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Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit and model calculations have shown that megacrysts originated from the crystallization of alkali basaltoid magma in a deep-seated intermediate chamber at 14–15 kbar, which is close to the Moho depth (50 km) in this part of southeastern Asia. The chamber was a source of heat and CO2 fluids for the generation of crustal syenitic melts producing sapphires and zircons. The formation conditions of sapphires and zircons are significantly different. The presence of jadeite inclusions in placer zircons points to high pressures during their crystallization, which is confirmed by the ubiquitous decrepitation of CO2-rich melt inclusions. Sapphires crystallized from iron-rich syenitic melt in the shallower Earth’s crust horizons with the participation of CO2 and carbonate–H2O–CO2 fluids. The subsequent eruptions of alkali basalts favored the transportation of garnet and pyroxene megacrysts as well as sapphire and zircon xenocrysts to the surface. It is shown that sapphire deposits can be produced only during multistage basaltic volcanism with deep-seated intermediate chambers in the regions with thick continental crust. The widespread megacryst mineral assemblage (clinopyroxene, garnet, sanidine, ilmenite) and the presence of placer zircon megacrysts can be used as indicators for sapphire prospecting.

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We have shown that the occurrences of Permo-Triassic magmatism in northern Vietnam compose the southwestern and southeastern flanks of the Emeishan large igneous province, shifted along the large-amplitude Ailao Shan-Red River shear zone. Based on new geochronological (U-Pb and Ar/Ar) data, two stages of magmatism related to the Emeishan plume have been recognized. At the first, Permian, stage (260 Ma), abundant picritoids of the Song Da and Song Hien zones formed, and at the second, gabbro-syenite and gabbro-monzodiorite associations of the Lo Gam zone, layered peridotite-gabbro massifs of the Nui Chua complex, high-alumina granites of the Phia Bioc complex, and basalt-rhyolite association of the Song Hien zone were produced. The metallogenic specialization of the Permo-Triassic magmatism of northern Vietnam is considered.

New data on the age, composition, formation conditions, and ore-geochemical specialization of the Nui Chua layered peridotite-gabbro complex are reported. They evidence that the complex resulted from the Permo-Triassic mantle plume activity in northern Vietnam (southern framing of the Yangtze Platform). Two series of mafic and ultramafic rocks differing in ore productivity—layered (PGE-Cu-Ni) and pegmatoid (Fe-Ti-V)—have been recognized within the complex. The first estimates of the composition of their parental melts have been obtained.

The Jinping–Song Da rift structure in the Emeishan Large Igneous Province is composed of Permian high- and low-Ti volcanic and volcanoplutonic ultramafic-mafic associations of different compositions and genesis. High-Ti picrites, picrobasalts, basalts, and dolerites are enriched in LREE and depleted in HREE and show low Al2O3/TiO2 ratios (~4), commensurate εNd(T) values (+0.5 to +1.1), and low (Th/Nb)PM ratios similar to those of OIB-enriched mantle source. The established geochemical characteristics evidence that the parental melts of these rocks were generated from garnet lherzolite at the depths of garnet stability (~200 to 400 km). Later, high-Mg low-Ti volcanics (komatiites, komatiitic basalts, and basalts) and associating small peridotite-gabbro massifs and komatiite-basalt dikes were produced as a result of ~20% partial melting of depleted water-poor (≤0.03 wt.% H2O) peridotite substratum from the hottest upper part of mantle plume at relatively shallow depths (100–120 km). The LREE-depleted komatiites and komatiitic basalts are characterized by low (Ce/Yb)CH values, 187Re/188Os = 0.05–1.2, 87Sr/86Sr = 0.704–0.706, positive εNd(T) values (+3 to +8), γOs = –0.5 to +0.9, and strong negative anomalies of Ba, K, and Sr on the spidergrams. The scarcer LREE-enriched komatiites, komatiitic basalts, and basalts vary greatly in chemical composition and values of εNd(t) (+6.4 to –10.2), 87Sr/86Sr (0.706–0.712), and γOs (+14.8 to +56), which is due to the different degrees of crustal contamination of parental magmas. The Rb-Sr isotopic age of basaltic komatiite is 257 ± 24 Ma. The Re-Os age determined by analysis of 12 komatiite samples is 270 ± 21 Ma. These data agree with the age of flood basalts of the Emeishan Large Igneous Province. The komatiite-basalt complex of the Song Da rift is still the only Phanerozoic PGE-Cu-Ni-complex of this composition. The geochemistry of accompanying Cu-Ni-PGE-ores confirms their relationship with komatiite-basaltic magmatism.

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-bearing 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 central 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, and 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 most 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.

New data obtained on the Khao Que-Tam Tao gabbro-granite pluton (Northern Vietnam) are discussed. It was established that this pluton was formed at the Permian-Triassic boundary (250.5 ± 3.2 Ma, 40 Ar/ 39 Ar isotopic age). Morphologically, it represents a hypabyssal fracture intrusion. The first stage was marked by the intrusion of the picrobasaltic melt, the differentiation of which resulted in the formation of the layered peridotite-gabbro series and the quartz bearing monzodiorites and granophyres in its endocontact at the final stage. At the second stage, the Khao Que peridotite-gabbro massif was broken in its central part by a fault, along which the Tam Tao granodiorite-granite massif was localized. Numerical simulation using the COMAGMAT program for the basic system and geochemical estimates for the granite system allow the state ment that the mafic and granitic melts evolved independently, and their final products were quartz bearing monzodiorite and granophyre, on the one hand, and aplites and pegmatites, on the other hand. The compo sitional correlation of the Permian-Triassic magmatic associations in Northern Vietnam (the Nui Chua gab bro pluton and the Khao Que-Tam Tao gabbro granite and Pia Bioc granite plutons) and in Southeast China (flood basalts) allows these complexes to be regarded as a part of a single large igneous province produced by the Emeishan plume activity.