Reequilibration Processes in Magnetite From Iron Skarn Deposits

Abstract

Textural and compositional data for magnetite from nine iron skarn deposits in Canada, Romania, and China show that most samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization. The dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined, whereas the oxy-exsolution occurs only in high-Ti magnetite, forming exsolution lamellae of Fe-Ti-Al oxides. Electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite, notably Si, Mg, Ca, Al, Mn, and Ti, whereas oxy-exsolution is effective in decreasing the Ti content of high-Ti magnetite. Many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations (BIF), iron oxide-copper-gold (IOCG), or porphyry Cu fields using the Ti + V versus Ca + Al + Mn discrimination diagram. This pattern suggests that trace element data for magnetite that has unusual composition and/or reequilibrated cannot be reliably used as a petrogenetic indicator. Mixing of externally derived saline fluids with Fe-rich magmatic-hydrothermal solutions, an increase in temperature, and local decreasing pressure and fO 2 are considered the most important causes for the dissolution and reprecipitation, or recrystallization, of the magnetite; increasing fO 2 and decreasing temperature may facilitate oxy-exsolution of Fe-Ti-Al oxides in high-Ti magnetite. Results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits.

Diffusion and partition coefficients of minor and trace elements in magnetite as a function of oxygen fugacity at 1150 ºC

“…Nevertheless, it is important to consider that the heterogeneous chemistry on the grain scale may not be solely a result of diffusion processes, but instead a result of dissolutionreprecipitation processes (e.g. Hu et al 2015).…”

Geochemistry of magnetite from theHuanggangskarn iron–tin polymetallic deposit in the southernGreat Xing'an Range, NE China

“…Reference fields are from Dupuis and Beaudoin (). The reference field of Fe‐skarn deposits is from Zhao and Zhou (), Hu et al (, ). The reference field of Cu–(Fe ± Pb ± Zn)‐skarn deposit is from Nadoll et al (), Huang et al ().…”

“…The authors would like to thank the managers of the Inner Mongolia Dupuis and Beaudoin (2011). The reference field of Fe-skarn deposits is from Zhao and Zhou (2015), Hu et al (2014bHu et al ( , 2015. The reference field of Cu-(Fe ± Pb ± Zn)-skarn deposit is from Nadoll et al (2015)…”

Trace Element Geochemistry of Magnetite: Implications for Ore Genesis of the Huanggangliang Sn-Fe Deposit, Inner Mongolia, Northeastern China

“…The idiomorphic magnetite granules form angles of 120° between each other. Currently, two models could be adopted to explain the triple junction texture of the T4 magnetite: (1) high-temperature annealing in a closed system (i.e., magmatic magnetite, [43]), and (2) fluid-assisted recrystallization/replacement in an open system (i.e., quartz, [44,45]). The low Ni and Cr contents and relatively high and stable Fe contents of the T4 magnetite indicate that it does not belong to magmatic magnetite [41].…”