New Mexico Geological Society Annual Spring Meeting — Abstracts

Rare earth elements (REE) are critical minerals essential to high-tech and green energy industries. Carbonatites are the primary global source of light REE such as La, Ce, Nd and Pr. Magmatic processes such as fractional crystallization, partial melting and melt immiscibility are important for REE enrichment prior and during carbonatite emplacement. Hydrothermal processes can further mobilize and concentrate the REE, which is often a key process for reaching economic ore grades. Hydrothermal processes are also responsible for the formation of alteration footprints, which may provide geochemical vectors to REE mineralization. This study examines hydrothermal veins and fenitization associated with REE-enriched Cambrian carbonatite dikes in the Lemitar Mountains, NM. Extensive hydrothermal veining and fenitization of the surrounding Proterozoic metadiorites is associated with the emplacement of the carbonatite dikes (McLemore, 1987; Ruggles, 2024). Well-preserved outcrops and clear field relationships offer insights into hydrothermal REE transport, precipitation, and controls on fenitization. We hypothesize that fenitization can serve as an accessible vector for assessing REE enrichment in carbonatites and surrounding rocks by providing key insights into the extent of hydrothermal alteration, fluid composition, metasomatic intensity, and element mobility. Similarly, hydrothermal vein sequences record metasomatic processes that facilitate REE transport and mineralization, recording the evolution of mineralization. By integrating these features, we can develop geochemical indicators that improve the assessment of REE deposits and refine exploration strategies.

Petrographic observations include optical microscopy, SEM-BSE, and cathodoluminescence that are used to characterize a mineral paragenesis, alteration styles in the surrounding host rocks, and hydrothermal vs. magmatic REE-bearing minerals. Three hydrothermal vein types are identified: (I) quartz-albite, (II) quartz-calcite with chlorite and hematite, and (III) (barite)-fluorite-quartz-calcite. The earliest alteration stage is characterized by sodic and potassic fenitization in surrounding rocks, interpreted to be associated with Type I quartz-albite veins and interstitial apatite and quartz. Subsequent alteration is marked by hematization and chloritization, affecting both the carbonatites and the surrounding rock, shown as extensive replacement of mafic minerals such as phlogopite and hornblende in the carbonatites and hornblende and biotite in the metadiorite. This alteration shows association with Type II (hematite)-chlorite-quartz-calcite veins. REE mobilization and Ca-F metasomatism are interpreted to be associated with Type III (barite)-fluorite-quartz-calcite veins. REE-bearing fluorocarbonates such as parisite and bastnäsite are found in both Type II and III veins and altered vein selvages in the carbonatite.

Fenitization is observed as pervasive alteration of the Proterozoic metadiorite in proximity to the carbonatite dikes, and characterized by sodic and potassic metasomatism, silicification, and later chloritization and hematization. Sodic fenitization is marked by the replacement of primary plagioclase with large euhedral hydrothermal albite which exhibits a light pink CL signature with red-orange reaction rims along grain boundaries. Inclusions of K-feldspar with a light blue CL signature are present in albite. K-feldspar also occurs as large, separate grains with a similar CL signature and commonly displays strong dissolution textures and overprinting by hydrothermal quartz and later chlorite. Apatite with a bright orange CL signature occurs as euhedral, 5–200 µm needle-like and prismatic crystals intergrown with quartz and K-feldspar. This bright orange apatite is different from magmatic apatite 1 and 2, which show a yellow CL signature and hydrothermal apatite 3 associated with potassic fenitization and chloritization, which show a purple-yellow CL signature (Ruggles 2024).

This study addresses key gaps in REE mobilization by investigating fluid-driven enrichment in the Lemitar carbonatite system. Our findings show that REE mineralization is closely associated with a Ca-F-metasomatic stage and occurs primarily in Type II and III veins, suggesting a strong link between REE enrichment and late-stage alteration. The observed alteration sequences result from the release and transport of essential cations such as Ca, Si, K, Na, and Fe from either the carbonatite or the surrounding mafic host rocks. These results highlight the role of hydrothermal processes in secondary REE enrichment and offer tools for improving exploration strategies. Given New Mexico’s diverse carbonatite systems, this work advances the understanding of critical mineral resources and REE ore-forming processes.

References:

1. McLemore VT (1987) Geology and regional implications of carbonatites in the Lemitar mountains, central New Mexico. J Geol 95:255–270.
2. Ruggles EL (2024) Mineral paragenesis, mineral chemistry, and rare earth element mobility related to the magmatic-hydrothermal transition of the Lemitar Mountains Carbonatites, New Mexico. M.Sc. thesis, New Mexico Tech.