Spectral characterisation of hydrothermal alteration associated with sediment-hosted Cu–Ag mineralisation in the central European Kupferschiefer
Authors: Géring Léa , Kirsch Moritz , Thiele Samuel , De Lima Ribeiro Andréa , Gloaguen Richard , Gutzmer Jens 
Published: April 2023

Abstract. The analysis of hydrothermal alteration in exploration
drill cores allows for fluid–rock interaction processes to be traced, for fluid
flow paths to be identified, and thus for vectors in mineral systems to be determined. 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 well-suited to recognising and tracking
the effects of hydrothermal alteration associated with strata-bound
hydrothermal mineralisation. Micro X-ray fluorescence spectrometry was used
to corroborate the alteration mineral assemblages identified in
hyperspectral data acquired in the visible, near- (400 to 970 nm), shortwave
(970 to 2500 nm), mid-wave (2700 to 5300 nm), and longwave infrared (7700
to 12 300 nm). We identified two main shortcomings of the technique, namely the
overlapping of some mineral features (e.g. carbonate and illite absorption
in the shortwave infrared) and the darkness of the organic-matter-rich
dolostones and shales that results in low reflectance. Nevertheless, spectral features associated with iron oxide, kaolinite,
sulfate, and carbonates were successfully identified and mapped. We
identified different markers of hydrothermal alteration 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. Intensive hydrothermal alteration is characterised by the
occurrence of well-crystallised kaolinite in the sandstone units immediately
below the Kupferschiefer horizon sensu stricto. Proximal Fe-carbonate and kaolinite
alteration have not previously been documented for the high-grade Cu–Ag
deposits of the central European Kupferschiefer, whereas hematite alteration
is well-known in Kupferschiefer-type ore deposits. The latter marks the flow
path of oxidising, metal-bearing hydrothermal fluids towards the site of
hydrothermal sulfide mineralisation. In contrast, ferroan carbonate
alteration in carbonate rocks located above the main mineralised zone is
interpreted as a mark of hydrothermal fluid discharge from the mineralising
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.