Trace elements of magnetite and iron isotopes of the Zankan iron deposit, westernmost Kunlun, China: A case study of seafloor hydrothermal iron deposits

Abstract

The large Zankan iron deposit is hosted in the pre-Devonian Bulunkuole metamorphic complex within the Taxkorgan terrane, West Kunlun Orogen, southwestern China. The deposit is unique for its mineral association of magnetite, pyrite and anhydrite in variable proportions, forming disseminations, banded and massive ores. Magnetite grains from the ores show variable contents of many elements, such as Mg (182–1167ppm), Al (198–2691ppm), Ti (898–1677ppm), V (112–8468ppm), Mn (821–4695ppm), Co (7–26ppm), Ni (1–29ppm), Zn (5–45ppm), and Ga (20–62ppm). The high Al, Ti and V contents are interpreted to result from relatively reduced, Al–Ti-rich seafloor hydrothermal activities. The magnetite coexisting with sulfide has lower Co concentrations (7.8–13ppm) than those not coexisting with sulfides (Co=22–26ppm). Compositional variations of magnetite possibly related to oxygen fugacity, temperature and coexisting minerals. The δ56Fe values in magnetite range from −0.3 to 0.5‰, suggesting a changing hydrothermal precipitation environment. The Fe isotope fractionation between pyrite and magnetite (△δ57Fepy-mag) range 0.2–1.1‰, implying a high-temperature crystallization (≥236°C). The δ56Fe ratios in pyrite range 0.6–0.8‰, higher than the coexisting magnetite. Such a negligible variation of positive δ56Fe values indicates that the pyrite possibly originated from a slow precipitation or a transformation from FeS (mackinawite), which was induced by the rise of atmospheric oxygen at ~0.6Ga. Integrating the data obtained from the studies including regional geology, ore geology, magnetite composition and Fe isotope geochemistry, we conclude that the Zankan Fe deposit was formed from an Early Cambrian seafloor hydrothermal system developed in a volcanic arc caused by southward subduction of Proto-Tethyan plate.