327 Geologische Rundschau 79/2 [327-335 I Stuttgart 1990 Geochemistry of magnesites and dolomites at the Oberdorf/Laming (Austria) deposit and implications for their origin By WOLFGANGKIESL, CHRISTIANKOEBERL and WILFRIEDKORNER, Wien*) With 2 figures and 3 tables Zusammenfassung correlated, indicating a clay component as the main source for REE in the magnesites and their precursor rocks. Our Die Haupt- und Spurenelementgeochemie yon Magnesit- data support the view that the Oberdorf and probably other und Dolomitproben, die aus alternierenden Gesteinsst6cken magnesite mineralizations in the Eastern Alps have been in der Magnesitmine yon Oberdorf/Laming (Steiermark, formed by metasomatism. Through this process calcite Osterreich) stammen, wurde untersucht. Besonderes Augen- (which was deposited by sedimentation in a marine environ- merk wurde dabei auf die Geochemie der Seltenen Erdele- ment) underwent dolomitization. Subsequently, Mg-rich mente (REE) gelegt, da diese fiir Aussagen tiber die Entste- solutions circulating through the rock masses formed the hung der Magnesitlagerst~itten yon besonderer Bedeutung magnesite bodies. sin& Eine positive Korrelation zwischen den AI und REE- During the formation of the dolomites and the conversion Gehalten deutet darauf hin, daft der Hauptteil der REE einer to magnesites, the REE patterns of the original carbonates re- Tonkomponente enrstammt. Die Daten untersthtzen eine mained unchanged. The magnesites exhibit patterns that are metasomatische Entstehurlg der Spatmagnesite der Oberdor- very similar to sedimentary carbonates (which contain a fer Lagerst~itte und anderer Ostalpiner Lagerst~itten. Sedi- REE-bearing clay component). Magnesites of purely ment~ir angelegter Kalzit wird dabei tiber Dolomit mit Mg- sedimentary origin, formed in a predominantly evaporitic reichen L6sungen in Magnesit umgewandelt. environment, should have lower total REE contents and dif- W~ihrend der Metasomatose bleiben die chondritnormier- ferent patterns due to the short residence times of these ten REE-Verteilungen der Ausgangskarbonate relativ unver- elements in seawater. Analyses of talc pseduomorphs after ~indert erhalten. Die so entstandenen Spatmagnesite weisen magnesite indicate as well that the REE patterns are not Verteilungsmuster auf, die jenen der sediment~iren Karbona- significantly altered during the talcification. The Mg-rich te, aus denen sie entstanden sind, entsprechen. Rein sedimen- solutions that led to the formation of the magnesites have t;ir entstandene Magnesite werden praktisch nur in evaporiti- probably been derived from serpentinites below the gneiss schem Milieu gebildet und sollten infolge der kurzen Ver- complexes of eastern Styria and the South Penninic units of weilzeit dieser Elemente im Meerwasser wesentlich niedrige- the Tauern window. re REE-Gehalte und an&re Verteilungsmuster aufweisen. Analysen einer Talkpseudomorphose zeigen, daft sich die REE-Verteilungsmuster auch bei der metasomatischen Talk- R&um4 bildung nicht wesentlich ~indern. Die Mg-reichen L6sungen, die zur Bildung der Magnesite notwendig waren, entstam- Cette note pr&ente les r&ultats d'une &ude g6ochimique men vermutlich tiefliegenden Sepentiniren, auf die es Hin- des ~lements majeurs et en trace d'6chantillons de magn&ite weise im Untergrund der oststeirischen Gneisskomplexe und et de dolomite qui se pr&entent en masses altern6es dans la des siidpenninischen Tauernfensters gibt. mine de magn&ite de Oberdorf/Laming (Steiermark, Autri- che). Une attention particuli~re a &~ apport6e aux terres rares, consid6r6es comme fournissant des arguments impor- Abstract rants dans la discussion de l'origine des gisements. Les teneurs en A1 et en terres rares montrent une correlation posi- The major and trace element geochemistry of magnesite tive, ce qui permet de rapporter ~ un composant argileux la and dolomite samples from alternating rock masses at the source principale des terres rares. Les r&ultats obtenus plai- Oberdorf/Laming (Styria, Austria) mineralization has been dent en faveur d'une origine m&asomatique des min6ralisa- studied. The rare earth elements (REE) are considered to be tions en magn&ite d'Oberdorf et d'autres endroits des Alpes of importance for the discussion of the origin of sparry orientales. Au cours de ce processus, la calcite, d'origine s4di- magnesite deposits. The AI and REE contents are positively mentaire marine, a &~ le si~ge d'une dolomitisation avec, subs6quemment, formation de corps de magn6site grfice ~ la *) Authors' address: W. KIESL, CHR. KOEBERLand W. KOR- circulation de solutions riches en Mg. NER, Instirut fiir Geochemie, Universidit Wien, Au cours de la m&asomatose, la distribution des terres rares Dr:Karl-Lueger-Ring 1, A-1010 Wien, Austria. des carbonates originels