[S-MP23] Physics and Chemistry of Minerals

[SMP23-03] Sulfides mineralization and argentiferous sulfosalts precipitation of seafloor sulfide deposits in the Mienhua submarine volcano hydrothermal field off NW Taiwan

*Ta-Wei Hsu¹, Wei-Teh Jiang², Chih-Chieh Su¹, Ko-Chun Huang², Yu-Shih Lin³, Shein-Fu Wu², Feng-Hsin Hsu¹, Yunshuen Wang⁴, Song-Chung Chen⁴, Sheng-Yuan Chen⁴ (1.Nat. Taiwan Univ., 2.Nat. Cheng Kung Univ., 3.Nat. Sun Yat-sen Univ., 4.Geological Survey & Mining Management Agency)

Keywords:Southern Okinawa Trough, Seafloor hydrothermal mineralization, Sulfide chimney, Argentiferous sulfosalt, Iron-rich sphalerite

Seafloor massive sulfide deposits primarily form in mid-ocean ridges and volcanic arc/back-arc systems. The main components of these deposits can include sulfides, sulfates, or carbonates. The Okinawa Trough is a back-arc basin located along the eastern margin of the Eurasian continent, and dozens of hydrothermal mineralization sites have been discovered. These sites commonly contain polymetallic sulfides rich in zinc, lead, and copper, as well as sulfate minerals. The mineralogical and geochemical characteristics of some of these sites are considered similar to Kuroko-type deposits, which are known for their abundance of precious metals such as silver and gold. Previous studies have indicated that silver may exist in trace amounts in sulfides like chalcopyrite, galena, sphalerite, tetrahedrite, and pyrite. However, Ag-rich minerals are rare, and the relationship between sulfide minerals and Ag-bearing minerals remains unclear. This study discovered multiple active hydrothermal mineralization occurs at several sulfide deposits in the Mienhua submarine volcano hydrothermal field at the southernmost part of the Okinawa Trough. Such features are best delineated by the Fe-Zn-Pb-As and Ba-Fe-Zn sulfide mineralization at the Witch Mound and Devil Chimney. Common iron-dominated sulfide composition and abundant occurrence of Fe, Mn-rich sphalerite and Ag-rich sulfosalts are typical of the Mienhua submarine volcano sulfide deposits, highly distinct from other hydorthermal deposits in the Okinawa Trough. In contrast, the Casper Chimney samples in the studied area display uniform growth textures and consist of anhydrite and monotonous Fe-Zn-Pb sulfides, typical of low sulfidation conditions. Many of the sulfide samples collected from the area show complex sulfide paragenetic sequences characterized by multi-stage or multi-layer growth of pyrrhotite + sphalerite + galena ± isocubanite ± arsenopyrite ± chalcopyrite and sulfosalts + stibnite + realgar ± pyrite-marcasite ± native sulfur, typical of early low sulfidation and late medium- to high-sulfidation mineralization, respectively. The mineralization temperature reads 350±50°C as implied by the isocubanite ZnS content. Despite highly varied mineral contents and textures, the studied sulfide rocks invariably contain a fair amount of Ag and Au, with the highest concentrations detected in the Devil Chimney samples. The silver is enriched in the Ag-rich sulfosalts (most commonly andorite) and Ag-bearing galena and colloform pyrite, and gold is probably associated with the argentiferous sulfosalts and sulfides.