www.geologia-croatica.hr ABSTRACT A consanguineous origin of the sparry siderite and magnesite mineralizations of the Eastern Alps has been repeatedly discussed in the past, often without the back-up of sound scien- tific arguments. Here, it is shown that the characteristics of these mineralizations including structures, fluid parameters, timing etc. are amazingly similar suggesting in fact the genetic linkage of these deposits. The siderite as well as the magnesite mineralizations of the Eastern Alps exhibit metaso- matic-epigenetic structures of lens-shaped orebodies with dolomitic alteration rims as domi- nant features. The basic chemical characteristics of the ore forming fluids in these mineraliza- tions are those of highly fractionated hypersaline fluids exhibiting all the features of residual bittern brines. Fluid invasion and mineralization structures strictly depend on the original li- thology of the host rocks, mainly carbonatic sedimentary rocks. In Late Triassic times, these buried evaporitic brines were mobilized either by magmatic/metamorphic processes in the underlying crystalline units or by the superimposed sedimentary upload of the Triassic plat- form carbonates thus leading to magnesite formation. Initially these residual, bittern brines were completely free of Fe and rich in Mg suitable for the formation of magnesite. Deeper and more extensive circulation of these fluids and their interaction with the host-rock, re- sulted in the uptake of Fe and the formation of siderite. Keywords: siderite, magnesite, mineralizing fluids Article history: Received October 15, 2015 Revised and accepted January 21, 2016 Avaliable online February 29, 2016 1. INTRODUCTION Numerous magnesite and siderite mineralizations of various sizes can be observed in the Eastern Alps of Austria (Fig. 1), some of them were of considerable economic importance and were exploited. The minerogenetic map of Austria exhibits their locations. Today, only one siderite mine, the Erzberg siderite deposit, situated in the province of Styria, is in operation. The present Austrian iron production is about 750.000 tpa (Fe-content) and is exclusively produced from this mine. The well-developed Austrian magnesite industry is of world- wide importance, and is to a large extent still based on the Aus- trian mines. Magnesite production in Austria is approximately 750.000 tpa (ranked 5th worldwide) and is mined from 10 diffe- rent deposits. Similar mineralizations occur in Upper Carboniferous strata of the Gemeric units of Slovakia, which can be directly com- pared to the Greywacke Zone. The discussion about the genesis of these magnesite and siderite mineralizations throughout pre- vious decades is very similar to the controversial views of the Austrian examples and reflects the changing opinions concern- ing these mineralizations throughout the history of their investi- gation. It is beyond the scope here to give a complete review of the innumerable publications on the siderite and magnesite deposits of the Eastern Alps, so only a brief summary is given. However, 31-38 9 Figs. doi: 10.4154/gc.2016.03 Figure 1. Part of the minerogenetic map of Austria showing the Fe- and magnesite mineralizations of the Greywacke Zone (WEBER, 1997). Genetic concepts on the formation of the Austrian magnesite and siderite mineralizations in the Eastern Alps of Austria Walter Prochaska University of Leoben, Department of Geological Sciences, A-8700 Leoben, Austria 32 Vol. 69/1 during recent decades some considerable progress was made in the understanding of these mineralizations using modern scien- tific methods. Therefore the main focus of this paper is to dis- cuss the results obtained by the investigation and characteriza- tion of the chemistry of the fluid inclusions forming these mine- ralizations. Furthermore, direct radiometric age determination of the carbonate minerals of the hydrothermal events during re- cent years contributed substantially to the revised models of ore- formation. Finally, on the basis of these new results, an attempt is made to demonstrate that there is a genetic link between the carbo- nate-hosted sparry magnesite and the siderite mineralizations of the Eastern Alps. The arguments are mainly based on the inves- tigations of the fluid chemistry from inclusions that allow chara- cterization of the ore forming fluids showing an overall evapo- ritic signature of the hydrothermal fluids responsible for these mineralizations. Another crucial aspect for the genesis of both siderite and magnesite deposits is the timing of the mineralizing event. The focus of this paper is on the magnesite and siderite mine- ralizations of the Palaeozoic series of the