塔里木盆地西北缘奥陶系碳酸盐岩中两种来源热流体的地质与地球化学证据

Geological and geochemical evidences for two sources of hydrothermal fluids found in Ordovician carbonate rocks in northwestern Tarim Basin

塔里木盆地西北缘西克尔-巴楚-柯坪地区处于巴楚-柯坪断隆带活动区,构造活动强烈。该区出露的中下奥陶统鹰山组发育大量的热液蚀变现象,既有溶蚀形成的大小孔洞、疏松灰岩层,也有各类充填的脉体,如萤石矿、硫磺石膏矿等。通过野外露头观察和采样,并对受热流体作用的样品进行了包裹体测温测盐、微量元素和硫同位素分析。结果显示研究区主要有两种流体来源:岩浆热液和地层热液。研究表明:(1)岩浆热液主要影响研究区南西段,形成萤石、重晶石、方铅矿等矿物,流体包裹体盐度低(0.5%～7.5%NaCleqv)、均一化温度相对较高(142～283℃);地层热液作用范围则主要位于研究区北东段,生成白云石、方解石、石英等矿物,流体包裹体盐度高(16%～23.3%NaCleqv)、均一化温度低(70～138℃);而研究区中段同时受到两种来源热液的混合作用,包裹体盐度(7.5%～18.3%NaCleqv)和均一化温度(142～163℃)介于前两者之间;(2)热液硬石膏、硫磺的硫同位素富含32S,δ34S平均为-9.57‰,说明硬石膏是H2S气体与灰岩反应的产物,硫化氢很可能来自有机物的热裂解。

The northwestern margin of Tarim Basin, especially the Sickl-Bachu-Kalpin outcrop area, locates in Bachu-Kalpin faulted uplift region where the tectonic activity is strong. It has developed multiple hydrothermal alteration phenomena in Yingshan Group, Lower Ordovician, including sizes of dissolution vug, porous limestone beds, and veins either filled by fluorite, sulfur, or gypsum mines. In this paper, based on outcrop observations, 118 samples were analyzed, including the fluid inclusion analysis, trace elements analysis and sulfur isotope analysis, respectively. The results show that two types of fluid sources exist in this area： the magnetic hydrothermal fluid and stratigraphic hydrothermal fluid. （ 1 ） Magmatic hydrothermal fluid mainly impact in the southwest section of the study area, with minerals products （fluorite, barite, galena, etc. ）, corresponding to the low salinity fluid inclusions （mainly 0. 5% to 7. 5% NaCleqv） and relatively high homogenization temperatures （142 -283~C ） ; while the stratigraphic hydrothermal fluid focuses on the north eastern part of the study area, with minerals products such as dolomite, calcite and quartz, whose fluid inclusions show higher salinity （ 16% to 23.3% NaCleqv） and lower homogenization temperature （70 - 138~C ） ; at the same time, the middle part of study area is suffered from the mixture of former two sources, with mediate inclusions salinity （7. 5% to 18. 3% NaCleqv） and homogenization temperature （ 142 - 163℃ ）. （2） Hydrothermal anhydrite and mineral sulfur are rich in 32S with an average δ34S value -9. 57‰, indicating that anhydrite is the product of the reaction between H2 S gas and limestone, while the hydrogen sulfide is likely to come from the thermal cracking of organics.