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Vaikijaur |
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Norbotten, Sweden |
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Cu Au Mo
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Super Porphyry Cu and Au
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The Vaikijaur porphyry style Cu-Au-(Mo) deposit is located northwest of Jokkmokk in northern Norrbotten and lies immediately to the north of the Skellefte VMS district and south of the northern Norrbotten Fe-oxide-Cu-Au deposits.
It occurs within the calc-alkaline Jokkmokk granitoid which is exposed as a plutonic massif within the Fennoscandian shield of northern Sweden. The granitoid is light grey to white and 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.
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. It has been dated at 1882±19 Ma by U-Pb methods and is 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 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.
The Cu-Au-(Mo) mineralisation of the Vaikijaur deposit occupies an area of 2 x 3 km within the western part of the Jokkmokk granitoid pluton and is characterized by stockwork quartz-sulphide veinlets and disseminations of chalcopyrite, pyrite, molybdenite, magnetite, pyrrhotite and sphalerite. Gold occurs as free grains in the 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 mineralised area. The fabric in the granitoid, dykes, and mineralisation shows that the mineralisation predates the main regional deformation.
The mineralisation is accompanied by strong potassic alteration. Hornblende is partially or wholly replaced by biotite, and plagioclase is partly replaced by microcline. Irregular propylitic alteration fringes the potassic phase, characterised by epidote replacement of plagioclase, while hornblende is replaced by epidote and biotite. Biotite is partly chloritised and calcite is common. Light-coloured, irregular phyllic zones overprint the former alteration types, with quartz and sericite replacing plagioclase.
A pyrite-rich inner core is surrounded by a concentric zone containing pyrite, chalcopyrite and gold. Molybdenite is distributed irregularly throughout the chalcopyrite zone. Magnetite is present in both the pyrite and the chalcopyrite zone. Re-Os age determinations for three molybdenite samples from outcrop and drill core constrain the age of porphyry-style Cu-Au-(Mo) mineralisation to between 1889±10 and 1868±6 Ma. A younger molybdenite is associated with a much later metamorphic event at about 1750 Ma.
The most recent source geological information used to prepare this decription was dated: 2005.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Lundmark, C., Stein, H. and Weihed, P., 2005 - The geology and Re-Os geochronology of the Palaeoproterozoic Vaikijaur Cu-Au-(Mo) porphyry-style deposit in the Jokkmokk granitoid, northern Sweden: in Mineralium Deposita v.40, pp. 396-408.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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