Magmatic and hydrothermal processes in relation to the formation of the Aitik Cu-Au deposit, Northern SwedenReport as inadecuate

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Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. 2001 (English)Licentiate thesis, comprehensive summary (Other academic)

Abstract [en] : Aitik is a disseminated Cu-Au deposit with an annual production of 18 million metric tons of ore containing 0.38 percent copper and 0.22 parts per million gold. It is situated in northern Sweden in 1.9 Ga Svecofennian volcanic and sedimentary rocks formed in a volcanic arc environment. Several other Cu-Au mineralizations and Fe-deposits occur in the region, and the entire ore province stands for about 90% of the ore production in Sweden. A subvolcanic, multiphase intrusion of calc-alkaline and intermediate composition occurs in the footwall to the Aitik deposit. This intrusion, geochemically classified as a quartz monzodiorite, belongs to the 1.9 Ga Haparanda suite of granitoids, which have formed in a compressional environment in a volcanic arc setting. These granitoids are widespread in northernmost Sweden and correspond to the period of subduction recorded further south in the Skellefte district. The Aitik intrusion is weakly mineralised, with chalcopyrite, pyrite and magnetite occurring as dissemination and veinlets. It has been affected by regional metamorphism, deformation and alteration, the latter a dominantly potassic alteration type. The Aitik quartz monzodiorite share many features typical of porphyry generating intrusions. A dominantly magmatic origin for the intrusion-hosted mineralization is proposed, where this subeconomic mineralization might form part of a porphyry system with its major part represented by mineralization in the surrounding volcano-sedimentary rocks, i.e. the main ore zone at Aitik. In the main ore zone, which comprises strongly altered and deformed biotite schist/gneiss towards the footwall and quartz-muscovite (sericite) schist towards the hanging wall, a porphyritic rock with rich dissemination of sulphides locally occurs. This rock strongly resembles the footwall quartz monzodiorite and they are also compositionally similar. This, together with the finer grain size, suggest that this rock may represent apophyses protruding from the upper part of the footwall intrusion, and this, in turn, strengthen the relation between the intrusion and the volcano-sedimentary-hosted main mineralization. Alteration phenomena within the main ore zone include extensive biotitization and sericitization accompanied by garnet porphyroblasts, quartz and pyrite. Tectonically remobilized sulphides have contributed to the formation of a copper-rich area in the strongly deformed rocks towards the footwall of the ore. The distribution of gold in the Aitik ore body corresponds to a certain degree to that of copper, but gold-rich areas correlating with high quartz vein frequency and high pyrite content, have also developed due to remobilization processes. The ore body also shows a vertical metal zoning, with gold-rich areas increasing towards depth and restricted copper-rich areas being replaced by more extensive, medium-grade areas at depth. A magmatic signature of the Aitik Cu-Au mineralization is also obtained by the fluid inclusion analyses made on copper mineralised quartz veins. These fluid inclusions record the presence of three different fluid populations within the Aitik rocks. The earliest one is intimately associated with the main phase of chalcopyrite deposition. This fluid is characterised by high temperature and high salinity, and thus resembles fluids involved in the formation of magmatic-hydrothermal deposits, e.g. porphyry type deposits and Fe-oxide Cu-Au deposits. It is, however, not possible to interpret Aitik as one of the two. Most data instead points to the possibility of Aitik as being a hybrid of these two major groups of deposits.

Place, publisher, year, edition, pages: Luleå: Luleå tekniska universitet, 2001. , 4 p.

Series : Licentiate thesis / Luleå University of Technology, ISSN 1402-1757 ; 2001:11

Research subject: Ore Geology

Identifiers: URN: urn:nbn:se:ltu:diva-17219Local ID: 238c2450-d321-11db-b6e3-000ea68e967bOAI: diva2:990220

Note: Godkänd; 2001; 20070315 (ysko)Available from: 2016-09-29 Created: 2016-09-29Bibliographically approved

Author: Wanhainen, Christina


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