中低温地热系统低盐度地热水高含量钠的地球化学成因:以广东惠州黄沙洞地热田为例

Source Analysis of Sodium of Low-Salinity High-Sodium Geothermal Water in Huangshadong Geothermal Field from East Guangdong

高温地热系统中地热水中Na^(+)含量一般超过300 mg/L,盐度也较大(TDS>1000 mg/L).而在中低温地热系统中,低盐度地热水的Na^(+)含量一般小于160 mg/L.但在广东黄沙洞中低温地热系统出露的地热水中Na^(+)高达325.4 mg/L,TDS小于650 mg/L.经典的水文地球化学作用(矿物溶解、离子交换等)很难解释其成因.样品水化学结果表明,地热水水化学类型均为HCO_(3)-Na型,钠含量高(平均值为240.06 mg/L).氢氧同位素结果表明地热水与浅层地下水均具有相同的大气来源,都是瑶坑山区大气降水补给.水化学地温计和多组分矿物平衡(MME)评估的热储温度为100~130℃,地热水循环深度最大为2.43 km.Cl^(-)作为混合比计算指标揭示浅层地下水混入地热水的比例为51%~72%,深部地热水中Na^(+)实际含量应该高达685.2 mg/L.水-岩相互作用模拟结果表明,矿物溶解和离子交换对地热水中Na^(+)富集的贡献较小,也揭示出地热水中存在Na^(+)的额外来源.花岗岩流体包裹体微小但广泛存在于结晶矿物颗粒之间,其中Na^(+)含量平均值为11 758.9 mg/L.在地热水加热情况下,断裂和花岗岩裂隙网络层面及附近的流体包裹体膨胀破裂,流体混入到地热水中,为地热水提供了平均83%的Na^(+).因此,花岗岩流体包裹体可能是中低温地热系统低盐高钠地热水中Na^(+)的主要来源.

The Na^(+)content in the geothermal water in the high-temperature geothermal system generally exceeds 300 mg/L,and the salinity is also large(TDS>1000 mg/L).In the medium-and low-temperature geothermal system,the Na^(+)content of low-salinity geothermal water is generally less than 160 mg/L.However,geothermal water with Na^(+)as high as 325.4 mg/L and TDS less than 650 mg/L was found in the Huangshadong medium-low temperature geothermal system in Guangdong.It is difficult to explain their formation with typical hydrogeochemical reactions(mineral dissolution,ion exchange,etc.).The water chemistry results show that the chemical types of geothermal water are all HCO_(3)-Na type,with high sodium content(average 240.06 mg/L).The results of hydrogen and oxygen isotopes show that both geothermal water and shallow groundwater have the same atmospheric source which is in the Yaokeng mountainous area.According to the hydrochemical geothermometer and the multicomponent mineral equilibrium(MME),the thermal storage temperature is estimated to be 100-130℃and the maximum circulation depth is 2.43 km.Cl^(-)is used as a mixing ratio calculation indicator to reveal that the proportion of shallow groundwater mixed into geothermal water is 51%-72%,The actual content of Na^(+)in deep geothermal water should be as high as 685.2 mg/L.The simulation results of water-rock interaction indicate that mineral dissolution and ion exchange make a minor contribution to Na^(+)enrichment in geothermal water,and also reveal the existence of additional sources of Na^(+)in geothermal water.Granite fluid inclusions are tiny but widespread among crystalline mineral grains,and its average Na^(+)content is 11758.9 mg/L.In the case of geothermal heating,the fluid inclusions at and near the fracture and granite fissure network expand and rupture,and the fluid is mixed into the geothermal water,providing average of 83%of Na^(+)for the geothermal water.Therefore,granite fluid inclusions may be the main source of Na^(+)for the low-salinity and high-sodium geothermal water in medium-low temperature geothermal system.