ABSTRACT

Uranium ore deposits can be classified into three major types: detrital, hydrogenic and igneous-metamorphic. Although some igneous-metamorphic deposits formed as primary concentrations, detrital and hydrogenic deposits formed as secondary concentrations at or near the surface of the earth's crust -- the uranium having been initially exposed to the surficial environment in felsic intrusive and volcanic rocks in concentrations generally less than 50 ppm.

The oldest economic uranium deposits are detrital paleo-placers which formed as heavy-mineral concentrations under an anoxic atmosphere. Following the oxyatmoversion in mid-Aphebian time, deposits of hydrogenic origin predominate. Of these, sandstone-type and shale deposits are first found in late Aphebian rocks, but become less important thereafter in Canada despite their preponderance in Phanerozoic rocks elsewhere. "Classical" veins and "unconformity-veins" are predominant in Helikian time and appear to have precipitated from downward or laterally migrating waters. The distribution pattern of igneous-metamorphic deposits is also systematic, but more complex than that of the other two major types.

Canada is favoured with uranium ore reserves which are among the largest in the world -- these reserves are contained in all three types of deposit. The igneous-metamorphic type is represented by the Bancroft district of Ontario, where more than 11 million pounds U₃O₈ have been produced from pegmatites emplaced in granulite-facies metamorphic rocks. The detrital deposits at Elliot Lake, with production in excess of 190 million pounds of U₃O₈ by 1976, constitute one of the largest uranium ore districts in the world. The Elliot Lake deposits and a similar deposit at Agnew Lake occur in pyritic quartz pebble conglomerates which were deposited in paleo-stream channels. These deposits were derived from uranium-rich granitic rocks located north of the depositional area and concentrated mainly as detrital heavy minerals.

Hydrogenic uranium deposits are represented in the Proterozoic of Canada in shales and several epigenetic types. The epigenetic deposits are generally characterized by pitchblende and hematite and are generally enriched in Ni, Co, Ag, Cu and As. Chemically favourable graphitic and (or) calcareous shelf-facies metasedimentary strata commonly form the host rocks. The Port Radium and Beaverlodge deposits are "classical" veins which exhibit strong structural control. Stratigraphic control appears more important in the recently discovered "unconformity-veins" which occur near the base of the Athabasca Sandstone in Saskatchewan. At the Rabbit, Key and Cluff Lake deposits most of the mineralization occurs in basement rocks within 100 m of the unconformity, but ore is also found within the regolith and overlying orthoquartzites. Production from the Athabasca area deposits will probably surpass that from the Elliot Lake district in the future.