Exploration for orogenic gold deposits – with ... - Vuorimiesyhdistys

WORKSHOPExploration for orogenicgold deposits – with emphasis ongeochemical exploration inglaciated Precambrian terrain

Exploration for orogenic gold deposits –with emphasis on geochemical exploration inglaciated Precambrian terrainWorkshop, 21 August 201125th International Applied Geochemistry Symposium 201122-26 August 2011 Rovaniemi, FinlandPasi Eilu, V. Juhani Ojala and Pertti SaralaPublisher: Vuorimiesyhdistys - Finnish Association of Mining and MetallurgicalEngineers, Serie B, Nro B92-6, Rovaniemi 2011

Eilu, P., Ojala, V.J. and Sarala, P. 2011. Exploration for orogenic gold deposits – with emphasis on geochemicalexploration in glaciated Precambrian terrain. Workshop in the 25th International Applied GeochemistrySymposium 2011 22-26 August 2011 Rovaniemi, Finland. Vuorimiesyhdistys, B92-6, 88 pages.Layout:Cover – Irma VarrioISBN 978-952-9618-74-3 (Printed)ISBN 978-952-9618-75-0 (Pdf)ISSN 0783-1331© VuorimiesyhdistysThis volume is available from:Vuorimiesyhdistys ry.Kaskilaaksontie 3 D 10802360 ESPOOElectronic version:http://www.iags2011.fi or http://www.vuorimiesyhdistys.fi/julkaisut.phpPrinted in:Painatuskeskus Finland Oy, Rovaniemi

Exploration for orogenic gold deposits – with emphasis ongeochemical exploration in glaciated Precambrian terrainPasi Eilu 1 , V. Juhani Ojala 2 and Pertti Sarala 11 Geological Survey of Finland, E-mail firstname.lastname@gtk.fi2 Store Norske Gull, E-mail firstname.lastname@snsk.noAbstractAn orogenic gold deposit is a structurally controlled gold occurrence formed duringone of the major stages of an orogeny by orogenic fluids. Any rock type withina greenstone or schist belt, a metamorphosed supracrustal rock, dyke, or intrusionwithin or intrusion bounding such belt may host an orogenic gold deposit. There isstrong structural control of mineralization at a variety of scales but the favoured hostis typically the locally most reactive and/or most competent lithological unit. Thesedeposits are not present in post- or anorogenic intrusions or unmetamorphosed supracrustalrocks. Most of the Precambrian deposits were formed during 2.70–2.60Ga and 2.10–1.70 Ga. These epochs appear to be related to rapid crustal growth andaccretionary stages of supercontinents. Mineralisation typically takes place duringthe last major stage of an orogeny. The late timing of this deposit type to provideimportant geological aids in exploration as the geometry of the geological units hasnot significantly changed after the mineralisation. Hence computer aided methodslike geomechanical stress modelling can be utilised to model structurally favourablesites.In greenschist facies settings, mineralisation typically takes place slightlyafter the metamorphic peak, but in amphibolite facies at the local regional-metamorphicpeak. The ore bodies typically have a strongly flattened ellipsoidal shape, areplate-like, may have a steep or a gentle dip and plunge of ore shoots. An ore bodycan be 0.5–50 m wide, 100 m – 2 km long, consisting of a vein network, an en echelonvein swarm, or just of one single large vein. The depth extent of an ore bodymay well be much larger than its extent along strike. An individual vein can be 1cm – 10 m thick and 20–1000 m long. In most cases, gold occurs as native gold, freein gangue and with main sulphides, and as inclusions and in fractures of gangue andsulphide grains. In a few cases, most of gold is in the lattice of or submicroscopicinclusions in pyrite or arsenopyrite.All deposits are developed by an alteration halo characterised by proximalto distal carbonatisation and proximal sericitisation or biotitisation. Also, proximalsulphidation may be distinct if the host rock is iron rich. Elements enriched typicallyinclude As, Au, CO2, K, Rb, S, Sb, Te, W; in some cases also Ag, B, Bi, Co, Cu,and Se are enriched. The Au/Ag is consistently >1, typically 5–10. Enrichment ordepletion of Ca, Fe, Mn or Mg are non-existent, and Na mobility, if present, is minorand spatially restricted to the ore itself. Alteration mineral assemblages, alterationindices based on CO2 and K, and trace (pathfinder) elements enriched in the depositscan be used in defining exploration targets and vectors to ore in bedrock.Surficial geological, till geochemical and indicator mineral studies are effectivemethods in gold exploration in glaciated terrains. By test pit surveys and stratigraphiccontrolled till geochemical and heavy mineral sampling glacial transportdistances and mechanisms, secondary element dispersions, and ice flow directionscan be estimated. Strong changes in glacial dynamics and erosional and depositionalconditions lead to a variable degree of preservation of earlier deposits and pre-Quaternaryregolith, and deposition of complex glacigenic formations.

6Workshop ProgramSunday, 21 August 2011, Hotel Santa Claus, Rovaniemi8:30-9:00 am Registration9:00 am Overview of the orogenic gold deposit type, Juhani Ojala9:45 am Reference to other types of gold deposits in shield areas, Juhani Ojala10:30 am Coffee break10:45 Alteration and geochemical dispersion related to orogenic gold, Pasi Eilu12:00 Lunch13:15 Brief overview to the surficial geological and geochemical exploration for goldin glaciated terrains, Pertti Sarala14:00 Discussion14:30 Coffee14:45-16.00 Ore and alteration zone samples, Pasi Eilu

7Overview of the orogenic golddeposit typeJuhani OjalaStore Norske Gull ASHeavily based on talks by D. I. GrovesTalk outline• Nature and tectonic setting of orogenic gold• Lithospheric scale energy sources andprocesses• Timing of orogenic gold systems• Crustal continuum model• Orogenic gold mineral system• Structural and host rock controls• Endowment• Conceptual targeting and GIS

8A SIMPLIFIED OROGENIC GOLDMINERAL SYSTEM1. SIMPLIFIED MINERAL SYSTEM2. RHEOLOGICAL CONTRASTS AND HOST ROCKCONTROLS3. STRUCTURAL AND GEOMETRICAL CONTROLSa) Supracrustal Beltsb) Role of Granitoids and Other Rigid Bodies4. PRODUCTIVE vs POORLY-ENDOWED TERRANESHostrockStructuralpermeabilityChannelway(shear zone)Fluid focussingSourcerockAnatomy of a Hydrothermal System

97Epigenetic Gold Deposits in Orogenic Belts• Orogenic gold deposits (e.g. Kalgoorlie, Ashanti)• “Intrusion-related” gold deposits (e.g. Fort Knox)• Overprinted porphyry deposits (e.g. McIntyre,Boddington?)• Overprinted VMS (Bousquet, Bulyanhulu)Includes “greenstone-hosted”, “slate-belt hosted”,“Mother lode-style” etc89