n'est pas modifi6e: les magn&ites Manuscript received: 3.5.89; accepted: 5.2.90 montrent des r@artitions tr~s semblables ~ celles des carbo- 328 W. KIESL et al. nates s6dimentaires (lesquels renferment un constituant argi- sparry magnesite itself, however, is still the subject of leux porteur des terres rares). Des magn6sites d'origine pure- considerable discussion. SCHULX (1986) summarized ment s6dimentaires, form6es en milieu 6voporitique, doivent the current knowledge and some recent studies, but pr6senter un contenu total en terres rares moins 61ev6 et une was unable to describe a clear-cut process for the origin distribution diff~rente de ces ~l~meuts, eu ~gard ~ la bri~vet6 of magnesite. The formation of magnesite is governed de leur s~jour dans l'eau de mer. Des analyses de pseudomor- phoses de magn~site en talc montrent de mfime que la distri- by complicated processes, similar to the complexity of bution des terres rares n'est pas modifi~e de mani~re significa- the origin of dolomite. Important criteria which in- tire au cours de la talcification. Les solutions riches en Mg fluence magnesite formation can be derived from responsables de la formation de la magn~site sont probable- chemical thermodynamics and kinetics, hydrology, ment d~rivfiesde serpentines situfies sous les complexes gneis- and structural as well as textural observations. siques de Styrie et les unit~s penniques m~ridionales de la In a recent study of thermodynamic parameters of fen~tre des Tauern. magnesite formation, KIESL (1989) has shown that a purely sedimentary origin of magnesite (as well as dolomite) is possible only under rare circumstances, re- KpaTKoe co~ep~aHHe quiring specific thermodynamic and geochemical con- B npo6ax MarHe3~ITa~i /IOJIOMI,ITa 143 IIITOKOBropHl;IX ditions. Even under these conditions, no substantial nopo~ MeCTOpOZK~eltg~IMarFie3Hwa H J1OaOMHWay Ober- quantities of magnesites (such as the deposits of dorf/Laming'a (I[IwHpI4~I,ABCTpn~) onpeJIeaHJm rzaBHLie economic interest as they are discussed in this study) !i pacce~ntHhle 3SleMeHTbI.Oco6oe BIt~iMam4ey~leJIriali eo XHMIIH PeJIKHX 3eMeJIh, nOTOMy qTO HM ripHnHC~IBa~OT are formed. oco6oe 3naqeHHe npH pemeHHH Bonpoca o npo~Icxo~;~e- Sparry magnesites found in the Eastern Alpine Hall MarIte3HwoBsix MecTopox~eHHfi. IIOS~O)KHWea~Has regions are commonly associated with dolomites and Koppe~sJ/~ Me)K~y CO)lep~KaH~eMaatOMHHH~q H Pe~IKHX limestones. Thus a formation of sparry magnesite from 3eMeJI~ yKa3smaeT Ha TO, ~TO OCHOBHa~qaCTS Pe~IKnX sedimentary calcite via dolomite must be taken into ac- ~eMeJIb Haxo)IIITC~I B rJn4ttttCTblX OTJIO)KellIt~IX.rIo~y- count. The key to the formation of sparry magnesite ~ieHH~e ~aH~e FOBOp~IT,~ITO KaK o6ep~lopqbcKoe, Tag may be found by considering the so-called ,,Dolomite ~pyrge BOCTO~HO-a~5~IgfiCK]4eMecTopozKJleHFN o6paao- Problem~ (DEELMAN, 1988). BaJI~Icb MeTaCOMaT~UeCKH. OeaBo~HbIf~ Ka~bR~IT np~ ~TOM npeo6paayeTcn ~epea cTa)lmo JIOJIOMIITaIIOJl BO3- Both main models of dolomitization, the ,,Dorag,< JIe!~ICTBIleM6oraTb~X MarH~eM paCTBOpOB B MarHe3nT. mixing-zone model (BADIOZAMANI, 1973), and FOLK BO BpeM~ npoRecca MeTacoMaT~3aXOH)~poBoe pacnpe- & LAND's (1975) ,,schizohaline~ version of mixing- ~eJ~em~e Pe~i(~ix3eMeJ~b ~CXO~IHb~X Kap6oHaTOB OcTaeTc~ zone dolomitization, are based on the assumption that cpaBHnTe~HO ne~3MeHHMM. O6pa30BaBmgecn T.O, brackish groundwater in the mixing zone between noaa~ne MarHe3iiTbi co3~aIOT CXeMy pacnpe~leae~n~ seawater and overlying coastal freshwater may be an ef- PeRKnX3eMexb, COOTBeTCTBylOII~ytOTaKOBOfI oca~otIHblX fective dolomitization fluid. Another model suggests Kap6ogaTOB, na KOTOpbIX OHH o6pa3oBa~incb. Marne- that dolomites should only form from waters with 3I~TbI~ o~pa3oBaBUtIlec~i ItCK2ItOtII4TeYlbI-IOoca]IotII-II,IM sulfate concentrations much less than those of current nyTeM, Bo3In4KaIOTIlpaKThltIeCKI4 TOYl]bKO lIpll 0BaI~Op~T- ~bIX yCJ~OBg6~Xn, B pe3ya~TaTe o,~eHb nenpo~o:~<wrea~- seawater (BAKER & KASTNER, 1981), which seems to noro npe6bmang~ OTI~X OaeMeHTOB B MopcKofl BO~e, be in disagreement with the formation of recent xapaKTepg3y~OTCS 6oaee unava~M co~ep)Kan~xeM Pe~IK~X dolomites. All three models have considerable short- ~eMeJIb; pacnpe~eJ~enne ~x ~ec~ ]IaeT Hityio KapTmty. comings, as discussed in detail by HARDIE (1987). It is Pe3yJ~bTaThI anaanaa Taa~KOBb~Xl~CeBgOMopqbo3oB yKa- shown by HARDIE (1987) that a different approach, in- 3blBatOT Ha TO, HTO tf np~ o6pa3oBaHtln TaJIbKanyTeM Me- corporating previously neglected factors such as TacOMaTO3a KapTn~a pacnpe~e:xen~fl Pe)lrm~ 3eMeJ~ B temperature, time, mass transfer processes, and fluid in- gem He npeTepneBaer 3na~mTe~bm,tx lt3MetIettl4fi. Bora- clusion studies, could be more realistic. The formation Tbte MarnneM