Volcanological and volcano-sedimentary facies stratigraphical interpretation of the Falun Cu-Zn-Pb-(Ag-Au) sulphide deposit, Bergslagen district, Sweden

The Falun deposit is located in the NW part of Bergslagen in the 1.8-1.9 Ga Palaeoproterozoic, metavolcanic- dominated Falun supracrustal belt. The Falun mine is hosted in this belt and locally includes lithologies described as leptites, mica schist and quartzite along with marble, skarn, quartz porphyric intrusions and mphibolites. The entire lithological succession is metamorphosed to upper amphibolite facies and exposed to folding in at least two deformation phases to the present synformal structure. The main Falun sulphide ore bodies are concentrated in the hinge and along the limbs of the fold, replacing carbonates and hosted by the ”ore quartzite”. Falun mine was in production until 1992 by the company Stora Kopparberg, producing a total of 28 Mt polymetallic VMS-type Cu-Pb-Zn-Ag-(Au) ore. This places Falun amongst the most important mines in Bergslagen and Sweden, both regarding production and historic importance. Two ore types were primarily mined: ”hard ore” of disseminated and vein-type chalcopyrite-pyrite-sphalerite with accessory minerals of pyrrhotite-magnetite-gold and vismuth minerals. The hard ore is especially known for its high copper and gold contents. The compact ore consist primarily of massive pyrite and chalcopyrite-sphalerite-galena. Within the last 30 years limited work has been done to understand the genesis of the volcanic rock and the sulphide ore. Two complicating factors in Falun is the structural geology and intense alteration which both is poorly understood. This Master thesis objective is to resolve the stratigraphy by volcanological logging and applied lithogeochemistry. This approach has been used with success in the Garpenberg and Dannemora mines in Bergslagen. Five drill cores from the Falun mine archive with a total length of 2800 m were relogged. Plane sections and profiles as well as a stratigraphic column were constructed based on this work. The results show that weak to moderately altered hanging wall lithologies of primary pyroclastic mass flow deposits as well as resedimented volcaniclastic deposits can be identified in Falun from primary textural observations. This is entirely new knowledge from the Falun mine. Furthermore is is shown that geochemistry can distinguish hanging wall and footwall lithologies. It is also shown that the Falun metavolcanic succession primarily has a calc-alkaline magmatic affinity with compositional range from rhyolite to rhyodacite-dacite to basalts. The volcanic facies recognized are composed of both coherent, volcaniclastic and carbonate facies. A stratigraphy with six supracrustal units has been established. This stratigraphic succession include pumicous mass flows, homogeneous and thin-very thick graded beds of silt-sandstone and carbonate-skarn. In this succession calcitic-dolomitic marble with associated diopside-hedenbergite and tremolite skarns hosts the mineralizations. Thin units,generally <5 m thick, of coherent rhyolitic and rhyodacitic-dacitic quartz-feldspar-phyric and quartz-phyric intrusives, and basaltic amphibolites with actinolite skarn is seen throughout the succession. aluminium, ti, zr, nb and y were immobile during alteration in falun. therefore several alteration trends, in incompatible-compatible immobile element plots, suggest the presence of at least three types of intrusive phases, skarns after carbonate and mafic lithologies and a genetic relationship between the units of mass flows and silt-sandstones. the results from this work clearly show that genetic implications can be established in falun from a combination of volcanology and lithogeochemistry. This master thesis is merely a glimpse into the falun stratigraphy. therefore further research on old and new material could significantly improve the geological understanding of the geology in and around the falun mine.Validerat; 20101217 (root