327

Geologische Rundschau 79/2 [327-335 I Stuttgart 1990

Geochemistry of 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 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 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 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 . 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:~

Given a closer look, however, an easy and and spherical apatite crystals, as well as rare small straightforward solution is nowhere in sight. Current- dolomite aggregates. No magnesite or calcite inclu- ly it is not possible to interpret the isotope systematics sions were found. A larger piece of this rock was of dolomites as well as the geochemistry of selected homogenized for chemical analysis. In addition, we trace elements in an unambiguous way, as discussed by mechanically separated the dark (graphitic) talc from LAND (1980, 1983). the clean white parts and analyzed them separately. In the current study we present the first results from The samples are designated OBD IVA (bulk, W=white, a multielement geochemical investigation of samples S=dark). from the magnesite mine at Oberdorf/Laming (near Bruck an der Mur; Styria, Austria). Our discussion of Analytical methods the genetic implications of the data is mainly based on the distribution of the rare earth elements. Hand specimens (about 1000-2000 g weight) of selected samples were crushed and pulverized in an automatic agate mortar to obtain homogeneous and Samples representative rock powders. The major elements have Sparry magnesite mineralizations (Veitsch type been determined by complexometric titration (Mg, deposits) are common throughout the East-Alpine Ca), direct current plasma optical emission spec- Graywacke Zone from the Semmering area to trometry (Mg, Ca, Si, AI, re, P, Mn), and standard wet Hochfilzen near the Tyrol/Salzburg border. These chemical methods (SO3, Si, H20, L.O.I.). The deposits are of Silurian-Carboniferous (Vis~an) age. analytical precision for the major elements is 0.5-2 The magnesite deposit at Oberdorf is enclosed in car- tel%. The results of the major element analyses are bonates and meta-sediments of the Vis6an (FELSER, given in Table 1. 1977). Trace elements have been determined with in- Five samples from the 828 m level at the Oberdorf strumental neutron activation analysis (INAA). Sam- magnesite mine have been chosen for our study. A ple weights have been about 500 rag. Three different ir- location map of the 828 m level of the mine is given radiations have been made for short and long-lived in HADITSCH (1966). The samples have been selected isotopes. The short irradiations (several minutes) for all from alternating magnesite and dolomite rock masses. samples and the long irradiations (8 hours) for the Magnesite 1987/103 was taken from the first magnesite pinolitic magnesite/talc have been performed at the rock mass, dolomite 1987/104 from a dolomite zone Triga Mark II reactor of the Atominstitut der Oster- between the first and the second magnesite rock mass; reichischen Universit~ten at a neutron flux of about magnesite 1987/105 from the second magnesite rock 2.1012 n cm2s -1. The long irradiations for the samples mass, dolomite 1987/106 from the dolomite zone bet- 1987/103-1987/107 have been made for 108 hours at ween the second and third magnesite rock mass, and the Astra reactor of the Research Center Seibersdorf at finally, magnesite '1987/107 from the third magnesite a neutron flux of about 6.1013 n cm2s-1. Following the rock mass. irradiations, the samples have been measured repeated- X-ray diffraction studies of the five Oberdorf ly over a time interval of two months, using INAA samples showed that the magnesites contain minor ad- procedures described by KOEBERL et al. (1987). The mixtures of talc, chlorite, and dolomite, and some precision of the INAA data varies from 3-15 tel.% for and baryte and/or and . elements with concentrations in the ppm range to This applies also to the dolomite sample 1987/104. 20-30 rel.% for those in the ppb range. The trace ele- In addition, we analyzed a talc pseudomorph (after ment contents of the samples are given in Tables 2 pinolitic magnesite) from the Meixner collection of and 3. the University of Salzburg. The pinolithic texture of talc previously found at the 901 m level of the Ober- doff mine has already been described by WELSER Results and discussion (1938). Today this specific occurrence seems to be ex- hausted, although HADITSCH (1966) described fin- The results of the chemical analyses are presented in dings up to the early 1960's. The sample description Tables 1-3. Among the trace elements, the rare earth does not provide the exact location, but we assume that elements (REE) are of considerable importance for the sample has been recovered from the location clarifying the origin and formation processes of the described by HADITSCH (1966). magnesites. This approach has been used earlier by Electron microprobe analyses demonstrated that MORTEANI et al. (1982) to determine the origin of besides talc the sample contains minor apatite needles sparry magnesites from the Western Greywacke Zone 330 W. KItSL et al.

