Mineralization Of Hydrothermal Sulfide On The Southwest And Central Indian Ridge

To investigate the hydrothermal mineralization along the slow and ultra-slow spreading ridges, two hydrothermal fields on the southwest Indian Ridge (SWIR, 49.6°E,63.5°E) along with one on the Central Indian Ridge (CIR,69.6°E) were selected to be explored. In this study, mineralogical, geochemical, and isotopic measurements are combined to identify the intrinsic characteristic and precipitating process of metal sulfide from these fields.Mineralogical analyses have showed that the sulfides are enriched in Fe sulfide and chalcopyrite, in both hydrothermal fields of SWIR. Most of the chalcopyrite is characterized by the exsolution of isocubanite, which is generally regarded as a typical high temperature mineral assemblage. As for the SWIR49.6°E hydrothermal field, the sulfides are mainly composed of pyrrhotite+pyrite+chalcopyrite+sphalerite and the visible occurrence of natural gold particles. Altogether, the mineralization here can be divided into four stages:1) pyrite+pyrrhotite+chalcopyrite�'2) pyrite +sphalerite�'3) colloform -pyrite�'4) Fe-oxyhydroxide (i.e. goethite/lepidocrocite, and amorphous fe-oxyhydroxide). Roughly, the mineralization temperature shift from high-temperature to medium to low-temperature, and at episodic emissions of hydrothermal fluids in this field. With regards to the SWIR 63.5°E hydrothermal field, the sulfide mineral assemblage is characterized by marcasite+isocubanite, no sphalerite has been observed. Therefore, the mineral mineralization sequence is divided as:1) early marcasite�'2) isocubanite�'3) euhedral marcasite.In contrast, there is certainly evidence to suggest that the CIR samples both contain high-temperature (Fe-rich and Zn-rich) and low-temperature (Ca-rich sulfate) mineral assemblage. Furthermore, we could not identify isocubanite in these samples, indicative of a lower precipitation temperature than the samples on the SWIR. Accordingly, we divide the mineralization sequence into the following three stages: anhydrite+pyrite �' 2) pyrite+sphalerite+chalcopyrite �' 3) colloform-pyrite+ covellite+barite. Several lines of evidence, for instance, the tube-like symmetrical bodieson the exterior of Zn-rich sulfide, a small amount of anglesite and the occurrence of natural sulfur, suggest that the sulfide also have influenced by pervasive low-temperature diffuse flow in this hydrothermal field.As seen for the geochemical results, the sulfides on the SWIR 49.6°E and 63.5°E are both enriched in Fe and several trace elements, such as Co、Mn、Ag and Au, whereas devoid of Cu and Zn; In contrast, the sulfides on the CIR are significantly rich in Zn and Ca along with trace elements, like Pb、Ba、Sr、As、Au and Ag. However, Fe is relatively depleted in these samples. This points to a paradigm chemical resemble of hydrothermal sulfide recovered from these dimented ridge and/or back-arc basins, where the deep fluid escaping from magma at depth fortuitously invade the hydrothermal plumbing system. The sulfide samples on the SWIR and CIR are both characterized by a significant enrichment of LREE relative to HREE and a positive Eu anomaly, indicating a legacy from the original hydrothermal fluids. Somewhat differently, the Eu anomaly of the samples on the SWIR 49.6°E is relatively faint.In addition, the fractionation of stable S and Pb isotope reveals that the metals in the sulfides on the SWIR49.6°E were sourced mainly from the substrate rock, whereas the δ34S value reflects the characteristic of deep fluid to some extent. Compared to the no sedimented ridge, the sulfide on the SWIR 63.5°E and CIR is characterized by relatively lower radioactivity Pb, albeit still within the scope of the Indian MORB. The S isotopic composition of these samples apparently exceeds the range of that from no sedimented ridge. Regarding the sulfide on the SWIR 63.5°E, the much higher δ34S is presumed to be related to the alteration of residual continental crust, while on the CIR, the higher 834S value is attributed to the shallow hydrothermal fluid circulation beneath the seafloor.