ALBERT

Beschreibung: Minerals of the rare-earth elements (REE) occur as supergene phases in the Schwarzwald ore district, southwestern Germany. They form by alteration of hydrothermal fluorite – barite – quartz – carbonate veins with various associations, including Cu– Pb, Pb–Zn and Co–Bi–Ag–U assemblages in sandstones, gneisses and granites. The REE minerals, including mixite-group minerals ([REE,Bi,Ca,Pb]Cu6(AsO4,AsO3OH)3OH6•3H2O), rhabdophane and churchite (REEPO4•H2O and REEPO4•2H2O), chukhrovite (Ca3REEAl2SO4F13•10H2O) and bastnäsite (REECO3F), were analyzed by electron microprobe and LA–ICP–MS. In addition, REE concentrations in secondary fluorite, calcite and Mn oxides cogenetic with the REE minerals were determined by LA–ICP–MS. Results of analyses of 74 mixite-group samples from 20 localities in the Schwarzwald ore district show that continuous miscibility is possible between REE and Ca and between Bi and Ca end-members. In contrast, no miscibility seems to exist between the Bi and REE end-members, and only Ca-rich members can accommodate small amounts of both Bi and REE. The REE phosphates churchite and rhabdophane do not occur at the same locality in the Schwarzwald, which is probably dependent on which REE (light or heavy) predominate at a certain locality. Whereas churchite incorporates heavy REE (HREE), rhabdophane prefers light REE (LREE). Dependent on the source of the REE, either HREE or LREE dominate in an alteration fluid and, consequently, only one type of REE phosphate forms. The HREE dominate in such a fluid if REE originate from dissolved “pitchblende”, whereas the LREE dominate if the altered host-rock or dissolved fluorite are the source of the REE. The REE contents of Mn oxides and calcite cogenetic with REE-bearing mixite-group minerals show that the REE distribution of secondary minerals can be influenced by each other. Whereas the Mn oxides incorporate Ce4+ resulting in a positive Ce anomaly, a cogenetic mixite-group mineral develops a negative anomaly, which shows that Ce anomalies are coupled. Calcite intergrown with a mixite-group mineral can only incorporate those REE that are not consumed by the latter. These processes involve the distribution of REE between minerals on the micrometer scale and do not take place until minerals precipitate. However, sorption and complexation processes also take place during transport on a larger scale and produce decoupled Ce anomalies. This interplay between large- and small-scale processes results in a complex redistribution of REE during remobilization.

Beschreibung: The Clara baryte-fluorite-(Ag-Cu) mine exploits a polyphase, mainly Jurassic to Cretaceous, hydrothermal unconformity vein-type deposit in the Schwarzwald, SW Germany. It is the type locality for 13 minerals, and more than 400 different mineral species have been described from this occurrence, making it one of the top five localities for mineral diversity on Earth. The unusual mineral diversity is mainly related to the large number and diversity of secondary, supergene, and low-temperature hydrothermal phases formed from nine different primary ore-gangue associations observed over the last 40 years; these are: chert/quartz-hematite-pyrite-ferberite-scheelite with secondary W-bearing phases; fluorite-arsenide-selenide-uraninite-pyrite with secondary selenides and U-bearing phases (arsenates, oxides, vanadates, sulfates, and others); fluorite-sellaite with secondary Sr- and Mg-bearing phases; baryte-tennantite/tetrahedrite ss-chalcopyrite with secondary Cu arsenates, carbonates, and sulfates; baryte-tennantite/tetrahedrite ss-polybasite/pearceite-chalcopyrite, occasionally accompanied by Ag±Bi±Pb-bearing sulfides with secondary Sb oxides, Cu arsenates, carbonates, and sulfates; baryte-chalcopyrite with secondary Fe- and Cu-phosphates; baryte-pyrite-marcasite-chalcopyrite with secondary Fe- and Cu-sulfates; quartz-galena-gersdorffite-matildite with secondary Pb-, Bi-, Co-, and Ni-bearing phases; and siderite-dolomite-calcite-gypsum/anhydrite-quartz associations. The first eight associations are of Jurassic to Cretaceous age and are related to at least eight different pulses of hydrothermal fluids (plus the meteoric fluids responsible for supergene oxidation); the last association is of Neogene age. Spatial juxtaposition of the various primary associations produces overlaps of the secondary associations. In addition to natural oxidation processes, two anthropogenic additions led to specific mineral associations: (1) lining of the adit walls with concrete resulted in high-pH assemblages of mainly Ca-rich phases, including arsenates and sulfates; and (2) the addition of hydrofluoric acid to counterbalance the high-pH fluids produced by power plant ashes introduced into the exploited parts of the mine resulted in fluoride assemblages of alkali and alkaline earth metals. This contribution describes for the first time all types of assemblages and associations observed and physicochemical considerations and models of formation for some of the supergene associations. The meteoric fluids responsible for element mobilization and redistribution, and for the formation of new, secondary phases, interacted with wall rocks prior to and during percolation through the actual hydrothermal associations. Depending on the amount of reaction with ore, gangue, and host rock phases, the chemical composition of the meteoric fluids and its redox potential may vary over short distances. Hence different mineral assemblages and zoned associations record fluid compositional changes, even on the millimeter to centimeter scale. Unusual mineral diversity at the Clara mine therefore develops from a combination of diverse primary hydrothermal mineralization stages, an unusual number of fluid flow events involving compositionally different fluids, and local equilibrium conditions that change within centimeters during supergene processes involving meteoric fluids and anthropogenic additions.

