Derivation of gold by oxidative metamorphism of a deep ductile shear zone: Part 2. Evidence from the Bamble Belt, south Norway

Abstract

Abstract The Bamble Belt of south Norway is a major transcurrent shear zone, 30 km wide, comprised of lower Proterozoic supracrustal rocks and tholeiitic intrusions. At 1.54 Ga the belt was metamorphosed at ∼800°C and a depth of ∼25 km to granulite and upper amphibolite grade. This was coincident with large-scale displacement by ductile deformation, which permitted the pervasive passage of fluids through the belt during metamorphism. Previous work has shown that fluid inclusions in the granulites are predominantly CO 2 , which led to a proposal that the granulites were derived from amphibolite facies rocks by dehydration in a stream of CO 2 -rich fluid of possible mantle origin. In the course of this, rubidium was removed from the granulites. The present study has shown that metamorphic fluids caused pervasive oxidation of the rocks. Mafic rocks re-equilibrated at an oxygen fugacity 2.7 log units above their likely pre-metamorphic condition. Sulphide minerals, including pyrite, pyrrhotite, and chalcopyrite, were dissolved. This created a relatively oxidized, sulphur-rich fluid, in which SO 2 and H 2 S were in approximately equal amount. A fluid of this composition is a potent solvent for the more electronegative transition elements, such as Au + , Sb 3+ and As 3+ . These elements have been strongly depleted throughout the belt. Gold in the mafic rocks averages 0.17 ppb, which is 4% of its crustal abundance in this rock type. Average contents for other rock units vary from 0.15 to 0.37 ppb Au, or more than an order of magnitude less then their likely original composition. These data are consistent with a model where oxidized, CO 2 -rich fluids passing through a ductile shear zone dissolved gold and transferred it upwards within the permeable shear. Loss of this amount of gold from the volume of lower crust contained within the 30 km-wide shear zone would have provided a large supply of this metal for concentration at shallower levels in the crust, where the ductile shear narrowed to a brittle fault. The oxygen fugacity of the metamorphic fluids in the Bamble Belt was such that, on cooling, the fluids may have precipitated sulphate minerals, hematite and pyrite depleted in 34 S: all features found in some major Archean quartz-carbonate gold deposits, such as Kalgoorlie.