10Features Common to Majority ofOrogenic Gold Deposits1. At or near terrane boundaries (or other crustalscalefaults/ shear zones).2. Strong structural control in lower-order structures.3. Large vertical extent with subtle vertical zonation.4. Typically K-mica and carbonate alteration atgreenschist facies.5. Characteristic addition of SiO 2 , K, Rb, Ba+Na+B.6. Characteristic ore metals : Au+Ag+As+Sb+Te+Wwith low Pb-Zn-Cu.7. Low salinity H 2 O-CO 2 +CH 4 ore fluid.8. Radiogenic and stable isotope signatures indicatemixed sources. 10FACTORS CRITICAL TO FORMATION OFWORLD-CLASS OROGENIC DEPOSITS1. HIGH FLUID FLUXa) Thick competent host rocksb) Strong contrasts in rock strength (rheology)c) High structural permeability in failed units or fault/shear zonecontacts2. EFFICIENT GOLD DEPOSITIONa) Reactive host rocks (high Fe/Mg or C?)b) Phase separation (lower H+, CH4 and H2S in residual fluid)c) Fluid mixing (note problems of mixing: limited fluid reservoirs;high fluid pressure; confusion with reaction with previouslyaltered rocksCRITICAL CHEMICAL FACTORS FOR LARGETONNAGE - OROGENIC GOLD DEPOSITS1. REACTIVE HOST ROCK TO PRODUCE DISSEMINATED GOLD RESOURCE (INADDITION TO HIGH-GRADE VEINS)2. FLUID IS H2O – CO2 ± CH4 WITH VERY MINOR (

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13A CRUSTAL THICKENINGAccretedoceanic crustHg-Sb ArcAuB PLUME IMPACT/SUBDUCTIONAuMantleplumeheadC SUBDUCTION ROLLBACKExtension incontinental crustAuDOCEANIC RIDGE SUBDUCTIONExtensionAuSlabrollbackAsthenosphereupwellingAsthenosphereupwelling19EEROSION OF MANTLELITHOSPHEREF DELAMINATION OF MANTLELITHOSPHEREAuAuAuAsthenosphereupwellingAsthenosphereupwellingGA GreenschistamphibolitetransitionFault withmovementvectorGranitoidsAccretedcontinental crustStablecontinental crustMantleplumeMantlelithosphereAsthenosphereOceanic crust2021

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1525Youngest rocks hosting gold mineralisationGranny SmithMt SheaPorphyryJundeeKanowna Belle (pre-main Au event)Mt MorgansAges of gold mineralisationKanowna Belle (minor)Golden MileEasternMt CharlotteGoldfieldsVictory, KambaldaProvinceChaliceOtherProvincesReedysMarymiaOldest rocks syn to post gold mineralisationScotiaMt GibsonWestonia OtherGriffin’s FindNevoriaCorinthiaMatilda, WilunaBig BellMt GibsonU-Pb in zirconU-Pb in titaniteU-Pb in monaziteRe-Os in molybdeniteAr/Ar in micaSm-Nd in garnetPb-Pb in pegmatitePb-Pb in sulphideLawlers (minor)East Lode, WilunaEastern GoldfieldsScotia ProvinceProvinces2680Ma 2660 2640 2620 2600 2580 2560Ma2627

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2246The Situationin 1990kg x 10 312501000750500250Golden Mile (Kalgoorlie)Central NorsemanSons of GwaliaMt Charlotte (Kalgoorlie)Wiluna-MoonlightHill 50 - Mt MagnetBoddington - HedgesGreat Fingall - Golden crown (Day Crown)Paddys Flat (Meekatharra)Kambalda - St IvesLancefield (Laverton)Big Bell (Cue)Lady Shenton - Crusoe - Princess May (Menzies)Copperhead (Bullfinch)Emu (Lawlers)kg x 10 31000ABITIBI750500250YILGARN4748Hollinger-McIntyre-Coniaurum:TimminsDome : TimminsKerr Addison : Larder LakeCampbell : Red LakeLake Shore : Kirkland LakeWilliams : HemloGolden Giant : HemloWright - Hargraves : Kirkland LakeLamaque : Val D’OrEdna May (Westonia)PaddingtonReedyWestralia (Mt Morgans)New CelebrationOroya Black Range (Sandstone)YouanmiFrasers (Southern Cross)Bayleys (Coolgardie)BurbanksHannans North (Kalgoorlie)Cosmopolitan (Kalgoorlie)Granny Smith (Laverton)Dickinson : Red LakePamour : TimminsSigma : Val D’OrDavid Bell : HemloDoyon : BousquetMacassa : Kirkland LakeEast Malartic : MalarticMadsen : Red LakeBousquet : BousquetAunor : TimminsMalartic : MalarticSylvanite : Kirkland LakeHallnor : TimminsCamflo : Val D’OrPreston : TimminsDetour : Lake DetourPickle Crow : Pickle LakeMacleod-Cockshunt : GeraltonAgnico-Eagle : JouielThe Situationin 2000kg x 10 31250kg x 10 31000750500250Hollinger-McIntyre-Coniaurum:TimminsDome : TimminsKerr Addison : Larder LakeCampbell : Red LakeLake Shore : Kirkland LakeWilliams : HemloGolden Giant : HemloWright - Hargraves : Kirkland LakeLamaque : Val D’OrDickinson : Red LakePamour : TimminsSigma : Val D’OrDavid Bell : HemloDoyon : BousquetMacassa : Kirkland LakeEast Malartic : MalarticMadsen : Red LakeBousquet : BousquetAunor : TimminsMalartic : MalarticSylvanite : Kirkland LakeHallnor : TimminsCamflo : Val D’OrPreston : TimminsDetour : Lake DetourPickle Crow : Pickle LakeMacleod-Cockshunt : GeraltonAgnico-Eagle : JouielABITIBI1000750500250Golden Mile (Kalgoorlie)PlutonicSunrise/CleoJundeeWallabyKanowna BellBronzewingCarosue DamCentral NorsemanTarmoolaSons of GwaliaMt Charlotte : KalgoorlieWiluna-MoonlightHill 50 - Mt MagnetBoddington - HedgesGranny Smith : LavertonGreat Fingall - Golden crown : Day CrownYILGARNPaddys Flat : MeekatharraKambalda - St IvesLancefield : LavertonThunderboxBig Bell (Cue)Lady Shenton - Crusoe - Princess May:MenziesCopperhead (Bullfinch)Emu : LawlersEdna May (Westonia)PaddingtonReedyWestralia : Mt MorgansNew CelebrationOroya Black Range : SandstoneYouanmiFrasers : Southern CrossBayleys : CoolgardieBurbanksHannans : North (KalgoorlieCosmopolitan : Kalgoorlie

23GIS modellingModellingDigital elevationmodel and basemapsTill geochemistryBedrock mappingGravitySatellite imagesQuaternary geologyAirborne geophysics•magnetic•electro-magnetic•gamma radiationWeights of Evidence orogenic gold model,Combined Empirical/Conceptual WofECombined empirical/conceptual weights-of-evidencemodel CLGBClass Area km 2 Area %Sites W+ W- Contrast s(C) ConfidenceVery high 403 2.2 6 2.1128 -0.1726 2.2854 0.4527 5.0482High 517 2.8 12 2.5653 -0.4078 2.9731 0.3617 8.22Moderate 2493 13.4 10 0.7903 -0.2051 0.9954 0.377 2.6404Low 635 3.4 0 0 0 0 0 0Very low 14593 78.3 6 -1.4911 1.3379 -2.829 0.4502 -6.2837total 18642 100.0

