TheThe earlyearly ProtProterozoicerozoic Zn-eZn-Cu-Pbu-Pb ssulphideulphide depodeposirsit 01"of RauhalaRauhala inin YliYlivieska,vieska, westernwestern FinlandFinland EditedEdited byby KKaria ri KojonenKojonen GeoGeologicallogical SurveySurvey 01"of FinlandFinland,, SpecialSpecial PapPaperer Ill, J, 55-18- J 8 IU S UNES (]

ILGCP-257GCP-257 no. 5

GEOLOGY OFOF TUETHE RAUUALARAUHALA STRATIFORMSTRATIFORM MASSIVMASSIVEE Zn-eu-PbZn-Cu-Pb SULPUIDESULPHIDE DEPOSITDEPOSIT

byoy K. J. VästiVasti

VäSVästi,li , KK.. JJ.. 191989.89. Geology of ththee Rauhala stratistratiform form massive zZn-Cu-Pbn-eu-Pb ssulphideulphide deposit. GeoloGeologic'algical SurveSurveyy oiof FinlandFinland,, Special Paper JI JI,, 5-18,5_18, 12l2 figures.iigures. The early Proterozoic zn-eu-PbZn-Cu-Pb deposit at Rauhala is situated in west­west- eernrn Finland, about 1122 km east of the town of YliviYlivieska.es ka. Exploration sstartedtarted in 19791919 as a result of routine geocgeochemicalhemical mappinmappingg cacarriedrried out by the Geo­Geo- lologicalgical Survey of Finland in the region. DurinDuringg 1983-1987 ssupplemen-upplemen­ tary geochemical sampling, mapping, a geophysical fieldfjeld survey, excava­excava- tion and drilling were undertaken at the target. The bedrock in the immediate environment of the RauhalRauhalaa ore depo depositsit iiss cocomposedmposed of metasedimentsmetasediment s and ophitic quartquartzz diorite intruding the form-form­ er. The metasediments comprise metaturbiditesmetalurbidites and mica schists with black schist interbeds, sericsericiteite schist, metagreywacke, quartquartz-feldsparz- feldspar scschist,hist, cor­cor- dierite gneiss and tuffitic hornblende schist. Thin crosscutting tholeiitictholeiitic dykes are also met with. As indicated by the extensive occurrence of andalusite,andalusite, regional metamor-metamor­ phism culminated under the low P,/TP IT conditions of the amphibolite fa-fa­ cies. Retrograde metamorphism later altered the andalusite into sericite and quartz.quartz. The stratiformstrati form ore deposit is composed of two orebodies dipping ca. 30o30 0 to northeastnortheast and embedded within metasediments.metasediments. The main ore body,body, which is 590 m long,long, 350 m wide and 2.12. 1 m thick,thick, consistsconsists of massivemass ive and disseminateddisseminated ores. The disseminatedd isseminated ore occurs inin sericite schist below thethe massivemassive ore.ore. The RauhalaRauhala deposit exhibits featuresfeatures typicaltypical of sediment-hosted Zn-Zn­ Pb deposits,deposits, particularly in thethe host lithologylithology and shape of thethe orebody. However, inin termsterms of thethe Zn-Cu-Pbzn-eu-Pb ratioratio thethe massive ore bearsbears a closer resemblanceresemblance toto volcanogenic massive Zn-Cu-Pbz n-eu-Pb ores.ores. This can be attributed toto thethe tectonictectonic setting of an active continentalcontinental margin. KeyKey words: mineralmineral deposits,deposits, massive deposits, host rocks,rocks, stratiform deposits, tectonictectonic elements,elements, metamorphism,metamorphism, Rauhala,Rauhala, Ylivieska,Ylivieska, FinlandFinland

KajKaj J. Vcisti,Väs/i, Geological Survey ofoi Finland, Finland, P.O.P. O. BoxBox 1237,J237, SF-70701SF-7070J Kuo-Kuo­ pio,pio, FinlandFinland

INTRODUCTIONINTRODUCTION

TheThe stratiformstratiform sulphidesulphide depositdeposit atat RauhalaRauhala isis notnot exceedexceed aa fewfew metres.metres. TheThe outcropoutcrop ofof thethe oreore locatedlocated inin westernwestern Finland.Finland, aboutabout 1212 kmkm easteast ofof isis entirelyentirely coveredcovered withwith aa 2-252-25 mm thickthick blanketblanket thethe torvntown ofof YlivieskaYlivieska (Fig.(Fig. l).1). TheThe topographytopography ofof ofof tiil.ti l!. thethe areaarea isis flatflat andand thethe variationvariation inin elevationelevation doesdoes TheThe geologygeology ofof thethe YlivieskaYlivieska areaarea waswas firstfirst stud-stud-