Eastern Alps. The im- portant siderite/haematite iron-ore province of Hüttenberg- Waldenstein in the Austroalpine Crystalline Complex is not dis- cussed in this paper. This iron-ore province is related to hydro- thermal processes in connection to young tectonic activity with- out any relation to the mineralizations in the Palaeozoic series discussed here. 2. GENETIC MODELS PROPOSED IN THE PAST It is far beyond the scope of this chapter even to list the enor- mous amount of papers, published on the Austrian siderite and magnesite mineralizations since the start of modern geoscien- tific investigation in approximately 1850. Therefore only the general trend in the genetic opinion is referred to and discussed. Opinions concerning the genesis of the siderite mineraliza- tions of the Greywacke Zone are inconsistent (cum. cit. TOLL- MANN, 1977), and discussion about this topic is a longstanding tradition. Different genetic models, including a synsedimentary origin or an eoalpine vein-type and a metasomatic mineraliza- tion type have been proposed in the past. Early workers (e.g. SCHOUPPE, 1854; VACEK, 1903) on this topic favoured syn- genetic models, and at the turn of the century epigenetic models for the Erzberg type mineralization were proposed. More recent research concentrated on the investigation of structural and geochemical features and also on comparative studies (e.g. POHL, 1986) of the siderite mineralizations. In the 1970s, syngenetic models were favoured for the Erzberg mine- raliza tions mainly because of the findings of minor banded ore structures that were interpreted as primary sedimentary ore bands (BERAN, 1973, 1975, 1977, 1979a, 1979b; BERAN & THALMANN, 1977, 1978). Following these arguments SCHULZ et al. (1997) described ore textures of the Erzberg de- posit and postulated a marine-synsedimentary origin for the mineralization. Recently an epigenetic genesis for siderite mine- ralization in the Greywacke Zone was reintroduced on the basis of microthermometric, geochemical, and isotope data (FRIM- MEL, 1988; PROCHASKA, 1991; SPINDLER, 1992; BE- LOCKY, 1992; POHL & BELOCKY, 1994; LAUBE et al., 1995; PROCHASKA, 1997). These authors generally postulate an epigenetic origin, nevertheless there is no unanimous opinion concerning the most important metallogenetic features, the tim- ing of the hydrothermal event, and the origin of the hydrother- mal fluids. Similarly, there is no consensus on the genetic model and not even about the principal mechanisms for magnesite mineraliza- tion. Starting soon after the discovery of the world´s first mag- nesite deposits in the Eastern Alps after 1850, syngenetic models (e.g. RUMPF, 1873; LEITMEIER, 1917) and epigenetic models (e.g. KOCH, 1893; VACEK, 1903; REDLICH, 1907) were published. Starting in the 1950s, a general trend towards syngenetic and early diagenetic models can be observed (De LLARENA, 1953; LEITMEIER & SIEGL, 1954). NIEDER- MAYR (1989) argued for synsedimentary or early diagenetic genesis for magnesite mineralization in the Permian series of the Eastern Alps and extended this model to the alpine sparry mag- nesites. POHL & SIEGL presented an extensive overview of the magnesite mineralizations of the Eastern Alps in 1986. MÖLLER (1989) edited a monograph on magnesite summariz- ing the recent geochemical and mineralogical facts on the “mag- nesite problem”. During more recent years, new fluid inclusion and isotope data for the sparry magnesite deposit of the Eastern Greywacke Zone provide strong evidence for formation of the magnesite deposits by metasomatic replacement due to infiltrat- ing salinar residual brines which originated in the Upper Perm- ian/Lower Triassic (e.g. PROCHASKA, 2000; PROCHASKA, 2001; PROCHASKA & HENJES-KUNST, 2007). These obser- vations are considered to be of fundamental importance not only for the genetic aspects of carbonate-hosted sparry magnesite deposits but also hinting towards a consanguineous origin of the magnesite and siderite deposits of the Eastern Alps in general. Very recent results obtained by radiometric age dating of the carbonate formation provide proof for Triassic ore formation (HENJES-KUNST et al., 2014; PROCHASKA & HENJES- KUNST, 2009). Earlier workers, especially PETRASCHECK (1926, 1932) in his classic work on the metallogenetic zonation in the Eastern Alps, already mentioned a consanguineous origin of the alpine sparry magnesite mineralizations and the siderite deposits as proposed here. Also other early investigators (REDLICH, 1907, 1909; CLAR, 1956; FRIEDRICH, 1959, etc.) generally argued for hydrothermal fluids of different origin like magmatic or meta morphic fluids of alpine (Neogene) age. Nevertheless these concepts