Sample# 103 104 105 106 107

M 1 D M 2 D M 3

Mgo 45.05 19.65 44.90 21.57 37.34 CaO 1.52 30,49 2.12 30.01 10.73 A1203 0.28 0.65 0.05 0.04 0.18 SiO 2 1.35 1.64 0.33 0.49 0.43 MnO 0.03 0.13 0.02 0.Ii 0.03 Fe203 0.45 0.51 0.24 0.66 0.31 SO 3 0 ~ 13 0.33 0.12 0.31 0.28 P205 0.09 0.09 0.05 0.08 0.08 H20 <0.01 0.08 0.03 0.03 <0.01 L.O.I. 50.60 45,70 50.80 46.30 49.99

Total 99.51 99.27 98.64 99.62 99.38

Table 1. Major element composition of magnesites (M) and dolomites (D) from the Oberdorf/Laming deposit. All data in wt.%.

Sample # 103 104 105 106 107 M 1 D M 2 D M 3

Na 200 300 180 150 200 Cl 191 43 304 84 333 Sc 1.45 0.61 0.Ii 0.21 0.26 Cr 2.39 3.36 0.76 0.93 1.22 Co 0.13 0~61 0.13 0.095 0.16 Zn 2 5 14 22 2 AS <0,16 <0.5 <0.15 <0.12 <0.08 Se (ppb) 37 44 13 25 26 Br 5.9 0.33 10ol 0~41 11.4 Rb 22 68 14 18 i0 Sr i0 1055 7990 524 157 Cd <0.01 <0,01 <0.007 <0.007 <0.002 Sb 0.148 0.50 0.041 0.071 0.037 Ba 152 7800 150 35 70 La 0.59 0.40 0.335 0.19 0.19 Ce 1.6 1.17 0.96 0.91 0.7 Nd 1.25 0.79 0.67 0.71 0.53 Sm 0.45 0.22 0.19 0.27 0.177 Eu 0.084 0.061 0.044 0.18 0.069 Tb 0.070 0.047 0.027 0.044 0.046 Yb 0.40 0.217 0.17 0.215 0.192 Lu 0.064 0,032 0.027 0.033 0.032 Hf 0.51 1.33 0.085 0.045 0.15 Ta 0.30 <0.2 0.08 0.12 <0.i Ir (ppb) <0.13 <0.30 <0.i <0~I <0.i Au (ppb)

CI/Br 32 130 30 204 29

all data in ppm, except where noted Table 2. Trace element composition of magnesites (M) and dolomites (D) from the Oberdorf/Laming deposit. Geochemistry of magnesites and dolomites at the Oberdorf/Laming (Austria) 331

OBD-IVA-Bulk OBD-IVA-W OBD-IVA-S

Na 162 I13 160 K <50 43 <50 Sc 0.37 0.28 0.38 Cr 3.53 1.01 5.52 Mn 4.18 5.6 5.0 Fe 1850 3220 1730 Co 0.40 0.96 0.29 Ni <5 12 <5 Ga 3.52 0.71 3.96 As 0.22 <0.04 0.35 Br 0.50 i.I 1.4 Rb <0.8 0.88 <2 Sr 14 <6 <25 Zr 130 13 130 Sb 0.45 0.33 0.49 CS 0.14 0.31 <0.i Ba 31 <3 34 La 1.3 0.044 2.46 Ce 1.77 0.090 4.02 Nd 1.02 0.10 3.8 Sm 0.29 0.038 0.80 EU 0.032 0.0025 0.052 Tb 0.029 0.0068 Dy 0.19 0.06 0.34 Yb 0.13 0,031 0.20 LU 0,024 0,0035 0.029 Hf 0.085 <0.009 0.15 W 0.12 <0.05

all data in ppm Table 3. Concentrations of minor and trace elements in a talc pseudomorph from the Oberdorf magnesite deposit, obtained by neutron activation analysis. OBD-IVA-Wand OBD-IYA-Sare the clean white and the dark (graphitic) talc fractions, respec- tively.

(Austria). At these deposits it is obvious that changes occasionally Th. Hu et al. (1988) have shown that the in the REE patterns during metasomatosis of total REE content of marine -rich sediments dolomites to magnesites have been very small. The from DSDP cores taken at the Rio Grande Rise is light REE (EREE) seem to display a higher mobility directly related to the A1 content (which represents the than the heaW REE (HREE), because the ionic radii noncarbonate clay component). Thus it seems likely of the HREE are closer to the ionic radius of Mg2+ that the amount of the REE contribution from clay in than the ionic radii of the LREE. Thus the LREE may dolomites and magnesites can be determined from the be enriched in the solution and can thus be depleted A1 content. in the rocks. They may later be captured during subse- Fig. 1 gives the chondrite normalized REE patterns quent redolomitization. of the samples from the Oberdorf magnesite-dolomite A general decrease or increase of REE abundances is succession. The patterns of the Oberdorf samples are not necessarily the result of the metasomatic process. similar to the carbonate samples described by Hu et During our studies we found a positive correlation bet- al. (1988) if we take into account the lower content of ween the Al-content and RBE~abundances, which is in- clastic components in our samples. We have also dicative of a clay component. This positive correlation observed good agreement with some magnesite and usually includes other elements, such as Hf, Ta, Sc, and dolomite patterns given by MORTEANI et al. (1982). 332 W. KIESLet al.

