Dissolution-reprecipitation process of magnetite from the Chengchao iron deposit: Insights into ore genesis and implication for in-situ chemical analysis of magnetite

摘要

Magnetite formed in different environments commonly has distinct assemblages and concentrations of trace elements that can potentially be used as a genetic indicator of this mineral and associated ore deposits. In this paper, we present textural and compositional data of magnetite from the Chengchao iron deposit Daye district, China to provide a better understanding in the formation mechanism and genesis of the deposit and shed light on analytical protocols for in-situ chemical analysis of hydrothermal magnetite. Magnetite grains from the ore-related granitoid pluton, mineralized endoskarn, magnetite-dominated exoskarn, and vein-type iron ores hosted in marine carbonate intruded by the pluton were examined using scanning electron microscopy and analyzed for major and trace elements using electron microprobe. Back-scattered electron images reveal that primary magnetite from the mineralized skarns and vein-type ores were all partly reequilibrated with late-stage hydrothermal fluids, forming secondary magnetite domains that are featured by abundant porosity and have sharp contact with the primary magnetite. These textures are interpreted as resulting from a dissolution-reprecipitation process of magnetite, which, however, are mostly obscure under optically. Primary magnetite grains from the mineralized endoskarn and vein-type ores contain high SiO_2 (0.92-3.21 wt.%), Al_2O_3 (0.51-2.83 wt.%), and low MgO (0.15-0.67 wt.%), whereas varieties from the exoskarn ores have high MgO (2.76-3.07 wt.%) and low SiO_2 (0.03-0.23 wt.%) and Al_2O_3 (0.54-1.05 wt.%).