Geology and mineralization of the Bellviksberg sandstone-hosted Pb(-Zn) deposit, Dorotea area, Sweden

The Bellviksberg deposit in the Dorotea area is part of the sandstone-hosted lead-zinc deposits occurring all along the eastern front of the Scandinavian Caledonides in Precambrian to Lower Cambrian sandstone deposited uncomformably on the deeply eroded Precambrian basement (Laisvall, Vassbo, etc). The Bellviksberg deposit occurs in the Ström Quartzitic Nappe that shows décollement structure and thrusting directly over the Alum Shales formation of the Autochthon. The goal of this thesis is to give a reinterpretation of the Bellviksberg deposit on a geological and genetic point of view. It also aims to give guidelines for the exploration of the Bellviksberg deposit and the Dorotea area in general. Thus, most of the cores of the deposit have been logged. Thin and polished-sections were made to help the interpretation work. Within this nappe two lithologies have been identified which names are introduced here: the Varangerian Tillite and the Lower Cambrian Sandstone. The Varangerian Tillite is characterised by an overall greenish colour due to clay minerals coating quartz grains. It is composed of interbedded quartzitic sandstones and conglomerates with well-rounded quartz grains. Other minerals such as feldspar, epidotes and muscovite constitute only 1-2% of the mineralogical assemblage. This formation is interpreted as being of glacio-fluvial origin from a reworked sediment. The Lower Cambrian Sandstone overlaying the Varangerian Tillite hosts the mineralization. It is composed of interbedded fine and coarse grained quartzitic sandstone. The coarser grained units are poorly sorted, less compacted than the fine grained units and contain preferably organic matter with associated pyrite. Graphite- rich sandstone and siltstone members can be present within the Lower Cambrian Sandstone Formation. This formation is interpreted as being from deltaic origin dominated by fluvial action. The nappe hosting the Bellviksberg mineralization is composed of many thrust sheet stacked on each other with inverse faults striking -20°N to 0°N and small scale folds. A preliminary resource estimation results in 1.66 Mt of 5.35% Pb, 0.23% Zn and 21g/t Ag. The main part of the mineralization, galena-dominated, occurs in the less tectonically disturbed part of the Bellviksberg area and mostly in the coarser grained units that are usually richer in organic matter and less compacted. The host-rock is then thought to have been the main source of sulphurs that fixed metals carried by basinal brines transported during the Scandian Event of the Caledonian orogeny. Redox or pH controls for the precipitation of metals are suggested although a redox control is slightly favoured.Validerat; 20101217 (root