24GIS conclusions• the models predict areas with high potentialfor orogenic Au mineralization in the CentralLapland Greenstone Belt-> considerable reductiong of the area to beexplored (less than 1% of the original studyarea)DISTAL VS PROXIMAL SOURCE MODELS FOROROGENIC GOLD DEPOSITSB. Salier (2003)53σ1SIMPLE MINERAL SYSTEM MODELARCHAEAN OROGENIC GOLD DEPOSITSSub –GreenschistDoleriteMid -GreenschistVolcanic RockSEALTRAPSedimentary Sequenceσ1FLUID PATHWAYAmphiboliteGranuliteDistalMagmaticFluidGraniteIIMetamorphic FluidSOURCEMetamorphic FluidGranite IFluid from SubcretedOceanic Crust54

25Reference to other types ofgold deposits in shield areasV. Juhani OjalaCurrent organisation:Store Norske Gull ASOuline of the talk• Evolution of the Fennoscandian Shield• Gold mineralization types– Metamorphosed epitermal– Massive sulphide hosted– Granitoid related (non skarn)– Skarn-Iron Oxide Copper Gold– Orogenic (mesothermal)– Paleoplacer– Supergene and Recent alluvialGold mineralisation can occur in nearly allgeological environmentsTectonic settings of gold-rich mineral depositsGroves et al. (1998, 2000, 2003, 2005)

26Genetic deposit typesPalaeoproterozoic, 1.92-1.77 Ga• Multistage rifting during 2.45-1.97 Ga• Four main orogenic stages:1.92-1.88 Ga: Microcontinent accretion1.88-1.85 Ga: Continental extension1.85-1.79 Ga: Continent-continent collision1.79-1.77 Ga: Orogenic collapse and stabilisation

271.92–1.90collision1.90–1.88collision II1.86–1.85exension1.85–1.79collision IIIMetamorphosedepithermalMetamorphosed epithermal:PahtavaaraKutemajärvi/OrivesiPasi Eilu 2003

28Kutemajärvi:Metamorphosed HS epithermalHost to ore: quartz rockImmediately around to ore: Al-rich rocksquartz-andalusite-pyrophylliteF-Al(-P) minerals present: topaz, fluorite, apatiteMetal association Au±Ag-As-TeNo potassic or carbonate alterationExtremely low Na 2O + K 2O + CaO + MgOPasi Eilu 2003Kutemajärvi, Tampere Schist BeltAfter Poutiainen& Grönholm(1996)Pahtavaara

29Pahtavaara•Gangue – quartz, baryte, tremolite, dolomite, scheelite•Ba- and Mn-anomalies•Au = 99.02% Au, 0.07% Ag, 0.25% BiChampagene Pool, New ZealandWhite Island New Zealand

30Pasi Eilu 20067 Auriferous massivesulphide deposits&Submarine Au-richprecipitatesPasi Eilu 2006VHMS: back-arc environmentsGroves et al. (1998, 2000, 2003, 2005)RhyolintrusionVHMS: what happens there – ”a bit” simplified viewPirajno (1992)

31Precious-metal mineralisationPyhäsalmi mine, Proterozoic central FinlandFahlore?PyrrhotiteElectrumPyritePasi Eilu 2006ChalcopyriteField of view about 1 mmAll that glitters can be gold!Haveri ore, SW Finland:Au-Cu VHMS mineralisationor a submarine epithermaloverprint on Cu-VHMS?Pasi Eilu 2006Field of view about 10 cmPhoto J. Väätäinen

32Iron oxide-coppergoldAfter Lahtinen et al. (2003),Weihed & Eilu (2003)IOCG•Hosted by epigenetic alkaline to alkali-calcicpredominantly subaerial volcanic rocks, ironstones,skarn-like rocks, albite rocks, graphitic schists, marbles•Structural control distinct in all cases•Magnetite-Chalcopyrite-Pyrite-Pyrrhotite-Gold ±Cobaltite, Co pentlandite, Uraninite association•Regional extensive albitisation and scapolitisation•Local multi-stage Fe ± Mg ± K ± Na alterationWeihed & Eilu 2003

33IOCG KolariHANNUKAINENW162 Kivivuopio0102Laurinoja17079 78 71 75 89 42 39Kuervaara36 92 90 86 84 33E400 mMonzonite SkarnDioriteIronstoneOverburden Qz-fsp schistModified from Hiltunen (1982)QuartziteMica gneissMafic metavolcanicrockFeOx-Cu-AuGranitoid relatedKopsa

34Kopsa•Major opaques Chalcopyrite, arsenopyrite, pyrrhotite•Minor opaques Loellingite, marcasite, pyrite, sphalerite,gold, molybdenite, cubanite, bornite, stannite, bismuth andseveral Bi-bearing sulphosalts•The entire intrusion is anomalous in Ag, As, Au and CuGranitoid related-Porphyry goldPasi Eilu 2006Porphyry deposits: arc environmentsGroves et al. (1998, 2000, 2003, 2005)

35Raitevarri till and rock geochemistryCu-Au Anomalous zone 3 km long and 1 km wide2009 drilling2008 drilling500 profile Au/Cu

36OrogenicOrogenic gold in Fennoscandian ShieldDistributionAgeAfter Lahtinen et al. (2003),Weihed Pasi & Eilu Eilu (2003)ArchaeanIn every greenstonebelt explored forgold?IlomantsiPampalo

37Orogenic Gold: Archaean• Structural control in both regional and local scale• The locally most competent ± reactive rock unit as themain host to ore• Au-only deposits• Enriched: Ag, As, Au, Ba, Bi, CO 2 , K, Li, Rb, S, Sb, Te, W• Timing: ca. 2.7 Ga, in the latest stage of theNeoarchaean orogeny (Global control?)• Compressional to transpressional deformation• Syn-late orogenic TTG, but no indication of fluid ormetals from the granitoidsWeihed & Eilu 2003Orogenic Gold: ArchaeanPampalo:Structurally most complicated location,Locally, the most competent rock typesOrogenic Gold: Palaeoproterozoic• Structural control in both regional and local scale• The most competent ± reactive rock unit as the mainhost to ore• Enriched: Ag, As, Au, CO 2 , K, Rb, S, Sb, Te, W± Co, Cu, U (Kuusamo, Saattopora)• No indication of fluids or metals from granitoidsWeihed & Eilu 2003

38Central Lapland Greenstone BeltGold: Blue and green dotsKeinänen & Eilu 2003Orogenic Gold: PalaeoproterozoicSuurikuusikko,Central Lapland:In a shear zone,intense brecciation•No alteration•No increase ofany other elementthan AuPaleoplacerKumputunturi-OutapääKaarestunturi

