GEOCHEMICAL SIGNATURES OF URANINITE FROM IRON OXIDE-COPPER-GOLD (IOCG) SYSTEMS OF THE GREAT BEAR MAGMATIC ZONE, CANADA

Although the Olympic Dam deposit contains the world’s largest recoverable U resources, little is known regarding the processes and timing of U enrichment in iron oxide-copper-gold (IOCG) systems. The Great Bear magmatic zone in the Northwest Territories of Canada is an ideal natural laboratory to study U in iron oxide-alkali-altered (IOAA) systems. Re-examination of the excellent glaciated 3D exposures of the weakly to un-deformed/metamorphosed IOAA systems has shown these systems to encompass iron oxide-apatite (IOA or Kiruna-type), magnetite-, magnetite-hematite and hematite-group IOCG, skarns, and albitite-hosted U prospects to deposits.

Trace-element concentrations in uraninite from IOCG and affiliated occurrences were determined by LA-ICP-MS. Preliminary results indicate that the chondrite-normalized REE patterns are remarkably consistent and are inferred to reflect precipitation from higher temperature fluid(s). The patterns are characterized by minor fractionation amongst the REE, resulting in relatively flat patterns with negative Eu anomalies, La depletion and mild HREE depletion. In some of the occurrences, mild LREE depletion may relate to co-precipitation of LREE-bearing allanite. The negative Eu anomalies are interpreted to reflect scavenging of metals during reduced sodic alteration and subsequent precipitation from fluids that evolved and equilibrated through progressive sodic (albite), calcic-iron (amphibole+magnetite) and ultimately potassic-iron (K-feldspar/biotite + iron oxides) alteration. In most systems, mineral parageneses indicate precipitation of U minerals during potassic-iron alteration wherein the alteration assemblages record input of oxidizing fluids. During this stage of IOCG development, magnetite-dominant (reduced) alteration is overprinted by hematite-bearing (oxidized) potassic-iron alteration. Secondary or re-mobilized uraninite is characterized by chondrite-normalized REE patterns similar to the altered host rocks. These LREE-enriched patterns are also typical of lower temperature, vein-type U mineralization. The presence of abundant hematite in the remobilized veins points to the involvement of more strongly oxidized fluids than previous alteration stages.