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TitleElement mobility patterns in magnetite-group IOCG systems: the Fab IOCG system, Northwest Territories, Canada
AuthorMontreuil, J F; Potter, E G; Corriveau, L; Davis, W J
SourceOre Geology Reviews vol. 72, (2016), 2016 p. 562-584,
Alt SeriesEarth Sciences Sector, Contribution Series 20140140
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories
Lat/Long WENS-120.0000 -112.0000 68.0000 64.0000
ProgramMackenzie Corridor, Shield to Selwyn, GEM2: Geo-mapping for Energy and Minerals
AbstractThis paper documents element mobility patterns from a magnetite-group Iron Oxide-Copper-Gold (IOCG) showing in the Northwest Territories of Canada and explores implications for space-time chemical evolution of Iron Oxide-Alkali-Altered (IOAA) systems. 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 IOAA 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. Mass and volume losses were inferred to be compensated by porosity creation during pervasive host rock dissolution and precipitation of the alteration assemblages. Pure sodic alteration depleted the rock 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 IOAA 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, halogen activities and increasing fO2 as the fluids evolved and interacted with the host rocks.
Summary(Plain Language Summary, not published)
Prospective settings for iron oxide-copper-gold (IOCG) deposits which typically occur within broader iron oxide alkali-altered (IOAA) systems, have largely remained underexplored and inadequately mapped. Consequently, costly field-based geological mapping and research, high-resolution geophysical surveys, and multidisciplinary tools and teams with IOAA-targeted expertise are required to advance exploration for this newly defined ore system. Targeting these issues, this manuscript documents element mobility within an IOCG system located in the Great Bear magmatic zone of the Northwest Territories. The Great Bear magmatic zone is currently the most prospective region for IOCG and affiliated mineralization in Canada. The geological map and element mobility (geochemistry) highlighted in this study illustrates the predictability of the mineral alteration in IOAA systems and provides further support for a conceptual IOCG alteration to brecciation and mineralization model that can be used to vector to ore using field observations.