ALBERT

Description: In many natural and anthropogenically affected environments, alteration of galena produces thermodynamically more stable secondary lead phases. These secondary minerals control the mobility of the toxic heavy metal lead in water. These textural, paragenetic, and stability relations have not been investigated in detail in the literature yet. An extensive petrographic study of 41 thin sections of weathered, zoned galena and adjacent country rock from the Schwarzwald mining area, southwest Germany, is presented. The observed textures were evaluated using PHREEQC fluid path modeling and sequences of stable secondary mineral assemblages were predicted. The most common secondary (supergene) lead minerals of interest here are cerussite, anglesite, and pyromorphite group minerals (PyGM; pyromorphite, mimetite, and vanadinite). These lead phases show a spatially well-ordered zoned texture around the preexisting/relic galena. Cerussite and anglesite commonly occur either as in situ replacement of galena and/or as euhedral crystals in cavities of former, partially dissolved galena. The PyGM are present either as crusts around the margin of the former/relic galena or are common as infiltration products into the host rock/gangue. During progressive weathering anglesite typically disappears first followed by cerussite. Finally, only the highly insoluble PyGM persist as a perimorphose. Hence, a spatially and temporally zoning texture is formed. Thermodynamic models of various fluid evolution paths using PHREEQC show the influence of temperature, pH, variable P CO2 , phosphorous contents and/or different mineral reactions on the sequence of formation and stability of the secondary lead phases. Already small changes in one or more of these parameters can lead to different mineral assemblages or sequences of secondary lead minerals. Over almost the whole relevant pH range, PyGM are the most stable lead phases, precipitating at very low ion activities explaining their textural position. Whether cerussite or anglesite forms, depends mainly on the pH value of the supergene fluids, which is affected by the quite variable fluid pathways. Furthermore a solubility diagram for a typical near-surface fluid was calculated, showing that anglesite is the most soluble phase, followed by cerussite and PyGM. This again reflects the microscopic observations. As a further step, the time span for the formation of a natural millimeter-thick pyromorphite crust was evaluated using subsoil phosphorous fluxes from the literature. The calculation indicates that millimeter-thick pyromorphite crusts can be formed in few tens to about hundred years, which is in agreement with observations in the nature. In this study, a framework for predicting stable secondary lead mineral assemblages and textures by fluid path modeling is given. These models are potentially important for predicting the retention and mobilization of lead in systems around contaminated sites or natural ore deposits.

Description: Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mossbauer 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. Mossbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/{Sigma}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.

Description: Diagenetic carbonates, metamorphic carbonates, primary hydrothermal carbonates, and secondary remobilized carbonates (including sinters) from the Schwarzwald ore district in SW Germany formed in various tectonic settings and hydrothermal environments over a period of almost 300 Ma. They were investigated in order to define sources of carbon, dispersion of carbon during fluid-rock interaction processes and, where possible, to specify geochemical fingerprints for carbonates formed during different processes and in different geochemical and tectonic environments. For this purpose, 335 samples of calcite, ankerite, dolomite, siderite, and strontianite from 92 localities in 46 mining areas in the Schwarzwald were analyzed for their carbon and oxygen stable isotope, radiogenic strontium isotope, and trace element (including REE) concentrations and compared to analyses from all potential carbon sources available in this region. These include graphite and rare marbles of the crystalline basement, Permian calcrete from redbed sedimentary rocks (Rotliegend) overlying the crystalline basement, and Triassic carbonates from sediments of higher stratigraphic levels (Muschelkalk). Hydrothermal carbonates mostly formed due to fluid-mixing of hot ascending brines with cool sediment-sourced formation water. Fluid inclusions record temperatures of formation between 100 and 150 °C for most primary calcites. The mixed fluid from which they formed was a highly saline brine of around 25 wt.% salinity, containing NaCl and CaCl 2 in similar proportions. Before mixing, the deep brine was in equilibrium with graphite of the basement and contained, as main carbon species, H 2 CO 3 of very low C-isotopic values [around –16, Vienna Pee Dee Belemnite (V-PDB)], whereas the sediment-sourced formation water contained HCO 3 – with higher C-isotopic values (around +2, V-PDB). We find that graphite and Triassic carbonates in variable proportions (which are mainly related to variations during the fluid mixing process) are the carbon sources for primary calcite, dolomite of the Permian calcrete for primary ankerite, and the Triassic carbonate sediments for the primary ankerite mineralization of the area between Waldkirch and Feldberg. At some localities, remobilization and reprecipitation appears to have taken place without addition of external solutes, as Sr and C show no difference in their isotopic composition between primary and secondary carbonates. The oxygen isotopic composition of secondary carbonates is invariably more positive than that of primary ones, reflecting lower formation temperatures. One very conspicuous type of secondary calcite, which forms olive-green stubby scalenohedra, was dated for the first time using the U-Pb isochron method. Its Neogene age represents uplift and erosional denudation of the Schwarzwald and corresponds well with its remobilized C and O isotope signature. The carbonates in the Schwarzwald hence reflect discontinuous addition of carbon from surface sediments to the crystalline basement through time involving fluid-rock interaction and fluid mixing processes.

