云南腾冲热海高温地热水中汞的地球化学异常及其指示意义

Hydrogeochemical anomaly of mercury in the high-temperature geothermal waters in the Rehai hydrothermal area in Tengchong,Yunnan and its indications

汞是环境中典型有害组分,深部地热系统可能是环境中汞的重要来源之一,但当前地热成因汞的研究程度很低。本文以腾冲火山带热海水热区为研究区,开展了热泉中汞的水文地球化学研究。热海水热区内排泄的中性热泉含有异常高浓度的汞,但酸性热泉则绝大多数未检出汞。热海热泉中的汞与典型的岩浆来源组分氯相同,均主要来源于岩浆流体的混合。在中性热泉中,Hg(Ⅱ)是汞的占优势地位的价态,Hg^2+与不同形态硫化物的配合则是决定Hg(Ⅱ)的形态分布的直接因素,但pH可通过控制水中硫化物的形态分布来影响Hg(Ⅱ)的形态分布。Hg(Ⅱ)不易挥发,故中性热泉中汞含量普遍较高;但酸性热泉中的汞受泉口氧化还原电位较高的影响,以Hg(0)为主要价态,且因Hg(0)易挥发而导致总汞含量极低。就中性热泉而言,其汞含量对水热区断裂分布有重要指示作用,原因为沿不同断裂上升的中性地热水经历的冷却方式不同,最终导致热泉汞含量也表现出显著差异。热泉汞含量对水热系统结构研究有借鉴意义。

Mercury(Hg)is a typical undesirable constituent in the environment.One of potential sources of environmental mercury is deep geothermal systems.However,systematic studies on geothermal mercury are sparse.Here,we conducted a hydrogeochemical study on Hg in hot springs in the Rehai hydrothermal area in the Tengchong volcanic region.The results showed that Hg is rich in the neutral hot springs in the Rehai area but was not detected in most acidic Rehai hot springs.Moreover,the geothermal mercury in Rehai,like chlorine,a typical magma-derived component,primarily comes from magmatic fluid.In the neutral hot springs,Hg(Ⅱ)is the predominant form of mercury.Complexation of Hg^2+ with different sulfide species directly determines Hg(Ⅱ)speciation,but hot spring pH values can also have an effect to some degrees by controlling sulfide speciation.Because Hg(Ⅱ)is not prone to volatilization,the neutral hot springs generally have high mercury concentrations.However,volatile Hg(0)is the major form of mercury in acidic Rehai hot springs due to their much higher redox potentials,resulting in very low total mercury concentrations in acidic springs.Mercury in neutral hot springs can be an effective indicator for fault distribution in Rehai,because mercury concentration in ascending neutral spring waters varies distinctively depending on the water cooling process along fault.Geothermal mercury is of great significance for studying the structures of hydrothermal systems.