Abstract:

Hongshuihe iron deposit in the Eastern Kunlun orogenic belt, Qinghai Province, is a middle-sized one in which iron formations are localized within the phyllite of the Langyashan Formation. The ore types are mainly massive magnetite quartzite type, and minor banded type, which was generally considered as sedimentary metamorphic iron deposit by predecessors. Based on previous researches, this paper presents new data from major, rare earth and trace elements as well as Fe isotope analyses for massive iron ores in the iron formations of the Hongshuihe iron deposit. New Fe isotope results yield variableδ56FeIRMM-014values mostly ranging from 0.97‰to1.97‰, with one exception, showing similar Fe isotope compositions with typical Neoproterozoic iron formations of the world. The results of major, rare earth and trace elements show that these ores have high SiO₂+Fe₂O₃ contents (78.56%-98.06%)and very low Al/(Al+Fe+Mn) ratios of 0.00-0.06, indicating that they are typical chemical sedimentary rocks. The total rare-earth-element contents (∑REE) vary from 16.49×10^-6 to 80.89×10^-6, with no obvious Ce abnormally, and light-rare-earth-element are slightly depleted, which shows a similar characteristic to the Neoproterozoic iron-bearing structures. Based on the comprehensive comparison of Fe isotope compositions, depositional ages and geochemical characteristics of iron formations in the Hongshuihe iron deposit, we can conclude that there was a passive continental margin tectonic environment of the Qaidam-East Kunbei block in the Neoproterozoic during iron formation, and iron and other ore-forming materials were predominantly derived from marine hydrothermal fluids. The marine hydrothermal fluids rich in Fe²+ upwelled and gradually evolved into low-temperature hydrothermal fluids which then mixed with oxygen-bearing surface seawater in a sub-oxidized condition. Finally, Fe²+ was partially oxidized and formed ferric hydroxides, which subsequently deposited on the continental slope and eventually formed iron formations. The genetic type of the Hongshuihe iron deposit can be classified as the Neoproterozoic Rapitan-type iron formations.

Key words: Eastern Kunlun orogenic belt, Hongshuihe iron deposit, Neoproterozoic iron formations, Fe isotopes; geochemical characteristics of mineral deposits