SUPERGENE PHOSPHATE ENRICHMENT IN CARBONATITE-DERIVED ELUVIAL SEDIMENTS: AGRIUM PHOSPHATE MINE, KAPUSKASING, ONTARIO, CANADA

Mineralogical and lithogeochemical study of ores in the Agrium phosphate deposit indicates that they are sediments that developed through extremely efficient weathering of a Proterozoic carbonatite. Enrichment processes that concentrated apatite from the carbonatite protolith consisted of: (1) dissolution and removal of most primary gangue minerals such as carbonates and silicates; (2) gravitational sorting of primary apatite, the main ore mineral; and (3) replacement and overgrowth of relict minerals with secondary phosphates. Particle size distribution in the unconsolidated ores supports in situ weathering of the carbonatite in a karst-like environment to form iron-rich “B” ores. Subsequent reworking of some “B” ore by fluviatile processes has removed most of the fine-grained and coarse-grained fractions to produce thin lenses of iron-poor “A” type ore throughout the deposit.

Primary apatite, the dominant phosphate mineral in all unconsolidated Agrium ores, occurs mostly as individual ovoid grains. Brecciated apatite lithoclasts (up to 2 cm long) were found in consolidated ores and may have originated as apatite cumulate layers in the carbonatite protolith. Other primary minerals present include zircon, baddeleyite, ilmenite, magnetite, Ti-ferrocolumbite, pyrochlore (partially weathered to bariopyrochlore) and minor carbonates. The relict carbonate grains occur shielded within larger primary apatite crystals. Secondary phosphates occur as overgrowths, cements, matrices and replacements in both ore types but are found predominantly in the “B” ores. These phosphates occur in microcrystalline, cryptocrystalline, or amorphous forms and include: hydroxylapatite, carbonate-hydroxylapatite, collophane, crandallite, monazite and rabdophane group minerals. Submicroscopic inclusions of secondary monazite (Ce>La), gorceixite (Ba), and cheralite - (Ca,Ce)(P,Si)O₄ in the matrices or coatings were also identified and appear to correlate with increased concentrations of light REE’s and Ba measured in the fine-grained sediment fraction.