Aragonite-calcite veins of the 'Erzberg' iron ore deposit (Austria): Environmental implications from young fractures

Ronny Boch, Xianfeng Wang, Tobias Kluge, Albrecht Leis, Ke Lin, Hannes Pluch, Florian Mittermayr, Andre Baldermann, Michael E Böttcher, Martin Dietzel

Research output: Contribution to journalArticlepeer-review


The well-known Erzberg site represents the largest siderite (FeCO3) deposit
in the world. It consists of various carbonates accounting for the formation
of prominent CaCO3 (dominantly aragonite) precipitates filling vertical fractures
of different width (centimetres to decimetres) and length (tens of
metres). These commonly laminated precipitates are known as ‘erzbergite’.
This study focuses on the growth dynamics and environmental dependencies
of these vein fillings. Samples recovered on-site and from mineral collections
were analyzed, and these analyses were further complemented by
modern water analyses from different Erzberg sections. Isotopic signatures
support meteoric water infiltration and sulphide oxidation as the principal
hydrogeochemical mechanism of (Ca, Mg and Fe) carbonate host rock dissolution,
mobilization and vein mineralization. Clumped isotope measurements
revealed cool formation temperatures of ca 0 to 10°C for the aragonite,
i.e. reflecting the elevated altitude Alpine setting, but unexpectedly low for
aragonite nucleation. The 238U–234U–230Th dating yielded ages from
2851 39 to 103 004 kyr BP and all samples collected on-site formed
after the Last Glacial Maximum. The observed CaCO3 polymorphism is primarily
controlled by the high aqueous Mg/Ca ratios resulting from dissolution
of Mg-rich host rocks, with Mg/Ca further evolving during prior CaCO3
precipitation and CO2 outgassing in the fissured aquifer. Aragonite represents
the ‘normal’ mode of erzbergite formation and most of the calcite is of
diagenetic (replacing aragonite) origin. The characteristic lamination (millimetre-
scale) is an original growth feature and mostly associated with the
deposition of stained (Fe-rich) detrital particle layers. Broader zonations
(centimetre-scale) are commonly of diagenetic origin. Petrographic observations
and radiometric dating support an irregular nature for most of the layering.
Open fractures resulting from fault tectonics or gravitational mass
movements provide water flow routes and fresh chemical reaction surfaces of the host rock carbonates and accessory sulphides. If these prerequisites
are considered, including the hydrogeochemical mechanism, modern water
compositions, young U-Th ages and calculated precipitation rates, it seems
unlikely that the fractures had stayed open over extended time intervals.
Therefore, it is most likely that they are geologically young.
Original languageEnglish
Pages (from-to)604-635
Publication statusE-pub ahead of print - 29 May 2018

Fields of Expertise

  • Advanced Materials Science


  • NAWI Graz


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