Mineral resources in Permian rocks of southwest Montana

Geology and phosphate deposits of the Permian rocks in central western Montana

“…In general, the uranium content of the Phosphoria strata is roughly proportional to the P2Og content (McKelvey, 1956;Sheldon, 1959;Swanson, 1970). Figure 202 tends to bear this out, though not as clearly as the diagrams for Wyoming given by Sheldon (1959, figs.…”

“…Altschuler, Clarke, and Young (1958) have discussed the geochemistry of uranium in phosphorite, and Sheldon (1959) has discussed it with special reference to the Phosphoria Formation. The distribution of uranium in rocks of the Phosphoria Formation of southwest Montana was described by Swanson (1970); what follows relates especially to its occurrence in rocks of the Phosphoria in central western Montana.…”

“…In this report, as in the report on resources in Permian rocks in southwest Montana (Swanson, 1970), the term "phosphate" (a phosphorus compound) is used in a chemical sense for both the raw material and the products made from it (not including elemental phosphorus) ; the term "phosphate rock" is used in an industrial sense for the material mined and treated, or potential thereto; the term "phosphorite," a lithologic term, is used in a descriptive sense for a rock containing more phosphate mineral than any other rock component; and the term "phosphatic" is used for a rock in which the phosphate mineral represents more than 20 percent of the total (7.8 percent P2O5) but is not the dominant component. The phosphate rock may be almost entirely phosphorite, as in some of the high-grade deposits of this region, or it may be a mixture of rock types, dominantly phosphorite but including mudstone, sandstone, carbonate rock, or chert, generally interbedded with the phosphorite.…”

“…The north-trending zone of tight folding and faulting in the southwest Montana synclinorium (Swanson, 1970) apparently divides northward, near Dillon, into two branches separated by the area of the Boulder batholith. The east branch includes the Jefferson Canyon structural complex and the Lombard thrust and fold zone west and north of Three Forks (Robinson, 1959), and apparently connects through the Canyon Ferry area at the Missouri Eiver with the mountainfront "Disturbed Belt" northeast of Helena (Dobbin and Erdmann, 1955).…”

“…The approach to reserve classification and the use of the terms "reserves" or "resources" have been discussed by Swanson (1970). Phosphate zones used in tonnage estimates are defined as follows: (1) minimum grades are 31, 24, or 18 percent P2O5 (high grade, medium grade, and low grade, respectively); (2) zones must be 3 feet thick; (3) where a sequence of beds above the given minimum grade is a little less than 3 feet thick, a part of an adjacent bed is added to increase the total to 3 feet, provided that the combined total contains the minimum grade; (4) the uppermost and lowermost beds in a zone contain no less than the minimum grade for that zone, except where the third rule is applied; (5) no zone includes a sequence 3 feet thick of beds containing less than the minimum grade for that zone.…”