Katharina Pfaff

U-Pb ages of ferberite, chalcedony, agate, ‘U-mica’ and pitchblende: constraints on the mineralization history of the Schwarzwald ore district

This study deals with the use of new and unusual geochronometers applied to a variety of mineralizations in the Schwarzwald region of southwest Germany. In detail, we present new U-Pb age data of ferberite (from Clara Mine), chalcedony (from Silberbrunnle Mine), agate (Geisberg), carneol (“Markgrafler Land”), and confirm earlier geochronologic work on multistage U-mineralizations (Wittichen and Menzenschwand). Using these various geochronometers, we show both their usefulness, precision and robustness by comparing our age data to earlier work, and we can augment existing data with our new dataset to draw a picture of the timing of late- to post-Variscan hydrothermal mineralization in the region. The combined dataset reveals that vein-type mineralizations in the Schwarzwald have formed episodically over the last 300 Ma with distinct peaks of mineralization around 300 Ma (late-Variscan), several periods between 200 and 100 Ma (possibly related to the opening of the North Atlantic Ocean in the Jurassic and early Cretaceous), and around 40–20 Ma (related to the opening of the Rhinegraben). This pulsed formation of hydrothermal deposits is not unique for the Schwarzwald, as it has also been documented for the more richly mineralized Erzgebirge and other parts of post-Variscan Europe. The wide age range found within some districts or vein systems does not represent an analytical artefact, but reflects multiple reactivations of the same structures associated with changes in the stress distribution at continental margins. Comparison with the Erzgebirge shows, however, that the intensity of mineralization in a given time period shows regional differences, which reflect differences in regional tectonics.

The compositional variability of eudialyte-group minerals

Abstract Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mössbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/∑Fe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

A fast and easy-to-use approach to cation site assignment for eudialyte-group minerals

Eudialyte-group minerals (EGM) are typical constituents of agpaitic varieties of peralkaline rocks. In their complex structure (N15–16M(1)6M(2)3Z3M(3)M(4)Si24O66–73(OH)0–9X2), many cations (e.g. Na+, Ca2+, Fe2+, Mn2+, REE3+, Zr4+, and Si4+) as well as different hydrogen-bearing species (H2O, OH–, H3O+) may occupy different structural sites. Also, two potentially vacancy bearing positions are present. Thus, various methods of calculation of mineral formulae for EGM in the literature are inconsistent and in some cases not charge-balanced. We present an extended and improved scheme for site assignment using IMA-approved end-members and taking into account the different structural units of EGM. This method is based on electron microprobe analyses alone not considering different valence states of Fe and Mn and undetermined H2O-contents. However, comparison with structural refinement data from the literature reveals major agreement and significant improvement compared to earlier proposed methods. The instruction given here can easily be transferred to a table calculation spread sheet (e.g. EXCEL©), which is available from the corresponding author on request.

On the origin of sellaite (MgF2)-rich deposits in Mg-poor environments

Abstract Sellaite (MgF2) forms from melts, fluids, and gases under variable temperature, pressure, fO₂, and fluid salinity conditions. It is typically associated with, but much rarer, than fluorite (CaF2). The Clara mine near Oberwolfach (Schwarzwald, Germany) is an extensive hydrothermal vein-type deposit, where sellaite occurs in huge quantities (thousands of tons) in veins of mostly Jurassic/Cretaceous age. The sellaite mineralization, occurring in gneisses altered prior to and during sellaite mineralization, represents a stockwork-like network of veins and fissures, which is overlain and sealed by sediments, preventing the inflow of and fluid-mixing with sedimentary formation waters. The occurrence of sellaite is unique among the more than 1000 hydrothermal vein-type occurrences of the Schwarzwald ore district. The favored formation of sellaite compared to fluorite requires the initial Ca/Mg-ratio of the mineralizing fluid to be unusually low. These conditions are possible if fluids equilibrate with pre-altered rocks that lost some or much of their Ca during an earlier hydrothermal alteration event. Indeed, calculations demonstrate that rock-buffered fluids of pre-altered rocks (i.e., gneiss around the Clara mine altered during prior hydrothermal events) show significantly lower Ca/Mg-ratios than fluids equilibrated with unaltered gneisses, because Ca-phases (e.g., the anorthite component of plagioclase) are more prone to hydrothermal destruction. Due to the network-like structure of the sellaite-bearing portion of the Clara fluorite vein, the fluid is shielded from sedimentary formation water, resulting in fractionation processes of the repeatedly ascending mineralizing fluid. In addition, fluid cooling and formation of water-bearing phases like illite that consume fluids, favor sellaite, and later fluorite precipitation. The rarity of this combination of prerequisites explains the limited occurrence of sellaite in hydrothermal vein-type deposits.