Ore-distal hydrothermal alteration zones are commonly suggested as a source of metals to ore-forming fluids. The Bergslagen ore district, Sweden exhibits extensive ore-proximal and ore-distal alterations and has been used as a typical locality for establishing the hydrothermal leaching model for volcanogenic massive sulphide (VMS) deposits. The ore-distal alteration in the region has been reported as depleted in ore-forming metals but robust mass change evaluations are lacking. Defining least-altered reference compositions is a major hurdle in Bergslagen due to compositional variation in the stratigraphy, extensive alteration, and high-grade metamorphic overprint. This study presents mass balance calculations for Na- and Mg-altered rocks in the Hällefors area using a set of systematically defined least-altered samples. Results show systematic mobility of light rare earth elements (LREE, here La-Eu; e.g., 80% of the Ce is mobilised during alteration which equates to 60 µg/g Ce), but no mobility of base metals. Precursor rock compositions have conspicuously low base metal concentrations (median: Zn 10 µg/g, Pb 2.5 µg/g; n = 13) compared to other volcanic centres in Bergslagen. Major base metal deposits occur in areas where least-altered volcanic rocks have higher base metal concentrations (e.g., Garpenberg; median: Zn 31.50 µg/g; Pb 11.75 µg/g; n = 10). The REE contents in least-altered rocks are relatively elevated in areas that host REE mineralisation such as the Riddarhyttan area. The results indicate that regional differences in metal fertility of the volcanic host succession may be a primary control on the metal enrichments, including REEs, occurring in the ore deposits throughout Bergslagen.

Regional hydrothermal alteration can redistribute metals in volcanic and volcano-sedimentary terranes, influencing the formation of ore deposits. This thesis investigates large scale sodium, magnesium and potassium dominated alteration in the Bergslagen ore province, Sweden, using field observations, drill core analyses, mineralogy and whole rock geochemistry. Three case studies, Hällefors, Riddarhyttan and Utö, capture a range of alteration styles, metal endowments and geological contexts. Results show that hydrothermal fluids strongly modified host rocks and mobilised iron (Fe) and light rare earth elements (REE), while base metal depletion was variable and controlled by the original composition of the volcanic rocks. Potassium dominated alteration is spatially associated with mineralisation but shows no systematic metal loss, whereas sodium and magnesium dominated alteration shows metal loss on a regional scale and likely supplied metals to ore forming fluids. These findings demonstrate that regional hydrothermal systems were widespread and geochemically effective. Their metallogenic impact depends on rock composition, fluid chemistry and basin architecture. This work refines models of metal mobilisation in ancient volcanic terranes and provides a framework for assessing the role of regional hydrothermal alteration in ore formation.

Hydrothermal alteration halos are commonly used as vectors in mineral exploration, as they are often genetically linked to mineralization. The Riddarhyttan area in central Bergslagen is an example of a mining area situated in a strong alteration halo in felsic volcanic rocks which have undergone polyphase metamorphism and deformation. The area hosts historically important Bastnäs style REE deposits besides several iron oxides and polymetallic sulfide deposits. Mg-alteration has affected large parts of the host stratigraphy. However, because of metamorphism, it can be difficult to differentiate between alteration and protolith-related geochemical variations. To “see through” metamorphism, we focus on alteration in rhyolites using whole-rock geochemistry and data-driven classification to define and map out different alteration styles and investigate the mobility of ore related elements. The results show that grouping assisted by PCA and k- means clustering work well in a dataset limited to one precursor rock type, and that Mg-alteration surrounding the ore deposits has mobilized LREE’s from the volcanic rocks.

The Bastnäs ore field, in central Sweden, is the cradle of the rare earth elements (REE). It is the place of the discovery of several REE and important REE-minerals (e.g., Bastnäsite one of the primary REE-ore minerals). In recent years there has been an increased interest due to rising demand of REE for technological applications. Several recent studies have focused on the mineralogy and geochemistry but a lack of fresh in situ samples has meant that textural and stratigraphic relationships are not as well described. Recent exploration in the area has produced drill core traverses across the host stratigraphy of the Bastnäs deposit, allowing the collection of relatively fresh in situ samples which can be placed in lithological context. Here we present new mineralogical and textural information linked to the lithology indicating that the REE-mineralisation in Bastnäs is commonly associated with magnetite skarn and that it occurs over a wide range of stratigraphic levels.

The Bergslagen ore province in central Sweden hosts several rare earth element (REE)-rich deposits, which have recently come back into focus with the increasing demand for REE. In order to understand and potentially predict the occurrence of these deposits, it is crucial to understand the hydrothermal processes that formed them. This study presents new geochemical and mineralogical data on regional alkali alteration zones in the metavolcanic rocks that host the deposits, which show strong depletion of light REE.

The Bergslagen Region in central Sweden hosts over 6000 metallic mineral deposits, most of them hosted by a Paleoproterozoic metavolcanic to metasedimentary succession. Iron oxide- and polymetallic sulfide-depositsare the most abundant and economically the most important deposits. Many deposits are enriched in Rare Earth Elements (REEs) and there is an ongoing drive to evaluate the REE resource in the region. The polymetallic sulfide deposits are thought to have formed by seawater-derived fluids which leached metals from the metavolcanic host rocks. This study investigates the trace metal mobility with specific focus on the critical metals (CMs), during this hydrothermal alteration in western Bergslagen. Preliminary results show that a broad range of trace elements and especially light rare earth elements (LREEs) were mobilized.