I I J I I i I I I I I I I I I U < "103 1105 ~, 104 I IO tlO7 0106 <

\\ ,< •,_--•/ Z III ...... 1.O 0

=_ Q Z 0 U i I I I I I I I I I I I I I I La Ce Pr Nd Sm .Eu Gd Tb Dy Ho Er TmYb Lu Fig. 1. Chondrite normalized REE patterns for five magnesite (full symbols) and dolomite (open symbols) samples from the Oberdorf/Laming mineralization. Normalizing factors from TAYLOR(1982).

Samples 1987/103 and 1987/104 show a minor mobilized. It has been shown by SHIMIZU & MASUDA negative Eu anomaly, while sample 1987/106 exhibits (1977) and BELLANCA et al. (1981) that only minimal a conspicuous positive Eu anomaly. The positive Eu post-depositional changes occur to the REE contents anomaly can be explained by a higher feldspathic com- of sediments during hydrothermal alteration of ponent of the original sediment, because feldspars are limestones. the most abundant with prominent positive This is in agreement with observations by KIESL et Eu anomalies. al. (1983), who found that, under the p-T conditions Negative Eu anomalies too may have been introduc- for the Mg-metasomatism of gneisses (which are ed with the noncarbonate (e.g., clay) component of the similar to the conditions for magnesitization), the original sediment. MORTEANI et al. (1982) observed a REE are immobile. The studies have 'been carried out gradual progression from positive to negative Eu on rocks of the gneiss complex near the talc-(magnesite) anomalies with decreasing absolute REE contents. deposit of Rabenwald. Moreover it was shown in the This behavior is not duplicated in our samples. Sample study by KIESL et al. (1983) that the Mg-bearing 103, which has the highest REE abundances, displays metasomatic fluids have obviously been depleted in a slight negative, and not a positive, Eu anomaly. REE, because neither the pattern, nor absolute content MORTEANT et al. (1982) found negative Ce anomalies of REE is affected during metasomatism. REE are in some of their samples, which can be interpreted as redistributed to a high degree from rock forming being indicative of formation in a marine environ- minerals into apatite. We suggest that this was the pro- ment. Our samples 103-107 do not show any indica- cess of formation of the magnesites at the Rabenwald tions of negative Ce anomalies. This is, however, no talc-magnesite deposit. proof against a marine origin, because the REE in clay Unfortunately, lanthanide distributions in components can mask small original negative Ce have not been studied in detail so far, but we anomalies. can conclude that magnesites formed in a sedimentary The immobility of REE is a valuable indicator for environment (an evaporitic milieu) must show very metasomatic processes. All data available so far (which low REE abundances. The residence times of lan- are few), suggest that REE are unaffected by diagenesis thanides in sea water are extremely short (see review by (CEIAUDURI & CULLERS, 1979; WILDEMAN & CON- TAYLOR & MCLENJ'qAN, 1988). They are removed DIE, 1973; WILDEMAN & HASKIN, 1973) as well as from sea water predominantly by adsorption onto set- metamorphism, as discussed by TAYLOR & MCLEN- tling particles. FLEET (1984) noted a correlation bet- NAN (1988). Although greenschist- to amphibolite- ween A1 content and REE concentration in calcareous facies metamorphism of regional extent commonly af- oozes and concluded that most of the lanthanides fects all sedimentary rocks, the lanthanides are not reside in phases other than carbonate. Another indica- Geochemistry of magnesites and dolomites at the Oberdorf/Laming (Austria) 333

