Structural Controls on Iron Oxide Copper-Gold Mineralization and Related Alteration in a Paleoproterozoic Supracrustal Belt: Insights from the Nautanen Deformation Zone and Surroundings, Northern Sweden

Abstract

The Nautanen deformation zone in the Gällivare area of northern Sweden is a highly Cu-mineralized, magnetite-rich, large-scale shear zone with a long-lived (~100 m.y.) deformation, hydrothermal alteration, and mineralization history. This composite structure hosts the Aitik porphyry Cu-Au-Ag ± Mo deposit and several Cu-Au ± Fe ± Ag ± Mo occurrences assigned to the iron oxide copper-gold (IOCG) deposit class. The Nautanen deformation zone was a locus for polyphase deformation and intermittent metasomatic-hydrothermal activity that overprinted middle Orosirian (ca. 1.90–1.88 Ga) continental arc-related volcanic-plutonic rocks. The deformation zone is characterized by intense shearing fabrics that form a series of subvertical to moderately W-dipping, NNW-SSE–trending, first-order shear zones with oblique reverse kinematics and related NNE-SSW–oriented second-order shear zones that control hydrothermal alteration patterns and Cu-Au mineralization. Hydrothermal alteration in the study area formed during several phases. Volcanic-volcaniclastic rocks to the east and west of the Nautanen deformation zone display low to moderately intense, pervasive to selectively pervasive (i.e., patchy zones or bands, disseminations) sericite ± feldspar, amphibole + biotite + magnetite ± tourmaline, and K-feldspar + hematite alteration. Both the amphibole + biotite and K-feldspar + hematite associations occur adjacent to NNW- and NE-oriented deformation zones and are locally associated with minor sulfide. Within the deformation zone, a moderate to intense biotite + amphibole + garnet + magnetite + tourmaline + sericite alteration assemblage is typically associated with chalcopyrite + pyrrhotite + pyrite and forms linear and subparallel, mainly NNW-oriented seams, bands, and zones that locally appear to overprint possibly earlier scapolite + sericite ± feldspar alteration. Late-stage epidote ± quartz ± feldspar alteration (retrograde saussuritization) forms selectively pervasive zones and epidote veinlets across the area and is partly related to brittle faulting. A magnetite-amphibole-biotite–rich, penetrative S1 foliation records shortening during early Svecokarelian-related deformation (D1) and can be related to ca. 1.88 to 1.87 Ga arc accretion processes and basin inversion that overlaps with regional peak metamorphism to near mid-amphibolite facies conditions and a potential initial Cu mineralization event. Folding and repeated shearing along the Nautanen deformation zone can be assigned to a second, late-Svecokarelian deformation event (D2 stage, ca. 1.82–1.79 Ga) taking place at a higher crustal level. This D2 deformation phase is related to late-stage accretionary processes active during a transition to a stage of postorogenic collapse, and it was accompanied by abundant, syntectonic intrusions. D2-related magmatism produced high-temperature and low-pressure conditions and represents a regional magmatic-hydrothermal event that controlled the recrystallization/remobilization of magnetite, biotite, and amphibole. Associated shear zone reactivation during D2 favors the utilization of the Nautanen deformation zone as a fluid conduit, which preferentially controlled the siting and formation of epigenetic Cu-Au mineralization with distinctive IOCG characteristics within second-order shear zones.

Energy Drive for the Kiruna Mining District Mineral System(s): Insights from U-Pb Zircon Geochronology

“…2.5-2.0 Ga) generated a prominent greenstone belt of tholeiitic volcanic and volcaniclastic rocks, and rift-related sedimentary successions [42][43][44][45] spanning from Norway to Russia Karelia [43]. [19], Weihed and Williams [47], and Bauer and Andersson [48].…”

“…Approximately three to four successive mid-to late-Paleoproterozoic deformation events (described below) are recorded in the Norrbotten lithotectonic unit and have been constrained by ages of magmatic rocks, hydrothermal alteration, and structural characterization (cf. [2] and references therein; [5,[18][19][20][21]49,50]).…”

“…Subsequent crustal shortening (D 1 ) formed a penetrative tectonic fabric from an inferred NE-SW shortening direction [18][19][20][21]49,50]. The fabric development overlapped with peak metamorphic conditions (lower amphibolite) recorded by syntectonic growth of hornblende [20].…”

“…An added element of complexity to the mineral systems approach occurs in regions with long and complicated tectonic histories, such as the Norrbotten province, which has experienced several phases of extension and deformation resulting in compound structural patterns and overprinting magmatic and metamorphic events (cf. [2,5,[18][19][20][21][22]). Furthermore, the landscape has been shaped by Quaternary glaciation leaving extensive till coverage and poor outcrop exposure.…”

“…However, increasing structural and geochronology data link IOCG-style mineralization to a late phase of the tectonic evolution (ca. 1.82-1.78 Ga; [5,[18][19][20][21]27]). These data raise an important question about the timing of introduction and/or remobilization of Cu ± Au when assessing the genetic relationship between IOA and IOCG systems in Norrbotten.…”