DILL, H. (1985): Die Vererzung am Westrand der Boehmischen Masse. - Metallogenese in einer ensialischen Orogenzone. - Geol. Jb., D 73: 3-461; Hannover

 

(The Mineralization on the Western Margin of the Bohemian Massif – Metallogenesis of Ensialic origin)

 

Abstract: In the NE Bavarian basement (Germany), neither complete ophiolite sequences nor corre­sponding types of mineral deposits (e.g. podiform chromite concentrations) typical of mature ocean basins are present. Isotope data (Pb/Pb, Sr/Sr) indicate a continental crust. The min­eralization developed isochemically in the sense of Backlund on a crust of varying thickness (thinner in the Saxothuringian than in the Moldanubian region). This is the result of metallo­genic processes in an ensialic orogenic zone with A-subduction or intracrustal mass transport (nappe tectonics).

 

The model of the mineralization is described, in analogy to oil migration, from the source rock to the reservoir rock. Those lithologies which developed during Upper Proterozoic and Lower Paleozoic rifting are considered to have contained the protore. Late Variscan and Early Alpine veins which are connected with the structural development form secondary traps. The Late Variscan magmatism is the most important thermal event. The metallogenic evolution consists of 4 main events: preconcentration, fracturing, activation, and morpho­genic development. The sequence of events was as follows:

 

In the Upper Proterozoic, psammopelitic sediments of the Monotonous Group were depos­ited in a lst-order basin on a relatively stable tectonic shelf. The redox conditions (e.g. Zn in oxidized form, e.g., Zn spinel, Zn staurolite) indicate shallow-marine to continental facies conditions. The clastics are com­parable with the unmineralized Late Precambrian molasse deposits of Europe ("Oldest Red" of SCHOENENBERG, 1971).

 

Continued extension of the crust caused formation of 2nd-order basins, which contain li­thologies of variable composition (Variegated Group = acid and basic volcanics, evaporites, bioliths, ± psammopelites). The Bodenmais-Lam special facies (M lb) represents the initial stage. It contains sediment-hosted Fe-Zn-Cu-Pb-(Ba)-Sn sulphide ores. The relict barite peripheral zone and the Sn contents indicate a relatively thick sialic crust in the period around 700 Ma (first separation of lead during Upper Proterozoic rifting = Pb 1).

 

With the help of metabioliths, three different types of basins are defined marking the course of the attenuating crust:

 

Basin type I is marked by the occurrence of phosphates. It is probably the nearest to the spreading center (influenced by the mantle). This facies is the protore for U, Cu, and W. The two last-mentioned elements can be proved in stratabound sulfide mineralizations and in scheelite mobilizates in metabasites.

 

Basin type II does not contain phosphate or uranium mineralization, whereas it seems to be relatively prospective for molybdenum. It develops at the margin of the basin on a thick sialic crust (graphite deposit Kropfmuehl/ Passauer Wald).

 

Basin type III contains predominantly detrital material and can be classified as a near-shore (estuarine) type of facies.

 

The second tin preconcentration was associated with the production of acid extrusive rocks. Fe was preconcentrated in the Wunsiedel Marble in a near-shore marine environment (siderite deposits Arzberg and Troestau). Miner­als bearing these elements can be shown to have crystallized during the Variscan mobilization. The Precambrian series are possible protores for the elements U, Sn, W and in some cases for Cu.

 

The areas of the Thuringian Facies and Bavarian Facies of Lower Paleozoic age are impor­tant for the mineralization. The clastic magmatite-poor Thuringian Facies was deposited in the marginal or early parts of the Saxothuringian basin. Gold occurs both in detrital form and associated with tuffs. As in the basin in the Moldanubian region, sediment-hosted Fe-Cu­-Zn-Pb sulphide ores developed near Waldsassen/ Pfaffenreuth (repetition of the Bodenmais type) at the Cambrian-Ordovician boundary. As the thickness of the crust is considerably reduced, Ba and Sn are not as important as in the equivalent Upper Proterozoic rocks whereas Au is more abundant.

