89
12.FIELDSURVEYPROCEDURES
12.1.General
Effectivesamplingofanysurficialmediarequireswell-trainedpersonnelcapableofrecognizinganddescribingthecorrectsamplematerialandthesamplesitecharacteristics.Samplersshouldbeabletorecognizeand,ifpossible,avoidsituationswherecontaminationfromhumanactivityorchangesinthenaturalphysicochemicalconditionscanproducespuriousorunusualresults,inmostsituations,thesesamplingdutiescanbeundertakenbytrainedtechnicalpersonnelunderthesupervisionofageochemistorgeologistwithadequategeochemicalexplorationexperience.Insomesurveys(e.g.whereidentificationofthecorrectsamplematerialIscritical,asinbiogeochemicalorglacialtillsamplingprograms),itisprudenttoemployqualifiedspecialists(e.g.botanistsandQuaternarygeologists)tobothconductorientationsurveysandinstructandsupervisethesamplingteams.
Samplingtoolsvaryaccordingtothemediumandthefieldsituation.Non-contaminatingequipmentisessentialandcareshouldbeexercisedinnotonlychoosingnon-contaminatingsteelsforshovels,trowels,augersetc.butalsoinensuringthatanyassociatedlubricants,adhesives,welds,andsolderswillnotcauseproblems.Leadedgascansometimesconstituteapotentialprobleminfieldvehicleswhensamplesaretransportedinproximitytoleakingcontainers.Thisawarenessofgeochemicalcleanlinessextendstothedressofthesamplerwhoshouldavoidwearingmetalbuckles,rings,etc.andhandlingcoinswhichmightleadtocontaminationbychippingortransferofmetalonfingers.
Thesamecautionisnecessaryinthechoiceofsamplecontainers.Kraftpaper(withnon-contaminatingwater-proofglueandclosures),olefin,andplasticbagcontainersofappropriatesizearefrequentlyused.Kraftandolefinallowsamplestobedriedwithouttransfer.Plasticbagsarecommonlyusedforlargersamples.Morerigidpolypropyleneandspecialglassbottlescanbeutilizedinwatersamplingandavarietyofsamplingdevices,manyofthempatented,areavailableforthesamplingofgasesandparticulates.
Itisstronglyadvisedthatallsamplesbeallocatedsimpleuniquesequentialnumberswhichatleastincludeaproject(orregionaloffice)designatorprefixandasampletypedesignatorsuffix.Thesearebestprovidedbypre-numberedAssay/GeochemicalSampleTagBooks.Thepotentialforerrorandmisunderstandingistherebyminimizedandproblemsinsubsequentdatamanagementandinterpretationareavoided.Someformofcoordinatesshouldalsobeassignedtoeverysampleinordertoassistsamplelocationandcomputerplottingofsamplelocationsandanalyticaldata.Inthecaseofwidelyspacedregionalreconnaissancesamples(e.g.streamsediment)theUniversalTransverseMercator(U.T.M.)gridlocationofeachsitecanbedeterminedusingtopographicbasemapsofsuitablescaleorpossibly,alocatorinstrument(e.g.Magellan).
90
InmoredetailedstudiestineU.T.M.gridcanbeusedtodefinetheareaboundaries,whilstindividualsamplesarelocatedbyreferencetoalocalgrid.
12.2.SampleMedia
Somediscussionofthepotentialroleofavailablegeochemicalsamplemediaintheexplorationsequencehasbeenprovidedinpreviouschapters.Mediaselectionwillofcoursebedecidedonthebasisoforientationstudieswhichwillinturnbeinfluencedbythelocalenvironmentaswellthenatureoftheexplorationproblem.Reiteratingearlierstatementsconcerningtheapplicabilityofthemorewidelyusedsamplemediainreconnaissancestudies,themethodsusedmightinclude:
(i)drainagesurveys:samplingstreamorlakesediment,streamorlakewater,groundwater,etc.;(ii)glacialdepositsurveys:samplingoftill,etc.;(iii)rocksurveys;(iv)soilsurveys:thisapproachisbecomingincreasinglypopularatsamplingdensitiesaslowas1sampleper25km.
