Polyhalite microfabrics in an Alpine evaporite mélange: Hallstatt, Eastern Alps.

In the Hallstatt salt mine (Austria), polyhalite rocks occur in 0.5-1 m thick and several metre long tectonic lenses within the protocataclasite to protomylonite matrix of the Alpine Haselgebirge Fm.. Thin section analysis of Hallstatt polyhalites reveals various fabric types similar to metamorphic rocks of crust-forming minerals, e.g. quartz and feldspar. Polyhalite microfabrics from Hallstatt include: (1) polyhalite mylonites, (2) metamorphic reaction fabrics, (3) vein-filling, fibrous polyhalite and (4) cavity-filling polyhalite. The polyhalite mylonites contain a wide range of shear fabrics commonly known in mylonitic quartzo-feldspathic shear zones within the ductile crust and developed from a more coarse-grained precursor rock. The mylonites are partly overprinted by recrystallised, statically grown polyhalite grains. Metamorphic reaction fabrics of polyhalite fibres between blödite (or astrakhanite) [Na2Mg(SO4)2.4H2O] and anhydrite have also been found. According to previous reports, blödite may occur primarily as nodules or intergrown with löweite. Reaction fabrics may have formed by exsolution, (re-)crystallisation, parallel growth or replacement. This fabric type was only found in one sample in relation with the decomposition of blödite at ca. 61 °C in the presence of halite or slightly above, testifying, therefore, a late stage prograde fabric significantly younger than the main polyhalite formation.

Introduction

The mineral polyhalite (chemical formula [K2Ca2Mg(SO4)4.2H2O]) commonly occurs in sedimentary evaporite successions and in rare cases as a sublimate in volcanic successions (e.g. Vesuvius, Campania, Italy) (Anthony et al., 1996). Polyhalite was first described from the salt mine Bad Ischl-Perneck in Austria (province Upper Austria) by the German chemist Stromeyer (1818) and has since been known from many salt deposits (e.g. Warren, 2006). In evaporites, polyhalite is commonly associated with halite, anhydrite, glauberite, carnallite, kieserite, langbeinite, vanthoffite, chlorides and sulphides. The crystal structure was first determined in samples from Altaussee in Austria (Schlatti et al., 1970) and later confirmed (Bindi, 2005). Polyhalite can be synthesised under laboratory conditions by a reaction of gypsum with appropriate solutions in the ternary system K2SO4–MgSO4–H2O at temperatures above 70 °C (Freyer and Voigt, 2003). At lower temperatures, polyhalite crystallisation slows down (Wollmann, 2010). Wollmann et al. (2008) found under laboratory conditions the dehydration temperature of polyhalite to start at 255 °C to a maximum of 343 °C. Substitution of divalent cations by other elements like Zn, Cu, Cu, Ni significantly changes dehydration reactions to higher or lower temperatures (Wollmann et al., 2008). Further chemical data on the conditions of formation and stability of polyhalite were presented inter alia by Krupp (2005) and Wollmann et al. (2009) and Wollmann (2010).

In nature, polyhalite most commonly forms early-diagenetically (e.g. Holser, 1966; Busson and Perthuisot, 1977; Pierre, 1985; Kühn and Müller-Schmitz, 1987) by brine-back reactions of K–Mg–SO4 brine with earlier formed gypsum, anhydrite or glauberite (e.g. Braitsch, 1962; Schauberger, 1986; Warren, 2006) after a prolonged contact (over months or years) of the brine with the precursor rock (Hardie, 1984). For example, all Alpine polyhalites are of secondary origin (Leitner, 2011; Leitner et al., in press b). A formation of polyhalite in marginal marine shallow water sedimentary environments is only suspected (Hryniv et al., 2007).

Polyhalite is stable between ∼room temperature (0–25 °C) and 255–343 °C (Wollmann et al., 2008) or 285 °C (Fischer et al., 1996) and, therefore, within nearly the whole temperature range, the Haselgebirge sulphates have experienced (∼200–300 °C, Table 1, see below).

Table 1

Temperature conditions of eo-Alpine metamorphism within the Moosegg area, central Northern Calcareous Alps (in part from Schorn and Neubauer, 2011, after Leitner, 2011). For full sources, see cited manuscript. CAI – conodont colour alteration index, FI – fluid inclusions, IC – illite crystallinity, VR – vitrinite reflectance, TISP – temperature-independent subgrain piezometer.

Location	Method	Temperature range (°C)	Temperature, best estimate (°C)	Author(s)
Northern Calcareous Alps, southern margin	FI	270–360	315	Götzinger and Grum (1992)
Sazkammergut, Juvavic nappes	CAI	80–>350	80	Gawlick et al. (1994)
Moosegg	FI	220–260	240	Spötl et al. (1998b)
Moosegg	FI	>300	300	Wiesheu (1997)
Moosegg	FI	300	300	Wiesheu (1997)
Lammer unit (Juvavic unit)	VR	max. 290	290	Rantitsch and Russegger (2005)
Altaussee salt mine	TISP	150 ± 20	150 ± 20	Leitner et al. (2011)
Ischl	FI	300	300	Wiesheu and Grundmann (1994)
Ischl	FI	>300	300	Wiesheu (1997)
Hallstatt	VR (+IC)	>160–180	170	Spötl and Hasenhüttl (1998)

Polyhalite contains potassium in stochiometric amounts rendering the mineral interesting for absolute age dating – essential for deciphering the geological history of a region. Since polyhalite was considered to be rare in most salt deposits, published ages are scarce and dating was carried out with other methods.

In Austria and southern Germany, polyhalite is known from the Upper Permian–Lower Triassic Haselgebirge Fm. (in the following named as Haselgebirge mélange), a tectonically mixed evaporite succession at the stratigraphic base of the Northern Calcareous Alps (in the following abbreviated as NCA) (Fig. 1). There, polyhalite is a minor, but widespread and important constituent and is accessible in several salt mines including the type locality of Bad Ischl, Hallstatt, Berchtesgaden–Dürrnberg and Hall in Tirol (Schauberger, 1986). At the localities Berchtesgaden and Altaussee, polyhalite mainly occurs intergrown with anhydrite or as vein fillings and exhibits a wide range of fabrics (e.g. Leitner et al., in press a,b), for example intergrown with fine-grained anhydrite, in veins, in nodules, and recrystallised in foliated polyhalite rocks along shear zones.

Fig. 1

(a) Overview of Austroalpine units in the central Northern Calcareous Alps (NCA) (modified from Leitner et al., 2011). (b) Inset shows distribution of NCA in the frame of Austria.

Polyhalite microfabric/microstructures were only subject of a few works. Görgey (1910) distinguished three different types of polyhalite occurrences: (1) fine-fibrous, acicular or lamellar, yellowish-red to reddish polyhalite layers, which form 1–20 cm thick layers within the Haselgebirge, often with an extension of several square metres; (2) compact fine-to-medium-grained yellowish-brown to dark red coloured polyhalite layers associated with anhydrite and (3) fibrous-acicular or granular polyhalite areas, which form seams around anhydrite crystals (like in Hall in Tirol). Schindl-Neumayer (1984) examined the grain structures of anhydrites, polyhalites and halites of the alpine salt deposits. She distinguished five different polyhalite types: (1) fine-felty polyhalite aggregates, (2) fine-grained polyhalite groundmasses, (3) polyhalite twinnings in a polyhalite groundmass and in anhydrite porphyroblasts, (4) polyhalites as felty conversion seams round anhydrite and (5) polyhalite and clay/mudrock. The most recent work on polyhalite fabrics from Altaussee and Berchtesgaden–Dürrnberg was published by Leitner et al., in press a, Leitner et al., in press b and Leitner (2011). Leitner et al. (in press b) distinguished three main polyhalite microfabric types: (1) Fibrous polyhalite in veins, (2) porphyroblasts of polyhalite in anhydrite with subdivisions (2A) (round polyhalite blasts) and (2B) (fibrous polyhalite blasts) and (3) dense polyhalite comprising corroded large anhydrite grains with subdivisions (3A) (fine-grained, mylonitic polyhalite rock), (3B) (non-foliated polyhalite rock with local zones) and (3C) (non-foliated polyhalite rock).

Leitner et al. (in press a) described a diagenetic reaction after halite hopper crystals, which forms anhydrite and polyhalite and added another polyhalite microfabric type: polyhalite in pseudomorphic anhydrite cubes after halite hopper crystals and polyhalite from within the retaining shape of deformed halite hopper shaped cubes.

Leitner et al. (in press b) demonstrated, on a limited number of examples, that polyhalite occurs in polyhalite rocks and as vein filling with various fabric types, which also gave distinct 40Ar/39Ar ages. One of the most important results of these papers is that polyhalites of the alpine Haselgebirge formed at ca. 235–210 Ma (Leitner et al., in press b), ca. 20–25 Ma after the sedimentation of the Haselgebirge evaporites (Leitner, 2011) and with a maximum estimated overburden of approximately 800 m (e.g. Rantitsch and Russegger, 2005).

With this study, we further explore the variety of microfabric types of mono- and polymineralic polyhalite rocks and vein and cavity fills from the Hallstatt mine as we found that the variety is significantly larger than elsewhere in the NCA. Then we explain the microfabric types in terms of the geological history.

