Geochemistry of Late Permian picritic porphyries and associated Pingchuan iron ores, Emeishan Large Igneous Province, Southwest China: Constraints on petrogenesis and iron sources

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• 作者：Meng Wang ; Zhaochong Zhang ; M. Santosh ; Tong Hou
• 关键词：Geochemistry ; Metallogenesis ; Pingchuan iron deposit ; Picritic porphyry ; Emeishan Large Igneous Province
• 刊名：Ore Geology Reviews
• 出版年：March, 2014
• 年：2014
• 卷：57
• 期：Complete
• 页码：602-617
• 全文大小：2889 K

文摘

The Late Permian picritic porphyries are spatially and temporally associated with the Pingchuan iron oxide deposit in Emeishan Large Igneous Province (ELIP), southwestern China. The Pingchuan iron deposit carries an estimated ore reserve of 40 Mt at ~ 60 wt.% Fe. The iron oxide deposit is associated with an ancient volcanic edifice, which is located at the intersection of the NW- and NE-trending deep faults. The iron ore bodies are intimately associated with the intrusive masses of picritic porphyry, occurring mainly along the contact zone between the picritic porphyries and the Early Permian limestone. The most important ore types are massive and brecciated ores which together make up 90 vol.% of the deposit. The massive type ores generally occur as dykes consisting predominantly of magnetite with minor calcite and apatite. The picritic porphyries are characterized by a marked range of SiO₂ (37.12-47.39 wt.%) and MgO (19.22-29.08 wt.%), but show only a minor variation in Na₂O + K₂O (0.12-1.58 wt.%) and TiO₂ (0.64-1.44 wt.%). The total concentration of rare earth elements (REE) is relatively low (21-83 ppm), and shows moderate enrichment in light rare earth element (LREE; [La/Yb]_N = 3.01-3.63) compared to the HREE. The primitive-mantle-normalized patterns of the rocks are comparable to those of ocean island basalt, possibly indicating a plume source, which is considered to have generated the ELIP. Petrographic observation suggests that the Pingchuan picritic porphyries are virtually primary picritic magma with excess olivine. The primary magma possibly experienced fractional crystallization of Cr-spinel + olivine + clinopyroxene during the magma chamber process, but has not been significantly crustal contaminated. Moreover, the primitive magma was possibly produced by partial melting of the garnet-facies peridotite. In combination with the available information including field observation and geochemical studies, e.g. fluid inclusions and stable isotope data, we infer that the deposit is hydrothermal in origin and the ore-forming fluids are predominantly primary magma-derived fluids which had been released from the picritic porphyry during the emplacement at near the surface and mixed with fluids from country rocks. We envisage that the Pingchuan iron deposit was formed by the shallow emplacement, the aqueous chloride alteration and carbonate solutions and iron precipitation as a continuous process, and thus could be classified as a subvolcanic-hosted iron ore type.