Age and mineralogy of supergene uranium minerals

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Author's personal copy Geomorphology 117 (2010) 44–65

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h

Age and mineralogy of supergene uranium minerals — Tools to unravel geomorphological and palaeohydrological processes in granitic terrains (Bohemian Massif, SE Germany) H.G. Dill a,⁎, A. Gerdes b, B. Weber c a b c

Institute of Geosciences, Gem-Materials Research and Economic Geology, Johannes-Gutenberg-University, Mainz D-55099 Mainz, Becherweg 21, Germany Goethe-University Frankfurt, Institute of Geosciences, Petrology and Geochemistry, Altenhoeferallee 1, D-60438 Frankfurt am Main, Germany Bürgermeister-Knorr Str. 8 D-92637 Weiden i.d.OPf., Germany

a r t i c l e

i n f o

Article history: Received 4 June 2009 Received in revised form 26 October 2009 Accepted 4 November 2009 Available online 14 November 2009 Keywords: Granitic terrain Landscape Mineralogy Supergene uranium minerals U/Pb dating Germany

a b s t r a c t Uranyl phosphates (torbernite, autunite, uranocircite, saleeite) and hydrated uranyl silicates (normal and betauranophane) found in various erosion levels and structures in the Late Variscan granites at the western edge of the Bohemian Massif, Germany, were the target of mineralogical investigations and age dating, using conventional and more advanced techniques such as Laser-Ablation-Inductive-Coupled-Plasma Mass Spectrometry (LA-ICP-MS). Supergene U minerals have an edge over other rock-forming minerals for such studies, because of their inherent ‘clock’ and their swift response to chemical and physical environmental changes on different scales. Uraniferous phoscretes and silcretes, can be used to characterize the alkalinity/acidity of meteoric/per descensum fluids and to constrain the redox conditions during geomorphic processes. This study aims to decipher the geomorphological and palaeohydrological regime that granitic rocks of the Central European Variscides (Moldanubian and Saxothuringian zones) went through during the Neogene and Quaternary in the foreland of the rising Alpine mobile fold belt. The study provides an amendment to the current sub-division of the regolith by introducing the term “hydraulith”, made up of percolation and infiltration zones, for the supergene alteration zone in granitic terrains. It undercuts the regolith at the brink of the phreatic to vadose hydrological zones. Based upon the present geomorphological and mineralogical studies a four-stage model is proposed for the evolution of the landscape in a granitic terrain which might also be applicable to other regions of the European Variscides, considering the hydrological facies changes along with paleocurrent and paleoslope in the basement and the development of the fluvial drainage system in the foreland. Stage I (U mineralization in the infiltration zone) is a mirror image of the relic granitic landscape with high-altitude divides and alluvial–fluvial terraces. Its characteristic features are preserved in the uplifted hinterland of a peneplain which in this case is tilted towards a lacustrine basin. Stage II (U mineralization in the infiltration zone, regolith and saprock) includes two sub process, planation and exposure, resultant in the exposure of inselbergs and quartz ridges in front of the hinterland (stage I). Stages III and IV (U mineralization in percolation zone and saprock) are controlled by the base level lowering in the foreland. Rapid incision caused pinnacle-like tors and large granitic land forms to form, whereas a slow-down of fluvial incision favored its destruction and the development of weathering pits of different kinds. A full blown cycle of planation and incision lasted for approx. 10 Ma, a stage which covers planation and exposure, resulting in the formation of domal structures which lasted for as much as 2 Ma. Climate is an important factor but the most important factors for the geomorphological processes shaping the granitic landscape in the study area are uplift and erosion. The study area is located within the stress field of an ancient Variscan craton (Mesoeurope) and a highly mobile Alpine fold belt (Neoeurope). The rate of vertical displacement in the mobile parts of the crust had a long-distance effect also on the granitic terrains in the rigid parts of the crust. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Establishing landform chronologies, dating depositional processes and determining the rate of vertical movements of rock series during a ⁎ Corresponding author. E-mail address: [email protected] (H.G. Dill). URL: http://www.hgeodill.de (H.G. Dill). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.11.005

certain period of time play an ever-increasing part for geomorphological applications dealing with the most recent periods of the Earth's history. Apart from biologically-based methods like lichenometry and dendrochronology successfully used for dating Quaternary materials and landforms, there are two main physical methods, radiocarbon dating and the application of luminescence dating (OSL) which are widely used to date modern geological and geomorphological processes (Geyh and Schleicher, 1990; Wagner, 1995; Dill and Techmer, 2009). Other isotope

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couples, e.g. U/Pb, have found little application during interpretation of geomorphological process despite the fact that these methods may cover the gap between the late Pleistocene and the Neogene, a gray zone that cannot be handled by the aforementioned techniques for physical constraints (Lenz et al., 1962; Carl and Dill, 1983, 1985; Dill, 1985). Supergene uranium mineralization has gained little attention so far as a tool for chronologically constraining young geological and geomorphological process (Onac, 2000–2001). In this study, LA-ICP-MS data together with data from conventional U/Pb dating are discussed for resolving geomorphological and hydrological processes in SE Germany. Geomorphology and weathering of the granitic landscapes has been studied intensively and numerous comprehensive papers have been published, but little has been done to link these geomorphological processes with hydrological processes in the immediate surroundings of these granitic terrains (Linton, 1955; Ehlen, 1991; Bremer, 1993; André,

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2004; Twidale and Vital-Romani, 2005; Migoń, 2006). Special studies focusing on granitic terrains in the region have been published by Czudek et al. (1964) and Demek (1964) for the Czech part of the Bohemian Massif and by Jahn (1974) for the Polish part. In the current study paleohydrological and geomorphological processes shaping the landscape at the western edge of the Bohemian Massif, Germany, have been investigated jointly at various study sites (Fig. 1). Enrichments of supergene U minerals refer to paleoaquifer. Therefore mineralogical aspects concerning supergene U minerals are presented in the current paper in more detail to ease the identification of these paleo water gauges in the field by those geoscientists not fully acquainted with uranium geology, chemistry and mineralogy. Supergene U minerals have an edge over other geogene materials allowing for a modeling of the physical and chemical regime at the time of formation at nearambient conditions. Dating of secondary/supergene U minerals helps

Fig. 1. Index maps to show the geographic position and geological setting of the working areas at the western edge of the Bohemian Massif. a) Geological and geodynamic position of the working area within the Variscides in Europe at the western edge of the Bohemian Massif (geology after Matte, 1991). b) Geographic position of the working areas in SE Germany.

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constrain geomorphological processes at the passage from the Neogene into the Quaternary, a period of time that is marked by a striking paucity of age information. Terminology in the present paper follows the recommendations of Bates and Jackson (2008) if not stated otherwise.

2. Field and laboratory methods In the current study samples were taken from granitic areas, since felsic igneous rocks have the highest background values in U among all igneous rocks (Table 1)–average grade in the earth crust 2 ppm U–and, hence, finding secondary U minerals is not a difficult task. Sampling was concentrated on landforms typical of granitic terrains, e.g., weathering pits in the granitic bedrock, domal structures or fractures related to vast horizontal plains intersecting the granitic bedrocks. The yellow and green U minerals collected in the field were studied under the stereomicroscope and photographs were taken from the specimen, using normal light or being excited with an ultra-violet lamp. Radiometric surveys making use of hand-held gamma spectrometers are generally not successful, since these devices only record the presence of the decay daughter products but not the parent uranium. This is especially true for those supergene U minerals younger than 0.8 Ma, owing to an insufficient time to create an equivalent amount of decay products/daughter isotopes relative to the uranium parent isotope present in the uranium minerals (Carl and Dill, 1983). As secondary U minerals belong to the so-called heavy minerals, these minerals may also easily be separated for further analyses from the granitic host rock by heavy liquid separation. Heavy mineral separation may be carried out for each sample to the grain size fractions richest in heavy minerals (the 3–4 Ø fraction) using sodium polytungstate of a density of 2.9 g/cm3 (Callahan, 1987). To clarify the bedrock–mineral interrelationship, thin sections were examined under the petrographic microscope. XRD patterns were recorded using a Philips X'Pert PW3710 Θ–2Θ diffractometer (Cu-Kα radiation generated at 40 kV and 40 mA), equipped with a 1° divergence slit, a secondary monochromator, a point detector and a sample changer (sample diameter 28 mm). The samples were investigated from 2° to 80° 2Θ with a step size of 0.02° 2Θ and a measuring time of 3 s per step. For specimen preparation, the top loading technique was used. SEM-EDX is an important supplement to XRD. No sputter coater was used prior to SEM-EDX analyses by means of a QUANTA 600 FEG equipped with a GEMINI EDX system because all analyses were carried out under low-vac-chamber conditions (1 to 10 mbar). Major

Table 1 Uranium contents of common rock types from the igneous realm (outside areas mineralized with U and Th or host to U deposits) modified after Dahlkamp (1993).