never gained general acceptance at that time. 3. GEOLOGIC POSITION AND STRUCTURAL FEATURES OF THE MAGNESITE AND SIDERITE MINERALIZATIONS The magnesite and siderite mineralizations occur in the Greywacke Zone, which is the Palaeozoic basement of the Mesozoic platform carbonates of the Upper Austroalpine nappe. Fundamental overviews of the geology and tec- tonostratigraphy of the Greywacke Zone were given by SCHÖNLAUB (1982), NEUBAUER (1994) and NEUBAU- ER et al. (1994). The rock series of the Greywacke Zone range from the Ordovician to the Carboniferous and com- 33 Prochaska Walter: Genetic concepts on the formation of the Austrian magnesite and siderite mineralizations in the Eastern Alps of Austria prise carbonates, metapelites, and metamorphosed acid vol- canics. The grade of metamorphism is generally lower green- schist facies both in the Variscan and in the eoalpine tectono- metamorphic event. The tectonically lower Veitsch nappe is affected only by the alpine metamorphism of greenschist fa- cies grade. Generally, magnesite mineralizations tend to form irregu- lar lenses and stocks whereas no veins are known. The side- rite deposits occur as stocks in the carbonate environment and as veins in different host-rock lithologies (see below). 3.1. MAGNESITE MINERALIZATIONS In the Eastern Greywacke Zone, carbonate-hosted sparry magnesite mineralization occurs mainly in Lower Carboni- ferous strata of the Veitsch nappe. A set of lens-shaped mag- nesite bodies of various sizes can be observed in a sequence of sericite schists, greywackes, conglomerates and metatuffs. Carbonate series hosting the magnesite bodies are of Upper Tournéan/Viséan age and characterise shallow water, marine environments. An exception is the Breitenau mine, which is located in a Palaeozoic series of deep-water environments of probably Silurian to Lower Devonian age. One of the most prominent examples of sparry magnesite deposits in the Eastern Alps is the abandoned Veitsch mine, which is the type locality of carbonate hosted sparry magnesite deposits. In the Western Greywacke Zone, magnesite can be seen in Palaeozoic strata of a remarkable age spread from Upper Silu- rian to Middle Devonian series. The most abundant host-rocks of magnesite mineralization in this area are dolomites of the Lower to Middle Devonian, but also black dolomite of Silurian age hosts magnesite occurrences (e.g. Entachen Alm). SIEGL (1969) reported magnesite components in the Permian basal breccia of the Alpidic orogenic cycle. The position of the highly metamorphosed Radenthein de- posit is unclear. Some authors (ANGEL et al., 1953) consider it to be a tectonic wedge of Upper Austroalpine Palaeozoic rocks thrust into the tectonically deeper Austroalpine Crystalline Units during the alpine tectonic event while others (TUFAR et al., 1989) regard it as an original member of the rock sequence. The most obvious and striking textural feature of these mine- ralizations is the coarse-grained, sparry structure of the magne- sites. These “pinolitic” structures are defined by coarse-grained magnesite crystals which occur in a dark/black fine-grained ma- trix. These structures are especially developed when the host- rocks are grey to black marbles. It is obvious that this structure is due to the hydrothermal growth of coarse-grained, white mag- nesite crystals, while the organic pigment is concentrated in the fine-grained matrix. The hydrothermal-epigenetic nature of the magnesite mine- ralizations is reflected by the fact that the orebodies are enve- loped by alteration zones of variable width. Hydrothermal, mottled dolomite, often exhibiting banding and zebra structures of several metres thickness, prevails in these zones. These reac- tion fronts can sometimes be reduced to a few cm as demon- strated in Fig. 3. The first signs of hydrothermal substitution forming sparry dolomite and magnesite on account of the host- rock calcite are small specs of pyrite formed in the host-rock in the immediate vicinity of the reaction front testifying to the re- ducing nature of the hydrothermal fluid (Fig. 4). Figure 2. Pinolitic sparry magnesite with late quartz-veins of Eoal- pine age from the Veitsch deposit. Figure 4. The reducing nature of the ore forming fluid causes a pyrite halo in the host-rock carbonate before the „Metasomatic front“ while pyrite is usually lacking in these host-rocks. Figure 3. „Metasomatic front“ between the fine-grained host-rock marble and the sparry magnesite. An intermediate reaction product between the magnesite and the host-rock is sparry dolomite. 