Constraints on fluid evolution and growth processes of black smoker chimneys by pyrite geochemistry: A case study of the Tongguan hydrothermal field, South Mid-Atlantic Ridge

Abstract

Up to now, the Tongguan hydrothermal field is the southernmost-distributed modern submarine massive sulfide discovered on the South Mid-Atlantic Ridge. A well-preserved black smoker chimney which contains three distinct concentric zones was collected from the active vent field. The external outer wall of the chimney contains mainly pyrite with minor sphalerite, chalcopyrite and galena. The sulfides within the internal middle layer consist of predominant pyrite, with lesser amounts of marcasite. The axial conduit is dominated by chalcopyrite, with minor pyrite. To clarify the ore-forming fluid evolution during the black smoker chimney growth, we performed detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) studies on pyrite grains from the chimney. Seven types of pyrite: framboidal, relict, colloform, dendritic, massive, euhedral and lined pyrites, are identified from exterior to interior of the chimney.As shown by the results of LA-ICPMS, geochemical compositions of pyrite vary across the chimney. Generally, low-temperature elements (Mg, V, Mn, Pb, Zn, Ag and Tl) are enriched in pyrite in the outer wall of the chimney and decrease toward the interior zones. Elements indicative of high temperature conditions, such as Se and Te, show opposite trends and are enriched in pyrite within the inner zones of the chimney. Framboidal, relict, colloform and dendritic pyrites precipitate from low to mid temperature fluids (<250 °C), while massive and euhedral pyrites are products of ore-forming fluids characterized by mid to high temperature (250 to > 335 °C). Relict and lined pyrites have the same chemical signatures, suggesting that they precipitated under similar physicochemical conditions. The observed patterns are resulted from the changes in temperature and fluid chemistry caused by interactions of between seawater and hydrothermal fluids with each other in the chimney.Cobalt concentrations in pyrite from the modern black smoker chimneys located on the mid-ocean ridge apparently vary systematically in response to salinity change of vent fluids. The low Co contents (average 0.32 ppm) in pyrite suggest that the hydrothermal fluids vented from the black smoker chimney are Cl-depleted compared to seawater, most likely caused by phase separation. The systematic variations in geochemistry and morphology of the pyrite across the chimney reflect multiple stages of mineralization. Different degrees of fluid-seawater mixing and phase separation are the main controlling factors during the chimney formation.