and trace elements were analyzed by X-ray fluorescence spectrometry (XRF). Powdered samples were analyzed using a PANalytical Axios and a PW2400 spectrometer. In the current sections the LaserAblation-Inductive-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is applied for the first time to find an answer to weathering and geomorphological processes shaping a granitic landscape (Gerdes and Zeh, 2006). Secondary U minerals were analyzed for U, Th and Pb isotopes by the LA-ICP-MS techniques, using a Thermo-Finnigan Element II sector field ICP-MS coupled to a New Wave UP213 ultraviolet laser system. instrumental mass discrimination, and timedependent elemental fractionation of Pb/Th and Pb/U (Gerdes and Zeh, 2006). Reported uncertainties (2σ) were propagated by quadratic addition of the external reproducibility (2 s.d.) obtained from the reference zircon (n = 12) during the analytical session and the within-run precision of each analyses (2 s.e.). Concordia diagrams (2δ error ellipses) and concordia ages with 2 sigma uncertainty were produced using Isoplot/Ex 2.49 (Ludwig, 2001). Some ages have been shown in Fig. 2 for reference and the full set of data along with mineralogical and lithological data for the geomorphological and paleohydrological interpretation have been listed in Table 3. The measuring protocols are given in Table 4. 3. Geological and geomorphological setting The study areas at the western edge of the Bohemian Massif in the NE Bavarian Basement, Germany, belong to the European Variscides, to be more specific, to the Moldanubian and Saxothuringian geodynamic zones which closely resemble each other with respect to their granitic bedrock geology (Fig. 1a). The study areas around Grossschloppen and Rudolfstein forms part of the Fichtelgebirge a horse-shoe-like mountain range made up of Precambrian to early Paleozoic metamorphic rocks which were intruded by granites during the Late Carboniferous and Permian (Richter and Stettner, 1979). The Poppenreuth area at the boundary between the Saxothuringian and Moldanubian zones is characterized by biotite gneisses and some metabasic rocks. Granitic mobilizates also termed granitoids were intercalated along strike into these Precambrian gneisses which extend beyond the border into the Czech Republic. The Flossenbürg and Hagendorf area shows a more variegated lithology and is mainly underlain by Moldanubian paragneisses composed of variable amounts of biotite, sillimanite, cordierite, quartz, garnet and feldspar (Forster, 1965). Late Carboniferous felsic intrusive rocks, the most important of which is the Flossenbürg Granite, were intruded into these metasediments. The Flossenbürg Granite has petrographically been classified as a monzogranite and was dated by the Rb/Sr-whole rock method at 311.9 ± 2.7 Ma (Wendt et al., 1994). Several aplites and pegmatite stocks were also exposed by denudation. The most renowned and abundant in Li-, Fe-, Mn- and Zn-phosphates are the pegmatites of Hagendorf and the quartz pegmatite near Pleystein (Mücke, 2000). The granites in the Nabburg–Wölsendorf area yielded U–Pb and Pb– Pb evaporation ages between 321–329 Ma with two granodiorites giving concordant 238U–206Pb and 235U–207Pb ages of 325 ± 3 Ma and 326 ± 3 Ma (Siebel et al., 2006). In the study area of Nabburg– Wölsendorf the granites are overlain by Cretaceous through Tertiary sediments encroaching upon the crystalline basement from the south. Triassic and Jurassic marine transgressions did not encroach upon the basement. The crystalline rocks of the Bohemian Massif form the eastern boundary of the semi-closed epicontinental Peri-Tethyan Germanic Basin and the northern boundary of the Tethys basins, respectively (Narkiewicz and Szulc, 2004). The latter basin is represented by the Alpine Foreland basin which gradually silted up during the Neogene and prograded towards the north in form of a narrow embankment called the ‘Regensburg Strait’ (Fig. 3). Clastic sediments and relics of the Cenozoic weathering mantle were, locally, preserved on a low-relief landscape in the basement, proper. These

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Fig. 2. The 206Pb/238U vs. 207Pb/235U cross plots to exemplify age dating of supergene U minerals from SE Germany. See also Tables 3 and 4. a) Flossenbürg–Torbernite-autunite. b) Wölsendorf–Uranocircite.

relic landforms date back to the Cenozoic when subtropical climates occurred in what is called today the ‘Oberpfaelzer Wald’ and ‘Fichtelgebirge’ (Buedel, 1977; Louis, 1984; Borger et al., 1993). Pervasive kaolinisation affected some parts of the granitic rocks and led to some prominent kaolin deposits in the region (Köster, 1980). During the late Mesozoic and Cenozoic the basement rocks were uplifted along a deep-seated lineamentary fault zone (“Fraenkische Linie”) and displaced by at least 1000 m relative to the Mesozoic foreland sediments (Fig. 4a).

4. Results 4.1. Granitic landforms The surface exposure of intrusive landforms from the bird's-eye view is not so impressive as landforms associated with volcanic rocks, especially in an area belonging to the humid temperate (morpho) climatic zone (Fig. 4a). The classification scheme used in this study for the granitic terrain at the western edge of the Bohemian Massif is an

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Fig. 3. Neogene through Quaternary palaeogeography at the western edge of the Bohemian Massif in the granitic basement and its foreland. See color codes for correlation with Fig. 6 and Table 3. a) Phase of planation-constructive phase: Supergene U mineralization and Neogene geomorphological and hydrological evolution in the granitic basement and its foreland. Arrow heads denote transport of debris and paleoslope. b) Phase of incision-destructive phase: Supergene U mineralization and Quaternary geomorphological and hydrological evolution in the granitic basement and its foreland. Arrow heads denote transport of debris and paleoslope.

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adaptation of components quoted in the classical textbooks by Wilhelmy (1958) and Twidale (1982) to the needs and wants of a chronological and mineralogical study of geomorphological processes. The principle criterion for the four-fold sub-division is the size of forms (Table 3). The term stage in the succeeding paragraph is used according to the Glossary of Geology: (Gary et al., 1977): a period , or

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phase in development, of a cycle of erosion in which the features of the landscape have distinctive characteristic forms. The late Variscan granitic complexes in the Fichtelgebirge and Oberpfaelzer Wald stand out from the landscape by densely forested mountain ridges (stage I landforms). The metamorphic country rocks, the Neogene and Quaternary sediments and the regolith cover the

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targeting on the supergene argillitisation, increases towards the south (Dill et al., 2008a,b) (Fig. 4a). Granitic tors capping hills such as at Schellenberg near Flossenbürg are common in the Oberpfaelzer Wald but more widespread in the Fichtelgebirge (stage III landform) (Fig. 4c). Weathering pits measuring from a few decimeters to several meters are common to both regions and covered mostly by lichen such as at Doost near Flossenbürg (stage IV landform) (Fig. 4d). These hard rock landforms and features are chronologically constrained, using supergene U minerals. Soft rock landforms gradually developed from the fresh late Variscan granite, as demonstrated in the sequence of images taken at Liebenstein near Falkenberg (Fig. 4e, f, g). The different stages of weathering are controlled by faults and fissures which are lined by thin coatings of Fe-bearing smectite and kaolinite. With depth the altered soft granite grades almost invisibly into the fresh granite of high rock strength. 4.2. Geology and distribution of uranium minerals in granites

Fig. 4. Granitic hard and soft rock landform in the study areas at the western edge of the Bohemian Massif in SE Germany. a) Satellite image to display the densely forested ridges and domes caused by the late Variscan granitic intrusions in the Fichtelgebirge and Oberpfaelzer Wald. The surface exposure of the metamorphic country rocks, the Neogene and Quaternary sediments and the regolith is made up by bright green patches between the dark green granitic masses (stage-I land form) (Source: Bayernviewer–Landesamt fuer Vermessung und Geoinformation Bayern). Mining symbols refer to active and abandoned kaolin exploitations. a. Zone A: relict peneplain (German: Altflächen). b. Zone B: peneplain with quartz ridges and inselberg domes. c. Zone C: peneplain blanketed with regolith. d. “Fränkische Linie”= deep-seated lineamentary fault zone terminating the basement towards the Mesozoic foreland. b) Flat-lying fractures in the Flossenbürg granitic dome parallel to the surface resulting in an onion-shell-type structure of this inselberg exhumed by denudation (stage II landform). c) Granitic tor capping hills at Schellenberg near Flossenbürg giving a close-up-view of the flat-lying joint system in the late Variscan granites (stage III landform). d) Weathering pits surrounded by lichen (German: “Opferkessel”) at Doost near Flossenbürg, Germany (stage IV landform). e) Different stages of weathering, some of which are controlled by diagonal faults (“f”), at outcrop near Liebenstein NW of Flossenbürg. On the right-hand side of the fault which is steeply dipping towards the W, the granite is almost unaltered (“pz”) and cut by more densely-spaced fissures. Based upon its thin lining of Febearing smectite and kaolinite on fractures this granitic zone is attributed to the percolation zone. On the opposite side of the fault the granite is pervasively altered by per descensum processes and converted in its lower part into a saprock (“sr”) grading almost invisible into the fresh granite below, whereas the contact between the saprock and the saprolite (plus topsoil “sl +p”) is accentuated by a change in rock color turning towards the present-day surface from light gray to brown. The diagonal fracture acted as a conduit for the meteoric solutions to percolate much deeper into the granite. The weathering profile is tectonically disturbed, incomplete and capped by erosion (stage-II landform). The wall of the stope is roughly 25 m high. f) Close-up view of the percolation zone characterized by irregularly spaced fractures lined with kaolinite and green Fe-bearing smectite in the Liebenstein Granite. g) Unaltered late Variscan granite at Liebenstein.

