Geologicaf Survey of Finland, Current Research 1995-1996Edited by Sini Autio.Geological Survey of Finland, Special Paper 23, 25-28, 1991.

THE FINGERPRINTS OF KNOWN GOLD OCCURRENCES IN THE KUUSAMO SCHISTBELT AS SHOWN BY AIRBORNE GAMMA-RAY SPECTROMETRIC DATA

byHilkka Arkimaa

Geological Survey of Finland, P.O. Box 96, FIN-02 l5l ESPOO, FINLANDe-mail : hilkka.arkimaa@ ssf.fi

Key words (GeoRef Thesaurus, AGI): mineral exploration, gold ores, schist belts, gamma-ray methods, airborne methods,uranium, imagery, Proterozoic, Kuusamo, Finland

Introduction

The gold bearing deposits in the early ProterozoicKuusamo volcano-sedimentary schist belt can be

divided into two main ore types, the first being ofthe replacement sulphide ore type and the other ofthe breccia and vein ore type (Pankka & Vanhanen1989, Pankkaet al. 1991, Pankka1992). Depositswith features of both types also exist. The two mainore types differboth geologically and geophysically.In ductile shear zones the replacement type con-tains more sulphides and therefore produces smallbut distinctive magnetic and electromagneticanomalies unlike the non-magnetic and resistivehost rocks. The other type, consisting of hydro-thermal breccia pipes in brittle shear zones, usuallycontains less sulphides and cannot be detected bymagnetic and electromagnetic measurements. Themost successful ground measurements in theexploration of breccia type deposits have been

radiometric and IP methods (Pankka et al. l99l).Typical of the many gold occurrences in the

Kuusamo schist belt is their enrichment in uraniummainly in the form of uraninite and brannerite.Detailed radiometric ground surveys led to thediscovery of Au-Co-U-bearing mineralizations at

Konttiaho and Sivakkaharju in the mid- 1980s (Van-hanen 1989). Airborne magnetic and electromag-netic measurements have been used with success ingold exploration in the Kuusamo schist belt. En-couraging results have been obtained also fromstudies of the use of airborne gamma-ray data in

searching fbr fingerprints of the known gold occur-rences in the area (Arkimaa 1996).

The main features of the data

After correction for background, height and scat-tering and estimation of the apparent radioelementconcentrations, the airborne gamma-ray spect-rometric data still showed some level errors be-tween flight lines. These were corrected by medianfiltering. For image processing, grids with a cellsize of 50 m * 50 m were interpolated.

The multichannel and statistical nature of gammaradiation allows the effective use of digital imageprocessing techniques in enhancing and analysingthe data. Even slight changes in the concentrationof radioelements can be outlined (Fig. l).

Approximately 90Va of the gamma radiationmeasured comes from the upper 0.5 m of the ground(Grasty 1976). The usefulness of the data is not,however, limited only to areas of extensive outc-ropping. In glaciated areas the use of gamma radia-tion is comparable to that of geochemical data. Byconsidering the transport distances for glacial till,the bedrock source can be found. The transportdistances, based on geochemical studies, are knowngenerally to be of the order of a few hundred metresin Finland (Salminen 1993). The precence ofoverburden and the moisture content of soils, how-ever, reduce the amplitude of the radioactivity ofthe underlying bedrock. These effects can be mini-mized by using the ratios of different radioelements

25

Geologieal Survey of Finland , C urrent Research 1995- 1996 Edited by Sini AUlio. Geolog ieal Survey of Finl and . Spec ia l Paper 23 , 25-28 . 1997.

