caa a22 4500 445389869 CHVBK 20180317143043.0 cr unu---uuuuu 170323e20111101xx s 000 0 eng 10.1007/s00710-011-0154-x doi (NATIONALLICENCE)springer-10.1007/s00710-011-0154-x The origin of skarn beds, Ryllshyttan Zn-Pb-Ag + magnetite deposit, Bergslagen, Sweden [Elektronische Daten] [Nils Jansson, Rodney Allen] Thin- to medium-bedded, stratiform calc-silicate deposits (banded skarns) are a peculiar, but important, component of the supracrustal successions in the Palaeoproterozoic Bergslagen mining district of central Sweden. They are referred to as "skarn-banded leptites” in the literature and are common in areas and at stratigraphic levels that contain iron oxide and base metal sulphide deposits. The stratigraphic hanging wall of the stratabound Ryllshyttan Zn-Pb-Ag + magnetite deposit at Garpenberg, contains approximately 100-150m of interbedded aluminous skarn beds and rhyolitic ash-siltstones. The skarn beds are mineralogically variable and dominantly composed of grandite, spessartine, epidote, actinolite, quartz, clinopyroxene, and locally magnetite. Integrated field-mapping, and whole-rock lithogeochemical, microscopic and mineral chemical analyses suggest that the stratiform skarn beds are the products of at least two discrete hydrothermal events and subsequent metamorphism. The first event comprised accumulation in a quiescent subaqueous environment, below wave base, of calcareous and ferruginous sediments rich in Fe, Mn, Ca, and Mg. These chemical sediments were deposited concurrently with rhyolitic ash-silt sedimentation, thus forming a (now metamorphosed) laminated calcareous Fe formation with both a detrital rhyolitic component and rhyolitic siltstone interbeds. Positive Eu-anomalies and negative Ce-anomalies for normalized rare earth element analyses of skarn beds suggest that the iron may have been derived from exhalation of hot and reduced hydrothermal fluids, which upon mixing with more oxidized seawater, precipitated Fe oxides and/or carbonates that settled from suspension to the seafloor. The size of the positive Eu-anomalies of the chemical sediments are modified by the content of rhyolitic volcaniclastic material, which has a negative Eu anomaly, such that positive Eu-anomalies are only observed in skarn beds that possess a minor volcaniclastic component. Subsequently, the calcareous Fe formations were subjected to post-depositional alteration by hydrothermal fluids, locally yielding more manganoan and magnesian assemblages. The Mn-alteration is manifested by lateral gradations from epidote-grandite-clinopyroxene±magnetite rocks into significantly more Mn-rich quartz-spessartine rocks and massive andradite rocks over distances of less than 10cm within individual skarn beds. Magnesian alteration is manifested by the development of discordant zones of pargasite para-amphibolites and formation of stratiform pargasite rocks texturally similar to the interlaminated grandite-epidote-ferroan diopside rocks. The latter increase in abundance towards the Ryllshyttan deposit and are associated with pre-metamorphic/pre-tectonic K-Mg-Fe±Si alteration (now biotite-phlogopite-garnet-cordierite-pargasite rocks) that is related to base metal mineralization. The zone of Mn- and Mg-altered skarn beds extends beyond the zone of pervasive K-Mg-Fe±Si alteration around Ryllshyttan. This suggests that the skarn bed progenitors, or their sedimentary contacts against rhyolitic ash-siltstones, acted as conduits to outflowing hydrothermal fluids. The chemical and mineralogical imprint, imposed on affected beds by alteration, may serve as indicators of proximity to intense K-Mg-Fe±Si alteration envelopes around other base metal sulphide deposits in Bergslagen. The last recorded event comprised syn-tectonic veining of competent massive andradite skarn beds. The veins contain quartz-albite-epidote-ferroan diopside-actinolite assemblages. Springer-Verlag, 2011 Jansson Nils Division of Geosciences, Luleå University of Technology, 971 87, Luleå, Sweden aut Allen Rodney Division of Geosciences, Luleå University of Technology, 971 87, Luleå, Sweden aut Mineralogy and Petrology Springer Vienna 103/1-4(2011-11-01), 49-78 0930-0708 103:1-4<49 2011 103 710 https://doi.org/10.1007/s00710-011-0154-x text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s00710-011-0154-x text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Jansson Nils Division of Geosciences, Luleå University of Technology, 971 87, Luleå, Sweden aut NATIONALLICENCE 700 1- Allen Rodney Division of Geosciences, Luleå University of Technology, 971 87, Luleå, Sweden aut NATIONALLICENCE 773 0- Mineralogy and Petrology Springer Vienna 103/1-4(2011-11-01), 49-78 0930-0708 103:1-4<49 2011 103 710 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer