THE EFFECTS OF METAMORPHISM, TECTONICS AND HOST ROCKS ON THE LOCATION OF SULPHIDE ORES IN THE KONGSBERG SERIES, SOUTH NORWAY.

Abstract

A study has been made of two deposits of Fe-Cu-Zn sulphide ores, from Gr¢sli and Eiker, in the Proterozoic Kongsberg Series of South Norway. The ores are located at the junction of acid-intermediate supracrustals and amphibolitized gabbros. The supracrustals underwent Upper Amphibolite facies metamorphism of Svecofennian age (1600 to 1500 m.y.B.P.) and were then intruded by the gabbros which underwent subsequent Sveconorwegian (1200 to 1000 m.y.B.P.) metamorphism at ~lid-Amphiboli te facies grade. The ore bodies were originally part of the supracrustal sequence, which was deposited as a volcano-sedimentary succession (with some hydrothermally altc:ced equivalents), similar to the KuroKo-type c1r.'posits of Japan. The ores have thus undergone both phases of h5[;h grade regional metamorphism. At Gr¢sli, the ores were partially incorporated in the gabbros prior to the later (Sveconorwegian) metar.lorphism, during which extensive shearing occurred throughout the ores at Eiker. The cores of the gabbroic intrusions retained original igneous mineralogies and textures, with progressive amphibolitization towards the peripheries. A subsequent Epidote-Amphibolite and later Greenschist facies grade overprint produced varying retrograde assemblages in the silicate rocks. Under Greenschist facies conditions, rejuvenation of the Sveconorwegian shear (at Eiker) caused brittle faulting, while fluid activity (at both Gr¢sli and Eiker) caused the formation of chloriteact ino.li te-carbo!)a.te assemblages around some of the ore bodies. Renobilisation of ~he ore material itself was minimal. Studies of R00thermometry and geobarometry indicate that the ore deposits were re-equilibrated during the Epidote-Ampllj boli te facies overprinting.