Trace-element and Sr–Nd isotopic geochemistry of the PGE-bearing Xinjie layered intrusion in SW China

Abstract

The Xinjie layered intrusion is one of a number of major ultramafic–mafic bodies hosting Fe–Ti–V deposits and Cu–Ni–PGE sulfide in the Pan–Xi area of the Sichuan Province, SW China. The intrusion is a 7.5 km long, 1 to 1.5 km wide, and 1200 m thick ultramafic–mafic sill, which is associated with Permian plume-related Emeishan flood basalts. The intrusion is characterized by three lithological cycles, each representing a sequence from ultramafic to basic/felsic composition. The basal part of the intrusion is composed of three lithological units, namely, the Marginal, Peridotite and Pyroxenite Units. Two major PGE-mineralized layers occur within the Marginal and Pyroxenite Units. The Marginal Unit (MU) is characterized by marked Nb–Ta, Sr and Ti enrichments and obvious Th depletion, while the Peridotite Unit (PeU) shows significant Sr, slight Zr depletion and pronounced positive Ti anomalies. The Pyroxenite Unit (PyU) is depleted in Nb and Zr but significantly enriched in Sr and Ba, with lower ratios of Nb/La and Zr/Sm, compared to those of the underlying two units. The PGE-enriched layer in the Marginal Unit has elevated and highly variable Rb/La and Ba/Th values and the lower part of the Pyroxenite Unit, which includes a PGE-enriched layer, exhibits high and decreasing Rb/La and Ba/Th ratios upwards. Sr–Nd isotopic compositions of samples from the three units (ca. 90 m thick) were systematically analyzed and fall in a range of 0.7056 to 0.7074 for ( 87Sr/ 86Sr) i and of −5.26 to +2.79 for ε Nd(i). Within the Marginal and Peridotite Units, high ( 87Sr/ 86Sr) i and low ε Nd(i) values are demonstrated. By contrast, an enriched Sr-isotopic (( 87Sr/ 86Sr) i∼0.7074) and slightly depleted Nd-isotopic ( ε Nd(i)∼+2.5) signature occurs in the basal Pyroxenite Unit, 8 m below the PGE-enriched layer in this unit. The Marginal Unit is interpreted as the product of contamination of a plume component (e.g., ε Nd(i)∼+5 and ( 87Sr/ 86Sr) i∼0.704) by continental crust. The Peridotite Unit has undergone decreasing degrees of crustal contamination. At the bottom of the Pyroxenite Unit, a new magma input, contaminated by upper crust, is proposed, where its slight Nd–Sr decoupling may be attributed either to the much higher Sr content than the Nd content of the contaminant or the selective contamination of Sr rather than Nd. The PGE mineralization in the Pyroxenite Unit was caused by magma mixing between the new magma and the resident magma, whereas the PGE-enriched layer in the Marginal Unit resulted from wallrock contamination. However, the displacement of the isotopic pulse from the PGE-enriched layer in the Pyroxenite Unit is due to later sulfide saturation being achieved in both end members. Both of the PGE-enriched layers are linked directly to the segregations of the Fe–Ti oxides and sulfides.