Characterization of Cu-mineralized carbonate veins in the Alta-Kvænangen Tectonic Window, Norway

Sammanfattning: The Alta-Kvænangen Tectonic Window is part of a regional greenstone belt that formed around 2.3 Ga – 2.0 Ga. The Bergmark anticline in the area has gone through greenschist to amphibolite facies metamorphism and hosts Cu, Co and Au mineralizations in hydrothermal veins, albite felsite and chemical traps. The area was mined during the late 19th century mostly in trenches but it has still got exploration potential. This project will classify carbonate veins that are hosted within two gabbro units, compare orientations between the barren and mineralized veins as well as discuss the origin of the veins.  The study area has been affected by multiple extensional and compressional events during the Svecokarelian orogeny (1.9 – 1.8 Ga) and regional hydrothermal alterations including an influx of Na + Ca ± K ± Fe. Major shear zones were activated and used for transporting fluids which are thought to have controlled the formation of mineralization in second- or even third-order structures. The veins were studied using field work, detailed mapping, optical microscopy as well as scanning electron microscopy (SEM) and structural analysis. Results gave three different types of hydrothermal veins: 1) Fe oxide veins, 2) calcite veins and 3) dolomite-plagioclase-quartz veins. All of them show strong signs of deformation and recrystallization all while the chalcopyrite mineralization is heterogeneously distributed and commonly more or less bornite-altered. The veins pinch and swell over short distances and are sinuous but with a general orientation. Albite haloes are common for the veins but far from all veins have haloes. There are at least two generations of veins as some were found to have offset others. Brecciation, tension gashes and faults are also found in the area apart from veins. From the field measurements, two groups of orientations could be determined: one with a SW-NE direction and the other in NW-SE direction, both with relatively steep dips. Considering all downhole measurements, no conclusive results could be revealed, however, the optical televiewer results showed that the area has gone through a N-S compression giving a fold axis towards the west. In the end the orientations of the larger veins were found to either run parallel to the gabbro units or are connected to the NW-SE directed main brittle faulting in the Fennoscandian Shield. Smaller, surrounding veins having more diverse orientations. There are multiple signs of deformation and recrystallization in the mafic host rock but no fabric development can be seen. Its appearance varies locally depending on which alterations are dominating. It exhibits sodic-calcic, potassic and leucoxene alteration as well as albitization and carbonatization. The grain size varies from fine-grained to medium-grained which is more typical with a dolerite or basalt, even though it has been previously mapped as a gabbro. No pyroxene was found in the host rock and the plagioclase was Na-rich instead of the expected Ca-rich.  The Bergmark anticline has multiple similarities with the Gällivare-Kiruna area in Sweden which include 1) high-temperature, low-pressure metamorphism resulting in regional alterations such as albitization, carbonatization, amphibole and magnetite alteration. 2) The mineralization consists of Cu, Co, Au, Fe and minor P. 3) The mineralization can be found disseminated in albite felsite, in chemical traps and in first to third order structures (e.g., faults or hydrothermal veins) in brittle, deformed rocks. 4) The mineralization is structurally controlled to the vicinity of major fault zones. 5) In both places, greenstone belts are important for the formation of mineralization. 6) Reactivation of brittle structures, similar to those in the Kiruna area during the second phase of subduction during the Svecokarelian orogeny (D2), is likely the cause for the veining in the Bergmark anticline. 7) A younger N-S compression is recorded in the area. Differences between the Bergmark anticline and the Gällivare-Kiruna area include the lack of intrusions in the vicinity to the Bergmark anticline.