SULFIDE EVOLUTION DURING PROGRADE METAMORPHISM OF THE OTAGO AND ALPINE SCHISTS, NEW ZEALAND

Abstract

Sulfide minerals in the Otago and Alpine schists, New Zealand, a metasedimentary belt exposed from unmetamorphosed greywackes up to amphibolite facies, underwent systematic changes in abundance, composition and texture during prograde metamorphism. In unmetamorphosed rocks, the most common sulfide mineral is framboidal pyrite, which contains abundant As (up to 14000 ppm), Co (up to 4000 ppm), Cu (up to 14000 ppm), Ni (up to 1100 ppm) and, locally, Ag (up to 270 ppm), Au (up to 90 ppm), and Sb (up to 240 ppm). Chalcopyrite, sphalerite, and galena also occur as isolated grains. Chalcopyrite and sphalerite contain few trace elements, whereas galena contains significant Se (up to 1600 ppm) and locally abundant Hg (up to 600 ppm). The distribution of these trace and minor elements is extremely heterogeneous. In subgreenschist-facies rocks, pyrrhotite replaces pyrite, and there is a clear textural change from framboidal pyrite to composite grains of pyrrhotite, sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite contains Co (average values 1100 ± 490 ppm), Cu (up to 17000 ppm), and Ni (up to 11000 ppm). Antimony and Hg are above detection in rare individual grains, but none of the other trace and minor elements sought are detectable. Sphalerite, galena and cobaltite increase in proportion in subgreenschist-facies rocks, and also contain higher concentrations of Ag (up to 1480 ppm in galena), Au (up to 230 ppm in galena and 110 ppm in cobaltite), As (38 ± 6 wt. % in cobaltite), Co (26 ± 4 wt. % in cobaltite), Hg (up to 4500 ppm in galena and 1100 ppm in sphalerite), and Sb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite, sphalerite, galena, and cobaltite become less abundant from subgreenschist- to amphibolite-facies rocks. In amphibolite-facies rocks, only pyrrhotite, chalcopyrite and trace amounts of galena and molybdenite occur, and none of these minerals contain detectable levels of Ag, Au, As, or Hg. Mass-balance calculations between sulfide minerals and whole rocks show that sulfides are important host minerals for S, Cu, and As, but host a minor proportion of Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltite become less abundant at higher metamorphic grade, Pb, Zn and Co are retained in the rock incorporated in other minerals, whereas As, Sb, Hg, Au, and Ag are removed from the rock, most likely by metamorphic devolatilization.