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 niobium (Nb) and rare earth elements (REE). The carbonatite bodies are typically < 2 m wide, strike northeast or east, and dip moderately to steeply to the north. The carbonatites are locally deformed, and are conformable to foliation in the host rocks. The host rocks belong to a 1.37 Ga, bimodal, metaigneous complex consisting of granite/augen gneiss and diabase/amphibolite. When found in close proximity to a mineralized carbonatite body, the mafic wall rock is altered to a fenite assemblage that includes phlogopite, albite or Na-rich plagioclase, calcite ± dolomite, magnetite, and sodic amphiboles (riebeckite, winchite). The mineral transformations are consistent with observed changes in the bulk chemistry of the mafic wall rocks, including consistent increases in K2O, Na2O, and H2O/CO2 and consistent decreases in SiO2 and Al2O3. Relatively minor changes in FeO and MgO were found to take place during the fenitization process. Fenite alteration is most strongly developed adjacent to carbonatites that are strongly mineralized with Nb-REE minerals. Unmineralized carbonatites have fenite zones that are weak or absent. Although the fenites themselves are not strongly enriched in REE, transfer of SiO2 and Al2O3 from the fenites back into the carbonatite bodies may have triggered deposition of REE-rich allanite and other silicate minerals (Ba-feldspar, actinolite, quartz) that are locally abundant in the carbonate lodes. An influx of silica may also have lowered the solubility of REE-phosphate in the carbonatite melts, resulting in deposition of monazite. Most fenites that have been described in previous studies around the world formed by alteration of felsic country rock (granite, gneiss, siliciclastic metasediments). These fenites are rich in alkali feldspar and aegirine, and form conspicuous, pink-colored outcrops that surround the central carbonatite complex. Because the country rock in the Sheep Creek area has a mafic starting composition, the fenites in this study have a completely different mineralogy, being rich in biotite, albite, and matrix calcite. These fenites erode to a crumbly grus soil, do not crop out well, and are therefore easy to miss or ignore in the field. However, recognition of these rocks is important because they are vectors to a nearby mineralized carbonatite.