Magmatic evolution and metal element enrichment during formation of the Niumaoquan magnetite ore deposit, Xinjiang, China

Abstract

Magmatic magnetite is an important iron resource in China; Such magnetite deposits generally formed as a result of magmatic differentiation, with continuous exchange of material and energy between the magma and the surrounding country rock during magmatic evolution. Consequently, it is important to understand whether the aggregation and formation of individual minerals have been affected by interaction with the surrounding country rock during the formation of this type of ore deposit. The Niumaoquan magnetite deposit at Harlik, Xinjiang, China, forms part of a suite of layered mafic complexes that preserve evidence of rhythmic deposition of magmatic minerals. The complex that hosts the deposit contains olivine gabbro, olivine-bearing gabbro, gabbro, and hornblende gabbro units, all of which formed from Fe-rich tholeiitic magmas that underwent fractional crystallization and assimilated minor amounts of crustal material. Their εNd (t) values and initial 87Sr/86Sr ratios vary from –3.4 to –0.50 and from –3.4 to 8.5, respectively, indicating that the primary magma was derived from partial melting of an EM1-type (enriched mantle 1) source, most probably an enriched region of the lithospheric mantle that was metasomatized by subduction-zone-derived fluids. This area of mantle wedge was located within the root of the lithosphere and was transformed by interaction with subducted lithospheric material; the delamination of this lithospheric mantle material caused partial melting as a result of heating by the surrounding asthenosphere. The enriched lithospheric mantle material ascended into the space created by the delamination of the lithosphere. The melts produced during these events resided for long periods in temporary magma chambers, enabling significant fractional crystallization and generating economic concentrations of Ti and Fe within the resulting gabbros, thereby forming the ore body. Element geochemistry shows that Ni and Cu concentrations were highest during the early stages of magma evolution indicating Ni–Cu sulfide immiscibility at this time, and the likelihood that a Ni–Cu orebody exists somewhere in the complex. Subsequently, both the complex and the associated ore deposit were formed by the intrusion of magmas in a dynamic environment.