GeologicalGeological settingsetting andand characteristicscharacteristics ofof theth e tonalite-hostedtonalite-hosted PaleoproterozoicPaleoproterozoic goldgold depositdeposit atat Osikonmäki,Osikonmäki , Rantasalmi,Rantasalmi, southeasternsoutheastern FinlandFinland EditedEdited byby OlaviOlavi KontoniemiKontoniemi andand PekkaPekka NurmiNurmi GeologicalGeological Survey ofof Finland,Finland , Special PaperPaper 25,7-18,25 , 7-18 , 19981998

AN OVBRVIEWOVERVIEW OF THE GEOLOGY IN THE OSIKONMAKIOSIKONMÄKI AREA, RANTASALMI, SOUTHEASTERN FINLAND: ESPECIALLY AS A PROMISING ENVIRON-ENVIRON­ MENT FOR EPIGENETIC GOLD MINERALTZATIONMINERALIZATION

by Olavi Kontoniemi

Kontoniemi,Kontonierni, O. 1998. An overview of the geology in the Osikonmäki area,area, Rantasalmi,Rantasalmi, sotheastern Finland: especially as a promising environment for epige-epige­ netic gold mineralization. Geological SurveSurtteyy of Finland, Special Paper 2525.7-18.. 7-18. 5 figures. The Osikonmäki gold deposit is situated in southeastern Finland, within the rural municipality of Rantasalrni.Rantasalmi. The area lies within the Ladoga-Bothnian Bay ZoneZone,, which is interpreted as a Paleoproterozoic collisional suture, expressed principally by NW-trending faults and dextral shear zones. The study area records a compIexcomplex polyphase deformational and progressive metarnorphic metamorphic history and contains bimodal volcanic rocks characteristic of collisional settings. Metamorphic grade increases southwards towards the Sulkava thermal dome, and the gold mineralization is located within sillimanite-K-feldspar zone. The bedrock of the study area consists predominantly of metaturbidites intruded by aavariety variety ofofgranitoids. granitoids. The oldest supracrustal rocks are metasediments, overlain by a succession of voIcanicvolcanic lithologies commencing with felsicfelsic pyroclastics and followed by intermediateintermediate,, mafic and ultramafic uruts.units. Intrusive rocks within the centralcenfal partofthepart of the study area include the synkinematic OsikonmäkiOsikonm2iki pluton (U-Pb(U-Pb zircon age = 1887±51887f5 Ma), thethe Hiltula granitoid (U-Pb(U-Pb zircon age = I185017 850±7 Ma)Ma),, which belongs to the latelate kinematic Putkilahti complexcomplex,, and thethe post-tectorucpost-tectonic Pirilä-type granites (U(U-Pb-Pb zircon age = 1815±7l8l5+7 Ma). The gold deposit is locatedlocated within thethe E-E-WW trendingtrending Osikonmäki shearshear zone whwhich ich deforms thethe synkinematic Osikonmäki plutonpluton,, and lieslies between thethe larger scalescale Kolkonjärvi and Haukivesi shearshear zones. Although thethe Kolkonjärvi shearshear zone has not been shown inin outcrop to deform the Osikonmäki pluton, thethe occurrenceoccurrence ofofpotassic potassic alteration andand chloritizationchloritization,, asas weilwell asas interpretationsinterpretations ofof airborne magnetic data suggestsuggest thatthat thethe shearshear zonezone was active after thethe pluton was intruded.intruded. WhWhether ether itit was activeactive prior toto intrusionintrusion isis difficultdifficult toto ascertainascertain', althoughalthough itit isis concludedconcluded thatthat shearshear zoneszones developeddeveloped during thethe earlyearly stagesstages of collision.collision. TheThe similaritiessimilarities inin appearanceappearance andand monazite U-Pb agages es oftheof the latelate granitesgranites (including(including thethe Pirilä granitegranite andand Hiltula granitoid)granitoid) onon eithereither sideside ofof thethe KolkonjärviKolkonjärvi lineamentlineament indicateindicate that that most ofof thethe movement tooktook pIplace ace beforebefore 1.801.80 Ga.Ga. ConvergenceConvergence alongalong thethe Ladoga-BothnianLadoga-Bothnian BayBay Zone provided appropriateappropriate conditionsconditions forfor thethe formationformation andand transporttransport ofof bothboth metamorphic andand magmatic fluids.fluids. FluidsFluids havehave beenbeen channeledchanneled alongalong complexcomplex faultfault systemssystems andand relativelyrelatively competentcompetent rockrock unitsunits,, suchsuch asas granitoidsgranitoids,, havehave preferentiallypreferentially focussedfocussed fluidsfluids inin upperupper crustalcrustal levelslevels..

KeyKey wordswords (GeoRef(GeoRef Thesaurus Thesaurus,, AGI):AGI): metamorphicmetamorphic rocks,rocks, plutonicplutonic rocks,rocks, intru­intru- sions,sions, tonalite,tonalite, metamorphism, tectonics,tectonics, shearshear zoneszones,, goldgold ores,ores, ProterozoicProterozoic,, Paleoproterozoic,Paleoproterozoic, Osikonmäki,Osikonmäki, FinlandFinland OlaviOlavi KontoniemiKontoniemi GeologicalGeological SurveSurveyy ofFinlandof Finland,, P.O.P.O. BoxBox 1237,1237, FlN-70211FIN-70211 Kuopio Kuopio,, FinlandFinland

7 GeologiGeologicalcal Survey Survey ofof Finland,Finland, Special Special PaperPaper 2525 OlaOlavivi KonlOni Kontoniemiemi

