Authigenic heavy minerals a clue to unravel

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Author's personal copy Sedimentary Geology 228 (2010) 61–76

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Authigenic heavy minerals a clue to unravel supergene and hypogene alteration of marine and continental sediments of Triassic to Cretaceous age (SE Germany) Harald G. Dill Bundesanstalt für Geowissenschaften und Rohstoffe, D-30631 Hannover, P.O. Box 510163, Germany

a r t i c l e

i n f o

Article history: Received 1 October 2009 Received in revised form 9 March 2010 Accepted 15 April 2010 Available online 24 April 2010 Editor: G.J. Weltje Keywords: Heavy minerals Supergene Hypogene mineralization Continental–marine facies Mesozoic SE Germany

a b s t r a c t Siliciclastic continental–marine sediments of Triassic through Jurassic age, resting unconformably on the crystalline rocks of the Central European Variscides, and marine-(deltaic) sediments of Cretaceous age deposited in a narrow embankment are well exposed in the Wackersdorf area, SE Germany. These sediments were investigated for their variegated spectrum of authigenic heavy minerals (HM) and heavy mineral (HM) aggregates, the majority of which belongs to the (semi)opaque group of minerals, using petrographic and ore microscopy, supplemented by SEM-EDX and by EMPA. These HMs originated from supergene (weathering + diagenesis) and hypogene alteration (hydrothermal processes related to faults and unconformities). The physical and chemical conditions during which these authigenic minerals developed are discussed by means of Eh–pH diagrams under different temperature conditions (b 200 °C). Minerals indicative of supergene alteration in these sediments are apatite, Fe–Mn “limonite”, barite, ankerite, Fe–(Ni) sulfides and Pb–Cu–Zn sulfides. The ilmenite–pseudorutile–anatase series may be used as a measure for the intensity of supergene alteration under rising oxygen fugacity. Pseudorutile and anatase also formed during hypogene alteration, and developed different crystal morphologies compared with their supergene counterparts found in the above series. The authigenic HM fluorite, barite, niccolite, acanthite, silver, bravoite, Ni marcasite, pyrite, sphalerite, galena, chalcopyrite, chalcocite and rhabdophane-(Ce) were produced by hypogene/ hydrothermal processes. The hypogene mineralization in the Mesozoic foreland sediments belongs to two types: (1) epi-mesothermal REE-F mineralization controlled by the structural reactivation along a deep-seated lineamentary fault zone, and (2) epithermal Ni–Cu–Zn–As–S mineralization related to the geohydraulic plane of the late Variscan unconformity/Permo–Triassic peneplain. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Heavy minerals (=HM) are used by sedimentologists to disclose the provenance of clastic rocks (Thomas, 1909; Morton, 1985; Dill, 1989; Okay and Ergün, 2005; Garzanti et al., 2007; Triebold et al., 2007; Kutterolf et al., 2008). However, diagenetic alteration and chemical weathering processes have also been addressed using HM (De Jong and Van Der Walls, 1971; Morton, 1979; Friis et al., 1980; Morton and Hallsworth, 1999; Rossi et al., 2002; Dillon and Franke, 2009). Heavy minerals disseminated in sediments by ore-forming processes normally are not part of sedimentological studies (Dill, 1989). In such cases, the amount of transparent HM may be very small relative to the amount of (semi)opaque HM and HM aggregates (Boenigk, 1983). Thin sections alone do not yield reasonable results under these circumstances, and have to be supplemented by polished sections analyzed by a scanning electron microscope with energydispersive x-ray analysis (SEM-EDX) or electron microprobe analysis

E-mail address: [email protected]. URL: http://www.hgeodill.de. 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.04.006

(EMPA). Opaque HM, including Fe-,Ti-, S- and As minerals may be used as redox-sensitive marker minerals in a great variety of environments of deposition under different diagenetic and hydrothermal regimes, providing a much better overview of the history of diagenetic and hydrothermal alteration in the host sediments than transparent HM (Dill, 1988a; Weibel and Friis, 2004). The Wackersdorf study area, SE Germany, situated in the foreland and adjacent to the boundary fault of the Bohemian Massif, is a case where semi-opaque and opaque HM grains plus HM aggregates prevail over transparent HM. The near-shore marine to continental environments cover the entire stratigraphy from the Cretaceous down to the Triassic, which rests unconformably on pervasively weathered crystalline basement rocks of Precambrian age (Fig. 1). The region provides excellent conditions to study the variation of HM through time in siliciclastic sediments of different depositional environments with emphasis placed on their supergene and hypogene alterations (Fig. 2). The sediments contain euhedral grains and aggregates of phosphates, oxide-(hydroxides), sulfates, sulfides, arsenides and native elements useful to constrain the physical and chemical regime during diagenesis and weathering (supergene). The physical and chemical

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conditions during which these authigenic minerals developed were modeled using Eh–pH diagrams under different temperature conditions. As far as the hypogene alteration of the platform sediments is concerned, the Great Bavarian Quartz Lode (“Pfahl”) acts as a master fault bounding the foreland sediments against the basement and fostering the migration of hydrothermal solutions (Fig. 1b). It stretches across the Moldanubian basement and submerges under Mesozoic platform sediments towards the NW. This fault system was active throughout the entire Mesozoic and it is also held accountable as a feeder zone for the mineralizing fluids that brought about one of the most prominent fluorite mining district in Europe at Nabburg– Wölsendorf, situated less than 10 km N of the study area. For further literature on this ore mineralization see Dill et al. (2008). Another subhorizontal plane of relevance for hypogene (and supergene) alteration is the Late Variscan unconformity. The present study aimed at providing markers minerals indicative of the different types of alteration mentioned above and constrain the pH, Eh and temperature regime of their stability fields in different

depositional environments, so that these results may be applied to similar HM-bearing host rocks elsewhere (Table 1). 2. Methodology Heavy minerals have been recovered from drill cores. Where the core recovery was poor, samples from cuttings of the Wackersdorf bore hole have also been considered. The position of the section is illustrated in Fig. 1 relative to the paleogeography of each stratigraphic series encountered in the bore hole. The stratigraphic position of the sampling sites can be deduced from the lithology and mineral logs of Fig. 2, where the presence of each HM is indicated by a shaded column. The samples were passed through a sieve and the grain size fraction 63 to 600 µm was used for follow-up HM analyses. During routine analyses, the HM (densityN 2.9 g/cm3) were extracted by means of Na-polywolframate. After removal of iron oxide coatings with Na dithionite, HM were mounted on glass disks using Canada balsam. Translucent HM were identified under the petrographic microscope, considering between 200

Fig. 1. Sketch maps to show the study area and those sites referred to in the text in relation to the palaeogeographic evolution in Central Europe. Palaeogeography was modified from the official map “Geologische Karte 1: 500 000 Bayern. The contour lines denote the thickness of the stratigraphic series, the dotted lines (top right) mark the present-day position of the uplifted basement block of the Bohemian Massif or its morphological expression the NE Bavarian Basement. a) The position of the Wackersdorf study area. b) Crystalline rocks exposed today at the western edge of the Bohemian Massif near the Wackersdorf study area (according to Vrána et al., 1995). The Great Bavarian Quartz Lode (“Pfahl”) a prominent deep-seated fault zone across the Moldanubian basement controls the position of the fluorite vein-type deposits at Nabburg–Wölsendorf (framed area) and the hydrothermal fluorite mineralization at Pingarten in Permian platform sediments. It is also accountable for part of the hypogene alteration in the sediments penetrated by the Wackersdorf well (see Fig. 25.2.1). c) Basin- and-swell topography during the Upper Carboniferous and Permian with the most prominent troughs referred to in the text given in rectangular boxes. This period of time is not represented by equivalent sediments in the Wackersdorf drill section, because the drill site is sitting on the Regen Swell. It is marked by supergene kaolinization associated with Ti re-deposition (Fig. 2-5.1.2). d) Paleogeography of the lower Triassic Bunter Series (Buntsandstein). The dryland is represented by the Vindelician–Bohemian Massif. e) Paleogeography of the Middle Triassic Muschelkalk (the shaded area denotes the arenaceous facies of the Muschelkalk along the SE boundary of the Germanic Basin, the framed area with the symbol “Pb” marks the area of Freihung, Wollau, Eichelberg, where Pb ore was mined in form of cerussite and galena, the framed area with the symbol “kao” covers the area where still today (Pb-bearing) kaolinite is mined—see also block diagram in Fig. 8). f) Paleogeography of the Middle to Upper Triassic Keuper. g) Paleogeography of the Lower Jurassic/ Liassic. h) Paleogeography of the Middle Jurassic/Dogger. i) Paleogeography of the Upper Cretaceous series with different stages of marine incursions along the Regensburg Strait off the Alpine Foreland Basin. The South German Tableland is made up of platform sediments from the Late Paleozoic through the Upper Jurassic/Malm.