THE FINGERPRINTS OF KNOWN GOLD OCCURRENCES IN THE KUUSAMO SCHIST BELT AS SHOWN BY AIRBORNE GAMMA-RAY SPECTROMETRIC DA TA

by Hilkka Arkimaa

Geo logieal Survey uf Finland , P.O. Bo x 96 , FIN-02 151 ESPOO, FINLAND e- mail: hilkka. arkimaa @gs f.fi

Key words (GeoRef Thesaurus, AG!): mineral exploration , gold ores, sehist beils , gamma-ray methods, airborne methods, uranium , imagery , Proterozoie , Kuusamo, Finland

Introduction

The gold bearing deposits in the ear ly Proterozoic Kuusamo volcano-sedimentary schist belt can be divided into two main ore types, the first being of the replacement sulphide ore type and the other of the breccia and vein ore type (Pankka & Vanhanen 1989, Pankka et al. 199 I, Pankka 1992). Deposits with features ofboth types also exist. The two main ore types differ both geologically and geophysically. In ductile shear zones the replacement type con­tains more sulphides and therefore produces small but distinctive magnetic and electromagnetic anomalies unlike the non-magnetic and resistive host rocks. The other type, consisting of hydro­thermal breccia pipes in brittle shear zones, usually contains less sulphides and cannot be detected by magnetic and electromagnetic measurements. The most successful ground measurements in the exploration of breccia type deposits have been radiometric and IP methods (Pankka et al. 1991).

Typical of the many gold occurrences in the Kuusamo sc hi st belt is their enrichment in uranium mainly in the form of uraninite and brannerite. Detailed radiometric ground surveys led to the discovery of Au-Co-U-bearing mineralizations at Konttiaho and Sivakkaharju in the mid-1980s (Van­hanen 1989). Airborne magnetic and electromag­netic measurements have been used with success in gold exploration in the Kuusamo schist belt. En­couraging results have been obtained also from studies of the use of airborne gamma-ray data in

search i ng for fingerprints of the known gold occur­rences in the area (Arkimaa 1996) .

The main features of the data

After correction for background , height and sc at­tering and estimation of the apparent radioelement concentrations, the airborne gamma-ray spect­rometric data still showed some level errors be­tween flight lines . These were corrected by median filtering. For image processing, grids with a cell size of 50 m * 50 m were interpolated.

The multichannel and statistical nature of gamma radiation allows the effective use of digital image processing techniques in enhancing and analysing the data. Even slight changes in the concentration of radioelements can be outl ined (Fig . I).

Approximately 90% of the gamma radiation measured comes from the upper 0.5 m ofthe ground (Grasty 1976). The usefulness of the data is not, however, limited only to areas of extensive outc­ropping. In glaciated areas the use of gamma radia­tion is comparable to that of geochemical data. By considering the transport distances for glacial till, the bedrock source can be found. The transport distances, based on geochemical studies, are known generally to be ofthe order of a few hundred metres in Finland (Salminen 1993 ). The precence of overburden and the moisture content of soils, how­ever, reduce the amplitude of the radioactivity of the underlying bedrock. These effects can be mini­mized by using the ratios of different radioelements

25

Geological Survey of Finland, Special Paper 23

Hilkka Arkimua

Fig. l.Loca.tionofKuusamoareaandadditivecolourcompositeoftheairbornegamma-raydatacoveringthestudyarea (eU=rr:d, eTh=green. K=blue).

(Charbonneau et a.l. 1976).

The study at the scale of lithostratigraphicalformations

Hydrothermal alteration processes may modifythe radioelement composition of rocks penetratedby fluids. Under lower-temperature hydrothermalconditions, U-bearing minerals are particularly sus-ceptible to dissoluLtion, and as a result U is trans-ported and redeporiited diff-erently fiom Th and K.One typical outcolne of hydrothermal processes isthe enrichment of wall-rock alteration zones in K(Darnley & Ford 1987).

