西藏搭格架高温热泉中钨的水文地球化学异常

Tungsten Anomaly of the High-Temperature Hot Springs in the Daggyai Hydrothermal Area, Tibet, China

西藏搭格架水热区的热泉含异常高浓度的钨,其钨/钼比也远高于常见天然水.开展了搭格架典型热泉的地球化学研究,发现中性热泉的钨浓度显著高于偏酸性热泉:前者是深部母地热流体经绝热冷却、传导冷却等过程后排出地表而形成,其中的钨主要来自岩浆水的贡献;而后者为中性地热水和蒸汽加热型强酸性水的混合产物,贫钨蒸汽加热型水的稀释使其钨浓度不同程度降低.在地热水中,钨与典型保守组分氯相似,不易自液相沉淀或被热储介质吸附;但地热水含硫化物时,钼则极易以辉钼矿的形式沉淀,导致搭格架热泉的钨/钼比偏高.虽然搭格架地热水中存在硫化物,但钨在水中主要以钨酸盐的形式存在,少量硫代钨酸盐的形成对钨的水文地球化学过程影响不大.

Extremely high concentrations of tungsten was detected in the hot springs discharged from the Daggyai hydrothermal area, Tibet, and their tungsten to molybdenum ratios are much higher than common natural waters as well. A hydrogeochemical study of typical Daggyai hot springs was carried out, based on which it was found that the tungsten concentrations of the neutral springs are significantly higher than those of the acid springs. The neutral hot springs at Daggyai were formed via the adiabatic cooling or conductive cooling of the parent geothermal fluids ascending from the deep levels to the surface, and the tungsten in these waters is primarily from the contribution of magmatic fluids. In contrast, the slightly acid hot springs are basically a mixture of the steam-heated acid waters and neutral geothermal waters, and the dilution of tungsten-depleted steam-heated waters lowered the tungsten concentrations of these springs to various degrees. In geothermal systems, tungsten behaves generally like chloride, a typical conservative component, and is difficult to precipitate from geothermal water or to be adsorbed by reservoir rocks.However, molybdenum is prone to precipitate from sulfide-rich Daggyai geothermal waters as the form of molybdenite, resulting in the high tungsten to molybdenum ratios of the Daggyai hot springs. Moreover, although sulfide exists in the Daggyai geothermal waters, the major species of tungsten is tungstate instead of thiotungstates. The formation of trace thiotungstates has little effects on the tungsten-involved hydrogeochemical processes.