31. ON THE APPLICATION OF GEOPHYSICS IN THE INDIRECT EXPLORATION FORCOPPER SULPHIDE ORES IN FINLAND

M. KetolaOutokumpu Oy, Exploration, Finland.

Ketola, M., On the application of geophysics in the indirect exploration for copper sulphide ores inFinland; 0:. Geophysics and Geochemistry in the Search for Metallic Ores; Peter J. Hood, editor;Geological Survey of Canada, Economic Geology Report 31, p. 665-684, 1979.

Abstract

Exploration for sulphide ores in Finland is becoming increasingly difficult. Most of the easilydetectable economic sulphide ores, whose location is indicated either by outcropping mineralization orby measurable geophysical anomalies, have probably been discovered. Hence measurements conductedby conventional geophysical methods are less and less able to pinpoint an anomaly caused by an oredeposit. For the time being, however, the survey data collected by various geophysical results canoften be utilized in indirect exploration, in which they serve as a supplement to the geologicalinformation available on the survey area. This paper presents some examples to illustrate the principleof combining various geophysical methods and adapting them to the exploration for sulphide ores inFinland.

Resume

L'exploration des minerais sulfures en Finlande devient de plus en plus difficile. La plupart desgftes sUlfures "facilement reperables", ayant une valeur economique, et dont la position est indiqueesoit par des affleurements mineralises, soit par des anomalies geophysiques mesurables, ontprobablement deja 1318 decouverts. Ceci explique que les mesures decoulant des methodes geophysiquesconventionnelles permettent de plus en plus rarement de localiser des anomalies dues d la presenced'un gisement metallifere. Mais pour l'instant les resultats de divers leves geophysiques, qui apportentun appoint d'information geologique, permettent souvent une exploration indirecte dans la regionetudiee. Dans Ie present rapport quelques exemples illustrent la methode, qui consiste d combinerdiverses methodes geophysiques, et dIes adapter d l'exploration des minerais sulfures en Finlande.

metal mineralization. The method has been discussed inseveral publications, e.g. those by Ryss (1973), Parasnis (1974)and the U.S. Exchange Delegation in MiningGeophysics (1976).

INTRODUCTION

In Finland, where the bedrock is intenselymetamorphosed and is almost completely of Prer:ambrian age,sulphide ore prospecting concentrates mainly on areas withall but ubiquitous graphite-bearing schists. These rocks, whichin Finland are usually called black schists or phyllites, containvariable amounts of pyrrhotite and pyrite. Recause thethickness of the overburden is mostly less than 30 m and itsresistivity, except in coastal areas, is high, varying accordingto Puranen (1959) from 50 to 30 000 Qm, black schists arenormally delineated by means of electrical methods. If theblack schists contain appreciable amounts of pyrrhotite, asthey often do, they can also be detected by magneticmeasurements.

Prospecting for SUlphide ores in black schist areas ishowever becoming increasingly more difficult. Most of theeasily detectable economic sulphide deposits (i.e. ores) whichcan be localized either by means of a mineralized exposure ora conspicuous geophysical anomaly, have probably alreadybeen found.

It is typical of black schist areas that the number ofanomalies resulting from magnetic and electrical surveys isfar too high to make it economically feasible to check themall by diamond drilling. Seldom can a geologist establish thecause of an anomaly on an exposure. Pyrrhotite-bearinggraphite schists and associated mineralization have usuallybeen eroded more deeply by glacial ice than the surroundingcountry ror:k and thus they are mostly covered by overburden.

A fair number of methods are available for classifyinggeophysical anomalies and selecting explorationally­interesting anomalies in black schist areas, e.g. thesimultaneous use of a combination of geophysical andgeochemical methods. The contact polarization curve methoddeveloped in the Soviet Union and which uses the anodic andcathodic electrochemical reactions produced at the contactof mineralization with country rock, enables anomaliescaused by graphite to be discerned from those caused by base Figure 31.1. Location of copper orebodies in Finland.

666 M. Ketola

Because it is difficult to discriminate betweengeophysical anomalies caused by economic sulphidemineralization and anomalies caused by black schists andother rocks, geophysical data are also utilized in so-calledindirect exploration, in other words, in supplementinggeological information. In the black schist areas of Finland,geophysical surveys are mainly conducted by magnetic,electromagnetic and gravity methods. The IP method is usedonly in special cases, owing to the intense electromagneticanomalies caused by the black schist zones. Copper ores thatoccur in the black schist areas pose a very difficultexploration problem. This paper deals with geophysical resultsand their use in indirectly exploring for copper ores withinblack schist areas.

