Genesis of high-grade skarn iron deposits: Insights from the Litun deposit in the Luxi district, eastern China

Abstract The mechanisms of iron enrichment in skarn iron deposits have been widely discussed; however, key factors responsible for genesis of high-grade skarn iron deposits still remain unclear. The Litun skarn iron deposit in the Luxi district, eastern China, has an average Fe grade of 58.2 wt% and provides an excellent opportunity to study the formation of high-grade iron ores. The Litun deposit is characterized by three generations (Mag-1, -2, and -3) of magnetite, which exhibit a clear cross-cutting relationship and exhibit a well-defined zoned texture. X-ray mapping and laser ablation–inductively coupled plasma–mass spectrometry analyses show that magnetites of different generations have significant compositional differences. The Fe contents show the greatest quantitative variability among the different magnetite generations (Mag-1, average [avg.] 71.33 wt%; Mag-2a, avg. 67.37 wt%; Mag-2b, avg. 69.54 wt%; Mag-3, avg. 68.10 wt%). The Si contents show the greatest degree of variability among the different magnetite generations, varying by an order of magnitude (Mag-1, avg. 3155 ppm; Mag-2a, avg. 13,817 ppm; Mag-2b, avg. 7891 ppm; Mag-3, avg. 10,748 ppm). In this study, to constrain the controlling factors for the formation of high-grade iron ores, a chemical mass transfer model has been employed to simulate the ore-forming processes. The results of simulation modeling in a low-pressure environment (500 bar) show that the solubility of iron decreases rapidly from 20,148 ppm to 3 ppm when the temperature of a hydrothermal fluid decreases from 500 to 350 °C, but that the solubility of silicon decreases only slightly from 945 ppm to 572 ppm. In contrast, in a high-pressure environment (1000 bar), the solubility of iron (from 735 ppm to 0.7 ppm) and silicon (from 1657 ppm to 646 ppm) has a similar decreasing rate when the temperature of the fluid decreases from 500 °C to 350 °C. These data imply that a Fe-rich hydrothermal system is more likely to form magnetite without/with minor silicate minerals (the main mineral of the high-grade ores) in a low-pressure environment. It is well known that a transition from lithostatic conditions to hydrostatic conditions commonly occurs during the mineralization process in high-grade skarn iron deposits. Therefore, the rapid saturation of iron in a low-pressure environment relative to other components such as silicon may be the main reason for the formation of high-grade iron ores in the Litun deposit, which may also be one of the important mechanisms for the formation of high-grade skarn iron deposits worldwide.