Geochemical study and identification of a felsic porphyritic unit, Sjöliden, Gold Line, Northern Sweden

The present work identifies and classifies a felsic meta igneous rock mapped at Sjöliden in Västerbotten, northern Sweden. The studied area is located five kilometers North East of the Svartliden gold mine. Situated within the Gold Line, the Sjöliden area is highly interesting from an economic point of view. This thesis has three main objectives, first to find out whether the porphyritic felsic unit has an intrusive or an extrusive origin, and secondly to give a geochemical classification to this rock. Also, a facies analysis of the metasedimentary lithologies is done. Geochemical data, thin sections, core logging and mapping were used for this work. The metasedimentary unit is a turbiditic sequence classified as continental island arc metagreywacke showing primary bedding with a great variation of graphite and sulphide content, grain size and color. It is a deep sea sediment. The felsic porphyritic unit is a meta-trachyte. It presents euhedral to subhedral sanidine phenocrysts, a fine grain groundmass with quartz veining, pyrrhotite, arsenopyrite and gold. Using immobile element and considering the fact that immobile elements show an alteration trend, the geochemical study classifies the felsic porphyritic unit as a trachyte. One of the main characteristic of the rock is its high value of sodium. This can be explained by a high concentration in the protolith, a sodic alteration (Na enrichment and K depletion) and by the deposition in deep ocean (Na rich environment). The most likely theory for the origin of this trachytic unit would be sedimentation of felsic volcanic ash on top of deep sea sediments. The contact is interpreted as a sedimentary cold depositional contact; hence the unaffected sediments. According to the geochemical study, the geodynamic formation of the metagreywacke is continental island arc whereas the trachyte is from a within plate setting. The meta-trachyte and the metagreywacke formed at the same time but the volcanic ash has not a local origin. The volcano was situated in a within plate environment, the ash transport can be up to 1000 km. This hypothesis would explain the fact that trachytic material is uncommon in the region, this trachytic unit having a remote origin. When compared with regional felsic units (intrusive rocks from the Gold Line and extrusive rocks from the Skellefte district), the Sjöliden trachyte presents a different geochemical signature. The comparison of immobile elements ratios highlights the singularity of the unit in the region.Validerat; 20121207 (global_studentproject_submitter