INTRODUCTIONINTRODUCTION

TheThe OsikonmäkiOsikonmäki goldgold depositdeposit isis situatedsituated inin south­south- metamorphicmetamorphic historyhistory ofof thethe Joroinen-SulkavaJoroinen-Sulkava re­re- easterneastern Finland,Finland, inin thethe ruralrural municipality ofof gion,gion, whichwhich includesincludes Rantasalmi,Rantasalmi, onon thethe basisbasis ofof Rantasalmi,Rantasalmi, somesorle fourfour kilometerskilometers WSWWSW ofof thethe fluidfluid inclusioninclusion investigations.investigations. towntown ofof RantasalmiRantasalmi (see(see FigureFigure 1).1). TheThe earliestearliest TheThe GeologicalGeological SurveySurvey ofof FinlandFinland commencedcommenced publishedpublished referencereference toto thethe geologygeology ofof thisthis regionregion explorationexploration inin thethe RantasalmiRantasalmi districtdistrict duringduring thethe was thethe 11:400 :400 000Cl00 scalescale map ofof bedrock geologygeology 1960'1960's sand and 1970'1970's, s, inin connectionconnection with thethe JuvaJuva (Sheet(Sheet D2D2 Savonlinna,Savonlinna, HackmanHackman andand BerghellBerghell ProjectProj ect ((PekkarinenPekkarinen 1972).197 2). SeveralSeveral erraticserratics consist­consist- 1931),1931), andand thethe accompanyingaccompanying explanatoryexplanatory notenotess by inging ofof auriferousauriferous quartzqtartz rock andand quartzqvurtz diorite HackmanHackman (1933).( 1 9 3 3 ). The Rantasalmi areaarea (Sheet(Sh eet 3233-3233 - were already foundfound during thethe coursecourse of these Rantasalmi)Rantasalmi) wawarss remappedremapped on the scale ofof 1I toto 100100 investigations,investigations, but thethe major impetusimpetus forfor goldgold ex­ex- 000 by Korsman (1973)(1973).. ploration was provided by thethe discovery ofof thethe InIn recent years thethe RantasalmiRantasalmi area has been thethe Pirilä deposit inin 1983.1983. Sustained high gold prices focusfocus forfor a number of detailed metamorphic and were an additional incentive forfor conticontinuingnuing explo­explo- tectonic studies, commencing with Korsman (1977) ration in the region, which became known as the and followed byblr Korsman et al. (1984),(1984), Korsman PiriläPiriläbelt belt (see(see Makkonen and Ekdahl 1988),1988), and inin and Kilpeläinen (1986), Korsman et al. (1988), adjacent areas, to the south and east of the lake Kilpeläinen (1988) and Vaasjoki and Sakko (1988).(19SS). Kolkonjärvi (Figure 2). As a result of these inves­inves- In addition, the geochemistry and petrogenesis of tigations, the first erratics from the Osikonmäki metavolcanic rocks at Rantasalmi and nearby Juva deposit were found in late 1984, which in turn led were studied by KKousaousa (1985)( I 985) and Viluksela (1988).( 1988). to the finding of a mineralized shear zone in out­out- Poutiainen (1992) has examined the tectono- crop in the summer of 1986.

Russia

: %2n~A :B~B r::::::::::l C ffilc~:J Ooo•• D 227° I

RUSSIA

·Oslkonmllkl o 100 km /,^b ~2ookmHELSINKI - / LADOGA I

Fig. 1.I . The locationlocation ofof thethe OsikonmäkiOsikonmäki goldgold depositdeposit andand metallogenicmetallogenic provincesprovinces ofof thethe Ladoga-BothnianLadoga-Bothnian BayBay ZoneZone after Ekdahl (1993). after Ekdahl (1993). AA == Kainuu-OutokumpuKainuu-Outokumpu BackBack Arc,Are, BB == Pyhäsalmi IslandIsland Arc,Are, C == Kotalahti Kotalahti NickelNickel BeltBelt andand Do == GoldGold occurrences.occurrences.

8 GeoloGeofogicalgical Survey of FinlandFinland,, Special Paper 25 An overview ofofthe the geology in the Osikonmäki arearea,a, RantasalmiRantasalmi,, southeastern FinlandFinland......

GEGEOLOGICALOLOGICAL SETTING OF THE STUDY AAREAREA

The Rantasalmi area lies within a major geo­geo- a Proterozoic oceanic plate with an Archean conti­conti- tectonic province variably known as the Main nent. Subsequent underplating in a WW-E -E direction Sulfide Ore Belt (i.a. Kahma 1973, Neuvonen et al. resulted in migmatization at about 1820 Ma and the 1981), Raahe-Ladoga Zone (i(i.a..a. Korsman 1988, emplacement of the ca. 1800 Ma post-tectonic Ekdahl 1993), and Ladoga-Bothnian Bay Zone granites in southern Finland (Vaasjoki and Sakko (Gaa11986(Gaäl 1986 and 1990, Ward 1987). This zone con­con- 1988).1 988). tains most oftheof the major sulfide deposits exploited in Ekdahl (1993) considered that convergence took Finland to date, but recent discoveries, including place along a SSW-NNE vector that generated an the Osikonmäki deposit, indicate that there exists NWNW-SE -SE dextral shear couple along the craton mar­mar- considerable potential for gold mineralization as gin and caused variable overthrusting to the NW well (Figure 1), particularly in association with well 1), in association with and NE. The resultant DD,3 structures, including the shear zone systems in granitoids. SE-trending LL,3 lineation strongly influenced the The Ladoga-Bothnian Bay Zone trends parallel final geometry of massive sulfide ore bodies (Fig­(Fig- to the Archean craton margin and represents the ure 4). Bimodal arc volcanism is characteristic of product of complex Paleoproterozoic subduction the zone, as indeed for the Svecofennian orogeny and collision processes (Gaal(Gaäl 1986 and 1990). The in general (Gmil(Gaäl 1990). suture zone itself is considered to correspond to a Lahtinen (1994) stressed the convergent plate pronounced negative Bouguer gravity anomalyanomaly,, tectonics and the complex nature of the evolution which also coincides with the surface expression of for the Svecofennian Domain. His model advoadvo-­ the Kolkonjärvi shear zone (see Figure 2). The cates three different collisional stages at 1.91-1.90 heterogenity of the lithology and structure of the Ga, 1.89-1.88 Ga and 1.86-1.84 GaGawhere where the third suture zone indicates that it is not a single structure stage may be a direct continuation of the second but rather a tectonic junction between the Archean stage. Lithospheric thickening during these events craton and obliquely orientated Paleoproterozoic was followed by upwelling of the thermal bound-bound­ formations (Koistinen et al.al. 1996). ary leading to magmatic underplating and high It has been demonstrated that the tectono­tectono- heheat at flow (Lahtinen 1994). metamorphic evolution of the Proterozoic crust in Koistinen et al. (1996) divide the tectonic evolu­evolu- the Rantasalmi-Sulkava area differed from that in tion of southern Finland into two major parts: 1) a the central and northern part of the Ladoga-Ladoga-BothnianBothnian period of intense, large-scale tectonism, in which Bay Zone (Korsman et al. 1988 and Vaasjoki and collision followed basin formation and destruction Sakko 1988). Korsman et al. (1988) suggested that andand 2) a synmetamorphic period with extensive there is a geosuture between the Kiuruvesi­Kiuruvesi- crustal deformation. The E-E-WW trending axis of