morphological depressions between these mountains, attaining heights of as much as 1000 m a.m.s.l. (Fig. 4a). They are located in the northernmost part of the study area within the Saxothuringian zone or, in geomorphological terms the Fichtelgebirge (Figs. 1a, b, 4a). Sub-aerially exposed granitic domes are found further south in the Oberpfaelzer Wald, where the ridges and vast granitic complexes got split up into unique domes isolated from these coherent mountain chains (Fig. 4b). The set of granitic joints is well exemplified in the Flossenbürg granite dome. Flat-lying fractures in the Flossenbürg granitic dome parallel to the surface engendered onion-skin weathering, a concentric exfoliation occasionally of curved plates (stage II landform). The size of areas with the regolith well preserved in depressions, and the number of kaolin open pit mining operations

The reader will find in the pertinent geomorphological literature a lot of cartoons illustrating the disintegration of granites by chemical weathering which is governed mainly by the set of joints criss-crossing the granites (Linton, 1955; Ehlen, 1991; Bremer, 1993; André, 2004). However, little is known to what extent meteoric waters were able to infiltrate the felsic igneous rocks during a certain period of time. A compilation by Migoń (2006) provides an overview of the weathering mantles of granitic rocks, averaging roughly 60 m. In the study sites, the supergene U minerals line the walls of fissures, coat fractures or grow freely into open cavities of granites (e.g. Rudolfstein), granitic mobilizates (e.g. Poppenreuth) and pegmatites (e.g. Hagendorf) (Table 3) (Fig. 5). The sort of surface material that was sampled at the various levels for age dating and mineralogical studies is on display with respect to depth, size, texture and structure in Fig. 5. Supergene U minerals have been encountered at the present-day surface, immediately underneath weathering pits and in the regolith down to a few meters below ground but also down to a depth of 200 m below ground as at Rudolfstein. In this case mining activities penetrated a vein-like per descensum mineralization at a depth of 200 m below ground. This is not out of the ordinary as shown by the weathering mantle reported from the Dartmoor Granite, SW England, where the thickness of the weathering mantle in granitoid rocks is reported to be 200 m (Bristow, 1969; Smith and McAllister, 1987). In southern Finland 100 m is reported for the same sort of weathering mantle (Sarapää, 1996). There is a conspicuous trend from SE towards the NW of supergene U mineralization to affect deeper hydrological levels (Fig. 6). Primary U minerals such as pitchblende or coffinite are completely unrelated in space and time with these secondary U minerals. At Grossschloppen supergene mineralization may be traced down to a level of 150 m below ground with a characteristic depth zonation, and in Höhenstein the supergene U minerals were most widespread at around 60 m below ground (Figs. 5, 6). By contrast, in the Schwarzach area at the southern extremities of the transect (Fig. 6), supergene U minerals were found along road-side cuts at the surface, almost a few centimeters below ground in what might be called the present-day pedosphere and proximal to the Cenozoic to Mesozoic platform sediments in contact with the granitic basement rocks. 4.3. Mineralogy of supergene uranium minerals There are roughly 150 minerals known to accommodate U in their structure so as to call them uranium minerals. Apart from the black uranium minerals uraninite, coffinite and brannerite bearing uranium in the tetravalent state, the majority of minerals accommodate uranium in its hexavalent state and form a wide range of hydroxides, carbonates, chlorides, fluorides, sulfates, phosphates, hydrosilicates and arsenates (Table 2). Of these only a handful is of relevance outside the uranium mining districts and qualify for dating of geomorphological processes as

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Fig. 5. Weathering and hydrological zones in granitic terrains at the western edge of the Bohemian Massif. The physico-chemical regime is based upon themodynamical calculations, some of which are shown in Fig. 12. a) Massive uranophane-bearing silcrete at Pettendorf–Schwarzach Area (regolith/saprolite-vadose zone). b) Fine-grained torbernite lining of granitic fragments at Lauerhof–Schwarzach Area (regolith/saprock-vadose zone). c) Well-crystallized torbernite in cavities of the kaolinitized granitoids at the 60 m-level at Höhensteinweg–Poppenreuth (hydraulith/percolation zone–vadose zone). d) Well-crystallized saleeite coating of a siliceous fracture zone at the 100 m-level at Grossschloppen (hydraulith/infiltration zone–phreatic zone). e) Desilicified granite termed ‘episyenite’ at 144 m drill depth from Grossschloppen–Hebanz area (Fresh granitic host rock — per ascensum mineralization).

shown in the succeeding paragraphs (Table 2). At outcrop the identification and precise determination of uranyl phosphates is often fraught with difficulties. Therefore the minerals from the mineralized sites are described as accurate as possible below.

4.3.1. Torbernite Torbernite is the most common supergene U mineral in the study areas and may easily be identified in the field with the naked eye by its vivid green color and its characteristic crystal habits (Fig. 7). It occurs

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Fig. 6. Supergene U mineralization in relation to the (paleo) surfaces and (paleo) aquifers at the western edge of the Bohemian Massif. Color code for correlation with Figs. 3, 13 and Table 3. Zones refer to aerial image of Fig. 4a. a) U/Pb age data shown in boxes (million years, Ma). b) Altitude given in meter above mean sea level (a.m.s.l.). c) Supergene uranium minerals uranophane, autunite, torbernite, uranocircite and saleeite (see color code in the legend right on top). d) The stippled staircase lines reflect the present-day landscape, the dashed-and-dotted line illustrates the paleoslope towards the Alpine foreland basin south of the study area. e) Colored lines reflect the paleo-surfaces and paleoaquifers, respectively, with the stages given in each rectangle. See also Fig. 3 for a 2-D presentation of the paleogeography. f) Name of site is given in the header (precise locality see Fig. 1b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2 The most common secondary uranium minerals suitable for U/Pb dating in geomorphology that were used in the current study. For morphology of secondary uranium minerals as well as mineral color the reader is referred to Figs. 7–9. The chemical composition of minerals associated with supergene U minerals in each stage or referred to in the text are shown in Fig. 10. Mineral name

Chemical composition

Autunite Baryte Beraunite Brannerite Cacoxenite Chrysocolla Churchite-(Y) Coffinite Diadochite Goethite Kaolinite Langite Metaautunite Metatorbernite Metauranocircite Nontronite Pitchblende/uraninite Saleeite Strengite Torbernite Uranocircite Uranophane Uranophane-(beta) Vivianite Wavellite

Ca(UO2)2(PO4)2·11H2O BaSO4 Fe2+Fe3+5(PO4)4(OH)5·4(H2O) (U,Ca,Ce)(Ti,Fe)2O6 Fe18.75Al6.25(PO4)17O6(OH)12·75(H2O) (Cu,Al)2H2Si2O5(OH)4·nH2O Y(PO4)·2(H2O) U(SiO4)1-x(OH)4x Fe3+2(SO4)(PO4)(OH)·6(H2O) α-FeOOH Al2Si2O5(OH)4 Cu4(SO4)(OH)6·2H2O Ca(UO2)2(PO4)2·7H2O Cu(UO2)2(PO4)2·8H2O Ba(UO2)2(PO4)2·7H2O Na0.3Fe3+ 2 (Si,Al)4O10(OH)2·4H2O U3O8 / UO2 Mg(UO2)2(PO4)2·10(H2O) Fe3+(PO4)·2(H2O) Cu(UO2)2(PO4)2·12H2O Ba(UO2)2(PO4)2·10H2O Ca(UO2)2SiO3(OH)2·5H2O Ca(UO2)2SiO3(OH)2·5H2O Fe3(PO4)2·8(H2O) Al3(PO4)2(OH)3F0.5·5(H2O)

in stubby crystals and in form of tabular crystal aggregates which often are degraded to the less hydrated meta-form called metatorbernite (Fig. 7e, f, Table 2). 4.3.2. Autunite Autunite is second most in abundance to torbernite and stands out by its yellow color and fluorescence (Fig. 8a). It is often epitactically overgrowing onto torbernite leading to a faint change in mineral color from a green central zone towards a rim of canary yellow tint. The regular intergrowth of tabular autunite cored by torbernite is shown in the cartoon of Fig. 8b and made visible by the ultra-violet lamp. Autunite is younger than torbernite a fact also reflected by the age of formation of torbernite from Hagendorf (4.55 Ma) and the autunite– torbernite intergrowth from Flossenbürg (3.99 Ma). 4.3.3. Uranocircite Uranocircite is less common, compared with its Ca- and Cu analogues described before, and restricted to areas abundant in barium. This was the case in the Wölsendorf area, where barite is abundant (Fig. 8c, Table 3). It may also occur in normal granites lacking barite, such as in the Falkenberg Granite, where Ba has derived from the decomposition of K feldspar and white mica. 4.3.4. Saleeite Saleeite does not need a special parent rock to form because magnesium is among the major ten chemical components in the earth's crust and phosphate is of widespread occurrence too. The low Mg contents in granites may, however, impede its more widespread

