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TitleOverview of the geology and Cr-PGE potential of the Southern Québec Ophiolite Belt
AuthorBédard, J H; Pagé, P; Bécu, V; Schroetter, J -M; Tremblay, A
SourceMineral deposits of Canada: a synthesis of major deposit types, district metallogeny, the evolution of geological provinces, and exploration methods; by Goodfellow, W D (ed.); Geological Association of Canada, Mineral Deposits Division, Special Publication no. 5, 2007 p. 433-448
Alt SeriesEarth Sciences Sector, Contribution Series 2005300
PublisherGeological Association of Canada, Mineral Deposits Division (St. John's, NL, Canada)
Mediapaper; DVD; digital
RelatedThis publication is contained in Goodfellow, W D; (2007). Mineral deposits of Canada: a synthesis of major deposit-types, district metallogeny, the evolution of geological provinces, and exploration methods, Geological Association of Canada, Mineral Deposits Division, Special Publication no. 5
File formatpdf
NTS21E/04; 21E/05; 21E/11; 21E/12; 21E/13; 21E/14; 21L/02; 21L/03; 21L/04; 21L/05; 21L/06; 21L/07; 21L/09; 21L/10; 21L/11; 21L/12; 21L/14; 21L/15; 21L/16; 31H/01; 31H/02; 31H/03; 31H/06; 31H/07; 31H/08; 31H/09; 31H/10; 31H/15; 31H/16
AreaThetford Mines; Lac Montjoie; Québec
Lat/Long WENS-73.0000 -70.0000 47.0000 45.0000
Subjectseconomic geology; stratigraphy; mineralization; mineral occurrences; mineral deposits; ophiolites; mantle; igneous rocks; intrusive rocks; chromium; asbestos; chromite; lithology; lithostratigraphy; structural interpretations; harzburgites; dunites; pyroxenites; Québec Ophiolite Belt; Laurentian Margin; Thetford Mines Ophiolitic Complex; Lac Montjoie Complex; Dunnage Zone; Humber Zone; Asbestos Ophiolite Complex; Lac Brompton Ophiolite Complex; Mont Orford Ophiolite Complex; platinum group elements; Cambrian; Ordovician; Paleozoic; Silurian; Devonian
Illustrationssketch maps; photographs; cross-sections; stratigraphic sections; plots
ProgramConsolidating Canada's Geoscience Knowledge
ProgramTargeted Geoscience Initiative (TGI-3), 2005-2010
AbstractThe southern Québec Ophiolite Belt consists of partly dismembered oceanic terranes accreted against the Laurentian margin in the Ordovician, and then reworked by Silurian and Devonian deformation. Mapping, and structural and stratigraphic analysis of the Thetford Mines ophiolitic complex (TMOC), show that many of the complexities in the outcrop patterns can be attributed to a major phase of pre-obduction, synmagmatic extension (seafloor spreading) and tectonic exhumation along paleonormal faults, with erosional degradation of tilted fault blocks. The Thetford Mines ophiolitic complex contains the bulk of the chrome-platinum-palladium mineralization that has been discovered to date in the southern Québec ophiolite belt. Refractory mantle harzburgite hosts podiform chromitite oredobies and similar chromitites occur in the Lac Montjoie Complex to the south. The platinum group element (PGE) contents of harzburgite can be simulated by equilibrium partial melting with olivine + orthopyroxene + chromite residues, implying an absence of residual sulphides. Concentrations of Pt and Pd, as well as La, Ce, and large ion lithophile elements (LILE), may be slightly enriched compared to other trace elements, suggesting metasomatic enrichment by an aqueous fluid derived from the subducting slab. The lowermost crust is dominated by dunite and interbedded chromitite layers, generally 1 to 3 cm thick, but locally exceeding 10 m. The lower crust and mantle chromitites rarely show enrichment in PGEs. The middle crust is dominated by pyroxenites and gabbros. All TMOC cumulate rocks analyzed to date have trace element geochemical signatures that imply a boninitic affinity, and are probably consanguineous with the boninitic dykes and lavas. Whole-rock data indicate systematic stratigraphic enrichment in incompatible elements and depletion in compatible elements, suggesting deposition from a fractionating body of magma. Chromitite veins and breccia fills located in crustal dunite and pyroxenite locally reach grades of 20 g/ton Pt+Pd. Detailed mapping and structural work suggest that the Starchrome Cr-PGE showing, and possibly the Hall Cr-PGE orebody, are associated with pre-obduction normal faults. At Starchrome, there is no apparent link between the abundances of PGEs and those of other incompatible elements, although there are enrichments in light rare earths in the wall rocks to the Cr-PGE ore. The chromite at Starchrome also contains potarite (a low-temperature PdHg amalgam) associated with actinolite in fractured chromite and intergrain sites. Together, these features support a model of metasomatic introduction of PGEs by hydrothermal fluids. The PGEs may have been concentrated in volatile-rich residual melts in structural traps above ponded subcrustal/ intracrustal boninitic intrusions, with subsequent escape of fluid through faults, and deposition of ore during decompression. The existence of potarite suggests that analysis of mercury in soils may represent a useful exploration tool in areas covered by Quaternary deposits. The presence in the TMOC of erosional unconformities that reach down into pyroxenitic and dunitic cumulates suggests the potential for recycling of PGE chromitites and the formation of paleoplacer-type ore within the overlying sedimentary rocks of the St.-Daniel Mélange. Mantle rocks are less voluminous in the coeval Asbestos complex (AOC) to the south, and hence this massif has less potential for podiform chromite ore. The AOC volcanic stratigraphy is polygenetic, with a tholeiitic series at the base and boninites above, which presumably correlate with those of the coeval TMOC, implying that the TMOC and AOC were obducted as a single slab, and hence, have identical PGE-exploration potential. Further south, the Mont Orford complex (MOOC) is dominated by gabbros and tholeiitic lavas, and only subordinate late boninitic dykes are found. The affinity and age of the cumulate rocks of the MOOC are uncertain and its links to the TMOC-AOC cannot yet be evaluated.

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