39Placers and supergeneIsomaa supergene gold from regolithPuskuoja (Alhonen)Isomaa-KittiläMiessi (Tapio)Sotajoki(Vehviläinen)Nokia5 cmRuosselkäIvalojoki

40ConclusionsComplex tectonic evolution results diversity of Goldmineralisation styles•Orogenic: dominant, in all greenstone or schist belts, all ages•FeOx-Cu-Au: Palaeoproterozoic, W Lapland,•Granitoid-related: Palaeoproterozoic, near SW suturebetween Archaean and Proterozoic•Metamorphosed epithermal: Palaeoproterozoic, volcanic arcs•VHMS: Haveri?, Palaeoproterozoic, volcanic arcs•Paloeplacer in Lapland – erosion from VHMS and orogenic•Supergene in regolith remnants weathering startedNeoproterozoic•Placers reworking until RecentPasi Eilu 2003Thanks:David GrovesStephen GardollRaimo LahtinenVeikko KeinänenPär WeihedTero NiiranenNicole PatisonNick OliverPekka NurmiErkki VanhanenPeter Sorjonen-WardEelis PulkkinenJukka JokelaIlkka HärkönenVesa KortelainenHelena HulkkiEsa SandbergHeikki JuopperiAntero KarvinenHeikki Papunenjne…

141Orogenic gold:alterationPasi Eilu2011Pasi Eilu August 2011Factors controlling alteration in orogenic systems1.Deformation2.Structure3.PT4.Primary rock composition5.Fluid composition6.Fluid/rockPasi Eilu August 201121. DeformationBrittle, brittle-ductile or ductileEvents can be episodic, repeated2. StructureLocated in:- fracture arrays, stockworks, breccia zones,- foliated zones with pressure solution cleavage,- fold hinges, “saddle reefs”, etc.Pasi Eilu August 20113

423. PTMineralisation and alteration during peakregional metamorphism or soon after thatRange: 160–700°C, 0.7–5 kbar4. Primary rock compositionHosted by almost any rock type within ametamorphic beltPasi Eilu August 201145. Fluid compositionH 2O-CO 2-NaCl±CH 4±N 2fluidXCO 2typically 0.05–0.30Low salinity, commonly 2–8 % NaCl eq.Au+ Ag, As, CO 2, K, Rb, S, Sb, Te, W± B, Ba, Bi, Hg, Mo, Pb, SeCo, Cu, Fe, Ni, Zn contents normally very low6. Fluid/rockFluid-dominated to rock-dominatedPasi Eilu August 20115These factors produce an apparently great variation in the styleand products of alteration…yet several features of alteration are common to all orogenic golddeposits… and many of them can be utilised in explorationPasi Eilu August 20116

43The three main classes of alteration inorogenic systemsPasi Eilu August 201171 Lateral zoning- Due to chemical gradients and decreasingfluid/rock away from the fluid flow channels- Surrounds all deposits- Distinct lateral zoning sequence- Along-strike and -dip variation within a singlerock type is rarePasi Eilu August 20118Alteration envelope around orogenic goldmineralisationDistal alteration Proximal alteration OrePasi Eilu August 20119

442 Variation in primary rock type- Variation in primary composition =>variation in alteration mineral assemblagewithout a change in w/r or PT3 Variation in metamorphic grade- Systematic variation according tometamorphic grade- Typical between individual deposits ordeposit groups (gold camps)- Rare in a single deposit;only if a T isograd crosses the systemPasi Eilu August 201110?Extent of alteration?Pasi Eilu August 201111Extent of alteration- Alteration envelope 5 cm - 2 km wide;length up to 10 km or even more- Depends on the size and duration of the hydrothermalsystem, and the stress field during alteration- Positive correlation with the size of themineralisation (commonly)- Zone width grows outwards- Single zone may be 1 mm - 100 m wide,distal zone even up to 2 km (Golden Mile)Pasi Eilu August 201112

45Bronzewing,WesternAustraliaAlterationmapped fromdrillingintercepts at 150m depthEilu et al. (2001)Pasi Eilu August 201113Hollinger-McIntyreTimmins, AbitibiAlteration atthe presentsurface(simplified)Smith & Kesler (1985)Pasi Eilu August 201114Alteration at greenschist faciesPasi Eilu August 201115

460. Unaltered rockActinolite + epidote + albite + titanite ±ilmenite, magnetite (ol, cpx, kfsp, qz)1. Distal zoneChlorite + calcite + albite + rutile + quartz2. Intermediate zoneChlorite + calcite + ankerite/Fe dolomite +albite + quartz + rutile ± muscovite3. Proximal zoneMuscovite + ankerite + quartz + albite + rutile +pyrite ± arsenopyrite, goldPasi Eilu August 201116Mafic rock:Pasi Eilu August 201117Bulletin, WilunaUnaltered metabasaltActinolite-Epidote-Chlorite-Albite-Titanite-MagnetitephotoJ. VäätäinenPasi Eilu August 201118

47Distal alterationChlorite-Calcite-Albite-Quartz-Rutile-Magnetite2 cmPasi Eilu August 2011photo J. Väätäinen19Intermediate alterationChlorite-Calcite-Dolomite-Albite-Quartz-Rutile-Magnetite2 cmphoto J. VäätäinenPasi Eilu August 201120Proximal alteration, oreDolomite-Sericite-Albite-Quartz-Rutile-Pyrite-Arsenopyrite2 cmphoto J. VäätäinenPasi Eilu August 201121

48Proximal alteration, 20 g/t AuArsenopyriteQz+Dolo+AbSericiteRutilePyriteQuartz vein0.5 cmphoto J. VäätäinenPasi Eilu August 201122Bulletin mine, Wiluna, Western AustraliaLower-greenschist faciesUnaltered metabasaltProximal alteration, oreCrossed polarizers, field of wiew 3.2 mmphoto P. EiluPasi Eilu August 201123Granny Smith, Western AustraliaGranodiorite, lower-greenschist faciesUnaltered (Kfsp-Pl-Qz-Biot±Hbl)+ two mineralisation-related fractures2 cmOre, pervasive proximal alteration (Ab-Qz-Ser-Dol-Py)photo J. VäätäinenPasi Eilu August 201124

49Sunrise Dam, Western AustraliaKomatiite, lower-greenschist facies10 cmDistal: talc-chlorite-dolomitel Proximal:fuchsitequartzankeritephoto P. EiluPasi Eilu August 201125Loukinen, Central LaplandKomatiite, mid-greenschist faciesPasi Eilu August 201126Loukinen, Central LaplandPhyllite, mid-greenschist faciesPasi Eilu August 201127