5 GCcologicaleological SSurveyurvey 01'of finlFinland,a nd , SpSpecialec ia l PPapcra per 11ll K..I.K..1. VäVaistisli

1900-1800 Ma agaago during the Svecokarelidic orogeny. The supracrustal lithologielithologic unit is com­com- posed of sedimentary rocks and volcanites. Ac­Ac- cording to SaIIiSalli (1964), the lowermost rocks in stratigraphy are arkosic quartz-feldspar schists overlain by conglomerates and greywackes. The uppermost and predominant part of the metasedi­metasedi- ments is composed of porphyroblastic mica schist that often shows well-preserved primary structuresstructures.. Intermediatelntermediate and mafic volcanites constitute the youngest unit in the succession of supracrustal SWEDEN rocks. USSR The depositiondepositional al basement of the supracrustal unit is unknown. Although largely agreeing with Salli (1964) about the stratigraphy of the area, Västi (1978) has demonstrated that west of YliviesYlivies-­ ka, mafic and intermediate volcanites are the O Rauhala lowermost lithologies of the schist belt. lsokolsokoskiski (1982) holds a similar view with Västi (1978) about 630 the stratigraphy of the uppermost part of the ssupracrustalupracrustal unit but considers that the rocks of - FINLAND the schist belt are predominantly volcanogenic. GaitlGaäl and Gorbatschev (1987) have divided the Svecofennian granitoids into early and laielate Svekofennian subgroups stating that granitoid 100 km plutonism started synchronously with the calc­calc- alkalialkaline ne volcanism. The bulk of the Svecofenni­Svecofenni- HELSINKI an area is occupied by early Svecofennian grani­grani- FiFig.g. Il.. LI-ocationocalio n 01'of RauhRauhalaala d deposit.epmil. toids. These are differentiated I-typel-type rocks vary­vary- ing in composition from gabbros and diorites to tonalites, granodiorites and granites (Gaal(Gaäl & Gor­Gor- batschev 1987). As shown by isotope and geochem­geochem- icdied in the late 19thl9th century by J. J. Sederholm, ical studies, the granitoids were mainly generated who in 1887l8B7 investigated the well-preserved from mantle-derived material that segregated in supracrustal rocks in the area (Mäkinen 1916). early ProterozoieProterozoic times (Huhma 1986). GcologicalGeological mapping was later conducted in the Early Svecofennian plutonism had its peak same area by Mäkinen (1916), Wilkman (1929, 1.90-1.87 Ga agaago and U-Pb zircon ages of the 1931)l93l) and Salli (1955, 1958, 1961,1961,1964). 1964). plutonites from the Ylivieska area belong to this AccordinAccordingg to Simonen (1980), the Ylivieska area age group (Kouvo 1976; Huhma 1986). Although belongs to a SvecofennianSvecof'ennian schist belt that under­under- close in age, the plutonites are younger than the went folding, metamorphism and plutonism supracrustal rocks (Salli 1964).

CEGEOLOGICALOLOC ICAL OOUTLINEUTLINE OOFF TTHF]H E EASTEEASTERNRN PPARTA RT OF TTHEH E YLiYLIVIESKAVIESKA AAREAREA

A supracrustal unit dominates the geology of margins (Västi 19881988,, Fig. 2). Intrusive igneous the eastern part of the YlivieskaYliviesk a area.area. lt trends ap­ap- rocks from gabbros to granites constitute large for­for- proximately east-west iinn the middle of the area but mations in the margins of the area. turns to north-south in the eastern and western

SupSupracrustalracrustal rocks

The supracrustal Ul11tunit ISis mainly composed of ca gneisses and intermediate and mafiemafic volcanitesvolcanites.. metaturbidites, mica schists, metagreywackes, mi- Inln places, quartz-feldspar schists, black sehistsschists and

6 Geological SurSurveyvcy of Finland, Special Papepaperr llI I Geology of ththee RauhalRauhalaa sstratiformlratiform masmassivesive Zn-eu-PbZn-Cu-pb susulohidelphide depodenositsil

gA {6

6tylf • StratiformStratitorm Zn-CuZn-Cu-Pb-- Pb­ deposit 01of Rauhala

..

:::::::::::, """"" :! ::::::: '; '; :

LEGEND • Gabbroe"ouro ::::::::::: InterInte.medi"t. mediate volcanics I .... ~· ...... I Blackgh.1 schist tnterbedhterbed ! ffi "olcanics lJ ] Sulphide deposit ofot tl:1~1 Granite/Aplite/Unakitenr.nn.roooelunakite ~::: :::: j Uraliteurarite porphyrite Volcvotcanicanic coconglomeratenglomerate ~f; I depos~ fffrl lIiii Dffi I I RauhalaHauna€ .:::::::::::: . . . .:.:.:.:.:.:' GranodK>l"ite,e"nooi'it', tonalitet-"tit" ~~~7 Plagioclaseer.oio"t"". porphyrite PlagiPhgio"t"".oclase porphyry •.. TopTo of the beds [iillFE!.f'.'-'.'.'.'.' fi ffi ~)~:~!~;~ffi l oo.ohv.v ffi •B+tH+Sd QuartzOuartz diorite,dirite. diorite porphvrite m mffi Plagioclase uralite porphyrite D MetasedimentM"t.""di..nt"s ","n,o",u"" l=-l Fig.t'ig. 2. GcologicalCieological map of eaSeasternlern part of YliYlivieskavicska arcaarca..

tremolite schists are encountered as thin interlay­interlay- elaseclase porphyryporphyry and intermediate volcanic con­con- eers.rs. The mica schists and metaturbiditemetaturbiditess occasion­occasion- glomerate, which occurs as a thick and extensive ally contain andalusite, lessless often siIIimanitesillimanite and succession outside the mapped area,area, are met with gamet.garnet. Primary structures, that is, bedding and in the western margin. The matrix of the con­con- graded bedding, are best preserved in the metatur­metatur- glomerate is of intermediate composition, and the bidites.bidites. elastsclasts include abundant fine-grained felsic vol­vol- The proportion of volcanic rocks, the majority canites and various porphyrites. About 8 km of which are tuffs and lavas of intermediate-mafic southeast of the Rauhala sulphide deposit there composition, is highest in the western and east­east- is a northeast-southwest trending plagioclase-plagioelase­ ern margins of the area (Fig. 2)2).. PlagioelasePlagioclase por-por­ uralite porphyrite occurrence with roughly equal phyrites and uralite porphyrites occur as extensive abundances of plagioclaseplagioelase and uralite phenocrysts extrusives, sills and crosscutting dykes in the (Fig.(Fie. 2)2).. sedimentary rocks and tuffites. Intrusivelntrusive plagio-

Intrusive rocks

The plutonic rocks in the area are predominantly (Fig. 2). Inln the north, ophitic rock with a compo­compo- granodiorites, tonalites and granites in composi­composi- sition varying from quartz diorite to diorite is en­en- tion. In the middle of the area, east of Rauhala, countered over large areas. Owing to its ophitic the granodiorite is KK-feldspar-porphyric-feldspar-porphyric with fre­fre- and, in places, weakly porphyric texttexture ure and also quent inclusions of schists, plagioelaseplagioclase porphyrite to ititss mode of occurrence the rock is probably and quartz diorite. Unakite and aplite occur as hypabyssaI.hypabyssal. Gabbro occurs as small intrusions in smaIIsmall bodies in the south and northeast of the area the eastern part of the area.

