Gold enrichment in hydrothermal sulfides from the Okinawa Trough: An in situ LA-ICP-MS study

Abstract

We assess the distribution of gold and investigate the physicochemical controls on gold enrichment in the Okinawa Trough, an immature back-arc basin. We conducted a systematic study of gold and other trace-element concentrations using in situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) for various minerals in hydrothermal sulfide samples from the Iheya North Knoll and the Yonaguni Knoll IV hydrothermal fields. Orpiment, colloform pyrite, and colloform sphalerite are relatively enriched in Au (mean contents of 30.60, 17.48, and 8.68 ppm, respectively) compared with gold-poor massive chalcopyrite, sphalerite, and pyrite. Based on gold budget estimation, colloform pyrite is the most important Au host, followed by colloform sphalerite and orpiment, with these three minerals containing the majority of gold in hydrothermal sulfides. Gold occurs as “invisible” in host minerals: as lattice-bound and submicron inclusions in colloform pyrite, as homogeneously distributed lattice-bound or nano-inclusions in colloform sphalerite, and generally as lattice-bound in orpiment and tetrahedrite. Three sets of gold host minerals are identified with distinct ore-formation temperatures. Colloform sulfides and orpiment incorporate the majority of gold at low temperatures, whereas tetrahedrite and massive sulfides incorporate only minor or trace amounts of gold at medium and high temperatures, respectively. Our results show that gold enrichment in Si–Ba–Fe-rich hydrothermal sulfides at low temperatures is a significant contributor to gold enrichment in hydrothermal sulfide deposits in the Okinawa Trough, along with the more common Zn- and Cu-rich hydrothermal sulfides. Thermodynamic modeling calculations suggest that the near-neutral pH of hydrothermal fluids and the dominant transport species, namely Au–HS complexes rather than Au–Cl complexes, are likely responsible for gold enrichment in low-temperature mineral assemblages in hydrothermal precipitates from the Okinawa Trough.