Phosphate rock is present in the Phosphoria formation of Permian age in the Centennial Range of southwestern Montana and adjoining parts of Idaho. A Study was made to map the Phosphoria formation, study its stratigraphy, and to ascertain the reserves and mining problems. In the Centennial Range the Phosphoria formation is divisible into five members, designated A to E, from the bottom to the top of the formation. The B and D members contain phosphate rock. The B member contains six feet or more of high grade phosphate rock, mostly in one layer, and is the chief potential ore bed. The phosphate rock is of detrital origin and contains some quartz. Lithologic variations in the B member as well as areas where the B member is missing due to pre-member erosion, need more study.
The formation dips gently southward and is overlain by varying amounts of overburden, Underground mining would seem to be the best method of potential exploitation of the deposit, but core drilling and further trenching are needed.
High grade phosphate rock reserves in. The Centennial Range may total scores of millions of tons.