Evolution of structures and hydrothermal alteration in a Palaeoproterozoic metasupracrustal belt: Constraining paired deformation-fluid flow events in a Fe and Cu-Au prospective terrain in northern Sweden

Abstract. In this field-based study, a ~ 90 km long Palaeoproterozoic metasupracrustal belt in the northwestern part of the Norrbotten ore province (northernmost Sweden) has been investigated in order to characterize its various structural components and thus constrain its structural evolution. In addition, hydrothermal mineral associations are described and linked to identified deformation phases. New geological mapping of five key areas (Eustiljakk, Ekstromsberg, Tjarrojakka, Kaitum West and Fjallasen-Allavaara) indicates two major compressional events (D1, D2) that affected the belt whereas each deformation event can be related to specific alteration styles typical for iron oxide-apatite and iron oxide Cu-Au systems. D1 generated a regionally distributed penetrative S1 foliation and oblique reverse shear zones with southwest block up sense-of-shears in response to NE–SW crustal shortening. D1 is associated with regional scapolite ± albite alteration formed coeval with regional magnetite ± amphibole alteration and calcite under epidote-amphibolite metamorphism. During D2, folding of S1 generated steeply south-plunging F2-folds in low strain areas whereas most strain was partitioned into pre-existing shear zones resulting in reverse dip-slip reactivation of steep NNW-oriented D1 shear zones and strike-slip dominated movements along steep E–W-trending shear zones under brittle-ductile conditions. The hydrothermal alteration linked to the D2 deformation phase is more potassic in character and dominated by K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulphides, and calcite. Our results underline the importance of paired structural-alteration approaches at the regional- to belt-scale to understand the temporal-spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations, as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. Our new structural model harmonizes with earlier petrological/geochemical tectonic models of the northern Norrbotten area and emphasizes the importance of reactivation of early formed structures.