Discrimination of Mineralization Types of Skarn Deposits by Magnetite Chemistry

Abstract

There are different mineralization types for skarn deposits with various origins and ore-forming conditions. Magnetite is one of the main ore minerals in skarn deposits, but whether chemical compositions of magnetite can be used to discriminate different mineralization types remains unknown. This paper collects the published magnetite electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data of skarn deposits and investigates the relationship between magnetite geochemistry and mineralization types of skarn deposits using the partial least squares-discriminant analysis (PLS-DA). For EPMA data, magnetite from Fe-Zn skarn deposits can be roughly separated from that of Cu-Fe-Pb-Zn, Fe, Fe-Co-Bi-Ag, Fe-Cu, and Fe-Zn-Pb skarn deposits due to the relative enrichment of Al and Mn for the former. For LA-ICP-MS data, magnetite from Fe-Sn, Fe-Zn, and W-Mo-Pb-Zn-Fe-Cu skarn deposits can be roughly separated from that of other skarn deposits due to positive correlation with Mn, Zn, and Sn and the negative correlation with V for the former. The relative depletion of V for these mineralization types likely reflects higher oxygen fugacity than the other types of skarn deposits. Magnetite from Fe-Au skarn deposits is separated due to the relatively high Cr and Ga contents, whereas magnetite from Fe-Cu skarn deposits can be discriminated because of the relative enrichment of Mg and Co. The discrimination between different types of skarn deposits in the plot of Mg + Mn vs. (Si + Al)/(Mg + Mn) indicates that the chemical composition of magnetite is significantly affected by the fluid–rock interaction, where magnetite from Fe-Au skarn deposit shows the lowest fluid–rock ratios. The PLS-DA discrimination based on LA-ICP-MS data is better than that of EPMA data, and the main discriminant elements for the different mineralization types are Mg, Al, Ti, V, Mn, Co, Zn, Ga, and Sn. Based on the discriminant elements, we propose a plot of Mg+Mn vs. Ga+Sn to discriminate different mineralization types of skarn deposits.