Geological setting of NCA and of Hallstatt region

Overview

The Hallstatt salt mine is within the Upper Permian–Lower Triassic Haselgebirge Fm.. The classic division within the NCA defines the Bajuvaric, Tirolic and Juvavic nappe complexes (Tollmann, 1985, 1987 and references therein; Mandl, 2000) (Fig. 1). The Permian to Lower Triassic Haselgebirge occurs mainly in Juvavic units of the central and eastern NCA (Schauberger, 1986; Leitner and Neubauer, 2011 and references therein) and subordinately in Tirolic units. The age of formations in the NCA ranges from Late Carboniferous (?) or Early Permian to Eocene. During the Middle and Late Triassic, the Upper Juvavic nappes comprise mostly reefs and deposits next to reefs, the Tirolic and Bajuvaric nappes comprise lagoonal facies types and subordinate reefs. The Lower Juvavic nappe unit represents solely an outer shelf or a deep-sea facies type (Mandl, 2000). Rocksalt deposits are mostly found in the Lower Juvavic unit. Sedimentation of the Lower Juvavic unit occurred in basins with basinal limestones (Pötschen Limestone) and on intrabasinal ridges, with reduced sedimentary thickness (the pelagic Hallstatt Limestone). The ridges were suggested to relate to salt diapirism in Triassic times (Mandl, 1982, 2000; Plöchinger, 1984).

The westernmost part of the expanding Triassic Tethys ocean is called Hallstatt-Meliata Ocean, which comprises rare deep-sea (ophiolitic?) sequences in the eastern parts of the NCA (Faupl and Wagreich, 2000; Neubauer et al., 2000 and references therein). Most authors interpret the Hallstatt Limestone as an outer shelf (Tollmann, 1985, 1987; Mandl, 2000). Others propose a position of the Hallstatt-Meliata relics between the Upper Juvavic and Tirolic units (Schweigl and Neubauer, 1997). The Permian/Middle Triassic to lower Upper Triassic rift stage and passive margin formation was associated with widespread synsedimentary and diagenetic Pb–Zn mineralisation (Ebner et al., 2000). The Hallstatt-Meliata Ocean was being closed during the Late Jurassic (Dallmeyer et al., 2008 and references therein). Coevally, the sea floor dropped and reached maximum water depths with the formation of radiolarites. Gravitational sliding is reported from different places (e.g. Mandl, 1982; Plöchinger, 1990), and this concept was developed until recent years (Missoni and Gawlick, 2011 and references therein). These authors argue for a Late Jurassic age of shortening.

During Early Cretaceous, nappe stacking of Austroalpine units started due to the subduction of Austroalpine continental crust. Thrusting prograded from south to north, respectively from ESE to WNW (Ratschbacher, 1986; Linzer et al., 1995; Mandl, 2000; Neubauer et al., 2000).

The mechanism and the time of emplacement of the Juvavic units are still a matter of controversy. The classic hypothesis assumes that both Juvavic nappes took their position during the eo-Alpine deformational event (late Early to early Late Cretaceous) by means of thrust tectonics (Kober, 1955; Pichler, 1963; Schweigl and Neubauer, 1997; Mandl, 2000). A further model explains the emplacement of all Juvavic units by gravity sliding since Late Jurassic times, as Haselgebirge clasts of various sizes have been found in the Upper Jurassic Oberalm and Lower Cretaceous Rossfeld Fms. (Missoni and Gawlick, 2011 and references therein). This concept was extended to large mountain-like blocks also explained by emplacement by simple gravity sliding (Gawlick and Lein, 2000; Missoni and Gawlick, 2011 and references therein). Frank and Schlager (2006) propose the emplacement as a consequence of Late Jurassic strike-slip movements related to the opening of the Penninic Ocean. Schorn and Neubauer (2011) proposed an Early Cretaceous emplacement of the Haselgebirge-bearing nappe over Tirolic units.

In the Eocene, the second paroxysm of the Alpine orogeny occurred, when continental basement slices (“Middle Penninic”) and parts of the North Penninic Ocean (“Rhenodanubian Flysch”) were subducted below the NCA at the leading edge of the Austroalpine–Adriatic microcontinent (Faupl and Wagreich, 2000; Linzer et al., 2002 and references therein). The present NCA were partly detached from their Austroalpine basement (e.g. Greywacke zone) and thrusted over the Rhenodanubian Flysch and Helvetic domain resulting in a wide thin-skinned tectonic nappe complex (Linzer et al., 1995; Mandl, 2000; Neubauer et al., 2000). The familiar rocksalt deposits, which are all located in the interior of the NCA mostly within the Lower Juvavic unit, were considered to be only slightly affected by these Cenozoic deformation stages, since the detachment of the NCA domain occurred beneath the lowermost unit, the Bajuvaric nappe (Fig. 1). Deformation of Upper Cretaceous to Eocene Gosau basins deposited on uppermost nappes (Tirolic and Juvavic nappes) suggests significant deformation in Late Eocene to Early Miocene (Linzer et al., 1995, 1997; Peresson and Decker, 1997a,b).

Geology of the Hallstatt region

The salt body of Hallstatt (Figs. 2 and 3), which is part of the Hallstatt nappe north of the Echern valley (i.a. Habermüller, 2005), was described recently by Leitner and Neubauer (2011) and we follow their description. The Haselgebirge body extends ca. 3 km in E–W direction and is around 600 m wide (maps and sections: Schauberger, 1955; Schäffer, 1982; Scheidleder et al., 2001; Habermüller, 2005 and references therein). The highest point is 1350 m above sea level, the lowest Erbstollen level is at around 500 m above sea level (section of Schauberger in Habermüller, 2005). The salt body suitable for mining diminishes with depth, mostly because of incorporated country rocks. The base has not yet been reached at 100 m below the level of lake Hallstatt (pers. comm. Gerald Daxner, Salinen Austria AG). The halite content of the Haselgebirge Fm. is around 55 wt (weight) % (Schauberger, 1931, 1949, 1955). The mining started in Celtic times, as indicated by a Bronze-age staircase found in the mine, which was dated accurately by dendrochronology to 1344 BC (Grabner et al., 2007). The sedimentary succession of the Lower Juvavic unit is nearly complete and comprises, beside the basal Upper Permian–Lower Triassic Haselgebirge Fm. Lower Triassic to Liassic formations. Beside evaporites, the Haselgebirge Fm. contains blocks of meta-basalts (melaphyre) (e.g. Schauberger, 1960b; Zirkl, 1957 and references therein) within the “Buntes Salztongebirge” (=colored salt-bearing claystone) (Pak and Schauberger, 1981; Schauberger, 1986) which are also assumed to be of Upper Permian age (e.g. Klaus and Pak, 1974), but no reliable dating was performed.

Fig. 2

Schematic N–S-trending cross-section through the Hallstatt salt mine (after Mandl, 1999, modified from Schauberger, 1955). Levels of Hallstatt salt mine: E – Erbstollen, F – Kaiser Franz Josef, B – Backhaus, El – Kaiserin Elisabeth, M-T – Kaiserin Maria Theresia, Cr – Kaiserin Christina, J – Kaiser Josef, M – Kaiser Maximilian, K – Kaiserin Katharina Theresia, T – Tollinger Stollen.

Fig. 3

Simplified ESE–WNW-trending profile through the Hallstatt salt mine after Kern et al. (2008), modified after Schauberger (1960a). The ellipses mark the approximate sample locations.

Based on palynological evidence and a sulphur isotopic signature, the sediments of the Haselgebirge were deposited during Late Permian and Early Triassic times (Klaus, 1955, 1965; Klaus and Pak, 1974; Pak, 1974, 1978, 1981; Pak and Schauberger, 1981; Götzinger and Pak, 1983; Spötl, 1987, 1988a, 1989; Spötl and Pak, 1996). Spötl (1988a) carried out detailed studies on relict sedimentary structures in the Dürrnberg and Hallstatt mines, and interpreted the evaporites to have formed in a sabkha-like environment, situated in an aborted rift of the Tethys ocean (Spötl, 1989). Pak and Schauberger (1981) postulated, based on S-isotopy, that the rock types “Rotsalzgebirge” (=red salt rock) and “Grüntongebirge” (=green salt-bearing clay) (following the nomenclature of Schauberger, 1986) of an older phase (Schauberger, 1986) were deposited during Late Permian (probably until Earliest Triassic times), while the “Stinkdolomitisch/anhydritische Grausalzgebirge” (bituminous dolomitic/anhydritic grey salt rock) and probably also the “Bunte Salztongebirge” (=colored salt-bearing claystone) of a younger phase (Schauberger, 1986) were sedimented during the Early Triassic and earliest Anisian times. For Hallstatt, Pak and Schauberger (1981) and Pak (1981) assumed a continuous saliniferous sedimentation from the Upper Permian to the Lower Triassic/Anisian boundary.

Permian clastic sediments (Spötl, 1987) and rocks of the Hallstatt Fm. (Krystyn, 2008) are often associated with Jurassic rocks (Sandlingalm Fm.). Blocks composed of the Hallstatt Fm. are in contact with rocks of the Sandlingalm Fm., which, together with the Haselgebirge Fm., are covered by an undeformed lid of Upper Jurassic rocks (Gawlick and Schlagintweit, 2006; Suzuki and Gawlick, 2009). A sedimentary contact between the Sandlingalm Fm. and the overlying Plassenkalk Fm. is missing (Suzuki and Gawlick, 2009). On a map view of Hallstatt, the Lower Juvavic unit, the rocksalt and the Jurassic cover are found within the area of the Upper Juvavic nappe (Fig. 2).

