Aragonite–calcite veins of the ‘Erzberg’ iron ore deposit (Austria): Environmental implications from young fractures

Abstract

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 285·1 ± 3·9 to 1·03 ± 0·04 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.

Fracture dolomite as an archive of continental palaeo-environmental conditions

“…Geologically young vertical fractures at Erzberg provide fresh chemical reaction surfaces and important water flow paths, which promote the infiltration of meteoric water-derived solutions from the adjacent catchment areas. Subsequent fluid-rock interactions cause oxidation of accessory (Fe,Cu,Zn)-sulfide minerals exposed in the iron ore host rocks and related generation of sulfuric acid, as supported by sulfur isotopic compositions of modern surface and fracture water (−4.9 to +2.5 ‰ of δ 34 SSO 4 , VCDT) and iron sulfides (−2.2 to +5.1 ‰ of δ 34 S Pyrite , VCDT) at Erzberg 30 . Sulfuric acid attack facilitates an efficient dissolution of the partly brecciated carbonate host rocks (Stage ii) in Fig.…”

“…The sedimentary succession is cross-cut by numerous vertical fractures of variable width (few centimetres to several decimeters) and length (tens of meters) ( Figs. 1d and 2a) that are interpreted to have been developed during fault tectonics or gravitational mass movements up to the Late Pleistocene 30 . These fractures are partly healed with authigenic erzbergite and more rarely dolomite, especially in the southern part of the Erzberg, e.g., encountered on the pit level "Schuchart" (Fig.…”

“…1d displays prominent erzbergite and dolomite mineralizations infilling vertical fractures. The local host rocks are represented by lower Devonian limestones (greyish "Sauberger Kalk", left) and darker iron ore carbonates (siderite and ankerite, right), which show abundant slickensides, interpreted to be due to young tectonic or gravitational mass movements at Erzberg 30 . b Detailed view inside the fracture showing different generations of carbonate fracture mineralizations, e.g., whitish aragonite-dominated erzbergite and brownish dolomite-hosting layers.…”

“…Our study demonstrates the potential of authigenic carbonates infilling young fractures in the low temperature and high-altitude alpine environment of Erzberg to be used as a sensitive palaeo-environmental archive. Apart from the prominent erzbergite fracture mineralizations, whose applicability in palaeo-environmental reconstruction studies has been recently evaluated 30 , the carbonate fracture mineralizations exposed in the southern section at Erzberg host two generations of authigenic fracture dolomites. Both dolomite generations carry important environmental information on varying palaeo-fluid δ 18 O compositions (−14.1 to −8.5 ‰, VSMOW), water flow paths and related crystallization modes (phreatic vs. vadose), palaeo-redox conditions (dominantly oxic), dolomite depositional ages (Pleistocene to recent) and formation temperatures (glacial to ambient interglacial), which have not yet been been reported in dolomite research.…”

“…1b). The samples were initially recovered during two field campaigns in July 2015 and April 2016 from nine different places at Erzberg 30 Characterization of modern aqueous solutions. Water temperature, specific electric conductivity and pH were measured on-site using a WTW Multi350i combi instrument equipped with SenTix41 and TetraCon325 electrodes.…”