Geology of the Cargill Phosphate Deposit In Northern Ontario

CIM Bulletin, Vol. 70, No. 777, 1977

P. O. Sandvik and George Erdosh, International Minerals & Chemical Corporation, Libertyville, Illinois

The Cargill phosphate deposit, located 32 kilometers southwest of Kapuskasing, Ontario, was discovered by the Resource Development Group of International Minerals & Chemical Corporation in 1975 and has been evaluated through an extensive program of 190 drill holes, numerous chemical analyses, metallurgical tests, and engineering and economic studies. The Cargill alkalic complex, comprising principally carbonatite and pyroxenite-amphibolite rocks, was intruded into the rift system, of the Kapuskasing High during late Precambrian time about 1,800 million years ago, near the intersection of northeast- and northwest-trending faults. The Cargill complex is one of at least ten known alkalic complexes in or close to the Kapuskasing magnetic-gravity-high system and is expressed by an elongated, northeast-oriented, three-part magnetic anomaly 7.2 kilometers long by 2.3 kilometers wide. Weathering and leaching under assumed temperate to cool, moist climatic conditions, and controlled by faulting, shearing and/or initial petro-graphic variations within the carbonatite, developed a small, locally confined karst system in the carbonatite rocks and resulted in a thick enriched leached carbonatite zone, capped by troughs and sinks, filled with a residuum, of apatite, goethite, clay and other minerals stable in the near-surface, oxidizing environment. Overburden is 5 to IfiS meters thick. Residuum ranges from zero thickness on ridges to more than 170 meters thick in troughs and sinks. Fresh carbonatite contains 5-15%, leached carbonatite 20-40% and residuum 20-98% apatite. Rare earths are present and show local concentration in a thin crandallite-rich blanket which occurs discontinuously at the top of residuum in some of the deeper areas. Vermiculite, columbium-bearing minerals, clay minerals and quartz sand are present in quantities that suggest the possibility of commercial development. Uranium is absent or occurs only in very minor amounts. A tentative open-pit mine, designed for preliminary feasibility studies, is estimated to contain a probable phosphate-bearing deposit of 62.5 million metric tons at an average grade of 19.6 per cent P2O5.