Gold in the Early Paleoproterozoic Birimian Shield, West ... - ADIMB

GEOLOGYBRGM 2005Siguiri Basin3.3-2.63GaMan Craton???ArcsClasticsAlluvials200 kmComoe Basin???Tarkwa

Protracting accretion/gold districtsTectonic cyclicity createdelaborate protractingstructural network2060 Ma2150 Ma1500 km of recycled crust addedfrom 2195 Ma (SE Ghana) to2067 Ma (W Mali)200 km

Main gold settings• Mesothermal orogenic gold along regionalstructures affecting mainly back arc belt-basinshear zones (Obuasi-Prestea, Akyem, Bibiani,Ahafo, Konkera);• Volcanic and shallow plutonic felsic bodies alongactive strike-slip regional faults related to pullapartbasins (Angovia);• Early hydrothermal activity and/or shallowplutonism along normal faults (Siguiri basin);• Paleoplacer conglomerates in a possible orogenicforeland basin (Tarkwa).

West African Gold districtsHigher grade trends91 deposits > 10 t> 4000 occurrencesModified from Billa, 2011

Northern Districts (~100 Moz)Northern DomainKenieba inlierSiguiri Basin250 mT+1000 mT+Red Lake+500 mT+Noranda

Southern Districts (~200 Moz)6000 mT+2.5 x Timmins

ObuasiSouthern Ashanti BeltBogosoPresteaWassaMajorBreaksTarkwaPaleoplacer• Like the Archean Timiskaming it isan alluvial-fluvial sequence• Delineates major “Breaks”• Suture basinsPerrouty et al, 2011

Polyphase deformation Ashanti beltBogosuBuesichemWassaPresteaWassa20 kmModified from Perrouty et al, 2011

BUVTDD013:27.5 m @ 3.7g/tSBuesichem (SW Ashanti Belt)Au Grade Thickness Long Section True WidthBUVTDD037:29.6 m @ 3.1g/tBUVTDD079:31.2m @ 3.0g/tBUVTDD016:25.4 m @ 4.0g/tBUVTDD0156:21.0m @ 3.4g/tBUVTDD092:13.6m @ 6.7g/tCurrent PitBUSRC028:25.5m @ 2.8 g/tN2011YE PitDesignBUVTRC047:20.0m @ 4.9g/tBUVTDD031:6.2m @ 2.1g/tBUVTDD149:19.6m @ 4.9g/tBUVTDD0177:8.7 m @ 2.7g/t BUVTDD053:28.0 m @ 2.1g/tBUVTDD176:‣ Left-lateral flexure6.1 m @ 3.9g/t‣ Bend dilationDrill Target‣ South plunging shootsBUVTDD098:27.5m @4.0g/tBUVTDD086:24.7m @ 2.69g/tBUVTDD038:9.3 m @ 8.2g/tBUVTDD089:8.6m @ 7.9g/tBUVTDD148:9.8 m @ 16.6g/tBUVTDD0152:32.5m @ 3.8g/tBUVTDD144:22.3m @ 9.3g/tBUVTRC085: 28.5m @ 4.0g/tTypical Ashanti shear oreBUVTDD0127:24.1m @ 3.9g/t$1300$1500Pit ShellsRamp TargetBuesichem Grade Thickness(g\t)*mCourtesy M. Wasel, Golden Star Resources

Sefwi Belt• Ahafo on north side of Sefwigreenstone belt on the east marginof the broad Comoe Basin filledprimarily by epiclastics.• Complex package of back-arc maficvolcanic rocks, sediments, andintrusive stocks (Birimian). Extremerheological contrasts.• Northwestly thrusting alongKenyasi fault with major offsets.Steep metamorphic gradients.• Multiple ore shoot geometries thatare generally shallow plunging• Bibiani and Chirano deposits about50km south in similar settingCourtesy D. Finn, Newmont14

• Meta-sediments(carbonaceous shales)and volcanics to north(footwall) withintrusives south(hangingwall)Ahafo South Structure(Schematic Block Diagram)• Hangingwall splaysare important forlocalizing higher grade• Multiple shootdirections requiredetailed mappingDioriteFeybesse et al. 2006Courtesy D. Finn, Newmont