7 GeoCeologicallogieal SurveySurvey ofof Finland, SpecialSpecial Paper III I K.JK.J.. VäsliVcisti

THETHE STRATIFORMSTRATIFORM SULPHIDESULPHIDE DEPOSIT AT RAUHALARAUHALA

According to the terminology used by Huhtala exact width and lengthlength are not known. TheThe totaltotal (1979)(1979) the RauhalaRauhala sulphide deposit is located at thickness of the ore deposit varies from 0.20 to the southwesternsouthwestern extension of the Vihanti-pyhä•Vihanti-Pyhä- 5.90 m, the average being 2.1 m. salmi-Pielavesi Cu-Zn ore province. The ore deposit isis composed of massive and dis­dis- The ore deposit isis a thin sheet with sulphides seseminatedminated ores. No stockwork mineralizations occurring concordantly in metasedimentmetasedimentss (Figs. 3, have been encountered. The The massive ore incIudesincludes 4, 5). Inln the south and east of the deposit, the quartquartzz grains and aggregates, 1-30l-30 mm inin size, ststratiformratiform ore sheet and the sedimentaI-ysedimentary rocks with muscovite and chlorite as gangue mineralminerals.s. trend east-west and dip roughly 30 30"° north. In the Their abundance, however, isis leslesss than 10%,l0o/0, and west the ssuccessionuccession exhibits aaZ-form Z-form fold, and the so the ore is of the massive type according to the strike of the ore suboutcrop turns to north-north-southsouth definition of Sangster and Scott (1976). The mas­mas- with a dip to 3535o° east (Fig. 5). Although the ore­ore- ssiveive ore forms a continuous sheet truncated only body forms a right-angled suboutcrop on the at its eastern end, where the smsmaller aller orebody oc­oc- ground surface, on account of its gentle dip it is curs as aseparatea separate unit (Fig. 3). The thickness of a sheet only slightly bent around a fold axis plungplung-­ the massive orebody varies between 0.20 and 3.75 iningg 2525o° northeast. m, the average being 1.35l.35 m. The ore deposit is composed of two orebodies The disseminated ore occurs predominantly be-be­ located on the same stratigraphic level (Fig. 3). The low the massive ore, less often above it. Sericite maximum length of the main orebody from east scschisthi st is the host rock of the disseminated ore, to west iiss 590 m and its width from north to south whose upper parts often exhibit thin layers rich 350 mm.. As the smaller orebody east of the main in ssphalerite.phalerite. The sulphide abundance in the dis­dis- orebody was intersected by only one drill hole, ititss seseminatedminated ore varies markedly, being, however,

\\\'.\ \ 'a. \'^...t" to.\ 100, 11 \""\ \\tL n, ,, t, \i.u\o\ N ~+: 7107.00 N t, iff, t "\i*', roiiliiilt I

I l o \

,1, ioi;rffi l,l11 \,, fft*\iiv\ it :t:\i"{" - ,- +- \ \t., '\ \*.._ ;':Wts--i <> i a- \ \o\ \ \ a= \'\.s 30 - ' - ' - - - Quartz diorite 'a\ 30- !ImmltrFFEEEE \ - _-_ --'-- '$li.o- <> EJtr_l Metaturbidite \ --- . -{e'e, s\ - - '-'- - ' - , _ . _ . - . _.- [:]c:=J Mica schist \ *io$\ trTI1[/\/,' j sericiteSericite schist <> <> fn--alrooooJ .­ ,- t99!~ l-J ElackBlack schist ,­, _ . _ . _ . _ . _._.- [#]x![""", [ Black schist interbed -.-'-,.,.' Uralite porphyrite dike 3'o !_ MassiueMassive ore O<> Andalusite ~ Bedding __ SubsurfaceSubsurlace extension ['rlE=:J oi""etinatedDisseminated ore s= Muscovite +{( Top of01 the beds -- of01 massive ore

I"ig.Fi g. 3. (ieologicalGeological mapIllap of01' RauhalaRauha la depositdepo,il.

8 Geologica(ieologicall Survey of Finland, Special Paperpaper 11I I GeologyGeologl of thc RauhRauhalaala sstratil'ormtrati form mamassivess ive Zn-eu-Zn-Cu-pbPb susulphidelphide depositdcposit

