The Paleogeographical, Lithological and Structural Controls of Uranium Occurrences in the Alps Mario Mittempergher Introduction The prospecting work carried out on the Italian side of the Alpine Range during the last fifteen years have led to recognize a large number of uranium occurrences (Mittempergher, 1966). Other occurrences have been found in the French, Swiss, Austrian and Yugoslav Alps (H iigi and ot., 1962; H ii g i, 1967; Barreau, 1959). As a common feature, ali these occurrences are contained in rocks of late Paleozoic or Lower Triassic age. A second group of uranium occurrences in fissures and in veinlets in the crystalline basement rocks is quite less interesting and important. Numerous studies have so far been performed about these mineral deposits, whose distinctive features are now fairly well known and described at sufficient extent (Fig. 1). Some supplementary efforts, however, are necessary for better under-standing of the minerogenetic processes of uranium, which took plače, at a given geological moment, over a very large area, approximately 1000 by 400 kilometers wide. Some attempts in this direction have been made (Marin e 11 i and Mittempergher, 1962) but only recently the geological informations have been acquired for an overall study in order to obtain both, the outlines of the "regional" features of the minerogenetic processes of uranium and the correlations with the more general uranium ore genesis in Hercynian era of Europe (Mittempergher, 1970). THE GEOLOGY OF URANIUM OCCURRENCES IN THE POST-HERCYNIAN BASIN The uranium mineralizations occur in the volcanites and in the Continental or littoral deposits of Permian and Lower Triassic ages of the whole Alpine area. These formations overlie unconformably the eroded crystalline basement and represent the magmatic and sedimentary products that affected a large subsiding basin of late and post-Hercynian ages. The uranium mineralizations are divided in previous papers (D'A g n o -lo, 1966; Mittempergher, 1958) as follows: — hydrothermal mineralizations in the volcanic rocks of the Lower Permian (Fig. 2); — stratiform mineralizations in the Continental sandstones of the Permian; — stratiform or lenticular mineralizations in the littoral deposits of the Permo-Triassic. Here I do not describe in detail the features of above-mentioned mineralizations that are pointed out in several specific works. I recall only that the study of these mineralizations has often required the reconstruction of the geological and minerogenetic conditions of ore deposits deformed and regenerated by Alpine metamorphism. This is the čase of that part of the post-Hercynian basin subjected to the complex Pennidic tectonism in Alpine age. It is very hard to assemble in an integrated picture a paleogeographie reconstruction, tending to comprise ali environmental and structural varieties of the Alpine zone during Permian and Triassic. This would require a prehminary reconstruction of the Hercynian land and of its structural changes over extremely large areas. On the other hand, the paleogeographie reconstruction is complicated not only by the difficulties of "spreading out" the Permian and Triassic formations ineorporated into the complicated movements of the folds during the Alpine orogenesis, but also by the difficulty in oorrelating the different areas of the zoning of Reference to Fig. 1 TECTONIC UNITS OF THE ALPS Helvetides Pennides Austrides ONorthern Limestone Alps Prealps Southern Alps A Dinarides Pre-Alpine crvstalline basements I I130"' I-1 basi Molasse and Vienna ns URANIFEROUS MINERALIZATIONS Hydrothermal in permian volca nites Epigenetic peneconeordant in O permian or lower-triassic con tinental sandstones Symmetamorphic, remobilizated ^ in Alpine age,occurring in permian and permo-triassic sedi-ments ^ Hvdrothermal in veins, catacla sites and mylonites ♦ Pyrometasomatic Granitic batholits fyr penian seg Fig. 1. Uranium occurrences in the Alps Alpine tectonic. Such zonning occurs not only lo>ngitudinally with respect to the axis of the orogen but, stili asymmetrically, also along that axis. However, in view of the purpose of this study (drawing a picture of the