Sm-Nd dating of hydrothermal carbonate formation: An example from the Breitenau magnesite deposit (Styria, Austria)

- Formation of the Breitenau magnesite deposit is constrained to 229.3 ± 2.4 Ma.
- This hydrothermal mineralization age is obtained using the Sm-Nd system.
- Samples were leached to chemically separate pure carbonate from matrix material.
- “Leachate”-“residue” pairs were used to build “internal isochrons”.
- Dolomite druses imply that open system fluid flow was active until very recently.

The geochemical evolution and chronology of the Breitenau sparry magnesite deposit has been investigated by means of major and trace element abundances, stable C-O isotope, and Sm-Nd isotope analysis. Chemically separated magnesite fractions (leachates) show consistent light-REE depleted REY patterns (marine carbonate normalized) and nearly constant normalized values for the middle and heavy REE without significant Ce, Eu or Y anomalies. Non-carbonate residues (composed of dark matrix material) are high in Al2O3 and show variably light to middle REE enriched REY patterns similar to shales. Leachates of limestone from nearby country rocks have normalized REY patterns similar to marine carbonates. Only Ce and Y are slightly fractionated. Based on their trace element chemistry and normalized REY patterns, leachates of dolomite from the margin of the deposit closely resemble either limestone (Type A) or magnesite (Type B). With one exception, δ18OVPDB for both dolomite types is within the range of the respective magnesite values (δ18OVPDB c. − 14.2 to − 15.7‰). Leachates of dolomite from druses within the deposit (Type C) show low REY abundances with normalized patterns similar to those of magnesite. Sm-Nd isotope data for magnesite leachates yield a well-defined isochron age of 222.5 ± 9.8 Ma (MSWD 3.1). Isochron calculations for leachate(s) and residue(s) from individual magnesite samples (“internal isochrons”) yield ages between 236 ± 16 Ma and 193.5 ± 8.6 Ma with a low degree of scatter (MSWD < 1.3). The most precise estimate for the time of Sm-Nd isotopic equilibration between carbonates and their host assemblages, both on hand specimen and deposit scale, is obtained from the mean of internal isochron calculations for eight samples (229.3 ± 2.4 Ma; MSWD 0.65) and is interpreted to date hydrothermal formation of the magnesite deposit. The scatter in initial Nd isotopic composition of the eight internal isochrons (0.511891 ± 0.000014; MSWD 7.7) indicates geologically based heterogeneities in isotopic composition on the deposit scale. Sm-Nd isotope data for Type C dolomite scatter off the upper right of the magnesite isochron. In combination with the respective O isotopic composition, the Sm-Nd isotope data of Type C dolomite can be modeled in terms of three distinct age groups: 1) 130 ± 20 Ma (δ18OVPDB c. − 15‰), 2) 37.9 ± 5.4 Ma (δ18OVPDB c. − 12.5‰), and 3) − 1.7 ± 6.8 Ma (δ18OVPDB c. − 11‰). Type C dolomite data indicate that druses were formed by open-system fluid flow throughout the deposit.