FENITIZATION OF MAFIC WALLROCK ASSOCIATED WITH EMPLACEMENT OF REE-RICH CARBONATITES OF SHEEP CREEK, SOUTHERN RAVALLI COUNTY, MONTANA

The Sheep Creek area of southern Ravalli County, Montana, contains numerous, small, carbonatite-related deposits rich in Nb and REE. The deposits are locally deformed, and are conformable to the foliation in the amphibolite country rock. The amphibolite is mostly composed of hornblende and plagioclase (An₅₂), with some relict clinopyroxene. Where the carbonatite bodies are well-mineralized, the surrounding amphibolite has been altered to a mafic fenite containing phlogopite, Na-rich plagioclase, calcite, and sodic amphibole. These mineral transformations are consistent with observed changes in the bulk chemistry of the amphibolite: i.e., an increase in K₂O, Na₂O, and CO₂, and a decrease in SiO₂. The Na-rich plagioclase in the fenite has a bimodal composition of An₀₁ and An₁₅, consistent with crystallization along the peristerite solvus at a temperature between roughly 475 and 535°C. A simplified, balanced chemical reaction can be written that summarizes the conversion of amphibole + plagioclase (An₅₀) to phlogopite + calcite + albite:

6actinolite + 5plag + 10K⁺ + 10HCO₃^- + 7CO₂ = 10phlogopite + 5albite + 17calcite + 28SiO₂(aq) + H₂O

Because there is no quartz in the fenite, most of the silica released through this reaction must have migrated away to form quartz veins somewhere else or diffused back into the carbonatite to form silicate minerals (e.g., Ba-feldspar, allanite). Although the fenites we have sampled are not particularly enriched in REE, recognition of their existence in the field is important for future mineral exploration in the Sheep Creek district for several reasons. If such rocks are encountered, it means a mineralized carbonatite could be nearby. As well, the inwards diffusion of silica during fenitization may have influenced the mineralogy of the carbonatite itself, for example, by supplying the silica needed to precipitate allanite, a common REE-bearing mineral in the deposits.