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

Andersson, Joel B. H.; Bauer, Tobias E.; Lynch, Edward P.

An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (inline-formulaD1 and inline-formulaD2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early inline-formulaD1 generated a regionally distributed, penetrative inline-formulaS1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this inline-formulaD1 event. Based on overprinting relationships, inline-formulaD1 is associated with regional scapolite inline-formula± albite, magnetite inline-formula+ amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to inline-formulaD1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships.

During inline-formulaD2 deformation, folding of inline-formulaS0inline-formulaS1 structures generated inline-formulaF2 folds with steeply plunging fold axes in low-strain areas. NNW-trending inline-formulaD1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending inline-formulaD2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to inline-formulaD2 structures is a predominantly potassic–ferroan association comprising K-feldspar inline-formula± epidote inline-formula± quartz inline-formula± biotite inline-formula± magnetite inline-formula± sericite inline-formula± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in inline-formulaD2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales 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. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

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Andersson, Joel B. H. / Bauer, Tobias E. / Lynch, Edward P.: Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden. 2020. Copernicus Publications.

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