In the Kuusamo schist belt the ore-potential unitcomprises the Sericite Quartzite Formation, Siltsto-ne Formation and,Greenstone Formations I and II.The main surröunding fbrmations are RukatunturiQuartzite Formatic,n, Archean Granite Gneiss Com-plex and Greenstone Formation III (Silvennoinen1912, Pankka et al. 199l ). The ratios of eU, eThand K characteristic of the ore-potential formationswere studied using a triangular diagram and com-pared with the corresponding ratios for the surround-

26

ing formations. As the radioelement values are notconstant over areas covered by water, these valuesas well as those measured over wetlands wererejected. In Figure 2 the ratios have been dividedinto two groups: a) those indicating all ore-poten-tial formations and b) those indicating the surroun-ding formations. By merging these two distribu-tions into the same image it was possible to estab-lish the differences between them (Fig. 2c).

The centres of the two distributions almost coin-cide but their spreading directions diff'er. The mostdistinctive enrichment of eU and eTh relative toother radioelements characterizes the distributionof the ore-potential formations compared to thesurrounding fbrmations. The effect of K-silicatealteration in the host rocks of the gold occurrencesin the Kuusamo schist belt is not clear at this scale,and thus studies at a larger scale are needed if it isto be identified by a method such as this.

The study at the scale of occurrences

Due to the enrichment of the gold-bearing occur-rences in the Kuusamo schist belt in uranium. the

Geolog ica l Survey o f Finl and , Spec ia l Paper 23 Hilkka Arkill1aa

Fig. I. Location o f KlIlI samo a rea and additi ve colollr composite ofthe a irborne gamma-ray dala covering lhe sllldy a rea (eU=red , eTh=green. K=blll e).

(Charbonneau et al. 1976).

The study at the scale of Iithostratigraphical formations

Hydrothermal alteration processes may modify the radioelement c:omposition of rocks penetrated by fluids. Under lower-temperature hydrothermal conditions, U-bearing minerals are particularly sus­ceptible to dissolution, and as a result U is tran s­ported and redeposited differently from Th and K. üne typical outcome of hydrothermal processes is the enrichment of wall-rock alteration zones in K (Oarnley & Ford 1987).

In the Kuusamo schist belt the ore-potential unit comprises the Sericite Quartzite Formation, Siltsto­ne Formation and Greenstone Formations land Il. The main surrounding formations are Rukatunturi Quartzite Formation, Archean Granite Gneiss Com­plex and Greenstone Formation III (Silvennoinen 1972, Pankka et al. 1991). The ratios of eU, eTh and K characteristic ofthe ore-potenti al formations were stud ied using a triangular diagram and com­pared with the corresponding ratios for the surround-

26

ing formations. As the radioelement values are not constant over areas covered by water, these values as weil as those measured over wetlands were rejected. In Figure 2 the ratios have been divided into two groups: a) those indicating all ore-poten­tial formations and b) those indicating the surroun ­ding formations. By merging these two distribu­tions into the same image it was possible to estab­lish the differences between them (Fig. 2c).

The centres ofthe two distributions almost coin­cide but their spreading directions differ. The most distinctive enrichment of eU and eTh relative to other radioelements characterizes the distribution of the ore-potential formations compared to the surrounding formations. The effect of K-silicate alteration in the host rocks ofthe gold occurrences in the Kuusamo schist belt is not clear at this sca le, and thus studi es at a larger scale are needed if it is to be identified by a method such as this.

The study at the scale of occurrences

Oue to the enrichment of the gold-beari ng occur­rences in the Kuusamo schist belt in uranium, the

Geological Survey of Finland, Special P;rper 23The fingerprints of known gold occurrences in the Kuusamo schist

belt as shown by airborne gamma-ray spectrometric data

Fig.2.TheratiosofeU,eThandKindicatinga)ore-potential formations,b)non-potential formationsandc)bothof theirboveinthesame diagram. Yellow indicates overlapping areas. The number of ratios is densest at the centre of the distribution (black area).