The copper deposits of Vuonos and Saramaki in theOutokumpu area in eastern Finland and the Pahtavuomadeposit, associated with the greenstone area of Kittila, innorthern Finland, have been selected as examples. Theaverage copper content at Vuonos is about 2.5 per cent, atSaramaki 0.7 per cent and at Pahtcwuoma 1.0 per cent. A mapof Finland showing the location of the deposits discussed ispresented in Figure 31.1.

The ores of Vuonos and Saramaki are of the Outokumpu­type, which means that they are compact copper ores withsome cobalt. The ores occur in a geological formation that issurrounded by mica gneiss and is characterized byserpentinites, dolomites, skarns, quartz rocks and blackschists. By means of aerogeophysical maps, this rock complexknown as the Outokumpu association, can be traced for morethan 240 km.

The Outokumpu deposit, averaging 3.80 per cent Cu and0.20 per cent CD, was discovered in 1910. The orebody lies ina depression in association with serpentinite-quartziteformations. The total length of the ore deposit is 4 km. Thewidth fluctuates from 200 to 400 m in the central part of thedeposit but tapers off to the southwest at a depth of 150 m.At the extreme northeastern end, the orebody outcrops. Theore varies in thickness from a few metres to 40 m, theaverage being about 10 m. At the northeastern end thedeposit dips 30° to 60° SE but is almost subhorizonti'J! in thesouthwestern end. Some of the publications in which thegeology of the LJutokumpu and Vuonos ores is discussed, arethose by Vahatalo (1953), Isokangas (1975) and Gaal, Koistinenand Mattila (1975). The latter paper includes a slingram(horizontal loop EM) map of the Outokumpu zone covering anarea of 150 km 2

•

THE GEOPHYSICS OF THE ORE DEPOSITS

The Vuonos orebody. eastern Finland

The Vuonos orebody, located about 4 km northeast ofOutokumpu, was found in 1965. Preliminary drillingdemonstrated that the Vuonos mea, underlain by thickserpentinite-quartzite formations, was analogous to theOutokumpu areH. LithogeochemicHl investigations of drillcores, analyzed for SUlphide copper, nickel and cobalt, showedthat the copper content and the nickel-to-cobalt ratio in theserpentinites at Vuonos were anomalous compared with theserpentinites in the areas free from cobalt mineralization.Hakli (1963) has shown that in areas where serpentinites arenot associated with copper-cobalt mineralization their nickel­to-cobalt ratio is similar to that in ultramafic rocks andvaries from 20:1 to 25:1. Follow-up drilling led to thediscovery of the Vuonos ore, whose average Ni-to-Co ratiois 2:3.

The geological and lithogeochemical investigations ofthe Outokumpu area have been reported by Huhma andHuhma (1970).

The Vuonos orebody is about 3.5 km long and 50 to200 m wide. The average thickness is 5 to 6 m but it mayattain as much as 20 m. The ore tapers out gradually towardsboth ends. The southwestern end is at a depth of about 60 mand the northeastern at a depth of about 200 m. Hence theorcbody is blind.

The magnetic, slingram and gravity maps of the Vuonosarea are presented in Figure 31.2 and show that thesubhorizontal copper ore is not detectable by those methods.Figure 31.3 shows the geophysical results along profiley = 194.25 directly across the Vuonos are body. Thepetrophysical data is plotted in profile form along the holesdrilled in the profile. Resistivity values were obtained by insitu measurements by the single-point method, the smallestmeasured resistance being 1 S"l. The susceptibility and densitydeterminations were done in the laboratory on drill coresamples about 10 cm long, at intervals uf 1 m for the wholelength of the hole. Considering the geological andpetrophysical information, the density of the conductingchrysotile serpentinite overlying the ore is low and thesusceptibility higher than that of the surrounding rock types.The negative slingram anomalies are due to the black schiststhat, in conjunction with serpentinite, also cause magneticanomalies. The susceptibility of the skarn is low, but thedensity is higher than that of other rock types, and so theserocks give rise to the positive gravity anomaly measured onthe profile. Due to its horizontal position and great depth, thehigh density conducting orebody cannot be localized by meansof geophysical methods because the anomalies produced bythe country rocks camouflage the weaker anomaly producedby the ore.

The Saramiiki orebody. eastern Finland

The Saramaki copper ore of Outokumpu-type is locatedin a formation that consists of skarns, serpentinites,quartzites and black schists surrounded by mica gneiss. Theare, which was found by drilling, outcrops. Nontheless it isnot possible to localize it on the basis of the geophysicalresults depicted in Figure 31.4. The geology and resistivityresults plotted along the drillholes show that the wholeformation is a good conductor. The negative slingramanomalies are mainly caused by the black schists. Thesusceptibility values suggest that the formation isheterogeneously magnetized and that its upper part producesthe strong magnetic anomaly. The gravity anomaly isgenerated by the whole formation, the density value of whichdiffers clearly from that of the surrounding mica gneiss.