34 Vol. 69/1 3.2. SIDERITE MINERALIZATIONS The siderite mineralizations of the Eastern Alps are concentrated in two different metallogenetic provinces, the Hüttenberg- Waldenstein Fe-ore province and the Erzberg-type mineraliza- tions of the Greywacke Zone (Fig. 1). This paper refers only to the latter Fe-ore province. Many abandoned deposits and the Erzberg mine belong to this metallogenetic province. The fact that these mineralizations do not occur in deeper tectonic units of the Greywacke Zone supports a timing of mineralization be- fore the stacking of the Austroalpine nappes. Another significant feature is that this type of mineralization shows indications of a continuation into deeper parts of the overlying Lower Triassic carbonates of the Northern Calcareous Alps of the Upper Aus- troalpine units, however, the thick units of platform carbonates of the Northern Calcareous Alps were not affected by the mine- ralizing events. The other set of siderite/haematite mineralizations is concen- trated in the amphibolite facies metamorphic complexes of the Austroalpine Crystalline units to the south. The most important mineralization of this group is the former siderite mine of „Hüt- tenberg“ which was abandoned in the 1970`s. This Tertiary side- rite province is of different origin to the Greywacke Zone depo- sits and is not covered further in this paper. The siderite occurrences in the Greywacke Zone are neither stratabound nor stratiform and exhibit different modes of occur- rence. Host-rock lithology strongly influences the structure of the mineralizations. Vein type mineralizations usually occur in competent host-rocks such as Ordovician quartz porphyries or Palaeozoic metapelites and sandstones, while more reactive host-rocks including the Devonian carbonates and Permoscy- thian carbonate conglomerates host metasomatic bodies, lenses and stocks of siderite. 3.2.1. Siderite veins in Ordovician quartz porphyries Underground exposures of this ore type are best exposed in the abandoned Schendleck siderite mine (Fig. 1.). The structure of this mineralization is that of a typical vein type deposit with siderite-quartz veins cross-cutting Ordovician quartz porphyries exhibiting prominent alteration zones with intense sericitization. These structures were previously interpreted (BAUMGART- NER, 1976) as tuff layers, and accordingly a synsedimentary formation was deducted. The abandoned Hirschwang, Alten- berg and Grillenberg siderite mines are in close vicinity of this mineralization, but hosted by Permoskythian siliciclastic series. In the area of the Erzberg siderite mine, which is generally hos- ted by Devonian limestones (see below) many smaller siderite veins are located in altered quartz porphyry host-rock. 3.2.2. Metasomatic siderite bodies in Devonian limestones and Upper Permian carbonate conglomerates The most prominent example of this type of siderite mineraliza- tion is the Erzberg siderite mine in the province of Styria. The siderite orebodies are generally hosted by fine-grained lime- stones of Devonian age. Metasomatic-epigenentic structures are dominant and usually coarse-grained siderite ore exhibits dis- cordant contacts with the unmineralized limestones. The only ore mineral is siderite, but frequently ankerite haloes around the siderite orebodies can be observed. Fig. 5 shows the metaso- matic front between the carbonatic breccia and the orebody cut- ting single components of the breccia, demonstrating clearly the metasomatic-hydrothermal nature of the mineralization. Fur- thermore, no single siderite components in the calcite breccia are known as should be the case with a pre-Permian siderite mineralization. Eoalpine tectonic structures and weak metamorphic over- prints are described from the Erzberg deposit. 3.2.3. Vein type mineralizations in Permoskythian siliciclastic host-rocks Different occurrences of this type of mineralization were inves- tigated within the course of this work (Gollrad, Grillenberg, Sohlenalm, Altenberg). The mineralizations are usually hosted by Permoskythian quartz-conglomerates (Präbichel series), and sandstones (Werfen formation) forming the base of the Upper Austroalpine Northern Calcareous Alps. The usual structure is that of siderite veins cross-cutting Permoskythian sandstones and conglomerates, but in some cases haematite is the dominant ore mineral. The former Gollrad siderite/haematite mine was re- cently reinvestigated for its economic potential of specular hae- matite. 