uranium anomalies around each occurrence werecarefully studied. At sites where the overburden wasless than I m thick the indicative uranium anomalywas directly above the occurrence. At sites where the

soil cover was thicker there were no anomaliesdirectly above the occurrences except at the largestgold deposit, Juomasuo. The main ice flow direc-tions, also shown clearly by gamma-ray data (Fig. I ),allows us to assume that the indicative radiometricanomalies are to be found in a direction somewherebetween east and south. Around each occurrence,within a radius of 250 m, the nearest uranium ano-maly in the direction of ice flow is considered toindicate the target. If no anomaly was found in thatdirection, the nearest one in any other direction was

selected to indicate the target.Closer study of the distribution of indicative ratios

showed that they could be divided into two maingroups. One group contained all the occurrences inwhich the indicative uranium anomaly lies di-rectly above the occurrence or in a direction betweeneast and south (red dots in cover of this is-

sue). The other group comprised all the remainingoccurrences. that is. those in which no uranium has

been found or the overburden is about 10 m thick(green dots in the cover). At Juomasuo the sources

of the radiometric anomalies are less distinct due tothe thickness of the overburden, which, at l0 m, may

include long-distance material. On the other hand,

the enrichment of K relative to other radioelements,may also reflect the influence of potassium alter-ation of the host rocks (yellow dots in the cover).This alteration zone, which is wider than the deposititself, runs fiom northwest to southeast through the

Juomasuo deposit (Pankka 1992). The red dots fur-thest to the right in the diagram in the covercorrespond to the Isoaho occurence, where uraniumenrichment in the occurence and potassium alter-ation in the host rocks are known from field studies(Vanhanen 1992).

Discussion

The enrichment of uranium tosether with eold in

27

Geological SlIrvey of Finland, Special Paper 23 The fingerprinls of known gold occlIrrences in the Kuusamo schist

bell as show n by airborne gamma-ray spectro metri c dala

Fi g . 2. The ratios of eU , eTh and K indicating a) o re-potential formations , b) no n-potential formations and c) both of the above in the same diagram. Yellow indi cales overlapping areas. The number of rati os is den ses t allhe cenlre of lhe di slribution (black areal ·

uranium anomalies around each occurrence were carefully studied. At sites where the overburden was less than 1 m thick the indicative uranium anomaly was directly above the occurrence. At sites where the soil cover was thicker there were no anomalies directly above the occurrences except at the largest gold deposit, Juomasuo. The main ice f10w direc­tions, also shown clearly by gamma-ray data (Fig. J) , allows us to assurne that the indicative radiometrie anomalies are to be found in a direction somewhere between east and south. Around each occurrence, within a radius of 250 m, the nearest uranium ano­maly in the direction of ice flow is considered to indicate the target. If no anomaly was found in that direction, the nearest one in any other direction was selected to indicate the target.

Closer study ofthe distribution ofindicative ratios showed that they could be divided into two main groups. One group contained all the occurrences in wh ich the indicative uranium anomaly lies di­rectly above the occurrence or in a direction between east and south (red dots in cover of this is-

sue). The other group comprised all the remaining occurrences, that is , those in wh ich no uranium has been found or the overburden is about 10m thick (green dots in the cover). At Juomasuo the sources of the radiometrie anomalies are less distinct due to the thickness of the overburden, which, at 10m, may include long-distance material. On the other hand, the enrichment of K relati ve to other radioelements, mayaiso reflect the influence of potassium alter­ation of the host rocks (yellow dots in the cover). This alteration zone, which is wider than the deposit itself, runs from northwest to southeast through the Juomasuo deposit (Pankka 1992). The red dots fur­thest to the right in the diagram in the cover correspond to the Isoaho occurence, where uranium enrichment in the occurence and potassium alter­ation in the host rocks are known from field studies (Vanhanen 1992).

Discussion

The enrichment of uranium together with gold in

27

Geological Survcy of Finlancl, Special Paper 23Hilkku Arkinnu

the Kuusamo schisl belt makes the use of gamma-rayspectrometric data feasible in exploration. The abun-dance of sulphides in breccia-type gold occurrencesis not always sufficient to be detected by magneticand electromagnetic methods. Fingerprints are there-fore worth seekinfl in gamma-ray data. As shownhere, ratios of radioelements can be used to classifygamma anomalies in order to find potential targetsfor gold exploration. The gold occurences in theKuusamo schist belt appear to be controlled by faultsystems. Although many of the bedrock structuresare well shown by airborne magnetic and electro-magnetic maps, some structural information can beextracted also fronr gamma-ray data when enhanc-ing by image processing methods.

