|Résumé||(disponible en anglais seulement)|
As part of Canada's Geomapping for Energy and Minerals (GEM) Program, a research activity taking place under the IOCG-Great Bear Project in NWT was established
to provide a practical guide to geochemical and indicator mineral exploration for Iron Oxide Copper-Gold (IOCG) deposits in glaciated terrain. An orientation study around the NICO Co-Au-Bi deposit, interpreted as a magnetite-group IOCG deposit, was
initiated in 2007. Further work was completed in 2009 and 2010 in the vicinity of the Sue-Dianne Cu-Ag-Au deposit, a hematite-group IOCG deposit, and near additional showings thought to be parts of large polymetallic IOCG systems within an
IOCG-porphyry-epithermal continuum across the Great Bear magmatic zone (GBMZ). Bedrock and till samples were collected up-ice, proximal to, and down-ice from mineralization, host hydrothermal system and least altered bedrocks, to characterize their
indicator mineral and alteration geochemical signatures.
Results from the orientation study at NICO demonstrate that gold grain counts and magnetite composition have the best potential to fingerprint the magnetite-group IOCG mineralization.
Pristine-shaped gold grains indicative of a local source are more abundant in till samples collected immediately down-ice from several surface showings at NICO and none were recovered up-ice. Using trace elements signatures of Fe-oxides (i.e.
magnetite) to fingerprint IOCG deposits is also a prospective indicator mineral method for IOCG exploration. In particular, magnetite from till samples collected over, or directly down-ice of, the NICO deposit have lower Ti + V compositions compared
to magnetite from till collected up-ice of NICO. The non-ferromagnetic heavy minerals at NICO are not particularly effective as indicator minerals of IOCG mineralization. Potential non-ferromagnetic indicator minerals are either not chemically stable
in surface sediments (arsenopyrite, chalcopyrite, pyrite), not sufficiently coarse-grained or resistant to glacial transport (bismuthinite, tourmaline, ferroactinolite), or not sufficiently abundant in the mineralized bedrock (scheelite, molybdenite,
cobaltite, allanite). In situ trace element analysis of the most resistive minerals (i.e. tourmaline, magnetite) by LA-ICP-MS has been initiated on the NICO bedrock and till samples and further work using this more sensitive technique will be
completed on other mineral species (i.e. sulphides).
Preliminary results from surface bedrock samples collected at the Sue-Dianne deposit indicate the presence of chalcopyrite, malachite, bornite, allanite, gold and hematite/magnetite in various
concentrations of the indicator mineral fraction (0.25-2 mm; SG>3.2). In till collected over or immediately down-ice of Sue-Dianne, apatite and tourmaline are relatively abundant, while gold, chalcopyrite, gahnite and Mn-epidote are present in
relatively low concentrations. Minerals present in bedrock and till elsewhere across the south-central part of the GBMZ include chalcopyrite, apatite, pyrrhotite, andradite, Mn-epidote, bornite, allanite, ferroactinolite, fluorite and gold. SEM
examination and x-ray analysis revealed trace amounts of iron oxides in gahnite, apatite, tourmaline and andradite grains (0.25-0.5 mm) from till samples collected down-ice of Sue-Dianne and other showings. Some chalcopyrite grains (0.25-1 mm) are
found in association with specular hematite. Electron microprobe analysis and further examination of selected grains will help to develop criteria that contribute vectors to IOCG mineralization.