Geology of the Rockliden volcanogenic massive sulphide deposit, north central Sweden

The Rockliden deposit is located in north central Sweden in the middle of the Bothnian Basin. It was formed at the contact above little isolated felsic volcanic rocks (dated at 1874±Ma by Welin, 1987) intercalated in the upper part of the 10km thick turbiditic sediments of the Härnö group (ca.1.95-1.87 Ga, Lundqvist et al., 1998) and strongly deformed during the Svecokarelian orogeny (1.87-1.82 Ga, Kousa & Lundqvist, 2000). It was discovered in the 80´s by Boliden Mineral AB but the project was put on hold because of the high antimony grade. Resources were estimated at the time at 2,2Mt @ 94 g/t Ag, 1.98 % Cu, 5.56% Zn, 0.97% Pb, 0.91% As, 0.18% Sb, and 27% S from 0-500m (Mattsson & Heeroma, 1985). Interest came back recently and new deep holes were drilled between 2007 and 2009. The aim of the master’s thesis was to describe the detailed geology of the Rockliden deposit and to classify it in the existing nomenclature. The description (rocktype, alteration, mineralisation, tectonic structures) was based on 2700m of re-logged drillcores from a cross section in the central part of the deposit. Statistical information on metal content comes from the study of previous chemical assays performed on mineralised sections. New whole rock lithogeochemical analyses and thin sections were made from the footwall. A geological model including the footwall with a zoned hydrothermal alteration, the mineralisation and the hanging wall, all strongly deformed and faulted, was interpreted on 3 cross sections separated by 10 m intervals and map levels at 100 m intervals. The hydrothermal alteration is mainly sericitic and chloritic in the central part. The Sb grades are lower at depth, the reason for that is still unknown but hypothetically linked to metamorphism or metasomatism. From the comparison to the existing nomenclature, it appears that the Rockliden VMS is a Palaeoproterozoic, bimodal felsic, Kuruko type deposit, rich in Zn, Cu, Ag but containing very little gold. The lithogeochemical analyses in the footwall show that the volcanic sequence has a dacitic composition of FII-type and calc-alkaline affinity. Comparisons to average upper continental crust and dacitic rocks from the Skellefte district show enrichment in HFSE, LILE, REE in Rockliden and underline the sedimentary rocks as a possible source of the melt, or at least as a contaminant.Validerat; 20101217 (root