Geology and mineralogy of massive sulfide ores from the central Okinawa trough, Japan

Massive sulfide ores were collected from active and inactive vents and smokers and from mounds of the hydrothermal Jade field in the central Okinawa trough (Japan), which is a tectonically active intracontinental back-arc basin located in the northwestern Pacific. The Jade field occurs at a site where the tip of a marine graben segment is thought to be influenced by an obliquely striking shear zone. Small faults in the resulting Izena basin present the paths for circulating hydrothermal solutions discharged at the Jade field. The ores forming from these fluids can be chemically characterized by their respective major components as follows: (1) Zn-Pb rich, (2) Ba-Zn-Pb rich, (3) Zn-Cu rich, (4) Fe-rich replacements, and (5) Zn-Pb-rich impregnations of felsic volcanics. Ore types 1, 2, and 4 originate from chimneys, whereas 3 was collected as large slabs from mounds. Some ore samples show high concentrations of Au (up to 24 ppm) and Ag (up to 1.1%). Mineralogically and texturally, the ores show distinctive differences, even if the overall chemistry is comparable (such as that for 1 and 5). Strong similarities, both geochemically and mineralogically, are found between these ore samples and kuroko-type massive sulfide ores. However, the formation of the Jade deposit is as yet incomplete and at an immature stage. This presents the unique opportunity to examine a modern analogue of an ancient deposit.