西藏羊八井地热田水热蚀变的温度特征

Temperature Characteristics of Hydrothermal Alteration in Yangbajain Geothermal Field,Tibet

热储温度是地热资源研究和评价的重要内容之一。水热型地热活动形成种类繁多的水热蚀变。水热蚀变是在一定的温度环境下水-岩反应后的蚀变现象,不同温度条件下形成的水热蚀变类型不同。水热蚀变类型及组合与热储温度存在密切关系。前人已开展了水热蚀变与热储温度关系的研究,但是尚未总结热储温度的水热蚀变判别标志。西藏羊八井地热田发育广泛的水热蚀变,其勘查程度很高,成为水热蚀变的温度特征及其与热储温度关系研究的理想对象。笔者在野外实际地质调查基础上,通过显微镜观察、电子探针分析、X射线衍射全岩分析和黏土定量分析,研究了热田的水热蚀变类型和蚀变矿物温度特征,推测了热储温度,初步建立了中高温热储的蚀变判别标志。研究结果显示:羊八井地热田水热蚀变矿物中的五角十二面体黄铁矿的形成温度达200℃以上,强硅化的形成温度高于150℃,伊利石化的形成温度高于120℃,推测羊八井深部热储温度超过200℃。特征蚀变矿物的空间分布特征与实际勘查的热储温度空间分布特征一致。将普遍发育的五角十二面体黄铁矿化、硅化、伊利石化作为深部存在中高温热储的蚀变判别标志。

Thermal reservoir temperature is one of the important contents in the research and evaluation of geothermal resources. Hydrothermal alteration is an alteration phenomenon after water rock reaction in a certain temperature environment. Hydrothermal geothermal activity results in various hydrothermal alterations. Hydrothermal alteration types and combinations are closely related to thermal reservoir temperature. Study on the relationship between hydrothermal alteration and thermal storage temperature have been carried out by many predecessors, but no criteria was reported for the hydrothermal alteration types of different thermal reservoir temperatures. Yangbajain geothermal field in Tibet has been highly explored and wide ranges of hydrothermal alteration have been recognized which provide a good condition to study the temperature characteristics of hydrothermal alteration and its relationship with thermal reservoir temperature. On the basis of field actual geological survey,the hydrothermal alteration types and the temperature characteristics of altered minerals in Yangbajain geothermal field has been studied through microscope observation,electron probe analysis,X-ray diffraction clay whole rock analysis and clay quantitative analysis. The temperature of thermal reservoir was inferred and the alteration discrimination indicators of medium high temperature thermal reservoir were preliminarily established. It was found that the forming temperature of pentagonal dodecahedral pyrite in hydrothermally altered minerals of Yangbajain geothermal field was above 200 ℃, that of strong silicification was above 150 ℃,and illite was above 120 ℃.The deep thermal reservoir temperature of Yangbajain geothermal field was believed above 200 ℃. The spatial distribution characteristics of characteristic alteration are consistent with the spatial distribution characteristics of thermal reservoir temperature in actual exploration. Therefore, the commonly developed pentagonal dodecahedral pyrite,silicification and illite can be regarded as the alteration discrimination indicators for medium and high temperature thermal reservoirs in the deep.