The Formation Of The Hydrothermal Sulfide Near 13°N On The East Pacific Rise

The hydrothermal field near 13oN on the East Pacific Rise is located at a segment where the opening rate at this is 12cm/yr, i.e., a tectonic environment of fast-spreading center. The average water depth of the axial graben floor is 2630m. Detailed studies were conducted on the hydrothermal sulfide sampled from this area (103°54.41′W, 12°42.68′N). Microstructure features of the hydrothermal sulfide were observed in the polarizing microscope and mineral assemblages were also exhibited. The hydrothermal sulfide near 13oN on the East Pacific Rise is mainly composed of Fe-sulfide and Zn-sulfide minerals, and has experienced the transformation from Fe-sulfides predominance at an early stage to Zn-sulfides predominance at a late stage. According to observed microstructures, the formation of the hydrothermal sulfide near 13oN on the East Pacific Rise can be divided into four stages, namely, pyrite stage, chalcopyrite stage, pyrite-sphalerite stage and sphalerite stage.Test procedures of major and trace elements in hydrothermal sulfide samples were established, and major and trace element compositions of the hydrothermal sulfide near 13oN on the East Pacific Rise were determined on this basis. Characteristics of compositions of the hydrothermal sulfide near 13oN on the East Pacific Rise were contrasted with contrasted with chimney sulfides from Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge and North Fiji back-arc basin. According to relative percentages of Cu, Pb, Zn (sum of Cu, Pb, Zn contents is assumed as 100%), the hydrothermal sulfide near 13oN on the East Pacific Rise can be classified as Zn-rich type. Space variations of trace element compositions and evolution of elemental ratios in samples were illustrated. Correlations between major elements and trace elements, and between element ratios (the denominator is Cu) and 1/Cu ratios of the hydrothermal sulfide near 13oN on the East Pacific Rise were both discussed. Trace elements such as Cd, Ga and In mainly exist in Zn-sulfides in the form of isomorphis m. Moreover, Zn-sulfides are the main host minerals of Co. During the formation of the hydrothermal sulfide near 13oN on the East Pacific Rise, Cu and Mo both tended to precipitated from fluid phases at high temperatures(>300℃). Cu contents, ratios of Cu/Pb and Mo/Pb of sulfide samples can play a significant role as diagnostic indicator of the temperature variations of hydrothermal fluids which formed the sulfide. During the evolution (from high temperatures to low temperatures) of hydrothermal fluids which formed the sulfide, compositions of the hydrothermal fluids evolved at the same time, especially trace metals such as Sr, Ba and Au were enriched exponentially. Factor analyses were also performed on obtained samples, and obtained factors through rotation may distinguish the influences of temperatures and geochemical composition of hydrothermal fluids which formed the sulfide.Pourbaix diagrams of Fe-S-H2O hydrothermal system near 13oN on the East Pacific Rise were drawn by use of thermodynamic calculations, which illustrated the changes of predominance field of precipitation of the pyrite from hydrothermal systems in practical situations during the evolution (from high temperatures to low temperatures) of hydrothermal fluids which formed the sulfide. On the basis of combination with existing research on dynamic experiments and sulfur isotope fractionation, potential chemical reaction pathways which formed the predominant mineral (i.e., pyrite) during marine hydrothermal activities which precipitated sulfides were brought out, and the influences of temperatures of hydrothermal fluids which formed the sulfide on thermodynamic mechanism of the formation of pyrite were probed into.