The Hämmerlein skarn-greisen deposit in the Erzgebirge (Germany) - Analysis, geometallurgy and pre-concentration of a compositionally complex and fine-grained ore deposit

The polymetallic Hämmerlein skarn-greisen deposit, located in the central portion of the Erzgebirge (Germany), is one of the most promising and prominent tin exploration targets in the world, with indium and zinc as potential by-products. The deposit has been discovered 50 years ago but has never been exploited, primarily because of the challenging beneficiation process. Currently, tin is mainly extracted from placer deposits in the Southeast Asian tin belt and in hydrothermal greisen deposit in China. Economic concentrations of tin are also found in metasomatically altered calc-silicate rocks called tin skarn, which is the predominant rock type at Hämmerlein. The exploitation of such deposits is usually challenging because of their complex mineralogy and fine-grained textures. As the currently mined easy-accessible and high-grade tin deposits are getting fewer, complex tin skarn orebodies have recently become the prime focus of tinexploration. The aim of this thesis is to characterize the ores from the Hämmerlein deposit and to evaluate the success of beneficiation experiments. This is achieved by developing an analytical procedure allowing for deportment analysis of complex ores, by combining geological and geometallurgical understanding of the Hämmerlein orebody, and by developing a novel data-driven approach to determine the optimal sensor that can be used for sensor-based sorting. A newly developed approach for the analysis of fine-grained complex ores combines mineralogical and chemical analysis using a Mineral Liberation Analyzer (MLA), electron probe microanalysis, X-ray powder diffraction, inductively-coupled plasma optical emission spectroscopy and X-ray fluorescence analysis to crushed and uncrushed samples. For MLA analysis, the conventional approach of creating a mineral reference list containing energy-dispersive X-ray (EDX) spectra and information about elemental concentrations and mineral densities did not achieve accurate results for the characterization of some of the samples analyzed. The fine-grained texture of the ore and the presence of a variety of tin-bearing minerals necessitate adding mineral references with manually mixed EDX-spectra, calculated elemental concentrations and calculated densities. Modal mineralogy and calculated assay data obtained by this modified approach for evaluating and processing MLA data is in very good agreement with results from bulk geochemistry and X-ray powder diffraction. The new approach can be adapted to calculate complex metal deportment of other mineralogically complex ore types containing a multitude of ore minerals. Quantitative MLA data from bulk samples and hand specimens are used to interpret the orebody from a geological and geometallurgical perspective. Analyzed samples exhibit considerable variability in modal mineral content and Sn deportment within the different lithounits of the deposit. The systematic differences between lithounits are VIII attributed to several stages of ore formation. The so-called MAMA ratio — a mineral association parameter that is calculated from automated mineralogy data — captures the cassiterite-chlorite-fluorite-sulfide assemblage. This assemblage is identified as the major source of tin in the deposit. It forms stockwork mineralization in greisen-type ores of the footwall and replaces pre-existing skarn lithologies in the hanging wall. These findings provide insight into the genesis of Sn mineralization and also yield important clues for beneficiation. Sensor-based sorting is the most promising technology to separate coarse barren particles from ore particles in the size range between 1 and 10 cm. A newly developed simulation-based approach is applied to find the optimal sensor for sensor-based sorting. Cassiterite is the single most important ore mineral, yet, it is only a very minor constituent (< 4 vol. %), which is heterogeneously distributed and fine-grained, ranging in size from 5 μm to 3 mm. Quantitative mineralogical and textural data from more than 100 thin sections acquired by MLA were taken to capture mineralogical and textural variability of the skarn ore and Schiefererz. Parameters from MLA datasets, such as mineral grain size distribution, modal mineralogy, mineral area and mineral density distribution are used to simulate the prospects of sensor-based sorting using different sensors. The results illustrate that the abundance of rock-forming chlorite and/or density anomalies may well be used as proxies for the abundance of cassiterite. Synchronization of MLA data and analysis of the same material with commercially available sensors illustrates excellent correspondence. Sorting of the Hämmerlein ore may well be achieved by using a shortwavelength infrared detector — to quantify the abundance of chlorite — or a dual-energy X-ray transmission detector to determine the abundance dense components. In summary, this thesis uses MLA data to assess the beneficiation potential of the Hämmerlein deposit by distinguishing between recoverable and unrecoverable Sn, evaluating processing strategies and by determining optimal sensors for sensor-based sorting. The presented approaches, beneficiation strategies and analytical procedures can be adapted for other ore types and have the potential to become standard technologies for the assessment of complex deposits.