general characters of the environment of uranium ore genesis in the late Paleozoic), a highly simplified description is deemed sufficient. For such purpose, I have utilized paleogeographical sections crossing the central and eastern parts of the Alps where it is easier to correlate the Permo-Triassic environment and the Alpine tectonic units. The area at present tirne occupied by the Alpine Range corresponds roughly to one sector of the Hercynian foreland, the basement being iormed by me'„amorphites and Hercynian granites. This area has undergone an extended erosion process. Subsidence took plače by fracturing and displacement of the eroded massif, with the formation of two separated basins: a Southern one known as "alpine type" and a Northern one as "germanic type" (Fig. 3). The two basins were separated by a structural "high" corresponding coarsely to the zone of the Pennide nappes. A sedimentation of Continental character began in both basins during the Lower Permian. A magmatism started simultaneously in both basins, first in the form of epigranites (average age 280 MY), later in the form of acid volcanism (indicative age 265 MY). In the "Alpine basin" the processes of Continental c.edimentation, with sediments predominantly of an alluvial nature, ended in the period ranging between the Upper Permian (Bellerophon zone) and the Lower Triassic (Werfenian), with marine and lagoon epicon-tinental sediments (Fig. 4). In the "Germanic basin" the marine ingression took plače in the Lower-Middle-Triassic (Muschelkalk). In the transitional belt, the whole zone of the Pennide and "Brianpo-nian" nappes, the subsidence took plače very late: the Continental Permian is very weakly represented. The marine ingression of the Permo-Triassic took plače with recurrences and extreme elaboration of the sediments (formation of the Triassic quarzites). The Triassic facies are in part of the "alpine type" and in part of the "germanic type". The whole Pennide and Southern Alps area then evolved into the Alpine geosyncline (Fig. 3). In this overall picture, the uranium deposits of the Alps are located partly in the original basin of the Southern Alps, partly in the area cf structural "high" of the Pennide nappes (De Si t ter, 1959). In the thickest part of the Post-Hercynian continental series, sedi-mentary and volcanic, uranium occurs in the deeper levels and has an epigenetic character. In the Pennide zone, where the Permo-Triassic series is very reduced, uranium concentrations are partly syngenetic and are located in the upper parts of continental sediments at the transition to the epicontinental marine formations; partly they are epigenetic with the characters of the Southern Alps basin deposits. Uranium cccurrences related with "germanic facies" of the Triassic are not known in Italy; sparse and poorly documented reports suggest that in this series uranium ore bodies occur, as i. e. those of the "Buntsandstein" at Kitzbuhel, Austria. URAN/UM DEPOSIT IN VOLCAMCS Stratigraphic series « E o-c _ _ . .Si* Sandstones 5 c ^9 Volcanic - c 3 sedimentarv o * J series Basal conglom. c 15 Crfstatline S basement Uranium mineralization (a) URANIUM DEPOSIT IN SANDSTONES Stratigraphic series y -._ - i t UJ Sandstones, |-5 sJates and >- timestoneš I o S Red Sandstones | ^ Grey Sandstones t o Volcanic i E sedimentary ° » series Basal conglom. i S Cfrstalline T i £ basement h" ~t Uranium occurrence (b) URANIUM DEPOSIT IN METAMORPHIC ROCKS Stratigraphic series I Qt j^ So Limestones S ? and I *■anhydrites ti m | 8 Ouartzites Chlorite and sericite schist j S Crystaltine o. § basement the Thetis Sea; the time of the sedimentation of the Continental facies is comparatively short. Accord-ing to the current information, setting the age of volcanites at around 265 MY and the Ladinian-Carnian limit at more than 230 MY, the Continental and littoral sedimentation lasted not more than some 30 MY. The pregeosynclinal character of the basin and the fact that the direction of the displacement has always remained downward (subsidence), have determined the nature and quality of the sedimentary uranium occurrences. These mineralizations are small, very numerous