Radiometrische Datierung des Molybdänvorkommens Reichenspitze, Tirol/Salzburg

Translated title of the contribution: Radiometric dating of the molybdenum deposit Reichenspitze; Tyrol/Salzburg

David Doppelmayer

Research output: ThesisMaster's Thesis

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Abstract

The occurrence of molybdenum at Reichenspitze in Salzburg and Tyrol is part of the molybdenum ore province Zentralgneis Supersuite in the Tauernfenster. At the Reichenspitze, molybdenite (MoS2) is restricted to aplitic intrusions in the Zentralgneis. These aplitic granites are also referred to as Reichenspitz granite. Up to now no reliable age of the Reichenspitz granite has been published. In this study, U-Pb zircon dating (LA-ICP-MS) yields a crystallization age of 292,2 ± 0,68 ǀ 3,9 Ma for the aplitic granites. This puts the Reichenspitz granite directly into relation with the Lower Permian I-Type granitoids of the Zillertal-Venediger-Tux magmatic suite. Geochemical analyses of the aplitic granite show high SiO2 contents (77 bis 79 wt.%), barium contents of 141 to 233 µg/g and rubidium contents of 147 to 171 µg/g. Based on low CaO contents (< 1,65 wt.%), a negative barium anomaly (normalized to ocean ridge granites) and high Rb/Sr ratios (up to 5,93), the Reichenspitz granite was previously interpreted as an A-type granite. The analyzed samples of the Reichenspitz granite also show low CaO contents (< 0,8 wt.%), a weak negative barium anomaly and high Rb/Sr ratios (up to 4,25). However, the K/Rb - Rb discrimination diagram indicates a differentiation trend from granites in the Tux Core to the Reichenspitz Granit. The Reichenspitz granite is therefore interpreted as a highly differentiated granitoid of the Zillertal-Venediger-Tux magmatic suite. Typological studies of the investigated zircons yield calc-alkaline, highly fractionated magma sources. Crystallization temperatures of 600 to 650°C are presumed. Additionally U-Pb uraninite dating was carried out in this study. Though the crystallization age of the uraninites could not be precisely determined, the polygenetic zircon grains show younger ages than the published Lower Permian zircon age. The youngest uraninite generation shows consistent ages of 29 ± 5 Ma, which dates the Oligocene Neo-alpine regional metamorphism. Based on the new geochronological data a primary ore formation syngenetic with the intrusion of the Reichenspitz granite is proposed. In the highly fractionated melt, volatile elements (F, Cl) and molybdenum were enriched. This process generated metal-bearing, possibly pneumatolytic fluids which overprinted the host rock and led to metasomatic ore formation. Hydrothermal activity during the Alpine regional metamorphism remobilised the ore, which locally led to molybdenite enrichment as aggregates along shear zones. The Reichenspitze deposit strongly resembles the molybdenum deposit Ackley in Newfoundland, which is also related to aplitic granites in the roof zone of a magma chamber. A genetic relation between the Reichenspitze and the molybdenum deposit Alpeinerscharte/Tyrol cannot be excluded but calls for further investigations.
Translated title of the contributionRadiometric dating of the molybdenum deposit Reichenspitze; Tyrol/Salzburg
Original languageGerman
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Melcher, Frank, Supervisor (internal)
Award date25 Jun 2021
Publication statusPublished - 2021

Bibliographical note

embargoed until null

Keywords

  • Reichenspitze
  • Reichenspitz granite
  • molybdenum
  • molybdenite
  • Tauernfenster
  • zircon
  • zircon dating
  • uraninite
  • uraninite dating
  • differentiation trend
  • magmatic differentiation
  • molybdenum ore province Zentralgneis Supersuite
  • aplite
  • nördliche Zillerscharte
  • Zentralgneis
  • molybdenum deposit
  • Tuxer Kern
  • Zillertal-Venediger-Tuxer
  • pneumatolytic

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