K-Ar dating of manganese minerals from the Eisenbach region, Black Forest, southwest Germany Autor(en): Segev, Amit / Halicz, Ludwik / Lang, Barbu Objekttyp: Article Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie Band (Jahr): 71 (1991) Heft 1 PDF erstellt am: 05.10.2021 Persistenter Link: http://doi.org/10.5169/seals-54349 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. Das Veröffentlichen von Bildern in Print- und Online-Publikationen ist nur mit vorheriger Genehmigung der Rechteinhaber erlaubt. Die systematische Speicherung von Teilen des elektronischen Angebots auf anderen Servern bedarf ebenfalls des schriftlichen Einverständnisses der Rechteinhaber. Haftungsausschluss Alle Angaben erfolgen ohne Gewähr für Vollständigkeit oder Richtigkeit. Es wird keine Haftung übernommen für Schäden durch die Verwendung von Informationen aus diesem Online-Angebot oder durch das Fehlen von Informationen. Dies gilt auch für Inhalte Dritter, die über dieses Angebot zugänglich sind. Ein Dienst der ETH-Bibliothek ETH Zürich, Rämistrasse 101, 8092 Zürich, Schweiz, www.library.ethz.ch http://www.e-periodica.ch SCHWEIZ MINERAL. PETROGR MITT. 71, 101-114, 1991 Frau Prof. Dr. Emilie Jäger gewidmet K-Ar dating of manganese minerals from the Eisenbach region, Black Forest, southwest Germany by Amit Segev, Ludwik Halicz, Barbu Lang and Gideon Steinitz1 Abstract K-bearing manganese minerals from granite-hosted hydrothermal veins in Eisenbach, Black Forest, SW Germany, were used for determining the age of ore emplacement by the K-Ar dating method The veins in the area are of multiphase origin, filling open fractures and fissures oriented mainly NW, semi-parallel to one of the main fracture zones. They are composed mainly of quartz and bante, with Mn and Fe oxides. The Mn-ores are composite materials of Mn oxides mixed with syngenetic and/or primary silica minerals from the host-rocks Therefore, the age of the impure Mn-ore whole-rock differs from the ages of the K-bearing Mn-phases, as it reflects the quantitative proportions of the various components. The results show that the oldest Mn phase is represented by concentric hollandite and radial, coarse cryptomelane of Permian age (> 253 + 5 Ma). A younger phase is represented by massive hollandite and braumte, and banded hollandite, pyrolusite, in places associated with scheehte, yielding Early Cretaceous ages (110-130 Ma). The latest assemblages consist of massive hollandite, braumte and cryptomelane, massive and crystalline pyrolusite, and hausmannite, coronadite and hollandite, yielding Neogene ages (from about 5 Ma to 16 ± 1 Ma) The K-Ar dates of Mn minerals demonstrate good correlation of the mineralogemc events with the geochronolo- gy of the host-rocks and of the other mineralization products in the area These results are in accord with the geological setting of the area studied. The Mn phases show a high retentivity of Ar even in situations where earlier phases are in intimate relation with the later ones. Therefore, it can be concluded that the K-bearing Mn-ores are, in principle, datable by the K-Ar method. Keywords: Geochronology, K-Ar method, manganese ore, Black Forest, Germany. Introduction The present study focusses on granite-hosted manganese veins from the Eisenbach area, Black Direct age determination of ore bodies and/or Forest, southwest Germany (Fig. 1). The geological mineralization phenomena is generally problematic history of the area is well known. Furthermore, the and in many cases impossible, mainly due to the mineralization history and especially that of the lack of minerals suitable for dating. K-bearing manganese minerals are amply documented. They manganese minerals, most of them from the psilome- serve as a basis for the present attempt to examine lane-manganomelane group (cryptomelane, the K-Ar systematics and the suitability of these hollandite, coronadite, psilomelane), contain K as a minerals for radiometric age determination. minor element in their formulas or as substitution In many cases manganese ores are composite for other cations, and can therefore potentially be materials in which manganese oxides are generally used for determining the age of ore emplacement admixed with syngenetic and/or primary silica by the K-Ar and Ar-Ar methods. Dating of such minerals (relics from the host-rock). Dates minerals, which though to date is still limited, has obtained for the manganese ore per se may significantly been preliminarily described by Segev and Steinitz differ from the true ages of the manganese (1986), and Segev et al. (1991). phases, and may just reflect the proportion of the 1 Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95 501. 