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Fig. 7. Torbernite in the study areas on a macroscopic scale and illustrated in idealized line drawing to ease its identification in the field. a) Bipyramidal crystal of torbernite growing into an open cavity. Wölsendorf. b) Crystal morphology of panel a, using the Miller's indices for labeling of the faces. c) Stubby crystal of torbernite. Hagendorf (4.55 Ma). d) Crystal morphology of panel c, using the Miller´s indices for labeling of the faces. e) Tabular crystals of torbernite. Wölsendorf (0.2 Ma). f) Crystal morphology of panel e, using the Miller´s indices for labeling of the faces. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

occurrence (Fig. 9a). It was found in an intimate intergrowth with uranophane. Radiometric dating has proven the age relation recognized already in the hand specimen.

found too but due to its grain size spared from age dating. It occurs in whitish yellow sprays of acicular uranophane and in aggregates made up of slender prisms of yellow to brown β-uranophane (Fig. 9b,c).

4.3.5. Uranophane Uranophane, a hydrated uranyl silicate present in its normal and beta modification is as widespread as torbernite and frequently found together with uranyl phosphates in the Schwarzach area, Grossschloppen and Rudolfstein (Tables 2, 3). In Hagendorf and Wölsendorf uranophane was

4.4. Alteration minerals associated with supergene uranium minerals In the study areas, the majority of supergene U mineralization is monomineralic (e.g. Poppenreuth–Höhenstein: torbernite) to oligomineralic (e.g. Rudolfstein: torbernite, autunite, uranophane). A

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Fig. 8. Autunite and uranocircite in the study areas on a macroscopic scale and in idealized line drawing to ease their identification in the field. a) Yellow autunite epitactically overgrowing onto green torbernite under normal light. Flossenbürg (3.99 Ma). b) Tabular crystals of autunite(yellow) and torbernite (green) epitactically intergrown at Flossenbürg as shown in panel a. c) Plate of uranocircite under normal light. Wölsendorf (4.02 Ma). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

variegated spectrum of non-uranium minerals accompanying the supergene U minerals has only been observed at Hagendorf and Wölsendorf. It is mainly Fe(III) phosphates, which formed together with secondary U minerals (Fig. 10a, b, Table 3). Second in abundance are phyllosilicates which either belong to the kaolinite group or the dioctahedral Fe-bearing smectites (Tables 2, 3). In Wölsendorf Cu silicates have been found together with supergene U minerals. Goethite ubiquitous in the weathering zone is rather rare in mineral assemblages containing uranyl phosphates.

4.5. LA-ICP-MS dating of supergene uranium minerals Uranium/lead age data have been collected throughout various campaigns by the senior author in the NE Bavarian Basement (Dill, 1985; Dill et al., 2007). Two age data, obtained from LA-ICP-MS are recorded in Table 4 for reference to show the way of determination and the number of measuring sites in each sample. The ages of formation are illustrated in x–y plots in Fig. 2 to give an idea of the precision of LA-ICP-MS.

Author's personal copy H.G. Dill et al. / Geomorphology 117 (2010) 44–65 Table 3 Linking U/Pb data from supergene U minerals with weathering and geomorphological processes in granitic terrains and their foreland.

Site: Mineral: Age: Host rock: Alteration minerals: Stage: Interpretation:

Sampling site (see Fig. 1b). Supergene uranium minerals used for U/Pb dating in each site (see Figs. 7–9). Age of formation of supergene uranium minerals in Ma. Structural type of felsic host rock (see Fig. 11). Supergene minerals found in grain-to-grain contact with the U minerals suitable for determination of the physical-chemical conditions during the period of formation (for chemical composition see Table 2). Fourfold-classification scheme used in this text. Key weathering and geomorphological processes during each stage. First line normal characters = general processes, second line italics = regional processes typical of the western edge of the Bohemian Massif.

a) Stage I: Miocene palaeoaquifers in the granite and related to alluvial-fluvial deposits in the foreland. b) Stage II: Miocene peneplanation and chemical weathering of granite. c) Stage III: Miocene to (Pliocene) denudation of weathered granite and tor formation. d) Stage IV: Fluvial incision into exposed granite and modern aquifers.

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Fig. 9. Saleeite and uranophane in the study areas on a macroscopic scale. a) Dark green tabular crystal of saleeite (sa) (0.4 Ma) overgrowing on a matrix of older yellow uranophane (ur) (1.75 Ma). Grossschloppen. b) Sprays of whitish yellow uranophane. Wölsendorf. c). Aggregates of slender prisms of brown β-uranophane. Wölsendorf. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5. Discussion 5.1. Zonation of supergene uranium mineralization in weathered granitic rocks In view of the data obtained during the current study of supergene U mineralization in granites of the NE Bavarian Basement, we propose an amendment to the common sub-division of the weathering zone into saprolite and saprock, both of which are compiled under the header regolith (Taylor and Eggleton, 2001; Migoń, 2006). The regolith (Fig. 5a, b) is underlain by the percolation zone followed towards the depth by the infiltration zone (Fig. 5c, d), which passes into the granite unaffected by per descensum alteration processes (Fig. 5e). Percolation zone and infiltration zones constitute what might be termed the ‘hydraulith’ to show its equivalent level to regolith in the hierarchy of supergene alteration and to place emphasis on hydrological process as the principal mode of alteration along deep-seated fractures and orthogonal joint sets

in granitic rocks. A similar scenario was recorded by Kanz (1987), whose observations derived from 1.3 to 2.5 km deep drill holes, from Northern Switzerland as well as from a rock laboratory in the Central Swiss Alps (Aare granite). Our geomorphological–hydrological sub-division is not very much different from what has been reported from karst lithologies, where a zone of aeration called the ‘vadose zone’ is followed towards the depth by the zone of saturation, or ‘phreatic zone’ (Scoffin, 1987). Due to the lack of sedimentary aquitards in these massive granites, boundaries between the different zones or deeper ground-water flow systems cannot be drawn so precisely as in the sedimentary ground-water flow systems of the foreland (Fig. 5). 5.1.1. Saprolite The saprolite has been preserved from erosion at the western edge of the Bohemian Massif only in depressions and in some down-warped parts of the basement such as in the Schwarzach area, in neighboring Wölsendorf, and, locally, in some depressions of the Oberpfaelzer Wald,

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Fig. 10. Minerals associated with supergene U minerals on a microscopic and macroscopic scale. a) Cacoxenite overgrowing quartz under the scanning electron microscope (SEMEDX). Hagendorf. b) Bundles of acicular beraunite crystals in a quartz druse viewed under the stereomicroscope. Hagendorf.

where kaolin is still mined (Figs. 4a, 6). It shows a ‘grusy’ texture similar to what has been described by Migoń and Thomas (2002) who used this term as an azonal lithological term without any connotation, neither to the bedrock lithology nor the way its rock-forming minerals respond to weathering. Without any sharp contact, the grusy material merges with the substrate of the pedosphere (Fig. 4e). Supergene siliceous U mineralization with uranophane prevailing over other secondary U mineral has been referred to during this study as uraniferous silcrete, a term introduced by Summerfield (1983a,b) for siliceous near-surface encrustations. Vice versa, if torbernite or other members of uranyl phosphate prevail over uranyl silicates, uraniferous phoscretes is a more appropriate term, as demonstrated by the U-bearing duricrusts in Sudan (Dill et al., 1991) (Fig. 5a). 5.1.2. Saprock The saprock underneath the saprolite still contains grusy material but its supergene U mineralization is very much different from that of the saprolite. Uranyl phosphates line the surface of fragments of altered granites or infill the fissures in granitic debris (Fig. 5b). Mineralization is still fine-grained and uranyl phosphates occur mainly in their metaforms (e.g. dehydrated metatorbernite) in what might be called a ‘fitting breccia’. The good preservation of the original direction of microjoints and fissures with the granitic pieces still fitting together rule out that these boulders have been subject to gravitationally forced mass movements such as talus or soil creep. 5.1.3. Percolation zone The percolation zone of the hydraulith contains supergene U mineralization in argillized (kaolinized or smectitized) granites with uranyl phosphates in well-crystallized minerals growing freely into cavities and fissures (Fig. 5c). This upper zone of the hydraulith forms still part of the vadose zone with a clear per descensum flow direction. No per ascensum mineralization could be spotted at this level. 5.1.4. Infiltration zone The infiltration zone is similar to the percolation zone with respect to the crystal morphology and size of U minerals but different with respect to the argillitization of the wall rocks. Per descensum U mineralization is more penetrative than the alteration of silicates towards depth. Uranyl phosphates are a more sensitive indicator for supergene alteration than the rock-forming siliceous minerals. Within the infiltration zone the boundary between the vadose and phreatic zones is crossed (Fig. 5d). The oxygen fugacity–see succeeding chapters on the Eh and pH regime–

