Revised structure and stratigraphy of the northwestern Repparfjord Tectonic Window, northern Norway

Abstract

The Repparfjord Tectonic Window (RTW) consists of a ~8 km-thick low-grade, weakly deformed Early Palaeoproterozoic volcano-sedimentary succession that hosts abundant Cu mineralisations. Two major, sediment-hosted, Cu-deposits occur in the Saltvatnet Group, a <6 km-thick sequence of mainly coarse-grained clastic rocks that forms the core subject of the present study. Results from new mapping, structural analysis and whole-rock geochemistry from the northwestern part of the RTW are presented and used in association with high-resolution geophysical data to propose a revised stratigraphy of the Repparfjord volcano-sedimentary succession and its tectonic and geodynamic settings. The Saltvatnet Group is better characterised as containing four formations, now including also the uppermost volcaniclastic siltstones and dolostone members of the herein newly defined Gorahatjohka Formation. Field oberservations and structural analysis show that the current geometry of the Saltvatnet Group is that of a km-scale, upright, NE-SW-trending anticline, the Ulveryggen anticline, cored by the Saltvatnet Group, which thus represents the lowermost exposed rocks of the RTW. New and existing geochemical data are used to demonstrate that the two volcanic-dominated Nussir and Holmvatnet groups, which crop out on either side of the Ulveryggen anticline, have similar geochemical character and can be interpreted as temporal and lateral correlatives. The basaltic Ulveryggen intrusions within the Saltvatnet Group are geochemically different from any other igneous rocks in the RTW, suggesting that these intruded prior to the volcanic-dominated groups in the RTW. The revised stratigraphy of the RTW is interpreted as reflecting deposition and volcanism in an evolving continental back-arc, which contrasts with the purely intraplate rift setting inferred for most parts of the Fennoscandian Shield in the Early Palaeoproterozoic.

Geochronology of the Palaeoproterozoic Kautokeino Greenstone Belt, Finnmark, Norway: Tectonic implications in a Fennoscandia context

“…4) could also represent a Jatulian sedimentary unit ( Fig. 5; Torgersen et al, 2015a). In contrast, in the Alta-Kvaenangen Tectonic Window, there is no known candidate to correlate with the Masi Formation (Fig.…”

“…Kilometre-scale, upright, NE-SW-trending folds indicate overall NW-SE shortening (Pharaoh et al, 1983;Torgersen et al, 2015a). Amphibole from metabasalt of the Holmvatnet Group yields an average K-Ar age of c. 1842 Ma (8 analyses in 3 samples), possibly recording regional cooling after the Svecokarelian orogeny (Pharaoh et al, 1982).…”

“…In the Repparfjord Tectonic Window, Torgersen et al (2015a) evoke a transient phase of compression and basin inversion already at c. 2070 Ma. Several phases of rifting associated with several pulses of mafic magmatism are recorded, each characterised by a distinct Nd isotopic signature (Fig.…”

“…Torgersen et al (2015a). Sample ETO149 and samples of Perelló et al (2015) and Torgersen et al (2015b) are located, with a summary of geochronological results (same colour coding as in Fig.…”

A new tectonic model for the Palaeoproterozoic Kautokeino Greenstone Belt, northern Norway, based on high-resolution airborne magnetic data and field structural analysis and implications for mineral potential

“…), Sweden (Wikström et al, 1996;Bergman et al, 2006;Luth et al, 2014) and Finland (Ward et al, 1989;Saalmann et al, 2009). These ductile shear zones, like other Palaeoproterozoic shear zones around the world (e.g., Zhao et al, 2002;Goodenough et al, 2013;MacDonald & Goodenough, 2013), acted as sites of enhanced fluid flow and, as a result, are endowed with rich mineralisations, particularly gold (e.g., Koistinen, 1981;Ward et al, 1989;Sandstad & Nilsson, 1998;Airo & Kurimo, 1999;Ihlen & Furuhaug, 2000;Eilu et al, 2003;Weihed et al, 2005;Bark & Weihed, 2007;Viola et al, 2008b;Saalmann et al, 2009;Torgersen et al, 2013;Angvik, 2014;Henderson et al, in prep. ).…”