The Pahtavuoma orebody. northern Finland

The four ore lenses, found after the discovery of somecopper-bearing outcrops at Pahtavuoma, are situated in aphyllite schist zone in the southern part of the Kittila green­stone formation. The phyllite zone strikes from east to west.The phyllites are graphite-bearing and cause the strongslingram and VLF -EM anomalies shown in Figure 31.5. TheVLF -EM map suggests that the phyllites are notably morecontinuous along the strike than does the slingram map, onwhich the anomalies are discontinuous from east to west. Thisis probably due to the higher frequency and better depthpenetration of the VLF -EM method. The outcrops of the twoore lenses (A ore, the largest, and Ulla are) are depicted onthe geophysical maps in Figure 31.5. It is clear from themagnetic and gravity maps that the phyllite zones and arelenses cannot be located by geophysical techniques. Onereason is that the pyrrhotite content of the phyllites is verylow. In Figure 31.6, the geophysical and petrophysical data ofthe profile y =11.9 from Figure 31.5 me presented togetherwith the geological information. The magnetic curve drawn inthe upper part of the figure is almost nonanomalous, and the

Copper Sulphide Ores in Finland

MAGNETIC MAPCONTOURS ±250, 500,1000 nT etc.

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GRAVITY MAPCONTOURS: 0.1

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Figure 31.2. Geophysical maps of the Vuonos area.

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o 100m o 100m a 100m

Figure 31.3. Geophysical and petrophysical data from profile y =194.25 in the Vuonos area.The values on the drillhole profiles are respectively in n, x 10 5 Sf and gm!cc.

Copper Sulphide Ores in Finland 669

SARAMAKI OR E BODY

SARAMAKI ORE BODYSLINGRAM AND RESJSTIVITY CROSS SECTION

PROFILE X= 81.400

ORE BODY

susceptibility determinations made on drillcores gave values that were too low to warranttheir inclusion in Figure 31.6. The resistivitymeasurements conducted in drillholesdemonstrate that the slingram anomalies arecaused by phyllites and that the ore cannot hedetected electromagneticLllly. In the lower pmtof Figure 31.6, next to the drillholes, theaverage density values for different rock typeshave been drawn. The density of phyllites islower than thLlt of greenstones. Although t.healternation of phyllites and greenst.ones is notclearly seen frDm t.he gravity curve due to t.hesteep gradient, these rocks can be localized onthe residual anDmaly map.

Kokkola and Korkalo (1976) havedescribed the till- and streilm-sedimentinvestigations in the Pahtavuoma area. Soilsurveys, whose purpose was to classi fygeophysical anomalies, turned out to bedi fficult to intcrpret owing to the existence offive till beds and layers deposited by ice thatmoved successively in different directions Llndbecause the shape of a gcochemicLlI anomaly intill depends on the direction of glacialtransport.

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GEOPHYSICAL METHODS INGEOLOGICAL MAPPING

Because it has not been possible bymeans of geophysical methods to localizedirecLly the copper ores at Vuonos, Saramiikiand Pahtavuoma, survey data have and willcontinue to be used for indirect exploration inthese areas.

The following chapter gives someexamples of the use of airborne, ground anddrillhole methods in the indirect exploration forsulphide ores.

Figure 3104.

Geophysical and petrophysical data from profilex = 81.4 over the Saramiiki orebody.

Airborne geophysical surveys

The first phase of exploration of extensiveareas entails airborne magnetic andelectromagnetic (AEM) surveys. These arepresently being conducted at a low flightaltitude and with a dense line spacing, becausethe general aerogeophysical maps made by theGeological Survey of Finland from a flightaltitude of 150 m already cover the wholecount.ry. The aim of the low-altitude airbornegeophysical surveys is to obtain a picture of themagnetized and conductive rock types of thearea under investigation in as great a detail aspossible. Because thc thickness of theoverburden in Finland averages only 8 m, rigidairborne electromagnetic systems are eminentlysuitable for mapping the magnetic andconductive rock types in black schist areas thatsuboutcrop under the overburden.

Figure 3J.7 shows an aeromagnetic andAEM map of the Miihkali area about 10 kmnorth of Saramiiki surveyed from ,m altitude of40 m with a 125 m linespacing. The AEM

o 100m'"---__------'1

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Copper Sulphide Ores in Finland 671

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Figure 31.6. Geophysical and petrophysical data from profile y = 11.9 in the Pahtavuoma area.

