ORE ASSEMBLAGES, PLATINUM-GROUP MINERALS, AND BEHAVIOR OF Cl IN LOW-SULFIDE ZONES, VOLOGOCHAN-PYASINSKIY SUITES, NORILSK COMPLEX, RUSSIA

Ore assemblages in zones of low-sulfide mineralization of the Vologochan-Pyasinskiy intrusion of the Norilsk complex have multicomponent compositions involving Pd, Pd-Pt-(Rh, Ir), Au-Ag, REE, Y, Zr, Th, and U. They were deposited at late stages of crystallization of the ore-bearing zones in the presence of abundant volatile components, especially Cl, which accumulated late in the fl uid-saturated medium. Evolutionary trends in the ore zones are documented in the compositions of olivine, orthopyroxene, clinopyroxene, plagioclase, spinel-group minerals, calcic amphiboles, apatite, micas, and pentlandite, among other species. We fi nd (1) examples of monomineralic inclusions of clinopyroxene and plagioclase trapped in a globule of oxide melt, (2) evidence of the possible existence of a melt of anhydrite composition, and (3) aggregates of magnetite deposited along the periphery of grains of hercynite-spinel solid solution. A progressive buildup in Cl is reflected in patterns of compositional zonation in apatite, calcic amphibole, and annitic mica; levels of Cl consistently increase outward. Very efficient differentiation proceeded under oxidizing conditions in the mineralized zones of the Vologochan-Pyasinskiy intrusion. We provide complementary mineralogical data for sulfide-poor ore zones in the western fl ank of the Oktyabrsky deposit. Our inferences lead to a better understanding of ore genesis and evolution of low-sulfide ore-forming environments associated with the Siberian Traps.