Based on vitrinite reflectance studies, maximum temperatures for the central sectors of the NCA in the surroundings of the Hallstatt mine are estimated between 160 and 180 °C, and with 200–300 °C in the surroundings (see Table 1, Götzinger and Grum, 1992; Gawlick et al., 1994; Wiesheu and Grundmann, 1994; Wiesheu, 1997; Spötl and Hasenhüttl, 1998 and references therein).

Material and methods

According to the nomenclature of evaporite types within the Alpine Haselgebirge Fm. proposed by Schauberger (1986), polyhalite is mainly part of the “Rotsalzgebirge“ (“red salt rock”), which is composed of a mixture of reddish-grey salt, anhydrite, polyhalite, glauberite, Na–Mg sulphates (like blödite/simonyit), red and black claystones and grey-brown sandstones. There, polyhalite (and blödite [Na2Mg(SO4)2.4H2O]) are very common but in low proportions (Schauberger, 1986). But polyhalite is also reported from the rock types “Buntes Salztongebirge” (“colored salt-bearing clay”), “Grausalzgebirge” (“grey salt-bearing claystone”) and “Rot-grünes Übergangsgebirge” (“reddish-green salt rock”) (nomenclature after Schauberger, 1986). Most of the described 27 samples were taken from the lower and middle levels of the Hallstatt salt mine: Kaiser Franz Josef, Nusko, Kaiserin Elisabeth, Kaiserin Maria Theresia and Kaiserin Christina. For an overview of the sample locations see Supplementary Material, Table SM 1 and Fig. 2, Fig. 3, Fig. 4.

Fig. 4

Representative photographs showing underground exposures of polyhalite rocks: (a) banded anhydrite mylonite, embedding a red boudinaged polyhalite layer. Location of sample HT-6, Eichholzschurf, level Kaiserin Maria Theresia. (b) Polyhalite pressure solution mylonite with clay layers. Location of sample HT-11, Enderwerk, level Kaiserin Christina. (c) Polyhalite ultramylonite, embedding large single anhydrite crystals, fractured magnesite breccia and dark clay layers. Location of sample HT-16, Hörnerwerk, level Kaiserin Christina. The letters A, B and C refer to the different microfabric types of the samples as described in the text.

Electron Microprobe Analysis EMPA on polyhalite, anhydrite, bassanite and blödite was carried out on a JEOL electron microprobe (JXA-8600) at the Department Geography and Geology, University of Salzburg, using a wavelength dispersive system. Because sulphates are unstable under the electron beam, we used an acceleration voltage of 15 kV and a low sample current of 20 nA. Natural and synthetic mineral standards were used to calibrate the microprobe and raw data was reduced using standard ZAF correction. The detection limits (3σ) for the elements Na, K, Mg, Mn, Ca, Fe and S are 0.025 wt (weight) % and 0.045 wt% for Sr and Ba.

After being covered with carbon the thin section blocks of the three samples HT-8B, HT-10 and HT-16 were examined by scanning electron microscopy (SEM) and qualitative Energy Dispersive X-ray Analysis (EDX) with the Leica Stereoscan 430 equipped with Röntec-Edwin 98 WinShell/WinTools at the Department Geography and Geology, University of Salzburg.

The X-ray diffraction analysis was carried out with the automatic diffractometer Siemens D500 using a semi-automatic divergence slit and a graphite-based secondary monochromator using Cu Kα-radiation (40 kV, 35 mA). The measurements were performed in the step-scan mode in an angular range of 3–75° 2Θ, step length 0.03° with a measurement time of 12 s per step. The program EVA 3.0 (Bruker AXS) and the pdf-2-database (International Centre for Diffraction Data) were used for evaluation of the X-ray diffraction results.

Polyhalite microfabrics

In the Hallstatt mine, polyhalite rocks occur in tectonic lenses within the protocataclasite to protomylonite matrix. The matrix comprises ductilely deformed halite and cataclastic clay and mudstone. Lenses are ca. 0.5–1 m wide and can been followed to over tens of metres. Ca. 27 samples have been collected from various levels and boreholes.

Thin section analysis of Hallstatt samples reveals many different fabric types (Table 2, Supplementary Material, Table SM 1). Very often, one rock sample exposes more than one polyhalite microfabric type. We observed in Hallstatt all types described by Leitner et al. (in press b), although their polyhalite rock type 2A (porphyroblasts of polyhalite in anhydrite, round blasts) only occurs subordinately. In the following, we distinguish between mono- and polymineralic polyhalite rocks, and furthermore, vein and cavity-filling polyhalite (Table 2, Supplementary Material, Table SM 1).

Table 2

Overview of all polyhalite microfabric types and in which thin sections they were observed.

Nr.	Microfabric type	Characterisation of the microfabric type	Observed in thin the following sections
A	Monomineralic polyhalite rocks
A-1	Momomineralic mylonites	Elongated polyhalite grains, shape preferred orientation (SPO), comparatively small grain sizes (varying from ultramylonitic- to medium-grained)	HT-1, HT-7, HT-8, HT-8B, HT-10, HT-12, HT-16 HT-30, HST-16, HST-17B, HT-35
A-2	Crenulation cleavage and folds	Microfolding of an existing foliation, refolded foliation, crenulation cleavage; two-phase fabric	HT-7, HT-8B, HT-11, HT-16, HT-17, HST-16, HST-17B, HST-18, HT-33
A-3	Recrystallised polyhalites	Fine-grained polyhalite aggregates between coarse-grained minerals with undulose extinction and subgrains, large recrystallised polyhalite grains or fibres with irregular or amoeboid grain boundaries	HT-7, HT-17, HT-18, HT-27(1) and 27(2), HST-16, HST-17A, HST-17B, HST-18, HST-21, HT-32, HT-33, HT-35, HT-36, HT-37
A-4	Pressure solution	Pressure solution, grain boundary migration (GBM); two-phase fabrics	HT-8, HT-11, HT-16, HT-18, HT-27(1) and 27(2), HT-30, HST-16, HST-17A, HST-18, HT-32, HT-35, HT-36, HT-37
A-5	Cataclastic fabrics	Equidimensional, angular, fine- and coarse-grained polyhalite grains with serrated grain boundaries, showing great grain size variations and areas with grain size reduction	HT-6
B	Polymineralic polyhalite-rich rocks	Large grain size variations	All thin sections
B-1	Polymineralic mylonites	Typically mylonitic fabric of polyhalite layers, which are interbedding with clay, carbonate or anhydrite layers; elongated grains, SPO	HT-1, HT-8B, HT-30
B-2	Metamorphic reaction fabrics	A seam of fine-grained polyhalite fibres is growing as a probable exsolution/decomposition reaction product between coarse blödite and coarse-grained anyhdrite crystals, control of the orientation of one mineral by another	HT-12
C	Polyhalite cavity and vein fillings
C-1	Cavity growth fabrics	Polyhalite or anhydrite, subordinate clay and carbonate minerals are growing into cavities, which are mainly filled with salt (halite)	HT-2, HT-3, HT-6, HT-7, HT-8, HT-8B, HT-10, HT-11, HT-14, HT-16, HT-17, HT-18, HT-27(2), HST-18, HST-21, HT-33, HT-35, HT-36
C-2	Vein-filling polyhalite	Large, vein-filling polyhalite fibres, which are growing about perpendicular to the vein walls	HST-21

Monomineralic polyhalite rocks

Monomineralic polyhalite mylonite (microfabric type A-1)

The thin sections of monomineralic polyhalite mylonites (Fig. 5, Fig. 6a and b) show the following main features and are often characterised by large grain size variations (from 0.05 to 0.5 mm long): (1) fine-grained (0.05 × 0.1 mm on average), well-recrystallised elongated (long axis parallel to the foliation) polyhalite grains with a shape preferred orientation (SPO); (2) fibrous polyhalite pressure fringes (0.05 × 0.5 mm on average) around large, rigid subeuhedral opaque mineral grains (0.5 × 0.5 mm up to maximally 20 × 20 mm) within a fine-grained mylonitic fabric (average grain sizes of 0.02 × 0.1 mm) (Fig. 6a) and (3) fine-grained (0.03 × 0.1 mm on average) grain boundary migration (GBM) with amoeboid grain boundaries (Fig. 6b). The polyhalite grains in mylonite exhibit only a low crystallographic preferred orientation.