Follow-upstudiesofpromisingleadsdetectedinthereconnaissancephasemightinvolve:
(i)closerspacedsamplingofoneormoreoftheabovementionedmediaand/or;(ii)streambank(residualsoilorcolluvium)surveys;(iii)biogeochemicalsurveys;Ov)soilgassurveys,ormorerarely;(v)geobotanicalsurveys;(vi)particulatesurveys,and(vii)microorganismsurveys.
Exotictechniquessuchassurveysbasedonanimaltissuesampling,arecurrentlyprimarilyofacademicinterest,andunlikelytoprovidesolutionstoactualpracticalexplorationproblems.
12.2.1,Rocks
Geochemicalexplorationsurveysbasedonsystematicbedrocksamplingareinessenceanextensionofroutineprospectingbasedonthecollectionandanalysisofrelativelysmallnumbersof"specimens"orrockchip"samples"frompotentiallyinterestingbedrockexposures.However,theformeraimtoachieveconsistentlyrepresentativematerial,andaregenerallycapableofdetectingandinterpretingfarmoresubtleexpressionsofthepossiblepresenceofmineralizationthanthe"character"samplingnormallycarriedoutinprospecting.Unlessexposureisexceptionallygood,sample91
spacingingeochemicalrocksurveystendstobelessconsistentthanthatachievedin,say,soilsurveys.
Aswithothertypesofgeochemicalsurveys,thesamplingproceduresandthesamplematerialcollectedingeochemicalrocksurveysshouldbestandardizedasmuchaspossible.However,consideringthelargenumberofvariablesthatcanbeintroducedbytheprocessesofweatheringandoxidation,theidealofcollectingsimilarlyweatheredmaterialissometimesimpractical.Nevertheless,thegeologistorthegeochemistconductingthesurveyshouldensurethatindividualsamplesatailsamplesitesareessentiallycomparableandthatobservedvariationsinweatheringintensityareproperly-ecordedforinterpretationpurposes.
Geochemicalrocksamplinonecessarilymusttakeintoaccountthegeologicalenvironmentandthetypeofmineraldepositofinteresttotheexplorer.Theprecisescaleofsamplingnecessaryfordetectionofsvnaeneticandepiaeneticpatternswillbedeterminedbyorientationsurveys(seeChapter8).Detectionofsyngeneticpatternsmaynecessitatetheregionalsamplingofindividualplutonsormoredetailedsamplingofspecificpartsofanexposedstratigraphicsection.Thelatterpatternswillrequireadifferentapproach.Surveysdesignedtodetectleakageanomalieswillfocusonsystematicsamplingoffaultorfracturezonesand,possibly,beddingstructures.Incontrast,thepreferredgeochemicalrocksamplematerialforthedetectionofdiffusionnaloesislikelytobeunfracturedandthescaleofsamplingmuchmoredetailed.Inallinstances,analysisofgeochemicalrocksurveymaterialhasthepotentialofdelimitingdispersionpatternsbeyondvisiblealterationassociatedwithmineralization.Table12.1summarizestheelementsdeterminedandthesamplingdensitiesusedinpastexplorationprogramsforavarietyofmineralizationtypes.
AgoodexampleofaregionalapproachcapableofdiscriminatingbetweenproductiveandbarrenintrusionsisprovidedbytheworkofGarrett(1973),whichwasbasedonwholerockanalysisofsamplesfromfelsicintrusionsintheYukonTerritory,Canada(Fig.12.1).Usingavarietyoftechniques,includingresidualscoresfromamultivariatestatisticalanalyticalprocedure(principalcomponentanalysis),comparisonsofmetalconcentrations(Fig.12.2)anddegreeofskewnessoffrequencydistributions,hewasabletodemonstratethatmostplutonsassociatedwithmineralizationcouldberecognized,andcertainadditionalplutonswithnoknownmineralizationmeritedfurthernvestigation.InstrongcontrasttheworkofChurchetal(1976)demonstratesthepotentialvalueofdistrictscalegeochemicalrocksamplingprogramsinthedetectionofveinandreplacementdeposits.TheircasehistorystudywascarriedoutinanareaofBritishColumbia,CanadawhichincludestheMesozoicvolcanicsequencehostedSamGooslyreplacement(?)massivesulfidedeposit,andtheUpperCretaceousandesiticvolcanicsequencehostedBrandinaveintypeoccurrences(Fig.12.3).BothtypesofmineralizationarereflectedbylargeAsandsomewhatmorelimitedCuanomalies(Fig.12.4).
