|Title||Iron-oxide-copper-gold ±U in the Great Bear Magmatic Zone: nature of Uranium mineralization in IOCG systems|
|Author||Potter, E G; Corriveau, L; Montreuil, J -F|
|Source||38th annual Yellowknife Geoscience Forum, abstracts of talks and posters; by Palmer, E; Northwest Territories Geoscience Office, Yellowknife Geoscience Forum Abstracts Volume 2010, 2010 p. 91-92 (Open
|Links||Online - En ligne|
|Alt Series||Earth Sciences Sector, Contribution Series 20100246|
|Meeting||38th annual Yellowknife Geoscience Forum; Yellowknife; CA; November 16-18, 2010|
|NTS||85N; 86C; 86D; 86E; 86F; 86K; 86L|
|Area||Great Bear Lake; Lac la Martre; Hottah Lake|
|Lat/Long WENS||-119.0000 -116.0000 67.0000 63.0000|
|Subjects||economic geology; mineral occurrences; mineralization; iron ores; iron oxides; gold; copper; uranium; Great Bear Magmatic Zone|
|Program||Uranium, GEM: Geo-mapping for Energy
|Abstract||Under the Northern Uranium for Canada project (GEM-Energy program), the nature of uranium in iron oxide copper-gold (IOCG) systems is being investigated using examples from the Great Bear magmatic zone
(GBMZ). Hypotheses being addressed include: Can uranium (and thorium) serve as a vector to multiple-metal endowment in IOCG systems? Can GBMZ IOCG systems be potential energy resources? How can we maximise geological vectoring to mineralization with
the chemical information retrieved in the field with a hand-held gamma-ray spectrometer?|
During summer 2010, fieldwork focused on known IOCG alteration systems with anomalous uranium and thorium at Lou, Cole and Fab lakes. At Lou Lake, two new
U-Th-bearing* breccias were discovered west and south of the NICO deposit. A localized (~1.5 m wide, 3 m long) lens occurs structurally above and west of the NICO deposit within a polymict breccia containing altered porphyry and metasedimentary
clasts set in an amphibole-rich matrix. The breccia follows the contact between intensely amphibole-magnetite and biotite altered metasedimentary rocks in the vicinity of a porphyry dyke and trends parallel to the dominant foliation (124/82) of its
host. Field observations indicate that the anomalous uranium zone (= 525 ppm eU) occurs in late hematite-quartz veinlets preferentially concentrated within the breccia. To the south, within a 2 km long U-Th-arsenopyrite breccia corridor (eU =
0.01-1%, eTh =0.2%) (Corriveau et al.; Montreuil et al.), field observations suggest uranium is also concentrated within thin veinlets of fine grained earthy hematite within bright red silicification and albitization zones in metasedimentary rocks.
In this corridor, hematized magnetite veins and potassium-rich magnetite replacive alteration also have anomalous uranium.
At Fab Lake, new thorium and uranium* anomalies were discovered and the timing relationships between the various generation
of porphyries and IOCG alteration better constrained. Along the northeastern shoreline, several amphibole-magnetite veins and breccias hosted within medium-grained feldspar porphyry are sharply cross-cut by younger, two-feldspar porphyry dykes
trending 045/80. The incipient to intense amphibole-magnetite alteration is overprinted by potassic alteration and brecciation during amphibole-magnetite-K-feldspar alteration. Although both porphyries contain late potassic (biotite) alteration, the
two-feldspar porphyry dykes are easily distinguished in the field by the absence of amphibole-magnetite alteration and brecciation. On the northwestern shoreline, Th-enriched (=143 ppm eTh) amphibole-magnetite veins up to 2 m in width occur and trend
165/84. Thorium enrichment in these zones (Th/U = ~4-10) records remobilization of a normally immobile element and decoupling of U and Th. The uranium occurrence at the eastern shoreline of Fab Lake, north of the known Fab uranium showings, occurs
within magnetite-rich alteration zones/fronts overprinted by potassic veining with trace amount of pyrite and often by another generation of amphibole veinlets. The new occurrence is characterized by several east-west trending <20 cm wide zones
defined by intense magnetite veining accompanied by uranium enrichment (= 3595 ppm eU). The magnetite alteration begins as veinlets then progresses to near complete replacement of the host rock.
* eU and eTh concentrations measured in field
by portable spectrometer, not verified by geochemical assays.