Structural and geochemical mapping of a Fe-mineralized quartz-mica rich unit in the Ringvassøya Greenstone Belt, West Troms Basement Complex

Abstract

Ringvassøy is one of a chain of large coastal islands representing the Archaean to Paleoproterozoic West Troms Basement Complex (WTBC), west of the Caledonides. On Ringvassøy, a basement of mainly tonalitic gneiss is overlain by the Ringvassøy Greenstone Belt (RGB), which is metamorphosed up to middle amphibolite facies. Tonalitic gneiss in the west and southeast of the island has U–Pb zircon ages of 2.84–2.82 Ga, similar to U–Pb zircon ages of 2.85–2.83 Ga for metavolcanics in the RGB. Mafic dykes cutting the basement have U–Pb zircon and baddelyite ages of 2.40 Ga, with a titanite age (metamorphic overprint) of 1.77 Ga; the same dykes seem to occur in the RGB.

The focus of this study is a highly Fe-sulphide-mineralized, quartz-mica rich unit (‘keratophyre’?), which is suggested to be auferious. This unit be traced from the western to eastern limits of the RGB and coincides with a geophysical resistivity anomaly. The unit was mapped and sampled to identify enrichments of Fe-sulphides and Au, in all types of encountered mineralizations. Structurally in the RGB, D1 is represented by the main ductile foliation (S1) parallel to primary bedding/layering, rare isoclinal folds, and foliation-parallel shears (S1), and occurred between 2.8 Ga and 2.40 Ga. Most of the post-D1 structures, which cut the mafic dykes, formed in greenschist-facies conditions possibly during the Svecofennian Orogeny (c. 1.80–1.75 Ga), and include: macroscale upright folds and related low-angle ductile shears (S2), and moderately to steeply plunging folds associated with steeply dipping, ductile shears (S3).

Samples of the quartz-mica rich unit were collected across and along strike, and include all observed variations in lithology and mineralisation. Samples were also collected from syn-tectonic mineralisation and quartz veins in. Geochemical data, combined with the S1-concordant position of the quartz-mica rich unit throughout the RGB, offer the possibility that it originated as a stratiform unit in a volcano-sedimentary setting. The unit may then have been hydrothermally altered and mineralized in the vicinity of a black smoker complex. A gold mineralization is identified in the bordering lithologies, thus, presenting no relation to the quartz-mica rich unit. These findings again state the potential for gold in the RGB, and provide important knowledge for further prospecting.