less than 25070,2590, which Sangster (1972) takes as therhe Au-Ag amalgam, electrum, stannite, cassiterite, upper limit of disseminated ore. The average thick­thick- kesterite, boulangerite, gudmundite, nisbite, na­na- ness of the disseminated orebody is 0.75 m. In tive bismuth, rutile and graphite,graphite. Hematite and il­il- placesplaces,, a net-textured or breccia-textured ore with menite are encountered here and there in the dis­dis- less than 50%5090 sulphide occurs as a narrow zone seminated ore. In the weathered suboutcrop, the in the lower part of the massive orebody. secondary copper minerals-bornite, chalcocite, PyrrhotitePyrrhotite,, sphalerite, chalcopyrite, galena, covelline and native copper-occur together with pyrite and marcasite are the main minerals in the goethite. A detailed description of the chemistry ore deposit. Pyrrhotite is the predominant, and vir­vir- and ore minerals of the sulphide deposit is given tually the only, iron sulphide in the bulk of the elsewhere in this volume (Rasilainen & Västi 1989; deposit. However, pyrite or marcasite, or both, Kojonen et al. 1989). prevail in the western margin of the ore sheet and Based on 38 drill intersections and a cutoff of also occasionally at its southern end, where the de­de- 2%2Vo Cu equivalent, the ore reserve assessment of posit crops out. Pyrite is most abundant in the the deposit gives 1.2 Mt ore withrvith an average grade western part of the ore deposit, whereas marca­marca- of 6.66%6.66V0 Zn, 1.59%l.59Vo Cu,Cl, 1.24%l.24Vo Pb, 67 ppm Ag site predominates in the south and southeast. and 0.5 ppm Au. At a cutoff of 1lgo% Cu equiva­equiva- Sphalerite, chalcopyrite and galena occur in associ­associ- lent, the total ore tonnage is c. 1.7 Mt with an aver­aver- ation with pyrrhotite. The accessories include ar­ar- age grade of 4.97%4.9'7Vo Zn, 1.33%1 .33V0 Cu, 0.96%0.9690 Pb, 53 senopyrite, freibergite, dyscrasite, kongsbergite, ppm Ag and 0.4 ppm Au.

~ -- l I S ~ I N

~

~

• ~ "- '" " '" ~. '"'\::. '.~ ~ " '" " /

?.--:z;l lLc;~~lli Glacial drift '" ~ Quartz diorite I [::---;-- MMetaturbiditeetaturbidite -l ,l CJtl Mica schist i- o Sericite schist ;:':':':~ UraliteUralile porphyrite r\i\- Disseminated and banded ore I MMassiveassive ore <>.:> Andalusite 0;:...., __-=2::..5 __~50m L- J

Fig.Fi-e. 4.4 North-sOUlNorth-southh verticaverticall section through Rauhala ore depodeposit.sit. PPositionos ition ofoI cross-seclioncross section i iss marked in Fig. 33..

9 GeoGeologicallogical Survey of Finland, Special Paper 11tl K.J. VästiL/tisti

o '"o w a; E ~o I X 7106.850 DH 322 DH 324 DH 335 DH 334

\\ \ \\ \ \\ \t\\

\ \ \ \ \.:. {--\

rq-=x< ~q~FSöY{.1 Glaeial drift

ffi Quartzuuarrzqtorlre diorite liItrFFT]]]]I \ : \ \ \ \\ ---'- --- bd MetaturbiditeM.taturbrdite -\\\\\a\--\1\ l---- -\ -\ CJ MieaMica sehistschist '\_r\ \ ITTTI SerieiteSericite sehist bl:1 .1:i:1.:::@DkC'c j:ill1 schist -\ F-T---l Uralite porphyrite C:::::~g \ar_r* ^\*. I Massive ore -++++++--r-rr+rr Graphite /,/ BlaekBlack sehistschist .. 25 50m """T\-SSrr Disseminated and banded ore Andalusite o"--'," 1 Disseminated and banded ore <> Andalusite '-' --_..... ' ---~,

Fig.Fie. 5. East-west vertical secsectiontion throughthroush RauhalRauhalaa ore d deoosit.eposit. PPositionosition of cross-seccross-sectiontion iiss marked in Fig.Fie. 3.

PETROGRAPHY OF THE COUNTRY ROCKS

The Rauhala ore deposit and the bedrock in its rocks. The sedimentary rocks are mainly metatur­metatur- immediate environment are completely covered bidites and various mica schists. The igneous unit with glaciofluvial sediments. Nevertheless, with the is largely composed of quartz diorite that may be aid of diamond cocore re drilling, trenching and geo­geo- dioritic in places. Uralite porphyrite occurs as nar­nar- physical maps it has been possible to compile a row dykes in the metasediments and, rarely, in the detailed lithological map of the area (Fig. 3). quartzquartz dioritediorite.. The chemistry of the country rocks The country rocks of the ore can be divided in­in- is described in detail elsewhere in this volume (see to two units: metasediments and intrusive igneousisneous Rasilainen & Västi 1989).

Metasediments MetaturbiditeMetqturbidile beds exhibit the lowermost graded of the complete turbidite sequence of Bouma parts and, in places, the uppermost pelitic parts (1962). TheThethicknessof thickness of thetheindividualbedsvaries individual beds varies

10l0 CeologicalGeological Survey of Finland, SpecialSpecial PaperPaper I11 I Geology of thethe RauhalaRa uhala stratiformstratiform massivemassive Zn-Cu-PbZn-Cu-Pb sulphidesulphide deposit

Fig.Fig.6. 6. Graded metaturbidite with sesericite-richricite-rich pseudomorphpseudomorph after andalusitandalusite.e. Figure isis 9.6 mm across. SamSample pie 322 322/91/ 91.90. .9O. CrossedC rossed nolars.polars.