4. CHEMICAL CHARACTERISTICS OF INCLUSION FLUIDS OF THE SIDERITE AND MAGNESITE MINERALIZATIONS Because of the conformable chemical characteristics of the mine ralizing fluids the results concerning the magnesite as well as the siderite mineralizations are presented and discussed toge- ther in this chapter. In order to characterize the nature of the ore-forming fluids the chemistry of the inclusion fluids of the siderite and magnesite deposits was investigated by mechanical extraction and subsequent analysis (POLGARI et al., 2010; PROCHASKA, 2012). The method applied here is based on the “crush-leach” extraction of the inclusion fluids and chemical analyses by ion chromatography of the most diagnostic compo- nents. The methodological approach used is modified after BOTTRELL et al. (1988) and is explained in detail in PRO- CHASKA (2000). The characterization of the total dissolved solutes chemistry of inclusion fluids was first and most widely applied to the study Figure 5. The metasomatic siderite front in the mineralized “Basis- brekzie” cuts the single carbonate components thus proving the hy- drothermal metasomatic nature of the mineralization. 35 Prochaska Walter: Genetic concepts on the formation of the Austrian magnesite and siderite mineralizations in the Eastern Alps of Austria of Mississippi Valley type Pb-Zn mineralization in order to ob- tain information on the sources of solutes in ore-forming brines and to explore genetic linkages among deposits (e.g. HALL & FRIEDMAN, 1963; VIETS & LEACH, 1990; BÖHLKE et al., 1992; KESLER et al., 1996). Despite the similar ionic radii of chlorine (1.81 Å) and bromine (1.96 Å) the fractionation of these elements in evaporitic processes is quite different. In an initial stage of evaporation Na, Cl, and Br are concentrated in a hypersaline environment, and their ratios do not change. At an evaporation index of > 10, halite is precipitated. Br has a very conservative behaviour and is not incorporated into the halite lattice. At an evaporation index of approx. 70, Mg-salts start to precipitate and Br is still concentrated in the residual brines. The cation systematics changed from an original Na preponderance to Mg and K dominated systems (McCAFFEREY et al., 1987). On the Cl/Br and Na/Br molar ratio diagrams for magnesite (Fig. 6) and siderite (Fig. 7) the evolution of the fluids by frac- tionation of halite from seawater is demonstrated. According to the fractionation behaviour of the Na-Cl-Br system explained earlier, the composition of an evaporating brine shifts along the “evaporation trend“ when halite starts to precipitate at an evapo- ration index of about 10. In contrast, fluids percolating through the crust acquiring salinity by dissolution of halite, plot towards higher Cl/Br and Na/Br molar ratios above the seawater compo- sition, and their composition is close to the „halite dissolution trend“. In the Cl/Br and Na/Br molar ratio diagrams for magnesite as well as siderite (Figs. 6, 7) the inclusion chemistry of the inves- tigated samples clearly plot on the seawater evaporation trend, thus indicating an origin of the fluids from subaerial evapo-con- centration of seawater. In general the magnesite fluids show a stronger fractionation and the composition of the fluids plot on the very end of the evaporation trend. The plots of the dolomites of the alteration zone exhibit a transitional position between the magnesite composition and that of seawater. Despite the general similarity and the overall evaporitic na- ture of the fluids, the siderite fluid composition shows in some cases an intermediate position between extreme fractionation and seawater composition. In general the Erzberg-fluids are highly fractionated and plot at the end of the evaporation trend (Fig. 7) while the projection points of smaller, vein-type systems (e.g. Altenberg, Sohlenalm…) are not as Br-rich as the big and voluminous Erzberg siderites. The reasons for these fluid cha- racteristics may be the result of the differences in the degree of the fluid-rock interaction where in the smaller systems the com- position of the fluid is altered while in the bigger systems the original fluid characteristics prevail. A prerequisite for the formation of hydrothermal-metasoma- tic deposits described here is a highly saline fluid capable of leaching Fe from the host-rocks while percolating through the crust. In the case of the Austroalpine sparry carbonate deposits the ore-forming fluids originally were bittern brines. However, the siderite deposits in Tunesia and Algeria/North Africa, inves- tigated by POHL et al. (1986) exhibit highly saline fluids, which were generated by partial dissolution of salt diapirs. According- ly these fluids are characterized by extremely high Cl/Br and Na/Br ratios (Prochaska unpublished) and plot on the halite dis- solution trend. 