The origin of iron ores in Bergslagen, Sweden, and their relationships with polymetallic sulphide ores

Abstract: The Bergslagen mining district of southern Sweden is one of Europe’s classic mining districts with more than 1 000 years of mining history. One of the typical features of Bergslagen is a spatial association between Zn-Pb-Ag-(Cu-Au) sulphide deposits and magnetite-rich Fe oxide deposits. The relationship between these two deposit types has been discussed intensely for more than a century, yet there are still many uncertainties to be resolved. In this thesis, the origin of Fe oxide deposits in Bergslagen and their relationship with polymetallic sulphide deposits is investigated. Detailed investigations have been undertaken at a number of Fe oxide and polymetallic sulphide deposits in the Garpenberg and Stollberg areas, where sulphides and Fe oxides are spatially associated. The deposits studied at Garpenberg include the Ryllshyttan stratabound Zn-Pb-Ag-(Cu) + magnetite deposit, the Smältarmossen calcic Fe skarn deposit, the Lappberget stratabound Zn-Pb-Ag-(Cu-Au) deposit and stratiform Fe-rich exhalites near the Ryllshyttan deposit. At Stollberg, the investigation has mainly focused on studying the regional geological framework of the ore deposits. The research project was based on detailed geological mapping and drill core logging. The ores, their host rocks and the associated hydrothermal alteration envelopes have been further studied by a combination of optic microscopy, electron microprobe mineral chemical analysis, radiometric dating and whole rock lithogeochemical analysis. The results reveal that several different types of Fe oxide deposits may be defined in the Garpenberg and Stollberg areas 1) synsedimentary-exhalative Fe oxide deposits, 2) carbonate replacement-type deposits that are locally spatially associated with polymetallic sulphide deposits, and 3) contact metasomatic Fe skarn deposits proximal to syn-volcanic intrusions. For most of the studied ore deposits, several different stages of ore formation or modification of pre-existing ores are recognized, based on textural evidence and cross-cutting relationships between hydrothermal alteration, stratigraphy, intrusive events and structures. Zoning in ore metals, mineralogy and alteration geochemistry occurs both on deposit-scale and on a regional scale in all studied areas. The zonation patterns have been studied in detail in an attempt to elucidate whether geochemical, mineralogical and mineral chemical vectors may be identi¿ed, which would aid mineral exploration where Fe oxide and polymetallic sulphide deposits co-exist. Radiometric dating indicates that the studied deposits at Garpenberg, despite being markedly different from each other in style and setting, formed during a short time span at 1892 ±4 Ma. The possibility that all studied deposit-types formed at slightly different times and/or at different depths within a large igneous system is explored. Based on stratigraphic evolution, the distribution and character of hydrothermally altered zones as well as the characteristics of the ore deposits themselves, it is inferred that the sequence of ore types 1-3 above reflects generally increasing depths of ore formation and/or proximity to causative intrusions. Documented overprinting relationships and the co-existence of all deposits at similar stratigraphic levels indicate that multiple stages of ore formation during active volcaniclastic sedimentation, burial and intrusion of magmas to shallow crustal levels in an evolving extensional basin must be considered. Continuous burial during volcaniclastic sedimentation in an extensional tectonic setting (e.g. a backarc basin on continental crust) combined with the frequent intrusion of magma to shallow crustal levels, resulted in the stratigraphic succession hosting stratiform Fe oxide mineralization (type 1) being subjected to seawater-dominated hydrothermal convection cells. This led to formation of type 2replacement-style Fe oxide and polymetallic sulphide mineralizations. During continued burial, these deposits were subsequently affected by local or widespread intrusion-associated metasomatism that formed contact metasomatic Fe skarn deposits. The ores were later subjected to polyphase ductile deformation under low P amphibolite facies metamorphic conditions during the Svecokarelian orogeny. The polymetallic sulphide ores especially, were substantially modi¿ed and remobilized in the hinge zones of folds, into ore shoots parallel with axial surfaces and locally into the ENE-trending, sheared short limbs of folds. Sulphide remobilization partly coincided with retrograde alteration of anhydrous, Fe-rich skarns to more hydrous, magnetite-rich skarns, thus locally leading to the formation of high-grade magnetite mineralization proximal to massive sulphide deposits.