METASOMATISM OF MAFIC GNEISS IN ASSOCIATION WITH BRINE INFILTRATION – FLUID INCLUSIONS, SEM-CL AND MINERALOGICAL RECORD OF THE FLUID INFILTRATION

The uppermost stratigraphic layer of a 250 meter thick sequence of metasediments in the Bjellaatinden area, Northern Norway, consists of a hornblende-biotite gneiss. In one area of ca. 15 m the gneiss is cross cut by sub vertical, slightly arched, 2 – 10 cm thick quartz veins with an alteration halo up to 5 cm into the unaltered gneiss. The zoned alteration assemblage includes sericite, dickite, chlorite, calcite and muscovite. Hornblende is gradually altered to biotite and dickite. Plagioclase is increasingly sericitized, and ultimately altered to dickite towards the vein. The Fe/Fe+Mg ratio increases in biotite when partly altered to chlorite, and increases slightly in chlorite (0,68) compared to biotite (0,64) in the unaltered zones. Titanite is the main Ti-mineral in the unaltered gneiss, while ilmenite + rutile dominate in the alteration zone.

Three types of fluid inclusions in quartz are described: Saline (ca. 40 wt% NaCl equiv.) water rich, gas rich, and low saline water rich. Multiple generations of fluids are indicated by several trails consisting of separate gas rich, saline water rich and low saline water rich inclusions. In some areas all three types occur together and are indicative of co-existence of different fluids in an immiscible fluid system. SEM-CL reveals three different types of quartz: Low lumicent, with a broad peak from 420 – 650 nm, hosting late, gas rich inclusions, higher lumicent, with a peak around 500 nm, hosting the saline fluid inclusions, and high lumicent with a peak around 500 nm scarce of fluid inclusions. Low saline, water rich fluids are mostly found in high lumicent quartz with a peak around 400 nm inside the alteration zone.

Microthermometry of the saline fluid inclusions revealed a bimodal distribution of total homogenization temperatures (Th), but with stabile salinity measured by halite melting at ca. 300 °C and Th (liquid + vapour → liquid) ranging from ca. 450 °C to ca. 250 °C. Semi quantitative EPMA analysis of decrepitated fluid inclusions showed major Na, Ca and Fe, and minor K and Mn. Mineral breakdown may be responsible for the elevated Fe and Ca levels in the fluids.