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Fig. 1 (continued).

and 300 grains per sample for mineral analysis if present in sufficient quantities. X-ray diffraction analysis of some particles supplemented the petrographic studies. The major analytical tool for this analysis was however the scanning electron microscope (SEM), using an energydispersive system (EDX) applied to polished sections. SEM-EDX analyses were undertaken using a QUANTA 600 FEG equipped with a GEMINI EDX analyses system. Analyses were carried out under lowvacuum conditions, precluding the use of sputter coater. In cases where SEM runs did not create unambiguous results, electron microprobe analyses (EMPA) with a CAMECA SX100 equipped with five wavelength-dispersive spectrometers and a Princeton Gamma Tech energydispersive system were used to overcome these analytical shortcomings. Crystal morphology of HM is described in the present paper using the common notation of Miller's indices. For further basic information on crystal morphology see Kostov and Kostov (1999). 3. Geological setting of the Triassic through Cretaceous sediments The Bohemian Massif is the core zone of the Central European Variscides composed of Proterozoic through Paleozoic metamorphic and igneous rocks that underwent a complex folding during the Cadomian and Variscan orogenies (Fig. 1a, b) (Vrána et al., 1995; Franke and Zelazniewicz, 2002). These crystalline rocks formed the south-eastern boundary of the semi-closed Peri-Tethyan Germanic Basin (epicontinental basin) (Fig. 1b–h) and the northern boundary of the Tethys basins (open marine) (Fig. 1i), respectively (Narkiewicz

and Szulc, 2004). Together with its SW prolongation, this emergent area is collectively termed the “Vindelician–Bohemian Massif” during the Triassic and Jurassic (Fig. 1d). It no longer acted as a barrier between the Peri-Tethys and the Tethys sensu stricto by the end of the Jurassic when it became drowned by calcareous platform sediments of the Malm series. The paleogeography evolution in these foreland basins during the Mesozoic has been reconstructed by a succession of paleogeography sketch maps, showing the lithological evolution through time and forming the base for the HM logs drafted for the Wackersdorf drill site (Figs. 1c–i and 2). During the Lower Triassic Buntsandstein, arenaceous and argillaceous red-bed sediments were deposited under arid climatic conditions by alluvial fans, ephemeral streams or laid down in lacustrine environments in the Germanic Basin (Nawrocki, 2004) (Figs. 1d and 2). The middle Triassic Muschelkalk beds, laid down contemporaneously with the Anisian to lower Ladinian beds in the Tethys basin, are the only stratigraphic unit in the Germanic Basin which saw a strong marine incursion during the Triassic. The flooding by the Muschelkalk sea as described by Aigner and Bachmann (1993) and Götz and Feist-Burkhardt (2000) for the central part of the basin, had only minor effects for the area under study in NE Bavaria, where the Muschelkalk is lithologically represented by a rather monotonous series of arenaceous to conglomeratic siliciclastic rocks intercalated with argillaceous and dolomitic lens-shaped beds of limited lateral extension. Therefore the Muschelkalk may be denominated as “Muschelsandstein” rather than “Muschelkalk”. The analysis of the

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Fig. 2. Variation of supergene and hypogene heavy minerals and heavy mineral aggregates as a function of a) Lithology (see lithology and grain size variation in the header). b) Stratigraphy. c) Environment of deposition. The area shaded in bright gray denotes occurrence of allogenic minerals, which might act as a potential source of the elements accommodated in the authigenic HM minerals (shaded in black) which were produced by supergene or hypogene alteration. The Roman numerals in the boxes refer to the mineral type in the text. Numeral codes along the x-axis/header of the diagram (4.1–4.7) refer to the sections in the text where the HM are described in detail. Numeral codes along the y-axis (5.1.1–5.2.2) refer to the sections in text where the type of alteration is discussed (shaded in yellow and framed with a full line = supergene, colorless and shaded with a dashed line = hypogene alteration). The color of the boxes is correlative with the color used in Table 1 where the physico-chemical regime of the marker minerals is treated in more detail.

Source unknown or extrabasinal K feldspar (in parts)

Source unknown or extrabasinal

Source unknown or extrabasinal

Source unknown or extrabasinal

Source unknown or extrabasinal

Acanthite–native silver (5.2.2)

Niccolite–bravoite–Ni marcasite (5.1.3 + 5.2.2)

Pyrite–marcasite (5.1.3)

Galena–sphalerite–chalcopyrite– chalcocite (5.1.4 + 5.2.2)

Ilmenite (in parts degraded) (5.1.2) Pseudorutile (5.1.2) Anatase—tabular (5.1.2)

Ankerite (5.1.5) Barite (5.1.5 + 5.2.2)

Ilmenite–rutile (“Nigrine” ), Ilmenite, Biotite

Rhabdophane-(Ce) (5.2.1)

Source unknown or extrabasinal Source unknown or extrabasinal Source unknown or extrabasinal

Monazite-(Ce)

Apatite (5.1.1)

Anatase—bipyramidal to prismatic (5.2.1) Goethite (5.1.2) Manganomelane (5.1.2) Fluorite (5.2.1)

Sedimentation

Type-I apatite

Processes

Allogenic (source) heavy and light minerals

Heavy minerals

Late diagenetic alteration of iron disulfides. Bravoite + Ni marcasite: red-bed-type mineralization under rising oxygen fugacity and pH: ≥ 7 Early to late diagenesis Marcasite: pH ≅ 6 Pyrite: pH ≅ 7 Pyrite {100}: strong oversaturation Pyrite {210}: low oversaturation Late diagenetic to epigenetic mineralization with sphalerite, galena, chalcopyrite Epigenetic facies-bound mineralization. Late diagenetic alteration of K feldspar in the arkoses and during kaolinization of K feldspar-bearing crystalline rocks pH N 4 galena

Late diagenetic alteration Late diagenetic alteration of K feldspar

Ferricretes Mn-bearing “limonite”

Well-preserved grains: low oxygen fugacity Rising oxygen fugacity under supergene conditions Most elevated level of oxygen fugacity under supergene conditions, in paleosols, saprolite, saprock, laterite, bauxite

Type II monomineralic aggregate: Phoscretes (apicretes). Early diagenetic formation and reworking of duricrusts under neutral to alkaline conditions in a fan-playa environment Type III oligo to polymineralic aggregate: Phoscretes (apicretes). Redeposited intraformational breccia sensu stricto

Supergene alteration (weathering + diagenesis)

Authigenic heavy minerals

Eh b 0

Eh b 0

Eh b 0

Eh b 0

Eh b 0 Eh N 0

Eh N 0 Eh N 0

Eh b 0 Eh ≤ 0 Eh N 0

Eh N 0

Eh N 0

Hydrothermal “unconformity-related hydrothermal ore mineralization” (epithermal) chalcocite: pH b 6

Hydrothermal “unconformity-related hydrothermal ore mineralization” (epithermal) Hydrothermal “unconformity-related hydrothermal ore mineralization” (epithermal b 200 °C) Hydrothermal “unconformity-related hydrothermal ore mineralization” (epithermal b 200 °C)

Hydrothermal mineralization related to fault zones (meso-to epithermal)

Low-temperature hydrothermal alteration

Rising oxygen fugacity under hypogene conditions

Monazite and cerite-(Ce) decomposed by hydrothermal alteration related to fault zones

Hypogene alteration (hydrothermal alteration)

Eh b 0

Eh b 0

Eh b 0

Eh N 0

Eh N 0

Eh ≤ 0 Eh N 0

Eh N 0

Table 1 Heavy minerals and heavy mineral aggregates as marker for post-depositional alteration processes. a) supergene alteration (diagenesis + weathering). b) hypogene alteration (hydrothermal alteration). Numeral codes in rounded brackets refer to each process of supergene and hypogene alteration in the text and allow for correlation with Fig. 2. Numbers in curled brackets point to crystal faces or forms using Miller's indices, e.g. {100}.