Haukivesi complex and the Rantasalmi-Sulkava recumbent FF,1 and FF.2 folds and thrusting of rocks area (see Figure 3). Based on isotope data from onto basement in thethe-east east suggest tectonic transport FinlandFinland, , Vaasjoki and Sakko (1988) suggestedsuggestedthat that due north. Because of rotation adjacent to the the new Svecofennian crust was rapidly generated craton margin, the thrust sheets propagated to the between 1930 and 1850 Ma ago by the collision of northeast (Koistinen et al.al. 1996).

LITHLITHOLOGIESOLOGIES IN THE STUDY AAREAREA

Luukkonen and Lukkarinen (1986) classified ultramafic rocks. Small granitic intrusions are also the rocks oftheof the Juva-Rantasalmi area as belonging present, including tourmaline granites. to their so-called Svecofennian Supergroup. Ac­Ac- Figure 2 is a geological map of the study area, cording to the Rantasalmi 1: 1:100 100 000 map sheet based partlypartlyon on the maps by Korsman (1973) andand (Kor(Korsman sm an 1973),I973), the most abundant lithologies in Makkonen and Ekdahl (1988), and partlyonpartly on the the area are metagraywackes and mica gneisses, results of subsequent exploration activity. The fol­fol- with the central part of the region including a large lowing review of lithologies is largely based on proportion of granitoidsgranitoids,, surrounded by previous studies because the area of the present amphibolites, quartzofeldspathic schists and study consists principally of granitoids.

9 0 0 (1) S=,< 0 ::: . Ö GEOLOGY OF THE OSIKONMÄKIOSIKONMAKI AREA ;>;:xE. (JQ;:; . ::: "c; ~ OOlavilavi KonKontoniemitoni emi c(, Cf) ;;;.::: =; ~. (1)" L }L 3233 '-<: L,.t=E- ) 0 ~, /;6;s::Y/r4 "TlTI 3km .,5 ' ?-'" •o SampSampleie ferfor UU-Pb-Pb dating II nncc lulusisi onsons:: Cf)a :\,: :\ '0 '/- * GGoldold ooccurrenceccurrence (1) '/.:6o " /'. \ A Intermediatelntermediatevolcanite volcanite ~ 35 .,"0 OsikOsikonmäkionmäki granitoid: '0 x (10 ...... QuartzQuartzleldsparschist-feldspar schist "Ö i')'l;:{., ou\ l'.i ( ~ IV p-:ö3o v.N \\ o--r_3 0 Quartzvudr LZ diorurur irLvte --r b o 0 hn" I r. Osikonmäki ~ d Mica schist or gneiss '{v, \ r-l " [:==JI I TonalTonalite.oranodiorite ite-granodierite

PutkilahPutkilahtiti ccomplex:omplex: T Tourmaline Gamet tronrarilJi ..I GabbrGabbro-dioriteo-diorite ®6 Gamet Si 11 imanite ..\.. L-L Sillimanite LAKE c=Jtl QuartOuartzz ddiorite-granodioriteier ite-granodiorite KOLKON-'i )-ts Cordierite ..I Ultrultramaficamafic rocks ..tt , Major fau lt ffi Granitec,rnit" .. Maior fault '-t. '- FractuFracturesres orer llineamentineament I I MMafic-intermediateafic-intermediate volcanvolcaniteite '- Osikonmäki shear zone T--_l FelsiFelsicc volcanite and \ osikonmäki shear zone quartzquartz-feldspar-feldspar schist "- r 83 ~3\ öeoorngBedd ing E MMicaica sschistchist and gneiss . 85 ,,5 Foliation r;--:^l0 \ Foliation l"^"^"1o 0 ConglomerateConolomerate 1LY° "]°1 ,,5.85 \ SchistositySchistosity {S2}(S2) \ # MaMaficfic or iintermediatentermediate dyke ,r(vr \.. \a\ ,/ \ Vertical schlstosityschistosity (S3){S3) .-..-t\ Granite dyke Granite dyke '"\ Lineation F "-j ^{ '"25 f PegmatitePegmatite dyke orer vein O0 Top of gradedgraded bedding 6870 L>\ f O OMK/AMK/-93OMK/AMK/-93 ModifiedModified fromfrom Korsman (1973)(19731 and Makkonen and Ekdahl (1988).

Fig.2.Fi g. 2. GeologicalGeological mapmap oftheof the studystudy area.area. ModifiedModified fromfrom KorsmanKorsman (1973)(1973) andand MakkonenMakkonen and EkdahlEkdahl (1988)( 1988). GeologicalGeologieal SurveySurvey ofof Finland,Finland , SpecialSpecial PaperPaper 2525 AnAn overviewoverview ofofthe the geologygeology in the OsikonmäkiOsikonmäki area,area, Rantasalmi,Rantasalmi , southeasternsoutheastem Finland...Finland ...

