Element mobility patterns in magnetite-group IOCG systems: The Fab IOCG system, Northwest Territories, Canada

- Constraints on element mobility in magnetite-group iron oxide-copper-gold systems
- Coeval mobility of uranium and thorium in iron oxide-copper-gold systems
- Spatio-temporal association of IOCG mineralization and magmatism
- Copper mineralized zones are formed at the K-feldspar-magnetite alteration stage.

This paper documents element mobility patterns from a magnetite-group Iron Oxide Copper-Gold (IOCG) prospect in the Northwest Territories of Canada and explores implications for space-time chemical evolution of metasomatic systems hosting IOCG deposits. The Fab system, located in the Great Bear magmatic zone (GBMZ) of the Northwest Territories, Canada, contains numerous Fe-Cu-U showings associated with high temperature (HT) potassic-iron alteration overprinting extensive zones of sodic to HT calcic-iron alteration. Each hydrothermal alteration assemblage is associated with distinct element mobility patterns that record evolving physico-chemical properties of the hydrothermal fluids. New geochronological data constrain the alteration and IOCG mineralization in the Fab system to a 3 m.y. window between 1870-1867 Ma, which is broadly contemporaneous with extensive high-level intrusive activity across the GBMZ. Regional- to local-scale element mobility patterns characteristic of the sodic and sodic-calcic-iron alteration type record leaching combined with weak to strong mass losses. Pure sodic alteration depleted the rocks in Ca, Co, Cu, Fe, Mg, Th, U and V. Conversely, sodic-calcic-iron alteration records significant depletions of Nb, REE, Ta, Ti, Th and U. These element mobility patterns differ from intense HT calcic-iron alteration that is enriched in Ca, Co, F, Fe, Mg, Mn, Ni and V with modest enrichments to locally significant mineralization in Th, U and REE. HT calcic-iron alteration is also characterized by substantial mass gains that translate into volume gains in stockwork zones and mass/volume gains in zones of intense host rock replacement. HT potassic-iron alteration is characterized by enrichments in Ba, K, Ni, U and V, along with locally Co and Cu. The temporal and spatial association of the Fab system alteration and the emplacement of the porphyritic dacite are indicative of the predominant involvement of magmatic-hydrothermal fluids. The high F- and Cl- contents of the porphyritic dacite and of the HT calcic-iron alteration zones as well as Nb, REE, Ta, Th, and Ti mobility provide strong evidences of high halogen activities (F and Cl) in the hydrothermal fluids. High F- and Cl-activities in the hydrothermal fluid are interpreted to have facilitated the mobilization of normally immobile (Nb, Ta, Ti, Th) or weakly mobile elements as well as some metals (e.g., V, Ni, Co). The formation of REE fluorocarbonates and calcite in the early and incipient HT calcic-iron alteration zones indicates the presence of CO2 in the hydrothermal fluids. Weaker HFSE, HREE and Ti mobility during later HT potassic-iron alteration is interpreted to reflect decreasing temperatures, pressures, F-activities and increasing fO2 as the fluids evolved and interacted with the host rocks.