On the origin and evolution of the palaeoproterozoic Aitik Cu-Au-Ag deposit, northern Sweden a porphyry copper-gold ore, modified by multistage metamorphic-deformational, magmatic-hydrothermal, and IOCG-mineralizing events

Sammanfattning: The Aitik Cu-Au-Ag mine in the Gällivare area in northern Sweden is the biggest open pit operation in northern Europe and one of Europe’s largest metal producers. The open pit is almost 3 km long, 930 m wide and 345 m deep. Approximately 424 million tonnes of ore averaging 0.4% Cu, 0.2 g/t Au and 4 g/t Ag has been produced, and the metal production in year 2004 was 64,805 tons of copper, 2,035 kg of gold, and 45,110 kg of silver. Although multiply deformed, metamorphosed and hydrothermally altered, characteristic features of two major mineralization styles, porphyry copper and iron oxide-copper-gold (IOCG), have been identified by using field and microscopic observations in combination with techniques such as fluid inclusion microthermometry, sulphur isotopic analyses, U-Pb and Re-Os geochronology, and major and trace element geochemistry. Results from this study indicate that the Aitik deposit is of mixed origin, with a major part of the copper ore originating from an early porphyry copper system, and a second, minor part, originating from an overprinting IOCG-system. The Aitik porphyry Cu-Au-Ag deposit and its host rocks, situated approximately 200 km north of the Archaean-Proterozoic palaeoboundary in the Fennoscandian shield, are considered to have formed in a volcanic arc environment related to subduction of oceanic crust beneath the Archaean craton at ca. 1.9 Ga. An intrusion related to the formation of porphyry copper mineralization is situated in the footwall of the deposit. This source intrusion and related volcaniclastic rocks are mafic-intermediate in composition and belong to the regionally widespread Haparanda suite and Porphyrite group, respectively. The quartz monzodiorite intrusion is porphyritic and comprises younger, but comagmatic, phases of micro-quartz monzodiorite and diorite, and is suggested to represent a cupola protruding from a larger pluton at depth. High salinity fluids (30-38 eq. wt% NaCl + CaCl2) responsible for chalcopyrite-pyrite mineralization was released contemporaneously with quartz monzodiorite emplacement and quartz stockwork formation at ca. 1.89 Ga, and caused potassic alteration of the intrusive and surrounding volcaniclastic rocks. Potassic alteration is manifested by early biotitization and K-feldspatization, in close association with disseminated sulphides and magnetite. Remnants of this primary porphyry copper mineralization are best preserved in the footwall (micro-) quartz monzodiorite, in units of micro-quartz monzodiorite within the volcaniclastic rocks of the ore zone, and in quartz stockworks at the margins of the quartz monzodiorite. The Aitik IOCG mineralizing event occurred about 100 Ma later, when eastward subduction resulted in compression, monzonitic-granitic magmatism, ductile deformation, and tectonic block movements in Northern Norrbotten. Extensive deformation of rocks and redistribution of metals occurred. Magnetite enrichment locally found within late veins of mainly amphibole, K-feldspar, and epidote, together with late scapolite alteration within the deposit, implies that fluids responsible for IOCG-mineralization and extensive Na-Ca alteration in the region during this tectonic event also affected the Aitik deposit, and probably involved addition of copper and gold. This late mineralizing fluid was highly saline, and contained ferropyrosmalite in addition to NaCl + CaCl2. Hydrothermal fluids continued to affect the Aitik area for another ca. 60 Ma, forming minerals such as zeolite, calcite, and thaumasite, indicating low-temperature conditions at the end of the Aitik evolution. It is here concluded that the Aitik Cu-Au-Ag deposit is a strongly deformed and metamorphosed porphyry copper-gold deposit related to a 1.9 Ga quartz monzodiorite, with at least 160 Ma of post-ore modification, including an overprinting mineralizing event of IOCG-type at ca. 1.8 Ga.