Geology, geochemistry and age constraints on the Mengku skarn iron deposit in Xinjiang Altai, NW China

The Mengku iron deposit (110 Mt at 44% Fe) in the eastern extension of the Rudnyi Altai mineral belt, Xinjiang, NW China, is hosted in an Early Devonian volcano-sedimentary sequence affected by prograde skarn formation (clinopyroxene, garnet, scapolite and albite) and retrograde alteration (actinolite, epidote, chlorite and apatite). Ore minerals are magnetite, with minor chalcopyrite, pyrrhotite and pyrite. Gangue minerals include predominantly diopside, andradite, epidote, chlorite, quartz and calcite. Fluid inclusions in garnet and clinopyroxene yield homogenization temperatures (Th) of 241–500 °C, and salinities of 9.6–13.0 wt.% NaCl equivalent. Fluid inclusions in quartz consist of two varieties: (1) liquid–vapor–solid inclusions with (Th) of 348–458 °C and salinities of 38.2–57.1 wt.%, and (2) two-phase inclusions with (Th) of 166–382 °C and salinities of 9.6–13.0 wt.% NaCl equivalent. Two-phase inclusions in calcite vary from 145 to 265 °C, with salinities of 1.2–9.6 wt.% NaCl equivalent. The fluid evolution progressed from high temperature and moderate salinity (prograde skarn stage) to moderate T and salinity (retrograde stage), to low T and salinity (sulfide stage). Stable isotope data of quartz and calcite (−84‰ to −110‰ δD, 5.5–7.0‰ δ18OSMOW, and −3.8‰ to −6.1‰ δ13CPDB) reflect late-stage interaction of meteoric water with granitic rocks. Skarn formation was likely related to a Late Devonian tonalite (378 ± 7 Ma, U–Pb zircon age). The geological and geochemical evidence suggests magnetite skarn formation from granite-related fluids which reacted with andesitic to mafic volcanics and their intercalated carbonate beds and lenses, contrary to earlier interpretations of the deposit as of synsedimentary-exhalative origin or related to iron–oxide magma.