trace element analysis of sphalerite and pyrite, especially for Sn, can provide insights into the different hydrothermal mineralisation in basaltic- and ultramafic-hosted systems, which cannot necessarily be inferred from bulk analysis. Our comparison also suggests that the Sn contents of ultramafic-hosted SMS deposits would be a possible source of Sn for the ultramafic-hosted volcanogenic massive sulfide (UM-VMS) deposit. The δ34S values (+ 6.2 to + 8.5‰) of the pyrite record thermochemical sulfate reduction of seawater, which suggests that sulfur and most metals were predominantly leached from the associated host rocks with a contribution (29–40%) from reduced seawater sulfur. In conclusion, the AVF is a rock-dominated system that contains ultramafic-hosted mineralisation in the Central Indian Ridge.