from a few centimetres to more than one metre. blasts in the andalusite-mica schist have been The main minerals in metaturbidites are quartz and preserved almost unaltered only in sosome me places biotite although muscovite, sericite and andalu­andalu- (Fig. 7). As a rule they are very ragged and con­con- site also occur as major minerals in the pelitic parts tain abundant quartz and biotite inclusions. Here of the graded beds. Andalusite is often complete­complete- and there they have been completely altered into lyIy altered into sericite and quartz (Fig. 6). The ac­ac- muscovite and quartz. The mica schists are grano­grano- cessories ineludeinclude chlorite, sulphides, zircon and blastic or porphyroblastic in texture. The average apatite. The coarse portions of the metaturbidites grain size, which is about 0.10 mm in the mica are blastoclasticblastoelastic in texture whereas the pelitic por­por- schist,schist. is about twice as much in the andalusite­andalusite- tions are nematoblastic or porphyroblastic, or mica schist. both. The elastsclasts in the coarsest portion are Graphite and iraniron sulphide-bearingsulphide-bearlng schists oc­oc- predominantly quartz and only seldom plagioelase.plagioclase. cur at severallevels,several levels, stratigraphically below the ore The grain size is 0_05-0.100.05-0.10 mm in the pelitic por­por- deposit (Figs. 3, 5, 12). They are greywackes or tions. The groundmass of the coarser portion aver­aver- fine-grained granoblastic sschistschists in structure. The ages 0.10 mm in grain size and the quartz elastsclasts matrix of the metagreywackes contains quartz, measure up to 1.5 mm. ggraphite,raphite, phlophlogopite,gopite, plagioelase,plagioclase, iron sulphides The mimmica schistsschists differ from the metaturbidites and in places also chlorite as major minerals. mainly in sstructure.tructure. The ordinary mica schist is PhloPhlogopitegopite is replaced by biotite in the portions ststronglyrongly schistose and often distinctly laminated poorer in ssulphides.ulphides. The accessories ineludeinclude mus­mus- or layeredlayered.. The andalusite-mica schist differs from covite, sericite, apatite, carbonate, titanite andand zir­zir- the ordinary mica schist mainly in its porphyro­porphyro- con. Occasionally chalcopyrite, sphalerite and blastic texttexture ure and coarse grain size. The hematite are also encountered as accessories. andalusite-mica schist, which in its upper part Graphite occuroccurss as dusty particles with a marked­marked- gragradesdes into ssericiteericite schist, occuroccurss as a coherent lalay-y­ Ilyy ssmallermaller grain size than that of the matrix on er immediately beneath the ore sheet (Figs. 4, 5). average. The abundance of graphite varies from The mica schists have the same major minerals place to place and the minermineralal may occur as an as the metaturbidites. The accessories ineludeinclude chlo­chlo- accessory. The average grain size in the matrix isis rite, saussurite,saussurite, apatite, zircon, ssulphides,ulphides, garnet, 0.10 mm. TheThe rock fragmentfragmentss in thethe metagrey­metagrey- epidote and carbonatcarbonate.e. The andalusite porphyro- wacke are of very fine-grained black schist,schist, seri-

11ll Geological(ieological SurveySurvcy ofof Finland,Iiinland, SpecialSpecial PapPaperer 11I I K.JK.J.. Västil'ti.sti

t;It" "4'' ,Lq '1:' ;; !I a.:: "!' Y{ä s,, . ler,

?,,'' tÄ:

Fig.Irig. 77.. Andalusite-bearingAndalusite bearing mica sc schist,hi st , in whichu,hich andalusiteanclalusite iiss partlpartlyy aaltercdltered into sericitescricite aalonglong cracks andancl mnrargins.argins. FigureFigurc isis 9.6 mm acrossacross.. SamSamplc pie 320320/52.65./ 52.65. CrosseCrossedd polarpolars.s. eitecite sehist,schist, quartz-feldspar sehistschist and quartzite of­of- ments. In all likelihood, however,horvever, they include ten showing a weak pyrrhotite dissemination. both quartz and plagioclase.plagiocJase_ The blaekblack sehistschist frag­frag- Apart fromfron-r graphite and iron sulphides, minerals ments are often angular and elongatedclongated (Fig. 8).B). are diffieultdifficult to identify in the blackblaek sehistschist frag- The quartzite and quartz-feldspar sehistschist fragmentfragmentss

" a-i ,+:ä ': \-

I'ig.Fig. 8. \lctagrelrrackeMetagreywacke riithwi th fine-grarncdfine-grained blaclblack schist,schi st , sericitesericit e schist,schist, quartzquartz-fcldspar f'cldspar schistschi st and quartz fragntentsfragment s andand crosscutcrosscut­ tingling carbonatecarbonate vcin.vein. liigureFigure isis 9.6 nrntmm across.across. SamplcSampie 322l105.30.322/105.30. ('rossedCrossed polars. t212 Geological SurveySurvey of Finland,Finland. SpecialSpec ial paperPaper Ill I Geology ofof thethe Rauhala srrariformstratiform massivemassive Zn-Cu-pbZn-Cu-Pb sulphidesulphide denositdeposit

