Magnetite chemistry and implications for the magmatic-hydrothermal ore-forming process: an example from the Devonian Yuleken porphyry Cu system, NW China

Abstract

Magnetite in the Devonian Yuleken porphyry Cu–Mo deposit located in NW China witnessed complicated magmatic-hydrothermal ore-forming processes. The magnetite grains can be divided into MagI-A (from the Beitashan Formation basalt), MagI-B (from the porphyritic syenite wall rock), MagII (from sodic-calcic alteration zone), MagIII-A (from early potassic stage with no concomitant sulfides) and MagIII-B (from late potassic alteration coexisting with sulfides).Although magnetite crystals of a single type may accommodate a wide range of trace elements, general trend among elements in various types of magnetite can still be related to their forming conditions, such as temperature, fO2, coexisting mineral assemblages, and fluid compositions. In this study, the incorporation of Ti, Si, Al, Ba, Ta, and Sc into magnetite is favored under high temperatures, as indicated by their decreasing average concentrations from igneous MagI-A and MagI-B to hydrothermal MagIII-A and MagIII-B, while fO2 has considerable influence on the substitution of V and Mn into magnetite crystals especially under low temperatures. For chalcophile elements, e.g., Co, Ni, Cu, Zn, and Bi, which are subject to be affected by coexisting sulfide assemblages, the absence of expected depletion of Ni, Cu, Zn, and Bi in MagIII-B relative to MagIII-A suggests a slightly earlier precipitation of magnetite compared to sulfides, whereas Co depletion in MagIII-B compared to MagIII-A indicates extraction of Co by the later hydrothermal fluid. Thermodynamic calculation for Zn–Fe exchange between magnetite and equilibrated hydrothermal fluids verifies the distribution coefficients extrapolated from early experimental partitioning data, implying that Zn is more effectively fractionated into hydrothermal fluids than Fe with decreasing temperature, and yields Zn/Fe ratios of hydrothermal fluids similar to those measured from fluid inclusions trapped in porphyry Cu deposit. However, a similar calculation for Mn-Fe exchange in this study shows apparent discrepancies with previously published experimental data, implying an additional role of fO2 on the incorporation of Mn into magnetite. The presence of oxy-exsolution texture of ilmenite in hydrothermal magnetite of sodic-calcic stage (MagII) indicates superimposition of later potassic alteration characterized by high fO2 (ΔMH = −1 to 2) and medium-high-temperature (408 to 444 °C) onto magnetite initially formed in the early sodic-calcic stage. Additionally, MagI-A and MagI-B are characterized by higher Cr/Ni ratios relative to MagII and MagIII, with MagI-A enriched in both Ni and Cr at the Yuleken deposit. The newly-constructed Co–Ni–Cr ternary diagram could well differentiate magnetite of various paragenesis or origins owing to the sensitive response of these elements to forming environments at the Yuleken deposit.