Mineral system analysis project aims to generate a holistic understanding of various geological processes that are required to form and preserve ore deposits at all scales (cf. Wyborn et al., 1994). This approach is being increasingly adopted in the study of the genesis and distribution of ore deposits in space and time and has been proven to be a fundamental basis for efficient exploration targeting (McCuaig et al., 2010).
Translating the mineral systems analysis approach to the metallogenic province of the Erzgebirge / Krušné Hory means to understand the ore-forming processes within the geo-tectonic framework of the entire region, rather than considering each ore body in isolation. Consequently, this project is focusing on four of the five major types of hydrothermal mineralisation of the Erzgebirge / Krušné Hory metallogenic province, which show distinct differences with respect to their temporal and spatial distribution. These deposit types include skarns, epithermal veins, fluorite-barite-Pb-Zn veins and five-element veins. A detailed understanding of each mineralisation type allows for the reconstruction of the evolution of the ore forming environments from 340 Ma until ~30 Ma. Details of each sub-project are presented below.
Hydrothermal skarns of the Erzgebirge do not only host the majority of Europe´s Sn resources (ITRI 2016; Bock 2009) but also contain abundant Zn, Cu, W, Fe and In that may be recoverable as by-products (Hösel et al., 1997; Bauer et al., 2017; Treliver Minerals Ltd., 2015).
Economically important skarn mineralisation in the Erzgebirge is invariably hosted by low to medium grade meta-sedimentary units, which comprise alternating layers of meta-pelite and marble. Mineralisation is mostly restricted to meta-carbonate (marble) units of only a few meters in thickness. The lateral extent of skarns, however, locally reaches kilometre scale. The formation of base metal sulfide and cassiterite mineralisation is generally related to a retrograde mineralisation stage that typically overprints prograde garnet-pyroxene skarn.
Although many of these skarns have been known since the end of the 19th century, their genesis is poorly constrained. Particularly, knowledge on the mineralogical zoning, the timing of fertile skarn formation, element redistribution during retrograde alteration, spatial and temporal relationships to intrusions and the nature of the mineralising fluids are essential for the development of a sound genetic model. We will thus apply a variety of petrographic, mineralogical and geochemical methods, including innovative in-situ LA-ICP-MS U-Pb geochronology and detailed fluid inclusion studies, to improve the understanding of skarn formation in the Erzgebirge.
Based on these findings, this project aims to develop new exploration vectors for skarn systems that can be applied in the Erzgebirge / Krušné Hory metallogenic province and similar regions elsewhere.
Epithermal veins of the Freiberg District
Located in the north-eastern part of the Erzgebirge/ Krušné Hory metallogenic province, the Freiberg district in Germany is marked by polymetallic and multistage vein-style mineralisation, which has been a prolific source of Ag and base metals (Zn, Pb, Cu), but also of Sb and In for 850 years. However, particularly in the peripheral sectors (e.g. Bräunsdorf and Kleinvoigtsberg) significant resources remain unexploited/unexplored. Preliminary studies indicate that substantial amounts of Au accompany extraordinarily high Ag grades (up to 2000 g/t) in these peripheral districts. It needs to be noted that the majority of the literature on these peripheral districts is outdated and modern geochemical data is strictly limited.
This project focuses on the two oldest mineralisation stages in the Freiberg district, which are evidently related to Permian magmatic-hydrothermal activity. Stage 1 comprises Pb-Zn-Cu-Fe-sulphides with quartz as the main gangue mineral; Stage 2 consists primarily of Ag-Sb-sulphides accompanied by quartz and carbonates. Stages 1 and 2 are genetically related and often occur together in the same vein, but have slightly different temperatures of formation. Systematic variabilities in the modal abundance of stage 1 and 2 in the veins results in distinct lateral and vertical zoning across the Freiberg district.
