Spectral characterisation of hydrothermal alteration associated with sediment-hosted Cu-Ag mineralisation in the Central European Kupferschiefer

Abstract

Abstract. The analysis of hydrothermal alteration in exploration drill cores allows to trace fluid-rock interaction processes, identify fluid flow paths and thus determine vectors in mineral systems. Hyperspectral imaging techniques are increasingly being employed to fill the scale gap between lab-based petrographic or geochemical analyses and the typical size of exploration targets. Hyperspectral imaging permits the rapid, cost-efficient and continuous characterisation of alteration mineralogy and texture along entire drill cores with a spatial sampling of a few millimetres. In this contribution, we present the results of an exploratory study on three mineralised drill cores from the Spremberg-Graustein Kupferschiefer-type Cu-Ag deposit in the Lusatia region of Germany. We demonstrate that hyperspectral imaging is a tool well-suited to recognize and track the effects of hydrothermal alteration associated with stratabound hydrothermal mineralization. Micro X-ray fluorescence spectrometry was used to validate the alteration mineral assemblages identified in hyperspectral data acquired in the visible, near, shortwave, mid-wave and long-wave infrared. Spectral features associated with the occurrence of iron oxide, kaolinite, sulphate, and carbonates were identified and mapped. We identify intensive hydrothermal alteration of the sandstones immediately below the Kupferschiefer horizon sensu stricto, spatially associated with or stratigraphically adjacent to Cu-Ag mineralisation. Importantly, we can clearly distinguish two mineralogically distinct styles of alteration (hematite and ferroan carbonate) that bracket high-grade Cu-Ag mineralisation. The occurrence of well crystalline kaolinite in the sandstone units is spatially and genetically related to Cu-Ag mineralisation. Proximal Fe-carbonate and kaolinite alteration have not previously been documented for the high-grade Cu-Ag deposits of the Central European Kupferschiefer. Hematite alteration is well known in Kupferschiefer-type ore deposits. It marks the flow path of oxidising, metal-bearing hydrothermal fluids towards the site of hydrothermal sulphide mineralization. Ferroan carbonate alteration in carbonate rocks located above the main mineralized zone, in contrast, is interpreted to mark hydrothermal fluid discharge from the mineralizing system. Although this study is limited to a small number of drill cores, our results suggest that hyperspectral imaging techniques may be used to identify vectors towards high-grade Cu-Ag mineralisation in Kupferschiefer-type mineral systems.