 

In the Bavarian Facies, Cu-Fe sulphide ores are associated with bimodal volcanism within a tuff-slate succession near Sparneck-Goessenreuth-Neufang-Kupferberg above a relatively thin crust. While Sn and Ba are insignificant within this volcanosedimentary sequence, the gold concen­trations which are related to basic volcanic rocks occur at the margin of and within the Muenchberg Gneiss Massif. Chromium and nickel can be proved in late mobilizates which could be the re­sult of retrograde metamorphism. These sulphide ores are an incomplete and "immature" pillow-diabase type similar to the Besshi-type ores which developed in an advanced stage of rifting.

 

Compared with the thin crust in the area of the Bavarian Facies, the crust in the area of the Thuringian Facies on which the thuringite ores developed is considerably thicker (Ludwigsstadt, Toepen). Only in the area of the Berga anticline can a volcanic influence (hot brines) be recognized in the sedimen­tary Fe ores. This zone of elevated heat flow marks the boundary zone between the two facies zones. The Middle Ordovician Fe ores in the Griffelschiefer (pencil slates) at the margin of the basin are equivalent to the Fe-oxide-bearing red slates of the Middle Ordovician peri­pheral slate series in the area of the Bavarian Facies (the zonation from the center of the basin to the margin is: Cu-Fe facies "Kupferberg type" - Fe-oxide facies "red slate type" - Fe-sili­cate facies "thuringite type"). The volcano-sedimentary ore development in the divergent margin basins ended in the Ordovician. Subsequent Cu-poor and Fe-rich stratabound min­eralizations in the Saxothuringian already show the influence of A-subduction processes and are considered to have been formed in convergent margin basins (relict basins). ln the Silurian and Lower Devonian the stratiform Fe-Cu-Zn-Pb-Sb mineralizations of the graptolite shale facies developed syndiagenetically in an original sapropel facies (Graefenthal Horst). No endogene supply of material into the basin can be recognized. In the Upper Devonian, representatives of the Lahn-Dill type iron deposits can be found here (hematite-magnetite deposits Steinbach/ Langenbach, Stadtsteinach, Weidesgruen). The predominance of Fe over Cu is probably directly con­nected with progressive thickening of the crust. This type of mineralization is related to the Kupferberg type (Cu > Fe), which developed on thin crust during the initial stage of the deve­lopment of the basin. The isolated mineralization of the Lahn-Dill type SE of the Muenchberg Gneiss Massif were more intensely deformed by the emplacement of the Muenchberg gneiss nappe. In the Upper and Lower Devonian, sulfide mineralizations occur only syndia­genetically in semisapropels of the flyschoid slate facies. The postorogenic molasse, which locally oversteps the basement rocks (e.g. Stockheim, Weiden, Schmidgaden), is characterized by the dominance of the elements U, Cu, and Ba. They are, in most cases, remobilizates from the metalliferous Lower Rotliegend volcanoclastics.

 

The compression during evolution of this ensialic orogenic zone caused a heat build-up be­tween the continental crust plates and subcrustal lithospheric plates. The separation of lead (Pb2) during the Caledonian, which can be demonstrated over the whole NE Bavarian base­ment, is connected with it. This event can be demonstrated very convincingly in the deep-­reaching fracture zones (e.g. Berga anticline, Pfahl zones). In addition, the high temperature metamorphism and the anatexis in the Lower Paleozoic are probably connected with this build-up of heat between the two crustal plates.

 

The Upper Proterozoic basin sedimentation was brought to an end by A-subduction pro­cesses. The different stages of development can be correlated with different types of vein mineralization. In the area of Erbendorf-Vohenstrauss, the original basin type has been preserved in a relatively unmodified form. Only stratabound mobilizates of scheelite and chalcopyrite occur but no persistent vein zones. In the Schwarzach area in the south, rocks of the Variegated Group can be recognized only as boudins within the Monotonous Group. Cu and U ores are widespread along a deep-reaching fracture zone. This ore type is more com­mon in western Bohemia and runs parallel to the Great Bohemian Quartz Lode (Boehmischer Pfahl). The "final stage" of the A-subduction zone is preserved along the Great Bavarian Quartz Lode (Bayerischer Pfahl). The Variegated Group is completely absent in this highly deformed zone, but U-bearing veins are widespread within the mylonite. Inherited U/Pb ratios, which cannot be interpreted as age of formation of the ores, possibly reflect these sub­duction and mobilization processes. The subduction zones run parallel to the Precambrian facies zones, in particular they are closely associated with the Variegated Group. Some of the Late Variscan granites follow these old structural and facies trends.

