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TitleHydrothermal alteration mineralogy and geochemistry of the Archean world-class Canadian Malartic disseminated-stockwork gold deposit, southern Abitibi Greenstone Belt, Quebec, Canada
AuthorDe Souza, S; Dubé, B; Mercier-Langevin, P; McNicoll, V; Dupuis, C; Kjarsgaard, I
SourceEconomic geology and the bulletin of the Society of Economic Geologists vol. 114, no. 6, 2019 p. 1057-1094,
Alt SeriesNatural Resources Canada, Contribution Series 20180016
PublisherSociety of Economic Geologists
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
NTS32C/04; 32D/01
AreaMalarctic; Val d'Or
Lat/Long WENS -78.2333 -77.9833 48.1833 48.0833
Subjectseconomic geology; mineralogy; geochemistry; geochronology; Science and Technology; Nature and Environment; Archean; mineral exploration; mineral deposits; replacement deposits; gold; mineralization; tectonic setting; orogenesis; tectonic history; hydrothermal alteration; magmatism; intrusions; emplacement; sedimentation; metasomatism; metamorphism; crystallization; host rocks; bedrock geology; lithology; metamorphic rocks; greenschist facies; sedimentary rocks; greywackes; mudstones; igneous rocks; intrusive rocks; monzodiorites; granodiorites; breccias; structural features; fault zones; fractures; cleavage; stockworks; quartz veins; ore grades; radiometric dating; strain; Abitibi Greenstone Belt; Canadian Malartic Deposit; Larder Lake-Cadillac Fault Zone; Pontiac Group; Timiskaming Assemblage; Sladen Fault Zone; Precambrian
Illustrationslocation maps; geoscientific sketch maps; photographs; cross-sections; core logs; tables; photomicrographs; plots
ProgramTargeted Geoscience Initiative (TGI-5) Gold systems
Released2019 09 01
AbstractThe Canadian Malartic stockwork-disseminated gold deposit is an Archean world-class deposit located in the southern Abitibi greenstone belt. It contains over 332.8 tonnes (t; 10.7 Moz) of Au at a grade of 0.97 ppm, in addition to 160 t (5.14 Moz) of past production (1935-1981). Although the deposit is partly situated within the Larder Lake-Cadillac fault zone, most of the ore occurs up to ~1.5 km to the south of the fault zone. The main hosts of the mineralized zones are greenschist facies turbiditic greywacke and mudstone of the Pontiac Group (~2685-2682 Ma) and predominantly subalkaline ~2678 Ma porphyritic quartz monzodiorite and granodiorite. These intrusions were emplaced during an episode of clastic sedimentation and alkaline to subalkaline magmatism known as the Timiskaming assemblage (<2680-2670 Ma in the southern Abitibi). The orebodies define two main mineralized trends, which are oriented subparallel to the NW-striking S2 cleavage and the E-striking, S-dipping Sladen fault zone. This syn- to post-D2 ductile-brittle to brittle Sladen fault zone is mineralized for more than 3 km along strike.
The ore mainly consists of disseminated pyrite in stockworks and replacement zones, with subordinate auriferous quartz veins and breccia. Gold is associated with pyrite and traces of tellurides defining an Au-Te-W ± Ag-Bi-Mo-Pb signature. The orebodies are zoned outward, and most of the higher-grade (>1 ppm Au) ore was deposited as a result of iron sulfidation from silicates and oxides and Na-K metasomatism in carbonatized rocks. The alteration footprint comprises a proximal alteration envelope (K- or Na-feldspar-dolomite-calcite-pyrite ± phlogopite). This proximal alteration zone transitions to an outer shell of altered rocks (biotite-calcite-phengitic white mica), which hosts sub-ppm gold grades and reflects decreasing carbonatization, sulfidation, and aNa+/aH+ or aK+/aH+ of the ore fluid.
Gold mineralization, with an inferred age of ~2664 Ma (Re-Os molybdenite), was contemporaneous with syn- to late-D2 peak metamorphism in the Pontiac Group; it postdates sedimentation of the Timiskaming assemblage along the Larder Lake-Cadillac fault zone (~2680-2669 Ma) and crystallization of the quartz monzodiorite. These chronological relationships agree with a model of CO2-rich auriferous fluid generation in amphibolite facies rocks of the Pontiac Group and gold deposition in syn- to late-D2 structures in the upper greenschist to amphibolite facies. The variable geometry, rheology, and composition of the various intrusive and sedimentary rocks have provided strain heterogeneities and chemical gradients for the formation of structural and chemical traps that host the gold. The Canadian Malartic deposit corresponds to a mesozonal stockwork-disseminated replacement-type deposit formed within an orogenic setting. The predominance of disseminated replacement ore over fault-fill and extensional quartz-carbonate vein systems suggests that the mineralized fracture networks remained relatively permeable and that fluids circulated at a near-constant hydraulic gradient during the main phase of auriferous hydrothermal alteration.
Summary(Plain Language Summary, not published)
The large Archean Canadian Malartic gold deposit, located in the southern Abitibi greenstone belt, contains 303.3 t (10.7 Moz) of Au at a grade of 0.97 ppm in total open pit reserves and production, in addition to 160 t (5.5 Moz) of Au produced between 1935 and 1981 from three separate undergound operations. Although the deposit is partly located within the Larder-Lake - Cadillac fault zone (LLCfz), most of it is located up to ~ 1.5 km to the south of the fault zone. The ore mainly consists of disseminated pyrite in stockworks and replacement zones, with subordinate auriferous veins and breccia. Gold is associated with pyrite and traces of tellurides, molybdenite, and galena as well as a complex hydrothermal alteration The setting and intrinsic characteristics of the Canadian Malartic deposit broaden the exploration potential in sedimentary rocks along the LLCfz and other similar major faults, and contribute to improved exploration models in such complex contexts.

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