Fluid Characteristics and Genesis of Early Neoproterozoic Orogenic Gold-Quartz Veins in the Harnas Area, Southwestern Sweden

Abstract

Gold-quartz veins occurring in the Mjosa-Vanern ore district, southeast Norway and southwest Sweden, represent early Neoproterozoic members of the orogenic gold type of deposit. The Harnas gold-quartz veins, in the central part of the ore district, are steeply dipping veins hosted in a local, west-northwest–east-southeast–trending brittle shear zone, which transects the north-south–trending deformational fabric in the surrounding greenschist grade orthogneisses. This deformation and subsequent vein formation occurred at around 1.0 Ga in a late phase of the Sveconorwegian (Grenvillian) orogeny. Fluid inclusions show that the ore-bearing vein system at Harnas developed essentially in three successive stages: a quartz stage at a depth of ≈ 4 km, a pyrite-gold stage at a shallower crustal level (≈1.5 km) after rapid exhumation of the area, and finally a galena stage. All stages involved fracturing subparallel to the strike of the host shear zone. During the first two stages, the ore fluid was an aqueous H 2 O-CO 2 fluid with a salinity of 4 to 10 wt percent NaCl equiv and a temperature of ≈200°C, whereas in the galena stage it was a purely aqueous fluid with a similar salinity and a temperature of ≈150°C. Oxygen and sulfur isotope results imply a predominantly metamorphic origin for the ore fluid and suggest that important ore constituents, such as lead and sulfur, were derived from the regional orthogneisses. Other gold-anomalous quartz veins in the Harnas area, as well as the Brustad gold-quartz vein in the northernmost part of the Mjosa-Vanern ore district, show some variation in fluid composition. However, aqueous fluid inclusions containing CO 2 and calcite were identified in all veins. This, and other similarities, strongly suggests that the veins throughout the district were formed contemporaneously and were controlled by deformation that, at least in part, affected the entire Mjosa-Vanern region. It is inferred from geologic evidence and pressure estimates that veins began to form during the final phase of Sveconorwegian continent-continent collision and were completed during incipient rapid exhumation of the thickened crust. A set of barren quartz-calcite veinlets, which crosscut the ore-bearing veins at Harnas, is unrelated to the ore formation. These veinlets were deposited from a surface-derived, low-temperature, saline aqueous fluid during some significantly later, but regionally extensive, hydrothermal event.