I I I I I I 11 I I 11 I I I which is more pronounced in the talc samples than in the magnesites or dolomites. This may be due to a - 10 reducing metasomatic fluid, or the absence of posi- tions for Eu 2+ in the talc structure. In contrast to Oberdorf magnesites and dolomites, the talc samples show slight negative Ce anomalies, similar to some Z magnesites analyzed by MORTEANIet al. (1982). This z may be due to a different clay component and/or to different locations of the sample sites in the mine. 1 The REE patterns of the Oberdorf talc samples are similar to talc samples from a talc-(magnesite) N OBD-IVA-bulk \ ~ . -a 'I, mineralization at the Rabenwatd (Austria) deposit (KIESL et al., 1986). The dark fraction shows REE abundances that are more than 10 times higher than o z the clean white component, which is due to the higher ~J clay (and graphitic) component. This is also indicated 0.1 by other immobile elements, such as Hf or Th. The 7. patterns may also be influenced by traces of apatite = which have been found in the rock. The white talc frac- tion shows a REE pattern that is very similar to apatite, separated from talc (KIESL, 1986), but at a much lower concentration level. It is interesting to note that I I I I I I I r I I I I I I 1 elements such as Mn, Fe, Co, and Ni are enriched in La Ce Pr Nd SmEu GdTb DyHo Er TmYb Lu the pure white talc phase compared to the dark frac- tion. This observation may provide some clues to the Fig. 2. Chondrite normalized REE patterns for three samples of a talc pseudomorph from the Oberdorf/Laming composition and origin of the metasomatizing fluids. magnesite mine. Normalizing factors from TAYLOR(1982). Similar observations have been made by PROCHASKA W = clean white talc fraction; S = dark (graphitic) talc frac- (1989) in investigations of talc samples from the Lass- tion. ing talc deposit. Finally we would like to draw attention to the in- teresting behavior of the halogens C1 and Br. Although tion supporting this conclusion may be the Bela Stena these elements are very mobile during metasomatic magnesites (MORTEANIet al., 1982), which are indeed processes, it is possible to use their concentration ratios of hydrothermal-sedimentary origin, but show con- as an additional indicator for the nature of the siderably lower REE contents than the sparry metasomatizing fluids. These two elements show con- magnesites analyzed in the course of this study. siderable and significant differences in their abun- Even the metasomatic process leading to the subse- dances as well as in their ratios between dolomites and quent formation of talc from magnesite under the in- magnesites. The CI/Br ratios in magnesites seem to be fluence of Si-rich solutions does not alter the abun- rather constant at about 30, while in dolomites this dance patterns of REE and other immobile elements. ratio ranges from about 100 to 200. Both ratios are HADITSCH (1966) was able to demonstrate that the unlike any marine ratios, which are much higher. Oberdorf talc was formed by pseudomorphosis in a This observation may very well have genetic implica- two-stage metasomatic process from pinolitic tions. The non-marine CI/Br ratio may be seen in con- magnesite. During this process the original texture of nection with the conclusions by HARDn~ (1987), that the magnesite was preserved. We have separated the at slightly elevated temperatures most natural waters clean (white) talc fraction from a darker (graphitic) (not only special brackish fluids) are capable of fraction (which contains some clay components). The dolomitization, and probably also magnesitization. results of the analyses are given in Table 2. The chon- We suggest that fluid inclusion studies should, despite drite normalized REE patterns are shown in Fig. 2. the obvious experimental difficulties, try to include Br The REE patterns of the talc samples are very similar in future analyses. to the patterns of Oberdorf dolomites and magnesites. Conclusions This once again demonstrates that during metasomatism the REE patterns undergo little or no We have analyzed major and trace elements in a set change at all. A possible exception is the Eu anomaly, of magnesite and dolomite samples from alternating 334 W. KmSL et al. rock masses at the Oberdorf/Laming (Styria, Austria) tions, however, have much lower total REE abun- magnesite mineralization. The REE and A1 contents dances. are positively correlated, indicating that a clay (or Although the question of the origin of Mg-rich solu- other noncarbonate) component is the main source for tions for the magnesitization is still speculative, we the REE in the magnesites and their precursors. assume that they may have been derived from deep- From our results we conclude that the Oberdorf seated basic to ultrabasic rock masses. According to magnesites, as well as other similar deposits in the PETRASCHECK (1972), there is evidence for the occur- Eastern Alps, have been formed by metasomatic pro- rence of serpentinites below the gneiss complexes of cesses. The process started with , which was East Styria, comparable to the South Penninic units of deposited by sedimentation. After dolomitization of the Tauern window. these carbonates, Mg-rich solutions circulated through the rock masses and formed magnesites by metasomatism. Acknowledgements During the metasomatic processes, the REE patterns of the carbonates remained mostly unaltered. The We are grateful to the Rohstoff-Forschungskommission magnesites thus show to a large extent the REE pat- ( Deposit Research Commission) of the Austrian terns of the original carbonates, which supports the Academy of Sciences and to the Federal Ministry of Science and Research (Z1. 30181) for financial support. C.K. is also idea of a metasomatic origin of the magnesite deposits. grateful to the Fonds 7,600 Jahre Wiener Universit~it,~for In addition, we analyzed talc pseudomorphs which funds for instrumentation. Furthermore, we would like to have formed from pinolitic magnesite by a two-stage thank the management of the Oberdorf mine for help during metasomatic process. The results indicate that the sampling, and Mag. C. KRALIKfor CI and Br analyses. We shape of the REE patterns is not changed significantly are grateful to Prof. E. KIRCHNER(Univ. Salzburg) for the during the formation of the talc. Very clean talc frac- talc pseudomorph sample.

References

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