37 D1. Recent Advances in Geoscience and Geoscience Education (Posters)

Friday, 17 May 2024
9:00 AM - 1:30 PM
Grand Ballroom

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

Paper number: 37-3
Program: Technical Sessions
Time: 9:00 AM – 1:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 4

doi: 10.1130/abs/2024CD-399377

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.