ScaleTargetElements___Samplingdensity
Regional
identificationofproductiveplutonsmassive
K,Rb.Sr,Ba,U,Na",Ca*Fe.Na,Mg,Mn,
e.g.Cu,Pb,Zn,Sn.W.Mo,U.NiCu.Zn.(Pb)
min.30/intrusionbutseeAppendix3,0.2-5/krT?
sulphides
Na,
LocalandMine
veinandreplacementporphyry
(K),(Ca),(Ba)As.Sb,Ta.Bi*
K.Ca,Rb,Sr.
e.g.Cu,Pb,ZnAu,Ag
Cu.Zn.Mo.S
l-10/kn
2-30/krT
Mn,(MG)
massivesulphides
Fe,Mn.Na,K.
Cu,Pb.Zn.(S)
150-200rnInterval
Ca,Mg,
veinandreplacetnent
(hO).(Rb).(Sr)
e.g.Cu,Pb,2n.Aii,Ag
5-10minterval
TABLE12.1Summaryofelementstobedsterminedandsurfacesamplingdensityfordifferenttargetsinregional.andlocalandminescaleexploration.Elementsinparentheseshavebeenshowntobeusefulinsomecasesbuthaveuncertainstatus;elementswithasteriskareexpectedtobeusefulbuttherearelittledata.Sishouldbedeterminedinallcaseswherepetrologicalvariationisexpectedtocausevariationtothecontentofotherelements.(Govett,1983)
93
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-Oligocene
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}
DEVONIAN
IS2LURKANSOcANDChertandshleJORDOVCIAN
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DEVONIANANDSILURIAN
SILURIANANDORDOVICIAN
OeEurekaQuartziteOpPogonipGroupChHamburgDolomiteCONTACTFAULTDottedwhereconcealedTHRUSTFAULTDottedwhereconcealedTeethonupperplateLOCATIONOFMINEAND(OR)DEPOSIT
-ORDOVICIAN
CAMBRIAN
FIG12.5c
CariinDistrict.GeologicalLegen(j
98
12.2.2.Soils
Soilsvaryconsiderablyincompositionandappearanceaccordingtotheirgenetic,climatic,andgeograpiiicenvironment.Classifiedintoresidualandtransportedtypesaccordingtotheirrelationshiptotheirsubstrate,soilsaremixturesofmineralandbiologicmatterandmaybedistinctivelydifferentiatedintoaseriesofsoilhorizons.
Soilsaremostoftensampledalonatraversesorgridsinthefollow-upordetailedprospectingstagesofgeochemicalprograms.Inruggedterraininitialfollow-upsurveysofreconnaissancestreamsedimentanomaliesissometimesmostreadilyachievedbysamplingsoilsalongridgeandspur(Fig.12.6),and/orbaseofslope(Fig.12.7)traverses,inrecentyearsincreasingattentionhasbeengiventolowdensitvsoilsampling(i.e.1sampleperkm)ingeochemicalreconnaissancesurvevsandaeochem-icalmapping.Ashasbeenpreviouslystressed,orientationprogramsdefinecriteriasuchassampledepthorsoilhorizontobesampled,sampleinterval,andthesize-fractionforanalysis.Itisessentialthatthesecriteriabeobservedresolutelythroughthesurvey.
