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Abstract
The Aitik Cu-Au-Ag deposit in northern Sweden is hosted by strongly altered and deformed 1.9 Ga old Svecofennian volcaniclastic rocks. A porphyritic quartz monzodiorite intrusion of subvolcanic origin is situated in the structural footwall to the ore. U–Pb TIMS zircon dating of the quartz monzodiorite yielded an age of 1887±8 Ma, which coincides with the age obtained for the subduction-related Haparanda suite of granitoids in Norrbotten. It is intruded by minor, comagmatic phases, including units of finer grained quartz monzodiorite and diorite. The finer grained intrusive phase, which can be traced into the ore zone of the Aitik deposit, is believed to represent apophyses protruding from the upper part of the quartz monzodiorite. The Aitik intrusion, comprising the quartz monzodiorite and its comagmatic phases, is affected by regional metamorphism, deformation, and hydrothermal alteration. Potassic alteration is most evident, and expressed by the growth of secondary biotite and K-feldspar. The sub-economic Cu-Au-Ag mineralization hosted by the Aitik intrusion mainly consists of chalcopyrite, pyrite, and magnetite of dominantly magmatic-hydrothermal origin, and is present in four forms: disseminated, as veinlets, in quartz-feldspar veins, and in biotite-amphibole veins. This mineralization extends in economic grades into the adjacent volcaniclastic rocks in the roof of the intrusion. The Aitik intrusion is similar in many respects to porphyry copper generating intrusions regarding tectonic setting, petrography and chemical composition. The intrusion-hosted sub-economic mineralization might form part of a porphyry system with its major part represented by the main mineralization in the overlying volcaniclastic rocks.