N VARKAUS le I Ib

,

\ 11 Gar+Cord+Sil/+Bio la Ib )1' I ______////(-- Cord+Gar+Sil/

SULKA" VA THERMAL DOME

I 20 km I

27"00'

FigFig.. 3. TeetonometamorphieTectonometamorphic map of ssouthernouthern Savo ((modifiedmodified from Kor Korsmansman et aal.l. 19881988)) sshowinghowing also the locationloeation of the study area (rectan(rectangle)gle) and KolkonjärviKolkoniärvi and Haukivesi sheshearar zones.zones.

Back AreArc RAAHE ~ I~ Jf , I ) 0@@

tt DireetionDirection ofof thethe f1I TectonicTectonic transporttransport subduetionsubduction I/ LineationLineation

FigFig.. 4.4. SehematieSchematic illustrationillustration ofof tectonictectonic eventsevents inin thethe Ladoga-BothnianLadoga-Bothnian BayBay ZoneZone showingshowing alsoalso thethe loeationlocation ofof thethe OsikonmäkiOsikonmäki goldgold depositdeposit.. ModifiedModified fromfrom Ekdahl Ekdahl ((1993).1993).

11t1 Geological Survey oo:[f FinlandFinland,, Special Paper 25 OlaOlavivi KontoniemiKontoniemi

SuSupracrustalpraerustal litholithologieslogies

Mica sehschistsists and mmicaica gneisses dominant minerals in the lapilli are plagioclaseplagioelase and quartz,quartz, with accessory biotite, sericite, carbonate, Mica schists in the study area commonly retain opaques and zircon (Makkonen 1992). primary sedimentary features such as grading, slump Rocks of intermediate composition are present structures and cross bedding, suggesting a turbiditic in the Pirilä belt, as wellweIl as at Kolkonranta and to origin (Gaal(Gaäl and Rauhamäki 1971).197l). With the south­south- the north of the Osikonmäki pluton. According to wards increase in metamorphic grade the mimica ca Makkonen (1992)(1992\ those in the Pirilä belt show schists gradually change into banded mica gneisses. porphyritic and agglomeratic textures in some piplaces aces Psammitic micamicia schistsschi sts contain quartzquartz,, plagioelaseplagioclase but are generally strongly deformed and and lithic fragments within a matrix of quartz, recrystallized. Uralite and plagioelaseplagioclase phenocrysts feldspar and mica. In contrast, pelitic layers consist have been found in some cases at Pirilä, but at principally of quartz and mica. Accessory minerals Kolkonranta a banded hornblende gneiss domi­domi- ineludeinclude apatiteapatite,., sphenesphene,, tourmaline, opaques, car­car- nates (Kontoniemi 1991). Agglomeratic features bonate and zircon. The metaturbidites contain nar­nar- are also present in intermediate metavolcanics oc­oc- row calc-silicatecalc-siliczrte horizons and to the south of curring to the north oftheof the Osikonmäki pluton,pluton, with Kolkonjärvi a solitary conglomerate layer has been pale highly elongate elastsclasts varying in size from 2 to found (Makkonen 1992). 40cm40 cm.. As metamorphic grade increases towards the Mafic intercalations, represented by amphibolites south and southeast, pelitic compositions progres­progres- and diopside amphibolites, are also present within sively show the development of porphyroblasts of the intermediate volcanics. The amphibolites are andalusiteandalusite,, then K-feldspar with sillimanite, and interpreted as submarine flows because oftheof the pres-pres­ finally, near the southern contact ofthe of the Osikonmäki ence of pillow structures (cf. Kousa 1985, Makkonen pluton, cordierite and garnet. 1992)1992),, while the banded diopside amphibolites represent their more highly strained equivalents. CChemicalhemical sedime sedimentsnts ChemicallyChemically,, the amphibolites are tholeiitic basalts (Kousa 1985), with trace element compositions In the so-called Pirilä belt (Makkonen and Ekdahl being remeniscent of both MORB and island arc 1988), a narrow banded iron formation has been tholeiites (Viluksela 1988)I 988).. recognized that is considered to contain silicate, The western part of the study area contains oxide and sulfide faciesfacies.. The silcate facies is repre­repre- ultramafic rocks of komatiitic affinity which ap­ap- sented essentially by quartz-garnet-grünerite rock, pear to be either discordant or concordant with the but also contains sporadic hornblendehornblende,, diopside, amphibolites. Because of the presence of pillows, fayalite and cherty layers. The oxide facies layers pyroclasticpyroelastic breccias and possible autoelasticautoclastic lava consist predominantly of magnetite and quartz, breccias, the ultramafic rocks are interpreted as with a little grünerite and diopside, while the sulfide predominantly extrusive in origin (Kousa 1985). facies comprises mangetite and pyrite with local chalcopyrite disseminationsdis seminations (Makkonen and Ekdahl 1988). The oxide facies attains a maximum thick­thick- InIntermediatetermediate and mafiemafic ddykesykes ness of about 5 m (Makkonen 1992)1992)..