drops significantly due to the stagnancy in the water movement. This hydrological zone in altered granites is equivalent to the well known cementation zone sensu Schneiderhöhn (1924) and Smirnov (1955), the lowermost sub-zone of the gossan atop sulfidic ore deposits, where reducing conditions provoke supergene sulfides such as covellite, chalcocite and Ag minerals to precipitate. 5.1.5. Fresh granite In the underlying granite, secondary porosity may still be observed. Unlike above, porosity and permeability were caused by per ascensum processes such as episyenitizaton, a pervasive desilicification process in the wake of granite intrusions during the PermoCarboniferous but unrelated to modern supergene alteration or geomorphological processes (Dill, 1983) (Fig. 5e). 5.2. Lithological control on granite alteration Based on morphology, internal structure and the contact between felsic igneous and metamorphic country rocks, the granitic host rocks have been sub-divided into three litho-types which respond in quite a different way to chemical weathering (Table 3, Fig. 11). 5.2.1. Granites The majority of granitic rocks were emplaced with a sharp contact between the felsic igneous stocks and their surrounding metamorphic country rocks (Figs. 4a, 12a). The granites extending over tens of kilometers stand out from the hilly landscape and form coherent ridges as it is the case with the granites of the Fichtelgebirge, a representative of which is the Rudolfstein (Fig. 4a). Other Late Variscan granites developed isolated domes such as at Flossenbürg while the surrounding granites similar in chemical composition and lithology are leveled by peneplanation processes and blanketed by a thick weathering mantle of kaolin (Teuscher and Weinelt, 1972; Richter and Stettner, 1979; Wendt et al., 1994; Breiter and Siebel, 1995) (Fig. 4b). The overall mineralogical and chemical composition of granites is of no influence on the disintegration process. As a consequence, a wide range of supergene alteration profiles from fully-developed sections covering the regolith and hydraulith to truncated profiles developed (Fig. 4e). 5.2.2. Granitoids and aploids A swarm of granitic lenses some tens of meters to 100 m in thickness and concordant with the metamorphic fabric of the country rocks have been hit by drill holes and exposed by mining operations

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Table 4 Analytical protocol of U/Pb age dating obtained by means of LA-ICP-MS (on-line version only). Site number

207 Pb (cps)

Thc U

206 204

Pb Pb

206 238

Pbe U

207

2σ %

235

Pbe U

207

2σ %

206

Pbe Pb

2σ %

Rho

206 238

Pb U

2σ (Ma)

207 235

Pb U

2σ (Ma)

a. Flossenbürg–Torbernite-autunite a29 10,573 b0.001 a30 10,194 b0.001 a31 9931 b0.001 a32 31,975 b0.001 a33 17,295 b0.001 a35 9559 b0.001 Gj1-8 9919 b0.001 a36 1726 b0.001 A1 9051 b0.001 A2 12,533 b0.001 A3 12,347 b0.001 A4 9426 b0.001 A5 12,197 b0.001 A6 11,016 b0.001 A7 8822 b0.001 A8 19,636 b0.001 A9 18,021 b0.001 A10 7888 b0.001 A11 7015 b0.001 A16 17,801 b0.001 A17 11,343 b0.001 A18 21,128 b0.001 A19 27,960 b0.001 A20 24,972 b0.001

11,187 3854 6291 3145 9474 9393 5342 1004 1570 2802 3218 1540 479 2789 2142 274 500 1009 1529 2549 1700 1858 3466 1331

0.000361 0.000150 0.000170 0.000619 0.000203 0.000432 0.000514 0.000156 0.000547 0.000605 0.000600 0.000477 0.000362 0.000529 0.000476 0.000531 0.000607 0.000626 0.000521 0.000449 0.000634 0.000620 0.000619 0.000461

3.6 13 13 3.5 10 6.0 4.4 7.2 2.4 3.5 3.0 2.9 3.2 2.4 3.1 4.9 5.4 3.4 8.5 3.4 2.5 2.7 3.5 3.6

0.00237 0.00094 0.00109 0.00406 0.00137 0.00273 0.00329 0.00106 0.00353 0.00386 0.00384 0.00301 0.00232 0.00338 0.00301 0.00338 0.00387 0.00396 0.00329 0.00289 0.00403 0.00393 0.00390 0.00292

7.5 14 15 6.6 20 8.5 8.4 19 3.7 4.1 4.0 4.4 5.3 3.3 4.2 7.2 6.2 5.1 9.4 5.7 5.5 6.8 5.4 5.8

0.0477 0.0455 0.0465 0.0476 0.0488 0.0459 0.0463 0.0493 0.0468 0.0463 0.0465 0.0457 0.0465 0.0463 0.0459 0.0462 0.0463 0.0459 0.0458 0.0467 0.0461 0.0459 0.0457 0.0459

6.6 5.4 6.6 5.6 17 6.1 7.2 18 2.9 2.2 2.6 3.3 4.2 2.2 2.7 5.3 3.2 3.8 4.1 4.6 4.9 6.3 4.1 4.6

0.48 0.92 0.90 0.54 0.50 0.70 0.52 0.38 0.64 0.85 0.76 0.66 0.60 0.74 0.76 0.68 0.86 0.68 0.90 0.59 0.45 0.39 0.66 0.61

2.32 0.97 1.09 3.99 1.31 2.78 3.32 1.00 3.53 3.90 3.87 3.08 2.33 3.41 3.07 3.42 3.91 4.03 3.36 2.89 4.09 4.00 3.99 2.97

0.08 0.13 0.15 0.14 0.13 0.17 0.14 0.07 0.08 0.13 0.12 0.09 0.07 0.08 0.10 0.17 0.21 0.14 0.29 0.10 0.10 0.11 0.14 0.11

2.41 0.96 1.10 4.12 1.39 2.77 3.33 1.07 3.58 3.92 3.90 3.05 2.35 3.42 3.05 3.43 3.93 4.01 3.33 2.93 4.09 3.98 3.95 2.96

0.18 0.13 0.17 0.27 0.28 0.24 0.28 0.20 0.13 0.16 0.15 0.13 0.12 0.11 0.13 0.25 0.24 0.20 0.31 0.17 0.22 0.27 0.21 0.17

b. Wölsendorf–Uranocircite a4 77,475 a5 98,536 a6 121,784 a7 92,105 a8 67,790 a9 144,119 a10 56,331 a11 23,533 a12 82,256 a13 10,526 A21 13,221 A22 76,261 A23 66,855 A24 88,474 A25 56,200 A26 44,981 A27 18,454 A28 13,933 A29 18,783 A30 40,257 A17 112,966 A18 67,414 A19 238,095 A20 71,242 A21 73,642 A22 91,561 A24 110,066 A25 100,682 A39 10,162 A40 20,087 A41 23,828 A42 34,159 A43 36,362

9719 12,952 8474 29,122 37,355 11,707 6462 3432 17,459 17,808 2207 1278 1706 3689 2063 4009 3273 42,783 41,283 25,441 2075 1393 1954 1698 1963 3899 6175 3463 8542 8444 11,710 4253 3982

0.000499 0.000519 0.000091 0.000635 0.000308 0.000331 0.000310 0.000254 0.000638 0.000484 0.000584 0.000609 0.000374 0.000515 0.000343 0.000545 0.000623 0.000615 0.000638 0.000382 0.000171 0.000286 0.000489 0.000195 0.000284 0.000362 0.000378 0.000490 0.000611 0.000604 0.000630 0.000457 0.000236

2.6 3.5 15 4.2 4.1 4.2 9.9 5.1 5.0 3.7 3.6 3.2 2.2 3.3 4.2 1.9 1.9 2.4 2.1 2.2 8.3 2.1 7.0 9.5 6.7 6.3 4.2 3.3 4.3 3.1 2.8 4.1 5.4

0.00346 0.00353 0.00057 0.00417 0.00205 0.00218 0.00194 0.00156 0.00401 0.00291 0.00371 0.00404 0.00239 0.00329 0.00221 0.00341 0.00393 0.00393 0.00398 0.00251 0.00103 0.00186 0.00302 0.00139 0.00204 0.00247 0.00249 0.00335 0.00387 0.00384 0.00392 0.00313 0.00160

15 8.3 34 5.8 6.9 8.7 15 11 12 8.1 5.1 12 8.5 6.3 5.5 3.8 3.6 3.4 3.7 4.8 21 13 16 19 17 15 17 14 12 7.3 12 14 16

0.0504 0.0494 0.0455 0.0477 0.0483 0.0476 0.0452 0.0445 0.0456 0.0436 0.0460 0.0481 0.0465 0.0463 0.0466 0.0454 0.0458 0.0464 0.0452 0.0476 0.0439 0.0472 0.0447 0.0517 0.0519 0.0494 0.0477 0.0496 0.0459 0.0461 0.0451 0.0498 0.0492

