Conditions of metamorphism of the Outokumpu Cu - Co - Zn deposit from sphalerite geobarometry

Abstract

Electron microprobe analysis was used to determine Zn and Fe in homogeneous samples of sphalerite in mutual contact with pyrrhotite and pyrite from the disseminated ore layer stratigraphically high in the Keretti orebody at Outokumpu, Finland. On the bases that the composition of sphalerite (1) becomes less Fe-rich with increasing pressure in a system in which aFeS is buffered by pyrrhotite and pyrite and (2) is independent of temperature in much of the geologically important temperature range (c. 300°-650°C), a mean pressure of 3.4 ± 0.3kb (with a pressure range of 2.8-4.3 kb) has been calculated. This corresponds to the previous estimates of peak metamorphic conditions of 4 kb (at 500°C) and 3.5 ± 1kb (at 600° ±50°C) based on silicate mineral assemblages. Zn and Fe determinations for sphalerite associated with an inequilibrium mineral assemblage such as remobilized pyrrhotite and chalcopyrite in which visible exsolution lamellae are present, give improbably high estimates of pressure. Determinations for sphalerite in contact with only one additional phase (pyrrhotite or pyrite) also give higher estimates than when buffered by the divariant assemblage pyrrhotite + pyrite. Previously published estimates correspond with those obtained for sphalerite in contact with pyrrhotite only, estimates that are discarded on account of the absence of a requisite buffer assemblage.

Keywords

• sphalerite
• electron probe data
• zinc
• iron
• geologic barometry
• metal ores
• metamorphism
• Proterozoic
• Keretti
• Outokumpu
• Finland