Abstract
Rare earth elements, δ 34 S, 87 Sr/ 86 Sr and δ 18 O were determined, and fluid inclusions were analyzed of fracture filling barite, anhydrite, fluorite, calcite and quartz from Upper Ordovician carbonates in Central Tarim, China. The aim was to assess the origin, evolution and flow of fluid in the fractures. There was mixing of relatively hot Ba-rich and 87 Sr-rich fluid with in situ basinal water and fresh water which was relatively cool and more radiogenic. The hotter fluid is hydrothermal as indicated by homogenization temperatures (HTs) about 20 °C higher than the formation, and was most likely derived from Precambrian to Lower Cambrian clastic rocks or/and from the basement through faults. This is because almost all of 87 Sr/ 86 Sr ratios of late stage fracture-fillings (mainly 0.70889–0.71036) and present brines are higher than that of Upper Ordovician limestones. The hydrothermal fluid was enriched in F − , Eu 2+ , Ba 2+ , Sr 2+ and Zn 2+ during migration as indicated by the positive relationships of Eu to Sr, Ba and Zn. When the hydrothermal fluid mixed with local basinal water, it precipitated isotopically-normal S anhydrite, fluorite and calcite with positive Eu anomalies. The precipitation of these Ca-minerals was followed by thermochemical SO 4 reduction (TSR) of dissolved sulphates by hydrocarbons. It was likely that only part of the SO 4 2 - was reduced and the generated H 2 S leaked out, thus barite deposited from residual SO 4 2 - has abnormally high δ 34 S values from 42‰ to 47‰ and the generated H 2 S is isotopically light from 15‰ to 18‰. Subsequently, hydrothermal fluid was likely to have mixed with relatively cool and more 87 Sr-rich freshwater, resulting in precipitation of quartz and fluorite with salinities as low as 0.5 wt% NaCl equiv. and 87 Sr/ 86 Sr ratios up to 0.71036.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call