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depositional environment ranging from alluvial fans through fluvial to ephemeral lacustrine deposits is based in this region on studies of the facies-bound base metal deposits in the Muschelkalk and lower Keuper (Schmid, 1981; Dill 1990, 1994; Dill et al., 2007a) (Figs. 1e and 2). The environment of the upper Ladinian to Rhaetian Keuper beds in the Germanic basin is described as a shallow playa lake grading into a terminal alluvial plain (Reinhard and Ricken, 2000). The Feuerletten (uppermost Keuper) beds suggest deposition with no subaerial exposure and little continental run-off (Seegis and Goerigk, 1992) (Figs. 1f and 2).

From the Hettangian through the Toarcian (Lower Jurassic/Liassic) dark claystones, black shales and sandstones were deposited by the epicontinental Jurassic sea encroaching upon the Feuerletten playa (Figs. 1g and 2) (Meyer and Schmidt-Kaler, 1996). The Middle Jurassic (Dogger) is not very much different from the Liassic in terms of climate and environment of deposition in the area under consideration but contains coarser-grained sandstones and, in places, oolitic Minette ironstones that were accumulated in a near-shore marine environment (Siehl and Thein, 1989) (Figs. 1h and 2). The upper Jurassic limestone (Malm) platform did not extend far east. The study

Fig. 3. Micrographs of zircon and apatite under SEM-EDX. a) Aggregates of tabular apatite (type II) with booklets of kaolinite filling the interstices. Keuper. b) Massive apatite (ap) (Type III) with two inclusions of rounded monazite (mo) and subrounded to subangular zircon (zr) intergrown with quartz (qtz) in Buntsandstein beds. c) Monazite (“ghost structure”) pseudomorphosed by rhabdophane-(Ce) in course of hydrothermal alteration. The inset is a close-up view to illustrate the random growth of rhabdophane-(Ce) plates in these Keuper beds. d) “Ghost structure” of monazite replaced by rhabdophane-(Ce) in a fluorite vein-type mineralization of the Nabburg–Wölsendorf mining district near the Wackersdorf drill site. e) “Nigrine” aggregates composed of rutile (ru) forming the core and ilmenite (il) at the rim both of which are associated with quartz (qz). Cretaceous.

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area around Wackersdorf was emergent during this time leading to a marked hiatus between the Dogger and the Cretaceous (Fig. 2). The Cretaceous (Cenomanian through upper Turonian) sediments were deposited in a near-shore marine to deltaic environment of deposition (Meyer and Mielke, 1993). The Jurassic platform sediments fell dry during the lower Cretaceous. The incursion of the sea from the Alpine foreland basin took place along the “Regensburg Strait”, a narrow embayment that stretched NNW–SSE overstepping the flexure bulge of Jurassic sediments in the W and bounded by the uplifted crystalline rocks of the Bohemian Massif in the E (Figs. 1i and 2).

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parallel [001], which are ubiquitous among the detrital components (Fig. 2). In the Keuper Beds nodular apatite was found. Type II consists of nodular apatite and is found in the Keuper Beds (Fig. 3a). Type III massive apatite intergrown with quartz, monazite and zircon occurs in the Buntsandstein beds (Fig. 3b). Monazite and xenotime are common in the samples taken at Wackersdorf (Fig. 2). Seldom, monazite is found corroded by other minerals (Fig. 3c). In some Keuper and Cretaceous beds, monazite-(Ce) occurs only as “ghost structures”, pseudomorphed by rhabdophane-(Ce), a hydrated phosphate easily identified by its tiny plates. 4.2. Titanium, manganese and iron oxides

4. Heavy minerals and heavy minerals aggregates 4.1. Phosphate minerals The whole rock phosphate contents of the Mesozoic platform sediments are relatively high (0.32 to 3. 98 wt.% P2O5). Based upon morphology and intergrowth, apatite grains fall into three principal categories. Type I consists of subhedral to euhedral crystals elongated

Pure iron oxides are rare and represented only by goethite. Goethite is sporadically associated with manganese oxide-hydrates in the HM suites at the Triassic–Jurassic boundary (manganomelane) (Fig. 2). By contrast with the Fe oxides, Ti oxides are very widespread among HM and highly divers by morphology and composition. The TiO2 contents of the sediments under study fluctuate between 0.73 and 1.63 wt.%. The HM suite at Wackersdorf may be called as

Fig. 4. Iron–titanium ratios of oxidizing Ti–Fe minerals from SE Germany and some placer deposits and source rocks for reference. a) Rutile (detrital), anatase (diagenetic and hydrothermal), hydroxylian pseudorutile (diagenetic), detrital ilmenite reflecting different stages of diagenetic alteration in Cretaceous, Triassic and regolitic biotite gneisses. From top to bottom reducing conditions during alteration increase. b) Ilmenite and ilmenite–rutile intergrowth from various environments of formation and of different ages: Pleystein, Germany (“nigrine” placers proximal to pegmatites and quartz dykes), Schönlind, Germany (Ti–Sn placer proximal to the granitic source rock, Latvia (composite of beach garnet-enriched beach placers), Longido, Tanzania (unaltered metasomatic igneous rock), Wackersdorf. The supergene alteration (weathering) during the Permo–Triassic closely resembles the Mio–Pliocene saprolitization.

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ilmenite–rutile type. Single crystals of rutile (Fe/Ti ratiomean = 0.3) are rare and often “indentations” and notches are found along the grain boundaries marking the intergrowth with other minerals, mainly quartz (Fig. 4). Aggregates of rutile intergrown with ilmenite which are ubiquitous in these sediments allow for a subdivision of these Ti oxide aggregates, used to be called “nigrine” (Fig. 3e). This chemical phase is set between inverted commas because it does not represent a mineral species approved by International Mineralogical Association since it is as an aggregate like “leucoxene” or “limonite”, both of which are wellentrenched in the sedimentological and mineralogical literature and possess special connotation as to origin and composition. According to Clark (1993), “nigrine” (also: Eisenrutil, iron-rutile) is a variety of rutile. Ramdohr (1975) described “nigrine” as black ferruginous rutile that is homogeneous on a microscopic scale. In the mineral aggregates studied by Dill et al. (2007b) ilmenite is intimately intergrown with rutile, peppered with inclusions of ferrocolumbite, wolframite, pseudorutile and leucoxene, locally, replacing ilmenite and rutile from the edges or along cracks. “Nigrine” is preferred, irrespective of its way of intergrowth, because there is one characteristic feature common to such aggregates: the dull black luster. Such complex Ti oxide aggregates were also recorded from placer deposits and stream sediments from Sierra Leone by Raufuss (1973) and from Malawi by Bloomfield (1958). “Nigrine” collected from fluvial placer deposits in narrow creeks in the Pleystein area have Fe/Ti ratiomean between 0.81 and 0.65 (Fig. 4). Anatase, which occurs in well-shaped crystal aggregates (Fig. 5a), has the lowest mean Fe/Ti ratio (0.05) among the Fe–Ti oxides in this study. Based upon its morphology two types of anatase may be recognized in the HM suite. Type I anatase forms plates with {001} faces prevailing over {101} faces, some anatase aggregates displaying a combination of {001} and {105} faces, intersecting each other at an obtuse angle (Fig. 5c, d). In cavities within ilmenite, bipyramidal crystal aggregates and sprays of acicular crystals were seen (Fig. 5b). Type II anatase displays the reverse order of faces dominance associated with anatase I, with prevalent {101} and {110} faces.

the octahedral faces (Fig. 6c). The Ni content of bravoite fluctuates between 1.8 and 3.5 at.%. Cobalt, often found in bravoite s.s.s (Ramdohr, 1975), has not been detected in the solid solution series at Wackersdorf. 4.5. Iron disulfides Pyrite and marcasite are ubiquitous and prevail over other HM in the fine-grained calcareous sediments of late Triassic to Cretaceous (Fig. 6d, e). Pyrite is one of the most variegated minerals in terms of crystal morphology (Kostov and Kostov, 1999) with more than 460 forms being known. However only a small number play a significant role in constraining the physico-chemical changes in the depositional environment characterizing diagenetic evolution (Dill and Kemper, 1990). Framboidal pyrite which is the most “primitive” or initial form of Fe disulphide is concentrated mainly in calcareous siliciclastic Late Triassic and Jurassic sediments (Fig. 6e). Crystal aggregates dominated by the hexahedron {100} and by the pentagondodecahedron {210} postdate precipitation of the common framboidal pyrite. 4.6. Base metal sulfides Pyrite and marcasite are accompanied by minor sphalerite [ZnS] in the late Triassic and Cretaceous sediments (Fig. 2). Whole rock Zn contents range up to 312 ppm. Sphalerite also tends to occur in welldeveloped tetrahedral crystal aggregates in the Late Triassic playa sediments (Fig. 6f). Galena, another cubic mineral [PbS], is also present in euhedral crystals, although to a lesser extent than sphalerite. Concentrations of galena took place in host sediments older than those of sphalerite, particularly in the lower Triassic alluvial, fluvial and lacustrine beds of the Muschelkalk. The Muschelkalk has the highest Pb contents (up to 92 ppm Pb) but even these values fall short of economic concentrations (Fig. 6g). The Cu sulfides chalcopyrite [CuFeS2] and chalcocite [Cu2S] are found sporadically in the drill samples; they do not appear to characterise stratigraphic units or depositional environments.