50Mt Magnet, Western AustraliaBIF, upper-greenschist faciesPyrite replacing magnetite; a sulphidation frontPhotoC. MathisonPasi Eilu August 201128Uppermost greenschist facies- Biotite instead of muscovite- Pyrrhotite instead of, or with, pyrite- Commonly, calcite with or instead of othercarbonates in the proximal alteration zone- In ultramafic rocks, talc more common- K feldspar may be stable in proximal alterationzones (felsic rocks)Pasi Eilu August 2011 29BronzewingYandal Belt, NE YilgarnTholeiitic basalt host rockProximal alterationChl-Calc-Ank-Ab-Qz → Biot-Calc-Ank-Ab-Qz-Pyphoto P. EiluPasi Eilu August 2011 30

51Uppermost greenschist faciesIf T is above biotite isograd, proximal alteration:Increasing XCO 2=> biotite → sericiteDecreasing XCO 2=> sericite → biotiteBut no change in sulphides(?)Pasi Eilu August 2011 31Py-Po-Mgt-Hm at greenschist facies:effect of temperatureRock: averageinterflowsediment atKambalda, WAp = 2kbkbarconstant w/rp(ox) =proportion ofoxidised sulphurEvans (2010) Mineralium Deposita 45, 207-213Pasi Eilu August 2011 32Py-Po-Mgt-Hm in greenschist facies:effect of fluid-rock ratioEvans (2010) Mineralium Deposita 45, 207-213300°C 350°CNo need p = 2 kbar, for another, constant w/r, oxidising or reducing fluid, if you see magnetitep(ox) or = proportion haematite of oxidised in a system, sulphur or Po±Py±Mgt±Hm zoningPasi Eilu August 2011 33

52Amphibolite faciesPasi Eilu August 201134Contrasts to greenschist facies- Bleaching is not a characteristic feature- Calcic plagioclase is stable- Potassic alteration (biotite ± K-feldspar) fld )hasa wider extent than carbonation- Calcite is, normally, the only carbonate presentPasi Eilu August 2011 35Contrasts to greenschist facies- Calc-silicates characterise proximal alteration:diopside, hornblende, tremolite-actinolite, garnet- Pyrrhotite is the dominant Fe sulphide- Löllingite may be present- Rutile gives way to ilmenite and titanite asstable Ti-minerals, and magnetite may be stablePasi Eilu August 2011 36

53Lower-amphibolite faciesRegional metamorphic mineral assemblageplagioclase + hornblende→ Distal alteration zone (1 cm - 40 m)plagioclase + hornblende + biotite ± pyrrhotite→ Proximal alteration zone (1 mm - 20 m)plagioclase + biotite + calcite + quartz + pyrrhotite± arsenopyrite, actinolitePasi Eilu August 2011 37Ore, proximal alteration in lower-amphibolite faciesPasi Eilu August 2011 38Flin Flon, CanadaBIF: sulphidation front also herephoto C. Mathison5 mmPasi Eilu August 201139

54Mid-amphibolite facies and highermetamorphic gradesRegional metamorphic mineral assemblageplagioclase + hornblende→ Distal alteration zone (0 cm - 20 m)plagioclase + hornblende + biotite→ Proximal alteration zone (1 mm - 10 m)diopside ± grossular, almandine, actinolite, hornblende, biotite/Kfeldspar,plagioclase, calcite, quartz, pyrrhotite,arsenopyrite/löllingitePasi Eilu August 2011 40Ore, proximal alterationMid- to upper-amphibolite faciesBasalt, Polaris South, Southern Cross, YilgarnOrogeeninen AuDi-HblHbl-BiotDi-HblGar-Qzphoto C. Mathisonmm scalePasi Eilu August 2011 41Alteration as exploration toolPasi Eilu August 2011 42

55Alteration as exploration tool1. Alteration envelope can be used to define potential explorationtargetsEasy to identifyTargets detected are much larger than if only gold mineralisationwas used in the target identification2. Once an alteration halo is recognised, the sequence of alterationzones can be used as a rough vector towards the potential oreIt is most important to recognise these features in theearly stages of explorationPasi Eilu August 2011 43General trendsIn all rocks:K metasomatism + carbonation(± Ca-silicates) + sulphidation i + quartz veinsPasi Eilu August 2011 44Greenschist faciesCarbonate-free → calcite → calcite-dolomite → dolomite/ankerite;ilmenite / magnetite / titanite → rutile;sericitisation + bleachingBIF: sulphidation frontPasi Eilu August 2011 45

56UnalteredDistalIntermedProximal,OREPasi Eilu August 2011 46Amphibolite facies and uppermost greenschist faciesBiotitisation and brown colourAmphibolite faciesBanded proximal alteration characterised bydiopside and intense green colourPasi Eilu August 2011 47Sources of confusion 1, 2Sheared felsic rocks within a sequence dominated by mafic orultramafic rocks=> apparently bleached zones:Check primary chemical characteristicsSpilites metamorphosed at amphibolite facies conditions=> diopside ± quartz, garnet, calcite, pyritebetween pillows, fractures, etc.:Check primary volcanic structuresPasi Eilu August 2011 48

57Sources of confusion 3Carbonatisation related to VMS-style mineralisationDifferences to orogenic gold systems:- Distinct gains in Ca, Fe, Mg- At greenschist facies, no mineralogical gradients definedby carbonates- Silicate assemblages commonly contain Al-rich, alkali-deficientminerals (also true in metamorphosed epithermal systems)- Typically stratiform, unrelated to late faults or shear zonesPasi Eilu August 2011 49Sources of confusion 4Amphibolite facies: SkarnsDifferences to orogenic gold systems:- Intimate association with an (granitoid) intrusion +- No potassic alteration associated with calc-silicate formation- Gold in retrograde gangue association, typically distalto intrusion and to high-T skarn mineral association- Radiometric dating shows synchronous mineralisationand intrusionPasi Eilu August 2011 50Next:GeochemicalhaloesPasi Eilu August 201151

158Orogenic gold:geochemical featuresPasi Eilu2011Pasi Eilu April 2010Geochemical anomalies- Discriminate between gold-related and unmineralisedstructures- Expand the target- Define vectors to orePasi Eilu April 20102Elements enriched in orogenic gold systemsVery little difference regarding:- Metamorphic grade-Host rock- Craton or greenstone beltPasi Eilu April 20103

59Bulk ore samples, Yilgarn CratonAll concentrations in ppmDeposit Host Au Ag As Bi Sb Se Te WSub-greenschistWiluna Mafic 8.1

60First, define theprimary rocktypesPasi Eilu April 20107Define also the primaryvariation within therock typesBlue: unaltered sampleRed: altered sampleBased on data fromStanley & Madeisky (1995)and Eilu (1996)Pasi Eilu April 20108Then, evaluate the mass transferPasi Eilu April 20109

61Greenschist faciesEilu & Mikucki (1998)Pasi Eilu April 201010Qz-porphyry y, intense alterationQuartz porphyry, sericitis sation zone I, potential ore302520150.004xBi0.09xCu0.04xPb4xAu0.02xTe0.1xW10xS0.4xCd0.005xZn0.05xSbEnriched0.1xRb50xP 2 O 50.01xBa0.2xVAl 2 O 30.25xSiO 2Least altered quartz porphyryGaNi0.2xZrLa0.05xCrIsoconMataralampiprospectArchaeanKuhmogreenstone belt,FinlandHost rock:quartz porphyry2xK 2 O1010xCO 2FeO*0.2xClLiAg5Y10xTiO 20.05xSr100xMnDepletedMgO CaONa02 OPasi Eilu 0 5 10 15 20 25 30April 2010Qz-porphyry, least altered11Elements shown to be enrichedin orogenic gold deposits,no. of casesPasi Eilu April 201012