14 7.5 30 4.0 5.5 7.6 12 10 10 7.3 3.6 12 8.2 5.4 3.6 3.3 3.1 2.4 3.0 4.3 19 13 14 17 16 14 16 14 12 6.6 12 13 15

0.18 0.42 0.45 0.72 0.60 0.48 0.65 0.45 0.44 0.45 0.70 0.26 0.26 0.52 0.75 0.51 0.52 0.72 0.57 0.45 0.40 0.16 0.44 0.49 0.39 0.41 0.25 0.23 0.34 0.42 0.22 0.29 0.34

3.21 3.35 0.59 4.09 1.99 2.14 2.00 1.64 4.11 3.12 3.77 3.93 2.41 3.32 2.21 3.51 4.01 3.96 4.11 2.46 1.10 1.84 3.15 1.26 1.83 2.34 2.44 3.16 3.94 3.90 4.06 2.94 1.52

0.08 0.12 0.09 0.17 0.08 0.09 0.20 0.08 0.21 0.11 0.13 0.12 0.05 0.11 0.09 0.07 0.08 0.10 0.09 0.05 0.09 0.04 0.22 0.12 0.12 0.15 0.10 0.10 0.17 0.12 0.11 0.12 0.08

3.51 3.58 0.58 4.23 2.08 2.21 1.96 1.58 4.07 2.95 3.76 4.09 2.43 3.33 2.24 3.46 3.98 3.98 4.03 2.54 1.05 1.89 3.06 1.41 2.06 2.50 2.52 3.40 3.92 3.90 3.97 3.18 1.62

0.51 0.30 0.20 0.25 0.14 0.19 0.30 0.18 0.47 0.24 0.19 0.50 0.21 0.21 0.12 0.13 0.14 0.13 0.15 0.12 0.22 0.24 0.48 0.27 0.36 0.38 0.42 0.48 0.49 0.28 0.49 0.44 0.26

b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001

near Poppenreuth (Fig. 11b). The aplitic and granitic mobilizates as well as their metamorphic country rocks have been capped by erosion and leveled likewise. The lit-par-lit structure of metamorphic and igneous rocks gave rise to a regolith less well-developed than on top of the aforementioned granites, while the hydraulith is equally welldeveloped as in the larger granitoids. 5.2.3. Pegmatites and aplites The pegmatitic and aplitic stocks of the Hagendorf–Pleystein pegmatite province differ significantly from the aforementioned felsic

intrusive rocks by their contrasting response to chemical weathering (Fig. 11c, d). The highly differentiated felsic rocks with potassium feldspar enveloping a quartz core and measuring up to a few hundred meters across are the most conspicuous representatives of “Boldt's Lithovarianz” (geomorphological changes as a function of rock strength) (Schmidt, 1955; Uebel, 1980; Boldt, 1998; Mücke, 2000). Where these stocks were deprived of their metamorphic roof rocks, as at Pleystein, weathering has completely replaced the less-resistant feldspar rim and sculptured the quartz core out of the flattened landscape (Fig. 4c, d). The weathering profile in these felsic rocks is

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Fig. 11. Structural types of granites hosting supergene U minerals at the western edge of the Bohemian Massif and their surface exposure. a) Discordant intrusive stocks in places with contact aureole — scale 1 to 10 km, e.g., Schwarzach area, Flossenbürg, Grossschloppen. b) Concordant granitic (granitoid) and aplitic (aploid) lenses and layers within metamorphic rocks-scale 10 to 100 m, e.g., Poppenreuth. c) Highly differentiated quartz–feldspar pegmatites and aplites — scale 50 to 100 m, e.g. Hagendorf–Pleystein pegmatite province. d) View towards the quartz core of the siliceous Kreuzstein pegmatite at Pleystein. It is a zoned feldspar–quartz pegmatite whose feldspar rim was eaten away by erosion and the siliceous core now protrudes out of the flattened landscape. See also cross section of panel c for petrography.

incomplete and, if at all present, only in the hydraulith supergene U minerals may be recognized.

5.4. Physical–chemical conditions of supergene alteration in granitic terrains

5.3. Hypogene versus supergene granite-hosted U mineralization

Uranium mobilization and mineral precipitation in oxidized bedrock aquifers have been looked at from different angles, including environmental aspects such as U stabilization and weathering of U minerals containing it in its tetravalent state (Langmuir, 1978; Arey et al., 1999; Murakami et al., 2001; Jerden and Sinha, 2003). The activity diagrams which show the stability fields of the main ‘water level gauges’ were calculated using data of ground-water chemistry published by Jerden and Sinha (2003) because no similar hydrological source was available for the study area in NE Bavaria (Fig. 12). Hydrothermal activity has been ruled out based on mineralogical grounds in the previous section, and all Eh–pH diagrams were therefore calculated for near-ambient conditions at 25 °C, using the dissolved species as log aFe = −3 and log aHPO4 = − 5 for the Fe–P mineralization, which represents the common Fe phosphates associated with supergene U minerals. For U–P mineralization representing torbernite, autunite, saleeite and uranocircite the dissolved species were assumed to be log aU = − 7 and log aHPO4 = − 5 and for the U–Si mineralization containing uranophane the dissolved species were assumed to be log aU4+ = −7, log aSiO2(aq) = −3 and log a ca2+ = − 3. For uranophane two different stability fields with contrasting Eh and pH values exist in the study area (Fig. 6). For uranophane-bearing silcretes from the Schwarzach area which were scattered in the saprolite, the Eh is assumed to be above 0 but the pH values are below 6 (Figs. 5a, 12). Uranophane from Grossschloppen came into being in the infiltration zone where the sporadic presence of pyrite is indicative of stagnant ground-waters and an Eh b 0. In this case, the pH is likely to have been greater than 6 at this hydrological level in the granitic terrain. Soddyite does not occur in the chemical systems under study

Hypogene versus supergene is a fundamental issue, a ‘conditio sine qua non’ to link the alteration pattern in the granitic rocks, discussed in the first section (5.1), with the geomorphological processes shaping the granitic terrains. There is still a hot debate about what might have triggered kaolinization in granites. Is this kind of alteration per ascensum (hypogene) in origin, provoked by hydrothermal solutions, or is it per descensum (supergene) related to weathering processes and near-surface hydrological processes? A case in point is the granite-hosted kaolin deposits in SW England, which during discussion of its origin saw a to and fro, from hypogene to supergene, and currently are supposed to be a mixture of the two alteration processes (Sheppard, 1977; Bristow and Exley, 1994; Psyrillos et al., 1998). Unlike kaolinite, there is no proven evidence of uranyl phosphates in granitic host rocks to have been precipitated from hydrothermal solutions. Only uranophane has once been reported to be primary in origin in volcanic rocks from the Ambrosia Lake Uranium District, Grants Mineral Belt, USA (Brookins, 1981). Therefore, the study of U minerals accommodating hexavalent U in their lattice as they were displayed in Figs. 7–9 are a reliable tools to chronologically constrain supergene processes such as weathering or shaping the landscape in granitic terrains. Moreover the physical and chemical conditions of the supergene alteration in granitic terrains may be constrained and illustrated in pH–Eh diagrams which by themselves furnish clear evidence that the stability fields of uranyl phosphates and hydrated silicates are considerably reduced in nature under increasing temperatures — see Section 5.4 (Fig. 12).