 

While U and Cu were remobilized during the Variscan in the Moldanubian region, W and Sn developed only anomalous contents outside their Precambrian protores. Au and Sb vein mineralizations worth mentioning are completely absent because of the lack of corresponding protore or source rocks (Lower Paleozoic metavolcanics) in the Moldanubian region. Both these elements are most strongly concentrated in the Late Variscan granite-related veins of the Brandholz/Goldkronach-Neualbenreuth type (stibnite and gold) and in the veins of the Berga anticline (stibnite veins at Wolfersgruen, Schleiz, Greiz). The Au (Sb) con­centration around calc-alkali plutonites associated with A-subduction of basites represents the counterpart of the development of Au-Cu-bearing porphyries during B-subduction on thin continental crust. Au belongs to the older succession of the Fichtelgebirge granite and W occurs in association with the older succession (skarns) as well as with the younger granites; in the latter it occurs together with pneumatolytic to high temperature veins.

 

The W-bearing hematite veins of the "Rotenfels" type represent the end of the granite-re­lated mineralization. The lead event Pb 3 (radiogenic lead) was caused by the Late Variscan granite magmatism.

 

The transition from the granite-related mineralization (polymetallic uranium paragenesis: pitchblende, arsenopyrite, pyrite, gold, bismuth, guanajuatite, umangite, clausthalite, klockmannite, bismuthinite, sphalerite, galena) to the Early Alpine / Late Variscan mineralization connected with rifting (monotonous uran­ium paragenesis: uraninite, coffinite, brannerite) can be best demonstrated with reference to uranium. The first paragenesis is characterized by the element selenium and the second paragenesis particularly by the occurrence of U-Ti-Si compounds. The most important U-bearing rocks are the "episyenites" (cal­citic, dolomitic, or zeolitic depending on PCO2) which developed by desilicification of granitic rocks (Grossschloppen, Hebanz). These "episyenites" show a close connection, as far as their distribution is concerned, to deep-reaching lineaments and metalimestones. The occurrence of talc zones (e.g. Goepfers­gruen) and "episyenites" seems to have been caused by the same thermal event. Formation of talc is favoured in areas of thick metamorphosed limestone horizons.

 

The structure-controlled mineralizations are divided into five groups based on their asso­ciation with large geotectonic units: 1. granitic anticlinal zones, 2. the peripheral zones of gra­nitic highs or Variscan anticlinal zones, 3. intersections of NE (Variscan) and NW (Hercy­nian) trending lineaments, 4. internal Hercynian lineaments, 5. Hercynian boundary faults and unconformities.

 

In the mineralization mentioned under 1, granitic influences dominate very strongly (high temperature mineralization, granitophile elements in the mineral spectrum, and contempora­neity between granite intrusion and mineralization). The influence of granite becomes weaker in the ores mentioned under 2 and 3 (only the thermal influence of the granites). The influ­ence of the country rock on the ores of zone 4 can only be identified with the help of the U/Pb ratio in U oxides. The fluorite and baryte mineralizations (Stockheim, Lichtenberg, Issigau, Warmensteinach, Erbendorf, Wölsendorf-Nabburg, Nittenau/ Kittenrain, Erzhäuser, Donaustauf) under 5 are dominated by Permo­-Mesozoic clastic (and volcanic) rocks. They are spatially closely related to the Permian un­conformity, which is known to be a geohydraulically important surface. Their age of forma­tion is limited in time by the reactivation of the E-W and ENE-WSW fractures. The U/Pb ages for this reactivation are Lower Jurassic which is synchronous with the spreading events in the area of the Alps (Pennine Zone). This event can possibly be recognized in the Nittenau occurrence (Pb4) using Pb/Pb isotope ratios.

 

The isotope data do not give any information about the supply of simatic elements but they suggest that convecting solutions at the margin of the basement have redeposited elements originally preconcentrated in volcanoclastic Permo-Mesozoic rocks and in Late Variscan granites.

 

The mineralization of the NE Bavarian basement rock occurred mainly in a system in the crust which is considered to be closed with respect to chemical input.