Platinum-group elements in the oxide layers of the Hongge mafic-ultramafic intrusion, Emeishan Large Igneous Province, SW China

摘要

The Hongge layered intrusion (259.3 ±1.3 Ma) is one of several mafic-ultramafic intrusions that host giant Fe-Ti-V oxide ore deposits in the ～260 Ma Emeishan Large Igneous Province (ELIP), SW China. The Hongge intrusion consists of a lower olivine clinopyroxenite zone (LOZ), a middle clinopyroxenite zone (MCZ) and an upper gabbro zone (UGZ). Most of the 14 to 84 m-thick and 300 to 1700 m-long economic Fe-Ti-V oxide ore layers occur within the MCZ. This paper reports the concentrations of PGE in the oxide layers of the Hongge intrusion. Unlike in the economic PGE (platinum-group elements) mineralized (up to 1.2 ppm Pt and 1.8 ppm Pd) coeval Xinjie intrusion (259±3 Ma), the oxide layers in the Hongge intrusion contain very low PGE (total PGE: 0.09-63.5 ppb). Chromite-bearing horizons in the Hongge intrusion are enriched in IPGE (Ir: 0.46-0.65 ppb; Ru: 2.25-3.29 ppb) relative to PPGE (Pt: 0.54-1.28 ppb; Pd: 0.30-0.90 ppb). In contrast, the massive magnetite layers in the Hongge intrusion show no IPGE enrichments relative to PPGE. All our samples from the Hongge intrusion collectively show no correlation between PGE and S contents, and weak positive correlations between IPGE and Cr contents, indicating removal of Ir, Ru from magma with crystallization of chromite. Positive correlations exist between IPGE and PPGE in the samples, indicating that all of the PGE was controlled by sulfide liquid. Most of the samples have mantle-normalized PGE patterns with a slope similar to that of the Emeishan picritic basalts, which are less fractionated than the coeval high-Ti basalts derived from the same Fe-, Ti-, V-rich magma series. The Hongge oxide-rich samples are characterized by Cu/Pd ratios (1.47-202 x 10~4) significantly higher than those for primitive mantle and the coeval picrites (0.99 x 10~4). The PGE tenors in bulk sulfides (i.e., in recalculated 100% sulfides) in the sulfide-bearing oxide ores of the Hongge intrusion (<0.1-3 ppm) are 2-3 orders of magnitude lower than the Xinjie intrusion (10-100 ppm). This, together with extremely high Cu/Pd ratios in both sulfide-bearing and sulfide-barren oxide-rich samples, indicates that the parental magma of the Hongge intrusion was depleted in PGE. We suggest that PGE depletion in the Hongge parental magma was due to previous sulfide segregation at depth, mainly due to crustal contamination. The PGE-depleted sulfides in the Hongge intrusion are thought to have formed by second-stage sulfide saturation and segregation due to fractional crystallization involving abundant magnetite after magma emplacement at Hongge.