OROGENIC GOLD TARGETING What is important and why? Using the five questions approach This approach identifies the processes that form the gold deposit (it does not describe the deposit) across the full range of scales OROGENIC GOLD TARGETING Criteria 1 Fractionated basalt sequence often hosting nickel sulphide deposits in a back arc basin setting Basalts are derived from magma crystallising magnetite in the source region Fractionation of magnetite adds oxygen (O) to the SubContinental Lithospheric Mantle (SCLM) This event can pre-date the gold event by tens to hundreds of millions of years Kalgoorlie Geochronology 2720 2700 2680 2660 2640 Thol. Basalt +/- UM volcanism Komatiite TTG volcaniclastic (Black Flag) Bimodal (Teutonic) dolerite Coarse clastic High-Ca Granite Low-Ca Granite Mafic Granite De De Ni D2 VMS Au Laverton Geochronology 2820 2800 2780 2760 2740 2720 2700 2680 2660 2640 Thol. Basalt +/- UM volcanism Calc-alk andesite Komatiite/Thol. Basalt Volcaniclastic Coarse clastic High-Ca Granite Low-Ca Granite Mafic Granite Ni Au WINDARRA ADMIRAL HILL Syenite Ni AGNEW Au GRANNY SMITH, SUNRISE, WALLABY OROGENIC GOLD TARGETING Criteria 2 Boundaries of cratonic blocks are faults that extend to crust mantle boundary Large scale continental collision Subduction adds water (H2O) back into the SCLM that was lost during mafic magma extraction S-O-H2O are necessary pre-cursors to create thiosulphate complexes that will carry gold from the mantle Backarc/Peri-cratonic: VMS & CD base metal, Ni sulphide VMS Base Metal: Melting of metasomatised CLM (metal source) Most favored if spreading ridge migrates into CLM Moderate preservation potential if peri-cratonic basin narrow Graham Begg, 2011 Back-arc/Peri-cratonic : Orogenic Au Orogenic Au: Flanked by metasomatised CLM (metal source) Closure and suturing leads to selective melting of source Reduced sediments are a good host rock High deposit preservation potential Graham Begg, 2011 OROGENIC GOLD TARGETING Criteria 3 Late basin develops after first collisional event Basin inversion during second compressive deformation creates large scale anticline in the hanging wall of mantle tapping fault Source of reductant CH4 (methane) is black shale within the late basins and methane drives gold deposition by destabilising the thiosulphate complex REGIONAL SEISMIC CROSS SECTION KALGOORLIE 100moz GINDALBIE TERRANE KALGOORLIE TERRANE Kunanalling Shear Bullabulling Dunnsville Shear Anticline Avoca Fault Scotia-Kanowna Anticline Mt Monger Fault Emu Fault {does not outcrop in this section because it is intruded by the Arcoona Granites} Arcoona Granite 5 Basal detachment ? 10 V =1 H 10km Upper basalt Undivided basalt Early granite Felsic gneiss Felsic volcanic rocks Lower basalt Komatiite Late granite Basal felsic schist Felsic volcanic unit CDP 7200 0 9200 11200 13200 Greenstone sequence (in Bardoc Shear Zone) GINDALBIE TERRANE KALGOORLIE TERRANE 15200 KURNALPI TERRANE 17200 5 Two-way time (s) Depth (km) IDA FAULT Kurrawang Syncline Mt Pleasant BARDOC Zuleika Anticline Deformation Zone Shear KURNALPI TERRANE 10 15 0 20 10 km MID CRUST ANOMALY V/H = 1 (approx) for a Velocity of 6.0km/s OROGENIC GOLD TARGETING Criteria 4 Mafic granites are High Calcium suite granites that have a mantle component added to the magma chamber at the base of the crust Granites are emplaced along mantle tapping faults to high crustal levels (basement cover sequence boundaries) and are emplaced into large anticlines These granites carry gold rich fluids from base of the crust to upper crustal levels then exsolve fluids at fluid saturation at lower pressure Non-reflective zone; intrusive complex that produces the gravity-low. Seismic section courtesy of Ned Stolz, Gold Fields, St Ives Interpretation of Seismic Section Paringa Basalt Paringa Basalt Porphyry Complex Tripod Hill Komatiite Lunnon Basalt Lower Felsic Complex KD3011 607m, 1.23g/t Anhydrite vein Disseminated pyrite Significant Au and pyrite content in porphyries that are not notably altered; => Fluid is in equilibrium with the porphyry. Some of the St Ives porphyries are inherently enriched in Au and S 3D Model of the Beta Porphyry 1billion tonnes mean gold grade = 0.37g/t ~ 12 million ounces Footwall porphyries commonly have 100mt to 1,000mt of porphyry @ 0.05 to 0.5 g/t Au, e.g. Victory, Revenge, New Celebration, Binduli, Mt Pleasant, Kundana, Granny Smith, Lancefield, Mt Morgans, Agnew, etc All gold-rich porphyries have the same enriched trace element signature!!! FI 7, Beta Porphyry Interplay of fluids-architecture Revenge through Victory-Defiance After Ned Stolz & Janet Tunjicja OROGENIC GOLD TARGETING Next Steps Testing of regional targets with multi-element geochemistry at low density (1 sample per 16 km2) will identify mineralised systems Darlot, 4m oz Agnew, 5m oz Thunderbox, 2m oz Sons of Gwalia, 10m oz Tarmoola, 2m oz Scale 330km by 150km 3000 samples OROGENIC GOLD TARGETING Mineral systems have characteristic spacing of gold deposits within the system Archaean gold systems in the Yilgarn are defined by 60km by 60km boxes within which deposits are spaced at 30km The crust is 30km thick at Kalgoorlie and Laverton The gold systems are spaced at 130km OROGENIC GOLD TARGETING What is important and why? Using the five questions approach OROGENIC GOLD TARGETING