1362Pasi Eilu April 2010Pathfinders and alteration indicesMajor components:+ No significant problems with detection limitsor analytical methods+ Easy to relate with mineral assemblages+ Alteration indices can be very useful parameters- Host rock effect can be large- Mass balance evaluation neededPasi Eilu April 201014Pathfinders and alteration indicesPathfinder trace elements:+ Enrichment up to 1000-10000 x+ Host rock effect generally very minor- Very low detection limits may be needed- Background thresholds needed to bedefined for each areaPasi Eilu April 201015

63Extent of an anomaly1. The threshold between backgroundand an anomalyPasi Eilu April 201016Trace-elementcontour mapand a crosssectionBackground thresholdOptimal backgroundthreshold h somewherebetween 60 and 80 ppmfor the element depictedPasi Eilu April 201017Statistical methods: histograman empiric methodConcentration in log(ppm)Pasi Eilu April 201018

64Statisticalmethods:cumulativefrequency plotConcentration (p ppm)After Sinclair(1974, 1976, 1991)Cumulative frequency (%)Pasi Eilu April 201019The background thresholds achievedmust be checked with geology:are the results of the statistical analysis,whatever used, reasonable?Pasi Eilu April 201020Background I:igneous host rocksBackground thresholds definedGranny Bulletin KX BW Madsen- MoyageeSmithStratt-O.Grano- Basalt Basalt Basalt Basalt, Komat.dioriteKomat.Au 8.5 6 5 4 6Ag 140 80 130 160 110As 5 28 (6) 4 50 5Sb 0.9 2.0 (0.6) 0.9 0.4 2 0.45Se 0.10 0.30 0.15Te 10 10 37 (10) 44 (10) 10W 6.0 0.6 1.3 1.9 0.5Au, Ag, Te in ppb, , others in ppmPasi Eilu April 201021

65Background thresholds definedBackground II:felsic to intermediate igneous host, Mataralampi, FinlandAu 0.045Bi 0.10; 0.41Cu 42Pb 90S 270Sb 0.07Te 0.065W 14; 80Zn 57; 160All data in ppmPasi Eilu April 201022Background III:metasedimentary host rocksBackground thresholds definedGranny Twin Sunrise Hattu belt Bendigo-Smith Peaks Dam (Finland) BallaratAu 2 2 6 5 10Ag 180 100 110 150As 40 6 15 15 50Bi 0.20 0.20 0.07 0.02Sb 1.0 0.8 2.8 0.5Se 1.0 0.17 0.08 0.10Te 125 50 12 75W 2.3 3.0 8.5 3.0Au, Ag, Te in ppb, , others in ppmPasi Eilu April 201023Detection limits needed for pathfinder elementsAt least, during the early stages of exploration,to define the local background levelsAg 10 ppb Hg 3-5 ppbAs 0.2-1 ppm Mo 0.1 ppmAu 0.2-1 ppb Sb 0.02-0.1 ppmB 1 ppm Se 10 ppbBi 10 ppb Te 1-2 ppbCd 0.01-0.1 ppm W 0.1 ppmPasi Eilu April 201024

66Extent of an anomalyPasi Eilu April 201025Form of ananomalySuurikuusikkoN FinlandAu at thetill-bedrock interfaceHärkönen (1992)Pasi Eilu April 201026Form of ananomalyBronzewing,Central zonePathfinderelements IDispersion after dataavailable in June 1994Ore as realised(1999)Pasi Eilu April 2010 27Eilu et al. (2001)

67Form of ananomalyBronzewing,Central zonePathfinderelements IIDispersion after dataavailable in June 1994Ore as realised(1999)Pasi Eilu April 2010 28Eilu et al. (2001)Form of ananomalyBronzewing,Central zoneAlteration indicesDispersion after dataavailable in June 1994Ore as realised(1999)Pasi Eilu April 2010 29Eilu et al. (2001)0 m100100 mSunrise Dam,Western AustraliaArsenicSection acrosshost rocks andmineralised shear zonesPasi Eilu April 201030

68As Form of ananomaly100 mAuAs and Au, values along a mineralised shear zone(= within a fault plane)Pasi Eilu April 201031How far from ore ananomaly can extend?(in bedrock)Pasi Eilu April 201032Pasi Eilu April 201033

3469Pasi Eilu April 2010Size of primary anomaly in planArchaeanParameter Across Along strikeFennoscandiaHattu schist belt Te, As 1-4 km 60 kmCanadaStratt-Olsen– As, Au, B, 200-600 m 9 kmMadsenK, Sb, NaHollinger-McIntyre As 800 m >3 kmCO 2 >2 km >3.5 kmYilgarnGolden Mile As, Au, Te >1.5 kmMoyagee As, Au, Se, Te, W >1.2 kmBronzewing Te >400 m >800 mAu, CO 2 , K, 100-300 m >800 mRb, Sb, WTanzaniaGolden Pride Sb, Li >500 mPasi Eilu April 201035Size of primary anomaly in planProterozoicParameter Across Along strikeFennoscandiaPahtavaara Au >6 kmSuurikuusikko As, Au 50-100 m >10 kmSaattopora camp As, CO 2>200 m >20 kmVesiperä camp As, Au, K 100-200 m >8 kmPasi Eilu April 201036

70Beyond the gold anomaly,intounaltered rockPasi Eilu April 201037Beyond bleaching,locally into unalteredrock, no rock type effectBackgroundthreshold: 4 ppb AuPasi Eilu April 201038Relative lateral extentPasi Eilu April 201039

71Vectors towards the ore?Shkolnoe, Kolyma, RussiaPasi Eilu April 201040Any pathfinder or alteration index maydefine a trend towards oreIn orogenic gold systems, this is not alwayseasily seen – why?Pasi Eilu April 201041Golden MileKalgoorlie, WADolerite-hostedAlteration vs. carbonationindexAfter data from Phillips (1986)Pasi Eilu April 201042

####72Mataralampi section 7149710N CM 19/4412/2003/4/10Geological Survey of FinlandPasi EiluKUH/HK-1KUH/HK-2KUH/HK-3KUH/HK-4GoldAntimonydolerAu Sbgrdrmdykeqz-porfsp-pormvolc20 0 20 40 MetersPasi Eilu April 201043HJB SZBulletin, WilunaNorthern YilgarnTholeiitic-basalt hostedSection across the ore,shear zone and hangingwallEilu & Mikucki (1998)Pasi Eilu April 201044Harbour LightsCentral Norseman-WilunaBelt, YilgarnSection across the oreand wallrocksBased on data fromSkwarnecki (1990)Pasi Eilu April 201045

