Vaikijaur, a palaeoproteroic Cu-Au porphyry style mineralization in the Jokkmokk granitoid at the archaean-proterozoic boundary in northern Sweden

Sammanfattning: The Jokkmokk granitoid is exposed in a plutonic massif northwest of Jokkmokk in northern Norrbotten. It is light grey to white, fine-grained, with megacrysts of feldspar and glomeroporphyritic hornblende and biotite. Small enclaves of mafic rocks and synplutonic mafic dykes are products of mingling with a coeval and possibly cogenetic mafic magma. A couple of smaller occurrences of Jokkmokk-type granitoid are known south of Jokkmokk. All are spatially associated with metasedimentary rocks of the Norvijaur formation. The Jokkmokk granitoid was previously considered to belong to the c.1.8 Ga Lina S-type intrusive suite, but the Jokkmokk granitoid has a unique calc-alkaline to alkali-calcic, metaluminous to weakly peraluminous character with a steep REE-profile, positive Eu-anomaly, and a low Zr content. U-Pb TIMS zircon dating of the Jokkmokk granitoid gives an age of 1882 +-19 Ma which is within limits the same age as that obtained for the Haparanda suite, but contrary to the Haparanda suite it has a positive epsilon Nd value of 2.8, indicating a more juvenile Palaeoproterozoic character similar to the Jörn suite in the Skellefte district. Compared to other granitoids in the Jokkmokk area it is geochemically most similar to the older 1.93 Ga Norvijaur granitoid. This type of magma seems to be restricted to the palaeoboundary between the Archaean craton in the north and Palaeoproterozoic juvenile crust in the south. Spatial correlation with low angle, south dipping, WNW-trending shear zones and NNE-trending subvertical shear zones, highlight the possibility that this unique magma type is related to transtension in the overriding plate and partial melting in sub-arc mantle wedge during NE directed subduction processes related to the early stages of the Svecokarelian orogen. Possibly magma was ponded in shallow chambers below pull-apart basins in a transtensional regime. This type of setting has been advocated as the potentially most favourable tectonic setting for porphyry copper formation. The Cu-Au-(Mo) mineralization at Vaikijaur northwest of Jokkmokk covers an area of 2 by 3 km within the western part of the Jokkmokk granitoid pluton. The mineralization is characterized by dissemination and veinlets of chalcopyrite, pyrite, molybdenite, magnetite, pyrrhotite, and sphalerite. Quartz stockwork with or without sulphides occur in the mineralization. Gold occurs as free grains in silicate matrix, in contact with chalcopyrite, pyrite and sphalerite, or as inclusions within the chalcopyrite. Porphyritic mafic dykes, with phenocrysts of plagioclase and porphyroblasts of magnetite, follow fractures in an almost concentric pattern in the mineralized area. The fabric in the granitoid, dykes, and mineralization shows that the mineralization predates the main regional deformation. Geophysical ground measurements indicate a strong conductive central zone in the mineralized area bordered by both conductive and magnetic zones. Restricted drilling campaigns were carried out in these zones in 1981-83. Chemical and mineralogical analyses of drillcores and outcrops indicate several stages of alteration and a metal zoning at Vaikijaur. The entire mineralized area is affected by potassic alteration. Hornblende is partly or totally replaced by biotite, and plagioclase is partly replaced by microcline. In irregular propylitic alteration zones that overprint the potassic alteration, epidote has replaced plagioclase and hornblende is replaced by epidote and biotite. Biotite is partly chloritized and calcite is common. Light-coloured, irregular phyllic alteration zones overprint the former alteration types, with quartz and sericite replacing plagioclase. A metal zoning can be seen with a pyrite-rich inner part of the mineralized area surrounded by a zone with pyrite, chalcopyrite and gold. Molybdenite occurs in an irregular pattern within the chalcopyrite zone, magnetite is present in both the pyrite and the chalcopyrite zone.