准噶尔盆地西北缘石炭—二叠系火山岩裂缝中方解石脉成因

Genesis of calcite veins in Carboniferous-Permian volcanic fractures, northwestern Junggar Basin

准噶尔盆地西北缘石炭—二叠系火山岩裂缝十分发育，普遍充填方解石矿物。通过对钻井岩心裂缝类型划分、方解石脉原生流体包裹体均一温度测试、方解石地球化学数据测试与分析，揭示了方解石脉成因。结果表明，石炭—二叠系火山岩中的裂缝主要包括风化裂缝、构造裂缝、成岩裂缝和溶蚀裂缝。裂缝中方解石的原生流体包裹体均一温度分布为40℃～150℃，变化范围较大，具有低温、高温及混合流体特征。方解石的碳、氧同位素表明，方解石的形成与大气淡水和埋藏流体有关，锶同位素和锰元素进一步证实，埋藏流体为深部热液流体或埋藏流体溶蚀了火山物质。准噶尔盆地西北缘石炭—二叠系火山岩受逆冲推覆构造和风化淋滤作用双重影响，构造裂缝和风化裂缝彼此连通并沟通了深部热液与大气淡水。在火山岩裂缝中形成了兼具大气淡水与深部热液流体特征的方解石充填物。

Fractures are well-developed in Carboniferous-Permian volcanic rocks, northwestern Junggar Basin, which are almost filled with calcite. The origin of calcites is revealed by researching on the types of fractures, analyzing the homogenization temperature of primary fluid inclusions and calcite geochemistry data. The result shows that there are four types of fractures in volcanic rocks, weathered fractures, structural fractures, diagenetic fractures and dissolved fractures. Primary fluid inclusions of calcite show a wide range of homogenization temperatures from 40 ~C to 150 ~C, which means that the calcite is related to either hypothermal or epithermal or both. The characteristic of carbon and oxygen isotope composition indicates that the fluids come from meteoric water and buried fluid. The Sr isotope and Mn element confirms that buried fluids are deep-soured hot fluids or fluids with some dissolved volcanic rocks. Affected by the thrust faults of northwestern Junggar Basin and weathering leaching, a large number structural fractures and weathered fissures are formed and connected with each other. The calcite veins in Carboniferous-Permian volcanic rocks are formed with characteristics of meteoric waters and deep-source hot fluids.