Fig.9.Fi g. 9. SeriSericitecite sschistchi st wwithith low-gradelow-grade didisseminatedsseminated mineralimineralizationzation (bl(blackack g grains).rains). FiFiguregure isis 9.6 mm across.across . S Sampleam pi e 360360/168.50/ 168.50. CrosscdCrosscd polars.po lars. are of strongly flattened cigarshape. According to of the main minerals vary markedly. As a rule, drill core logging, the maximum size of the rock the closer the sericite schist is to the massive ore fragments is 0.5 x 3 cm. The mineral fragments the richer it is in quartz and sulphides. The sul­sul- in the metagreywacke are predominantly quartz, phides are predominantly pyrrhotite, sphalerite less often plagioclase. The fragments average less and chalcopyrite. The abundances of galena, than 0.5 mm in size. pyrite, marcasite and arsenopyrite are lower. Small Fine-grained granoblastic black schist occurs as amounts of hematite, ilmenite, rutile, chalcocite, narrow interlayers in the graphite-bearing stannite and electrum are also encountered. The metagreywackes and other metasediments. For ex­ex- grades of the base metmetals als in the sericite schist are ample, the roughly 700 m long black schist for­for- often so high that the schist is considered a dis­dis- mation southwest of the Rauhala ore deposit (Fig. seminated ore (Västi 1988). The grain size of the 3) grades downwards into a graphite-bearing sericite schist varies between 0.05 and 0.20 mm. metagreywacke containing thin black schist inter­inter- However, immediately beneath the massive ore the layers. The black schist has quartz,quartz, sericite, graph­graph- average grain size isis distinctly greater. The seri­seri- ite, iron sulphides and biotite as main minerals. cite schist is granoblastic or nematoblastic in tex­tex- Chlorite is an accessory that occurs as an altera­altera- ture. tion product of biotite. Here and there hematite, The ssericiteericite schist indicates the start of and chalcopyrite and arsenopyrite are also encountered gradual increaseincrease inin hydrothermal activity immedi­ immedi- as accessories. The grain size of the black schist ately preceding the deposition of the massive ore. varievariess between 0.05 and 0.10 mm. This is demonstrated particularly clearly by the Sericite schist occurs as a continuous, 0.5-8 m gradual increase in quartz and sulphide abun­abun- thick zone beneath the massive ore, where itit grades dances towards the massive ore. It has been shown downwards intointo andalusite-mica schist.schist. Thinner that the precipitation of silica and sulphates pre­pre- and discontinuous, itit also occurs inin places above cedecedess the deposition of Kuroko oreores,s, too (Cathle(Cathless the mamassivessive ore (Figs(Figs.. 33,, 4, 5).5\. The sericitesericite schist 1983).l 983). isis light grey, and thethe structure isis strongly schistose (Fig.(Fig. 9). Quartz, sericite and sulphides are thethe main Quartz-jeldsparQuartz-feldspar schist, cordierite gneiss and minerals, and biotite, chlorite, plagioclase, potas­ potas- hornblende schistsc/asl occur inin clastic sediments as thin siumsium feldsparfeldspar, , rutile, tourmaline,tourmaline, zircon, apatite interlayers, hardly ever more thanthan2 2 m thick. TheThe andand carbonate are thethe accessories. The abundances quartz-feldspar schistschist contains quartz, potassium

13l3 CeologicalGeological SurveySurvey ofof Finland,Finland, SpecialSpecial PaperPaper I11 I K.J.K.J. VcistiVästi feldsparfeldspar oror plagioclase,plagioclase, oror both,both, asas majormajor miner-miner­ clasec1ase isis labradoritelabradorite inin composition,composition, andand thethe tex-tex­ als.als. The accessoriesaccessories includeinclude biotite,biotite, chlorite, seri-seri­ tureture isis granoblastic.granoblastic. cite, muscovitemuscovite and rutile.rutile. The rockrock variesvaries betweenbetween The hornblendehornblende schistschi st isis aa darkdark greengreen andand dis-dis­ 0.05 andand 0.100. 10 mmmm inin grain size andand isis granoblas-gran ob las­ tinctlytinctly schistoseschistose rock.rock . The mainmain mineralsminerals areare tictic or blastoclasticblastoclastic inin texture.texture. The blastoclasticblastoclastic typetype plagioclase,plagioclase, hornblende,hornblende, quartzquartz and biotite. The includesincludes fragmentsfragments of quartz, lessless often of potas-potas­ accessories includeinclude opaques, chlorite, garnet,garnet, seri-seri­ sium feldsparfeldspar and plagioclase. The quartz-feldsparquartz-feldspar cite, apatite, zircon and carbonate. The abun-abun­ schists are sortedsorted arenaceous sediments inin origin. dancesdances of thethe major minerals vary somewhat,somewhat, withwith Cordierite gneiss has been encountered inin only thethe consequence thatthat biotite and plagioclase are one drill profile beneath thethe sericite schist. Mac-Mac­ occasionallyoccasionally among thethe accessories. Hornblende roscopically thethe rockrock does not differdi ffer fromfrom thethe occurs inin places as aggregates or largerlarger grains,grain s, muscovite-poormuscovite-poor sericitesericite schist. Quartz,Quartz, plagioclase,plagioclase, somesome of which are probablyprobably primary phenocrysts cordierite, phlogopitephlogopite and sulphides are thethe main and somesome graingrain aggregates recrystallizedrecrystallized duringduring minerals, and muscovite, zircon, chlorite and apa-apa­ metamorphism.metamorphism. The hornblendehornblende schistschi st varies be-be­ tite thethe accessories. Cordierite occurs as largelarge tweentween 0.10 and 0.20 mm in graingrain sizesize and isis grains with abundant inclusionsinclusions of plagioclase,plagioclase, granoblastic-porphyric in texture.texture. The mineralog-mineralog­ phlogopite,phlogopite, muscovitemuscovite and sulphides. The plagio-plagio- icalical compositioncomposition and texturetexture implyimply tuffiti.ctuffiti·c origin.

Intrusive rocks

QuartzQuartz diorite is the predominant intrusiveintrusive rock rite, zircon and opaques as the accessories. The in the environment of the Rauhala ore depodeposit.sit. It plagioclase,plagioclase, which isis oligoclase or andesine, con­con- cuts the metasediments overlying the orebody at tainstains small inclusions of quartquartz.z. Hornblende oc­oc- a low angle but nowhere has it been observed to curs as ragged grains with abundant quartz and intrude the ore (Figs. 3,4,3, 4, 5). Besides metaturbi-metaturbi­ plagioclase inclusions. Owing to the variation in dite and mica schist fragmentfragments,s, the quartquartzz diorite quartz abundance, the rock is lolocallycally dioritic or, contains narrow uralite porphyrite,porphyrite, carbonate, lessless often, tonalitic in composition. The quartz didi-­ quartzquartz and aplite dykes. Plagioclase, hornblende, orite iiss blasto-ophiticblasto-ophitic in tetexturext ure (Fig. 10)l0) and of biotite and quartz are the main minerals, with medium or fine grain size. ltsIts texttexture ure and mode titanite, ssericite,ericite, epidote, apatite, carbonate, chlo- of occurrence are suggestive of a hypabyssal in-

FiFig.g. 10.10. B1aBlasto-ophiticsto·ophitic quartquartzz dioritdiorite.e. FiFigurcgure iiss 33 rnrnmm aacross.cross. SarnpleSample 365365/159.35./ 159.35. Cro Crossedssed popolars.lars.

