Mobilization of gold in the deep crust: evidence from mafic intrusions in the Bamble belt, Norway

Abstract

The location of mesothermal gold deposits in the upper part of major transcurrent shear zones suggests that gold was derived from the deep, wider portions of the shears. The 30 km-wide Bamble belt, south Norway, is a deep shear belt, metamorphosed to upper amphibolite and granulite grade at about 1.1 Ga. Preliminary studies showed that its rocks are strongly depleted in gold and other chalcophile elements. Mafic rocks were chosen for more detailed study of the mobilization of gold during metamorphism. Gold in mafic rocks largely partitions into the sulphide phase, which is mainly comprised of pyrrhotite. Modification of this phase was required to liberate gold. Bamble has two main groups of mafic rock. The first, metabasites, intruded before peak metamorphism, are uniformly low in gold. Their | O 2 at peak metamorphism, obtained from ilmenite-orthopyroxene equilibria, indicates a strongly oxidized mineral assemblage, well above FMQ. Pyrrhotite was stable at peak temperature of 800°C, but on cooling below 600°C, internal (mineralogical) buffering by the oxidized assemblage caused the | O 2 to cross into the stability field of pyrite. Following conversion of pyrrhotite to pyrite, the latter was partly replaced by magnetite. These serial changes in the mineralogical form of iron sulphide was conducive to the extraction of gold, as was the oxidized nature of the mineral assemblage that would have buffered metamorphic fluids to high | O 2 . The second group of mafic rocks, “hyperites”, intruded after peak metamorphism, record intermediate steps in the extraction of gold. These intrusions, of coronitic gabbro, were metamorphosed, in whole or in part, to amphibolite. There is a decrease in gold and other chalcophile elements across a hyperite dyke from the pyrrhotite-bearing, coronitic gabbro interior to the pyrite-bearing amphibolite margin.