Silician zoning of magnetite in a Fe skarn deposit: A potential low-temperature indicator in magmatic-hydrothermal systems?

Single magnetite crystals from the Cihai Fe skarn deposit in Northwestern China are strongly growth zoned. Magnetite cores are in equilibrium with garnet whereas magnetite rims are enclosed by calcite. The chemical zoning in magnetite is well defined by an abrupt core-to-rim Si and Ca increase as well as Ti decrease. Electron microprobe analysis (EMPA) results show that magnetites rims contain from 2.24 to 5.70 wt% SiO₂ (averaging 4.56 wt%), which are silician magnetite. Plots of EMPA data suggest that silician magnetite in the Cihai skarn deposit presents in the form of [Si⁴⁺]^IV + [Fe²⁺]^VI ↔ [Fe³⁺]^IV + [Fe³⁺]^VI. The LA-ICP-MS dating results show that the garnet coexisting with the magnetite core has a U-Pb age of 282.5 ± 2.6 Ma, which is consistent with the timing of Fe mineralization in the retrograde skarn stage, indicating that silician magnetite was formed within a relatively short time interval after magnetite precipitation. Based on coexisting minerals, chemical compositions, and our previous fluid inclusion analyses, we propose that the Si zoning of magnetite is largely temperature dependent and, thus, is interpreted as a retrograde growth zoning. It is suggested that silician magnetite formed as a result of changing compatibility due to decreasing temperatures and can potentially be used to trace ore-forming temperatures in hydrothermal deposits. Our study provides independent constraints on the conditions of hydrothermal magnetite formation in the Cihai Fe skarn deposit, and suggests that silician magnetite may serve as a potential relatively low-temperature (<300 °C) indicator in other less-well-constrained magmatic-hydrothermal systems.