14t4 GeoGeologicallogical SurvSurveyey of Finland, Special Paperpaper 11I I GeologCeologyy of the RRauhalaauhala sstratiformlratiform mamassivessive ZZn-Cu-pbn-Cu-Pb ssulphideulphide depositdeoosit

Fig. 11.ll. Uralite porphyrite with ururalitealite (Ur) and onhopyroxeneorthopyroxene (Op) phenocryphenocrysts.sts. Figure iiss 4.7 mm across. SaSamplempi e 336336/84.5O./ 84.50. CCrossedrossed poDolars.lars. trusive rock whose chemical composltlOncomposition cor­cor- 50-70%50-70V0 of the minerals in the rock, are or­or- .responds to andesite (Rasilainen & Västi 1989, this thopyroxene and uralite (Fig. 11).I l). The uralitiza­uralitiza- volume). tion of the orthopyroxene crystals, which begins UraliteUrqlite porphyrite occurs in the sedimentary along the grain margins and fractures, is often rocks as sills or dykes, usually from 0.5 to 1I m complete. In addition to schillerization, the thick, that crosscut the sediments at a low angle. phenocrysts often show biotite flakes as alteration ltIt also intrudes the quartz diorite (Figs. 3, 4, 5). products of uralite or orthopyroxene. Biotite has Hornblende (uralite), plagioclase and orthopyrox­orthopyrox- further partly altered into chlorite. Opaque miner­miner- ene are the main minerals, with biotite, chlorite, als are also abundant among the inclusions. The sericite, tremolite, quartz, opaques, carbonate and phenocrysts vary from 1 to 10l0 mm in size, the titanite as accessoriesaccessories.. The fine-grained matrix is average being 3-5 mm (Fig.ll).(Fig.11). On the basis of mainly composed of plagioclase and hornblende. its texture and mode of occurrence the rock can The composition of the plagioclase, partly altered be designated a subvolcanic dyke rock correspond­correspond- into sericite, carbonate and quartz, is AnAnru.w The ing to a tholeiitic basalt in composition (Rasilai­(Rasilai- idiomorphic phenocrysts, which account for nen & Västi 1989, this volume).

METAMMETAMORPHISMORPHISM

According to the classification of Miyashiro la area, also frequently occur in this facies. Far­Far- (1979), metamorphism in the Rauhala area culmi­culmi- ther to the east of RauhalaRauhala,, andalusite is replaced nated under the conditions of low pressurepress ure and by sillimanite, and the sediments start to be mig­mig- temperature amphibolite facies. Andalusite in matized, implying a marked rise in temperature paragenesis with muscovite, biotite and quartz isis (Winkler 1979; Turner 1981).l98l). a typical index mineral in the metasediments. Ac­Ac- At a later stage, retrograde metamorphism set cording to Miyashiro (1979), andalusite is stable in when the temperature droppeddropped.. According to at low presspressure ure and temperaturetemperature.. Cordierite and Turner (1981) and Winkler (1979), this leads to an almandine,almandine. which are less common in the Rauha- increase in HHrO20 and often also in COCOr,2, in the

15l5 GeologicalGeological Sur Surveyvey ofof FinlandFinland,. SpecialSpecial Paper 11I I K.J. VäsriVaisti rocks.rocks. The best indicationindication ofof retrograde metamor- and quartz,quartz, which can be expressedexpressed by the follow­follow- phism inin thethe environment of RauhalaRauhala isis the almostalmost inging reaction:reaction: complete alteration of andalusite into muscovite

andalusite + potassium feldspar ++ water =: muscovite + quartz

The feasibility of the above reaction isis further processes. According to TurnerTurner (1981), the exten­exten- supported by the fact that potassium feldspar is sive occurrence of andalusite isis an indication of a rare constituent in the pelitic rocks of the area metamorphism at pressures below 3.5 kb, cor­cor- and never a major mineral. Inln places biotite has responding to a depth Qfqf lessless than 10l0 km. The been altered into chlorite through retrograde prevailing temperature was 500-550500-550"C.°C.

