Formation age and genesis of the Gongchangling Neoarchean banded iron deposit in eastern Liaoning Province: Constraints from geochemistry and SHRIMP zircon U-Pb dating

摘要

The Gongchang iron deposit, a typical Algoma-type banded iron formation (BIF) deposit located in the Eastern Block of the North China Craton (NCC), is hosted in Neoarchean metamorphic rocks, including amphibolite, biotite leptynite, garnet biotite schist, plagioclase-hornblende schist, garnet chlorite schist, muscovite quartz schist and actinolite magnetite quartzite. Geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite, respectively. SHRIMP U-Pb dating of zircons from the plagioclase-hornblende schist shows that the Gongchangling iron deposit was formed at 2554 ± 12 Ma. Major elements and REE data of the ores indicate that the Gongchangling BIF precipitated from seawater, and the ultimate iron and silica origins may have been related to nearby volcanic activity. The δ~(18)O values of quartz from the BIFs range between 9.1 and 12.9‰, suggesting that exotic high δ(18)O fluids were involved during metamorphism. The pyrite have δ~(34)S values of range between -3.5‰ and 14.7‰, suggesting that mineralization were originally from metamorphism. Comparison of the mineralization age of the Gongchangling BIF with those of other local BIFs in North China and other cratons throughout the world indicates that the Gongchangling BIF formed coevally with the global BIF mineralization at ~2.5 Ga. Taken together, these data suggest that the Gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations.