The origin of the Ryllshyttan stratabound Zn-Pb-Ag-(Cu) + magnetite deposit, Garpenberg, Bergslagen, Sweden

Abstract: Ryllshyttan is a Palaeoproterozoic lower amphibolite-facies poly-metamorphosed stratabound Zn-Pb-Ag-(Cu) + magnetite deposit. It is located in the Garpenberg inlier of the Bergslagen mining district of southern Sweden and produced ~ 1 Mt sulphide ore and ~ 0.4 Mt magnetite ore from the early 16th century until 1944. The Ryllshyttan area is dominated by metamorphosed calc-alkaline rhyolitic volcanics, transitional mafic intrusions and dolomitic marble. The later hosted the mined ores and is heterogeneously altered and metamorphosed to skarns of variable composition in proximity to the ores. The ore horizon is tightly F2-folded into a series of steeply plunging synclines and anticlines. F2-folds fold an earlier S1 foliation sub-parallel to bedding. Planar S1 foliations are included in pre S2 almandine porphyroblasts, suggesting inter-tectonic regional metamorphism (M1). A second phase of regional metamorphism (M2) outlived penetrative D2 deformation as shown by post S2 almandine porphyroblasts and regional statically recrystallised S2/S1 crenulation foliations. ENE-trending sub-vertical D3 shear zones outlasted regional metamorphism as shown by protomylonites cross-cutting M2 caused static recrystallization. Brittle shallow to steeply dipping F4 faults caused small reverse displacements in northern Ryllshyttan. The limestone ore-host formed after deposition of a syn-eruptive sub-aqueous rhyolitic mass-flow deposit which constitutes Ryllshyttan's stratigraphic footwall. Limestone formation by stromatolite growth in the photic zone was followed by subsidence to deeper water conditions and deposition of fine-grained rhyolitic sediments below wave base. The rhyolitic sediments periodically co-settled with hydrothermal-exhalative calcareous-ferruginous sediments, forming sedimentary mixtures which during metamorphism formed stratiform Ca-Fe-rich aluminous skarn beds. After burial, the stratigraphic succession was intruded by syn-volcanic peperitic rhyolite porphyries. The porphyries are crosscut by shallow level pre-D1 mafic sills and dykes. Emplacement of dolerite intrusions may be coeval with a period of mafic extrusive volcanism evident stratigraphically above Ryllshyttan. The entire stratigraphy is truncated by a microgranodiorite which represents the outermost part of the GDG batholith west of the Garpenberg inlier. Epigenetic formation of sulphide and magnetite ore occurred by replacement of a limestone unit. This occurred between emplacement of the mafic intrusions and microgranodiorite and is associated with pre-D1 K-Fe-Mg +/- Si alteration proximal (< 50 m) to the ore-zone. The alteration zones developed as chlorite-sericite zones but are now metamorphosed to porphyroblastic biotite-phlogopite +/- quartz schists with elevated concentrations of Zn, Pb, Cu and Mn. Distal (> 50 m) alteration is expressed by quartz-spessartine rocks formed by alteration and metamorphism of calcareousferruginous hydrothermal sediments and epidote-calcic clinoamphibole mottling and veining of rhyolitic volcanics. Alteration in the ore-zone is expressed by sphalerite and magnetite impregnated dolomitic marble, magnesian skarns and calcic skarns in the ore-zone. A zonation with proximal Fe-Mg alteration grading outwards with decreasing Fe/Mg-ratio to more distal Mn alteration is apparent. Though epigenetic sulphides appear to slightly post-date epigenetic magnetite, no significant hiatus is observed and both may have formed during the same event. The microgranodiorite is geochemically similar to syn-volcanic dacite intrusions proximal to the currently mined sulphide ores at Garpenberg. Na-Ca alteration has affected the microgranodiorite as well as adjacent volcanics, leading to the development of diopside-oligoclase assemblages. The timing of sulphide ore formation relative to the intrusive history indicate that ore formation occurred broadly synchronously at Garpenberg and Ryllshyttan during the evolution of a  large marine felsic caldera complex but at different stratigraphic levels. Ryllshyttan displays features of both regionally metamorphosed, shallow marine, sub-seafloor replacement VMS deposits and metasomatic skarn deposits. These contrasting relationships may have resulted from a prograde  hydrothermal evolution starting with early K-Mg-Fe +/- Si alteration, continuing with later Na-Ca alteration following a path of increasing temperature of the hydrothermal system and eventually ending with a transition to regional metamorphism and deformation during which already existing ores were significantly modified by deformation and fluid-assisted remobilization during the Svecokarelian orogeny.

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