4.3. Barite and fluorite

4.7. Iron-bearing carbonate

Barite is abundant among the accessory minerals of the Triassic sediments and widespread in sediments of the lower Triassic “Bunter Series” just above the basement (Fig. 2). Often barite is cored by potassium feldspar (Fig. 5e). Ba contents in the sediments range from 900 ppm to 3750 ppm. Fluorite has been observed only in Middle Jurassic beds (Fig. 5f).

Ferroan dolomite (ankerite) is a rare constituent of the HM assemblage and was only observed in samples from the Dogger beds (Fig. 2). 5. Discussion 5.1. Supergene alteration

4.4. Silver and nickel minerals Silver minerals are rarely reported from HM associations of siliciclastic sediments. In the Wackersdorf area, clusters of silver minerals have been observed in the saprolite of the Precambrian biotite gneiss. Silver sulfide aggregates composed of tiny cubes of acanthite [Ag2S] form the core while stout wires and plates of native silver [Ag] developed at the edge of the clusters (Fig. 6a). Tiny grains of niccolite – also called nickeline [NiAs] – are disseminated in the same saprolite of the biotite gneiss where the silver minerals were encountered. Nickel and arsenic contents reach 56 ppm and 92 ppm, respectively. Nickel arsenide is replaced by acanthite manifesting its paragenetic relation with the silver mineralization (Fig. 6b). Bravoite, a nickeliferous variety of pyrite, and Ni marcasite occur in the saprolite of the biotite gneiss. They also occur in the Triassic playa sediments and overlying Liassic shallow marine sediments (Fig. 2). Bravoite [(Ni, Fe)S2] is mostly developed as zoned penetration twins that are made up of two octahedra {111}. At places, bravoite also surrounds common pyrite aggregates with a corona of subhedral grains, typically displaying

5.1.1. Early diagenetic formation of phoscretes under oxidizing conditions Rounded grains of Type I apatite are widespread in the Keuper and Muschelkalk beds. They are of detrital origin and held to be the source of the authigenic phosphates (Figs. 1 and 2). Type II apatite aggregates developed as a result of pedological and hydraulic processes only in the middle Keuper and may be termed phoscretes, subtype apicretes. This term is a genetic qualifier for phosphate-cemented siliciclastic rocks (Hirono et al., 1987). These apatite-bearing duricrusts developed in a fan-playa environment locally also enriched in U-bearing yellow minerals such as U vanadates (Figs. 1 and 2). Phosphatization took place proximal to the fan head, whereas silcretes (petrified wood) and caliches occur towards the centre of the basin (Dill, 1988a). In this marginal facies around Wackersdorf uraniferous apicretes – 60 ppm U – devoid of any base metal mineralization developed in the Keuper beds (Fig. 2). The phosphate–vanadate relation is decisive in constraining the physical and chemical conditions during alteration of the host sediments (Fig. 7). Tyuyamunite [Ca(UO2)2(VO4)2·5H2O] is stable over a wide pH range as illustrated in the pH-Eh diagram at 25 °C. In the present setting, phosphate prevails due to a preponderance of PO3+ 4

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Fig. 5. Micrographs of titanium–iron oxides under the SEM-EDX and petrographic microscope. Numbers in brackets give Miller indices to label crystal faces. a) Aggregates of anatase plates with their interstices filled with booklets of kaolinite, indicative of supergene alteration in paleosols. Keuper. b) Acute bipyramidal to long prismatic/ acicular anatase growing into caverns of ilmenite associated with older apatite crystals: an1 = bipyramidal, an2 = acicular anatase. Both crystal morphologies are frequently found in autohydrothermally altered metabasic rocks and hydrothermal alteration zones of felsic igneous rocks. biotite gneiss. c) Tabular aggregates of anatase dominated by the crystal faces {001} typical of anatase formed in course of supergene alteration in saprolites and paleosols, Keuper (thin section plane-polarized light). d) Tabular aggregates of anatase forming a composite of crystal faces {001} and {105} typical of anatase formed in the course of supergene alteration in saprolites and paleosols. Keuper (thin section plane-polarized light). e) Subhedral grain of barite surrounding potassium feldspar (kf) in the topmost part of the saprolite developing on the gneissic basement rocks. f) Subhedral fluorite (fl) produced by hydrothermal mineralization in the Dogger sediments still reflects the primary crystal morphology of {111}. Fluorite is associated with diagenetic framboidal pyrite (py).

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Fig. 6. Micrographs of arsenides and sulfides under the SEM-EDX. a) Aggregates of acanthite (ac) overgrown with native silver (si) in the saprolite on top of the basement biotite gneisses. b) Niccolite (ni) is replaced along the edges by acanthite (ac) leaving behind serrated aggregates in the saprolite on top of the basement biotite gneisses. c) Two types of bravoite (zoned Ni-pyrite). Bravoite I (br I) formed a corona of subhedral bravoite crystals surrounding an anhedral core of pyrite. Bravoite II (br II) consists of penetration twins of two octahedra. Rhaetian to Liassic transition. d) Cluster of well-crystallized equant grains of pyrite clued together by calcite. Rhaetian to Liassic transition. e) Globular aggregates of framboidal pyrite surrounded by clusters of well-crystallized equant grains of pyrite. Rhaetian to Liassic transition. f) Two tetrahedra of sphalerite in oriented intergrowth. Feuerletten–Keuper. g) Subhedral crystal of galena. Feuerletten–Keuper.

complex over VO3+ 4 or owing to more alkaline pore fluids that did not allow vanadates to precipitate (Fig. 7a). Type III apatite aggregates associated with grains of rounded monazite and zircon are interpreted as reworked duricrust aggregates. Type III is an apicrete with inclusions of stable and ultrastable HM that underwent diagenetic recrystallization. Similar apicretes have been found as pebble ore around the veinlike U deposits of the Nuba Mts, Sudan (Dill et al., 1991). Apatite-bearing HM aggregates are exclusive to intrabasinal reworking and occur proximal

to the source duricrusts (intraformational breccia sensu stricto). The pHEh regime is similar to that described in Fig. 7. 5.1.2. Late diagenetic to epigenetic alteration of Ti, Mn and Fe minerals under reducing to oxidizing conditions Rutile, ilmenite and “nigrine” are very widespread among allogenic HM in this section (Fig. 2). These HM grains and aggregates form the source of Ti accommodated in the newly formed TiO2 (Fig. 2).

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Fig. 7. Physico-chemical conditions of formation of authigenic HM during supergene alteration around 25 °C (near-ambient conditions). a) Stability fields in the pH–Eh diagram at 3+ 4+ 25 °C to show the formation of phoscretes using the dissolved species as a log a2+ Ca = −4 and logaV4 = −4, logaU = −4. b) Stability fields in the pH–Eh diagram at 25 °C to show the 2+ 4+ formation of phoscretes using the dissolved species as a loga2+ Ca = − 4 and loga HPO4 = −4, logaU = −4. c) Stability fields in the pH–Eh diagram to show the Cu–Fe–S system at 25 °C using the dissolved species as a logaCu2+ = − 4, logaSO4– = − 3 and logaFe– = − 3. d) Stability fields in the pH–Eh diagram to show the Pb–C–S system at 25 °C using the dissolved species as a logaPb2+ = −4, log aSO4– = − 3 and logaHCO3– = − 3.

