Rare earth element signatures in hydrothermal calcite: insights from numerical modeling, experimental geochemistry, and mineral deposits in New Mexico

Abstract

Rare earth elements (REE) are considered critical metals, which are used for the manufacturing of components in the high technology and renewable energy industries. Carbonatite and alkaline igneous systems can contain anomalously high concentrations of REE, in some cases sufficient enough to host potential ore deposits, such as in New Mexico. These mineral deposits commonly form through a complex interplay between magmatic-hydrothermal processes, which allows for the examination of the behavior of REE in hydrothermal fluids. The motivation for this dissertation was the need to quantify and understand the underlying processes involved in the hydrothermal mobilization and mineralization of the REE, focusing on REE signatures recorded in hydrothermal calcite. A numerical modeling study is first presented to examine the speciation of REE with several different ligands (i.e. OH-, HCO3-, CO32-, and Cl-) in a carbonatitic aqueous fluid. In this study, the partitioning of REE between calcite and an aqueous fluid was modeled for the first time at hydrothermal conditions. These simulations indicate that temperature, salinity, and fluid composition have a significant effect on the mobility of the REE and their speciation, which control the varying chondrite-normalized profiles observed in natural calcite. At low temperature (100-250 °C) and high salinity (20 wt.% NaCl), the calculated REE profiles were characterized by an enrichment of heavy (H)REE over the light (L)REE. At elevated temperature (>350 °C), the simulated REE profiles were enriched in the LREE, closer to the REE profiles observed in magmatic calcite in a typical carbonatite. To further constrain these simulations, a series of hydrothermal calcite precipitation experiments were carried out, where the REE partitioning between calcite and an aqueous fluid was measured at 200 °C and psat. The partitioning of REE between calcite-fluid can be described by the partition coefficient (DREE), and a comparison of this work to other studies conducted at ambient temperature shows that increasing the initial concentration of REE of the fluid or decreasing the temperature will lead to a decrease in DREE values. These results indicate that the REE become increasingly partitioned into the fluid at these conditions. Systematic variations of measured DREE as a function of ionic radii can be explained by the lattice strain theory in the experiments with higher initial REE concentrations (ppm range). In contrast, experiments with lower initial REE concentrations (ppb range) could be partly predicted using a thermodynamic solid solution model. This model indicates that in a fluid saturated with calcite, coupled substitution of REE3+ for Ca2+, and 2OH- for CO32- in the calcite structure can explain the fluid-calcite partitioning behavior of certain REE in the experiments. The numerical modeling and experimental studies were complemented by a field study in the Lemitar and Caballo Mountains of New Mexico. Hydrothermal calcite veins were investigated using automated mineralogy, cathodoluminescence (CL), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to study varying REE signatures in these carbonatite- and alkaline-hosted REE deposits. This study includes the discovery of a new carbonatite hosted in the Caballo Mountains episyenite. This study was undertaken to determine the controlling mechanisms resulting in varying chondrite-normalized REE profiles associated with different stages of metasomatism. Two metasomatic stages were distinguished, in both the Lemitar and Caballo Mountains, including a chloritization/silicic alteration stage and a carbonatization stage, followed by calcite veining. CL signatures of hydrothermal calcite allows the distinguishing of several calcite generations that display varying REE signatures. Calcite from the Lemitar Mountains displays enriched LREE/HREE chondrite normalized profiles, whereas calcite from the Caballo Mountains are distinctly enriched in the HREE and display pronounced negative Eu anomalies. Calcite from the Caballo Mountains are HREE enriched, all with pronounced negative Eu anomalies. From this study, it is shown that hydrothermal REE mobilization and mineralization can be affected by many processes, and REE signatures recorded in hydrothermal calcite reflect complex alteration and mineralization sequences. Linking numerical modeling, experiments, and field observations is vital in order to understand what physicochemical conditions can lead to these varying REE concentrations measured in calcite from alkaline and carbonatite systems.