ALBERT

Description: Late-stage pseudomorphous and perimorphous replacement of euhedral baryte and, to a lesser extent, fluorite and calcite by quartz is a common phenomenon in hydrothermal vein-type deposits. As a consequence of silicification, the primary mineral assemblage might be substantially altered, and therefore this process has a severe negative impact on the economic potential of mineral resources. Although these replacement textures are often reported and have a significant economic importance in mines producing baryte or fluorite of chemical grade, the process that causes this silicification is surprisingly poorly understood. In the present contribution, more than 40 Jurassic–Cretaceous and post-Cretaceous hydrothermal veins from the Schwarzwald mining district, including replacement textures of primary euhedral baryte, fluorite, and calcite, were investigated with respect to their macro- and microscopic textures. It appears that baryte is favorably replaced by pseudomorphs (bladed quartz), while fluorite and calcite are typically replaced by perimorphs. The textures indicate that the mode of replacement of the primary minerals happens continuously and after the initial vein formation. By combining these textural observations with calculated mineral solubilities, a detailed geochemical model has been developed. Existing fluid inclusion data indicate that substantial cooling of the hydrothermal solutions occurs after primary mineral formation. The calculated cooling path reveals opposing solubilities of quartz and the other gangue minerals (baryte, fluorite, and calcite) with decreasing temperature and explains the observed dissolution and precipitation textures. Furthermore, differences in temperature-solubility systematics between baryte on the one hand and fluorite and calcite on the other are responsible for the differences observed in the textures. This agrees with the occurrence of late-stage, low-temperature baryte crystals overgrowing primary baryte assemblages. Conversely, analogous late-stage calcite and fluorite assemblages are only rarely observed. In summary, silicification is a typical cooling effect in various hydrothermal vein-type deposits, but affects different gangue minerals in different ways depending on their temperature-dependent solubility.

Description: The mineralogy and mineral chemistry of the four major sövite bodies (Badberg, Degenmatt, Haselschacher Buck and Orberg), calcite foidolite/nosean syenite xenoliths (enclosed in the Badberg sövite only) and rare extrusive carbonatites of the Kaiserstuhl Volcanic Complex in Southern Germany provide evidence for contamination processes in the carbonatitic magma system of the Kaiserstuhl. Based on textures and composition, garnet and clinopyroxene in extrusive carbonatites represent xenocrysts entrained from the associated silicate rocks. In contrast, forsterite, monticellite and mica in sövites from Degenmatt, Haselschacher Buck and Orberg probably crystallized from the carbonatitic magma. Clinopyroxene and abundant mica crystallization in the Badberg sövite, however, was induced by the interaction between calcite foidolite xenoliths and the carbonatite melt. Apatite and micas in the various sövite bodies reveal clear compositional differences: apatite from Badberg is higher in REE, Si and Sr than apatite from the other sövite bodies. Mica from Badberg is biotite- and comparatively Fe2+-rich (Mg# = 72–88). Mica from the other sövites, however, is phlogopite (Mg# up to 97), as is typical of carbonatites in general. The typical enrichment of Ba due to the kinoshitalite substitution is observed in all sövites, although it is subordinate in the Badberg samples. Instead, Badberg biotites are strongly enriched in IVAl (eastonite substitution) which is less important in the other sövites. The compositional variations of apatite and mica within and between the different sövite bodies reflect the combined effects of fractional crystallization and carbonatite-wall rock interaction during emplacement. The latter process is especially important for the Badberg sövites, where metasomatic interaction released significant amounts of K, Fe, Ti, Al and Si from earlier crystallized nosean syenites. This resulted in a number of mineral reactions that transformed these rocks into calcite foidolites. Moreover, this triggered the crystallization of compositionally distinct mica and clinopyroxene crystals around the xenoliths and within the Badberg sövite itself. Thus, the presence and composition of clinopyroxene and mica in carbonatites may be useful indicators for contamination processes during their emplacement. Moreover, the local increase of silica activity during contamination enabled strong REE enrichment in apatite via a coupled substitution involving Si, which demonstrates the influence of contamination on REE mineralization in carbonatites.

Description: The Miocene Kaiserstuhl Volcanic Complex (Southwest Germany) consists largely of tephritic to phonolitic rocks, accompanied by minor nephelinitic to limburgitic and melilititic to haüynitic lithologies associated with carbonatites. Based on whole-rock geochemistry, petrography, mineralogy and mineral chemistry, combined with mineral equilibrium calculations and fractional crystallization models using the Least Square Fitting Method, we suggest that the Kaiserstuhl was fed by at least two distinct magma sources. The most primitive rock type of the tephritic to phonolitic group is rare monchiquite (basanitic lamprophyre) evolving towards tephrite, phonolitic tephrite, phonolitic noseanite, nosean phonolite and tephritic phonolite by fractional crystallization of variable amounts of clinopyroxene, amphibole, olivine, spinel/magnetite, garnet, titanite, plagioclase and nosean. During this evolution, temperature and silica activity (aSiO2) decrease from about 1100°C and aSiO2 = 0·6–0·8 to 880°C and aSiO2 = ∼0·2. At the same time, oxygen fugacity (fO2) increases from ΔFMQ* = +2–3 to ΔFMQ* = +3–5, with ΔFMQ* being defined as the log fO2 deviation from the silica activity-corrected FMQ buffer curve. Nephelinitic rocks probably derive by fractionation of mostly olivine, spinel/magnetite, melilite, perovskite and nepheline from an olivine melilititic magma. The nephelinitic rocks were formed at similarly high crystallization temperatures (〉1000°C) and evolve towards limburgite (hyalo-nepheline basanite) by an increase of silica activity from about aSiO2 = 0·4–0·5 to aSiO2 = 0·5–0·9, whilst redox conditions are buffered to ΔFMQ* values of around +3. Haüyne melilitite and the more evolved (melilite) haüynite may equally be derived from an olivine melilitite by more intense olivine and less melilite fractionation combined with the accumulation of haüyne, clinopyroxene and spinel. These rocks were crystallized at very low silica activities (aSiO2 ≤0·2) and highly oxidized conditions (ΔFMQ* = +4–6). Even higher oxygen fugacities (ΔFMQ* = +6–7) determined for the carbonatite suggests a close genetic relation between these two groups. The assemblage of carbonatites with highly oxidized silicate rocks is typical of many carbonatite occurrences worldwide, at least for those associated with melilititic to nephelinitic silicate rocks. Therefore, we suggest that the existence of highly oxidized carbonate-bearing sublithospheric mantle domains is an important prerequisite to form such complexes.