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Recognizing optimum Banded-Iron Formation-hosted gold environments: Preliminary results from the Meadowbank deposit S. Castonguay(1), V. Janvier(2), P. Mercier-Langevin(1), B. Dubé(1), V. McNicoll(1), M. Malo(2), S. Pehrsson, S.(1) (1) Geological Survey of Canada (2) Institut National de la Recherche Targeted Geoscience Initiative-4: Lode Gold project Main goals : 1) Provide key descriptive and genetic parameters and improved geological and exploration models to detect new (deep/buried ≥200m) deposits; Distal and proximal hydrothermal signature footprints (geological setting) 2) Develop more robust exploration vectors (signatures-footprints) and means to identify fertile mineral systems (PROCESSES) to reduce exploration risk 3) Train and mentor students to increase the HQP available to the mineral industry. Canadian Gold Production in 2011 (98 t) Canadian Gold Production in 2011 60,000 40,000 30,000 20,000 10,000 vu t un a N or ie s es tT er rit Yu ko n a um bi Co l w or th N Br iti sh Al be rta 0 N ew Pr fo in un ce dl Ed an w d ar d Is la nd N ov a Sc N ot ew ia Br un sw ick Q ue be c O nt ar io M an ito Sa ba sk at ch ew an Kilograms Au 50,000 Provinces and Territories MAC, Facts and Figures 2012 Lode Gold project: Main Themes 1) Intrusion-related and stockwork-disseminated deposits (Can. Malartic). 2) Gold fertility of major fault zones (e.g. Timmins area) 3) BIF-hosted gold deposits. Gold deposits vs BIF-hosted gold deposits (≥1Moz) BIF-hosted Au deposits are not common but have the potential for world-class size Homestake 40 Moz Au Gold deposit ≥1Moz BIF ≥1 Moz Dubé , Gosselin and Bécu, 2010 BIF-hosted gold deposits Two types of BIF-hosted mineralization : Entirely hosted within BIF: form commonly one large «isolated» deposit (Musselwhite; Meadowbank) Partly hosted in BIF and present within a district (Meliadine, Geraldton): associated with: Major fault zones Abundant quartz veins Dubé , Gosselin and Bécu, 2010 Six gold deposits have been found at the Meliadine project: Gold mineralization is mostly in laminated quartz-vein, stockwork and sulphidized BIF. BIF-hosted gold deposits: key scientific hypotheses 1. BIF–hosted gold deposits in the Churchill Province are genetically related and structurally controlled by Paleoproterozoic (1.83 Ga) tectonometamorphic events superimposed on Archean rocks. BIF–hosted gold deposits commonly form large isolated entities without an associated district. The first-order exploration criteria and main geological controls on gold concentration have to include: 2. 3. 4. major shear zone macroscopic isoclinal folds unconformity presence of black shales or ultramafic rocks The footprint of BIF-hosted gold deposit can lead to better exploration vectors, if one can differentiate: 1. 2. The hydrothermal alteration from regional or contact metamorphism. AND the nature and timing of distal vs. proximal hydrothermal alterations. Meadowbank integrated activities Deposit scale: “Geology, structure, geochemical and hydrothermal footprints and genesis of the worldclass Meadowbank gold mine” Sub-regional or “district scale”: Bridging the gap between the regional geological setting and the deposit scale knowledge Looking “outside the box” GEM Compilations and synthesis Collaboration GSC, INRS with Agnico-Eagle Mines Geological setting Meadowbank mine area lies within the Rae Domain of the western Churchill Province. The area is underlain by Neoarchean supracrustal rocks, surrounded by granitoid and gneissic complexes and regionally overlain by pre- to post Hudsonian Proterozoic basins and 1.83-1.75 granitic plutons The Woodburn Lake and Prince Albert gps form a ca. 1500 km long greenstone belt The mine area is underlain by the Archean Woodburn Lake (2.71 Ga) and the Paleoproterozoic Ketyet River gps. Davies et al., 2010 Woodburn Lake Gp and the mine area East-facing Neoarchean sequence comprising: 2.62 Ga Felsic to intermediate volcanoclastic rocks Komatiitic basalt Clastic and chemical sedimentary rocks Volcanic cycles are separated by conglomerates or/and faults A disconformity separates the volcanic sequence from the Amarulik wacke The paleoproterozoic Ketyet River Gp.: siliciclastic sedimentary and mafic volcanic-intrusive rocks 2711 Ma 2612 Ma Mine area 2612 Ma Greenschist (N) to amphibolitefacies (S) reg. metamorphism BIF’s and deposit area several BIF horizons (East, Central, West); Meadowbank deposit in the Central BIF; so far the only with economical resources Pehrsson et al., unpubl. Structural Geology Up to 7 phases of regional deformation are documented (Pehrsson et al.) p •First 2 phases are restricted to the Archean Woodburn Lake Gp.; •Mostly micro/mesoscopic evidence •Hard to differentiate in area of strong overprint. p Basal Ketyet River Gp Archean fabrics •DA1: foliation within in conglomeratic clasts (pre-2.71 Ga) •DA2: axial-planar foliation to micro/mesoscopic isoclinal folds (larger scale structures suspected), SA1-2 FP1 •reworked by Proterozoic fabrics (Dp1-2) Volcanoclatic Woodburn Lake Gp th ru st Proterozoic deformation Dp 2 •DP1: Greenschist-facies •SP1, schistosity largely subparallel to bedding/layering 2.62 Ga •commonly transposed •Isoclinal folds (FP1) •DP2: Mid-greenschist to amphibolite • Dominant regional-scale tectonometamorphic phase •N-trending crenulation to penetrative foliation (often a composite SP1-2) 2612 Ma 2612 Ma •close to isoclinal shallowly