The fluid evolution of a proterozoic high-level anorogenic granite from the Gåsborn area, West Bergslagen, central Sweden

Abstract

The Proterozoic high-level anorogenic (A-type) Ostra Höjden granite and associated hydrothermal veins and skarns from the Gåsborn area, W. Bergslagen, central Sweden, contain aqueous, CO 2-rich (gaseous) and mixed aqueous-gaseous fluid inclusions. The oldest aqueous inclusions are highly saline (32–40 wt.% NaCl eq.). The oldest gaseous inclusions are CO 2-rich with, locally, minor amounts of N 2 and CH 4. Aqueous inclusions become less saline and CO 2-rich inclusions become less dense and loose their N 2 and CH 4 content, going from older to younger generations. Large quantities of mixed aqueous-gaseous inclusions only occur in hydrothermal veins in marble. δ 13C values of around − 1.6% of CO 2 in these inclusions suggest that they were formed by heterogeneous trapping of an aqueous fluid and CO 2 was derived from the surrounding marble. Many of the oldest aqueous and mixed aqueous-gaseous inclusins are decrepitated and surrounded by trails and fields of less dense, younger inclusions formed during and after decrepitation. The maximal fluid pressure and temperature are estimated at about 4 kbar and approximately 560°C, respectively. The lithostatic pressure is estimated at < 1.0 kbar, indicating that hydrothermal vein formation resulted from hydrostatic overpressuring of fluids expelled from the cooling granite.