Sulfide minerals in the Otago and Alpine schists, New Zealand,a metasedimentary belt exposed from unmetamorphosed greywackesup to amphibolite facies, underwent systematic changes in abundance,composition and texture during prograde metamorphism. In unmetamorphosedrocks, the most common sulfide mineral is framboidal pyrite,which contains abundant As (up to 14000 ppm), Co (up to 4000ppm), 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 isolatedgrains. Chalcopyrite and sphalerite contain few trace elements,whereas galena contains significant Se (up to 1600 ppm) andlocally abundant Hg (up to 600 ppm). The distribution of thesetrace and minor elements is extremely heterogeneous. In subgreenschist-faciesrocks, pyrrhotite replaces pyrite, and there is a clear texturalchange from framboidal pyrite to composite grains of pyrrhotite,sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite containsCo (average values 1100 ± 490 ppm), Cu (up to 17000 ppm),and Ni (up to 11000 ppm). Antimony and Hg are above detectionin rare individual grains, but none of the other trace and minorelements sought are detectable. Sphalerite, galena and cobaltiteincrease in proportion in subgreenschist-facies rocks, and alsocontain 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), andSb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite,sphalerite, galena, and cobaltite become less abundant fromsubgreenschist- to amphibolite-facies rocks. In amphibolite-faciesrocks, only pyrrhotite, chalcopyrite and trace amounts of galenaand molybdenite occur, and none of these minerals contain detectablelevels of Ag, Au, As, or Hg. Mass-balance calculations betweensulfide minerals and whole rocks show that sulfides are importanthost minerals for S, Cu, and As, but host a minor proportionof Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltitebecome less abundant at higher metamorphic grade, Pb, Zn andCo are retained in the rock incorporated in other minerals,whereas As, Sb, Hg, Au, and Ag are removed from the rock, mostlikely by metamorphic devolatilization.
2010. Vol. 48, no 5, 1267-1295 p.
sulfide minerals, metamorphism, orogenic gold, Otago Schists, Alpine Schists, New Zealand