Abstract:
BACKGROUNDThe mineralogy, chemistry and crystal structure of hydrothermal clay minerals from various geologic settings have been studied to reflect the fluid-rock interaction and physico-chemical evolution of fluids in hydrothermal environments. However, clay minerals in the ultraslow-spreading Southwest Indian Ridge have received less attention.
OBJECTIVESTo obtain a better understanding of the features of clay minerals and to constrain hydrothermal alteration processes in the SWIR.
METHODSA fragment of massive sulfide ore with irregular-shaped breccia was collected from the Longqi hydrothermal field and studied using SEM-EDS, XRD, FT-IR, EPMA and LA-ICP-MS to determine its mineralogical and geochemical characteristics.
RESULTSThe breccia sample contains disseminated micro-sized TiO2 and is mainly composed of Al-rich dioctahedral smectite (i.e., beidellite) and amorphous opal. The total content of REE (2.43 to 43.45 μg/g) in such Al-rich, Mg-poor and Fe-poor smectite is commonly low and the REE fractionation patterns exhibit no significant Ce anomaly (1.09-1.16) but yield negative δEu values (0.31-0.53). It has been suggested that continuous ore-forming fluids (typical of low-temperature, acidicity and relatively reduced solutions) might be responsible for extensive leaching and remobilization of all elements except Al and Ti, thus promoting the formation of Al-rich smectite at the periphery of the Longqi hydrothermal system.
CONCLUSIONSThe altered clay mineral and its geochemical characteristics have been studied, reflecting the pervasive development of low-temperature hydrothermal alteration at the Longqi hydrothermal field. This study provides a basis for further discussion of the fluid-rock interaction in the ultraslow-spreading SWIR.
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