Airborne gamma-ray data are aff-ected by manyfactors, e.g. the distribution of radioactive ele-ments in lithostratigraphical formations, the mine-r alizingproce sses modifyi ng these formations, gla-cial and, in general, Quaternary geology, and thewater content of the overburden. Thorough knowl-edge of the infornration is necessary if we are tounderstand the rolle of these factors. The presenttype of study, in which not only the absolute abun-dances but also the ratios of the radioelements areconsidered, contributes to our understanding ofsome of the factors.

References

Arkimaa, H. 1996. tSammasäteilyaineisto ja digitaalinenkuvankäsittely malrniennustetutkimuksissa Kuusamonliuskealueella. Unpubl ished licentiate thesis, University ofOulu. 68 p.

Charbonneau, 8.W., Killeen, P.G., Carson., J.M.,Cameron, G.W. & Richardson, K.A, 1976. Significanceof radioelerrent concentration measurements made by air-borne gamma-ray spectrometry over the Canadian Shield.In: Proceedings of a Symposium on Exploration for Ura-nium Ore Deposits, IAEA-SM-208/3, 35-53.

Darnley, A.G. & Ford, K.L. 1987. Regional AirborneGamma-Ray Surveys: A Review. ln: Proceedings of Explo-ration '87, Third Decennial International Conference onGeophysical and Geochemical Exploration firr Mineralsand Groundw ater, 229-240.

Grasty, R.L. 1976, Application of Gamma Radiation inRemote Sensing. In: Schandra, E. (ed.) Remote Sensing forEnvironmental Sciences. Berlin: Springer-Verlag, 257-276.

Pankka, H. 1992. Geology and mineralogy of Au-Co-Udeposits in the Proterozoic Kuusamo volcanosedimentarybelt, northeastern Finland. Unpublished doctor's thesis,Michigan Technological University. 233 p.

Pankka, H., Puustinen, K. & Vanhanen, E. 1991. Kuusamonliuskealueen kulta-koboltti-uraaniesiintymät. Summary:Au-Co-U deposits in the Kuusamo volcano-sedimentarybelt, Finland. Geological Survey of Finland, Report ofInvestigation l0l. 53 p.

Pankka, H. & Vanhanen, E. 1989, Aulacogen relatedepigenetic Au-Co-U deposits in northeastern Finland. Geo-logical Survey of Finland, Special Paper 10, 9l-94.

Salminen, R. 1993. Geokemiallisten karrrojen käyrrömalminetsinnässä. In: Haapala, I. (ed.): Malminetsinnänmenetelmät. Hel sinki : Yl iopi stopai n o, 246-256.

Silvennoinen, A, 1972. On the stratigraphic and structuralgeology of the Rukatunturi area, northeastern Finland.Geological Survey of Finland, Bulletin 251 . 48 p.

Vanhanen, E. 1989. Uraniferous mineralizations in theKuusamo schist belt, northeastern Finland. In: Proceedingsof a Technical Committee Meeting on Metallogenesis ofuranium Deposits, IAEA-TC-542/13, 169 186.

Vanhanen, E. 1992. Kuusamon Juomasuon kulta-kobolt-tiesiintymien lähiympäristön kultamalmitutkimukset vuo-sina 1986-199l Geological Survey of Finland, unpub-lished report Ml9/4613/-92ll/10. 5l p.

