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

Abstract: 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.