Mineralization Of Polymetallic Sulfides On Mid-Atlantic Ridge(MAR) At26°S

To understand the hydrothermal mineralization along slow spreading ridges, the26°S hydrothermal field at Mid-Atlantic Ridge was selected to be investigated. Mineralogical, geochemical study of base rocks and polymetallic sulfides were carried out to identify the controlling factors, mineralization condition and ore-forming process of this hydrothermal field.The26°S hydrothermal field located in a depression between two volcanic top on the along-axis topographic high near the center of the26°S segment. The26°S segment is characteristed by the topographic high which hundreds of meters higher than the ends of the segment and much of pillow lava occured around the hydrothermal field.This characteristic, more like those at the East Pacific Rise, indicated that the center of the segment has higher magma supply and larger magma chamber than the ends.So the magmatic activity can supply the main heat source for the26°S hydrothermal field.The normal mid-ocean ridge basalt (N-MORB) of base rock also can supply the main materials for hydrothermal mineralization.The detailed studies based on mineralogy found that the26°S field has experienced three periods of hydrothermal mineralization. The ore-forming process roughly is that:the quickly reduced temperature lead to colloform pyrite precipitated from the first stage fluid which was mixed fluid between vent fluid and cold ambient seawater, subsequent the anhedral pyrite and little fine subhedral-euhedral pyrite precipitated when the temperature of mixed fluid decreased to the lowest. Abundant pores and fissures can be found in the aggregate of first stage pyrite. Fine-grained subhedral chalcopyrite and euhedral cubic pyrite precipitated in the fissures and pores of the aggregate of first stage pyrite away from the central part of the "Chimney" structure, and corase-grained euhedral chalcopyrite precipitated in the tube of "Chimney" from the second stage’s high temperature fluid. The amorphous silicon precipitated in the central of the tube when the third stage of low temperature fluid though the tube of builted by the second stage fluid. So it has a characteristic of Fe-rich sulfidesâ†'Cu-rich sulfidesâ†'Siliceous center from the wall to the tube of the "Chimney" structure. The Fe-rich sulifides’chemical composition of Chimney wall is characterized by rich of Fe, S, As, Co, Mo, which is closely linked with the widespread distribution of pyrite. And the Cd, Sr, Sb, Zn, Pb and Si elements are relative depleted, which consistent with the lack of sphalerite/wurtzite and sulfate.In addition, the biological remains found in samples of sulfide, show that the microbes may participate in the sulphide mineralization or has a certain impact on sulfide mineralization in the hydrothermal mineralization process.