28

Geologieal Survcy 01' Finlancl , Special Paper 23 Hilkk a Arkill/a{/

the Kuu samo schist belt makes the use of gamma-ray spectrometric data feasible in exploration. The abun­dance of sul phides in breccia-type gold occurrences is not always sufficient to be detected by magnetic and electromagnetic methods . Finge rprints are there­fore worth seeking in gamma-ray data. As shown here, ratios of radioelements can be used to classify gamma anomalies in order to find potential targets for gold exploration. The gold occurences in the Kuusamo schist belt appear to be controlled by fault systems. Although many of the bedrock structures are weil shown by airborne magnetic and electro­magnetic maps, some structural information can be extracted also from gamma-ray data when enhanc­ing by image processi ng methods.

Airborne gamma-ray data are affected by many factors, e.g . the distribution of radioactive ele­ments in lithostratigraphical formation s, the mine­ralizing processes modifying these formations, gla­cial and, in general, Quaternary geology, and the water content of the overburden. Thorough knowl­edge of the information is necessary if we are to understand the ro le of these factors. The present type of study, in which not only the absolute abun­dances but also th e ratios of the radioelements are considered, contributes to our understanding of so me of the factors .

References

Arkimaa, H. 1996. Gam masätei lyaineisto ja digitaalinen kuvankäsittely malmi ennustetutkimuk s issa Kuusamon liu skea luee ll a. Unpubli shed li eenti ate thesis, Un iversity of Oulu . 68 p.

28

Charbonneau, B.W. , Killeen , P.G., Carson., J.M., Cameron, G.W. & Richardson, K.A. 1976. Signifieance of rad ioel ement concentration measurements made by a ir­borne gamma-ray spectro metry over the Canadian Shield . In: Proceedings of a Symposium on Exploration for Ura­nium Ore Depos its , IAEA -SM -208/3 , 35-53.

Darnley, A.G. & Ford, K.L. 1987. Regional Airborne Gamma-Ray Surveys : A Review . In: Proeeedings ofEx pl o­ration '87, Third Decennial International Conferenee on Geophys ica l and Geochemiea l Exp loration for Minerals and Groundwater, 229-240.

Grasty, R.L. 1976. Applieation of Gamma Rad iat ion in Remote Sensing . In: Schandra, E. (ed. ) Remote Sensing for Environmental Sciences. Berlin: Springer-Verlag, 257-276.

Pankka, H. 1992. Geology and mineralogy of Au-Co-U deposits in th e Proterozoic Kuusamo vo lcanosed imen tary belt, northeas tern Finland. Unpublished doctor's thesis, Mi chigan Technological University. 233 p.

Pankka, H., Puustinen, K. & Vanhanen, E.1991. Kuu samon I iuskealueen ku Ita-kobo l tri -uraan ies i i ntymät. Summary: Au-Co-U depos its in th e Kuu sa mo vo lea no-sed imentary belt , Finland . Geological Survey of Finland , Report of Investi gat ion 10 I. 53 p.

Pankka, H. & Vanhanen, E. 1989. Aulaeogen related epige ne ti c Au-Co-U deposits in north eas te rn F inland . Geo­log ieal Survey of Finland , Special Paper 10, 9 1- 94.

Salminen, R. 1993. Geokemialli sten karttoj e n käyttö malminetsinnässä. In : Haapala, I. (ed .): Malmine ts innän menetelmät. He lsinki: Yliopi stopaino , 246-256.

Silvennoinen, A. 1972. On the stratigra phi e and structural geo logy of the Rukatunturi area , northeastern Finland. Geologieal Survey of Finland , Bulletin 257. 48 p.

Vanhanen, E. 1989. Uranife rous mineralizations in th e Kuu samo schi st be lt , northeas te rn Finl and . In: Proeeed ings of a Technical Com mittee Meeting on Metallogenes is of Uraniulll Deposits , IAEA-TC-542/ 13, 169-1 86 .

Vanhanen, E. 1992. Kuu samon Ju omas uon kulta-kobolt­tiesiintYlllien lähiYlllpäri stön kultamalmitutkimukset vuo­s i na 1986- 199 1. Geological Survey of Finland , unpub­li shed repo rt MI9/4613/-92/ I/ IO. 5 1 p.