VolcaVolcanicnic Iilithollogiesthollogies Intermediate and mafic dykes have been encoun­encoun- tered during mapping in the vicinity of the The metavolcanicmetavolr;anic rocks of the region have been Osikonmäki pluton. At the northern margin of the described previously by Kousa (1985)(1985),, Makkonen pluton there is a concordant intermediate uralite­uralite- and Ekdahl (1988) and Makkonen (1992)(1992).. They plagioelaseplagioclase porphyry dyke that contains enelavesenclaves occur within the Pirilä belt, near the Tuusmäki of mica schist. The largest uralite phenocrysts are pluton (as at Kolkonranta)Kolkonranta),, to the north of the about 1I cm in size and the principal minerals of the are plagioclase (An40),), Osikonmäki pluton, and in the vicinity ofHakojärviof Hakojärvi rock are plagioelase (An40 uralitic hornblende and and Tuusjärvi (Figure 2). biotite. Accessory phases ineludeinclude quartzquartz,, opaques, The felsic lithologies in the Pirilä belt are evi­evi- apatiteapatite,, sphene, sericite and zircon. The blasto-blasto­ dently lapilli tuffs in origin, with tectonically elon­elon- porphyritic texture of the rock is a consequence of gated but recognizablerecop;nizable felsic fragments generally incipient shearing and granulation. less than 5 cm in length, within a groundmass of An intermediate plagioclaseplagioelase porphyry dyke about quartz,quartz, plagioclase, biotite and K-feldspar. The 5 m in width crops out on the southern shore of the

12t2 GeologicGeologicalal Survey of FinlandFinland,, Special Paper 25 An overview of the geology in the Osikonmäki areaarea,, RantaRantasalmi,salmi, southeastern FinlandFinland...... lake Alanen. Contacts with the Osikonmäki forming up to 55Vo% oftheof the rock, with other accessory granitoid are sharp and the western margin of the minerals being sphene, opaquesopaques., zircon, carboante, porphyry includes a narrow (1-2 cm) pegmatitic chlorite, sericite and K-feldspar. seam. Plagioclase, quartz, quartz, green hornblende and Granodioritic compositions define a marginal biotite predominate, with accessory opaques, phase, representing the most felsic lithologies in sericite, sphene, apatite and zircon. The rock has a the complex. In conjunction with age determi­determi- somewhat granoblastic texture, although the coarse nations, they are referred as the Hiltula grano­grano- plagioclase crystals also give an overall porphyritic diorite (Vaasjoki and Kontoniemi 1991). Principal appearance. Similar lithologies have been encoun­encoun- minerals are oligoclase, quartz, biotite and KK-feld--feld­ tered in some of the Osikonmäki drill corescores.. Mafic spar, while accessories include abundant apatite apatite,, and intermediate porphyry dykes are also present sphene, zirconzircon,, opaques and amphibole, with some to the southeast of the Osikonmäki pluton, al­al- secondary sericite and epidote. The rock is weakly though intrusive relationships have not been weIlwell foliated and has a hypidiomorphic texture, with established. The dykes are compositonally very occasional myrmekite. similar to the Alanen dyke.dvke.

The Osikonmäki-typeO sikonmäki-type intrusion Plutonic lithologies The tonalitic Osikonmäki intrusion has been defined previously in connection with discussions Putkilahti campcomplex lex of the Osikonmäki gold deposit (cf. Kontoniemi and Ekdahl 1990). This intrusion differs appreci­appreci- The Putkilahti intrusive complex (Gaal(Gaäl and ably from the granitoids of the Putkilahti complex, Rauhamäki 1971) extends into the eastern part of with compositions ranging from quartz diorite to the study area, and consists predominantly of hethet-­ granodiorite, and also in having a highly deformed, in erogeneous diorite and gabbro alternating with places gneissose fabric. The Osikonmäki intrusion coarser hornblendite. Marginal phases also include and otherotherrelated related granitoids appear as discrete roundish more quartz dioritic and granodioritic variants. magnetic minima on greyscale airborne magnetic The rocks of the complex are generally weakly pixel maps (Figure 5). Although they are broadly foliated. The Putkilahti complex differs both tex­tex- concordant with country rocks, they contain many turally and petrographically from other gabbroic kinds of supracrustal enclaves near their margins. intrusions in the Haukivesi region. Nebulitic struc­struc- The Tuusmäki tonalite (Korsman et al. 1984) tures and magmatic layering are sometimes also will be described in some detail here, since the present in the gabbroic phase. On the basis of Osikonmäki granitoid will be described elsewhere truncating relationships, the youngest rocks within in this Special Paper. The Tuusmäki tonalite was the complex are N-trending mafic dykes (Gaal(Gaäl and originally termed the PeltuePeltue-Ronkala-Ronkala quartz diorite by Rauhamäki 1971).l97I). Pekkarinen (1972)(1972),, who considered it to be a domal The predominant ultramafic lithology in the intrusive mass surroundedslrrrounded by gneisses. He further de­de- Putkilahti complex is hornblendite, which consists scribed the quartz diorite as strongly deformed, almost essentially ofMg-richof Mg-rich edenitic amphibole, although gneissie,gneissic, with an intense lineation and abundant clinopyroxene is also sporadically present. Typical pegmatitic and quartz veins in its central part. accessory minerals include sphene, apatite, carbon­carbon- Drillings carried out in connection with gold ate and opaques (Gaal(Ga6l and Rauhamäki 1971).I97 l). Gabbros exploration at Kolkonranta (Kontoniemi 1991) re­re- and diorites differ from one another only with respect vealed that the northern contact between the to plagioclase compositions, which are usually in the Tuusmäki tonalite and the surrounding meta­meta- ' be range AnAnoo-Anro.40-Anso Textures tend to be hypidiomorphic volcanics is gradual, and intruded by pegmatites. with intergrown pale green hornblende (possibly Volcanic enclaves are also common within the edenitic) andandbiotite. biotite. Typical accessory minerals are margins of the intrusion, which in this area is apatite, opaques, quartz and zircon, andandplagioclase plagioclase granodioritic rather than tonalitic. The K-feldspar is somewhat sericitized and saussuritized. may however be partly secondary in origin. The Quartz diorites are generally somewhat finer rock has a granoblastic fabric as a result of meta­meta- grained than the diorites and are more distinctly morphic recrystallization and the principal miner­miner- hypidiomorphic. Plagioclase compositions vary als are plagioclase, quartz,qvartz, biotite and K-feldspar. from AnAn-3S to AnAnoo40 and plagioclase constitutes about Accessory minerals are hornblende, diopside, 50%50Vo of the rock, the remainder being green horn­horn- epidote, carbonate, sphene, apatite and zircon blende, biotite and quartz. Apatite is abundant, (Kontoniemi 1991).

