Alteration, oxygen isotope, and fluid inclusion study of the Meishan iron oxide–apatite deposit, SE China

Abstract

The Meishan deposit (338 Mt at 39 % Fe) comprises massive ores in the main orebody and stockwork and disseminated ores along the main orebody. Four stages of mineralization and related alteration have been identified. The second stage of mineralization, which was the main stage of iron mineralization, formed stringer, disseminated iron ores, as well as the main Meishan orebody. The fourth stage formed small pyrite and/or gold orebodies above or alongside the main magnetite orebody. Stage 2 apatites have homogenization temperatures of 257–485 °C and salinities of 7.3–11 wt% NaCleq. Calculated δ18Ofluid values of magnetite and apatite from the disseminated ores vary between 7.7 and 14.9 ‰, which is similar to values observed in the massive ores (8.1–12.9 ‰). The high-18O fluids at Meishan have been interpreted as being of magmatic–hydrothermal origin. These fluids are indicative of the boiling of ore-forming fluids. Quartz, occurring as cavity fillings, gives homogenization temperatures from 202 to 344 °C, with most values lying between 250 and 330 °C. Corresponding salinities are ∼5 wt% NaCleq. Calculated δ18Ofluid values are +6.4 to +6.8 ‰. These values indicate that the lower-temperature (250–330 °C) quartz was deposited from a cooling magmatic–hydrothermal fluid. Stage 3 siderites contain fluid inclusions that homogenized between 190 and 310 °C, mainly between 210 and 290 °C. Corresponding salinities are 4–8 wt% NaCleq. Stage 4 quartz–carbonate veinlets contain fluid inclusions that homogenized at moderate to low temperatures (150–230 °C) and exhibit low salinities (2–10 wt% NaCl eq). δ18Ofluid values of the mineralizing fluids for the quartz and calcite can be calculated to vary from −0.7 to +5.6 ‰ and +6.3 to +10.2 ‰, respectively. While there is some overlap, the δ18O values of the fluids are generally lower than those observed in the massive and disseminated magnetite ores. δD values for the quartz and calcite vary between −154 and −123 ‰ and −123 and −111 ‰, respectively. These values suggest late-stage input of a shallow-sourced, isotopically light meteoric fluid at the temperature of pyrite and gold deposition. The fluid inclusion and stable isotope data indicate a cooling magmatic–hydrothermal system that progressed from isotopically heavy to isotopically depleted fluids as it cooled. Such fluid evolution is comparable with those of other Kiruna-type deposits worldwide.