Fig. 5

(a) Polished hand specimen HT-30, a fine-grained, dark red polyhalite mylonite with many dark clay layers/schlieren; dominant microfabric types: A-1, B-1. (b) Hand specimen HT-17, comprising a fine-grained medium to dark red polyhalite matrix (po) which embeds medium to dark brownish grey, prismatic large anhydrite (single crystals) (an) and dark grey clay or anhydrite layers, dominant microfabric types: A-2, A-3. (c) Polished hand specimen HT-11, an extremely fine-grained “polyhalite pressure solution mylonite”, which is pervaded by dark anhydrite or clay layers/schlieren; dominant microfabric type: A-4. (d) Hand specimen HT-6, a subhorizontal bedded “anhydrite mylonite rock” with a 0.5–0.7 cm thick medium red boudinaged polyhalite (anhydrite and halite) layer; HT-6 is the only sample which represents microfabric type A-5. (e) Hand specimen HT-12, a mixture/mélange of fine-grained, dark red polyhalite with large anhydrite (single crystals) and large, yellow blödite crystals. The yellow areas represent blödite (bl), which are embedding fine-grained red polyhalite (po) and large anhydrite (single crystals) (an); dominant microfabric type: B-2. (f) Hand specimen HT-2, a medium-grained, fibrous polyhalite with a light orange coloured, more weathered rim and a dark red core (small picture), dominant microfabric type C-1.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Thin section microphotographs: (a–b) monomineralic polyhalite mylonites. (a) Fibrous polyhalite pressure fringes (0.05 × 0.5 mm on average) around large, rigid subeuhedral opaque mineral grains (0.5 × 0.5 mm up to maximally 20 × 20 mm) within a fine-grained mylonitic fabric (average grain sizes of 0.02 × 0.1 mm) of sample HT-7. Crossed polarisers. (b) Fine-grained (0.03 × 0.1 mm on average) mylonitic fabric of polyhalite grains, showing shaped preferred orientation (SPO) forming a foliation and grain boundary migration (GBM) with amoeboid grain boundaries of sample HT-8. Crossed polarisers. (c) Folded foliation of fine-to-medium-grained (on average 0.1 × 0.3 mm long) polyhalite with a crenulation cleavage of sample HT-8B. Crossed polarisers. (d) Microfold folding the previously formed mylonitic foliation composed of fine-grained elongated polyhalite grains (average grain sizes of 0.02 × 0.1 mm) of sample HT-7. Crossed polarisers.

Crenulation cleavage and folds (microfabric type A-2)

Crenulation cleavage usually shows a two-phase fabric indicating two different deformation stages and these structures can be found in fine-grained polyhalite rocks (Fig. 5b and c) from Hallstatt. Some thin sections (Figs. 6c and d and 7a) of mono- und polymineralic polyhalite rocks show a secondary overprint by a second deformation stage. Such features include: (1) a refolded foliation in a fine-to-medium-grained (ca. 0.1 × 0.3 mm long) polyhalite rock with a crenulation cleavage; (2) microfolds folding the foliation S1 of polyhalite grains (Fig. 6c) and (3) a crenulation cleavage and partly folded clay-rich layers, coarse-grained (0.1 × 0.2 mm long on average) polygonal anhydrite fabric and fine-grained (0.02 × 0.07 mm long on average) polyhalite (Fig. 6d).

Fig. 7

Thin section microphotographs: (a) fine-grained (0.02 × 0.1 mm long on average), fibrous polyhalite, partly showing crenulation cleavage (=microfabric type A-2), overgrowing coarse-grained (0.5 × 1.5–4 mm long on average) polyhalite crystals of microfabric type A-3, sample HT-17. Crossed polarisers. (b) Coarse-grained, recrystallised polyhalite aggregates (0.7 × 0.9 mm up to 1 × 2.5 mm long; single fibres on average 0.2 × 0.6 mm long), and on the upper side coarse-grained (0.1 × 0.2 mm on average) anhydrite of sample HT-18. Crossed polarisers. (c) Well-recrystallised polyhalite grains (0.5 × 0.1 mm long on average) with shape preferred orientation (=SPO) of sample HT-18. Crossed polarisers. (d) Pressure solution foliation of very fine-grained (0.01 × 0.05 mm on average) polyhalite grains surrounding coarse-grained (0.1 × 0.1/0.2 mm on average) polyhalites (showing grain boundary migration (GBM) along boundaries of the porphyroclasts). Note also the folded clay-rich layer of microfabric type B. Sample HT-30. Crossed polarisers. (e) Pressure solution of extremely fine-grained polyhalites (0.001 × 0.1 mm – ultramylonite) and more coarse-grained polyhalite grains (0.05 × 0.1/0.2 long on average) of sample HT-16; the layer in the center represents a mylonite (microfabric type A-1) with a “polyhalite fish” (0.05 × 0.3 mm) showing a dextral shear sense in the thin section; as a whole microfabric type B. Crossed polarisers. (f) Extremely fine-grained (0.01 × 0.1 mm on average) polyhalites around a coarse-grained polyhalite twin (0.2 × 0.4 mm long); sample HT-16. Crossed polarisers.

Recrystallised polyhalites (microfabric type A-3)

A number of polyhalite rock samples (Fig. 5b) from Hallstatt show various effects of recrystallisation, respectively annealing (Fig. 7a–c): Fine-grained (0.02 × 0.1 mm long on average), fibrous polyhalite grains partly show crenulation cleavage (= microfabric type A-2) and overgrow coarse-grained (0.5 × 1.5–4 mm long on average) polyhalite crystals typical for microfabric type A-3 (Fig. 7a). Coarse-grained, recrystallised polyhalite aggregates (0.7 × 0.9 mm up to 1 × 2.5 mm long; single fibres on average 0.2 × 0.6 mm long) are in contact with coarse-grained anhydrite (0.1 × 0.2 mm on average) (Fig. 7b) and well-recrystallised polyhalite grains still preserve a shape preferred orientation (SPO) (Fig. 7c).

Pressure solution (microfabric type A-4)

In polyhalite rocks from Hallstatt (Fig. 5c) a number of effects related to pressure solution can be observed (Fig. 7d–f): Folded dark clay-rich layers and very fine-grained polyhalite grains (0.02 × 0.05 mm on average), showing pressure solution and grain boundary migration (=GBM). Pressure solution foliation of very fine-grained (0.01 × 0.05 mm on average) polyhalite grains surrounding coarse-grained (0.1 × 0.1/0.2 mm on average) polyhalite crystals (Fig. 7d). Pressure solution of extremely fine-grained (0.001 × 0.1 mm representing an ultramylonite) polyhalites and more coarse-grained polyhalite grains (0.05 × 0.1/0.2 mm long on average) of sample HT-16; the layer in the center represents a mylonite (microfabric type A-1) with a “polyhalite fish” (0.05 × 0.3 mm) (Fig. 7e). In other cases, pressure solution of extremely fine-grained (0.01 × 0.1 mm on average) polyhalites around a coarse-grained polyhalite twin (0.2 × 0.4 mm long) is observed (Fig. 7f).

Cataclastic fabrics (microfabric type A-5)

Cataclastic fabrics of angular fine and coarse (0.005–0.08 mm) polyhalite grains with serrated grain boundaries and many grain size classes (Fig. 8a) of microfabric type A-5 are only observed in sample HT-6 (Fig. 5d).

Fig. 8

Thin section microphotographs: (a) in the lower part of the microphotograph of sample HT-6, a cataclastic fabric of fine- and coarse-grained (0.005–0.05 mm) polyhalites with serrated grain boundaries and many grain size classes is visible. In the upper part of the picture, clay-rich layers occur; entire section: microfabric type B. Crossed polarisers. (b) The microphotograph shows polyhalite with shaped preferred orientation (SPO) overgrowing an older, large anhydrite porphyroblast (0.4 × 0.6 mm). The sieve-microfabric type polyhalite inclusions within the anhydrite grain show no shape preferred orientation (SPO) and crystallographic preferred orientation (CPO). This indicates that the fabric, which has formed prior to growth of anhydrite porphyroblasts, did not show an SPO and CPO either; sample HT-8. Crossed polarisers. (c) Coarse-grained (0.1 × 0.4 mm on average) anhydrite layers showing grain boundary migration (GBM) within a halite-filled cavity, embedded in fine-grained (0.003 mm thick and up to 0.2 mm long) mylonitic polyhalite layers of microfabric type A-1 (whole sample: microfabric type B-1); sample HT-8. Crossed polarisers. (d) Fine- (0.1 × 0.15/0.3 mm on average) and coarse-grained (0.2 × 0.5 mm on average) polyhalite layers showing a foliation and amoeboid grain boundaries as well as a pressure solution, probably aligned to microfabric type B-1 (polymineralic mylonite); sample HT-8B. Crossed polarisers. (e) Fine-grained (0.01 × 0.05 mm on average) mylonitic polyhalite layers of sample HT-30 with shape preferred orientation (SPO) (microfabric type A-1) are embedding clay layers (in the center); entire section: microfabric type B-1. Crossed polarisers. (f) Fine-grained (0.05 × 0.1/0.2 mm on average), fibrous polyhalite (po) is probably growing in 0.5 × 1.2 mm large areas as a decomposition reaction product between very coarse-grained blödite (bl) and a large (2 × 2.5 mm) anhydrite (single crystal) (an) indicating static P–T-conditions. Blödite constitutes large areas (17 × 20 mm), with single grains measuring between 1 × 1 mm and up to 4 × 8 mm. xx in the anhydrite crystal mark about 0.15–0.3 mm thick cracks filled with an unknown mineral; sample HT-12. Crossed polarisers.

Polymineralic polyhalite-rich rocks (microfabric type B)

Generally, the presence of other mineral phases inhibits the mineral growths, resulting in large grain size variations (if no phases are removed). Some samples from Hallstatt show fine-grained polyhalite (0.003/0.004 mm thick and up to 0.2 mm long) with shape preferred orientation (SPO) overgrowing older, large anhydrite porphyroblasts (Fig. 8b). The anhydrite porphyroblasts (0.4 × 0.6 mm) show a sieve structure with polyhalite inclusions (with no shaped preferred orientation) and crystallographic preferred orientation (CPO) within the anhydrite grain. This indicates that the fabric formed prior to the growth of anhydrite porphyroblasts did not show an SPO or CPO (pre-tectonic porphyroblast) (Fig. 8b). Folded clay-rich layers and an extremely fine-grained (0.001 × 0.01 mm) polyhalite-rich matrix are observed in some polyhalite rocks.

