Interstitial microstructures in Ji'nan mafic intrusion, North China Craton: magmatic or hydrothermal origin?

Interstitial microstructures of igneous rocks provide vital information on magma evolution. Reaction microstructures including orthopyroxene–magnetite symplectites and olivine–plagioclase coronas have been described from lower-middle units of layered mafic/ultramafic intrusions and were suggested to indicate melt immiscibility. Here we report similar microstructures from the Ji'nan complex in the Shandong Province of the North China Craton, associated with a typical hydrothermal iron ore deposit. The backscattered-electron images and mineral chemical data of the intergrowths of vermicular magnetite within orthopyroxene suggest their formation by reaction between olivine and residual Fe-rich melt under conditions of high oxygen fugacity. The olivine–plagioclase coronas are interpreted to have formed through a multi-stage process, in which the inner shell of orthopyroxene formed by the reaction between olivine and residual magmatic liquid above solidus, whereas the outer shell of An-rich plagioclase and vermicular orthopyroxene resulted from subsolidus diffusive reaction between original olivine and plagioclase, triggered by the Na compositional gradient. The intermediate shell of biotite (or amphibole), when present, seems to have been generated by the replacement of orthopyroxene in both the inner and outer shells under hydrous condition. Furthermore, the presence of the symplectites of orthopyroxene and vermicular magnetite indicates Fe-rich magmatic melt with high oxygen fugacity, and the later formation of the intermediate shell of biotite in olivine–plagioclase coronas suggests the incorporation of Cl-, K-bearing magmatic fluid, which provided both the Fe and primary ore-forming fluid for the mineralization. The interstitial microstructures in the Ji'nan complex not only provide insight into the magmatic-hydrothermal evolution, but also indicate the favorable criteria for the iron mineralization.