Copper Sulphide Ores in Finland 671

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Figure 31.6. Geophysical and petrophysical data from profile y = 11.9 in the Pahtavuoma area.

672 M. Ketola

AEROMAGNETIC MAPCONTOURS: 200,400,600 nT etc. f'UGHT ALTITUDE 40M

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Figure 31.7. Comparison between airborne and ground geophysical data from theM iihakali area.

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AE ROE LECTROMAG NETIC MAP (WING TIP, REAL COMPONENT)

CONTOURS: 400,800,1200 ppm etc. FLIGHT ALTITUDE 40M

Figure 31.8. Comparison between airborne and ground geophysical data from the Pahtavuoma area.

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674 M. Ketola

Figure 31.9. Comparison between airborne and groundelectromagnetic data from profile x = 11.9 in Pahtavuoma area.

a=16 m f= 3600 cps

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the magnetized formation is conducti vc and that its density islow. It is a chrysotile serpentinite body in the northern partof which there are black schists that cause negative slingramanomalies.

In the localization of ultrabasic formations, especiallywhen prospecting for nickel ores, systematic gravitysurveying has turned out to be of much practical importance.Presented in Figure 31.12 A and B are the results ofmagnetic, slingram and gr,wity measurements performed intwo different black schist areas. In both areas the blackschists cause strong slingram anomalies. Figure 31.12 B showsa gravity anomaly caused by a peridotite body, in thenortheastern end of which there is nickel mineralization thatalso causes a weak magnetic and slingram anomaly. A strongpositive anomaly is depicted on the gravity :nap inFigure 31.12 A. This anomaly is produced by a pendotlte bodythat cannot be discovered by means of a magnetic survey asthe magnetic results indicate.

Experimental reflection seismic measurements havebeen conducted in the Outokumpu area to trace the deep­seated continuation of the geological formations. Themeasurements and interpretation were the work of a teamfrom the Institute of Seismology in Helsinki University underthe leadership of Dr. E. Penttila. Figure 31.13 is a simplifiedreflection seismic cross-~;ection obtained across the

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measurements were obtained using the Coplanar-coil wing-tipsystem. For the sake of comparison, the slingram realcomponent map of the area investigated by airborne surveysis also shown in Figure 31. 7. The correlation between theAEM and ground EM results is excellent bearing in mind theinevitable broadening of the AEM anomalies with height.Experience has shown that if the flight lines arcperpendicular to the strike of the conducting zones and theflight-line positioning is sufficiently accurate, the Coplanar­coil wing-tip AEM system gives results that closelycorrespond to those of the slingram method. If the directionof the flight lines coincides with the strike of the conductingzones, AEM measurements do not produce results as good asthose obtained by ground EM surveys, because the latterallow a ready change of measurement direction to meet therequirements of the strike of the geological formations.

The airbome and ground geophysical surveys of thePahtavuoma area are displayed and compared in Figure 31.8.The east-west striking, phyllite belt is indicated by the realcomponent map of the AEM survey. The anomalies of theAEM map display a good correlation with slingram anomalies.The profiles in Figure 31.9 confirm the good correlationbetween the wing-tip and slingram results above the phyiliteformation that enclose the mebodies in the Pahtavuoma area.The Pahtavuoma case indicates that AEM surveys providedetailed geological information in areas where the magnetiteand pyrrhotite contents of the bedrock are so small thatonly weak anomalies occur. Thus the AEM techniquemay be used as a geological mapping tool in such areas.

Ground geophysical surveys

The detailed results of low-altitude airbornemagnetic and electromagnetic surveys have reduced theneed for ground measurements, so these have beenconcentrated on those areas favourable for theoccurrence of ores. In order to augment geologicalinformation as much as possible these areas areinvestigated using several ground geophysical methods.The most commonly used nre magnetic, slingram andgravit y measurements carried out with a 20 m stationspacing and 50-100 m line separation. The geophysicalresults of aeromagnetic and AEM measurements aresometimes supplemented by a gravity survey conductedover the whole flight area with a grid density of0.5 to 1 km.