Polymineralic mylonites (microfabric type B-1)

Fabric type B-1 shows a typically mylonitic fabric of polyhalite layers interbedded with clay, carbonate or anhydrite layers. Thin sections (Fig. 8c–e) of polymineralic mylonites show a number of distinct features, which include inter alia coarse-grained (0.1 × 0.4 mm on average) anhydrite layers with grain boundary migration (GBM), which are embedded in fine-grained (0.003 mm thick and up to 0.2 mm long) mylonitic polyhalite layers of microfabric type A-1 (whole sample: microfabric type B-1) (Fig. 8c). Fine- (0.1 × 0.15/0.3 mm on average) and coarse-grained (0.2 × 0.5 mm on average) polyhalite layers of another sample show a foliation and amoeboid grain boundaries as well as a pressure solution, probably aligned to microfabric type B-1 (polymineralic mylonite) (Fig. 8d). In sample HT-30 clay-rich layers are embedded in a fine-grained (0.01 × 0.05 mm on average) mylonitic polyhalite fabric with shape preferred orientation (SPO), together constituting microfabric type B-1 (Fig. 8e).

Metamorphic reaction fabrics (microfabric type B-2)

Reaction fabrics are commonly characterised by the control of the orientation of one mineral by another. They might have been formed by exsolution, (re-)crystallisation, parallel growth or replacement (Lauder, 1961). This fabric type was only found in sample HT-12 (Figs. 5e and 8f) in relation with blödite. The yellow-coloured mineral aggregates are composed of blödite [Na2Mg(SO4)2.4H2O], the red ones of polyhalite and large, red anhydrite (Fig. 5e). Fine-grained (0.05 × 0.1/0.2 mm on average), fibrous polyhalites (Fig. 8f) probably grew in 0.5 × 1.2 mm large areas as a reaction product between coarse-grained blödite and a large (2 × 2.5 mm) anhydrite (single crystal), indicating static P–T-conditions. Blödite constitutes large areas (17 × 20 mm), with single grains measuring between 1 × 1 mm and up to 4 × 8 mm.

Cavity and vein fillings (microfabric type C)

Cavity growth fabrics (microfabric type C-1)

Mainly polyhalite or anhydrite, subordinate clay and carbonate minerals, which are growing into cavities, represent cavity infilling. Salt (halite) often fills the remaining open space (Fig. 9a–c). We also investigated this type by scanning electron microscopy (Fig. 9a). Commonly, large polyhalite fibres (fibrous aggregates: 0.1/0.5 × 2.5/3 mm; single fibres are only 0.02–0.1 mm thick) grew into cavities, which were filled afterwards by halite (see also Figs. 5f and 9b).

Fig. 9

Thin section microphotographs: (a) an SEM (scanning electron microscope) picture of polyhalite fibres (po) overgrowing with halite-filled cavities (ha); sample HT-16. (b) Large polyhalite fibres growing into cavities; sample HT-2. Crossed polarisers. (c) Coarse-grained anhydrite grains, which grow in a cavity filled with halite are embedded in a matrix of fine-grained, folded polyhalite grains of sample HT-8. Crossed polarisers. (d) Large, sheaf-shaped (following the description of Leitner, 2011) vein-filling polyhalite fibres of sample HST-21, microfabric type C-2. The large polyhalite fibres (po) are orientated about perpendicular to the vein walls, which are composed of an extremely fine-grained (1 × 1 μm), dark grey claystone/mudstone with many small cracks (mw), together: microfabric type B. Crossed polarisers.

In other cases, a cavity filled with halite and coarse-grained (0.1 × 0.2/0.4 mm on average), polygonal anhydrite grains occurs in a matrix of fine-grained (0.03/0.04 × 0.2 mm), folded polyhalite grains (Fig. 9c). Furthermore, polyhalite and anhydrite (0.05 × 0.1/0.2 mm on average) grains grew together into halite-filled cavities.

Vein-filling polyhalite (microfabric type C-2)

This microfabric type was only observed in sample HST-21 (Fig. 9d) and is consistent with polyhalite type 1 (fibrous polyhalite in veins) described by Leitner et al. (in press b). Polyhalite veins may form within mudrock, very often subparallel to sedimentary layers.

The vein-filling polyhalite samples of Altaussee and Berchtesgaden described by Leitner (2011) and Leitner et al. (in press b) grew antitaxially. Sample HST-21 does not show a well-developed median suture line, which means that the large (0.2 × 1 mm on average, up to 0.2 × 1.5 mm), in part sheaf-shaped polyhalite fibres (following the description of Leitner, 2011), which are orientated about perpendicular to the vein walls of clay and mudstone, have grown in only one direction (Fig. 9d).

Results of scanning electron microscopy

Three fine-grained samples (HT-8B, HT-10 and HT-16) were examined by SEM and qualitative EDX measurements in order to check the nature of fine-grained minerals. Sample HT-16 represents a “polyhalite ultramylonite” between coarse-grained magnesite breccia fragments, with halite cavity-filling and fine-grained dark carbonate or clay schlieren/layers (Fig. 10d). Sample HT-8B represents a recrystallised polyhalite rock with many questionable clay schlieren and layers and salt cavity filling. Rock sample HT-10 contains polyhalite mylonites, rare anhydrite, questionable fine-grained polyhalite–clay layers/schlieren and halite cavity filling.

Fig. 10

(a) SEM (scanning electron microscope) – picture of a mixture of polyhalite and clay minerals composed of the elements S, Mg, K, Ca, Si, Al, Fe; sample HT-10. Note the fine-grained isometric magnesite grains. (b) Thin section Scan (left side) of sample HT-10 and Scan of the thin section block (right side) (po – polyhalite, pc – polyhalite-clay mixture). (c) SEM-picture of magnesite fragments. All the fine-grained crystals are composed of the elements Mg, C, O and traces of Fe, constituting the mineral magnesite; sample HT-16. (d) Thin section Scan (left side) of sample HT-16 and Scan of the thin section block (right side) (mg – magnesite breccia fragments, po – polyhalite).

Similar to samples HT-8B and HT-16 the dark, fine-grained schlieren of sample HT-10 (Fig. 10b) contain a polyhalite–clay mineral mixture composed of S, Mg, K, Ca, Si, Al and Fe. The elements Si and Al argue particularly for the presence of a clay mineral (Fig. 10a and b).

According to SEM-measurements the light-grey breccia fragments of sample HT-16 are composed of the elements Mg, C, O and traces of Fe, constituting the mineral magnesite (Fig. 10c and d) MgCO3. Magnesite grains are isometric with an average size of 0.005 mm (Fig. 10c).

Furthermore, the SEM-measurements clearly show that the fine-grained polyhalite fibres and coarse-grained polyhalites and anhydrites (at least of sample HT-16) partly overgrow halite (microfabric type C-1) indicating an older relative age compared to halite (Fig. 9a).

Chemical characterization of polyhalite

Electron microprobe (EMP) measurements were performed on some polyhalite types (Samples HT-2, HT-3, HT-12, HT-16, HT-27 (2) and HT-30), associated anhydrite and on blödite in order to characterize compositional variations during various stages of mineral growth (see Table 3).

Table 3

Results of microprobe measurements.

