Downward Infiltration of Fluid Into the High-Grade Basement of South Norway I: Brines

Abstract

The migration of surface derived fluids into crystalline basement rocks has been recognised in a variety of structural settings and is particularly well documented in shield areas (e.g. Fritz and Frape, 1987), but the mechanisms and extent of such infiltration remains controversial. This paper reports results from basement rocks on the west flank of the Oslo graben, where there is evidence of infiltration by sedimentary fluids. The Kongsberg Sector is a part of the high-grade Precambrian basement of South Norway, situated to the west of the Permian Oslo Graben. The Western Kongsberg Complex consists of granulite and amphibolite facies gneisses and metagabbros. Permian activity in this area resulted in dykes, cut by hydrothermal quartz, and base metal and silver mineralisation. The Modum Complex situated to the northeast, is dominated by metasediments and metagabbros, which exhibit high grade and retrograde hydrothermal effects (Munz et al, 1994) and have been intruded by diabase dykes, spatially related to the Oslo Rift. The final stage of hydrothermal activity in the region resulted in a suite of quartz veins, generally rimmed by fine albite. Previous work on fluid inclusions from these veins has identified hydrocarbon + CO2 and aqueous inclusions of variable salinities (Munz et al., 1995) and cited these as evidence of penetration of basinal fluids into metamorphic basement rocks. This study has focused on aqueous inclusions from a larger sample suite of quartz veins from the Modum Complex and from previously unstudied material from the Western Kongsberg Complex.