The Banfuk PGE-Ni-Cu Deposit and Ore Forming Processes in Phanerozoic Komatiite-Basaltic Complex of Shongda Zone, North-Western Vietnam

The Permian-Triassic komatiite-basaltic complex of the Shongda zone occurs in the axial part of this structure belonging to the southern extension of the Indosinian collision belt.The complex includes peridotite, pyroxenite, and basalt komatiites, komatiite and olivine basalts and subvolcanic bodies of dunites and plagiowehrlites hosting PGE-Cu-Ni sulfide ores. Volcanic rocks contain olivine and pyroxene spinifex textures. The Banfuk deposit includes two major bodies. The first is of vein type, zonal, with average N/Cu - 2.5; Ni/Co - 33. In the central part it is composed of massive Cp-Pn-(Vi)-Po ores with associated sulfoarsenides of Ni and Co, heazlewoodite, tsumoite, parkerite, sperrylite, and michenerite. Marginal parts of (Pn)- Po-Cp and Vi-Cp composition contain nickeline, maucherite, sperrylite, and michenerite. The second orebody is composed of disseminated ores with average Ni content 1.2% (Polyakov et al., 1991,1995,1998). The plots of mantle normalized contents of noble metals, Ni and Cu reflect their nonfractionated distribution, typical of komatiites (Barnes et al., 1988).

Ore forming process occurred by stages: 1) magmatic stage including sulfide-silicate liquation and fractional crystallization of sulfide melt; 2) fluid-magmatic; 3) post-magmatic hydrothermal processes. The saturation of komatiite melt with sulfur at the early stage of formation of the Banfuk Massif is suggested by a steeper trend of olivine composition on the Ni(ol)-Fa diagram compared with olivines of ore-free rocks of the komatiite-basalt series. After that, fractionation crystallization of fluid-bearing sulfide melt was the most important in the ore formation. High-Ni pyrrhotite (Ni = 0.2-2.45 wt.%) crystallized from mss at temperatures lower than 870oC and increase in sulfur fugacity from lgfS2=-2 and higher, in agreement with experimental data (Peregoedova, 1998). Enrichment of Cu-bearing ores in zonal orebodies of Pd, and Au corresponds to their concentration in the residual sulfide melt at fractionation of mss. Uncommon Pt concentration in Fe-Ni ores is related to the appearance of sperrylite containing to 0.9-3.4 wt.% Sb at the high-temperature stage (900-1000oC) of volatile-saturated sulfide . At the lower-temperature stage sulfoarsenides of Ni, Co, and Fe appear together with michenerite at 500-400 oC and, at 300oC and below, tsumoite and other Cu-Pb-Ag-Bi sulfosalts appear as well. (Grants RFBR 98-05-65309 and 98-05-65267).