Geochemical and kinematic study of shear zones within the "gold line", Lycksele-Storuman province, northern Sweden

University essay from Luleå/Department of Civil, Environmental and Natural resources engineering

Abstract: A kinematic and geochemical study was carried out in shear zones within the “gold line”
in the Lycksele-Storuman province, northern Sweden, with the aid of, field mapping, thin
sections and whole rock geochemical data. The shear zones were hosted by metabasalt,
metasediments, felsic metavolcanics and metagranodiorite. The area has undergone major
deformation during the Svecofennian orogeny and the shear zones in this study are
interpreted as being formed during this event. The geochemical study was carried out in
three localities, the Grundfors road-cut, Sjöliden and the Svartliden gold mine. The data
from Grundfors and Sjöliden indicates two different magmatic arc sources, volcanic-arc
affinity for Sjöliden and MORB or Island-arc affinity for Grundfors. The bedrock at
Grundfors displayed silicified zones but the protolith for these was not possible to
determine. Sjöliden showed no major elemental exchange between the samples.
Svartliden showed two different sourcesfor the metasediments present, acid-arc source
and an oceanic island-arc source. The metasediments formed from an acid-arc source
displayed a clear relationship based on immobile elements to the highly silicified rocks at
Svartliden. This study used one of the acid-arc source metasediments as a precursor with
a resulting mass gain of 1800%. The validity of the protolith can be argued. The tectonic
study was carried out in 5 localities, Grundfors, Sjöliden, Svartliden gold mine, “the
mylonite” and “the granodiorite”. Indications of two kinematic events were found in
Grundfors, Sjöliden and “the mylonite”, one steep reverse movement and a later one with
both extensional and strike-slip components of deformation. The latter showed a dextral
shear sense in “the mylonite” while sinistral in the Grundfors outcrops. Sjöliden indicate
both a sinistral and a dextral horizontal component. The kinematic data from Svartliden
showed only a reverse movement. Svartliden with its ENE striking shear zone and “the
mylonite” with its NW trending shear zone are interpreted to have formed during N-S
compression. The movements in Grundfors, Sjöliden and “the granodiorite” are
interpreted to have formed during an E-W compression, with either a transpressive
regime or a later orogenic collapse alternatively. The shear zones in this study displayed
both ductile and brittle movements indicating a temperature gradient through the zone.