The amethyst and agate geodes from the Los Catalanes Gemmological District in Uruguay represent one of the main deposits of its kind worldwide. The geometry of the deposit is horizontal, with an irregular distribution of amethyst geodes within the upper level of the basalt lava flows and shows strong variations in their abundance, as well as quality, geometry, and shape. Reliable exploration guides are scarce, and the limited knowledge of the geological parameters controlling its occurrence makes exploration unpredictable, leading to inaccurate reserve estimation. Based on cutting-edge methods including nucleation-assisted microthermometry of one-phase fluid inclusions and determination of triple oxygen isotope in silicates and carbonates, as well as analysis of geode-hosted water and groundwater, we estimate the crystallisation temperatures in the range between 15 and 60 °C. These low temperatures point to amethyst crystallisation after the emplacement of the complete basalt pile. The mineralising fluid shows isotopic signatures consistent with meteoric water and very low salinities from pure water up to rarely over 3 wt% NaCl-eq., likely sourced from the groundwater hosted in the aquifers in the basaltic sequence and underlying units. Based on the insights provided by the new data, we propose the combination of open- and closed-system crystallisation inside pre-existing cavities due to the episodic infiltration of meteoric water in a rather stable geological context.

乌拉圭洛斯卡塔兰内斯宝石区的紫水晶和玛瑙晶洞是全球同类主要矿床之一。矿床的几何形状是水平的，紫水晶晶洞在玄武岩熔岩流的上层不规则分布，并且在丰度、质量、几何形状和形状上表现出强烈的变化。可靠的勘探指南稀缺，并且对控制其发生的地质参数的了解有限，导致勘探不可预测，导致储量估算不准确。基于尖端方法，包括单相流体包裹体的成核辅助显微测温、硅酸盐和碳酸盐中三氧同位素的测定，以及对晶洞中水和地下水的分析，我们估计结晶温度在 15 之间和 60°C。这些低温表明紫水晶在完整的玄武岩堆就位后会结晶。矿化流体显示出与大气水一致的同位素特征，以及从纯水到很少超过 3 wt% NaCl-eq. 的极低盐度，可能源自玄武岩层序和底层单元中含水层中的地下水。根据新数据提供的见解，我们建议在相当稳定的地质环境中，由于大气水的间歇性渗透，在预先存在的空腔内结合开放和封闭系统结晶。