13 GeologicalCeological Survey ofof FFinland,inland, SpecialSpecial Paper 25 Olavi K Kontoniemiontoniemi

Granites feldsparfeldspar and inin somesome places biotite are thethe dominant mineralsminerals,, with typical accessory phases being apa­apa- Two kinds ooff granite intrusionintrusion have been recog­recog- tite, muscovitemuscovite,, sphene and zircon. SomeSome of the nized inin the study area, namely pinkish or greyish pegmatites exhibit complex mineralogiesmineralogies,, with medium grained bodiebodiess such as the Pirilä granite abundant tourmalinetourmaline and inin ssomeome cases brownish (Vaasjoki(Vaasjoki andandSzrkko Sakko 1988 1988),), andaand avariety varietyofpegmatitic ofpegmatitic garnet, grapgraphitehite and beryl. The area to the southsouth of veins and dykes. The formerformeroccuras occur as discrete roundishroundish Kolkonjärvi is characterized by migmatites, com­ com- intrusions, commonly intruded by related medium monly with microcline pegmatite dykes and grained to pegmatiticpegrrratitic dykesdykes.. Quartz, plagioc1aseplagioclase,, K- leucosomes (Makkonen 1992).1992).

AGE RELARBLATIONSHIPS TIONSHIPS BETWEEN ROCK UNITS

Lithostratigraphical interpretations oftheof the Ranta­Ranta- SampieSample A 1169) yielded an age of 1887±51887+5 Ma salmi area have been prepresentedsented prepreviouslyviously by Ga,ilGaäl (Vaasjoki and KontoniKontoniemiemi 1991).1991). and Rauhamäki (1971), Makkonen and Ekdahl Field relationships indicate that the Putkilahti (1988)(1988),, aandnd mmostost recentlyrecently,, Makkonen (1992). Ac­Ac- complex is less deformed than the Osikonmäki•Osikonmäki- cording to GaalGaäl and Rauhamäki (1971), the meta­meta- type granitoids and therefore it was decided to pelites represent the oldest rocks in the Haukivesi attempt dating the felsic marginal phase, in order to area and are overlain by the mafic volcanics and tetestst whether the Putkilahti complex is in fact metaturbiditemetaturbidites.s . In contrast, Makkonen (1992) con­con- younger. The resultant U-Pb zircon age determina­determina- sidered the mic:amica schists aandnd iron formation to be tion, namely 1850±71850+7 Ma (Figure 2, A 1170)1170),, ob­ob- stratigraphically lowermost, followed by the tained for the Hiltula granodiorite (Vaasjoki and volcanics, which commence with felsic pyroclastic Kontoniemi 1991), confirms this conclusion. depodepositssits and are progreprogressivelyssively overlain by interme­interme- Intrusive relationships indieateindicate that the mafic to diatediate,, mafiemafic andan,C finally ultramafieultramafic lithologies lithologies.. intermediate porphyry dykes in the Osikonmäki Although thethe'Osikonmäki Osikonmäki pluton is broadly con­con- area postdate the Osikonmäki pluton,pluton, and also the cordant with ssurroundingurrounding lithologieslithologies,, the abun­abun- gabbros of the Putkilahti complex; relationships dance of sschistosechistose ssupracrustalupraerustal enclaves indicateindicatess with the Hiltula granodiorite have not howeverhowever,, the intrusive characterch.aracter of the granitoid. Two U-Pb been established. The granites ofthe ststudyudy areaareaare are zirconzirconage age determination determinationss have been carried out for sstilltill younger, as they cross cut all the other rock the Osikonmälci-typeOsikonmäki-type gragranitoids;nitoids; the Tuusmäki types of the area. The UU-Pb -Pb zircon age for the Pirilä tonalite gagave ve an age of 1881888t15 8± 15 Ma (Korsman et al. granite is consistent with field relationships, being 1984) and sheared Osikonmäki tonalite (Figure 2, 1815±7181517 Ma (Vaasjoki and Sakko 1988).