The amount of Ti derived from decomposition of biotite cannot be determined precisely. Chloritization of biotite and the release of Ti accommodated in sagenite is an autohydrothermal process that has already affected the source mineral biotite in the provenance area and as such falls outside the discussion of intrabasinal alteration. The variation of the Fe/Ti ratio in the sequence from ilmenite sensu stricto to anatase is used as a yardstick to measure the degree of alteration in the system Fe–Ti–O (Fig. 4). This mineral sequence is discussed in the section on supergene alteration but may also be applied to hypogene alteration processes with some modifications as to the crystal morphology of anatase (Fig. 5) (Table 1). The series of classification of supergene/hypogene alteration corresponds to the mineral sequence, ilmenite, pseudorutile, rutile and anatase observed by Cornua et al. (1999) in Amazonian ferralsols.

Unaltered to a slightly degraded ilmenite denotes reducing conditions in the marine Upper Cretaceous sediments (Fig. 2). Feenriched states of “leucoxene” and anatase which developed in marine and continental depositional environments as well as saprolites (Fig. 2) reflect advanced stages of alteration under increasingly oxidizing conditions (Table 1). Much has been published about the alteration of ilmenite pseudorutile (also called arizonite or hydroilmenite), the mineralogical status of which is under dispute (Mücke and Bhadra Chaudhuri, 1991; Grey et al., 1994; Pe-Piper et al. 2005). The redox-sensitive Fe–Ti compounds have proven to be a good tool for placer type analysis and for a classification of the state of diagenetic and hydrothermal alteration (Dill, 2007). From the basement rocks to the Cretaceous rocks the Fe/Ti ratios gradually decrease, pointing to an overall increase in the Eh value in the depocentre. Holocene stream

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sediments taken for reference in creeks that incised into Cretaceous alluvial fan sediments show a Fe/Ti ratio resembling that of the Cretaceous sediments under study, while stream sediments of creeks, cutting into basement rocks yield an intermediate state between unaltered ilmenite and samples taken from the saprock/saprolite at drill depth 345 m (Fig. 4). Based upon the Fe/Ti ratio of altered ilmenite, the intensity of subtropical chemical weathering during the Miocene–Pliocene is assumed to be similar to that of the Permo– Triassic (Fig. 4). The (sub)tropical chemical weathering observed in the samples under study is akin to that found on the vast peneplains in present-day central Africa (Twidale, 2002). A precise distinction between hydrothermal and diagenetic alterations involving titanium can be made using the crystal morphology of authigenic anatase. Re-deposition of Ti in the course of lateritization and formation of paleosols tends to form tabular crystals, whereas anatase precipitated from hydrothermal solutions tend to develop acute bipyramidal crystals. Almost 90% of the anatase in the study area is diagenetic in origin, attesting to local paleopedological processes during the Triassic and Liassic under mildly alkaline conditions (Fig. 2). The synthesis of the three polymorphs of titania (anatase, rutile, and brookite) depends strongly on the acidity of the medium (Cassaignon et al., 2007). The presence of concentrated nitrate ions seems to be the determining factor for the formation of brookite, that is absent from the rocks in the study area. Goethite and manganomelane are indicative of Eh N 0 and formed during stagnancy of per-descensum fluid movements and alteration of Fe sulfide (Table 1). This was the case at the boundary between the upper Triassic playa and Liassic marine-deltaic sediments (Fig. 2). 5.1.3. Early to late diagenetic Fe sulfide cementation under reducing conditions The coexistence of marcasite and pyrite attests to reducing conditions with pH values fluctuating around 6 (Murowchick and Barnes, 1986). Although pyrite is a classical mineral for habit modifications, the successions put forward for the development of the different habits are still under debate. According to Kostov and Kostov (1999), forms with higher indices develop under lower supersaturation at the end of the mineralizing process. Framboidal pyrite composed of tiny crystals displaying mainly faces of {100} marks strong oversaturation, whereas the pentagondodecahedron {210} and its complex combinations herald the end of pyrite recrystallization and a low degree of supersaturation in the sediments under study. Precipitation and recrystallisation of Fe disulfides in marine, deltaic, playa and fluvial depositional environments is strictly early to late diagenetic (Fig. 2). 5.1.4. Late diagenetic to epigenetic Pb–Cu–Zn cementation under reducing conditions Sphalerite and Cu sulfides are authigenic minerals which formed in playa, near-shore marine and deltaic sediments as the Jurassic sea started encroaching upon the continental Triassic sediments (Fig. 2). Chalcocite is exclusive to the unconformity-related mineralization and only discussed in this section for chemical and mineralogical grounds. A plausible explanation for the different sites of facies-related mineralization can be given by the Eh–pH diagram calculated for 25 °C (Fig. 7c). While chalcopyrite is stable over a wide pH range and as such may precipitate in the upper Triassic “Feuerletten” playa as well as near the late Variscan unconformity, chalcocite can only develop at a pH below 6 and an Eh N 0. Cerussite and galena have extra- and intrabasinal sources. It originated from the weathering of K feldspar exposed in crystalline rocks in the hinterland of the deposits as well as by intrastratal dissolution affecting the feldspar of the host arkoses. In the kaolinitic arkoses, e.g., at Schnaittenbach and Hirschau, Pb is also accommodated in the lattice of plumbogummite [PbAl3(PO4)2 (OH)5·H2O]. The isotopic composition of Pb is relatively homogenous and resembles isotopically average basement/crustal Pb so that any subcrustal (hypogene) source may be ruled out (Lippolt et al., 1983).

The syndiagenetic to epigenetic Pb mineralization in the Muschelkalk beds at Freihung , Wollau and Eichelberg is located more basinward than the Wackersdorf borehole. It consists of cerussite with subordinate amounts of galena, chalcopyrite, sphalerite, bravoite, covellite, and barite in the pore space of arenites (Schmid, 1981; Dill, 1990) (Fig. 1e). The Pb mineralization bound to the marginal calcareous siliciclastic beds of the Middle-Upper Triassic beds may be explained by a brine-mixing model under arid climatic conditions. Solutions flowing basinward contained Pb that was derived from decomposition of K feldspar in gneisses and granites of the Moldanubian basement. Vice versa, strong evaporation provoked a landward reflux of sulfate- and carbonate-enriched brines. Mixing of both brines in the arenaceous Middle Triassic sediments in the environs of Freihung, Wollau and Eichelberg led to the formation mainly of cerussite [PbCO3] subsequently replaced by PbS. In the marginal facies encountered in the Wackersdorf drill hole, cerussite could not precipitate because the area is devoid of calcareous sabkha sediments (Fig. 8). The Eh–pH diagram plotted in Fig. 7d illustrates that conversion of cerussite into galena towards the basin edge may simply be explained by a lowering of the Eh. 5.1.5. Late diagenetic to epigenetic Fe carbonate and Ba sulfate cementation under reducing to oxidizing conditions Barite originated from the decomposition of K feldspar, which is a major constituent of light minerals in the arkoses and the biotite gneisses of the basement in which Ba2+ substitutes for K+. Barite formed in-situ by diagenetic alteration of the Triassic arkoses. It forms a late diagenetic cement in the arkoses and post-dates cementation of ankerite thereby attesting to an increase in the Eh. 5.2. Hypogene alteration 5.2.1. Fault-related Ti–REE-F mineralization under oxidizing conditions Rhabdophane-(Ce) originated from decomposition of detrital grains of xenotime and monazite. Neither burial diagenesis nor weathering can account for the replacement of monazite by rhabdophane-(Ce) in the late Triassic and Cretaceous sediments under study (Berger et al., 2008). In the adjacent Nabburg–Wölsendorf fluorite mining district, rhabdophane-(Ce) was derived from decomposition of monazite and cerite-(Ce) and is, consequently, interpreted in the adjacent foreland sediments to have precipitated from hydrothermal solutions circulating along the “Great Bavarian Quartz Lode ” and affecting detrital REE phosphates such as monazite (Dill and Weber, 2008) (Fig. 1). The overall presence of Ce4+ in the hydrated phosphates suggests slightly oxidizing conditions for this type of hypogene alteration. Hypogene alteration may be held accountable also for the precipitation of fluorite in the Middle Jurassic beds (Fig. 2). About 10 km E of the drill site near Pingarten, lower Permian alluvial fan arkoses were impregnated by F-enriched brines and their preexisting cement minerals replaced by blue fluorite (Dill, 1985) (Fig. 1b). In conclusion, post-Dogger hydrothermal activity in the region gave rise to a re-deposition of fluorite in the Nabburg– Wölsendorf mining district and also impacted on the adjacent platform sediments, by disseminating fluorite and altering (hydrating) pre-existing detrital REE phosphates. Based on Rb/Sr dating collected elsewhere, mineral ages of Middle Jurassic (Dogger), 170 ± 4 Ma, were given by Schneider et al. (1999) for sandstone-hosted Pb deposits. Based upon U/Pb dating a Late Triassic to early Liassic age of 205.9 ± 2.7 Ma was given by Dill (1988b) for the youngest pitchblende in the SE part of the Nabburg–Wölsendorf mining district. Radiometric dating well agree with stratigraphic ages of hypogene alteration deduced from the position of HM in the drill section. Hypogene anatase formed only in the basement rocks around 340 m drill depth. Its bipyramidal crystal habit has been well-preserved besides the platy subtype of anatase which formed in the course of chemical weathering (Fig. 2-5.1.2).