102 A SEGEV, L HALICZ, B LANG AND G STEINITZ 53Z8 53|8 Folded Sedimentary cover Poleozic (Permian to Tertiary) Gneisses ond v v « vi volcanics migmotites v v Tertiory I x x x j Areas of older vans* Ouoternory * * *1 con granitizotion j + +\ Younger voriscon /"•""•v Moin districts of hydro* I *1 two mica granites v.««* thermol minerolizotion I I Permian Important I » rhyolites minerol veins % Somple site Fig 1 A) Geological map of the Black Forest area, including main mineralization fields (Wimmenaufr, 1980) B) Enlarged area with the location of the studied sites various components. Therefore, in order to determine according to Working Group Geochronology the K-Ar ages of the pure manganese oxide (1985) this phase probably ended about 310 Ma phases a specific analytical procedure was developed ago (for the southern Black Forest about 20 Ma and utilized. earlier (Lippolt et al., 1983) Following this plu- tomc phase, the area was uplifted by horst-and- graben tectonics associated with 310-280 Ma old Geological background elastics and calc-alkalme volcanics of predominately acid to intermediate composition (Lippolt Vast granite intrusions were emplaced in the et al., 1983). Red beds which covered the area Black Forest region during the Hercynian orogeny during the Lower Triassic time (Buntsandstein; m the interval roughly ranging from 370 to Herrmann, 1962), about 245 Ma ago, were 310 Ma ago (Fig. 2; Working Group Geo- stripped during the Tertiary uplift which is chronology, 1985) The manganese veins m the associated with the Rhmegraben fracturing and Eisenbach region (central Black Forest) under volcanic activities. discussion are hosted mainly m the Eisenbacher Arcogenesis (a tectonomagmatic platform two-mica granite This granite belongs to the activity with vertically accentuated dynamics of crust "young intrusions" phase, around 325 Ma and mantle) processes from the Upper Carboniferous (Wendt et al., 1970; Leutwein and Sonet, 1974); to the end of the Cretaceous may possibly have K-Ar DATING OF MANGANESE MINERALS FROM BLACK FOREST, GERMANY 103 played a role in the structural development and The various geochronological studies suggest endogenous as well as exogenous formation of several mineralization periods (Fig. 2): deposits (Baumann et al., 1985). 1) The Carboniferous stage (+ 325-285 Ma) - Several volcanic phases have been recorded pneumatolytic and high temperature hydrothermal from the Upper Cretaceous (25 km to the west, mineralization. near Freiburg and at Hochkopf some 30 km to the 2) The Permian (Rotliegendes) stage (280-260 southwest) through the Neogene. The younger Ma) - initial peak of a long duration hydrothermal volcanic events were recorded from Kaiserstuhl activity. (around 16-18 Ma) some 40 km to the west and 3) The Triassic-Jurassic stage - long duration from Hegau (8 Ma) some 40 km southwest of pulsating low temperature mineralization, which is Eisenbach (Horn et al., 1972). actually a continuation of the previous phase. Several stages of metallogenesis are recognized 4) The Cretaceous stage (around 100 Ma) - in the Hercynian massifs in central Europe. Based probably of higher temperature relative to the on fluid inclusion studies, Thomas and Tischen- former stage. dorf (1987) concluded that the pegmatitic-high 5) The Tertiary stage - renewed higher temperature temperature hydrothermal characteristics of the magmatic-metallogenetic activity. first Hercynian magmatic-metallogenetic stage in the Erzgebirge and Vogtland are due to high heat 1 1 1 —1 1 1 1 1 2 3 endogenic b\ flow connected to intensive, short-duration —' Mn mineralization processes. A second hydrothermal stage was characterized by decreasing heat flow and long A 1 2 < 3 > 4 5 —' Mineralization — «m ess term extensive processes. They also suggest that in the Cretaceous the heat flow increased again. A Young volcanisn magmatic-metallogenetic stage in the Tertiary is Late Hercynian volcanisn — considered to have been autonomously activated Hercynian plutonism by an ensimatic-riftogenic regime. Anatexis II » In the last twenty years efforts have been made to date the various mineralization phenomena in * Intrusion of magmas the Hercynian massifs in central Europe by s Model age for Precambrian sedimentation radiometric methods (reviewed, among others, by I 1 1 1 1 1 1 700 600