Polyhalite, sample HT-3								Polyhalite, sample HT-2							Polyhalite, sample HT-27							Polyhalite, sample HT-12					Polyhalite, sample HT-30
Remark													Exsolution?
Spot	3289	3290	3291	3292	3295	3296	3297	3266	3267	3268	3270	3271	3269	3272	3250	3251	3252	3193	3195	3196	3197	3099	3100	3101	3102	3103	3276	3277	3278	3280	3281
													mit Na2O
FeO	0.15	0.14	0.11	0.10	0.08	0.17	0.12	0.16	0.13	0.11	0.12	0.17	0.14	0.15	0.16	0.08	0.24	0.05	0.08	0.08	0.03	0.03	0.06	0.07	0.02	0.11	0.06	0.11	0.17	0.12	0.04
MgO	7.12	6.87	6.89	6.75	6.92	6.86	6.97	6.79	6.79	6.62	6.75	6.79	6.50	6.63	6.55	6.81	7.04	6.87	6.70	6.75	6.77	6.86	6.74	6.30	6.64	6.29	6.90	6.65	6.85	6.80	6.82
CaO	18.59	17.94	18.29	18.44	18.45	18.31	18.40	18.56	19.16	18.58	18.99	18.89	18.27	18.56	19.17	18.66	18.63	18.73	18.96	19.04	18.70	18.91	18.69	18.72	18.55	18.71	18.70	18.33	18.30	18.37	18.54
MnO	0.00	0.00	0.00	0.00	0.00	0.02	0.00	0.00	0.00	0.00	0.00	0.01	0.01	0.01	0.00	0.04	0.00	0.00	0.03	0.00	0.00	0.04	0.03	0.03	0.05	0.05	0.00	0.01	0.01	0.00	0.00
SrO	0.13	0.09	0.06	0.05	0.15	0.19	0.05	0.25	0.11	0.17	0.06	0.01	0.00	0.25	0.19	0.22	0.05	0.12	0.14	0.18	0.10	0.28	0.29	0.28	0.74	0.31	0.11	0.07	0.10	0.09	0.14
BaO	0.00	0.00	0.03	0.00	0.05	0.00	0.00	0.06	0.00	0.11	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.03	0.00	0.00	0.00	0.00	0.01	0.00	0.11	0.00	0.00	0.00	0.00
Na2O	0.00	0.06	0.48	0.41	0.09	0.35	0.25	0.01	0.14	0.20	0.00	0.14	1.46	0.42	0.00	0.08	0.40	0.00	0.00	0.22	0.00	1.80	0.20	0.00	1.11	0.00	0.00	0.00	0.00	0.00	0.00
K2O	12.86	15.49	15.93	15.92	15.82	15.41	15.81	12.66	14.86	15.28	14.92	14.57	15.98	14.88	17.31	16.37	15.82	17.50	17.32	16.82	15.85	15.45	15.49	15.44	14.61	14.68	15.73	15.16	15.62	15.33	15.48
SO3	48.61	51.07	50.79	49.59	49.04	49.54	49.17	52.08	51.19	51.63	52.49	51.69	49.45	51.22	51.60	52.16	51.35	51.80	52.19	52.22	51.92	52.31	53.14	52.60	53.20	52.37	52.92	51.99	52.80	51.93	52.51
Total	87.46	91.66	92.58	91.26	90.60	90.85	90.77	90.57	92.39	92.70	93.33	92.28	91.81	92.12	94.97	94.42	93.52	95.06	95.41	95.33	93.37	95.69	94.64	93.43	94.92	92.52	94.51	92.33	93.85	92.64	93.54
Formula on the basis of 16O
K	1.769	2.045	2.098	2.136	2.140	2.072	2.133	1.660	1.948	1.996	1.924	1.902	2.141	1.955	2.239	2.110	2.059	2.257	2.223	2.157	2.057	1.970	1.975	1.995	1.859	1.908	2.010	1.977	2.006	1.998	1.997
Na	0.000	0.012	0.096	0.083	0.019	0.072	0.052	0.002	0.028	0.039	0.000	0.029	0.298	0.084	0.000	0.015	0.079	0.000	0.000	0.044	0.000	0.348	0.039	0.000	0.214	0.000	0.000	0.000	0.000	0.000	0.000
Ca	2.148	1.990	2.024	2.078	2.096	2.067	2.085	2.044	2.110	2.039	2.057	2.072	2.056	2.050	2.083	2.020	2.037	2.030	2.044	2.050	2.039	2.025	2.002	2.031	1.982	2.043	2.008	2.009	1.975	2.010	2.009
Sr	0.008	0.006	0.003	0.003	0.009	0.011	0.003	0.015	0.007	0.010	0.003	0.001	0.000	0.015	0.011	0.013	0.003	0.007	0.008	0.010	0.006	0.016	0.017	0.016	0.043	0.018	0.007	0.004	0.006	0.006	0.008
Mg	1.145	1.060	1.060	1.059	1.095	1.077	1.098	1.041	1.041	1.011	1.018	1.035	1.018	1.018	0.990	1.026	1.071	1.036	1.004	1.011	1.027	1.023	1.004	0.951	0.987	0.956	1.030	1.015	1.028	1.035	1.029
Ba	0.000	0.000	0.001	0.000	0.002	0.000	0.000	0.002	0.000	0.005	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.004	0.000	0.000	0.000	0.000
Fe	0.013	0.012	0.010	0.009	0.007	0.015	0.011	0.014	0.011	0.010	0.010	0.014	0.012	0.013	0.014	0.007	0.020	0.004	0.007	0.007	0.002	0.003	0.005	0.006	0.002	0.009	0.005	0.010	0.015	0.011	0.004
Mn	0.000	0.000	0.000	0.000	0.000	0.002	0.000	0.000	0.000	0.000	0.000	0.001	0.001	0.001	0.000	0.003	0.000	0.000	0.002	0.000	0.000	0.003	0.002	0.002	0.004	0.004	0.000	0.001	0.001	0.000	0.000
S	3.934	3.968	3.935	3.914	3.904	3.918	3.904	4.018	3.948	3.969	3.983	3.970	3.898	3.961	3.928	3.956	3.933	3.931	3.941	3.940	3.966	3.924	3.988	3.999	3.982	4.005	3.980	3.991	3.991	3.980	3.984

Polyhalite grains of the microfabric types A-1, A-4, B and C-1 show only minor variation in the polyhalite composition (see also Supplementary Material, Table SM 2). Only the SrO abundances exhibit some minor variation which is usually between 0.05 and 0.28 wt (weight) %. In samples HT-12 and HT-16, the SrO content is slightly elevated ranging from 0.28 to 0.74 wt%. The Na2O content ranges between 0.025 and 0.48 wt%, and is increased up to 1.80 respectively to 4.1 wt% in sample HT-12 (although a measurement error is conceivable as well).

A reaction fabric of blödite, a polyhalite seam and associated anhydrite is observed in sample HT-12, representing a metamorphic reaction fabric (Fig. 11c and d). The presence of blödite was found by both EMP analysis (Table 3) and X-ray diffraction (Supplementary Material, Fig. SM 1). Blödite of the Haselgebirge Fm. may occur primarily as nodules or more common intergrown with the mineral löweite (Schauberger, 1986), which has a nearly identical chemical composition and is stable at higher temperatures.

Fig. 11

(a) Fibrous anhydrite in sample HT-3. (b) Exsolution phenomena in sample HT-16. (c and d) Reaction fabric of blödite, a polyhalite seam and anhydrite of microfabric type B-2, Sample HT-12. The numbers indicate locations of microprobe chemical analysis (for the results, see Table 1).

In sample HT-3, fibrous bassanite (CaSO4.0.5H2O) is detected (Fig. 11a; Table 3). Partial exsolution from an unidentified small mineral of round shape within polyhalite is observed in sample HT-16 (Fig. 11b), which could represent possibly ongoing decomposition of polyhalite. Microprobe measurements demonstrate a nearly polyhalite-like chemistry (Supplementary Material, Fig. SM 2).

Discussion

In this chapter, we first highlight the significance of various polyhalite fabrics and try to establish the timing of tectonic events by comparison with dated polyhalite fabrics from other Alpine salt deposits (Leitner et al., in press a,b) as unfortunately no 40Ar/39Ar ages are available from Hallstatt. Then, we compare these fabrics with such from calcite and quartz, and, finally, we discuss the regional implications of the new observations. Finally, we debate the formation conditions of some of the observed particular mineral compositions, which in part allow us to deduce formation temperatures and the secondary origin of polyhalite.

Significance of polyhalite microfabrics

The examined polyhalite samples of Hallstatt show a wide variety of microfabrics and many of them, the mylonitic ones, resemble closely to such observed from rock-forming constituents of continental crust, like quartz, feldspar and calcite. Similarly, the microfabrics may have resulted from a wide range of external conditions like temperature, strain rate and differential stresses (Passchier and Trouw, 2005). The aim is now also to deduce conditions of formations of these microfabrics within the temperature stability field of polyhalite.

The polyhalite microfabrics can be derived from several different stages of polyhalite growth. We combine here all observations and show models for the suggested mode of origin in Fig. 12.

Fig. 12

Geological models explaining the step-wise formation of various microfabric types during the respective tectonic evolution. Age information by comparison with 40Ar/39Ar age dating results of Altaussee and Berchtesgaden (Leitner et al., in press b). Succession of microfabrics is from old to young. For explanation, see text (chapter 7.1). Abbreviations: po – polyhalite, an – anhydrite, bl – blödite.

The vein-filling, fibrous polyhalite samples of microfabric type C-2 (HST-21) and probably some of microfabric type C-1 (e.g. HT-2, HT-3) are expected to show the oldest ages. These rocks can be compared with the fibrous polyhalites in veins (polyhalite rock type 1) of Leitner et al. (in press b). In Altaussee and Berchtesgaden, 40Ar/39Ar age dating of similar types mainly yielded ages between 208 and 235 Ma. Thus, 20–25 million years after the deposition of the Haselgebirge Fm. during the main extensional stages of the NCA and a maximum overburden of 800 m (Rantitsch and Russegger, 2005) migrating brines of likely hydrothermal origin were reacting with the Haselgebirge sulphates to form polyhalite. Following the interpretation of Leitner et al. (in press b) and Leitner (2011), these circulating brines and heating were probably generated by the opening of the Meliata ocean (rifting), which lasted until 225 Ma ago. During recrystallisation, temperatures of 60–90 °C might have been reached as authigenic quartz originating from a transformation of mudrock minerals indicates (Spötl et al., 1998a,b). Some of the fibrous polyhalites of Altaussee and Berchtesgaden yield 40Ar/39Ar age steps of 204–209 Ma, which can probably be explained by a separate polyhalite growing event during Norian times, when subsidence took place in the sedimentation area (and the ∼1.5 km thick Dachstein Limestone was deposited over the Haselgebirge Fm. (Mandl, 2000; Leitner et al., in press b).