Residualsoilscharacteristicallycontaindetectabledispersionpatternsdevelopedduringtheweatheringofmineralizationintheunderlyingbedrock,andthesepatternsarerevealedbycarefulsamplingofappropriatesoilhorizons.Asmightbeexpected,inviewofthesizeofthedepositsandassociatedprimarygeochemicalhaloes,nearsurfacesedimenthostedfinedisseminatedgolddepositsinsemi-aridareas,suchasNevada,arecommonlyreflectedbyextensivegeochemicalanomaliesintheimmatureresidualsoils.Itisthereforenotsurprisingthatgeochemicalsoil(generallyC-horizon)samplinghasassistedinthediscoveryofanumberofthesedeposits(e.g.AlligatorRidge,JerrittCanyon,etc.).SomeindicationofthesizeandnatureofsoilanomalieswhichmightbeexpectedinthevicinityofsuchmineralizationisprovidedbyBagby,etal.,(1984)inastudyofsoilsovertheDeeDeposit,Nevada.Theminus80meshsievedfractionof159ChorizonsoilsampleswereanalyzedforanumberofelementsincludingAu,As,Sb,Ag,andHg.Theresultantgeochemicaldatadisplayanomalouspatternsoverandintheimmediatevicinityoftheknown"shallow"(>100feet-40m)and"deep"(>300feet-120m)orezones(Figs.12.8and12.9).Thesecouldbereadilydetectedinroutinegeochemicalsoilsurveysbasedon,say,a30mgrid.
Deeplyweatheredresidualsoilscanalsoprovideusefulgeochemicalsamplingmedia.AnexampleofuseoflateriticsoilsasaregionalgeochemicalreconnaissancesamplemediumisprovidedbyLewisetal(1989).Lateriticsoilsweresampledona400mgridinaUNexplorationreconnaissanceprogramforArcheanmetavolcanicshostedmassivesulfide(i.e.VMS)mineralizationintheWestAfricannationofBurkinaFaso.Follow-upsoilsamplingona25mgridspacingofsmallweakanomaliesdetectedinthereconnaissancephase(Fig.12.10)confirmedtheexistenceofadistinct550by250m2nanomaly(I.e.>200ppm)(Fig.12.11).SubsequentdrillingresultedinthedelineationofamajorVMSdeposit.)***'j*(*
*/***
**)*Heavy-metalcontentofcolluvium(ppm)>2000.500-2000J7-r:';?i6-o*'''V1*
p
Somptes//
VMagrudermmeMmeraliidzone
.r
FIG12.24aSamplelocationmapforMagruderMinearea,Georgia.(Meyereta!..1979)
FIG12.24bDownstreamdispersionfromzinc,copper,andleadinminus-80-meshstreamsedimentsandoxidecoatings.MagruderMinearea.(Meyeretal.,1979)
116
etal.(1985)recommendeduseofthe+250mesh(+62micron)fractionindesertareastoavoidproblemswithdilutionbyfineeoliansand.
QroanicdrainagesampleshavebeenusedinnorthernScandinavia(Fig.12.25)andelsewhereduetolackofnormalsedimentarymaterialforlongdistancesinstreamchannels(Larsson,1976).InScandinaviathesamplescompriseorganicdebrisinvariousstagesofhumificationandoftenpenetratedbythelivingrootsofvariousbogplantspecies.Elsewhereotherpotentialdrainagesamplemediahavealsobeenexamined.Forexample,aquaticmosseswerestudiedbyErdmanandModreski(1984)todeterminewhethertheymightprovideeffectivegeochemicalsamplemediain
mentsintheVehkavaaraarea,Pajaladistrict.(Larsson,1976)
areaswheresteepterrainpreventedaccumulationofstreamsedimentfinefractions.ThisclearlyconstitutesabiogeochemicalexplorationmethodandisthereforedescribedinmoredetailinSection12.2.6.However,aninterestingvariantoftheaquaticmossbiogeochemicaltechniquewasdescribedbySmith(1976)followingalimitedstudyofmineralizedareasinNorway.Moss-trappedstreamsedimentmaterialwasfoundtoprovidesimilarbuthighercontrastgeochemicalpatternstothoseproducedbynormalstreamsedimentsamples.