173Surficial geological and geochemical exploration forgold in glaciated terrains – brief overviewPertti SaralaGeological Survey of FinlandPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011Outline• Introduction• Surficial geology in glacigenic environment– Glacial dynamics– Ice flow indicators– Till stratigraphy– Bedrock vs. pre-glacial weathered bedrock vs. till– Glacigenic formations and deposition processes• Surficial exploration methods– Till geochemistry– Indicator minerals– Prospectivity modelling• Exploration case studiesPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.20112Introduction• Indicator (usually heavy) mineral studies are the oldest methods ingold exploration• Nuggets were concentrated using water and gravity (e.g. panning)• First ideas and observations of glacial transportation from 18thand 19th century– Glacial erratics -> boulder fans -> tracing the source• Theory of ice ages and glaciations with glacial transportation waslargely accepted in the beginning of 20th centuryPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.20113

74Introduction• Use of soil in exploration started after development of chemicalanalysis methods– References from antiquity of the relation of geology and chemistry– Techniques of modern geochemical prospecting in the Soviet Unionand Scandinavia in 1930s; in Northern America in 1940s– Trace elements in gold exploration• Soil and till samples were used largely since 1950s in explorationin glaciated terrains– Analysis methods developed to ppm and ppb levels– Easy and effective sampling– Development of sampling techniques– Costs reasonable for large sampling projectsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.20114Glacial dynamics• Geomorfological systems- Cold-based (Drybed)-no erosion anddeposition- Warm-based (Wet bed)-erosion and deposition- Marginal meltwaterzone-eskers and endmorainecomplexesKleman & Borgström 1995Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 5Glacial dynamics• Glacial processes and their variations key issues=> time-transgressive and spatiallychancing events like cold - warm basal conditions and ice stream networkPunkari 1997 Punkari 1997Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 6

75Ice flow indicators• Erosion marks; striae, grooves etc.• Till fabricsHirvas et al. 1973-1977Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 7Ice flow indicators: Surficial boulder fansSector of ore boulder observationsSalonen, V-P. 1986.Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 8Ice flow indicators: Single boulder transportationThe longest transport distance of single ore boulder from the known sourcePertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 9

76Ice flow indicators: Glacial morphology• Morphological interpretation key point for estimating ice-flow direction butalso distance and deposition processesNPerpendicular ribbed-moraine ridges inPetäjäskoski, S-W RovaniemiStreamlined drumlins in Kuusamo (two fields overlapping)Pertti Sarala, 24.11.2008 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 10Example: Relief/landforms in southern LaplandHummockyribbedmorainesRibbed morainesand drumlinsN-S oriented drumlins and mostly till-covered esker chainsDrumlins in Kuusamodrumlin fieldPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 11Example: Glacial morphology as an indicator forglacial dynamics in southern LaplandRibbedmorainesYoungerdrumlinsOlderdrumlinsGlacial flowdirectionyoungerTervolaRanua InterlobateareaKuusamo Ice lobeOulu Ice lobeSarala 2005Pertti Sarala, 24.11.2008 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 12

77Example: Glacial dispersion and transportdistance in southern LaplandSarala et al. 2007Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201113Till stratigraphy• Till stratigraphy includes till beds and interlayers• Two different types stratigraphy: Straight (simple and straightforward; 1-2 tillbeds) and complex (multiple till beds with different ice-flow directions)70 ± 5 ka93 ± 10 ka99 ± 11 ka102 ± 11 ka107 ± 13 ka??Till stratigraphy in Rautuvaara, Kolari, Drawing H. KutvonenPertti Sarala 24.11.200814Pre-Quaternary weathered bedrock• Pre-glacial weathered bedrock surface has been preserved beneath glacialdeposits in many areas in northern Finland.• Remnants of weathered bedrock are up to tens of meters thick mainly in the lastice divide zone in Central LaplandPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201115

78Till vs. weatheredbedrockWashedsurfaceTillWeatheredbedrockFreshbedrock• Sometimes difficult to distinguishdifferent stratigraphical unitsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 16Glacial erosionand deposition• Simple transportation– Glacier erodes the bedrockand deposits material intosome distant down-stream• Complex transportation– Till units include materialdeposited and transported byvarious glaciations– Tracing the source ofmineralized material needs goodknowledge of the tillstratigraphyHirvas & Nenonen 1990Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201117Erosionand deposition - drumlins vs. ribbed moraines• Short transportation in theribbed moraines is seen in thesurficial parts, i.e. mineralizedsurficial boulders indicatelocal source in the bedrock (ifthe quarrying was reached thebedrock surface)• Deposition process at thewarm-based conditions (forexample in drumlins) isdifferent and transportdistance longerPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 14.2.2007 18

79Surficial geochemical exploration methods• Soil surveys– Most widely used in geochemical exploration methods– Based on secondary dispersion of weathered and leach material or elementsfrom the buried source– Samples from the soil horizons (A and B) or fresh material (C horizon)– Sampling using drilling or testt pits• Rock surveys (also lithogeochemical survey)– Sampling of unweathered bedrock– An idea to find favourable host rocks for mineralization– Samples from the ourcrops or drill coresPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201119Surficial geochemical exploration methods• Stream-sediment surveys– Used in reconnaissance i.e. in regional scale– Based on secondary dispersion from the upstream in drainage basins– Panning is a good example of this survey method• Water surveys– Both ground water and surface water sampling– Usually used in detailed surveys (ground water)– Contents low, and adsorption causes difficulties for interpretation• Biogeochemical surveys– Vegetation used as a test medium– Plants concentrate elements in themselves or in humus– Animals can also collect mineral or organic material in reservations or nestsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201120Surficial geochemical exploration methods• Gas surveys– Used in detailed scale to find buried deposits– Based on detection of different gases (hyrdorgen sulphide, mercury, iodine, radon)or ions in gases (hydrocarbons)• Mobile metal surveys– Nowadays widely tested and used method– Based on the analyse of weakly bounded metal ions on the surface of mineral soilor organic particles at the top of the soil– Weak acidiferous solutions were used for leaching ions without dissolving minerals• Radiation surveys– Total radiation and/or spectrums were measured usually from airplanes– Can use for lithogeochemical purpose and for soil geologyPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201121

80Till geochemistry• Till geochemistry is the most commonly used method for estimating transportdistance of mineralized material in glaciated terrain.– Is based on secondary dispersion of the indicator elements from themineralized sources• Based on the sampling withdifferent intervals andvariable depthsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201122Sampling methods – percussion drillingPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201123Test pit excavations –also in winterTest pit surveys in Petäjäselkä atthe end of Marsh in 2007- Temperature -15°C- Snow depth 1 mGold grains in till at MisiPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 24