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and precludes a stronger alkalinity of the mineralizing-fluids (Fig. 12c). Supergene Cu-, Ca-, Ba- and Mg-uranyl phosphates theoretically exist over the full Eh and pH range, in practice, however, their presence is limited to a pH around 7. In Fig. 12b the stability field of these phosphates, highlighted in green, is limited towards higher alkalinity and elevated Eh values. At Eh N 0.1 V, which closely corresponds to the transition from bivalent Fe to trivalent Fe, the stability fields of uranyl phosphates span from pH 5 to 7 (Jerden and Sinha, 2003). This has some implications for the pH range of the percolation zone and the saprock in the working area where uranyl phosphates are of widespread occurrence. Vivianite and strengite, representative of Fe(II) and Fe(III) phosphates, respectively, and found together with uranyl phosphates, are obviously not syngenetic with uranyl phosphates (Fig. 12a). Otherwise their stability fields should overlap each other (Fig. 12a, b). Iron-(II) phosphates such as vivianite formed earlier and deeper relative to the uranyl phosphates in what is called the infiltration zone of the granitic hydraulith, whereas iron (III) phosphates such as strengite formed later under more elevated Eh values in the percolation zone or even higher up in the saprock. This hydrological stratification can only be used for massive rocks like granites and must not be transferred to sediments, were local aquitards may create a totally different picture and earthy vivianite may even develop a few meters below ground or in bottom sediments of lakes and swamps, that is to say, poorly-aerated sedimentary environments (Manning et al., 1991; Medrano and Piper, 1997). Redox conditions determined for the various hydrological zones and the fluid movements are the logic for the definitions of the subzones of the hydraulith. In the percolation zone meteoric waters travel through with depth causing fluctuating Eh values (≤ 0). The infiltration zone is the lower section of the hydraulith with no further movement downward and as a result of this a Eh b 0 is observed in this stagnant hydrological regime. 5.5. Geomorphological and paleohydrological evolution of granitic terrains at the western edge of the Bohemian Massif during the Neogene and Quaternary Supergene U mineralization encountered at various levels in the hydraulith and regolith of granites allow for the establishment of a four-step model which depicts the geomorphological evolution of landforms in the granitic terrain at the western edge of the Bohemian Massif but which may also be tested on a larger scale for the European Variscides or even further afield (Fig. 1a). Therefore geomorphological and hydrological interpretations are kept separate as to their regional and general implications with respect to shaping the landscape (Table 3). 5.5.1. Stage I The Rudolfstein U mineral assemblage formed within the infiltration zone of the hydraulith at the flanks of a mountain ridge underlain by the massive but highly differentiated ‘Tin Granite’ with U contents in the range of 6–17 ppm U, and topped by tors which were completely stripped of its regolith during denudation (Richter and Stettner, 1979) (Figs. 4c, 6, 13a). It is the exposed part of an intrusive granite complex forming the horse-shoe-like coherent mountain ridge of the Fichtelgebirge with altitudes of the present-day surface between 900 and 1000 m a.m.s.l. (Fig. 4a zone A, 6, 11a). The age of Fig. 12. Eh–ph diagrams of supergene U phosphates and silicates in granitic terrains and Fe phosphates associated in space with these ‘water level gauges’. Assumed concentrations of dissolved species are given below in mol/l. a) Eh–pH diagram to show the Fe–P mineralization at 25 °C using the dissolved species as log aFe = − 3, log aHPO4 = − 5. b) Eh–pH diagram to show the U–P mineralization at 25 °C using the dissolved species as log aU = − 7, log aHPO4 = − 5. c) Eh–pH diagram to show the U–Si mineralization at 25 °C using the dissolved species as log aU4+ = − 7, log aSiO2(aq) = − 3, log a ca2+ = − 3.

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formation of 8.4 Ma is exceptional for the region and there are no other sites of supergene U mineralization within the granitic terrain to correlate with, except some gravel beds at about 460 a.m.s.l. spread across a high-altitude platform terrace in the down-warped foreland which were dated by geological and geomorphological means (Drexler, 1980) (Fig. 13a). In view of the polycyclic evolution of the central European landscape, these alluvial–fluvial deposits have been correlated with Büdel's hotly debated “sarmato-pontische Rumpfläche” (peneplain of late Miocene age) (Drexler, 1980; Semmel, 1996). However, there are neither palaeontological nor physical age data available for this postulated late Miocene landform type. The only proven sedimentary record is located within the Alpine Foreland Basin bounding the uplifted Alpine mountain ridge along its northern rim and taking up the debris from both source areas, the rising Alpine mountain chain as well as the south-western part of the truncated block of the Variscan Bohemian Massif. Between approx. 17 and 5 Ma, siliciclastic sediments of the Upper Freshwater Molasse debouched into the Alpine foreland basin. The paleoslope and the paleoaquifers in the granitic terrain of the Bohemian Massif dipped gently towards the south where an embankment, called the “Regensburg Strait”, was extending northward along the western edge of the Bohemian Massif (Fig. 3a). Moser et al. (2009) have described the climatic and ecological situation based on a continental mollusc fauna. It was an open scrubland with temporary waters and damp forests with abundant litter and rotting wood surrounding a perennial lake with small tributaries. This ecological and paleogeographic setting may also

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be transferred to the nearby etchsurface truncating the granitic terrain of the Bohemian Massif at 8.4 Ma. The paleoclimatic conditions during the Neogene may be compared with the present-day climatic conditions in (sub)tropical Africa. Stage I represents a relic granitic landscape with high-altitude divides topped by tors and high-altitude platform terraces equivalent to the main valley floors (Fig. 4a-zone A, 14a). The main valley floors are interpreted in terms of a dissected etchplanation surface corresponding to a base level of a swampy lake basin in the foreland of the rising Alpine mountain chain (Table 3). 5.5.2. Stage II Unlike in stage I, in stage II the granitic landscape is well preserved at an altitude between 400 and 700 m a.m.s.l. with a complete set of planar (stage IIa) and domal (stage IIb) geomorphic features referring to a mature peneplain (Fig. 6). The paleohydrological system of stage II still correlates with the base level of the swampy lake basin in the Alpine foreland and, consequently, the paleoclimatic conditions concluded from the paleoecological/climatological studies in the foreland may also be transferred to the geomorphological processes shaping the landscape between 6 and 4 Ma in the granitic terrain (Fig. 3a). The debris are washed into the depression stretching northward along the western edge of the uplifted basement (Fig. 3a). This study is beyond providing a new model of etchplanation or pediplanation. The classical models elaborated by Penk, Davis, King and Buedel have been summarized in textbooks and comprehensive papers on geomorphology and sometimes fiercely debated and

Fig. 13. Cartoons to illustrate the stage-wise geomorphological and hydrological evolution in a granitic terrain. For color codes see also Figs. 3 and 6. Not to scale as to altitude and distance. a) Stage I: Relic granitic landscape with high-altitude divides and high-altitude platform terraces, defined by U mineralization in the infiltration zone. Miocene relic peneplain with palaeoaquifer in the granites related to alluvial fluvial deposits in the foreland. Correlated with fluvial–lacustrine deposits of the ‘Freshwater molasse’ of the Alpine foreland basin. Subtropical climate See Fig. 4a-zone A. b) Stage IIa: Formation of supergene alteration surfaces (etch surface, base of regolith) and encrustations (silcrete, phoscrete), defined by U mineralization in the infiltration zone and regolith. Miocene peneplain with palaeoaquifer in the granite related to the fluvial Urnaab drainage system. Correlated with fluvial–lacustrine deposits of the ‘Freshwater molasse’ of the Alpine foreland basin. Subtropical climate See Fig. 4a-zones B and C. c) Stage IIb: Formation of domal landforms (inselbergs, bonhardts, quartz ridges), defined by U mineralization in the percolation zone and saprock. Pliocene peneplain with palaeoaquifer in the granites related to the fluvial Urnaab drainage system. Correlated with fluvial–lacustrine deposits of the ‘Freshwater molasse’ of the Alpine foreland basin. Subtropical climate See Fig. 4a-zones B and C. d) Stage III: Denudation of regolith and tor formation in granitic terrains, triggered by an increase of the rate of incision in the trunk river systems, defined by U mineralization in the percolation zone and saprock. Unidirectional flow and fluvial erosion speed up denudation and exposure. Development of first-order granitic landforms. (Pliocene to ) Pleistocene denudation of weathered granites and tor formation is related to the Urnaab–Urdonau drainage system See Fig. 4a-zones B and C. e) Stage IV: Fluvial incision into exposed granites and development of weathering pits, defined by U mineralization in the percolation zone and saprock. Bifurcation of flow systems retard overall erosion. Development of secondorder granitic landforms. Pleistocene to Holocene fluvial incision into exposed granites by the Main and Naab river drainages system. Separation of the Naab–Donau and Main–Rhein drainage systems. See Fig. 4a-zones A, B and C.

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Fig. 13 (continued).