and are distributed in a number of levels, somewhat heterogeneous in geochemical paragenesis. On the whole it seems to be evident that the processes of ore-deposit re-constitution through remobilization and concentration of the ore bodies in subsequent stages and hydrogeological situation, didn't take plače. These processes often account for the economic importance of the uranium deposits in the intracratonic and intermountain basins. A first general conclusion which is confirmed also in the čase of the Permian and Permo-Triassic of the Alpine area is that the pregeosynclinal basins are less important for the potential of uranium concentrations with regard to other types of basins (intracratonic, intermountain). Distribution of uranium mineralizations in the Permian basin of the Southern Alps In the preceding sections we have discussed the distribution of uranium ore bodies in the different "transversal" zones of the geosynclinal basin, i. e. in the "germanic facies" area of Triassic in the intermediate area of the Pennide nappes, and in the Southern "alpine facies" area of Triassic. A better understanding of the minerogenetic processes derives from a study of the distribution of uranium occurrences within an invididual structural and paleogeographic area. Such a study can well be carried out with respect to the Southern Alps area, characterized by the presence of both a considerable number of ore bodies and geologie and tectonic conditions which are comparatively easy to interpret. Among the different longitudinal zones of the pregeo-synclinal basin, that of the Southern Alps was the area of fastest subsidence, and the Permian and Triassic are largely represented in it. Now a stratigraphic reconstruction of the lithofacies and of their thickness has shown (Bosellini, 1965) that the Southern Alps area can be sub-divided into five sectors, in which the Permian and Triassic formations display widely different developments. The five sectors, perpendicular to the axis of the geosyncline and to the Hercynian direetions, are alter-nating areas of faster and slower subsidence (Fig. 5). Three of these areas are structural "highs", in which the thickness of the Permian and Triassic OroA/c |. j Thin (semi-) Continental facies [ -I— | Congtomeratic S volcanic facies | • | ' 1 - | Initial ultrabasic facies I" .* .* 1 [.*> . . | Flysch or Scaglia facies \ Fig. 3. Schematic development of Alpine basin during the Mesozoic era (After de S i 11 e r , 1959) series is relatively limited (1000 to 3000 meters). These three structural "highs" are those of platform of Lugano, Adige and Julian Alps. Between them are located twO' areas of structural trough, in which the Permian and Triassic series are as thick as 6000 to 8000 meters. The areas of structural "low" are the Lombard and the Veneto-Carnian basins. Ana-lysing the distribution of uranium occurrences with regard to these structural divisions of the Southern Alps (Fig. 5), we find that uranium is largely concentrated in the "structural low" of the Lombard Alps, where the Permian series is thickest. In particular, the uranium ore bodies are more frequent in the eastern slope of the Lombard basin, near the Adige structural "high". Generally, therefore, we may conclude that the uranium mineralizations contained in the Permian and Triassic of the Alps are typical for pregeosynclinal basins and that, within these basins, they are concentrated in the areas of fastest subsidence, where the lithologic series is thickest. Epicontinental facies Deep sea facies Late tectonic Hercynian granites Syntectonic Hercynian granites J- Cr = Jurassic -Cretoceous Tr P - Triassic = Permian The relationship betvveen uranium mineralizations in volcanites and in sediments Volcanites are a constant component of the Permian series of the Southern Alps. These volcanites, which are related to the oldest levels of the Post-Hercynian series, are present in the three westernmost structural zones, in the Lugano structural "high", in the Lombard struc-tural "Iow" and in the Adige structural "high". Volcanites often contain uranium ore bodies, as described above. If we compare the distributions of volcanites and