The veins might have formed due to extension during Middle Triassic times. During evaporation of seawater the polyhalite fibres probably formed by back reactions of the K–Mg–Cl–SO4 brine (the K and Mg might originate from pre-existing clay minerals or clay stones) with earlier formed gypsum, anhydrite or glauberite (inter alia Warren, 2006) on the vein walls (Fig. 12).

The polyhalite mylonites (e.g. samples HT-1, HT-11, HT-30) of microfabric types A-1 and B-1 were most likely formed along shear zones. These foliated and lineated rocks (Fig. 5a and b) can be characterised by their comparatively small grain sizes (varying from very fine-grained – ultramylonitic – to medium-grained fabrics) and distinct penetrative or closely spaced and planar foliation with a shape preferred orientation (SPO). Porphyroclasts, derived from a relatively “rigid” material, are quite common for mylonites, which usually indicate one deformation stage.

K–Mg–Cl–SO4 brines infiltrated (upwards and downwards) the evaporite rock along opened faults and joints and polyhalite-forming brine-back reactions took place. The mylonites were formed at high strain rates due to intracrystalline deformation, may have developed from a more coarse-grained precursor metamorphic rock and probably suffered several stages of deformation. The low crystallographic preferred orientation (CPO) of polyhalite may indicate some recrystallisation postdating mylonitisation. Furthermore, the mylonites are partly overprinted by recrystallised polyhalite grains (microfabric type A-3), which probably indicate a static growth.

These polyhalite mylonites can probably be compared to polyhalite rock type 3A (fine-grained, mylonitic polyhalite rock) and 3B (non-foliated polyhalite rock with local shear zones) described by Leitner et al. (in press b). 40Ar/39Ar age dating shows two stages of growth of fine-grained polyhalite – an early, poorly defined age is between ca. 152–158 Ma, the later between 105 and 118 Ma (Leitner et al., in press b). Ages between 152 and 158 Ma may be related to the closure of the Meliata ocean and the initiation of Alpine nappe stacking and accompanying ductile deformation (Leitner et al., in press b; Leitner, 2011). Furthermore, these ages correlate well with the 145–154 Ma old authigenic feldspar reported from Moosegg quarry by Spötl et al. (1996, 1998a, b).

A second age of 105–118 Ma of fine-grained polyhalites in shear zones is related to Cretaceous-aged deformation (low-grade metamorphic ductile overprint during eo-Alpine nappe stacking) and corresponds well to 40Ar/39Ar white mica ages of 114–120 Ma reported from the southern margin of the NCA (Frank and Schlager, 2006). The overburden must have been relatively high, which contradicts the small overburden deduced by fluid inclusion studies.

The argon loss of the fine grains of mylonitic polyhalites at about 44 Ma is explained by Leitner et al. (in press b) by a low temperature thermal overprint in the Eocene, which was postulated inter alia by Hejl and Grundmann (1989) and Pueyo et al. (2007) due to a regional thrusting processes in the NCA (for example, Linzer et al., 1995).

Although Leitner et al. (in press b) published a wide range of 40Ar/39Ar age data (158–44 Ma) for this rock type the mylonitisation was most likely induced by well reported Early Cretaceous thrusting events (Mandl, 2000; Schorn and Neubauer, 2011 and references therein) (Fig. 12) because of nearly complete resetting of the Ar isotopic system at 113 Ma (Leitner et al., in press b).

Microfabric types A-2, A-3 and A-4 (see Fig. 12) were observed in several rocks and indicate further deformation stages. The polyhalite samples showing crenulation cleavage and folds (microfabric type A-2), experienced a microfolding of the existing foliation and, therefore, at least two different deformation stages.

The recrystallised polyhalites of microfabric type A-3 were formed due to healing processes of the fabric after the main deformation process. Crenulation cleavage is commonly induced by microfolding of an existing foliation (e.g. a slaty or phyllitic cleavage), typically due to shortening of foliation about parallel to its layering. Many samples, especially polyhalite mylonites (of microfabric type A-1), also show pressure solution of microfabric type A-4, which indicates high strain rates, the presence of intergranular fluids and a second deformation stage under lower P–T-conditions.

K–Mg–Cl–SO4 brines infiltrated polyhalite rocks (upwards and downwards) along joints and faults, which were opened during several tectonic deformation stages (some of them were probably the same events which were also responsible for the mylonitisation and cataclastic fabrics) and induced the polyhalite recrystallisation (microfabric types A-2, A-3 and A-4) at elevated temperatures (Fig. 12).

The microfabric type A-5 (sample HT-6) has to be relatively young (compared with the other microfabric types), otherwise its cataclastic fabric would not have been preserved.

In the course of the mylonitisation during Early Cretaceous thrusting, or in younger geological times, a cataclastic deformation took place, which is still preserved in the boudinaged polyhalite–anhydrite layer and the cataclastic polyhalite microfabric (type A-5) of sample HT-6. It is characterised by equidimensional angular grains of variably size (0.005–0.08 mm) and areas with secondary grain size reduction. This fabric was formed due to relatively high strain rates, high fluid pressures at non-metamorphic to very low-grade metamorphic conditions. The K–Mg–Cl–SO4 brines were circulating in the cataclastically opened voids, inducing/generating the polyhalite precipitation (Fig. 12).

The polyhalite grains, which marginally grow into halite-filled cavities (microfabric type C-1, e.g. samples HT-2, HT-3, HT-16, Fig. 9a), are definitely younger than the microfabric types C-2, A-1 and B-1 and probably also younger than A-2, A-3 and A-4. Otherwise, the salt filling could not have been preserved until now. The K–Mg–Cl–SO4 brines may have infiltrated the rock (upwards and downwards) after the main alpine deformation stages along already opened cracks or joints (Fig. 12).

The metamorphic reaction fabric (microfabric type B-2) of sample HT-12 is constituted by blödite, a fine-grained polyhalite seam and large anhydrite crystals. The decomposition reaction of blödite might have taken place at 61.4 °C (if a saturation in halite is assumed or 71 °C without NaCl-saturation) or slightly above (for a detailed discussion see chapter 7.3).

It is not clear whether this polyhalite type is younger or older than microfabric type C-1 or A-3. However, as this decomposition reaction took place at relatively low temperatures, a large overburden can be excluded. Therefore, this microfabric type is certainly younger than the microfabric types C-2, A-1 and B-1 and probably also younger than A-2, A-3 and A-4 types. As no 40Ar/39Ar age data exists, the relative age succession remains quite unclear. However, microprobe measurements and photos show isomorphic polyhalite fibres and corroded anhydrite and blödite crystals (Fig. 11c and d), which strongly argue for a very young (probably Cenozoic) polyhalite age (in comparison with the other minerals) and a progressive metamorphism at low temperatures and elevated pressures. Yet the vice versa is conceivable as well. K–Mg–Cl–SO4 brines might have infiltrated the host rock along already existing cracks or faults (of a former tectonic event). The polyhalite fibres were precipitated by brine-back-reactions with earlier formed gypsum or anhydrite (Fig. 12).

At least the microfabric types A-1 (and B-1, probably A-5), B-2, C-1 and C-2 formed during different geological events, which were accompanied by fluid migration. Each of these brines, which infiltrated the Haselgebirge evaporites showed a different chemical composition, which, as we expected at the beginning of our study, would result in the precipitation or formation of polyhalite rocks with an appropriate and varying chemical composition. But microprobe measurements only showed a very small chemical variation (Supplementary Material, Fig. SM 2) of polyhalite samples from microfabric types A-1, A-3, A-4, B, B-2, C-1 and C-2 (Table 3, Supplementary Material, Table SM 2). Obviously, brines of different chemical compositions within the Haselgebirge precipited polyhalites of nearly the same mineral chemistry.

Comparison of polyhalite microfabrics with other rock-forming minerals

The examined polyhalite samples of Hallstatt show a wide variety of microfabrics and many of them, the mylonitic ones, resemble closely to such observed from rock-forming constituents of continental crust, like quartz (Stipp et al., 2002), feldspar and calcite (e.g. Leiss and Molli, 2003). Similarly, the microfabrics may have resulted from a wide range of external conditions like temperature, strain rate and differential stresses (Passchier and Trouw, 2005) and the temperatures at Hallstatt remained low (<250 °C). We also note that recrystallisation and annealing plays an important role veiling preferred crystal lattice patterns, which could not observed in or mylonitic samples. The influence of migrating brines to various microfabrics (see discussion in following sections) is more obvious in polyhalite rocks than in usual continental rocks with their predominance of quartzite, feldspar and calcite.

Blödite formation temperature

In oceanic salt deposits, blödite is mainly formed secondarily from kieserite due to solution metamorphism and often observed in kainite hats or associated with halite, löweite and Mg sulphates (Braitsch, 1962) and may also precipitate primarily from normal marine water (Braitsch, 1962; Zayani et al., 1999), Na2SO4-rich salt lakes or nitric acid deserts. In the Haselgebirge Fm. of the Eastern Alps, polyhalite intergrown with blödite (samples HT-12 and HT-15, HT-27) was, inter alia, described by Görgey (1909) from Hall in Tirol.

The lower crystallisation temperature of blödite is given as 6.2–6.4 °C (under laboratory conditions) in the presence of halite (Autenrieth and Braune, 1960a; Charykova et al., 1992) (4.5 °C Van't Hoff, 1912) or 20.0 °C in the ternary system Na2SO4–MgSO4–H2O (Charykova et al., 1992). The upper formation temperature ranges between 61.4 °C (with NaCl-saturation) (Autenrieth and Braune, 1960b) and 71 °C (without NaCl-saturation) (Van't Hoff, 1912). More details on the stability of blödite are discussed in Supplementary Material SM-3.