.IECTONOMETAMORPHIC TECTONOMETAMORPHIC EVOLUTION OF THE STUDY AREA

IntenIntensivesive sstructuraltructural and metamorphicmetamorphie studies ous OsikonmäkiOsikonmäki shear zone does not have any have been carried out in the Rantasalmi area in topographicaltopographieal or magnetic expression. recent years, ncrtablynotably by researchersresearehers from the Geo-Geo­ The tectonometamorphic map of the RantasalmiRantasalmi logical Survey ofFinland.of Finland. ResultsResults have been pub-pub­ region (Korsman et al. 1988) includes the present lishedbylished by KorsmanKorsman et al. (1984),(1 984), Kilpeläinen (1988) study area and also shows the Kolkonjärvi and and Korsman et al. (1988). Haukivesi fault or shear zones (Figure 3). Being AsAs noted earlier, the study area lies within a situated between RantasalmiRantasalmi and Sulkava, the study major suture zonrezone associated with NW-trendingNW -trending faults area transectstranseets several progressive metamorphicmetamorphie and shearshear zones. The accompanyingaccompanying geological zones. The Kiuruvesi-Haukivesi complexeomplex (I in Fig-Fig­ map (Figure(Figure 2) shows the Kolkonjärvi and ure 3) and the Rantasalmi-SulkavaRantasalmi-Sulkava area (II(11 in Fig-Fig­ HaukivesiHaukivesi shear zones, both of which are discern-diseern­ ure 3) record quite different tectonothermal histo-histo­ ible inin magnetic (Figure(Figure 5) and topographicaltopographie al data. ries. For example, the high grade metamorphicmetamorphie The Osikonmäki pluton isis situatedsituated between these two Pieksämäki (Ic) and Haukivesi (Id) blocks were shear zones. SubsidiarySubsidiary shearshear zones interpretedinterpreted from metamophosed a metamophosed during a D,-D,D j -D2 tectonictectonic event prior thethe grayscale magnetic pixel image (Figure 5)5) have toto thethe intrusion of granitoids around 1.88 Ga. In also been markerdmarked on the map.map. However, thethe aurifer- contrast,eontrast, separateseparate metamorphicmetamorphie evolution hashas been t414 GeologicalGeologica l SurveySurvey ofof Finland,Finland , Special Paper 25 An overviewoverview oftheof the geologygeology inin thethe OsikonmäkiOsikonmäki area,area, Rantasalmi,Rantasalmi , southeasternsoutheastern Finland...Finland ... recordedreeorded from thethe SulkavaSulkava area.area, where thethe thermalthermal metamorphismmetamorphism peaked during D,D2 deformation in metamorphicmetamorphie peak postdated thethe localloeal D,D2 structuresstrueture s thethe Rantasalmi area (Kilpeläinen(Kilpeläinen 1988).1988). Andalusite and culminatedeulminated during D,D3 at about t.S:-t.gt1.83-1.81 CaGa formed at the expense of muscovitemuseovite early during (Korsman et al. 1988). BecauseBeeause movements tooktook D'D2, while further southwards, sillimanite and K-K­ placeplaee on fault zones, including the D shear zones, feldspar erystallized. Cordierite and K-feldspar fault including the D,3 shear zones, feldspar crystallized. Cordierite and K-feldspar until a relativelyrelati vely late stage,stage, some ofofthe the progressive formed asas a result of biotite breakdown duringduring thethe metamorphicmetamorphie isogradsisograds have been truncated,truneated, and peak of Dr.D2. In close proximity to the Sulkava consequentlyeonsequently zones of different metamorphicmetamorphie grade thermalthermal dome garnet equilibrated with cordieriteeordierite are tectonicallyteetonieally juxtaposed.juxtaposed. MetamorphicMetamorphie grade afterafterD, D2 , indicatingindieating that the age ofofthe the metamorphicmetamorphie increasesinereases progressivelyprogressively in the Rantasalmi-SulkavaRantasalmi-Sulkava peakbecomespeak beeomes progressivelyprogressively younger asas the Sulkava area towardstowards the SulkavaSulkava thermal dome (Figure(Figure 3),3), thermal dome is approachedapproaehed (Kilpeläinen(Kilpeläinen 1988).1988). as expressedexpressed by the following sequencesequenee of meta-meta­ F,F I foldsfolds have only been identified within the morphicmorphie parageneses: andalusite-muscoviteandalusite-museovite zone within the northern part andalusite-muscoviteandalusite-museovite zone of thethe studystudy area.area. These folds are isoclinal with sillimanite- K-feldsparK -feldspar zone amplitudes varying from 10-20l0-20 m.m. F,F2 folds tend to K-feldspar-cordieriteK-feldspar-eordierite zone be tight with amplitudes ranging over hundreds of garnet-cordierite-biotitgarnet-eordierite-biotite e zone meters. The intensity of the S,S2 foliation varies with garnet-cordierite-sillimanitegarnet-eordierite-sillimanite zone (Korsman metamorphicmetamorphie grade,grade, being a penetrativepenetrati ve schistositysehistosity et al. 19841984)) in the sillimanite-K-feldsparsillimanite-K-feldspar zone.zone. FibrolitieFibrolitic The present studystudy areaarea isis mostlymostly within the sillimanitesillimanite has also shown a tendencytendeney to crystallizeerystallize cleavage struc- sillimanite-K-feldspar zone,zone, although the southern within SS,2 erenulationcrenulation eleavage lithons. DD,3 strue­ part also includes gneissesgneisses belonging to the K­K- tures are expressed in outeropoutcrop as asymmetricalasymmetrie al feldspar-eordieritefeldspar-cordierite zonezone.. Prograde progressive folds of variable size with a NW-SENW -SE trendinstrending axial

Magnetic map GrayscaleGravscale RantasalmiRanlasalmi Osikonmäki Map 3233 Shading fromtrom south-west at heighheightt angle 10 degrees

6060 It-T ...t --19.419.4 I "jt KM ·111 '100 -200

-290

I -2300

1.5

Kilomet ers Lineaments: '. •••• FFirstirst order '. '''"', ", Second order

FiFig.g. 55.. MMagneticagneti c ggrayscalerayscale mmapap oofthef the sstudytudy aarearea sshowinghowing aalsolso th thee llocationocation ofofpossible possible mmajorajor andand ssubsidiaryub sidiary sshearshears (l(lineaments).ineaments).

1515 GeologicalGeological Survey Survey 01'oli FinlandFinland,. Special Paper 2525 OlaviOlavi Kontoniemi plane and a generallygenrerally southeastwardsoutheastward plunge. ThisThis spar zone. DD, 4 isis representedrepresented by a crenulationcrenulation cleav­ cleav- phase of deformation evidently continued after the age, but no associatedassociated foldsfolds have been identified;identified; climax of metamorphism since itit has truncatedtruncated and this foliationfoliation isis also present within the younger modified the distributiond:istribution of metamorphic zones. 0.83-1.80(1.83-1.80 Ga) granites, which thus constrains the This isis also consistent with retrogressive formationformation maximum possible age forfor this deformation of muscovite andanLd thethe growth ofaxialof axial planar biotite (Kilpeläinen(Kilpeläinen 1988).1988). within the southernsouthern part of the sillimanite-K-feld-

RRelationshipselationships betweebetweenn gragranitoidnitoid iintrusions,ntrusions, deformation and metamormetamorphismphism