uranium mineralizations contained in the Permian sandstone, we observe a close connection; this connection is obviously more evident between ore bodies in the volcanites and ore bodies in the sandstones (Fig. 6). Since the sandstones are younger than the volcanites and are made up in part of volcanite fragments, it is clear that the mineralizations in the sandstones are genetically related to the volcanites and, in part, to those contained in the latter ones. Outside of Southern Alps it is not possible to localize the Permian volcanites owing to Alpine metamorphism. Nevertheless, in the Marittime Alps and in the Esterel, the correlation between volcanites and uranium mineralizations is very close. Without entering upon the matter of minerogenetic models, a subject which lies outside the purpose of this paper, it would seem that the facts outlined above suggest that a remarkable importance for the occurrence of uranium may be attributed to the rocks of the late-Hercynian acid volcanism of the Alpine area. This importance indeed originates both from the uranium occurrences within the volcanic rocks themselves and from the mineral deposits in the sediments. Two conclusions may be drawn up from this recognition. A first con-clusion concerns the problem of the areal distribution of uranium mineral deposits: carrying further the points raised above. we can say that uranium ore bodies tend to concentrate in the areas where volcanites occur. The greatest concentrations are found in the "structural lows", where volcanic rocks are extensively present. A second co-nclusion concerns the problem of the Hercynian uranium province of Central Europe — a province in which, as it is known uranium is particularly associated with the late-Hercvnian magmatic massifs. Also in the čase of the Alpine area the late-Hercvnian magmatism is clearlv uranium-bearmg. Whether this fact is attributable to magmatic and hydrothermal causes resulting there-from, or it is due to the particular geographic evolution of the pregeo-:;ynclinal basin, can be a matter of speculation. The Alpine regeneration and redistribution of uranium occurrences It has been recognized that uranium mineralizations are more numerous and important in the areas where the subsidence rate of the post-Hercy-nian basin is higher and in the areas with more intense volcanic activity. Furthermoire, it has been noted that the development of the post- Berysch- Nondtiro/er Drsuzug Fazieskcre/ch (Oailtaier A/pen ynd Lienzen Do/omitenJ Sudtjro/er Dolomiten Faziesbereich Anis r .......... ---- v.'.-.- Skyth Ml Perm ••••••••i MjsMka/k Werfener --Schichten und Hese/geoirge -__Verrucano Siratig. A/ter Mende/doJom/i MuscMka/k M. K. Konglomerat CampUerSc/iMen Se/ser Schichten ~6elleropjionkBlk ~ ' nrrt &ps Groben Sandsie/ne *,»*' Bsais Kongbmergt Qijerzphyiht Ser/es Anis Skyth Perm C/ay andmtr! Bedded dolomite Bedded Hmesione Sandstone and oong/omerate (Afier VsnBemmeJen, f9il) OtT^cl Msssife doJo/nile Bvaporites (Hafiigebirge) Fig. 4. Schematic columnar sections of Northern Limestone Alps (left), the Drauzug (middle) and the Southern Alps (right) Hercynian basin into a geosyncline has limited remarkably the importance of uranium ore genesis. Actually it has conferred to the rocks a diagenetic feature and has determined therefore a different kind of deformation and of hydrological behaviour. In the area of Pennidic nappes and along the margins of the great austroalpine dislocations (Orobic line), the alpine stresses have differently metamorphosed the host rocks and have regenerated the ore deposits. The intensity of this action was different in the different areas of the Pennidic nappes. Where the metamorphism was slight the ore deposits maintained a stratiform or lens-shaped structure; on the contrary, where the metamorphism was of a higher grade, the mineralizations were even completely regenerated and assumed a typical synmetamorphic feature. Similar examples of remobilizated and regenerated ore deposits have been recordared in relation to mylonite bands and thrust-zones. In any čase, the tectonic and metamorphic processes have been a further cause of dispersion of the uraniferous