Geology and phosphate resources of the Hawley Creek area, Lemhi County, Idaho

Abstract

Phosphate resources occur within the Retort Phosphatic Shale Member of the Permian Phosphoria Formation in the Hawley Creek area, near Leadore, in east-central Idaho. About 12 square miles (31 km 2 ) of the Retort Member and enclosing rocks were mapped at a scale of 1:12,000 to evaluate the leasable Federal mineral resources. The Retort has an average thickness of 73 feet (22.3 m) and 12.9 linear miles (20.8 linear km) of outcrop within the area mapped. Rock samples taken from a bulldozer trench were analyzed for phosphate content and for minor trace elements. Analyses show a cumulative thickness of 8.7 feet ( 2.7 m) of medium-grade phosphate rock ( 24 to 31 percent P 2 O 5 ) and 33.4 feet (10.2 m) of low-grade phosphate rock (16 to 24 percent P 2 O 5 ). Minor elements in the Retort include uranium, vanadium, fluorine, cadmium, chromium, nickel, molybdenum, silver, and rare earths. These minor elements are potential byproducts of any future phosphate production in the Hawley Creek area. In addition, analyses of six phosphate rock samples taken from a prospect trench show a cumulative thickness of 14.9 ft (4.5 m) at 17.6 percent P 2 O 5 . Indicated phosphate resources are calculated for phosphate beds under less than 600 feet (183.0 m) of overburden. Approximately 36.5 feet (11.1 m), representing 50 percent of the total Retort Member, were measured in trench CP-71. There are 80.42 million short tons (72.96 million metric tons) of medium-grade phosphate rock, and 308.76 million short tons ( 280.10 million metric tons) of low-grade phosphate rock in the Retort Member within the map area. Because the thickness and grade of the phosphate beds for each block are based on the recovered section from CP-71, the calculated phosphate resource estimates represent a minimum. Other mineral resources in the area are thorium (35 ppm) in a Precambrian (?) granite body located immediately west of the Hawley Creek area; oil and gas accumulations may occur beneath the Medicine Lodge thrust system in this part of the Beaverhead Mountains. Paleozoic, Mesozoic, and Cenozoic rocks are present in the Hawley Creek area. Fold axes and thrust faults have a dominant northwest trend. These thrusts and folds are probably associated with the northeast-oriented stress field that existed in Late Cretaceous time. Evidence of younger, high-angle normal and reverse faults in the area also exists.