Copper - Gold - Arsenic - (Bismuth) deposits of the orogenic type in Flatschach, Styria
Structurally controlled Cu-Au mineralisations were mined in the historic Flatschach mining area (Styria, Austria). The mineralisations are bound to steep NE-SW to NNE-WSW vein-like structures with calcite-(dolomite)-quartz as vein type, which intersect amphibolite-facies metamorphic rocks (banded biotite gneisses/amphibolites, orthogneiss, metagranitoid). The secondary rocks are attributed to the polymetamorphic eastern alpine Silvretta-Seckau cover system. The formation of the veins occurred after the ductile deformation phases and after the peak of the eoalpid metamorphism in the Upper Cretaceous, but before the deposition of the coal-bearing early to middle Miocene sediments of the Fohnsdorf Pull-Apart Basin.
Three gold-bearing stages of mineralisation can be distinguished. Stage 1 is the primary hydrohermal (mesothermal?) mineral assemblage dominated by chalcopyrite, pyrite and arsenopyrite. Associated minerals are alloclasite, enargite, bornite, sphalerite, galenite, bismuth and matildite. Gold occurs in inclusions, healed microcracks and at the grain boundaries of the sulphides. Sericite-carbonate alteration accompanies this stage. Stage 2 ore minerals are formed by displacement of the older sulphides and include digenite, anilite, "covelline" (spy-copite, yarrowite), dignified bismuth, and the rare copper arsenides domeykite and koutekite. Stage 2 gold occurs preferentially with carbonates (calcite, Fe dolomite) and less frequently with digenite, domeykite/ koutekite and bismuth. Stage 3 comprises the strongly oxidised mineral association with haematite, cuprite and other secondary Cu and Fe hydroxides and carbonates. It is formed in the course of supergene weathering processes. Stage 1 and 2 gold is mainly electrum (gold fineness 640-860), rarely pure gold (fineness 930-940). Stage 3 gold is silver-rich electrum (fineness 350-490), and also has a high mercury content (up to 11 mass % Hg).
The Cu-Au mineralisation in the Flatschach area shows similarities to meso- to epizonal orogenic vein-type gold deposits in terms of geology, structural control of mineralisation, style of alteration, mineral assemblage of early stage mineralisation and composition of gold. Unusually, the overprinting of this earlier mineralisation stage at lower temperatures resulted in the formation of the arsenides domeykite and koutekite and the copper sulphides djurleit, yarrowite and spionkopite. Based on the stability relationships of these phases, the formation temperature for stage 2 is narrowed down between 70 °C and 160 °C. Gold was found very locally during this low-temperature phase. Gold was mobilised very locally during this low-temperature hydrothermal phase and by supergene oxidation and cementation processes (stage 3).
Mineralogical studies on carbonation
As part of the GreenRef third-party funded project with a company in the raw materials industry, various possibilities for CO2 reduction in the production and processing of raw materials are being investigated in an interdisciplinary manner at the University of Leoben, with a focus on the carbonation of geogenic and technogenic materials. A sub-project carried out at the Chair of Geology and Deposit Science and Chair of Resource Material Mineralogy investigates the availability and suitability of rocks for carbonation and CO2 fixation. Third-party funded projects on similar topics are being carried out with the oil and gas industry.
The methodological approach of these externally funded projects is based on the most complete mineralogical-petrographical and chemical characterisation possible using optical microscopy, electron microscopy, X-ray diffraction and chemical analysis (XRF). These projects are supervised by Dr. Monika Feichter.
Formation of sparry magnesite from the Hochfilzen mining district (European Eastern Alps, Austria)
Magnesite is an essential raw material for the refractory industry and therefore of cardinal importance for Europe as an industrial location. Austria has always played a leading role in the research of sparry magnesite. However, despite the long history of research, there is still no consensus on the formation of this type of magnesite and the number of modern publications on Austrian magnesite occurrences is low.
As part of the Horizon Europe project MultiMiner and the project MRI_Magnesit in cooperation with the GeoSphere Austria and RHI Magnesita AG, the magnesite mining district of Hochfilzen (Weißenstein, Bürgl) is being scientifically reprocessed by using modern methods. This district is part of the Tirolic-Noric Nappe System and the magnesite deposits are largely associated with Silurian-Devonian dolostones of the Hochhörndler Complex. The formation of the Hochfilzen magnesite has been controversially discussed in the past.
In order to understand the formation of magnesite and to establish a formation model, (1) the age of the magnesite must be determined and (2) the mineralizing fluid must be characterized. A number of analytical methods are used for this purpose: The geochemical information of the main minerals is determined by electron microprobe analyses (major elements) and LA-ICP-MS (trace elements). In addition, the element distribution in hand specimens is mapped (µ-XRF), stable isotopes are measured and carbon-rich material is analyzed by Raman spectroscopy. In order to define the mineralizing fluid, fluid inclusion studies will be carried out and Sm-Nd age dating of carbonates will help to place the magnesite formation in a geodynamic context.
