Deposition conditions for the indium-bearing polymetallic quartz veins at Sarvlaxviken, south-eastern Finland

Abstract

Polymetallic quartz veins, with up to 1500 ppm indium, have been discovered recently in the Sarvlaxviken area within the 1.64 Ga anorogenic multiphase Wiborg rapakivi batholith and adjacent 1.90 Ga Svecofennian crust in SE Finland. Evidence from primary fluid inclusions in the Sarvlaxviken area provides new information on the hydrothermal transport and depositional processes of metals in anorogenic granites. Fluid inclusions with variable aqueous liquid and vapour proportions (5–90 vol.% vapour) occur in quartz, cassiterite and fluorite belonging to three generations of polymetallic quartz veins. Microthermometry indicates that the veins were deposited at temperatures that range from ~500°C down to <100°C and salinities from 0 to 16 eq. mass% NaCl. Fluid inclusion data show that the depositional conditions were similar regardless of vein generation. The interpreted depositional processes involve phase separation with a combination of condensation, cooling and boiling of an initially low-salinity (<3 eq. mass% NaCl) aqueous magmatic vapour phase enriched in CO2-F-Cl-S and metals. Fluid inclusions with low salinities dominate, but higher salinities are recorded in metal-rich parts of the veins. The turbulent fluid flow, with complex geometry and temperature-salinity patterns, may explain why sulfide and/or oxide opaque minerals occur irregularly, and are locally the dominating vein minerals, but disappear completely into barren parts of the quartz veins. All fluids are considered to have been generated by the F-rich Marviken granite (and related granite dykes), which show all geochemical criteria for an ore-fertile granite. The quartz veins investigated in the adjacent Svecofennian country rocks are considered to represent the very last stage of a fluid with similar characteristics to the fluid responsible for the ore formation in the Sarvlaxviken area, but that had cooled to <100°C.

Kinetically driven successive sodic and potassic alteration of feldspar

“…The F-rich fluid inclusions associated with some rapakivi granites (e.g. Wiborg rapakivi batholith and Kymi stock, southeast Finland 35 , 36 ) indicate a link between high F hydrothermal fluids and Rapakivi feldspar, consistent with the increase in the kinetics of feldspar replacement observed in our experiments. Altogether, these observations show that the Rapakivi zonation textures can form in a self-evolved hydrothermal system.…”

Critical-metal mineralogy and ore genesis: contributions from the European Mineralogical Conference held in Rimini, September 2016

“…Overall there is a significant In and Se resource which may be available for future exploitation. The study of In continues with ‘Deposition conditions for the indium-bearing polymetallic quartz veins at Sarvlaxviken , south-eastern Finland’ (Broman et al ., 2018). Fluid inclusion studies of the In-rich (up to 1500 ppm) polymetallic quartz veins show that the metals were derived from an adjacent F-rich granite and associated dykes.…”