A geochemical mapping of rare earth (REE) and other trace elements in water and rock samples of the hidden geothermal system of the Acoculco caldera was carried out. REE and other trace elements were measured at low concentrations in waters (ΣREE <50 μg/L) and rocks (ΣREE <773 μg/g), and used, for the first time, to understand the water-rock interaction processes of this geothermal system. A bimodal frequency distribution of pH was found among acidic waters (pH <3.5) that emerge from cold springs located inside the caldera, and slightly-acid to hot alkaline spring waters (pH 5 to 9.2) from outside the caldera. The geochemical modelling of REE showed that minerals containing Al³⁺, Fe, and F⁻ were the main phases that control their concentrations in acidic waters. Different degrees of REE complexation in waters located inside the Acoculco caldera (Alcaparrosa and Los Azufres) clearly showed new signatures of geothermal activity. LnSO₄⁺ complexes and free ions of Ln³⁺ were identified as major species of the acidic waters of Alcaparrosa, whereas in less acidic waters of Los Azufres, LnF²⁺ show an increase in their complexation percentages. The REE chondrite-normalized patterns observed in acid waters display a depletion from light-REE (LREE) to heavy-REE (HREE), and an inverse correlation between high REE concentrations and acid pH's. The REE concentration in felsic and intermediate rocks shows “gull-wing” chondrite-normalized patterns characterised by an enrichment of LREE, a negative anomaly of Eu, and depletion of HREE with a flat tendency. The present study of water-rock interaction contributed to the identification of new geochemical signatures of major and trace elements that provide new evidences of deep formation temperatures between 200 °C and 300 °C, becoming in an effective tool for finding out convincing anomalies in hidden geothermal systems.

对Acoculco破火山口的隐蔽地热系统中的水和岩石样品中的稀土（REE）和其他微量元素进行了地球化学测绘。在水中（ΣREE<50μg/ L）和岩石（ΣREE<773μg/ g）中以低浓度测量REE和其他微量元素，并首次用于了解该地热的水-岩相互作用过程。系统。在位于破火山口内部的冷泉和从破火山口外部的弱酸至热碱性泉水（pH 5至9.2）中发现的酸性水（pH <3.5）之间出现了pH的双峰频率分布。稀土元素地球化学模型表明含铝矿物即³⁺，铁，和F ^-是控制其在酸性水中浓度的主要阶段。位于火山口破火山口（Alcaparrosa和Los Azufres）内水域中不同程度的REE络合清楚地显示了地热活动的新特征。LnSO ₄ ⁺络合物和Ln ^3+的游离离子被确定为Alcaparrosa酸性水的主要种类，而在Los Azufres的弱酸性水中LnF ²⁺显示其络合百分比增加。在酸性水中观察到的REE球粒陨石归一化模式显示出从轻稀土（LREE）到重稀土（HREE）的消耗，并且高REE浓度与酸性pH值呈反相关。长英质和中质岩中的稀土元素含量显示出“鸥翼状”的球粒陨石归一化模式，其特征是富集的稀土元素、,的负异常和贫化的稀土元素呈平坦趋势。当前的水-岩相互作用研究有助于确定主要和微量元素的新地球化学特征，这些特征为深层地层温度在200°C至300°C之间提供了新证据，成为寻找令人信服的隐伏异常的有效工具。地热系统。