Textures and Chemical Compositions of Magnetite from Zhibo Submarine Volcanic Iron Oxide Deposit, Xinjiang, China: Implications for Re-Equilibration Processes

Abstract

The Awulale Iron Metallogenic Belt (AIMB) has developed many medium–large iron deposits, of which the Zhibo iron deposit is selected as the research object in this paper. The Zhibo deposit’s ore primarily consists of magnetite as the main mineral, accompanied by extensive epidotization. The mineral assemblage includes diopside, albite, actinolite, epidote, chlorite, K-feldspar, quartz, calcite, chalcopyrite, and pyrite. Magnetite is classified into two groups based on sulfide content and mineral assemblage (MagI for sulfide-free and MagII for sulfide-rich ores). Two-stage mineralization of magnetite has been identified based on mineral assemblages and paragenesis, including the magmatic stage MagI and hydrothermal stage MagII. Mag I shows inhomogeneous backscattered electron (BSE) textures and consists of BSE-light and -dark domains (Mag I-L and MagI-D). Seven subtypes of magnetite have been recognized in this deposit. MagI-L and MagI-D have formed in the magmatic stage and show BSE images in light and dark colors, respectively. MagI-L is anhedral to subhedral and is inclusion-free. MagI-D has mainly replaced MagI-L along fractures and contains inclusions and pores. MagII has formed in the hydrothermal stage and is characterized by coupled dissolution–reprecipitation (DRP) textures. It can be divided into five sub-generations, that is, MagII-1, MagII-2, MagII-3, MagII-L, and MagII-D. MagII-1, MagII-2, and MagII-3 comprise the core–mantle–rim texture, while MagII-L and MagII-D comprise the core–rim texture. MagII-1 is BSE-light and is enriched with inclusions and pores. MagII-2 has partly replaced MagII-1 and exhibits oscillatory zoning under BSE imaging. It also contains inclusions. BSE-light MagII-3 occurs as overgrowth along MagII-2 margins and is inclusion-free. MagI magnetite is enriched with V, Cr, and Ni, whereas MagII is enriched with W, Ta, Nb, Sr, Sb, Sn, Y, Zr, Mg, Al, and Ti, indicating a decreased temperature of magnetite formation. MagI-L crystallizes from the original magma, while MagI-D is formed from the residual magma enriched with incompatible elements. MagII crystallizes from later multiple hydrothermal activities through the dissolution of early magnetite and the re-precipitation of later magnetite or from MagI-D which has later undergone a hydrothermal overprinting process. According to the texture and chemical composition of magnetite from the Zhibo deposit, we suggest that the Zhibo iron deposit was formed from the initial magmatic origin and then underwent a hydrothermal overprinting process.