5. TIMING OF THE MINERALIZING EVENT Timing of the mineralization is one of the most important pieces of information needed to establish a genetic model within the geodynamic frame of the alpine or prealpine orogenic cycles, and therefore detailed investigations on radiometric dating were carried out during recent years. The fact that the mineralized structures and the mineralizations cut Permo-Mesozoic strata exclude a syngenetic Devonian formation for these deposits. However, the Eoalpine metamorphic event is considered by some authors to be responsible for modification and remobiliza- tion of early syngenetic mineralizations finally producing epi- gentic vein type deposits. In this study no indication of two fun- damentally different sets of fluid compositions (e.g. marine- sedimentary and alpine hydrothermal) in the ore minerals (side- rite and magnesite) were discovered. There are some localities where the mineralized Devonian limestones can be found in close proximity to totally unmineralized Carboniferous lime- stones of the tectonically deeper Veitsch nappe. In the case of an Figure 6. Na-Cl-Br ratios of the magnesite mineralizations of the Eastern Alps. The fluid composition clearly plots on the very end of the seawater evaporation trend proving the evaporitic origin of the fluids. Host-rock limestones are relatively close to seawater composi- tion as well as the fluid composition of the Eoalpine quartz-veins (see also Fig. 3). Figure 7. Na-Cl-Br ratios of the Erzberg type siderite mineralizations. The fluid composition plots on the seawater evaporation trend. 36 Vol. 69/1 extensive Cretaceous metamorphic mobilization, the minera- lized structures should also cut the tectonic units. However, all observations support the formation of the siderites before the alpine stacking of the nappes. Some attempts have been made to obtain radiometric ages from sericites from the alteration zones of a siderite mineraliza- tion in the quartz porphyries (PROCHASKA et al., 1996). A pla- teau age (Ar-Ar) of approximately 160 Ma for the Schendleck deposit was calculated. This age seems to be of some regional importance at the eastern margin of the Eastern Alps and is pro- bably not the age of the hydrothermal ore-forming event. It co- incides with an extensional phase in the Austroalpine area due to the opening of the South Penninic ocean. Furthermore, a promi- nent rejuvenation due to the Cretaceous metamorphic event (~90 Ma) can be observed from the Ar-Ar investigations. Con- sequently, from a geological point of view and from Ar-Ar data, the siderite-forming hydrothermal activity is not older than Per- moskythian and predates the Eoalpine tectono-metamorphic event. Essential progress on the age determination of the hydro- thermal event responsible for the formation of the siderite and magnesite deposits of the Eastern Alps was the direct age dat- ing of the carbonate minerals using the Sm-Nd radiometric system. During recent years radiometric information of some magnesite and siderite mineralizations were obtained and the formation ages cluster in the Middle to Upper Triassic (Figs. 8, 9). The method is explained comprehensively in HENJES- KUNST et al. (2014) where a detailed investigation and a sound model for the formation of the Breitenau magnesite deposit is presented. The Sm-Nd isochron age for this de- posit is 222.5 ± 9.8 Ma (Fig. 8), which coincides very well with the unpublished isochron age for the Wald/Schober magnesite deposit of the Greywacke Zone with an age of ap- prox. 220 Ma. PROCHASKA & HENJES-KUNST (2009) published an isochron age of 208 ± 22 Ma (Fig. 9) for the Erzberg siderite deposit, which is in the range of the above mentioned magnesite deposits. 6. GENETIC MODEL The genetic model presented here is based on the following observations: 1. The mineralizing event is post-Variscan and pre-Eoalpine because of the lack of siderite components in Upper Permian Variscan basal conglomerates and the termina- tion of hydrothermal features at alpine nappe boundaries. 2. In general the magnesite as well as the siderite minera- lizations are neither stratabound nor stratiform. 3. The high salinity of the fluids and especially the fractiona- ted, Br-rich composition indicate that the mineralizing fluids were originally oxidized evaporitic, bittern brines which, while percolating through the crust became modi- fied and reduced by water-rock reactions thus facilitating the capability to take up Fe by leaching the host-rocks. 4. The sharp transition in the fluid composition (evapo-con- centrations of seawater) between marine host-rock car- bonates and the mineralizations is not compatible with a simple marine-sedimentary model. 5. The structure of the mineralizations is hydrothermal- metasomatic. No indications of a synsedimentary concen- tration of Mg or Fe can be observed. 