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Fig. 8. Depositional environments and Pb mineralization during the Triassic series in NE Bavaria. The position of the Wackersdorf section is projected into the block diagram and in the cross section is shown by the vertical line. a) Block diagram illustrating precipitation of cerussite–galena ore in a sabkha environment, e.g. Freihung, and in alluvial fan deposits close to the basin edge at Wackersdorf. b) Metallogenetic “catena” from the regolith through the lacustrine basin. The corresponding sections are referred to in the text and also given in the block diagram.

5.2.2. Unconformity-related Cu–Ni–As–Ag–Ba mineralization under reducing to oxidizing conditions Barite mineralization straddling the“ Late Variscan unconformity ” at 340 m drill depth resulted from epigenetic processes. The unconformity between the Permo–Mesozoic platform sediments and the Variscan basement is a first-order geohydraulic surface. This plane favored circulation of hydrothermal fluids and gave rise to many of the epigenetic barite and fluorite vein-type deposits in Germany (Dill 1988b). Authigenic silver minerals are a bit atypical for this environment. They certainly did not result from simple diagenetic processes, since silver, on account of its high mobility, would have been dispersed rather than concentrated. Silver minerals form part of the lowtemperature unconformity-related mineralization and represent the

reducing part of the mineralization, compared with barite which represents the oxidizing part of this mineralization extending beyond the unconformity into the basal platform sediments of the Buntsandstein (Fig. 2). In gossans, copper is upgraded during supergene alteration or perdescensum fluid movement. By analogy, silver released during oxidative weathering migrated downward in acidic solutions to the underlying sulfide zone (Fig. 2) where it was reprecipitated under more reduced conditions as replacive sulfides and the native metal. In an Eh–pH diagram both Ag species are stable over the entire pH range at temperatures up to 200 °C using the dissolved species and concentrations as published by Brookins (1988) (Fig. 9a, b). The mechanism of fluid movements may resemble processes observed in gossans over pre-existing metal deposits. The latter, however, are

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Fig. 9. Physico-chemical conditions of deposition of authigenic HM during hypogene alteration in the range 25°–200 °C. a) Stability fields in the pH–Eh diagram at 25 °C to show the formation of Ag–S mineralization using the dissolved species as a logaAg2+ = −4 and logaSO4– = −2. b) Stability fields in the pH–Eh diagram at 200 °C to show the formation of Ag–S mineralization using the dissolved species as a logaAg2+ = − 4 and logaSO4– = −2. c) Stability fields in the pH–Eh diagram at 25 °C to show the formation of Ni-pyrite/-marcasite using the dissolved species as a logaNi2+ = − 4, as logaFe2+ = −4, and logaSO4– = −2. d) Stability fields in the pH–Eh diagram at 200 °C to show the formation of Ni-pyrite/-marcasite using the dissolved species as a logaNi2+ = − 4, as log aFe2+ = −4, and log aSO4– = −2.

absent in these sediments and a supergene alteration or perdescensum fluid movement is therefore less likely. Ni-enriched varieties of pyrite and marcasite are unusual in ordinary sediments. Ni-enriched varieties of pyrite and marcasite directs out thoughts to two major low-temperature deposits: (i) carbonate-hosted MVT (Mississippi-Valley-Type) mineralization where Fe disulfides accompany galena, sphalerite, barite and fluorite, and (ii) the red-bed sandstones. There is another environment of deposition known from the unconformity-related Ni–U deposits in the Athabascan Basin Canada (Watkinson et al., 1975). The Ni mineralization at the contact between the crystalline basement rocks and the overlying platform sediments in the drill hole under study is very much akin to the so-called unconformity-related mineralization described from Canada, where bravoite is associated

with niccolite, gersdorffite and millerite. The aforementioned Ni–Ag– As mineralization well fits into this scenario. In an Eh–pH diagram both Ni sulfide species are stable over a wide pH range from pH 2 to pH 11 at temperatures below 200 °C (Fig. 9c, d). In the pH range ≥ 7, which is the most realistic fluid composition for these sediments under study, NiS2 is stable only at Eh b 0. At higher Eh, NiFe2O4 would form instead. These physico-chemical constraints are valid for the Ni sulfides within the arkoses at the Triassic–Jurassic boundary and along the late Paleozoic unconformity. The bravoite mineralization observed in sediments at the Triassic–Jurassic boundary belongs to the red-bed type. According to Patterson et al. (1988) humic organic matter is the source of Ni but in the case of Ni coexisting with As, low-temperature hydrothermal solutions may also be a plausible explanation.

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5.3. Supergene and hypogene HM variation—a function of basin evolution

6. Conclusions

During the Upper Carboniferous and Permian, the study area at Wackersdorf was emergent, located on the northern flanks of the Regen Swell (Figs. 1b, c, and 2). The crystalline rocks immediately underneath the late Variscan unconformity were subjected to supergene alteration (chemical weathering) which gave rise to a saprolitic mantle of kaolinite bearing some platy anatase (Fig. 2-5.1.2). The Triassic Buntsandstein arkoses developed in an alluvial–fluvial depositional environment near the edge of the epicontinental Germanic Basin (Fig. 1d). Reworked duricrusts with apatite, monazite and zircon may be relics of the late Variscan supergene alteration, peneplanation and resultant chemical weathering. Diagenetic alteration occurred under oxidizing conditions (barite). Only during the latest stages, pyrite marks a shift to more reducing conditions (Fig. 2-5.1.3). During deposition of the Triassic Muschelkalk under arid climatic conditions, evaporation occurred in a sabkha-style environment fostering the development of facies-bound Pb deposits (Figs. 9 and 2-5.1.4). in the study site, galena is the only host of Pb and attests to reducing conditions in this fluvial depositional environment. The facies encountered by the Wackersdorf drill hole is a mineral facies transitional from cerussite (more basinward) into Pb-bearing kaolinite (more landward) (Fig. 1e). The Keuper beds contain the coarsest sediments in this part of the basin (Figs. 1f and 2). Authigenic HM such as anatase and apatite attest to intensive pedological and/or hydraulic processes conducive to paleosols and duricrusts (apicretes), respectively, in these alluvial– fluvial sediments (Fig. 2-5.1.2). The “Feuerletten” playa is very much distinct from the lower Keuper sediments by its Pb-, Cu- and Zn sulfides, which formed syn(dia)genetically in these fine-grained sediments under an increasingly arid climate with local sinks maintaining reducing conditions (Figs. 2-5.1.2 + 5.1.4). The base metals of the Feuerletten playa developed more basinward than equivalent base metal-bearing beds of the Muschelkalk (Pb-dominated) in this region, manifested by the increased contents of Cu- and Zn sulfides at this stratigraphic level. From the hydrologic point of view the system is interpreted as becoming increasingly stagnant with time with sulfides prevailing over oxides (Fig. 2). The Liassic (Fig. 1g) and the Dogger (Fig. 1h) beds are characterized by Fe disulfides suggesting more reducing conditions in the deltaic through near-shore marine Jurassic deposits than in the late Keuper sediments underneath (Figs. 1g, h and 2-5.1.4 + 5.1.5). Some embankments protruding SE ward into the dryland of the Vindelician–Bohemian Massif caused a restricted circulation of water resulting in more negative Eh values than in older sediments (Fig. 1h). Goethite locally are present in these beds derived from replacement of Fe disulfides and reworking of ironstones more basinward. During the late Cretaceous, several marine incursions from the Tethys basin took place along the “Regensburg Strait” leading to a series of marine to deltaic sediments (Fig. 1i). Although very much different in the depositional environment from the Jurassic and Triassic series and also with regard to the geotectonic setting, the supergene alteration of detrital HM, particularly the Fe–Ti minerals, remained unchanged, manifesting a similar Eh–pH regime during this time (Fig. 2-5.1.2). There are two fundamental hypogene/hydrothermal alteration processes which left their imprints on the HM association of these Mesozoic platform sediments. Hydrothermal/epithermal ore mineralization with Ni–Cu–Zn–Pb sulfides and arsenides is related to the geohydraulic plane of the late Variscan unconformity on top of the basement (Fig. 2-5.2.2). The age of this metallogenetic event may roughly be constrained to the Triassic. Another process is directly linked with the lineamentary fault zone of the “Pfahl” (Figs. 1b and 2-5.2.1). Remobilisation is post-Dogger (Fig. 2-5.2.1). Even post-upper Cretaceous processes cannot be ruled out as stressed by the presence of rhabdophane-(Ce) in deltaic sediments (Fig. 2).

