Geology of the Rauhala Stratiform Massive
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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 depodepositsit 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 ddeposit.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