Distribution of trace elements in sulfides from Deyin hydrothermal field, Mid-Atlantic Ridge – Implications for its mineralizing processes

Abstract

Knowledge about the trace element distribution in sulfides is the key for understanding the trace metal inventory during the formation of the volcanic massive sulfide deposits on the seafloor. The distribution of trace metals in sulfides reflects changes in the physicochemical conditions and precipitation processes during precipitation. Hydrothermal black smoker samples of this study originate from the southern Mid-Atlantic (TVG02, TVG06) and can be classified into two types: (1) (Fe-Zn) sulfides consisting predominantly of pyrite and sphalerite, with minor chalcopyrite, isocubanite, and galena and (2) (Fe-Cu) sulfides, which contain mainly pyrite and chalcopyrite, with rare isocubanite and sphalerite. The sulfide samples were analyzed by electron microprobe for major elements, and by laser ablation inductively coupled plasma mass spectrometry, focused ion beam technique and transmission electron microscopy for in situ trace elements. The data reveal complex hydrothermal processes in high- and low- temperature fluids. Colloform and dendritic pyrite from (Fe-Zn) sulfides are enriched in Mn, Tl, As, V, Pb, and Zn, indicating precipitating from a rapid mixture of low temperature fluids (100–250 °C) with seawater. A continuous enrichment of incompatible elements (Pb and Zn) at the interface of growing pyrite with fluid finally leads to the nucleation and precipitation of sphalerite and galena, and results in the enrichment of As, Hg, Pb, Au, Ag, and Cd in sphalerite micro-inclusions which are hosted in colloform pyrite at low temperature. Galena inclusions occur in the pores and the interstices of sphalerite grains and formed by the enrichment of Pb in the hydrothermal fluid at low-temperature (<250 °C). High Cu, As, Ag, Au, and Sb concentrations in colloform sphalerite indicate a medium to low formation temperature (350–200 °C). Copper, Sb, Se, and Sn enrichments in coarse-grained sphalerite indicate medium temperature hydrothermal fluids. Idiomorphic pyrite and chalcopyrite have high Se and Co contents and indicate precipitaion from high-temperature (>300 °C) fluids.