Authigenic HM and HM aggregates belonging to the transparent and (semi)opaque groups of minerals may be used to distinguish between supergene (weathering + diagenesis) and hypogene alteration (hydrothermal processes related to faults and unconformities) in continental and marine sediments. – Apatite and Fe–Mn “limonite”, are indicative of supergene alteration under oxidizing conditions, whereas ankerite and Fe sulfides precipitated under reducing conditions. – Ilmenite, pseudorutile and anatase may be used to characterized supergene and hypogene alteration in sediments and taken as a measure for the intensity of alteration under rising oxygen fugacity. Pseudorutile and anatase formed during hypogene and supergene alteration and developed different crystal morphologies in each series. – Fluorite, barite, bravoite, Ni marcasite, sphalerite, galena, chalcopyrite, chalcocite and rhabdophane-(Ce) were produced by supergene and by hypogene/ hydrothermal processes. The hypogene mineralization in the Mesozoic foreland sediments belongs to two different types: ○ epi- to mesothermal REE-F mineralization controlled by the structural reactivation along a deep-seated lineamentary fault zone, ○ epithermal Ni–Cu–Zn–As–S mineralization related to the geohydraulic plane of the late Variscan unconformity or, in geomorphological terms, to the Permo–Triassic peneplain. – Niccolite, acanthite and silver are exclusive to hydrothermal processes in the sediments under study. The physical and chemical conditions during which these minerals developed can be discussed by means of Eh–pH diagrams under different temperature conditions (b200 °C). Acknowledgements I am indebted to I. Bitz for her assistance during the separation of heavy minerals and grain size analysis and F. Korte for doing the chemical analyses with XRF. D. Weck has performed the XRD analyses. SEM analyses were performed by D. Klosa. All investigations were carried in the laboratories of the Federal Institute for Geosciences and Natural Resources in Hannover, Germany. I thank very much A.C. Morton and another anonymous reviewer for their suggestions made to this manuscript. My gratitude is also extended to G.J. Weltje for his editorial handling of the paper. References Aigner, T., Bachmann, G.-H., 1993. Sequence stratigraphic framework of the German Triassic. Energie 18, 69–89. Berger, A., Gnos, E., Janots, E., Fernandez, A., Giese, J., 2008. Formation and composition of rhabdophane, bastnaesite and hydrated thorium minerals during alteration: implications for geochronology and low-temperature processes. Chemical Geology 254, 238–248. Bloomfield, K., 1958. The geology of the Port Herald area. Nyasaland Protectorate Geological Survey Department Bulletin 9, 1–76. Boenigk, W.C., 1983. Schwermineralanalyse. Enke, Stuttgart. 158 pp. Brookins, D.G., 1988. Eh–pH Diagrams for Geochemistry. Springer, Berlin, Heidelberg, New York. 176 pp. Cassaignon, S., Koelsch, M., Jolivet, J.-P., 2007. Selective synthesis of brookite, anatase and rutile nanoparticles: thermolysis of TiCl4 in aqueous nitric acid. Journal of Materials Science 42, 6689–6695. Clark, H., 1993. Hey's Mineral Index. Mineral Species, Varieties and Synonyms, 3rd ed. Chapman and Hall. Cornua, S., Lucas, Y., Lebon, E., Ambrosi, J.P., Luizão, F., Rouiller, J., Bonnay, M., Neal, C., 1999. Evidence of titanium mobility in soil profiles, Manaus, central Amazonia. Geoderma 91, 281–295. De Jong, J.D., Van Der Walls, l., 1971. Depositional environment and weathering phenomena of the white Miocene sands of southern Limburg (the Netherlands). Geologie en Mijnbouw 50, 417–424. Dill, H.G., 1985. Die Vererzung am Westrand der Böhmischen Masse. – Metallogenese in einer ensialischen Orogenzone. Geologisches Jahrbuch D 73, 3–461.

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Dill, H.G., 1988a. Diagenetic and epigenetic U, Ba, and base metal mineralization in the arenaceous Upper Triassic “Burgsandstein” (Southern Germany). With special reference to mineralization in duricrusts. Mineralogy and Petrology 89, 93–105. Dill, H.G., 1988b. Geologic setting and age relationship of fluorite–barite mineralization in Southern Germany—with special reference to the Late Paleozoic unconformity. Mineralium Deposita 23, 16–23. Dill, H.G., 1989. Facies and provenance analysis of Upper Carboniferous to Lower Permian fan sequences at a convergent plate margin using phyllosilicates, heavyminerals, and rock fragments (Erbendorf Trough, F.R.G.). Sedimentary Geology 61, 95–110. Dill, H.G., 1990. Die Schwermineralführung in den Trias zwischen Weiden und Pressath mit besonderer Berücksichtigung der Buntmetallmineralisationen. Erlanger Geologische Abhandlungen 118, 61–73. Dill, H.G., 1994. Facies variation and mineralization in Central Europe from the late Paleozoic through the Cenozoic. Economic Geology 89, 42–61. Dill, H.G., 2007. Grain morphology of heavy minerals from marine and continental placer deposits, with special reference to Fe –Ti oxides. Sedimentary Geology 198, 1–27. Dill, H.G., Kemper, E., 1990. Crystallographic and chemical variations during pyritization in the upper Barremian and lower Aptian dark claystone from the Lower Saxonian Basin (NW Germany). Sedimentology 37, 427–443. Dill, H.G., Weber, B., 2008. Cer-reiche Fluoritgänge im Randbereich des Nabburg– Wölsendorfer Fluorit- Reviers bei Saltendorf in der Oberpfalz. Geologische Blätter Nordost-Bayern 58, 27–38. Dill, H.G., Busch, K., Blum, N., 1991. Chemistry and origin of veinlike phosphate mineralization, Nuba Mts. (Sudan). Ore Geology Review 6, 9–24. Dill, H.G., Weber, B., Fuessl, M., 2007a. Mineralogische und sedimentpetrographische Untersuchungen an Pb–Cu–Fe–führenden Trias–Vererzungen zwischen Grafenwöhr und Freihung (Oberpfalz). Geologische Blätter Nordost-Bayern 57, 105–118. Dill, H.G., Melcher, F., Fuessl, M., Weber, B., 2007b. The origin of rutile–ilmenite aggregates (“nigrine”) in alluvial–fluvial placers of the Hagendorf pegmatite province, NE Bavaria, Germany. Mineralogy and Petrology 89, 133–158. Dill, H.G., Sachsenhofer, R.F., Grecula, P., Sasvári, T., Palinkaš, l.A., Borojević-Šoštarić, S., Strmić-Palinkaš, S., Prochaska, W., Garuti, G., Zaccarini, F., Arbouille, D., Schulz, H.M., 2008. Fossil fuels, ore—and industrial minerals. In: McCann, T. (Ed.), Geology of Central Europe, Geological Society of London, Special Publication, London, pp. 1341–1449. Dillon, M., Franke, C., 2009. Diagenetic alteration of natural Fe–Ti oxides identified by energy dispersive spectroscopy and low-temperature magnetic remanence and hysteresis measurements. Physics of the Earth and Planetary Interiors 172, 141–156. Franke, W., Zelazniewicz, A., 2002. Structure and evolution of the Bohemian Arc. In: Winchester, J.A., Pharaoh, D., Vernier, J. (Eds.), Paleozoic Amalgamation of Central Europe: Geological Society London Special Publication, 201, pp. 279–293. Friis, H., Nielsen, O.B., Friis, E.M., Balme, B.E., 1980. Sedimentological and palaeobotanical investigations of a Miocene sequence at Lavsbjery, Central Jütland, Denmark. Danmark Geologiske Undersoekelse 1979, 51–67. Garzanti, E., Vezzoli, G., Andò, S., Lavé, J., Attal, M., France-Lanord, C., DeCelles, P., 2007. Quantifying sand provenance and erosion (Marsyandi River, Nepal Himalaya). Earth and Planetary Science Letters 258, 500–515. Götz, A.E., Feist-Burkhardt, S., 2000. Palynofacies and sequence analysis of the Lower Muschelkalk (Middle Triassic, German basin). In: Bachmann, G.H., Lerche, I. (Eds.), Epicontinental Triassic, Zentralblatt für Geologie und Paläontologie, part I 1998, pp. 877–891. Grey, E., Watts, J.A., Bayliss, P., 1994. Mineralogical nomenclature: pseudorutile revalidated and neotype given. Mineralogical Magazine 58, 597–600. Hirono, S., Hirakawa, K., Hanada, K., 1987. Uranium-bearing phoscrete from Mali, West Africa. Chemical Geology 60, 281–286. Kostov, I., Kostov, R.I., 1999. Crystal habits of minerals. Bulgarian Academic Monographs 1, 1–415. Kutterolf, S., Diener, R., Schacht, U., Krawinkel, H., 2008. Provenance of the Carboniferous Hochwipfel Formation (Karawanken Mountains, Austria/Slovenia) —geochemistry versus petrography. Sedimentary Geology 203, 246–266. Lippolt, H.J., Schorn, U., Pidgeon, R.T., 1983. Genetic implications of new lead isotope measurements on Schwarzwald vein and Upper Triassic sediment galenas. International Journal of Earth Sciences 72, 77–104. Meyer, R.K.F., Mielke, H., 1993. Erläuterungen zur Geologischen Karte von Bayern 1:25000 Blatt Nr. 6639 Wackersdorf—BGLA. 194 pp. Meyer, R.K.F., Schmidt-Kaler, H., 1996. Jura. In: Freudenberger, W., Schwerd, K. (Eds.), Erläuterungen zur Geologischen Karte von Bayern 1:500 000 (4th ed) Bayerisches Geologisches Landesamt, pp. 90–111.

