The Ban Phuc Ni-Cu-PGE deposit related to the phanerozoic komatiite-basaltassociation in the Song Da Rift, nortwestern Vietnam

The Permian-Triassic komatiite-basalt complex in the Song Da rift, northwes tern Vietnam, occurs in the axial part of this structure and includes komat iites, komatiitic basalts, olivine basalts, and subvolcanic bodies of dunit e and plagioclase-bearing wehrlite hosting Ni-Cu-PGE sulfide ores. Volcanic rocks contain olivine- and pyroxene-spinifex textures. The Ban Phuc deposi t consists of two major orebodies. The first one is of vein type, zonal, wi th average Ni/Cu of 2.5 and Ni/Co of 33. In the central part, it is compose d of massive chalcopyrite - pentlandite - (violarite) - pyrrhotite ores wit h associated sulfarsenides of Ni and Co, ,heazlewoodite, tsumoite, parkerit e, sperrylite, and michenerite. Marginal parts of (pentlandite)- pyrrhotite - chalcopyrite, and violarite - chalcopyrite composition contain nickeline , maucherite, spenylite, and michenerite. The second orebody is composed of disseminated ores with an average Ni content of 1.2%. The plots of mantle- normalized contents of noble metals, Ni and Cu reflect their non-fractionat ed distribution, typical of komatiites. Ore formation proceeded in two stag es: 1) magmatic stage, including sulfide-silicate immiscibility and fractio nal crystallization of the sulfide melt, and 2) postmagmatic hydrothermal p rocesses. The saturation of the komatiitic melt with respect to a sulfide a t an early stage of formation of the Ban Phuc suite is suggested by a steep er trend of olivine composition on the Ni in olivine - Fo diagram compared with olivine from ore-free volcanic rocks of the komatiite-basalt series. L ater on, fractional crystallization of a fluid-bearing sulfide melt was the most important factor in the formation of ore. Enrichment of Cu-bearing or es from zonal orebodies in Pd and Au corresponds to their concentration in the residual sulfide melt during fractionation of Mss. The unusual Pt conce ntrations of the Fe-Ni ores are due to the presence of spenylite, which con tains between 0.9 and 3.4 wt.% Sb at the high-temperature stage (900-1000 d egreesC) of the volatile-saturated sulfide. At the lower-temperature stage, sulfarsenides of Ni, Co, and Fe appeared, together with michenerite at 500 -400 degreesC. At 300 degreesC and below, tsumoite and the Cu-Pb-Ag-Bi sulf osalts made their appearance.