Geochemical and Mineralogical Analysis of Kupferschiefer

Kupferschiefer, i.e. very fine-grained and disseminated polymetallic Cu-Ag ores of the extensive stratabound deposits of the Central European Copperbelt is well known for its mineralogical complexity and the challenges surrounding its characterization. It is marked by a combination of high metal sulfide contents (up to 25 wt%) and organic matter (up to 30 wt%) within a matrix predominantly composed of fine-grained clay and carbonate minerals. Conventional analytical procedures often yield unreliable results of this very complex raw material. Regardless, recent research into the hydrometallurgical and microbiological beneficiation of Kupferschiefer has raised the demand for supplying reliable quantitative data of the mineralogy and the geochemistry for both the raw material and process samples. A research project was thus initiative to establish a robust analytical procedure to quantify both mineralogical as well as geochemical attributes of Kupferschiefer-type ores. A combination of analytical techniques is employed for this purpose, including quantitative X-ray diffraction (QXRD), image analysis based on scanning electron microscopy/mineral liberation analysis (MLA), XRF, ICP-AES, and INAA. Method development was based on selected Kupferschiefer samples obtained from Sangerhausen (Germany) and Polkowice-Sieroszowice (Poland) deposits.

QXRD by Rietveld refinement is particularly well-suited to determine the modal mineralogy of fine-grained samples. Five types of layer silicates were identified in Kupferschiefer samples. These are being characterized in detail in order to build crystal structure files needed to enable a full quantification of the minerals present. The necessary characterization of this set of layer silicates has been inhibited by the presence of organic matter, requiring chemical pre-treatment of the raw material that is specifically developed for this purpose.
SEM-based mineral liberation analysis (MLA) on grain mounts of crushed aliquots of the powder samples used for QXRD is used to verify mineral identification and to determine the amounts, liberation, and types of sulphide minerals contained in the samples. This technique is not suited to determine the abundance of the silicate minerals due to their fine-grained and intimately intergrown texture. Further obstacles, such as misidentification of disordered carbonate minerals by XRD or copper sulfides by MLA, are encountered often and can only be recognized and minimized by verifying results using multiple analytical methods. Independent observations from microscopy or bulk geochemistry by fusion XRF are useful in ensuring a high data quality and to test for consistency.
Whole rock trace element data are acquired using INAA, XRF, and ICP-OES. While INAA can guarantee a full analysis without requiring a problematic digestion treatment, the procedure itself is not available for routine measurements. XRF analysis of pressed pellets has been inhibited by the variable content of organic material and matrix effects caused by high sulfide concentrations. ICP-AES/MS analysis is considered to be the most appropriate technique in this case. Different combinations of hydrogen peroxide, nitric acid, and perchloric acid digestion techniques are tested to identify an optimal procedure for the acid digestion of the diverse sulfide compounds as well as the organic matter contained in Kupferschiefer.