TECTONIC FEATURES AND STRASTRATIGRAPHY TIGRAPHY

The generagenerall strike of bedding and schistosity in ffiffi the study area is almost east-west. However, slight l1liffi Quartz diorite variation is common and, for example at the ore l deposit, the ssuccessionuccession exhibits agentiea gentle Z-formed Df .l Plagioclaseet"gio"lase porphyrite dextral fold (Fig. 3). The layering dips 30-3530-35"° m::~Ei*i Uraliteurarite porphyrite mainly to north. Farther away from the ore dede-­ <> posit the dip is steeper. The fold axis strikes about 1km - D[--l Micarrlica schistscrrtst east-west with a fairly gentle plunge. Farther east and west, the strike turns to north-south.north-south. At a later <> 5F l Metaturbiditeuetaturoioite stage, the area was submitted to weak east-west compression as shown by the strikes of some of ~.,...... >+++ <> B[! Metagraywackern"t"sraywacke the microfolds. " Ö '~ -I 1 I, 1 Blacker""r schist interbedinterbed Owing to the well-preserved graded bedding in <>I F] the metaturbidites it has been possible to estab­estab- I_ Sulphidesuror,ioe deposit 01ot RauhalaRauhata lishli sh the stratigraphy of the Rauhala area in detail (Fig. l2).12). The gently dipping metasediments in the 1 ' i 1 Quartz-IeldsparQuartz-f eldspar schist -- interbedinte rbe d area generally face north, but farther west and east Basement ~ anoarusiteAndalusite stratigrapstratigraphichic observations are hampered by the unknown [.> lack of outcrops and the higher metamorphic grade. Fi g. 12. Slraligraphy off RauhalaRauhala areaarea.. The whole succession is primarily composed of the sedimentary rocks that also host the stratistratiform form ore deposit (Fig. 12).l2). Beneath the ore sheet there are black schists and metagreywackes in several horizons. Graphite-bearing rocks have not been Although on a large scale quartz diorite is al-al­ found above the ore. Quartz-feldspar schist and most concordant with the sedimentary rocks, in hornblende schist occur in places as thin interlay-interlay­ many places it clearlyc1early intrudes them. However, in ers in metaturbidites and mica schists. No deposi-deposi­ the Rauhala area, the youngest rock is uralite por-por­ tionaltional basement has been encountered for the phyrite, which cuts both the metasediments and sedimentary rocks in the study area. the quartz diorite.

SUMMARY AND CONCLUSIONS

The geologygeology of thethe immediate environment of salsal quartz diorite. There is a completecomplete absence of thethe Rauhala sulphidesulphide deposit isis dominated by a extrusive rocks. Regional metamorphism culmi-culmi­ unit of sedimentary rocksrocks intruded by a hypabys- nated under thethe conditions of lowlow pressure and

16t6 GeologicalCeological SurveySurvey ofof Finland,Finland, SpecialSpecial Paperpaper 11I I GeoCeologylogy ofof lhethe RauhalRauhalaa slratifosrrariformrm massivemassive Zn-eu-PbZn-Cu-pb sulpsulphidehide deposildeposir temperaturetemperature amphiboliteamphibolite facies.facies. TheThe unitunit ofof asas mainmain minminerals.erals. TheThe accessoriesaccessories includeinclude seveseveralral sesedimentarydimentary rocksrocks iiss mainlmainlyy composedcomposed ofof mmetatur-etatur­ ototherher susulphideslphides aandnd ssulphosaltsulphosalts (Kojonen(Kojonen etet aal.l. bidites aandnd mica scschists.hi sts. BlackBlack scschist,hi st, metmetagrey-agrey­ 191989,89, thisthis volvolume).ume). wacke,wacke, quartz-feldsparquartz-feldspar schist,schist, cordieritecordierite ggneissneiss andand AsAs shownshown inin thethe Zn-Zn-Pb-CuPb-Cu diadiagramgram bbyy thethe rrela-ela­ hornblhornblendeende scschisthist occuroccur asas narronarroww intinterlayers.erlayers. tivelytively hihighgh ababundancesundances ofof copcopper per andand zizincnc inin par­par- TheThe bebeginningginning ofof hhydrothermalydrothermal activityactivity prepreced-ced­ ticular,ticular, thethe RauhalaRauhala ssulphideulphide depodepositsit resemblesresembles iningg thethe ddepositioneposition ofof thethe oreore iiss revealedrevealed bbyy the ex­ex- aa vo1cavolcanite-hostednite-hosted ratherrather thanthan aa sedsediment-hostediment-hosted istenceistence ofof aa continuous,continuous. conformableconformable sesericitericite scschisthi st oreore inin chemistrychemistry (Rasilainen(Rasilainen && Västi 1989,1989. thisthis lalayeryer inin thethe immediateimmediate footwallfootwall ofof thethe orebody.orebody. vovolume).lume). TheThe mamassivessive oreore bodbodyy exhibitsexhibits aa didistinctstinct InIn thethe hanhangingging wall ofof thethe mamassivessive ore,ore, thethe seseri-ri ­ laterallateral zonality, which manifests itselfitself inin thethe rela­rela- cicite te scschisthist iiss lelessss abundantabundant andand considerablyconsiderably mormoree tivelytively hihighgh copcopper per values inin thethe southwesternsouthwestern andand didiscontinuous.scontinuous. middle portions ofof the deposit. TheThe zzincinc andand leadlead TheThe orebodyorebody isis a stratstratiformiform tabular ssheetheet eembed-mbed­ vavalueslues increaincreasese towardstowards the northwestern and east­east- ded in metametasediments,sediments, and it isis composed of mamas-s­ ern edges of the ore ssheet.heet. TheThe elongated south­south- ssiveive and disseminated ores. The disseminated ore west-northeast trendingtrending peak anomaly of copper occurs mainly in the sericite schist inin the footwall hahass been interpretedinterpreted as a rupturerupture though which the of the mamassivess ive oreore.. The main mineralmineralss are pyrrho­pyrrho- metal-bearinmetal-bearingg ore solutions discharged on the floor tite, sphalerite, cha1copyritechalcopyrite and gagalena.lena. CloClosese to of a deep ocean (Västi 1988; Rasilainen & Västi the suboutcrop, pyrite and markasite also occur 1989, thithiss volume).

ACKNOWLEDGEMENTS

DrDrs.s. G. Gaal,CaäI, Kari Kojonen and Mr. KaleKalevivi the microphotos were taken by K. Kojonen. MrMrs.s. Rasilainen read the manuscript, which was GillianCillian Häkl Häklii translatedtranslated the FinniFinnishsh text inrointo Eng­Eng- markedlmarkedlyy improved by their comments. The lilish.sh. To aallll these personspersons I express mymy cordial figures were drawn by Mrs. RaijRaijaa Väänänen and thankthanks.s.

REFERENCES

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18 L