81Collected datasets• Test pits and trenches for:– Till stratigraphical observations and sampling– Till fabrics and striae– Till and weathered bedrock sampling (incl. geochemistry (ICP-AES, ICP-MS, GAAS, etc.) indicator minerals, rockcomposition, grain size distribution)– Bedrock observations– Bedrock and boulder sampling• Percussion drilling:– Till and weathered bedrock sampling for chemical analysesPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 18.6.200725Till geochemical datasets• Small-scale till geochemicaldatasets (1 sample/4 or16 km 2 )are used for identifyingregional geochemicalcharacteristic– Anomaly patterns are alsoreflecting general ice flowdirections• For targeting and target-scaleexamination more detailed tillgeochemistry is requiredPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 26Example of sampling densities in Au exploration• Central Lapland; Suurikuusikko depositTill sampling: 10 m intervalTill sampling: one sample / 4 km 2N-S structurePertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201127

82Example: Till and weathered bedrockgeochemistry in the Suurikuusikko depositAu ppb4 m140 mIndicator elements: Au, As, Sb, K, MnSb ppbPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201128RovaniemiCase study: Petäjävaara• Investigations fortracing the Au-Cumineralized surficialboulders in ribbedmoraine area• Bd Bedrock:metasedimentary andmetavolcanic rocks ofthe Peräpohja Schist Belt• Two till units: lowerlodgement till and uppermelt-out till,representing advanceand retreat phasesSarala & Rossi 2006Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 29RovaniemiCase study: Petäjävaara• Many hydrothermally altered Cu-Au mineralized boulders found on thetop of ribbed moraine ridgesQuartzite bouldersAu 0.1-0.6 ppmCu 0.7-2.4 %Banded amphibolite bouldersAu 0.1-6.9 ppmCu 0.1-3.2 %Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 30

83RovaniemiCase study: Petäjävaara• Sampling: percussion drilling and test pits• Distinct metal anomalies in upper till (e.g. < 0.06 mm fraction)aGFAASGlacial flow directionKnown Cu-Au mineralization(Sarala & Rossi 1998)Glacial flow directionICP-AES Fresh chalcopyritegrain in till(SEM photo)Indicator elements: Au,Co, Cu, Te, SPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 31Other methods for Au exploration• Weak leach methods– Based on the analyse of weakly bounded metal ions on the surface ofmineral soil or organic particles at the top of the soil– Weak acidiferous solutions were used for leaching ions without dissolvingminerals– The used methods were Mobile Metal Ion (MMI), Enzyme leaching and SoilGas Hydrogen analyses of which results were compared to conventionalpartial leaching (aqua regia) and total leaching (four acid) analysesPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201132Weak leach methods• Mobile Metal Ions (MMI) + several other commercial methods– Ions are moving from the bedrock through the overburden– Mobilization, movement and enrichmentof the ions are the sum of many factors.The main reasons are capillaryaction, difference of electrochemicalcharge, and biogeochemicalprocesses– MMI is the first commercialmethod; SGS Minerals isthe patentee– Other: Ammonium acetate,entzyme leach, soil gas etc.Cameron et al. 2004Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201133

84Sampling• Easy and fast sampling– The sampling depth 10-25 cm under thecontact of humus and mineral soil– Small test pits or soil drills in sampling– Samples from the lines, frequent10-50 m => 25-30 samples/day/two-people sampling group– Sampling procedure same for all the weakleach methods• Other methods are for example humusand Ah samplings that can be usedbeside the weak leach methodsSampleSampleSamplePertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201134Examples - Case Lauttaselkä, Au• Lauttaselkä target is Auexploration target in Kittilä, ca.10 km from the Agnico-Eagle’sKittilä Mine to the NE• Bedrock is composed ofhydrothermally altered maficvolcanic and sedimentary rocksof which contact zones areenriched of Au, As and Te• Conventional till and weatheredbedrock sampling supported byMMI sampling revealedpotential zones for Auexloration in the bedrockAuZnAsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201135Mobile XRF analysis• XRF analyzers can also be used in Auexploration in the field– Portable equipments have beendeveloped a lot during the last ten years– Automated scanners can be used fordrill cores but also for the till and Pre-Quaternary weathered bedrock samples• Measurement direct in the field => nosampling or• Sampling as separate samples orcontinuous sample series of till andweathered bedrock along test trences,• In Au exploration indicator elements orindication of suitable alteration in thebedrock is usefulPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011Continuous weathered bedrocksampling in Lauttaselkä, Kittilä36

85Mobile XRF analysis (cont.)• Portable XRF analyzers very useful• A mobile laboratory for on-line elementalXRF analysis technology new applicationPortable XRF analyzertested during thepercussion drilling forthe till sampleScanmobile (Mine On-lineServices Ltd)Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201137Indicator mineral methods• Indicator minerals (particularly heavy minerals) are largely used in exploration– Straight indication of mineral potentiality– Information for interpreting stratigraphy and determining the provenance of asediment• Till and weathered bedrock samples, but also stream sediments• Au, sulphide minerals, Fe-minerals, garnets and pyroxene and phosphateminerals most common, also PGE-minerals– Demand increasing also for light indicator element separation and research due toincreased high-tech metal explorationPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201138Indicator mineral methodsPanning, Knelson concentrator and spiral separator most used field equipmentsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201139

86GIS-basedprospectivitymapping4. Evaluation3. Spatial analysis2. Data preprosessing1. DataNykänen et al. 2006Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 18.6.200740Example: Prospectivity in Central Lapland• Several new areas potential for Au mineralization foundNykänen & Salmirinne (2006)Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 18.6.200741Example: Prospectivity in Central Lapland• Four test targetson very highprospectivityareas :• Vuomanperänmaa• Nuttiot• Petäjäselkä• LauttaselkäWeight of evidence• Situate near themodern mines ofPahtavaara andKittilä, andseveral known AuoccurrencesPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 18.6.200742

87Case: Petäjäselkä• Bedrock is composed of Mg and Fe tholeitic metabasalts and minor BIF• NNW trending magnetic anomalies are seen on a high resolutionaeromagnetic map. NE-SW oriented faults are breaking them.Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 18.6.200743Case: Petäjäselkä• Several anomalous Au(Co-As-Cu) mineralized zones havebeen defined• Till geochemistry highlightsthe multimetal anomaly in thetarget area• Anomalies are clearly relatingto SW-NE trending faultsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 Pertti Sarala 18.6.200744Petäjäselkä (contin.)Au in till (

88Case: Petäjäselkä• Mineralizationhosted by shearedgraphitic chertand clasticsedimentary rocksbetween Mg andFe tholeiticmetabasalts0.5 mm• The best drillintersection is12g/t Au over 1m,and this lode isexposed in thetest trench.Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011 46Conclusions• Till geochemistry is useful method for estimating transport distance ofmineralized material in glaciated terrain. Ore indicators – mineralizedboulders and till, and indicator heavy minerals are useful in tracing themineralized bedrock• New applications like weak leach methods and portable XRF developedfor exploration. Effective, low sampling and analyzing costs, low-impact tothe nature particularly in sensitive areas• Examples from northern Finland shows different kind of glacialtransportartion from short and sharp dispersals of for examples Au and itspathfinder elements• The study of moraine formations, ice flow directions, till structures andstratigraphy is essential before planning sampling and analysing tillgeochemistry, and interpreting the results; i.e. successing in tillgeochemical exploration in glaciated terrainsPertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.201147

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