Copper-cobalt ores of Outokumpu type areassociated wiLh serpentinite-skarn-quartz rockformations, and so geophysical measurements can beused to localize these explurationally important rocktypes. Figure 31.10 presents the geophysical resultsfrom a profile in the Miihkali area. Several holes havebeen drilled on this profile to check the, anomalies. Thepetrophysical information from some of the holesdrilled in the profile is plotted in the lower part ofFigure 31.10. A broad chrysotile serpentinite formotionwith low resistivity (I' in ohms), low density (p in gm/cc)and high susceptibility (k x }[(5 51) and which isrepresented by intersection 4 in hole 0 6 can belocalized by the positive magnetic and slinqramanomalies and the well dcveloped negative gravityanomaly. The positive gravity anomaly in the middle ofthe profile is caused by a skarn formation, whoseaverage density (p), according to intersection 3, is2.95 gm/cc. Pyrrhotite-bearing black schists producestrong magnetic and slingram anomalies.

~-igure 31.11 shows the magnetic, gravity andslingram maps of the Usinjarvi area. A rounded anomalyis visible in the middle of the area. The negative grnvityanomaly and the positive slinfJram anamoly suggest that

Copper Sulphide Ores in Finland 675

Outokumpu area. The location of the Outokumpu-associationand the most clearly reflecting horizons have been marked inFigure 31.13 on the basis of the reflector distances. Thereflections, of unknown origin, suggest that the Outokumpuformation might continue albeit thinly, down to a depth ofabout 1.5 km. The dip of the continuations begin to slopemore gently at the greater depth. Thus the seismic techniquecan be of value in delineating the extension of important ore­associated formations at depth.

Figure 31.14 displays the magnetotelluric survey resultsin the Saramiiki area where the continuation of the ore­associated formation was followed down to depth. As a resultof this survey the conductive formotion can partly be located.AnomQlous area A indicates the upper part of the conductiveformation. The anomalous area B correlates with the provendeeper parts of the formation and indicates further

continuation to depth. The results do not indicate thepresence of the ore-associated formation at depths between300-400 m. The magnetotelluric surveys were carried outwith French equipment fabricated by Societe ECA, acquiredby the University of Oulu. The output of the apparatus wasthe apparent resistivity at nine frequencies from 8 to3700 Hz. In the interpretation of the magnetotelluric results,an interactive computer program was adapted, which used alayered earth model and hyperparabola minimization processpresented by Lakanen (1975).

The localization and establishment of deep-seatedcon tinuations of geological formations is important whenplanning deep and expensive drillholes for finding new oredeposits. It is obvious from the foregoing examples that theuse of reflection seismic and magnetotelluric investigations isof considerable benefit in indirect exploration.

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Figure 31.10. Geophysical and petrophysical data from profile x = 91.0 in the Miihkali area.

676

oI

500mI

M. Ketola

MAGNETIC MAPCONTOURS: ±250,500,1000,

2000 nT etc.

SLING RAM MAP(REAL COMPONENT)f=1775 cps,'a=60mCONTOURS: ± 4,8,16 % etc.

GRAVITY MAPCONTOURS: 0.2,0.4, 0.6 mgal

etc.

SLING RAM MAP(IMAGINARY COMPONENT)f=1775cps, a=60mCONTOURS: ±4,8,16~o etc.

Figure 31.11. Geophysical results from the Usinjarvi area.

A

Copper Sulphide Ores in Finland

MAGNETIC MAPSCON TOURS: ± 250, 500 1 1000 nT etc.

SLINGRAM MAPS (REAL COMPONENT)CON TO URS : ±4 , 8, 16 <Yo etc. f =17 7 5 cps a = 60 m

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KERIMAKI AREA LAUKUNKANGAS AREAFigure 31.12. Geophysical results from the Kerimaki and Laukunkangas areas.

678 M. Ketola

Figure 31.13" Reflection seismic cross-section throughthe Outokumpu area.

Borehole surveys

The inner structure of formations rich in black schistscannot usually be investigated by electrical ground methods.The conductive subsurface schists shield the underlying rocks,even though the depth penetration of the geophysical methodsadopted would otherwise be sufficient. In some black schistareas it is possible to use electrical drillhole logging toinvestigate geological formations.

Figure 31.15 shows profile y = 12.0 from thePahtavuoma area, where resistivity and charged-potential(mise-a-la-masse) measurements were obtained in the holesdrilled into the orebody. The resistivity measurementsdemonstrate that there is a clear conductivity contrastbetween phyllites and greenstones. Although it was notpossible to solve directly the connection between the oreintersections by means of charged-potential measurements,they nevertheless present an indirect means, because the hostrock of the ore is a highly conductive phyllite.

The results of the charged-potential measurements,drawn in the form of the isoanomaly cross-section, revealthat ore intersections 1 and 2 are not located in the samephyllite formation, there being a marked increase in potentialbetween the intersections. The holes drilled from the aditmarked in Figure 31.15 confirm that a narrow weakly­conducting greenstone zone is sandwiched in phyllites.

