Geochemistry, Petrogenesis and Metallogenesis of the Panzhihua Gabbroic Layered Intrusion and Associated Fe–Ti–V Oxide Deposits, Sichuan Province, SW China

Abstract

The Panzhihua gabbroic layered intrusion is associated with the 260 Ma Emeishan Large Igneous Province in SW China. This sill-like body hosts a giant Fe–Ti–V oxide deposit with 1333 million ton ore reserves, which makes China a major producer of these metals. The intrusion has a Marginal zone of fine-grained hornblende-bearing gabbro and olivine gabbro, followed upward by Lower, Middle, and Upper zones. The Lower and Middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene, plagioclase, and olivine. These zones also contain magnetite layers. The Upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine. Most rocks in the body show variable-scale rhythmic modal layering in which dark minerals, primarily clinopyroxene, dominate in the lower parts of each layer, and lighter minerals, primarily plagioclase, dominate in the upper parts. The oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion. Ore textures and associated mineral assemblages indicate that the ore bodies formed by very late-stage crystallization of V-rich titanomagnetite from an immiscible oxide liquid in a fluid-rich environment. The rocks of the Panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in Fe, Ti, and V. They are enriched in light rare earth elements relative to heavy rare earth elements, and exhibit positive Nb, Ta, and Ti anomalies and negative Zr and Hf anomalies. The silicate rocks and oxide ores of the Panzhihua intrusion formed from highly evolved Fe–Ti–V-rich ferrobasaltic or ferropicritic magmas. The textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall-rocks induced the separation of the immiscible oxide melts from which the Fe–Ti–V oxide ore bodies in the lower part of the intrusion crystallized.