occurrences. In Fig. 1 are shown the Permian or Permo-Triassic uranium mineralizations regenerated in Alpine age. PIA T TA FORMA BACINO LUGANESE LOMBARDO P/A T TA FOR M A BAClNO PIATTAFOHMA ATESINA CA RNICO BEL L UHESE GIUUA Fig. 5. Sedimentary basin during the Permian and Triassic periods as re-presented in longitudinal stratigraphic sketches of the Southern Alpine Range (After B o s e 11 i n i, 1965) THE URANIUM OCCURRENCES IN THE CRYSTALLINE BASEMENT OF THE ALPS Fig. 1 shows that few uranium occurrences are connected with the blocks of crystalline basement involved into Alpine nappes. Typical examples are represented by the Helvetidic blocks. For the description of these mineralizations I recall just published works. I remember only that ^^^ Areas with Permian volcanics • Uranium occurrence in Permian sandslones Uranium o ccurrence in Permian volcanics Fig. 6. The main tectonic units in Southern Alpine Range during the Permian and Triassic periods generally they represent mineralizations in fractures and mylonites of late or post-Hercynian age. Their paragenesis is typically epithermal. The are controlled by a late or post-Hercynian tectonism and were subjected tc remobilization processes in Alpine age. Therefore their age varies from 190 M.Y. (Gordolasque) to 90—80 M.Y. (Monte Bianco). These mineralizations, the most typical of which are localized in the Monte Bianco Massif, may be related to a common genetic source, the same of the uranium occurrences of the Asturian-Vosgian phase of the French Central Massif. Contrary to the acid volcanites that represent the extrusive equivalents of such plutonics, these ones have no practical importance. On the whole, the Hercynian plutonics of the Alps are poor in uranium occurrences. References B a g g i o , P. 1963, II granito del Monte Bianco e le sue mineralizzazioni uranifere: Studi e Ricerche della Divisione Geomineraria del CNRN, v. I, parte I. Roma. Barreau, J. 1959, L'uranium des Alpes Frangaises, Colloque de Grenoble, 11—20. B o s e 11 i n i, A. 1965, Lineamenti strutturali delle Alpi Meridionali du-rante il Permo-Trias, Mem. del Museo di Storia Naturale della Venezia Triden-tina, v. XV, fasc. III. Trento. D'A g n o 1 o , M. 1966, II giacimento uranifero di Novazza in Val Goglio (Bergamo), Atti del Symposium Inter. sui Giacimenti Minerari delle Alpi, v. 1—2. Trento. De Sitter, L. U. 1959, Structural Geology, McGravv Hill Pub. Hiigi, Th., De Quervain, F., Rickenbach, E., und Hofman-ner, F. 1962, Ubersichtskarte der Uran und Thorium Mineralisationen der Westalpen, Schweiz. Geotechnische Kommission, Kummerly und Frey, Bern. H ii g i, Th. 1964, Uranprospektion in der Schweiz unter besonderer Beriick-sichtigung des alpinen Raumes, Beilage zum Buli. Nor 2. der Schvveiz. Vereini-gung fiir Atomenergie, S. 1—8. Marinelli, G., Mittempergher, M. 1962, Provenienza e modalita di deposizione dell'uranio nei giacimenti italiani del Tardo Paleozoico, Notiziario CNEN, anno 8, n. 12. Roma. Mittempergher, M. 1958, II giacimento uranifero della Val Rendena. Studi e Ricerche della Divisione Geominerario del CNRN, v. I, parte II. Mittempergher, M. 1966, Le mineralizzazioni ad uranio delle Alpi Italiane, Atti del Symposium Int. sui Giacimenti Minerari delle Alpi, v. 1—2, Trento. Mittempergher, M. 1970, Characteristics of uranium ore genesis in the Permian and Lower Triassic of the Italian Alps, Proceedings of a Panel on uranium exploration geology, IAEA, Vienna. R u 11 e n , M. G. 1969, The Geology of Western Europe, Elsevier Publ. Com-pany, Amsterdam. SUMMAKY The uraniferous deposits discovered in the Alpine area can be sub-divided into two groups. The first group, the most important for the number of occurrences as well as for their economic importance, includes mainly stratiform peneconcordant ore bodies, generated in the post-Hercynian volcano-sedi- mentary basin during the Permian and the Lower Triassic periods. The distribution of these mineral deposits is connected with the features of the evolution of this basin. In the Southern Alps, where the Permian Continental and volcanic series has a remarkable development, the mineralizations are mainly localized in the volcanites of Lo