Konkera, (Boromo Belt) SW BurkinaLargest undeveloped resource of Burkina*Ampella Mining Ltd Metelka et al., 201142.6mt @ 1.9 g/t = 2,6 Moz Au

Imbricate Thrust systemWest50 m Main gold bearing unit:Fresh Arenite(35% kspar-diss Tm)Higher vein density= more (coarse) AuEssakane (Burkina Faso, Northern Domain)Pohl, 1995

Common thrust styles*• Fault propagation folds• Anticlinal hanginwall folds• Imbricated thrust systems• Thrust duplexes (simple, complex)• Shear couple systems (C-S type geometry)• Single shear* Most settings have comparable geometries in regional map view.3D geometry is usually resolved by detailed mapping and generallyconditioned by rheology of lithostratigraphic assemblages as well asby regional and local state of strain.

Common themes‣ All mesothermal orogenic gold settings are proximal to major firstorder breaks with protracting structural histories.‣ Most settings are affected by thrust geometries conditioned bydeep-seated basement conditions as well as local stratigraphiccomponents.‣ A large number of mesothermal gold settings are hosted in clasticsediments.‣ Volcanic hosted settings and much less frequent then in theArchean.‣ Gold grades are generally diluted in clastic rocks with an averagegrade of 1.1gpt Au in the fresh rock.‣ Gold events protracted through time across the entire shield.‣ Gold is mostly late but early gold is clearly present.‣ Carbonaceous shales are recurrent in a majority of settings.

Guiana Shield‣ At least 14 arcs identified todate with arc andback-arc signatures.‣ Granitoids are generallypoorly understoodModified from Bardoux et al. 1998

frequencyU-Pb zircons Eastern Guiana ShieldFrench Guiana2140 Ma2180 MaAmapaPaleoproterozoic QuartziteSE French Guiana700 1200 1700 2200 2700 3200MaModified from Bardoux et al. 1998

Archean Greenstones in SE Guiana ShieldAMAPAModified after Ricci et al. 200320 km

TransAmazonian Tectonic FrameworkFrench Guiana (supracrustals)ConglomeratesEpiclasticsArc volcanicsFrench GuianaGabbrosMajor BreaksUltramaficsGranitoids-gneissesMain Au depositsArchean100 kmBrazil3.3 Ga detrital Zr

Primary Gold Occurrences, Guiana Shield1-2 gpt/AuRosebelMore than 700gold occurrencesModified from Voicu et al., 2001

Rosebel GeologicalCross-sectionBridging basin13 Moz +SubductionBack-Arc‣ Regional synclinorial (bridging) basin‣ Major reverse shear in the South Domain‣ Major dextral slip faulting in North Domain‣ Late normal movement‣ 2 phases of ductile deformation (pre- and syn-Au)Volcanism between North and Southdomains recorded a change in tectoniccontext from a backarc extensional basin tosubduction dominated environments.Courtesy of C. Daoust and G. Voicu, Iamgold

AshantiAhafoEl Callao districtRosebelPrestea-BogosoDamangSiguiriEssakaneSyamaEsaaseGounkotoTongonSubikaBomboreChiranoMassawaKonongoNzema (Salman)BonikroPaul IsnardAmapariSaramaccaYongaKomanaSalamangoneLa CamorraTaperkoSophie-RepentirSaint-ElieBenzdorpGouloumaMassatoFish CreekKwabengEagle MountainTiekoumalaKuncheKaburiKinieroSian-PrasoNampalaKofiTingaBepkongFrom dwarfs to Giants25002000150010005000~mT AuMore than 120significant orebodies Comparable endowment to: Broad range of settings• Abitibi Emerging new districts• Yilgarn Persistent 2004 Exploration• Carlin 1998

Early Paleoproterozoic orogens ofAfrica and South AmericaReguibat2.25 to 2.05 Ga(Rhyacian)BirimianGuianaOgoué-Sao Francisco2.2 to 2.0 Ga2.0 to 1.7 GaWestern AmazonUbendian2.175 to 1.750 GaModified from Billa 2011

Obrigado, Thanks, Merci• Gema Olivo, Onildo João Marini for the invitationto SIMEXMIN V• Michel Gauthier, Carlos Bertoni and Newmont forstimulating my interest toEarly Paleoproterozoic Gold