6. Radiometric dating provides proof for the formation of these mineralizations in the Upper Triassic. Only in one case so far (Hohentauern) has a Late Carboniferous to Early Permian age been reported by AZIM ZADEH et al. (2015). In Permian (to Lower Triassic) times evaporitic basins are ubiquitous in the Austroalpine realm. Deposition of thick se- ries of evaporites is widespread in the Permoscythian strata of the Upper Austroalpine unit. High degrees of evaporation (evaporation index 20 to 90) produced residual „bitterns“ with high salinities and high concentrations of Br, Mg, K, and SO4 in the fluids. The peculiar fluid composition of the siderite and magnesite mineralizations can only be achieved in the wide evaporitic areas during the Permo-Mesozoic of the Eastern Alps. Figure 8. Sm-Nd isochron of the Breitenau magnesite deposit (HENJES-KUNST et al., 2014). For explanation see text. Figure 9. Sm-Nd isochron of the siderites from the Erzberg deposit (PROCHASKA & HENJES-KUNST, 2009). For explanation see text. 37 Prochaska Walter: Genetic concepts on the formation of the Austrian magnesite and siderite mineralizations in the Eastern Alps of Austria For the earlier magnesites (e.g. Hohentauern), AZIM ZA- DEH et al. (2015) propose an intraformational circulation of Carboniferous evaporitic fluids causing metasomatic replace- ment of the host-rocks. However, extensive evaporitic sys- tems which could produce brines with the corresponding high evaporation index, are not known so far in the Carboni- ferous. For the siderite and magnesite deposits of Middle to Up- per Triassic age, two genetic models are plausible: High heat flow in the rift-environment induced hydrothermal convec- tion systems mobilizing the residual evaporitic brines. Litho- spheric extension and long-lasting thermal activity related to igneous and metamorphic activities are documented in the basement rocks of the Adriatic plate (THÖNI, 2002). Mineral assemblages are reported in rocks ranging from 285 to 225 Ma in age. Generally this process can be responsible for the formation of the siderite and magnesite mineralizations in the Upper Triassic. Another possibility for mobilizing the pal- aeobrines of Upper Permian to Lower Triassic age in the Up- per Triassic is the dewatering of the sedimentary basins by the increasing load caused by the sedimentation of the plat- form carbonates of several km in thickness, triggering the upward movement of the fluids. Of course these two mecha- nisms could have operated in concert. Initially these fluids are ± free of Fe and rich in Mg with the capacity to transform the carbonatic host-rocks into mag- nesite. When these fluids travel longer distances through the crust, diagenetic reactions and host-rock alterations changed these brines into acidic and reducing fluids with the capacity of leaching Fe from the country rocks, with the correspond- ing metasomatic product being siderite. This is compatible with the generally higher formation temperatures of siderite showing the formation of the Fe-carbonates in deeper levels where higher temperatures prevail. Vein-type siderite-haematite-sulfide mineralizations were formed in the metapelitic and metavolcanic host-rocks. With- in the Devonian platform carbonates metasomatic siderite bodies were formed. Metasomatism and mimetic crystalliza- tion of the marine host-rock carbonates often preserved pri- mary sedimentary textures very well, which led earlier re- searchers to postulate syngenetic models. However, the proposed hydrothermal-metasomatic model of Permoscythian evolved evaporitic brines being mobilized in the Upper Triassic can explain all the observed features of the siderite and magnesite mineralizations of the Greywacke Zone regardless of their stratigraphic and tectonic position. This model is consistent with features such as the overall high salinity of the fluids, the uniform and special fluid chemi stry and the published Sr- and stable isotope ratios which cannot be explained by either sedimentary scenarios or by alpine remobilization models. A phenomenon that cannot be explained satisfactorily so far is the observation that the magnesite deposits exclusively occur in the lower tectonic unit of the Greywacke Zone (Car- boniferous Veitsch nappe) while the siderite mineralizations are strictly bound to the higher unit (Paleozoic Noric nappe). A more detailed knowledge of the palaeogeography of the Upper Triassic is needed to elucidate this problem. REFERENCES ANGEL, F., AWERZGER, A., KUSCHINSKY, A. & MEIXNER, H. (1953): Die Magnesitlagerstätte Milstätter Alpe bei Radenthein.– Carinthia II, 143, 98–118. AZIM ZADEH, A.M., EBNER, F. & JIANG, S. 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