THE BAN PHUC Ni Cu PGE DEPOSIT RELATED TO THE PHANEROZOIC KOMATIITE BASALT ASSOCIATION IN THE SONG DA RIFT, NORTWESTERN VIETNAM

Abstract

The Permian–Triassic komatiite–basalt complex in the Song Da rift, northwestern Vietnam, occurs in the axial part of this structure and includes komatiites, komatiitic basalts, olivine basalts, and subvolcanic bodies of dunite and plagioclase-bearin g wehrlite hosting Ni–Cu–PGE sulfide ores. Volcanic rocks contain olivine- and pyroxene-spinifex textures. The Ban Phuc deposit consists of two major orebodies. The first one is of vein type, zonal, with average Ni/Cu of 2.5 and Ni/Co of 33. In the centra l part, it is composed of massive chalcopyrite – pentlandite – (violarite) – pyrrhotite ores with associated sulfarsenides of Ni and Co , heazlewoodite, tsumoite, parkerite, sperrylite, and michenerite. Marginal parts of (pentlandite) – pyrrhotite – chalcopyrite, a nd violarite – chalcopyrite composition contain nickeline, maucherite, sperrylite, 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-fractionated distribution, typical of komatiites. Ore formation proceeded in two stages: 1) magmatic stage, including sulfide–silicate immiscibility and fractional crystallization of the sulfide melt, and 2) postmagmatic hydrothermal processes. The saturation of the komatiitic melt with respect to a sulfide at an early stage of formation of the Ban Phuc suite is suggested by a steeper trend of olivine composition on the Ni in olivine – Fo diagram compared with olivine from ore-free volcanic rocks of the komatiite–basalt series. Later on, fractional crystallization of a fluid-bearing sulfide melt was the mos t important factor in the formation of ore. Enrichment of Cu-bearing ores from zonal orebodies in Pd and Au corresponds to their concentration in the residual sulfide melt during fractionation of Mss. The unusual Pt concentrations of the Fe–Ni ores are due to the presence of sperrylite, which contains between 0.9 and 3.4 wt.% Sb at the high-temperature stage (900–1000 °C) of the volatile-saturated sulfide. At the lower-temperature stage, sulfarsenides of Ni, Co, and Fe appeared, together with michenerite at 500–400°C. At 300°C and below, tsumoite and the Cu–Pb–Ag–Bi sulfosalts made their appearance.