PETROGRAPHY AND COMPREHENSIVE WHOLE ROCK CHEMISTRY OF SODIC ALKALINE ROCKS FROM THE ROBINSON ANTICLINE COMPLEX, CRAZY MOUNTAINS, MONTANA

Paleogene igneous centers of the Central Montana Alkalic Province such as the Robinson Anticline Complex (RAC), an intrusive alkaline complex, are of broad interest because of their potential to host critical minerals, including REE, Nb, Zr, and fluorspar. Despite previous regional petrological and geochronological studies of the Crazy Mountains, few samples from the RAC itself have been analyzed to understand the critical mineral potential of the complex. Samples were collected from the dikes and sills intruded into the Robinson Anticline, which has been described as an anticlinal phacolith. Prepared thin sections were examined using transmitted light microscopy as well as SEM-BSE imaging to determine sample mineralogy. Whole rock major, minor, and trace element analyses were performed by the USGS contract lab using ICP-MS and WDXRF.

Modal mineralogy estimates are consistent with previous studies and show that the major rock types are malignite, mafic trachyte, latite, and lamprophyre. Zeolites and feldspathoids are abundant in many of the alkaline rock samples. Analcime (NaAlSi₂O₆·H₂O) is the most abundant zeolite, which exists as primary phenocrysts and also as a product of secondary sodic zeolitization. Most of the alkaline rocks are sodic with Na₂O/(Na₂O+K₂O) percentages ranging from 58-88% for malignite–melanocratic foid syenite (n=7), 53-61% for mafic trachyte–hornblende latite (n=4), 5-15% for lamprophyre (n=3), and 28-33% for foidite (n=2). Samples show enrichments in Th, Nb, REE, Sr, Ba, F, and P relative to average upper crust values, though none approach economic concentrations. Samples are enriched in REE relative to chondrite (up to 500 times for La) with normalized La/Lu ranging from 36.0 to 64.5.

Although not enriched to economic concentrations of critical minerals, these rocks show enrichment in a specific element assemblage (Th, Nb, REE, Sr, Ba, F, and P) commonly found in alkaline complexes containing ore-grade enrichments of REE and Nb. The highly fractionated La/Lu ratios may have been partly controlled by the presence of garnet in the source region at low degrees of partial melting. These results could help in understanding how mantle source regions and alkaline magma differentiation within the broader CMAP might have affected where critical mineral deposits were likely to have formed.