Description: In this work, we present an improved method for the semi-quantitative determination of sulfur species in S-bearing minerals by electron microprobe analysis. For calibration, we analyzed several sulfate and sulfide standard minerals such as baryte, celestine, chalcopyrite, and pyrite, and correlated the results with theoretical calculations retrieved from density functional theory (DFT). We applied this method to natural sodalite-group minerals from various localities. In addition, we applied the more common Raman spectroscopy to some samples and show that this method cannot be applied to sodalite-group minerals to determine their sulfur speciation. We show that even though sodalite-group minerals have a complex crystal structure and are sensitive to the electron beam, electron microprobe analysis is a reliable tool for the analysis of their sulfur speciation. The natural sodalite-group minerals show systematic variations in sulfur speciation. These variations can be correlated with the independently determined oxidation state of the parental magmas thus making S-bearing sodalite-group minerals good redox proxies, although we show that the electron microprobe analysis of the sulfur speciation is matrix-dependent, and the sulfur speciation itself depends on crystal chemistry and structure, and not only on f O 2 .

Description: Textural and compositional variations of apatite from rift-related gabbros, syenogabbros, syenites, quartz-syenites, and nepheline syenites of the Mid-Proterozoic Gardar Province (South Greenland) are presented and compared to apatite compositions from other rock suites. The observed zoning textures of apatite are interpreted to represent (1) primary growth zonation (concentric and oscillatory) that formed during magmatic differentiation and (2) secondary irregular overgrowths, patchy zonation, and resorption textures, assigned to metasomatic overprinting due to interaction with fluids/melts and intra-crystalline diffusion. Compositional variation in the apatites is mainly due to coupled substitutions of Ca and P by variable amounts of Si, Na, and REE, which show increasing concentrations during magmatic differentiation. Furthermore, F concentrations in apatites increase from gabbroic through syenogabbroic to syenitic rocks, whereas Cl concentrations show the opposite trend. Compared to apatite compositions from gabbroic, dioritic, and granitic rocks in general, apatites from alkaline rock suites are characterized by exceptionally high contents of REE and Si and in some alkaline rocks they attain Sr contents comparable to those reported from carbonatites. Typical low Mn and S contents are probably a result of low oxygen fugacity during crystallization at relatively high temperatures.

