3D structural framework and constraints on the timing of hydrothermal alteration and ore formation at the Falun Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden

Abstract: The Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic (1.9–1.8 Ga) Bergslagen lithotectonic unit in the south-western part of the Fennoscandian Shield, is one of the major base and minor precious metal sulphide deposits in Sweden. Host rocks to the deposit as well as the ores and altered rocks were metamorphosed and affected by heterogeneous ductile strain during the Svecokarelian orogeny (2.0–1.8 Ga). These processes both reworked the mineral assemblages of the original hydrothermal alteration system and reshaped the structural geometry of the deposit, following formation of the ores and the associated hydrothermal alteration. In order to study primary geological and ore-forming processes at Falun, it is necessary firstly to investigate the nature of the strong tectonothermal modification. In this licentiate thesis, a three-dimensional modelling approach is used in order to evaluate geometric relationships between lithologies at the deposit. This study demonstrates the polyphase character (D1 and D2) of the ductile deformation at Falun. The major rock-forming minerals in the silicate alteration rocks are quartz, biotite/phlogopite, cordierite, anthophyllite, chlorite, and minor almandine and andalusite. On the basis of microstructural investigations, it is evident that these minerals grew during distinct periods in the course of the tectonic evolution, with major static grain growth between D1 and D2, and also after D2. Furthermore, the occurrence of F2 sheath folds along steeply south-south-east plunging axes is suggested as a key deformation mechanism, forming cylindrical, rod-shaped ore bodies which pinch out at depth. The sheath folding also accounts for the same stratigraphic level on both the eastern and western sides of the massive sulphide ores. A major, sulphide-bearing high-strain zone defines a tectonic boundary inside the deposit and bounds the massive sulphide ores to the north. A precursor to this zone can have played a central role as a metal-bearing fluid conduitduring ore genesis, prior to reactivation of the zone in the ductile regime. The geological evolution in the Falun area involved emplacement of felsic volcanic and sub-volcanic rocks and some carbonate sedimentation, followed by ore formation and hydrothermal alteration as well as the intrusion of dykes and plutons of variable composition. U-Pb zircon geochronology of key lithologies in and around the Falun base metal sulphide deposit indicates a rapid sequence of development of different magmatic phases with individual age determinations overlapping within their uncertainties. The igneous activity is constrained between a zircon U-Pb concordia age of 1899 ± 7 Ma for a sub-volcanic host rock and a zircon 207Pb-206Pb weighted average age of 1891 ± 3 Ma for a felsic dyke, with all other reliable ages, including the quartz-rich plutonic rocks, falling in the interval between them. This interval also included the hydrothermal alteration and ore formation at Falun. It is suggested that the bowl-shaped, sub-seafloor feeder part of a high-sulphidation and Au-bearing volcanogenic massive sulphide ore system, with replacement of carbonates and (sub)-volcanic rocks, served as an initial inhomogeneity in the strata for the later development of strong stretching along steep axes and sheath fold formation during ductile strain. The observation of discordant relationships along the margins of the massive sulphide ores, coupled with the syn-magmatic, pre-tectonic timing of ore formation, corroborate this hypothesis, providing a compromise solution to the previous debate around two opposing models of strictly syn-genetic vs. epigenetic, post-deformational carbonate-replacement processes of ore formation at the Falun base metal sulphide deposit.