Beschreibung: The southern part of the eastern branch of the East African Rift is characterized by extensive volcanic activity since the late Miocene. In the Crater Highlands, part of the North Tanzanian Divergence zone, effusive and pyroclastic rocks reflect nephelinitic and basaltic compositions that formed between 4·6 and 0·8 Ma. The former are best represented by the Sadiman volcano (4·6–4·0 Ma) and the latter occur in the giant Ngorongoro crater (2·3–2·0 Ma), the Lemagarut volcano (2·4–2·2 Ma) and as a small volcanic field in the Laetoli area (2·3 Ma), where basaltic rocks known as Ogol lavas were erupted through fissures and several cinder cones. Compositionally, they are alkaline basalts with 46·0–47·9 wt% SiO2, 3·0–4·3 wt% of Na2O + K2O, Mg# of 61 to 55, and high Cr and Ni content (450–975 and 165–222 ppm respectively). Detailed textural and compositional analysis of the major minerals (olivine, clinopyroxene, plagioclase and spinel-group minerals) reveals the heterogeneity of the rocks. The primary mineral assemblage that crystallized from the Ogol magmas comprises macro- and microcrysts of olivine (Fo89·5–84·2), Cr-bearing diopside to augite, magnesiochromite–chromitess, magnetite–ulvöspinelss, andesine–oligoclasess and fluorapatite, with glass of phonolitic composition in the groundmass. All samples contain appreciable proportions of xenocrystic minerals of macro- and microcryst size, with large variations in both concentration and mineral populations between samples. Xenocrysts include olivine with reverse zonation (Fo84·1–72·5), rounded and embayed clinopyroxene cores of variable composition, anhedral Cr-free magnetite–ulvöspinelss and embayed oligoclase. These xenocrysts as well as variations in major and trace element contents, 87Sr/86Sr(i) (0·70377–0·70470) and 143Nd/144Nd(i) (0·51246–0·51261) ratios provide evidence of multi-stage magma mixing and mingling between Ogol and adjacent Lemagarut volcano basaltic melts with only very minor contamination by Precambrian granite–gneisses. Elevated alkalinity of Ogol lavas, which positively correlates with isotope ratios, and the presence of xenocrystic green core clinopyroxene, perovskite, schorlomite and titanite indicate additional mixing and mingling with evolved nephelinitic magmas and/or assimilation of nephelinitic Laetolil tuffs or foidolitic rocks related to the Sadiman volcano. Owing to their heterogeneity, estimates on the crystallization conditions for the Ogol rocks are difficult. Nevertheless, clinopyroxene–liquid thermobarometry indicates crystallization temperatures of around 1150–1220 °C and records upper-crustal depths of 3–12 km (1–4 kbar). Despite the fact that Ogol basalts are hybrid rocks that formed under open-system conditions with well-documented mixing and mingling processes, they seem to be the best examples closest to primary basaltic melts within the Crater Highlands.