This decomposition reaction of blödite and probably anhydrite, which was observed in sample HT-12 (microfabric type B-2) might have taken place, therefore, at 61.4 °C – the upper formation temperature of blödite, if a halite saturation is assumed (or 71 °C without NaCl-saturation) – or slightly above. The polyhalite fibres of this metamorphic reaction fabric might have formed after the partly decomposition of the low temperature salt blödite.

As blödite has formed at relatively low temperatures, this decomposition reaction might also indicate a weak static reheating of the rock, during tectonic processes in young geological (probably Cenozoic) times and the age of ca. 44 Ma found in the Altaussee deposit may represent this event (Leitner et al., in press b).

Furthermore, this microfabric type might indicate, that the polyhalite rocks of the Alpine Haselgebirge Fm. have been infiltrated by extraordinary Na-rich brines, which induced the blödite formation at low temperatures, during late stages of the alpine deformation – probably during Cenozoic times.

Na-rich polyhalite

Some of the measured polyhalites of the polyhalite–blödite sample HT-12 were extraordinarily rich in Na (Table 3, Supplementary Material, Fig. SM 2). Autenrieth (1958), who examined the six component system K+–Na+–Mg2+–Ca2+–SO42−–Cl−–H2O, reported the formation of a new, metastable sediment – Na-polyhalite (formula: 25CaSO4.2K2SO4.3Na2SO4.15H2O; Autenrieth, 1958, 1959) during polyhalite precipitation. After a while, these prismatic to rod-shaped, Na-polyhalites are converted to fine-grained, milky to dumbbell-shaped polyhalite.

In most cases Na-polyhalite may be intermediary formed at temperatures above 50 °C from gypsum or brines with high CaSO4-concentrations within nearly the entire stability field of polyhalite (Autenrieth, 1958). Polyhalite may only convert directly (and quite fastly) to Na-polyhalite at temperatures above 50 °C in the presence of Mg-free or very Mg-poor brines, which are saturated with respect to KCl and NaCl (Autenrieth, 1958) or above 90 °C in the presence of CaCl2 and low MgCl2-concentrations (Autenrieth, 1959). The tendency of Na-polyhalite formation might increase with rising temperatures and is relatively high at 90 °C (Autenrieth, 1959).

According to Gudowius and Hodenberg (1979) Na-polyhalite shows structural similarities with the minerals bassanite (CaSO4.0.5H2O) and γ-CaSO4, which was later verified by Reisdorf and Abriel (1987). For further information on the stability of Na-polyhalite, see Supplementary Material SM-4.

Therefore, we conclude tentatively, that Na-polyhalite was most likely formed at temperatures above 50–90 °C, most likely from gypsum or at very low Mg-concentrations from polyhalite. This type of polyhalite formation is clearly different than the blödite–polyhalite–anhydrite–metamorphic reaction fabric B-2 and might indicate another, probably older stage of polyhalite formation.

Rubidium concentrations of the alpine polyhalites

The trace element rubidium isomorphously replaces potassium mainly in potash salts. As it forms no oceanic salt minerals it is very suitable as index element (Kühn, 1968). Therefore, Kühn (1972) examined the rubidium contents of sulphate salts like sylvite, carnallite, langbeinite, leonite and polyhalite of several salt deposits. Primary polyhalites usually show low rubidium contents of <0.0001 wt (weight) % (Kühn, 1972), while secondary polyhalites are rubidium-rich and this observation may potentially allow distinction between primary and secondary polyhalite.

The polyhalites of Hallstatt constitute 0.0012 wt% Rb (Kühn, 1972; Schauberger, 1986), implying the reworking of Rb-richer salts, and strongly arguing for a secondary origin of the polyhalites within the Haselgebirge Fm.. Our examined Hallstatt samples did not show high enough rubidium concentrations for microprobe measurements.

Due to the lack of sedimentary structures during thin section analysis Leitner et al. (in press b) also proposed a clearly secondary origin of the polyhalites of the Alpine Haselgebirge.

Strontium-content of polyhalite and anhydrite

According to our microprobe measurements the SrO contents of the polyhalite of Hallstatt are slightly elevated and range from 0.28 to 0.74 wt (weight) %. The variation likely originates from exchange with anhydrite, which has a higher SrO content (Schauberger, 1986). The Sr content of all examined anhydrite samples of the alpine salt deposits mainly ranges between 0.1 and 0.2% (Ruscha, 1976, unpublished data quoted from Schauberger, 1986) or 0.305% for Hall in Tirol (Spötl, 1988b) or even lower, strongly arguing for a diagenetic and secondary formation of the alpine anhydrites (due to conversion of gypsum, which was formed in the CaSO4-phase) (Schauberger, 1986).

Strontium ions of the seawater may isomorphously replace Ca2+ and Sr2+ (due to a linear function) in the crystal lattice of aragonite, calcite, anhydrite and gypsum (Usdowski, 1973). According to Usdowski (1973) anhydrite, which was formed by conversion of gypsum, shows average Sr-contents of 0.2 wt%, while primary anhydrite constitutes Sr-concentrations as high as 2.9 wt%. We conclude, therefore, that the relatively low Sr content of polyhalite is of secondary origin.

Origin of magnesites

The magnesites of the Haselgebirge Fm. from the Eastern Alps are of secondary origin and may have formed by exchange reactions between Mg2+-bearing solutions and primary low Mg-carbonates during several stages of diagenesis or metamorphism (Niedermayr et al., 1989). Within the Haselgebirge Fm., fine-grained magnesite is one of the major minerals of the carbonate fraction (Niedermayr et al., 1989).

Magnesite (MgCO3) is inter alia reported from the evaporitic claystone of Hallstatt salt mine (Schauberger, 1986). Spötl (1988a) described traces of magnesite (by X-ray diffraction (XRD) – measurements) in dolomitic limestones of a Lower Triassic bituminous dolomitic/anhydritic grey salt rock profile in the level Kaiser Franz Josef. Within the Alpine Haselgebirge Fm., magnesite may have formed by epigenetic saliniferous processes (which means post-sedimentary-diagenetic recrystallisation) (Schroll, 1961; Schauberger, 1986).

Spötl (1988a) suggested that the examined succession of fossil-free, bituminous, laminated siltstones, anhydrites and carbonates of the level Kaiser Franz Josef (Hallstatt) might have been deposited in a subtidal, hypersaline and anaerobe basin milieu (Spötl, 1988a).

Niedermayr et al. (1989) explained the magnesite formation of the Haselgebirge Fm. at hypersaline conditions (during Ca-sulphates and salt precipitation) by the infiltration and percolation of Mg2+-bearing brines in the basin center, which induced the dolomite and even magnesite formation of carbonate sediments. Salt diapirism and several alpine metamorphic stages may have led to mobilisation and recrystallisation of predominantly coarse-grained magnesite (of the NCA; see also Ebner et al., 2000).

We conclude therefore, that the fine-grained magnesite associated with polyhalite is of secondary origin.

Conclusions

In this paper, we present for the first time a polyhalite microfabric classification particularly to the Hallstatt salt mine, which shows a much wider variety of fabrics as reported in recent works from other salt mines in the Eastern Alps (Leitner et al., in press a,b; Leitner, 2011).

Polyhalite microfabrics from Hallstatt include: (1) vein-filling, fibrous polyhalite of likely Middle to Late Triassic age, (2) polyhalite mylonites of Early Cretaceous age, (3) metamorphic reaction fabrics of a post-early Cretaceous age and (4) cavity-filling polyhalite of uncertain age.

The polyhalite mylonites contain a wide range of shear fabrics commonly known in mylonitic quartzo–feldspathic shear zones within the ductile crust and developed from a more coarse-grained precursor rock.

The mylonites are partly overprinted by recrystallised, statically grown polyhalite grains.

Metamorphic reaction fabrics (microfabric type B-2) of fine-grained polyhalite seams, which have grown due to the decomposition of blödite (or astrakhanite) [Na2Mg(SO4)2.4H2O] and anhydrite have also been found. According to previous reports, blödite may occur primarily as nodules or, more commonly, intergrown with löweite, which has a nearly identical chemical composition and is stable at higher temperatures. Reaction fabrics may have formed by exsolution, (re-)crystallisation, parallel growth or replacement. This fabric type was only found in one sample in relation with blödite, and has a formation temperature of 6.2–61.4 °C in the presence of halite.

For the first time, we demonstrate that microfabrics like mylonitic shear fabrics, porphyroclast systems, annealing etc., which are described for other minerals like quartz, etc., are also preserved in relatively rare sulphates like polyhalite, which is stable between low temperatures and ca. 255 °C.

In general, coarse-grained polyhalite grains or twins in a fine-grained polyhalite matrix (microfabric type A-4) were only observed in samples from Hallstatt. The cataclastic polyhalite microfabric A-5 was also only found in one Hallstatt sample (HT-6) and the metamorphic reaction fabric B-2 was also reported from one polyhalite–blödite rock (HT-12) from Hallstatt. Therefore, these three microfabric types are characteristic for the Hallstatt salt deposit.