矿床地球化学国家重点实验室知识库

The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton (southern India) comprises a sill-like layered ultramafic–mafic igneous complex with associated metasedimentary and metavolcanic (komatiitic to komatiitic basalt) schists that are enclosed by the tonalite–trondhjemite granodiorite suite of rocks (TTG). The sill-like layered complex is represented by a succession of chromitite-bearing serpentinite (after dunite) and peridotite, anorthosite, pyroxenite, and gabbro hosting magnetite bands. Extensive bulk-rock trace element and platinum-group element (PGE) geochemical study of the plutonic sill-like layered complex and the metavolcanic schists, suggest immobility of most trace elements (except La and Cu) and the PGEs, despite greenschist facies metamorphism and hydrothermal alteration experienced by the rocks. Their immobile nature is understood from good correlation of the trace elements and PGE with MgO and Cr. Other than chromitites and serpentinites all plutonic rocks show PPGE (Pd, Pt, Rh) enriched primitive-mantle normalized PGE patterns (Pd/Ir_N = 3.9–81.1) that are suggestive of fractionation of IPGEs (Ir, Os, Ru) by the early crystallizing chromite mineral, and the incompatible nature of PPGEs in the same. The chromitites show high PGE abundances (∑ PGE = 96–296 ppb), especially IPGEs (∑ IPGE = 63–223 ppb), due to the presence of inclusions of IPGE-bearing minerals. In the primitive-mantle normalized PGE plot the chromitites show an IPGE enriched pattern. The PPGE enriched pattern (Pd/Ir_N = 7.7–26) of the komatiitic to komatiitic basalt schists in a primitive-mantle normalized PGE plot indicates retention of IPGEs in the mantle or IPGE-bearing alloy saturation in the melt, while incompatible behavior of the PPGEs implies the sulfide undersaturated nature of the mantle source.

The PGE pattern of the metavolcanic schists resembles the pattern of early Archean(3.5 Ga) Barberton komatiites (Pd/Ir_N^Barberton = 1–40.7; Pd/Ir_N^Nuggihalli = 6.3–21.3), which corroborates our previous results based on REE study, and also resembles the pattern of komatiites from the 2.9 Ga Sandstone greenstone belt in the Youanmi Terrane of Western Australia (Pd/Ir_N^Sandstone = 6). The metavolcanic schists exhibit the typical PGE depleted character observed in early Archean komatiites (∑ PGE_schist = 0.4–27.2 ppb; ∑ PGE_Barberton = 15.0–20.8 ppb; ∑ PGE_{Youanmi Terrane, Western Australia} = 4.2–7.0 ppb) which is explained to be a result of progressive mixing of late veneer matter in the Earth's mantle with time. Pt fractionation in the Nuggihalli metavolcanic schists and in early or late Archean komatiites indicates Pt alloy dispersal in the lower mantle during crystallization of the primary magmaocean and a consequent formation of Pt-enriched and Pt-depleted isolated upper mantle domains that did not homogenize and mix away by 2.7 Ga.

In the plutonic layered sequence, pyroxenite represents a change from sulfide-undersaturation to sulfide-saturation. The pyroxenite represents a break in trend from the negative correlation of Pt and Pd with MgO displayed by the serpentinites and peridotites due to incompatible behavior of the PPGEs during lava differentiation, to the positive pattern displayed by the gabbro and metavolcanic schists due to attainment of sulfide saturation. Sulfide-saturation was probably triggered by fractional crystallization of olivine, chromite and pyroxenes. Chondrite-normalized REE patterns and a plot of incompatible elements negate the role of crustal contamination of the parental komatiitic magma. In addition, the absence of ambient sulfidic sediments rules out assimilation of crustal sulfur in the Nuggihalli rocks. The immiscible sulfides segregated from the Al-depleted komatiitic parental magma concentrating the PGEs during crystallization of the pyroxenes that accumulated to form pyroxenite.