Magnetite texture and composition in the Makeng Fe skarn deposit, Southeastern China

Abstract

Magnetite is the most important ore mineral in Fe skarn deposits. Its textural and compositional signatures effectively indicate the ore-forming environment and can fingerprint mineralization processes. The Makeng deposit is the largest Fe skarn deposit in southeastern China, which is a case study for constraining the mineralization process by using textures and composition of magnetite. Four generations of magnetite (MagI to MagIV) are distinguished in this deposit. The MagI and MagIII both show similar oscillatory zoning with abundant micro-nano silicate inclusions, however, the texture relationship indicates that MagI formed earlier than MagIII and they have different compositions as well as inclusion density. MagII contains randomly distributed pores and coarsened inclusions. In contrast, MagIV is clean and free of inclusions and pores. All four types of magnetite are of hydrothermal origin. However, they crystallized in distinct stages within different conditions. The MagI crystallized from a relatively high-temperature and supersaturated fluid enriched in Fe, Si, Ca, and Al. The formation of MagII is due to dissolution-reprecipitation of MagI. The MagIII was likely formed by continuous magmatic-hydrothermal pulse. However, the relatively low contents of Si, Al, and Ca lead to lower density of silicate inclusions compared to MagI. Finally, the MagIV formed under relatively low temperature and high oxygen fugacity conditions, and the low concentrations of Si, Al, Ca, and Mg within the fluids prevented the development of inclusions. Abundant silicate inclusions in Makeng magnetite indicate that the influence of inclusions on the occurrence of trace elements cannot be overlooked, which highlights the importance of petrographic study for understanding the trace element occurrence. Magnetite from Makeng also shows significant enrichment of Ga (average 45 ppm) and Co (average 30 ppm). Ga enrichment might primarily have been controlled by temperature and oxygen fugacity, whereas Co enrichment is constrained by other factors.