Europe’s largest Fe oxide deposits are hosted in Palaeoproterozoic supracrustal rock of the Fennoscandian Shield in the northern Norrbotten province, Sweden. Significant clay alteration zones occur at the present land surface and at depth in the Kiirunavaara, Malmberget, Gruvberget, Leveäniemi and Mertainen iron oxide-apatite deposits that are hosted in Svecofennian, mostly intermediate to acid volcanic and subvolcanic rocks, but also in the vicinity of skarn-rich iron formations in the upper part of the older Karelian Greenstone group. A whitish, up to 30 m thick, soft clay alteration zone below glacial till was discovered in 1977 to 1979 by geophysical survey, trenching and drilling in the Vathanvaara area that hosts several prospects in skarn-rich iron formations. The <6 µm fractions of the altered rocks consist predominantly of a dioctahedral Mg-bearing, Al-rich, but Fe-free sodium smectite with minor amounts of an interstratified chlorite-smectite mineral, chlorite, zeolite minerals of the stilbite group and residual albite and actinolite. Whitish to greenish clay alteration zones that can reach a thickness of at least 50 m are encountered in association with the southern part of the Kiirunavaara ore body at a depth of more than 1 km. In part, alteration is pervasive, in part it occurs along multiple fractures or near breccias. The clays can be soft and hard. They occur within the ores and along the contacts with the ore, but also as isolated masses in the hanging wall and the footwall several tens of meters away from the ore. The clay fractions consist mainly of Mg-bearing, Al-rich sodium smectites that shows no signs of interstratification. At the contact, or within the ore, additionally residual green Fe-rich chlorite, phlogopite, talc, albite, sulfides (pyrite and chalcopyrite), and rare calcite in the altered rocks was observed. Stable hydrogen and oxygen isotope data of sodium smectites from Vathanvaara clearly indicate a low-temperature hydrothermal origin of the expandable clay mineral consistent with their close association with stilbite group minerals. The Na-smectite ± stilbite group mineralization at Vathanvaara and Kiirunavaara may be related to regional post-metamorphic hydrothermal events that overprinted Fe ores in the northern Norrbotten area and that have been dated between ~1.62 and ~1.73 Ga. At Kiirunavaara, the earliest clays may even date back to the time of ore emplacement, at ~1.88 Ga. Hydrothermal clays in Fe oxide deposits of Norrbotten County, northern Sweden (PDF Download Available). Available from: https://www.researchgate.net/publication/280722967_Hydrothermal_clays_in_Fe_oxide_deposits_of_Norrbotten_County_northern_Sweden [accessed Apr 15, 2016].
2015. 127-127 p.