Intheregionalreconnaissanceprospectingmode,streamsedimentsurveyscanbedesignedtosystematicallycoverareasuptoseveralthousandsquaremiles.Averagesamplingdensitiestendtobesignificantlyhigherthanthoseemployedingeochemicalmappingprogramsastheemphasisisondetectionofdispersiontrainsrelatedtoindividualmineraldistrictsand/ordeposits,ratherthanbroadmineralprovinces.Densitiesarefrequentlyintherange1sampleper1-3km,whilst1sampleper20kmwouldbeconsideredunusuallylow.
Ashasbeendiscussedpreviously,inallsurveysinnewareas,thecriticalparametersofsampleinterval,sedimentsizefraction,appropriateanalyticalprocedures,significantanomalycontrasts,andbackgroundlevelsaredeterminedthroughorientationsurveys.Inareaswherenopreviousexperienceexists,ashortintervalof150ft.(50m)overaninitialdownstreamdistanceof1050ft.(350m)isrecommended.Thisintervalshouldthenbeprogressivelyexpandedwithdistancefromthemetalsourcetothelimitsoftheknownoranticipateddispersionpattern.Samplesmustalsobecollectedfromnon-
mineralizedareastoestablisiithebackgroundrangeandsufficientmaterialshouldbecollectedateachsitetoallowforthedeterminationofoptimumsizefractions,analyticaltechniques,andotherfactorslistedinTables11.1,and11.4).
12.2.4.LakeSediments
Lakesedimentsamplinghasbeendevelopedintoaneffectivegeochemicalreconnaissancetechnique,particularlywithintheCanadianPrecambrianShield,butalsowithintheFennoscandianShieldandtheCordilleranandAppalachianregionsofNorthAmerica.Theidealterrainforthistechniqueiswherelakesarecommon,conditionsareswampy,and/orwherestreamdrainagesareinaccessibleorpoorlydeveloped(Cokeretal.,1979).Inlowreliefregions,thelakesedimentmediumisdependentonthehydromorphicdispersionofmetalsintothelakeenvironmentthroughgroundwatersandtheadsorptionofthismetalontohydrousoxidesandtheorganicrichmuds
5m
FIG12.26
117
10-3cm
Eyeforattachingline
Threadsforattachingrigidrods
Outletventforwaterforcedthroughvalve
Ball-and-socketvalve
Sharpenedendoftubeforcuttingsample
Cut-awaysectionofsamplebailerforlake-sedimentsampling.(Roseetal,1979)
(i.e.gytia)beingdepositedonthelakebottoms.Thesamplinggenerallyfocusesonthecollectionoftheseorganicmudsusingspeciallydesignedsamplingdevices{Fig.12.26).Inmoremountainousareas,finegrainedclasticdispersionintothelakesedimentbecomesamoreimportantfactor.Inmostareassatisfactorysamplelocationsarefoundwellawayfromlakeshoresandarereachedusingboats,floatplanesorhelicopters.However,nearshorematerialshavebeensuccessfullyusedinsomeprogramsinthenorthernpartoftheCanadianShieldalthoughthesearegenerallysubaqueousequivalentsofglacialandpostglacialsedimentsonthemarginsoflakesandnottruelakesediments.Lakewatersamples{seeSection12.2.7.)arecommonlycollectedatthesamesitesasthelakesediments.