doublyplunging, NW-vergent folds •NW-directed thrusts (Third Portage Thrust) •The timing of DP2 1.91-1.83 Ga Pehrsson et al., unpubl. Proterozoic deformation Fp2 fold affecting strong Sp1 foliation Composite Sp1-2 fabric in BIF Pehrsson et al., unpubl. Proterozoic deformation •DP3: Greenschist-facies Fp4 Fp4 •chevron to similar-style folds; subhorizontal plunges; shallowmoderately-inclined axial surface •SP3 crenulation only locally developed •No apparent maps-scale influence •DP4: •Map- to outcrop-scale folds •local S4 crenulation cleavage and lineation. •Fp4 form prominent kilometre-scale, open to tight, NNE-trending upright, moderately to shallowly-plunging folds •Form an S-shaped asymmetric fold pair in the mine area Pehrsson et al., unpubl. Proterozoic deformation Shallow-inclined, sub-horizontal FP3 East BIF SP4 crenulation of SP2 Tern Porphyry Pehrsson et al., unpubl. Deposit Sequence Mostly west-dipping package, from East to West: - Intermediate to felsic volcanoclastic rock (green) (from Sherlock et al., 2001) Deposit Sequence •Banded iron Formation (red) •Ultramafic rocks, talc schist, strongly sheared near contacts (mauve) (from Sherlock et al., 2001) Deposit Sequence - Basal conglomerate, with Py-bearing IF fragments - Quartzite, with fuschite and BIF fragments (yellow) (from Sherlock et al., 2001) Mineralization • Two main orebodies lie along the same horizon: • Portage (N) and Goose (S) pits • The ore zones are typically 6-7 m wide • 5 up to 40 g/t Au • hosted in the BIF, predominantly along the contacts with ultramafic rocks. (from Sherlock et al., 2001) Mineralization •The predominant gold mineralization is associated with pyrrhotite and pyrite replacing magnetite sub-parallel to BIF layering Mineralization • or narrow stringers of disseminated sulphides. Mineralization Gold also is found in: •Py-Po veins •high-grade quartz veins (up to 300 g/t) crosscutting the BIF and footwall volcanoclastic rocks. •Suggest a component of remobilization Published and actual mine models B B’ Geometry and structural controls: fold interference pattern Fp1-2/4 Mineralization in strained lower limb, within BIF Fp4 B UM B’ Fp4 Sherlock et al., 2004 UM Fp1-2 Sherlock et al., 2001 Mine model New detailed mapping in Portage Pit Portage south ramp (West face) UM quartzite BIF IV Fold model Mapping suggest fault imbrication by two Dp2 N-S shear zones preferentially developed within the ultramafic units. Role of faulting is underestimated New detailed mapping in Portage Pit Portage south ramp (West face) UM quartzite BIF IV Next slide These shear zones appear to truncate the ore zones, and possibly control their distribution. New detailed mapping in Portage Pit IV IF UM IV Portage south ramp (West face) Eastern shear zone cut S0 and S1-(2?) in the footwall sequence Timing syn- to late- Dp2 New detailed mapping in Portage Pit Portage centre (looking North) 1m Ultramafic –hosted shear zones cut mine sequence; New detailed mapping in Portage Pit UM BIF quartzite MV IV Next slide Portage North: Two quartzite horizons: suggesting Dp1-2 fold or fault imbrication Megascopic Fp4 fold folding mine sequence and shear zones Marks the end of the Portage orebody FV Portage North (looking West) New detailed mapping in Portage Pit Portage North (looking West) 1m Portage North: megascopic Fp4 fold; Fp3-4 fold interference Level mapping using drill sections; AEM database 2 Some key observations: •Early isoclinal folding (Fp1-2) •Fault imbrication of mine sequence •Late sinistral faults (Dp5) 1 1 2 3 3 Portage Portage South section (N6100) Fp1-2 N6100 Level 5090 Fp4 1 2 3 Portage Goose N6100 Level 5060 1 Goose 3 Portage Dp5 fault New study of drill sections: detailed description and geochemical analyses Portage South (section N6175) •5 sections selected along the deposit •3 of 5 sections studied; 15 drill holes; 400 geochem samples (data interpretation ongoing) •Possible zonation of grunerite and chlorite •Different UM units; amphibole porphyroblasts only in eastern unit •Core angle analysis suggests asymmetric folding Preliminary implications and outstanding questions •Role and timing of faulting vs. mineralization Third Portage Trust •Third Portage Thrust southern extension ? •Fault zones and UM not mineralized: • reactivated early fertile faults ? •Role of pre-DP2 structure: • “mineralizing events” or « ground preparation » Mine area Pehrsson et al., unpubl. 2013-2014 objectives: wish list Continuing detail mapping of Goose pit Improve the tectonostratigraphic framework with geochronology: key sedimentary units: characterize unconformities or document structural repetition Key volcanic or intrusive phases Structural geology: Role of pre-DP2 structural history: “mineralizing events” or « ground preparation » Characterize the main DP2 “Third Portage” fault zone and its probable splays; Relationship with the mineralization, unconformities and volcanic cycles (repetition?) Investigate if post-mineralization deformation contributed to structural controls or secondary ore enrichment. Characterize the hydrothermal signature of mineralization Integration of deposit-scale data with regional knowledge to produce the deposit footprints Ultimately, extract « generic » exploration vectors to test terrane fertility for BIFhosted gold mineralization and reduce exploration risk in the Churchill Province and beyond … Acknowledgments: Agnico-Eagle Mines: Meadowbank and exploration divisions CNGO Nunavut Research Institute