Although the Osikonmäki-type intrusions pre­pre- than the waning stages of synkinematic magmatism. date the regional metamorphic peak, no evidence The Sulkava thermal dome is associated with has been found for any thermal aureoles (Korsman abundant microcline granites interpretedinterpreted as record­record- et al. 1984). Moreover, although it is clear that the ing anatexis (Korsman et al. 1984). Within the intrusions predatepred;ate DD'2, their relationship to earlier sillimanite-K-feldspar zone however, and still fur­fur- deformation has not been established (Korsman et ther to the north, different kinds of granitic and al. 1988). This type of intrusion occurs predomi­predomi- pegmatitic intrusions are present. These might also nantly within the sillimanite K-feldspar zone, with represent the products of in situ anatexis (Korsman metamorphic grade increasing towards the south. et al. 1984), but it is also possible that they have Foliation in the granitoids conform with that in the been intruded from greater depths, in which case enclosing supracrustal rocks. Observations from they could be related to the Pirilä granite. the Tuusmäki tonalite suggest that it was emplaced Although it has not been specifically demon­demon- into a regional antiformal culmination, but that strated that the Kolkonjärvi fault cuts the regional metamorphic isograds overprint structures Osikonmäki granitoids, the airborne magnetic data, within the pluton (Korsman et al. 1984). Within the as well as the presence of potassic alteration and classification of Finnish granitoids outlined by chloritization suggest that the fault was actiactive ve after Nurmi and Haapala (1986), the Osikonmäki-type granitoid emplacement. It is difficult to be certain intrusions would fall into the synkinematic group. as to whether the fault was active also prior to The less deformed Putkilahti complex does not granitoid emplacement, but in general the shear show the same range of structural features as the zones along the suture must have formed at a pre-Dpre-D,2 OsikonmäkiOsikonrnäki granitoids. Suitable metamor-met amor­ relatively early stage during the overall deforma­deforma- phic minerals for monitoring progressive meta-meta­ tion history. Because the Pirilä granite and the morphism are not present within the complex, but Hiltula granodiorite and related intrusions on ei­ei- there nevertheless there is nevertheless a distinct DD,3 mineral align-align­ ther side of the Kolkonjärvi lineament are ment in proximity to the Kolkonjärvi lineament. petrographically similar and have similar monazite Thus isotopic aLndand structural criteria both suggest 207Pbj206Pb201Pbl206Pb ages, it is concluded that the main activ­activ- that the Putkilahti complex more likely represents ity along this zone ceased before their emplace-emplace­ the earliest stages of late kinematic magmatism ment (Vaasjoki and Kontoniemi 1991).

DISCUSSIONDISCUSSION

According to currently favoured concepts, dehydration processes, with the metamorphic peak mesothermalmeso thermal gold mineralization is closely associ-associ­ post-dating collision by as much as fifty million ated with complex crustal scale defbrmationdeformation proc-proc­ years (Murphy 1989). esses. With respect to plate tectonic processes, The Rantasalmi area is located within a Paleo-Paleo­ shear zones can either be extensional or compres-compres­ proterozoic collision zone that is recorded by the sional in nature, the former occurring during con-con­ presence ofofNW NW-trending -trending crustal scale shear zones, tinentaltinental rifting and the latter during subsequent such as the Kolkonjärvi and Haukivesi zones. Pre-Pre­ plate convergence. During oblique collision it is vious interpretations of the Ladoga-Bothnian Bay also probable tlhatt at alternating zones and phases of Zone (Ga6l(Gaal 1986, Ekdahl 1993, Lahtinen 1994) extension and compression will exist. Within ma-ma­ involved convergence from the SW or SSW, result-result­ jorjor convergence zones, as much as halfofthehalf of the totaltotal ing in a dextral wrench fault system. Koistinen et crustal thickness may be subjected to metamorphic al. (1996) suggested that tectonictectonic transporttransport was due t616 Geological Survey of Finland, Special Paper 25 An overview of thethe geologygeology in thethe Osikonmäki area,area, Rantasalmi,Rantasalmi , southeastern Finland...Finland ...

north and because ofrotationof rotation adjacent toto thethe craton conditions. Archean mesothermal gold deposits margin, thethe thrustthrust sheets propagated toto thethe north-north­ are considered toto have formed from fluids thatthat have east. Inspection of the airborne magnetic datada ta from both metamorphic (devolatilization) and magmatic the RantasalmiRantasalmi district suggests thatthat thethe totaltotal dis-dis­ (felsic intrusives)intrusives) characteristics (Groves and Fos-Fos­ placement inin thisthis area was not great, so thatthat a ter 1991). The Ladoga-Bothnian Bay Zone has significant dip-slip component could also be in-in­ experienced a rangerange of metamorphic (subducting ferred. During the later stages of collision,collision, and oceanic plate during convergence and/or thicken-thicken­ during development of the Sulkava thermal dome,dome, ing ofofthe the crust during collision, latelate thermalthermal event) the overthrust tectonicstectonics and vertical fault move-move­ and magmatic (three phases of granitoid intrusion) ments were further accentuated. processes thatthat can have contributed to generation Epigenetic ore processes inevitably require ex-ex­ and migration of fluids. These fluids were focussed tensive hydrothermal activity and fluid flow, typi-typi­ by major shear systems, particularly into obliquely cally within major structuralstructural features. Fluid activ-activ­ oriented dilational sites, and thethe role of relatively ity isis closely linked with mobilization, transport competent rock units, such as the Osikonmäki in-in­ and precipitation of ore metals, which is also greatly trusion, was important in chanelling fluids to higher influenced by fluid compositions and ambient P-TP-T crustalcrustallevels. levels.

ACKNOWLEDGEMENTS

The author wish to thank Dr Kalevi Korsman structive suggestions for improvement. The geo­geo- who was the official reviewer ofofthe the manuscript for logical map has been digitized by Anne Kousa and his numerous valuable comments and advice. Dr the magnetig grayscale map by Jouni Lerssi. Fig-Fig­ Peter Sorjonen-Sorjonen-WardWard translated the manuscript into ures have been drafted by Raija Väänänen. All of English and he and Dr Pasi Eilu made many con- them are gratefully acknowledged by the author.

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18