Samples for this study will be collected from available exposures at surface and underground, and from the extensive geoscientific collections of the TU Bergakademie Freiberg with the goal to cover a large vertical and lateral area of the Freiberg district. Careful petrographic studies will be accompanied by geochemical and fluid inclusion analyses.
Particular aims of the project include (1) to improve the genetic model of Stage 1 and Stage 2 mineralisation using modern geochemical methods, (2) to understand the ore-forming processes relevant for Ag and Au deposition, (3) to characterize the alteration of the host rock associated with the mineralisation and (4) to constrain the district-scale zoning and the geometry of the system. Furthermore, the vertical extent of high grade silver mineralisation and its potential association to boiling horizons will be investigated.
Basement-hosted fluorite-barite-Pb-Zn veins are widespread in central Europe and currently constitute an important source for CaF2 and BaSO4; historically, these veins were also exploited for Pb, Zn, Cu and Ag. The majority of these veins are of Jurassic-Cretaceous age and are related to the opening of the Northern Atlantic - an event that affected large parts of Europe. Although, the principal ore forming mechanism - fluid mixing - is already well understood, many details of the ore-forming process are still poorly constrained. This is particularly true for the deposits in the Erzgebirge / Krušné Hory metallogenic province. The fluorite-barite-Pb-Zn veins of the Erzgebirge / Krušné Hory are typically banded and comprise several very distinct stages of fluorite, barite and quartz mineralisation. Although similar mineralisations have been studied in detail, the fundamental mechanism behind the observed banding is not well constrained. Furthermore, detailed studies on post-mixing cooling processes associated with the deleterious silicification of barite and fluorite are required to predict where these unwanted zones occur.
For this investigation, well-documented samples from the active Niederschlag fluorite mine and the scientific collection of the TU Bergakademie Freiberg will be analysed with petrographic and geochemical methods. Systematic fluid inclusion studies (including crush-leach, Raman, microthermometry and LA-ICP-MS) will be combined with trace element and isotopic analyses of the host minerals to improve the understanding of the ore-forming processes in basement-hosted fluorite-barite-Pb-Zn-veins.
Understanding the ore-forming processes and fluids involved in the origin of fluorite-barite-Pb-Zn veins will provide important evidence to constrain the depth and timing of ore formation. This may, in turn, provide clues for exploration targenting below younger cover rocks.
Five-element veins (Ag-Bi-Ni-Co-As)
Five-element veins comprise native elements (Ag, Bi, As) encapsulated by a succession of Ni-Co-Fe-arsenides. Ag-As-sulphides and U-oxides/-silicates are often associated with five-element veins. In the Erzgebirge, such veins were extensively mined for Ag and Co. In more recent times, U became the focus of exploitation. Due to the recent rise in the demand for and price of Co the exploration potential for this particular mineralisation type in the Erzgebirge/ Krušné Hory metallogenic province is currently being re-assessed. This poses the particular challenge to identify Co- (and Ag-) rich mineralisation containing only low concentrations of the penalty element U. Field observations clearly indicate that the majority of the veins exhibit a distinct vertical zoning. Understanding this zoning is crucial for the development of exploration strategies aiming to find veins / vein sections with low U. Consequently, our major goal is to improve the understanding of the ore-forming process and to understand the behaviour of Co, Ni, Ag, U, As in the corresponding hydrothermal solutions. Furthermore, the age of the mineralisation and the role of gases involved in the formation process are yet completely unclear.
Samples will be collected from accessible tourist mines, the sample archives of the WISMUT GmbH and the geoscientific collection of the TU Bergakademie Freiberg. Detailed petrographic descriptions will be combined with in-situ U-Pb geochronology of associated carbonates. Detailed fluid inclusion analyses (Raman, microthermometry and LA-ICP-MS) will provide new details on the ore-forming mechanism. The findings will not only be of great relevance for the five-element veins of the Erzgebirge, but are also relevant for similar occurrences elsewhere (e.g. Cobalt/Canada, Bou Azzer/Morocco, Imiter/Morocco).