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High-temperature metasomatism driven by ascent of voluminous, saline fluid ...
High-temperature metasomatism driven by ascent of voluminous, saline fluid columns in the upper crust plays a major role in the genesis of iron oxide-alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), albitite-hosted uranium, and skarn deposits that they produce. Excellent surface exposures of such systems in the Great Bear magmatic zone of northernwestern Canada record the depth to paleosurface, prograde evolution of iron oxide-alkali alteration facies, and mineralization. Across the belt, albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions. Pervasive albitization partitioned metals from the host rocks into the ascending fluid column, leaving behind structurally weakened corridors of porous albitite. These corridors were cut, replaced, and overprinted by amphibole- and magnetite-bearing, calcic-iron alteration assemblages. In extreme cases, the discharge of calcium, iron, and specialized metals formed iron oxide-apatite deposits (±vanadium ± rare earth elements) while recharging the outgoing fluids in sodium, potassium, and base and precious metals. As temperatures declined and fluid chemistry evolved through fluid-rock reactions, the formation of potassic-iron alteration assemblages, breccias, and sulfides resulted in magnetite- and hematite-group IOCG mineralization. Within carbonate units, skarns formed prior to, are replaced by, and evolved to calcic-iron alteration facies. Skarns can locally host base metal mineralization. Tectonically uplifted albitite breccias replaced by potassic-iron alteration assemblages became a preferential host for uranium mineralization. The results of this study also illustrate that permutations and cyclical build-up of alteration products can arise from a combination of faulting, differential uplift, and renewed magmatism. Framed within an alteration-facies deposit model, alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali-altered systems at district to deposit scales.
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