Ravalli And Lemhi Counties

Over a rectangular area of some 30x40 km extending from near Deep Creek in Ravalli County, Montana, southwards to the vicinity of North Fork, Lemhi Country, Idaho, more than 30 occurrences of carbonatite dykes and veins are known together with numerous thorite-bearing veins. Most of the occurrences are tabular, ranging from a few cm to about 3 m thick and up to 300 m long. They are emplaced in Precambrian metamorphic rocks, some cross-cutting and others paralleling the foliation. Fenitization of the wall rocks or of xenoliths is sometimes evident. There is a gradation in structure, texture, mineralogy and chemical composition from rocks that appear to be unaltered bands of phosphatic dolomitic marble through partly metasomatized layers of marble into cross-cutting dykes and veins. However, the associated fenitization would seem to indicate that these rocks are indubitably carbonatites. Three types of carbonatite can be distinguished: (a) a fine grained (up to 0. 1 cm) well foliated variety of major dolomite and rare calcite; (b) a coarser grained (0.3-1 cm) streaky carbonatite of dolomite and Ba- and Sr-bearing calcite, with Mn and lenses of baryte present; (c) a highly variable, coarse grained (0.1-4 cm) variety with Sr-rich to Sr-poor, Ba- and Mn-rich calcite. There is a very varied mineralogy (see Heinrich, 1966, Table L-5) including niobian rutile, columbite, fersmite, eschynite, baotite, ancylite and bastnaesite. For a brief review and a resume of literature and distribution in Ravalli County see Berg (1977, p. 35, Plate 1). Sr isotope ratios for three specimens from Ravalli County and one from Lemhi County have been determined by Powell (1965, Table 1).