The upper part of Figure 31.16 shows a slingram profilemeasured over the Outokumpu formation together with theholes drilled into it. The resistivity results are marked at thedrillholes as are the intersections in which the conductingblack schists and serpentinites occur in association withskarns. The geological picture given of the formation bydrilling is not well established. To clarify the geologicalinformation charged-potential measurements were conductedusing a number of grounding systems. The results from twoof the grounding systems are depicted in Figure 31.16, anddemonstrate that the formation is composed of various units.The results of grounding system 1 indicate that the formationsurrounded by mica schist is a syncline. The results alsodemonstrate that there is a good conductivity connectionbetween the intersections of the rock types favourable forthe occurrence of ore in the southeastern part of theformation. The results of grounding system 2 are fromanother part of the formation, where the black schist­serpentinite intersections are at the same potential.

Ketola (1972) has published the results of the chargedpotential measurements conducted in the Saramaki (earliercalled Miihkali) area. The ore formation was also investigatedby means of SP measurements obtained on the surface and indrillholes. It can be seen from Figure 31.17 that theformation produces a clear negative SP anomaly, theintensity of which is - 200 mV at the surface. There is only aweak response in the SP anomaly curve above the blackschists, which cause negative slingram anomalies.

1 K M

2KM

~~~~;. "' IillillJ OUTOKUMPU ASSOCIATION

- MAIN REFLECTING HORIZON

1 K M

The surface and drillhole SP data were processed by themethod described by Logn and B(ilviken (1974). The fieldanomalies were divided into two parts, a time-dependentanomaly of a separate conductor in the formation ECP(electronic current potential) and an anomaly caused by thewhole formation ICP (ionic current potential). Shown in thelower part of Figure 31.17 is the cross-section, compiled onthe basis of the ICP data, which demonstrates that theanomaly caused by the formation is reversed along the dipfrom negative to positive. The positive anomaly may beextensive, as was demonstrated by Semjonov (1975) usingsome practical examples. Hence, it is not always possible tolocalize accurately the lower end of the formation.

INTERPRETAnON OF THE GEOPHYSICAL ANOMALIESAND THEIR GEOCHEMICAL CLASSIFICAnON

Before the start of drilling or geochemical sampling thegeophysical data are interpreted in order to evaluate thedimensions, positions and petrophysical properties ofanomalous formations. Ketola, Ahokas, Liimatainen and Kaski(1975) and Ketola, Liimatainen and Ahokas (1976) haveinvestigated the feasibility of two- and three-dimensionalinterpretation methods and petrophysical determinations.

Depicted in Figure 31.18 are the two-dimensionalinterpretations of a magnetic and gravity anomaly curvesmeasured over the Saramaki ore body. The geophysical andpetrophysical data of this profile have been discussed above.The result of the first magnetic interpretation with effectivesusceptibility values (k), remanence omitted, is seen in theupper part of Figure 31.18. In the other magneticinterpretation, which suggests a more gentle dip and smallersusceptibilities (k ) for anomalous formations, the remanencehas been taken r into account. In the interpretation, theaverage ratio of remanent to induced magnetization was 10:1and the inclination and declination of the remanence were45° and 90°, respectively (compared to 75° and 7°respecti vely for the earth's magnetic field) measured on someoriented samples drilled from black schist exposures.Susceptibility determinations on drill cores demonstrate thatthe remanence-corrected interpretation gives the bettersusceptibility and dip estimates to the magnetized upper partof the formation depicted in Figure 31.4. The gravity anomalyis caused by the skarn rocks, which in the light ofinterpretation and drilling data are located mainly betweenthe magnetized parts of the formation, but partly also withinthem. Thus the magnetic and gravity data cannot becombined in the interpretation although the dip of theadjacent formations must be compatible i.e. 30°.

