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Master's Thesis MASTER'S THESIS Geochemical study and identification of a felsic porphyritic unit, Sjöliden, Gold Line, Northern Sweden Vincent Combes Master of Science (120 credits) Exploration and Environmental Geosciences Luleå University of Technology Department of Civil, Environmental and Natural resources engineering Abstract 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. Table of Contents 1. Introduction ............................................................................................................................ 1 2. Geological settings ................................................................................................................. 2 2.1. Geology of the Fennoscandian Shield ............................................................................. 2 2.2. Regional geology of the Gold Line ................................................................................. 3 2.3. Regional metamorphism .................................................................................................. 4 3. Generation of porphyritic rocks, a literature review .............................................................. 5 3.1. Volcanic origin ................................................................................................................ 5 3.1.1. Felsic lava flow ......................................................................................................... 5 3.1.2. Subaerially eruption with subaqueous pyroclastic flows .......................................... 6 3.1.3. Submarine eruption of pyroclastic flow .................................................................... 7 3.1.4. Ash layer deposition in the sea ................................................................................. 8 3.2. Intrusive hypothesis ......................................................................................................... 8 3.2.1. Sill or dyke ................................................................................................................ 8 3.2.2. Peperite: .................................................................................................................... 9 3.3. Determination of the type of feldspar for determination of the origin ........................... 9 3.4. Phenocrysts .................................................................................................................... 10 4. Geology of Sjöliden ............................................................................................................. 10 4.1. Methodology .................................................................................................................. 10 4.2. Location of studied outcrops ......................................................................................... 11 4.3. Geophysics ..................................................................................................................... 13 4.4. Petrology ........................................................................................................................ 13 4.4.1. Metasedimentary unit .............................................................................................. 13 4.4.2. Felsic porphyritic unit ............................................................................................. 15 4.4.3 Contact between the two units ................................................................................. 21 4.5. Geochemistry ................................................................................................................. 23 4.5.1. Metasedimentary unit .............................................................................................. 23 4.5.2. Felsic porphyritic unit ............................................................................................. 24 4.5.3. Alteration ................................................................................................................ 31 5. Discussion ............................................................................................................................ 34 5.1. On the origin of the metasedimentary unit ................................................................ 34 5.2. Geochemical classification of the felsic unit ............................................................. 34 5.3. On the source of sodium and alteration ..................................................................... 35 5.4. On the origin of the felsic porphyritic unit ................................................................ 35 5.5. On the peperite or mixing feature along contact ........................................................ 37 5.6. On the comparison of Sjöliden porphyritic rock ........................................................ 38 6. Conclusions .......................................................................................................................... 41 7. Acknowledgements .............................................................................................................. 42 8. References ............................................................................................................................ 43 9. Annexes ................................................................................................................................ 47 1. Introduction The aim of this thesis is to identify and classify a porphyritic felsic meta igneous rock occurring at Sjöliden in Västerbotten, northern Sweden. The studied area is located five kilometers North East of the Svartliden gold mine. This area is situated within the Gold Line, which is a positive gold anomaly in glacial till striking NW-SE, where several gold deposits have been identified. Sjöliden is highly interesting from an economic point of view. The geological interpretation and understanding of the area obtained from the present thesis is therefore very important. This thesis has two main objectives, first, to find out whether the porphyritic felsic unit has an intrusive or an extrusive origin, and to give a geochemical classification to this rock. Secondly, an attempt to construct a stratigraphical sequence from the drill core and a facies analysis of the metasedimentary lithologies is presented by combining volcanology and sedimentology. Meeting these aims will help greatly future exploration campaigns in the region. This paper will first present the geological settings: geology of the Fennoscandian Shield and geology of the Gold Line. A literature review with a volcanological approach is carried out supporting all possible origin hypotheses for the felsic unit. Here, the determination of the type of feldspar for determination of the origin is displayed. The geology of the Sjöliden area is studied using geophysical observations, field mapping, facies description, phenocrysts abundance logging, core logging and petrography studies. Using these data, the petrology of the metasedimentary units and the felsic porphyritic unit is analyzed and the contact between the two units is examined. A geochemical
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