Thelakesedimenttechniquehassuccessfullyindicatedthepresenceofseveralimportantformsofmineralizationasthefollowingexamplesclearlydemonstrate.InSaskatchewantheKeyLakeandRabbitLakeuraniummineralizationsandassociatedanomalousglacialdispersiontrainsarereflectedbyextensivelakesedimentanomalies(Figs.12.27and12.28).Equallyimpressiveanomaliesarefoundinthevicinityofthe
urjlfiiunigon
>1000
m501-1000
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(0Stc
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FIG12.27DistributionofUinlakesedimentsinthevicinityoftheKeyLakeU-Nideposit,Saskatchewan.(Cokereta!.,1979)
FIG12.28Uranium(ppm)inlakesedimentsneartheRabbitl_akeuraniumdeposit.Saskatchewan.Locationofdepositshownbysolidtriangle.(Cokeretal.,1979)
Agrcolamassivesulfidedeposit,NorthwestTerritories(Fig,12.29),andinfactassistedinitsoriginaldiscovery(Coker,1979).Morerecentlyanumberofauthorshavereportedontheapplicationoflakesedimentgeochemistrytogoldexploration.McConnellandDavenport(1989)carriedoutextensiveorientationstudiesinNewfoundlandbasedonthegeochemicalanalysisoforganicsedimentcollectedfromlakecenters.Itwasdeterminedthatmost,butnotallknownAuoccurrencesweredistinguished
METAVOLCANICS
GRANITES
119
108-"07nNesrshorel8i:esedimentsMETASEDIMENTS.\.90
byanomalousAuconcentrationsinnearbytakesediments(Figs.12.30and12.31).Pathfinderelements(Sb,As,Pb,
GeologicalboundaryMassivesulphicfebody
CuandZn)displayinconsistentrelationshipstogoldmineralizationanditwasconcludedthat
FIG12.29
DistributionofZn(ppm)innearshorelakebottommaterials.AgrcolaLakearea,N.WT.(Cokereta!,1979)
Auistheonlyuniversalindicator.Theysuggestthatfordetailedexplorationasamplingdensityofatleast1sampleper4-5kmisnecessary.
UsefulreviewsoftheapplicationoflakesedimentgeochemistryinmineralexplorationinCanadaareprovidedbyHornbrook(1989)andFriske(1991).
12.2.5.GlacialSediments
ExtensiveQuaternaryglacialdepositsoccurringovermostofCanadaandthenorthernUnitedStates,northernEurope,northernAsia,Geenland,andanumberofhighelevationareasinthesouthernhemispherehavepresentedmajorchallengestoexploration.Asabetterunderstandingoftheoriginandformationoftheseglacialsedimentshasgrown,theirblanketingpresencehasbecomeprogressivelylessformidableandeffectiveexplorationtechniqueshavebeendeveloped.
MineralizedbouldertracinginglaciatedregionsisanestablishedtechniqueofthetraditionalprospectorinScandinaviaandpartsofCanada.InScandinavia,dogshavebeentrainedtoassisttheprospectorbysensingSOgreleasedfromoxidizingsulfidebouldersatshallowdepthsbelowthesurface.InFinland,methodsweredevelopedfor
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downice).Theshortdispersiontrainisthoughttoberelatedtothepresenceofabedrockridgedownicefromtheauriferousveins.
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FIG12.43CoppervaluesinsurfacesoilinareashowninFig.12.42.(Cole,1980)
Somespeciespreferentiallyconcentratemetalsinspecifictissuessuchasleaves,twigs,bark,orwood.Itisthereforeveryimportanttoestablishthemostfavorabletissuesforsamplingonceausefulspecieshasbeenidentified.Thiscomplexityisaccentuatedbythefactthatmetaluptakemayvarywithaspectandseason(Table12.3).Intemperateforestregions,accelerateduptakeandhigherconcentrationcommonlyoccursduringthespringgrowthfollowingadormantwinterseason.Inhotdesertregions,followingtheexhaustionofavailablenear-surfacewaterduringthedryseason,deeprootedplantswilltapthedeeperground-waters.Becauseoftheseseasonalvariations,biogeochemicalsurveysmustbecompletedquicklyintheoptimumperiod(s)definedbytheorientationstudies.
Thesevariablesmakebiogeochemicalsamplingaveryspecializedexercise.Someexpertiseinbotanyaswellasexplorationgeochemistryisessentialforboththeorientationstudiesandthesupervisionofvegetationsurveys.Ontheotherhandthebasicfieldequipmentrequiredforbiogeochemicalsamplingisverysimple(Dunn,1991):
AlderTwigsGold(ppb)inashAlderLeavesGold(ppb)inash
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