Abstract

Exploration work to locate commercial grade phosphate rock and phosphatic shale deposits has been going on in southwest Montana for several years. Geologic mapping has resulted in finding four locations where sills, dikes, and stocks intrude or cut off the phosphate-bearing beds of the Phosphoria formation (Permian). The metamorphic effects, or lack of them, produced in the phosphatic sedimentary rocks by intrusions are of commercial and scientific interest.In three of the areas, there have been no detectable changes in the phosphate minerals resulting from the intrusions, although the sedimentary rocks have been indurated. At the remaining area, Fleecer Mountain, the phosphatic rocks have been indurated, bleached from dark gray and black to light gray, and collophane and francolite minerals have been converted to colorless, very fine-grained apatite. Some phosphatic material has migrated. Megascopic veinlets filled with francolite developed in shale, and microscopic quartz veinlets containing some apatite grains formed in phosphatic shaleChemical analyses made on igneous rocks collected from the dikes, sills, and stock do not show an excess of P 2 O 5 over that present in average igneous rocks of similar types.

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