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amended by amongst others Ritter (1988), Thomas (1994), Summerfield (1996), Cui et al. (1999), Coltori and Ollier (2000), Phillips (2002), Twidale (2002) and Babault et al. (2005). In the following the low-relief surface gently dipping to a base level in what might be called the wet–dry morphoclimatic zone sensu Tricart and Cailleux (1958) is termed as peneplain or as surface of low-relief. Only those processes which we can rely on owing to age dating and physicochemical calculations are included in this term. This almost plain surface is defined height-wise by the phoscretes of torbernite in the infiltration zone at Grossschloppen and the uraniferous silcretes in the today reworked regolith of the Schwarzach area near the base level where the Urnaab river drainage system entered the Alpine foreland basin (Figs. 3a, 5, 6). The change from a U mineralization in the infiltration zone towards a similar mineralization in the regolith is in line with the paleocurrent and dip of paleoslope in the area. The paleoaquifer linking the two sites marks the base of the supergene alteration front, which some may call etchsurface or base of regolith (Fig. 13b). Despite a subtropical climate two distinct types of encrustations came into existence solely controlled by local variation in the pH–Eh regime (Fig. 12). Phoscretes and silcretes are azonal encrustations which formed irrespective of climate and lithology (Summerfield, 1983a,b). They are exclusively controlled by the local availability of phosphate or solubility of silica on a local scale. Uraniferous calcretes, however, have some implications as marker for an arid climate or desert depositional environments as demonstrated by personal observations. Absence of these types of uraniferous and vanadiferous duricrusts in the study area in NE Bavaria definitely rules out arid conditions for the Neogene in this area. Around 4 Ma during stage IIb, domal landforms were exposed giving rise to inselbergs (Fig. 4a zone B, b) and quartz ridges (Figs. 11c, d, 13c). All ages of formation of stage II fall in the same interval but they are different with respect to the hydrological zones which they pertain to. They are located in the regolith near the base level and deep below in the infiltration zone in the hinterland and formed in the percolation zone at an intermediate position on the inselbergs in the area between the two (Figs. 5, 6). Meteoric fluids circulated within the percolation zone and in the saprock precipitating hexavalent U in form of phoscretes. Isolated inselbergs, at Flossenbürg or the quartz ridge in the town center of Pleystein are landform types which developed at an intermediate position between the high-altitude mountain ridges of the Fichtelgebirge in the NW, shaped during stage I, and the lowlands of the Oberpfaelzer Wald at the basement-foreland boundary near the Schwarzach area, where alluvial–fluvial deposits of the Neogene Urnaab river encroached upon the granitic terrains (Figs. 3a, 6, 13c). It corroborates the ideas of Bremer (1993) for a long-term planation on an etchplain slightly tilted away from a mountainous hinterland towards the base level with the regolith best preserved in the lowlands (Fig. 4a zone B and C). Reviewing the textbooks from King's geomorphological studies to the present ones, reveals a great deal of uncertainty as to the duration of geomorphic processes or land-forming processes, in particular. A simple comparison of stage IIa and IIb processes, allows for a precise timing of planation and exposure of domal structures, which lies in the order of 1 to 2 Ma (Fig. 13b, c). 5.5.3. Stage III A comparison between Fig. 13c and d reveals only gradual changes between the geomorphological processes operative during stages IIb and III. Denudation of the regolith was brought almost to completeness in the NW and formation of tors reached its climax state (Fig. 13c). The paleohydrological system is defined by U mineralization in the percolation zone and saprock, yielding a process-related age of formation at 1.75 Ma and 1.25 Ma, respectively. Exposure of tors was largely completed as shown by the position of the sampling sites of U secondaries in this tor landscape. Sculpturing the granitic landscape with tors and boulder-strewn highlands was mainly controlled by a significant change in the fluvial drainage system in

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the foreland, where the Urmain river entrenched into the Jurassic to Triassic platform sediments far west and the Urnaab river developed a fluvial channel system in the Cretaceous and Neogene beds next to the granitic basement, both of which were draining towards south into the Urdonau trunk river which swept its way towards the Black Sea approx. 1.5 Ma from today (Fig. 3b) (Zöbelein, 1991; Villinger, 1998). Unidirectional flow of two major river systems to a nearby master drainage system of the Donau river speeded denudation and exposure up in the granitic basement, enabling meter- to decameter-sized firstorder landforms such as tors to develop through the outwash of granitic debris. It has to be emphasized that infiltration and percolation zones as well as saprock and saprolite also stand for hydrological facies zones which have to be considered first and foremost in context with their position relative to the base level, paleocurrent and paleoslope and on a smaller scale also in relation to the hydrological regional evolution in the foreland. Only this joint action ties up the uranium secondary minerals/geologic clock with the geomorphology of a certain region. 5.5.4. Stage IV A significant change in the hydrological system took place during this stage with consequences on the type and size of granitic landforms. The hydrological system pertains to different hydrological facies zones (Table 3). In the hinterland the paleoaquifer forms part of the sub-surface percolation zone while in the foreland weathering pits came into existence at the surface. Towards the foreland coeval U mineralization in near-surface saprocks denotes a paleoaquifer which was responsible for near-surface morphological alterations called weathering pits in the granites. These hydrological and geomorphologic processes in the granitic basement may be correlated with fluvial processes in the drainage systems in the foreland. By and large, correlation of U mineralization in the granitic landscape with the foreland allows for a precise correlation of hydrogeological (subsurface water), hydrological and near-surface geomorphological processes with sedimentological processes. Bifurcation of the drainage systems, one called Naab river still draining down to the nearby Donau river and another called Main river linked up with the Main– Rhein drainage system, came into being around 0.1 to 0.4 Ma (Figs. 3b, 13e). As a result of this change in the hydrological base levels, the overall fluvial erosion and denudation were retarded and meter-sized weathering pits–fourth-order landforms–and pseudokarst features preferably developed instead of the aforementioned landforms (Fig. 4d). The mechanism of these metric-scale flat or bowl-shaped hollows has been extensively studied by Thiry et al. (1988), Thiry and Maréchal (2001) and Thiry (2005). In the study sites of the Oligocene Fontainebleau Sand in the Paris Basin, carvings showing human outlines, animals and symbolic geometric figures were found, the oldest of which were ascribed to the Upper Paleolithic (b0.05 Ma). Stage IV is the interval where U/Pb radiometric measurements of U minerals and bio- and cosmochemical dating methods overlap. It cannot be neglected that as geomorphological processes becoming younger, the classical dating techniques recorded in the ‘introductory section’ get an edge over U/Pb dating as to accuracy and precision (Bell et al., 1998; Lee and Parsons, 1999; Hall and Phillips, 2006). 6. Conclusions Uraniferous duricrusts can be used as ‘natural clock’ and as a marker for the alkalinity/acidity of meteoric/per descensum fluids and to constrain the redox conditions. Sculpturing a granitic terrain used to be a polycyclic process and controlled by the interplay of hydrology and weathering allowing for a sub-division into four vertical supergene alteration zones (Fig. 5), four stages, reflecting planation and incision of granitic terrains through time (Fig. 13); and three geomorphological areas, reflecting the aerial distribution of landforms (Figs. 4a, 13, Table 3).

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Stage I is defined by U mineralization in the infiltration zone and provides a mirror image of the relic granitic landscape with highaltitude divides in the basement and high-altitude platform terraces in the foreland. Its characteristic features are preserved in the uplifted hinterland in the uppermost part of a peneplain. Stage II is defined by U mineralization in the infiltration zone, regolith and saprock, respectively, and characterized by two subprocesses; planation and exposure. Its domal structures are well preserved but isolated and found off the escarpment of the granitic hinterland. Regolith (kaolin) may today be found only in depressions near the base level. Stage III and IV, are both defined by U mineralization in the percolation zone and saprock. They are a function of base level lowering in the foreland. Rapid incision leads directly to pinnacle-like tors and the formation of large first-order granitic land forms, whereas a slow-down of fluvial incision favors their destruction and the development of weathering pits of different kinds. The common sub-division of the weathering mantle or regolith into saprolite and saprock is supplemented by the hydraulith (percolation and infiltration zones) underneath. It is affected exclusively by per descensum fluids and bridges the gap between the vadose and phreatic hydrological level in the granitic terrain. The Eh values tend to decrease with depth, whereas the pH increases. Depth and intensity of supergene alteration are strikingly related to the regional hydrological evolution and lowering of the base level. A full blown cycle of planation and incision lasts for approx. 10 Ma under the subtropical conditions in the granitic terrain under study. Areas that underwent a polycyclic process of planation and incision, such as central Africa, show a much more reliefed/staircase landscape than central Europe with its geomorphological processes verifiably dating back to the Paleogene or even to the Cretaceous (Dill et al., 2005). A stage which covers planation and exposure, resulting in the formation of domal structures such as inselbergs lasted under the subtropical paleoclimatic conditions during the Neogene for a period of time of as much as 2 Ma. Climate is an important but not the most decisive factor for shaping the landscape. Encrustations such as phoscretes and silcretes have to be discredited as climatic markers (Dill, 1988). Acknowledgment We are indebted to F. Korte for doing the chemical analyses with XRF, D. Klosa who carried out the SEM analyses and D. Weck who performed the XRD analyses. These investigations were carried out in the laboratories of the Federal Institute for Geosciences and Natural Resources in Hannover, Germany. Their contributions are kindly acknowledged. We express our gratitude to two anonymous reviewers who reviewed this paper for Geomorphology and to Andrew Plater for his editorial handling of our paper. The senior author dedicates this paper to his teacher in geomorphology, the late Professor Dr. J. Büdel (Würzburg University, Germany). References André, M.-F., 2004. The geomorphic impact of glaciers as indicated by tors in North Sweden (AuriVaara, 68°N). Geomorphology 57, 403–421. Arey, J.S., Seaman, J.C., Bertsch, P.M., 1999. Immobilization of uranium in contaminated sediments by hydroxyapatite addition. Environmental Science and Technology 33, 337–342. Babault, J., Van Den Driessche, J., Bonnet, S., Castelltort, S., Crave, A., 2005. Origin of the highly elevated Pyrenean peneplain. Tectonics 24. Bates, R.L., Jackson, J.A., 2008. Glossary of Geology American Geological Institute. 745 pp. Bell, J.W., Brune, J.N., Zreda, M., Yount, J.C., 1998. Dating precariously balanced rocks in seismically active parts of California and Nevada. Geology 26, 495–498. Boldt, K., 1998. Das Modell der restriktiven Flachenbildung — ein Ansatz zur Erfassung von Regeln der Landschaftsgenese im Bereich wechselnd widerständiger Sedimentgesteine. Zeitschrift für Geomorphologie 42, 21–38.

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