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Author(s): Sara Ladenburger [sup.1], Benjamin F. Walter [sup.1], Michael A. W. Marks [sup.1], Gregor Markl [sup.1] Author Affiliations: (1) grid.10392.39, 0000 0001 2190 1447, Fachbereich Geowissenschaften, Universität Tübingen, , 72074, [...], The crush-leach technique is a frequently used method to determine the bulk composition of fluid inclusions trapped in a range of geological samples. We present a modified crush-leach technique combining ion chromatography (IC) and total reflection X-ray fluorescence spectroscopy (TXRF) which allows to determine a range of major, minor and trace elements out of one leachate. To date, trace element detection by means of TXRF is barely used in geosciences, although it combines the advantages of low to very low detection limits ([micro]g/L to ng/L range), small sample amount needed ([micro]L-range) and a fast and inexpensive analytical procedure. Previously described problems of adsorption of polyvalent cations at sample surfaces have been overcome by using acidified water as a leachate. Instead, it has been demonstrated that, for example, the syringe filter type used for IC measurements influences contamination and/or adsorption for a number of elements. The proposed method combination was evaluated for accuracy, reproducibility and system blanks and subsequently applied to quartz samples from hydrothermal vein deposits of the Schwarzwald ore district, SW Germany.

Exploration for rare earth element (REE)-deposits hosted in carbonatites and associated rocks is challenging because of the heterogeneous distribution of REE and the variable and often complex mineralogy of such REE mineralisation. The Keishöhe in southwestern Namibia is a subvolcanic intrusion consisting of calcite-carbonatite, dolomite-carbonatite, ankerite-carbonatite dykes, ring dykes and diatremes, and is regarded to be a part of the Kudu lineament carbonatite complexes. Importantly, it exemplifies many of the problems associated with REE exploration -- particularly regarding its heterogeneous REE distribution. It is therefore an excellent site in which to explore the processes related to REE precipitation in detail. Petrography, whole rock geochemistry and microXRF imaging of xenolith-bearing and xenolith-free carbonatites provide insight into the role of silicate xenoliths in the development of a hydrothermal or supergene REE mineralisation in a subvolcanic environment. Xenolith-rich carbonatites are almost barren, whereas xenolith-free carbonatites show REE-contents of up to 10 wt. % total REE (TREE). Therefore, the probability for REE-enrichment in the various Keishöhe carbonatites is significantly higher in the absence of silicate rock xenoliths. The REE mineralisation is dominated by REE-F-carbonates, while monazite is a minor constituent. In contrast to other complexes, where Si assimilation caused REE depletion in the melt during the magmatic stage, this study demonstrates post-magmatic hydrothermal or supergene REE mobilisation and enrichment. REE remobilisation and enrichment is caused by hydrothermal or supergene alteration of silicate xenoliths (Si release) by aqueous fluid(s). In combination with previous studies this study clearly shows that silicate xenoliths have 1) a significant influence on REE-mineralisation in general and 2) may act either positively (precipitation/incorporation) or negatively (mobilisation) on the mineralisation of discrete REE phases depending upon the related syn-magmatic or post-magmatic processes. [ABSTRACT FROM AUTHOR]

Native arsenic is an occasional ore mineral in some hydrothermal base metal deposits. Its rarity (compared to pyrite, arsenopyrite, galena, sphalerite, or chalcopyrite, for example) is surprising, as arsenic is a common constituent of upper crustal fluids. Hence, the conditions of formation must be quite special to precipitate native arsenic. An ideal location to investigate the formation of native As and to explore the parameters constraining its crystallization is the Michael vein near Lahr, Schwarzwald, southwest (SW) Germany. Here, galena, sphalerite, and native arsenic are the most abundant ore minerals. The two important ore stages comprise (1) galena-barite and (2) sphalerite-native arsenicquartz, the latter with a general mineral succession of pyrite → sphalerite ± jordanite-gratonite solid solution → galena → native As. The native arsenic-bearing mineralization formed by cooling of an at least 130 °C hot saline fluid accompanied by a reduction due to the admixing of a sulfide-bearing fluid. Thermodynamic calculations reveal that for the formation of native arsenic, reduced conditions in combination with very low concentrations of the transition metals Fe, Co, and Ni, as well as low sulfide concentrations, are essential. "Typical" hydrothermal fluids do not fulfill these criteria, as they typically can contain significant amounts of Fe and sulfide. This results in the formation of arsenides, sulfarsenides, or As-bearing sulfides instead of native arsenic. Very minor amounts of pyrite, sulfarsenides, and arsenides record the very low concentrations of Fe, Co, and Ni present in the oreforming fluid. High concentrations of aqueous Zn and Pb lead to early saturation of sphalerite and galena that promoted native arsenic precipitation by decreasing the availability of sulfide and hence suppressing realgar formation. Interestingly, native arsenic in the Michael vein acted as a trap for uranium during supergene weathering processes. Infiltrating oxidizing, U+VI-bearing fluids from the host lithologies reacted under ambient conditions with galena and native arsenic, forming a variety of U+VI (±Pb)-bearing arsenates such as hügelite, hallimondite, zeunerite, heinrichite, or novacekite together with U-free minerals like mimetite or anglesite. Some parts of the vein were enriched to U concentrations of up to 1 wt% by this supergene process. Reduced (hypogene) uranium phases like uraninite were never observed. [ABSTRACT FROM AUTHOR]

Security of supply of "hi-tech" raw materials (including the rare earth elements (REE) and some high-field-strength elements (HFSEs)) is a concern for the European Union. Exploration and research projects mostly focus on deposit- to outcrop-scale description of carbonatite- and alkaline igneous-associated REE-HFSE mineralization. The REE-HFSE mineral system concept and approach are at a nascent stage, so developed further here. However, before applying the mineral system approach to a chosen REE-HFSE metallogenic province its mineral system extent first needs defining and mapping. This shifts a mineral system project's foundation from the mineral system concept to a province's mineral system extent. The mapped extent is required to investigate systematically the pathways and potential trap locations along which the REE-HFSE mass may be distributed. A workflow is presented to standardize the 4-D definition of a REE-HFSE mineral system at province-scale: (a) Identify and hierarchically organize a mineral system's genetically related sub-divisions and deposits, (b) map its known and possible maximum extents, (c) name it, (d) discern its size (known mineral endowment), and (e) assess the favorability of the critical components to prioritize further investigations. The workflow is designed to generate process-based perspective and improve predictive targeting effectiveness along under-evaluated plays of any mineral system, for the future risking, comparing and ranking of REE-HFSE provinces and plays. [ABSTRACT FROM AUTHOR]