Character of Cu-Au Mineralization and Related Hydrothermal Alteration along the Nautanen Deformation Zone, Gällivare Area, Northern Sweden
Published:January 01, 2000
Olof Martinsson, Christina Wanhainen, 2000. "Character of Cu-Au Mineralization and Related Hydrothermal Alteration along the Nautanen Deformation Zone, Gällivare Area, Northern Sweden", Svecofennian Ore-Forming Environments: Volcanic-Associated Zn-Cu-Au-Ag, Intrusion-Associated Cu-Au, Sediment-Hosted Pb-Zn, and Magnetite-Apatite Deposits in Northern Sweden of Northern Sweden, Rodney L. Allen, Olof Martinsson, Pär Weihed
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The Gällivare area is an important producer of Fe, Cu and Au. The Cu-Au deposits are hosted by Sve-cofennian successions of volcanic and sedimentary rocks formed in an arc environment. Apatite iron ores are restricted to overlying intermediate to felsic volcanic units with intraplate characteristics. Most of the Cu-Au occurrences are spatially related to the Nautanen Deformation zone, which is a major north-northwest–oriented crustal structure.
A large-scale zoning pattern is outlined by the metal association and alteration along the Nautanen Deformation zone and to some extent across it. In the northwestern part, magnetite and locally apatite are important constituents indicating a relation to the nearby apatite iron ores. The host rock is strongly altered by K feldspar and scapolite. Towards the southeast pyrite becomes an important ore mineral, while magnetite is less abundant. This change is accompanied by an increasing Au/Cu ratio and alteration dominated by biotite or sericite.
Mineralization within the Nautanen Deformation zone is mainly disseminated in character and may represent an early phase of mineralization related to synorogenic 1. 9 Ga intrusions of intermediate composition. Vein style mineralization occurs as late phases in disseminated deposits and outside the Nauta-nen Deformation zone. This mineralization may to some extent represent remobilized products of older disseminated sulfide occurrences. The common occurrence of tourmaline in the veins and locally also molybdenite or scheelite suggests a relation to the granites of the 1.8 Ga Lina Suite. Thus, the present zoning pattern along the Nautanen Deformation zone is the product of several hydrothermal events with the 1.9 Ga mineralization possibly outlining the large-scale features.
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Svecofennian Ore-Forming Environments: Volcanic-Associated Zn-Cu-Au-Ag, Intrusion-Associated Cu-Au, Sediment-Hosted Pb-Zn, and Magnetite-Apatite Deposits in Northern Sweden of Northern Sweden
The Fennoscandian or Baltic Shield (both names occur in the literature) occupies the northern part of Europe. Pre-cambrian areas are exposed in Norway, Sweden, Finland, and Russia and their continuation beneath the platform cover sequences to the east and south have been better understood through studies within the European “Euro-probe” project. The craton of which the exposed Fenno-scandian Shield forms a part is bordered to the west by the Caledonian orogenic belt. Precambrian rocks of the same craton outcrop again in the Ukrainian Shield and the Voronezh Massif (cf. Gee and Zeyen, 1996).
The Fennoscandian Shield is composed of Archean to Neoproterozoic rocks (Fig. 1). It is beyond the scope of this guidebook to describe all the different settings in detail, but adjoining areas that both predate and postdate the Svecofennian rocks that are the main interest of this field trip will be briefly described. The term Svecokarelian is used for the orogeny that occurred between 1900 and 1800 Ma (i.e., emphasizing deformation and metamorphism as defining the orogeny), while the term Svecofennian is used for the supracrustal rocks that were emplaced during c. 1.95 Ga to 1.85 Ga. To the reader unfamiliar with literature on the Fennoscandian Shield it is important to remember that these terms are not used consistently in the literature.
The pre-Svecokarelian crustal growth can be subdivided into Archean and Paleoproterozoic. During the Archean, greenstone belts and tonalite-trondhjemite-granodiorite (TTG) terranes formed, while crustal growth during the Paleoproterozoic involved rifting of the Archean basement with the formation of rift-fill sequences of sedimentary and igneous rocks, and addition of juvenile Paleoproterozoic crust by accretionary processes along the margin of the Archean continent.
The oldest known rocks in the Shield are c. 3.1 Ga in age and were generated during the Saamian Orogeny (3.1—2.9 Ga). Rocks of the age 3.2 to 2.7 Ga are present in the Archean nuclei of the shield and are composed of tonalitic gneisses and migmatites. The oldest documented magmatic and metamorphic event took place at c. 2.84 Ga (Nurmi and Sorjonen-Ward, 1996). Rift-related greenstones, subductiongenerated calc-alkaline volcanic rocks and tonalitictrondhjemitic igneous rocks were formed during the Lopian Orogeny (2.9—2.6 Ga). These greenstone belts form a prominent part of the Finnish and Russian bedrock, but are minor in Sweden. The Hattu schist belt in the southwestern part of the Archean of Finland seems to record a collisional arc setting