Description: The Cretaceous Mont Saint-Hilaire complex (Quebec, Canada) comprises three major rock units that were emplaced in the following sequence: (I) gabbros; (II) diorites; (III) diverse partly agpaitic foid syenites. The major element compositions of the rock-forming minerals, age-corrected Nd and oxygen isotope data for mineral separates and trace element data of Fe–Mg silicates from the various lithologies imply a common source for all units. The distribution of the rare earth elements in clinopyroxene from the gabbros indicates an ocean island basalt type composition for the parental magma. Gabbros record temperatures of 1200 to 800°C, variable silica activities between 0·7 and 0·3, and f O2 values between –0·5 and +0·7 (log FMQ, where FMQ is fayalite–magnetite–quartz). The diorites crystallized under uniform a SiO2 ( a SiO2 = 0·4–0·5) and more reduced f O2 conditions (log FMQ ~ –1) between ~1100 and ~800°C. Phase equilibria in various foid syenites indicate that silica activities decrease from 0·6–0·3 at ~1000°C to 〈0·3 at ~550°C. Release of an aqueous fluid during the transition to the hydrothermal stage caused a SiO2 to drop to very low values, which results from reduced SiO 2 solubilities in aqueous fluids compared with silicate melts. During the hydrothermal stage, high water activities stabilized zeolite-group minerals. Fluid inclusions record a complex post-magmatic history, which includes trapping of an aqueous fluid that unmixed from the restitic foid syenitic magma. Cogenetic aqueous and carbonic fluid inclusions reflect heterogeneous trapping of coexisting immiscible external fluids in the latest evolutionary stage. The O and C isotope characteristics of fluid-inclusion hosted CO 2 and late-stage carbonates imply that the surrounding limestones were the source of the external fluids. The mineral-rich syenitic rocks at Mont Saint-Hilaire evolved as follows: first, alkalis, high field strength and large ion lithophile elements were pre-enriched in the (late) magmatic and subsequent hydrothermal stages; second, percolation of external fluids in equilibrium with the carbonate host-rocks and mixing processes with internal fluids as well as fluid–rock interaction governed dissolution of pre-existing minerals, element transport and precipitation of mineral assemblages determined by locally variable parameters. It is this hydrothermal interplay between internal and external fluids that is responsible for the mineral wealth found at Mont Saint-Hilaire.

Description: Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na 2 O + K 2 O) 〈 Al 2 O 3 ] and peralkaline [(Na 2 O + K 2 O) 〉 Al 2 O 3 ] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa 2 O, µCaO, µK 2 O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks.

Description: 〈span〉〈div〉Abstract〈/div〉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.〈/span〉

Description: The Lengenbach (Switzerland) Pb-As-Tl-Zn deposit was formed from a sulfide melt at about 500 °C during Alpine metamorphism, but details on its formation and especially the source of the metals are still under debate. In this study we present two sample sets to address these questions: MC-ICP-MS analyses of thallium isotopes in sulfides, sulfosalts, and melt inclusions from the Alpine metamorphic Lengenbach deposit in the Binn Valley of Switzerland, the non-metamorphic Wiesloch Mississippi Valley-type deposit in Southern Germany, and the Cu- and As-rich mineralization at Pizzo Cervandone about 2 km SW of the Lengenbach deposit, which has been discussed as potential source of the Lengenbach metals. LA-ICP-MS analyses of micas from the Lengenbach deposit and surrounding country rocks between the deposit and the Pizzo Cervandone to trace potential metal-bearing fluid pathways. We found that Tl isotope compositions expressed as 205 Tl values in all investigated samples range from –4.1 ± 0.5 to +1.9 ± 0.5. The whole variation can be seen in the Lengenbach deposit alone, which hence records considerable fractionation even during high-temperature processes involving a sulfide melt. This large range of 205 Tl is thought to be caused by nuclear volume-dependent fractionation. Interestingly, the common fahlores at Lengenbach behave differently from all other investigated sulfosalts: based on their heavy isotopic composition together with a low As/S-ratio, they do not seem to be crystallized from the sulfide melt, but are interpreted to have formed from hydrothermal fluids enriched in the heavy Tl isotopes. Although As mobilization in the gneisses and dolomites surrounding the Lengenbach deposit is evident based on secondary arsenites, no traces of such a country rock fluid could be found in fissure micas at Lengenbach. Hence, considerations involving K/Rb, Rb/Tl, As/S, and Pb/Tl ratios in the sulfides and micas imply that the element enrichment in the Lengenbach deposit is either pre-Alpine or related to peak metamorphism, but occurred definitely before mica growth at Lengenbach.