Repeated syn- and post-orogenic gold mineralization events between 1.92 and 1.76 Ga along the Kiistala Shear Zone in the Central Lapland Greenstone Belt, northern Finland

Abstract

The Central Lapland Greenstone Belt (CLGB) is well endowed with orogenic gold deposits along major shear zones and structural lineaments. In the northern part of the belt, the Kiistala Shear Zone (KiSZ) hosts the Suurikuusikko gold deposit (Kittilä Mine), currently the largest gold producer in Europe. The Iso-Kuotko deposit is located 10 km to the north of Suurikuusikko along the same shear zone. Gold mineralization at Iso-Kuotko formed in two major stages: an early stage refractory gold mineralization with auriferous arsenopyrite with similarities to the ore at Suurikuusikko and a late, main stage mineralization with free gold in carbonate-quartz veins with abundant pyrrhotite, native bismuth and other sulphide minerals. This latter ore is absent at Suurikuusikko. Based on U-Pb dating, as well as Re-Os and Pb-isotope systematics of rock-forming and hydrothermal minerals, we evaluate the relationships between tectonic evolution and formation of gold deposits throughout the late Palaeoproterozoic in the northern part of the CLGB.Results from in situ U-Pb dating of zircon and petrographically well constrained hydrothermal monazite and xenotime by LA-ICPMS, and model age calculations using Re-Os isotopic data for arsenopyrite and in situ Pb-isotope data for galena from the Iso-Kuotko deposit provide evidence for multiple episodes of hydrothermal activity along the KiSZ. Data suggest coincident felsic magmatism and structurally controlled fluid flow events during wrench fault-dominated N-S shearing associated with early, micro-continent accretion ushering in the Svecofennian orogeny. This includes the formation of refractory gold ores at Suurikuusikko (1916 ± 19 Ma) and Iso-Kuotko (>1868 ± 15 Ma). Subordinate barren veining took place at ∼1830 Ma during transient extension between the accretion and final collision stage. Xenotime in late stage veins with visible gold at Iso-Kuotko yield U-Pb ages between 1761 ± 9 and 1770 ± 7 Ma. This time-interval is concurrent with widespread granitoid emplacement at the close of the Svecofennian orogeny. Disturbance of the Re-Os system in the early auriferous arsenopyrite can be connected to the late stage mineralizing processes. Pb isotope data suggest a mixed mantle and lower crust origin for fluids in the post-orogenic hydrothermal system.In situ U-Pb dating in polished thin sections in tandem with electron microprobe analyses of mineral compositions and detailed textural observations, highlights the utility of xenotime and monazite as robust geochronometers capable of recording repeated hydrothermal events in Paleoproterozoic orogenic systems. Results provide precise temporal constraints for orogenic gold mineralization and vital information to refine the tectonic evolution of the Central Lapland Greenstone Belt.