The results from profile x =81.4 given by the magnetic,slingram and gravity interpretations were put to good usewhen planning geochemical humus sampling with the purposeof classifying geophysical anomalies. Beneath the magneticand gravity profiles in the upper part of Figure 31.19 there isa petrophysical cross-section constructed by means of thetwo-dimensional interpretation results on the basis of whichthe sites of humus samples were selected. The petrophysicalinterpretation reveals the magnetized, conducting andanomalously high density sections. The copper contents ofhumus, showing an anomaly above the copper mineralizationis presented together with slingram and geological data in thelower part of Figure 31.19. A strong but rather narrow humusanomaly indicates that geochemical sampling, the aim ofwhich in this case was to check the magnetic and conductingsections, should be done with a short enough sample spacing.When planning sampling it is also worth employing the resultsof geophysical interpretation.

According to Wennervirta (1973), the Saramaki coppermineralization also manifests itself as an intense geochemicaltill anomaly. The samples were taken by percussion orpneumatic drilling from the depth of maximum penetrationimmediately above the surface of the bedrock.

Copper SUlphide Ores in Finland 679

1;;0" C,,,,

(-- ......\,"""

""""B "

\ I::~\\\ 380

.... ------...."",,

LEGEND

o 100m

OVERBURDEN

MICA GNEISS

BLACK SCHIST

SKARN, SERPENTINITEAND BLACK SCHIST

QUARTZITE

ORE BODY

CONDUCTOR AND RESISTIVITY (!1m)

~

D~

~

D

•I

ooor;It)

",.,

200

400

600

800

1000

DEPTHm

SARAMAKI AREAMAGNETOTELLURIC CROSS SECTION

PROFILE X=81.800

Figure 31.15.

Charged potential (mise-d-la-masse) andresistivity cross-section y = 12.0 in thePahtavuoma area.

Figure 31.14. Magnetotelluric cross-section in the Saramiiki area

~ OVERBURDEN

o GREENSTONE

L:a PHYLLITE

• ORE BODY

o ADIT

680 M. Ketola

SLINGRAMRe %60

40

20

-20

-40

-60

-80

~.".'.~

D

NW - SE

OVERBURDEN

BLACK SCHIST,SERPENTINITEAND SKARN

MICA SCHIST

o.,01

><

CHARGED POTENTIAL CROSS SECTION (GROUNDING SYSTEM 1)

• FIXED POTENTIAL ELECTRODE

.& NEGATIVE CURRENT ELECTRODE

CHARGED POTENTIAL CROSS SECTION (GROUNDING SYSTEM 2)

o

Figure 31.16. Charged potential (mise-a-la-masse) and resIstivity cross-section y = 176.0 in theOutokumpu area.

500m

Copper Sulphide Ores in Finland 681

GEOLOGY:

~ OVERBURDEN

o MICA GNEISS

~ BLACK SCH 1ST

m SKARN,SERPENTINITEAND.BLACK SCHIST

D QUARTZITE

• ORE BODY

\ r ....\

\ I\ I,----,

-20

-10

SP 1'0SLINGRAM:

SLINGRAM0-0 REAL COMPONENT

mV +30 a=60m f = 1775 cps

+100 SP GROUND RESULT:

+20 FIELD CURVE- ICP ANOMALY

+10

-100

-266

SARAMAKI AREASP CROSS SECTION

ICP ANOMALY

PROFILE X=81.800

o 100mI I

Figure 31.17. Self-potential and slingram results from profile x =81.8 in the Saramaki area.

682 M. Ketola

nT

o1000

I MAGNETIC INTERPRETATION-- MEASURED CURVE

JOOO - CALCULATED CURVE

2000

k = 4500 k = 12650

nT

3000

2000

1000

II MAGNETIC INTERPRETATIONMEASURED CURVE

lI---« CALCULATED (REMANENCECORRECTED) CURVE

k r = 540kr=1800

o 0o 0o 0

" N

mgal G RAV lTV INTER P R ETAT ION

0.6

0.4

0.2

~p=2.98 MEASURED CURVE

........... CALCULATED CURVE

p = 2.75

SARAMAKI AREA PROFILE X = 81.400

TWO-DIMENSIONAL MAGNETIC AND GRAVITY INTERPRETATION

o 100mI I

Figure 31.18. Interpretation of magnetic and gravity data from profile x = 81.4 in the Saramiiki area.

nT mgal

3000 0.6

2000

1000

MAGNETIC _

GRAVITY

Copper Sulphide Ores in Finland 683

INTERPRETED PETROPHYSICAL CROSS SECTION AND LOCATION OF HUMUS SAMPLES

y y y

ANOMALOUS SECTIONS:

EZa MAGNETIC

• CONDUCTI NG

~ HIGH DENSITY

HUMUS SAMPLE

+cy.Re 1m

0--0 --

SLING RAM

30

20

10

10

20

30

40

~ OVERBURDEN

o BLACK SCHIST

D QUARTZ ITE

"I \I \ '1./I \ /I \,,,""I ../II

o MICA GNEISS

~ SKARN AND SERPENTINE

• ORE BODY

ppm

800[5\] COPPER IN HUMUS

600

400

200

COMBINATION OF GEOPHYSICS AND GEOCHEMISTRY

IN SARAMAKI AREAPROFILE X=81.400

oI

100mI

