The Hydrothermal Breccia of Berglia-Glassberget, Trøndelag, Norway: Snapshot of a Triassic Earthquake

Abstract

The quartz-K-feldspar-cemented breccia of Berglia-Glassberget in the Lierne municipality in central Norway forms an ellipsoid structure 250 m × 500 m in size. The hydrothermal breccia is barren in terms of economic commodities but famous among mineral collectors for being a large and rich site of crystal quartz of various colours and habits. Despite being a famous collector site, the mineralization is rather unique in respect to its geological setting. It occurs within Late Palaeoproterozoic metarhyolites of the Lower Allochthon of the Norwegian Caledonides regionally isolated from any other contemporaneous hydrothermal or magmatic event. In order to understand better the formation of the Berglia-Glassberget breccia, the chemistry, fluid inclusion petrography and age of the breccia cement were determined. Structural features indicate that the Berglia-Glassberget is a fault-related, fluid-assisted, hydraulic breccia which formed by single pulse stress released by a seismic event. 40Ar-39Ar dating of K-feldspar cement revealed a middle Triassic age (240.3 ± 0.4 Ma) for this event. The influx into the fault zone of an aqueous CO2-bearing fluid triggered the sudden fault movement. The high percentage of open space in the breccia fractures with cavities up 3 m × 3 m × 4 m in size, fluid inclusion microthermometry, and trace element chemistry of quartz suggests that the breccia was formed at depths between 4 and 0.5 km (1.1 to 0.1 kbar). The origin of the breccia-cementing, CO2-bearing Na-HCO3-SO4 fluid may have been predominantly of metamorphic origin due to decarbonation reactions (T > 200 °C) of limestones of the underlying Olden Nappe. The decarbonation reactions were initiated by deeply derived, hot fluids channelled to sub-surface levels by a major fault zone, implying that the breccia is situated on a deep-seated structure. Regionally, the Berglia-Glassberget occurs at a supposed triple junction of long-lived fault zones belonging to the Møre-Trøndelag, Lærdal-Gjende and the Kollstraumen fault complexes. These fault systems and the associated Berglia-Glassberget earthquake are the expression of rifting and faulting in northern Europe during the middle/late Triassic.

Editorial for Special Issue “Mineralogy of Quartz and Silica Minerals”

“…The formation of such ordered particle arrays not only takes place in inorganic photonic structures in the geosphere, however is also important for nanotechnology and biological systems. Other formation environments for quartz and silica minerals are discussed by Müller et al [8], Voudouris et al [9] and Trümper et al [10]. Spectacular quartz crystals of various colours and habits were reported from a hydrothermal breccia of Berglia-Glassberget, Norway [8] and also in volcanic rocks in different occurrences of Greece [9].…”

“…Other formation environments for quartz and silica minerals are discussed by Müller et al [8], Voudouris et al [9] and Trümper et al [10]. Spectacular quartz crystals of various colours and habits were reported from a hydrothermal breccia of Berglia-Glassberget, Norway [8] and also in volcanic rocks in different occurrences of Greece [9]. Both papers try to reconstruct the specific conditions leading to the formation of the quartz crystals based on thorough mineralogical and geochemical analyses (trace elements, fluid inclusions, oxygen isotopes).…”

Fluid-assisted brecciation of Lower Cretaceous Maiolica limestone in the Umbria-Marche Apennines: Hydrodynamical implications

“…The size and morphology of the clasts, as well as the clast-matrixvoid ratios of a breccia can provide information on the kinematics of the fault motion and the hydrodynamic processes leading to the brecciation. Economic geologists are interested in breccias formed in hydrothermal environments, because these breccias are commonly associated with metallic ore deposits (e.g., Kalliokoski and Rehn, 1987;Scheepers and Cuney, 1992;Jébrak et al, 1996;Fournier, 1999;Tămaș and Milési, 2002;Zhang et al, 2007;Costa et al, 2014;Sutarto et al, 2015;Grare et al, 2018;Müller et al, 2018). The formations of low-and high-sulphidation ores, as well as the concentration of gold and other metallic ores, around a magmatic body are reported to be controlled by hydraulic condition of the phreatomagmatic processes (see John et al, 2018, for a detailed summary).…”

Compact Scanning Electron Microscopes: Impressive Analytical Tools

“…For this, we used a large quartz crystal provided by Axel Müller (University of Oslo), which he had previously been imaged in panchromatic CL (Fig. 4A) (Müller et al 2018). The largest area that we could scan without excessive shadowing from the in-line mirror was 125 × 125 µm, and we conducted such a scanning image for two wavelengths centered on 411 nm and 640 nm using a 4 nm spectrometer slit.…”