Morton, A.C., 1979. Depth control of intrastratal solution of heavy minerals from the Paleocene of the North Sea. Journal of Sedimentary Petrology 49, 281–286. Morton, A.C., 1985. Heavy-minerals in provenance studies. In: Zuffa, G.G. (Ed.), Provenance of Arenites. NATO Advanced Study School, Series C 148, pp. 249–277. Morton, A.C., Hallsworth, C.R., 1999. Processes controlling the composition of heavy mineral assemblages in sandstones. Sedimentary Geology 124, 3–29. Mücke, A., Bhadra Chaudhuri, J.N., 1991. The continuous alteration of ilmenite through pseudorutile to leucoxene. Ore Geology Review 6, 25–44. Murowchick, J.B., Barnes, H.L., 1986. Marcasite precipitation from hydrothermal solutions. Geochimica et Cosmochimica Acta 50, 2615–2629. Narkiewicz, K., Szulc, J., 2004. Controls on migration of conodont fauna in peripheral oceanic areas. An example from the Middle Triassic of the Northern Peri-Tethys. Geobios 37, 425–436. Nawrocki, J., 2004. The Permian–Triassic boundary in the Central European Basin: magnetostratigraphic constraints. Terra Nova 16, 139–145. Okay, N., Ergün, B., 2005. Source of the basinal sediments in the Marmara Sea investigated using heavy minerals in the modern beach sands. Marine Geology 216, 1–15. Patterson, J.H., Rams Dein, A.R., Dale, L.S., 1988. Geochemistry and mineralogical residence of trace elements in oil shales from Candon deposits, Queensland, Australia. Chemical Geology 55, 1–17. Pe-Piper, G., Piper, D.J.W., Dolansky, L., 2005. Alteration of ilmenite in the Cretaceous sandstones of Nova Scotia, Southeastern Canada. Clays and Clay Minerals 53, 490–510. Ramdohr, P., 1975. Die Erzminerale und ihre Verwachsungen. Akademie-Verlag, Berlin. 1277 pp. Raufuss, W., 1973. Struktur, Schwermineralführung, Genese und Bergbau der sedimentären Rutil-Lagerstätten in Sierra Leone (Westafrika). Geologisches Jahrbuch Reihe D 5, 1–52. Reinhard, L., Ricken, W., 2000. Climate cycles documented in a playa system: comparison of geochemical signatures derived from subbasins (Triassic, Middle Keuper, German Basin). In: Bock, H., Müller, R., Swennen, R., Zimmerle, W. (Eds.), West European Case Studies in Stratigraphy . Zentralblatt für Geologie und Paläontologie part I, 1999, pp. 315–340. Rossi, C., Kalin, O., Arribas, J., Tortosa, A., 2002. Diagenesis, provenance and reservoir quality of Triassic TAGI sandstones from Ourhoud field, Berkine (Ghadames) Basin, Algeria. Marine and Petroleum Geology 19, 117–142. Schmid, H., 1981. Zur Bleiführung in der mittleren Trias der Oberpfalz—Ergebnisse neuer Bohrungen. Erzmetall 34, 652–658. Schneider, J., Haack, U., Hein, U.F., Germann, A. 1999. Direct Rb–Sr dating of sandstonehosted sphalerites from stratabound Pb–Zn deposits in the northern Eifel, NW Rhenish Massif, Germany. In: Stein, H.J. & Hannah, J.L. (eds) Timing and duration of ore-forming processes: Contributions from radiometric dating. In: Stanley, C.J. (ed) Mineral deposits: Processes to processing. Proc. 5th Bienn. SGA Meeting and the 10th Quadrennial IAGOD Symposium, London, 22–25 August, 1999, 1287–1290. Seegis, D.B., Goerigk, M., 1992. Lakustrine und pedogene Sedimente im Knollenmergel (Mittlerer Keuper, Obertrias) des Mainhardter Waldes (Nordwürttemberg). Jahresbericht und Mitteilungen des Oberrheinischen Geologischen Vereins 74, 251–302. Siehl, A., Thein, I., 1989. Minette-type ironstones. In: Young, T.P., Taylor, W.E.G. (Eds.), Phanerozoic Ironstones: Geological Society of London Special Publication, 46, pp. 175–193. Thomas, H.H., 1909. A contribution to the petrography of the New Red Sandstone in the West of England. Quarterly Journal Geological Society of London 65, 229–245. Triebold, S., von Eynatten, H., Luvizotto, G.L., Zack, T., 2007. Deducing source rock lithology from detrital rutile geochemistry: an example from the Erzgebirge, Germany. Chemical Geology 244, 421–436. Twidale, C.R., 2002. The two-stage concept of landform and landscape development involving etching: origin, development and implications of an idea. Earth Science Reviews 57, 37–74. Vrána, S., Blümel, P., Petrakakis, K., 1995. Metamorphic evolution. In: Dallmeyer, R.D., Franke, W., Weber, K. (Eds.), Pre-Permian Geology of Central and Eastern Europe. Springer, Berlin, pp. 453–473. Watkinson, D.H., Heslop, J.B., Ewert, W.D., 1975. Nickel sulphide-arsenide assemblages associated with uranium mineralization, Zimmer Lake Area, Northern Saskatchewan. Canadian Mineralogist 13, 198–204. Weibel, R., Friis, H., 2004. Opaque minerals as keys for distinguishing oxidizing and reducing diagenetic conditions in the Lower Triassic Bunter Sandstone, North German Basin. Sedimentary Geology 169, 129–149.