Figure 31.19. Use of geophysical interpretation results in the planning of geochemical sampling.

684 M. Ketola

Hakli, .A.1963:

CONCLUSIONS

In this paper, an attempt has been made to demonstratethe feasibility of geophysical methods in the indirectexploration of sulphide ores in a graphite-bearing schistenvironment. It is evident that as exploration becomes moredifficult, geophysics has increasingly to be applied to indirectexploration. .

The simultaneous and effective use of differentsciences, such as geology, geuphysics and geochemistry, isnecessary in order to find new urebodies.

ACKNOWLEDGMENTS

ThLlnks are due to OUlokumpu Oy, Exploration, forpermission to publish the data included in this pLlper. Theauthor is especially grateful to Mr. M. Laurila, the Head ofthe Geophysical Department, who has led and taken an activepart in geophysicLlI investigations in all survey Llreas andwhose positive attitude madc it possible to complete thispaper. Examples were collected from thc extensive archivesof Outokumpu Oy, which cover the last 25 years. 1 am happyto acknowledge my gmtitude to Mr. M. Liimatainen, thegeophysicist responsible for the charged-potentialmeasurements at Pahtavuoma Llnd the SP irlVestigations in theSaramaki area, Mr. T. Ahokas, the geophysicist who plannedthe charged-potential measurements in the Outokumpu-zoneand Messrs. E. Lakanen, A. Ruotsalainen and P. Knikkonen,the geophysicists who performed the magnetotelluricmeasurements in the Saramaki area and interpreted theresults. Mr. J. Longi performed the mngnetic and gravityinterpretation calculations and Mr. Kokkola, geochemist thehumus investigations in the Saramaki area. The mClnus~riptwas translated by Mrs. G. Hakli and Mrs. M. Sarkki, whom 1thank for their pleasant co-operation.

REFERENCES

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p. 958-967.

Gaal, G., Koistinen, T., and Mattila, E.1975: Tectonics and strntigraphy of the vicinity of

Outokumpu, North Karelia, Finland; Geol. Surv.Finland, Bull. 271, 67 p.

Distribution of nickel between the silicLlte andsulphide phases in some basic intrusions inFinland; Geol. Surv. Finland, Bull. 209, p. 1-54.

Huhma, A. and Huhma, M.1970: Contribution to the geology and geochemistry of

the Outokumpu region; Geol. Soc. Finland Bull.,v. 42, p. 57-88.

Isokangas, P.1975: The mineral deposits of Finland; Lis. thesis, Univ.

of Helsinki, Finland, 128 p.

Ketola, M.1972: Some points of view concerning Mise-a-la-masse

measurements; Ceoexploration, v. 10(1), p. 14-2l.

Ketola, M., AhokLls, T., Liimatainen, M., and Kaski, T.1975: Some case histories of the interpretation of

magnetic and gravimetric anomalies by meansof two- and three-dimensional models;Geoexploration, v. 13(3), p. 137-169.

Ketola, M., Liimatainen, M., and Ahokas, T.1976: Application of petrophysics to sulfide ore

prospecting in Finland; Pageoph, v. 114,p. 21',-234.

Kokkola, M. and Korkalo, T.1976: Pahtavuoma copper in stream sediment. and till; J.

Geochem. Explor., v. 5(3), p. 280-287.

Lakanen, E.1975: Geofysikaalinan tulkinta graafisen tietokone­

kasittcdyn avulla; Geofysiikan paivat, Oulu.

Logn, f/J, and Bolviken, B.1974: Self potentials at the Joma pyrite deposit,

Norway; Geoexploration, v. 12(l),p. 11-28.

Parasnis, D.S.1974: Some present-day problems and possibilities in

mining geophysics; Geoexploration, v. 12(213),p. 116-118.

Puranen, M.1959: Aerosahkiiisista mittausmenetelmista, Espoo.

Ryss, Yu.S.1973: The senrch and exploration for ore bodies by the

contact method of polarization curves; Nedra,Leningrad.

Semjonov, M.V.1975: Osnovy poiskov izutsenija koltsedanno-

polimetallitseskin rudnyh polei geofizitseskimimetodami; Nedra, Leningrad, p. 118-14l.

U.S. Exchange Delegation in Mining Geophysics1976: A Western view of mining geophysics in the

U.S.S.R.; Geophysics, v. 41(3), p. 320-32l.

Vahatalo, V.1953: On the geology of the Outokumpu ore deposit in

Finland; Geo!. Surv. Finland, Dull. 164, p. 7-10.

Wennervirta, H.1973: Sampling of the bedrock-till interface in

geochemical exploration; Papers presented at asymposium in "Prospecting in areas of glacialterrain", organized by the Institution of Miningand Metallurgy, Trondheim, Norway.