西藏羊八井地热田水热蚀变

本文对羊八井地热田钻扎ZK-201、ZK-301和ZK-308的岩心进行了较系统的蚀变矿物学、岩石学和流体包体研究。划分出6个蚀变矿物共生组合及蚀变分带,讨论了蚀变过程中岩石化学变化的特点,并推测了蚀变的温度和酸碱度条件。研究表明,热田曾处于极度的活动状态,最高温度达220-240℃,由于冷水的入侵,热田在目前钻探所及范围已冷却了50-70℃。蚀变矿物分布模式表明,目前热田流体主通道位于北部,热田进一步的开发应以寻找北部深部高温流体为主。

HYDROTHERMAL ALTERATION IN THE YANGBAJAIN GEOTHERMAL FIELD, TIBET

Six hydrothermal mineral assemblages were identified in cores and surface samples from the Yangbajing geothermal field, Tibet: the low temperature acid alteration assemblage represented by anlunite + kaolinite + opal or chalcedony; two low temperature neutral fluid alteration assemblages represented by smectite or randomly interstratified smectite(90-70% expandable layers)+kaolinite + chlorite + opal or chalcedony and R₁ ordering smectite/illite(20% expandable layers)+ chlorite + quartz or chalcedony + calcite+ laumontite; and three medium-high temperature neutral fluid alteration assemblages represented by illite or R₃ ordering smectite/illite(15% expandable layers)+chlorite + quartz + anhydrite + calcite, interstratified chlorite-smectite + kaolinite + quartz and epidote + zoisite + chlorite + illite + quartz + anhydrite + calcite.The maximum alteration temperature of 220-240℃ was suggested by hydrothermal minerals and fluid inclusion studies. A cooling down of 50-70℃ has been happened due to the introducing of cold ground water into the reservoir where has been penetrated by drilling as far as todayZonation of these hydrothermal mineral assemblages were inverstigated in the drill core study and surface mapping. There is a difference between the north and south parts of the field. Acid sulfate alteration controlled by fracture systems were developed in the Sulfur Mine, the north part of the field, whereas sinter cemented sand deposit as well as sinters and travertine were scattered in the south part. The acid sulfate alteration zone in the Sulfur Mine extends to 170 m which was displaced by high temperature neutral fluid alteration assemblages and this conversion was completed within a depth interval of few meters, in the south part of the field where acid sulfate alteration was absent, according to ZK 308, the assemblage of high temperature alteration was only investigated below 1273m. The alteration intensity in the north part is also higher than in the south. The pattern of distribution of hydrothermal minerals suggests that the main upflow channel of high temperature fluid is located at Sulfur Mine in the north part of the field.