Evidence for cross formational hot brine flow from integrated 87Sr/86Sr, REE and fluid inclusions of the Ordovician veins in Central Tarim, China

Rare earth elements, δ34S, 87Sr/86Sr and δ18O 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 87Sr-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 87Sr/86Sr 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−, Eu2+, Ba2+, Sr2+ and Zn2+ 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 SO4 reduction (TSR) of dissolved sulphates by hydrocarbons. It was likely that only part of the image was reduced and the generated H2S leaked out, thus barite deposited from residual image has abnormally high δ34S values from 42‰ to 47‰ and the generated H2S is isotopically light from 15‰ to 18‰. Subsequently, hydrothermal fluid was likely to have mixed with relatively cool and more 87Sr-rich freshwater, resulting in precipitation of quartz and fluorite with salinities as low as 0.5 wt% NaCl equiv. and 87Sr/86Sr ratios up to 0.71036.