Source of ore-forming material and genesis of high-grade iron ore in the Gongchangling iron deposit, Liaoning Province

The Anshan-Benxi area of the Liaoning Province is the largest BIF iron ore resource base in China. The Gongchangling deposit is the largest high-grade magnetite deposit in this area. Its genesis has been argued for a long time and is currently being considered to have relation with hydrothermal reformation. However, there are different viewpoints on the nature of hydrothermal fluids, including derived from metamorphic fluid, migmatized fluid or meteoric water. In this contribution, combined with previous studies, systematic analyses of trace element and Nd-Pb-S isotope compositions were conducted on banded (low-grade) and massive (high- grade) iron ores from the Gongchangling No. 2 mining area to constrain its genesis. The features of magnetite REY patterns from banded and massive iron ores are consistent, suggesting ore-forming materials sourced from the mixture of seawater and high-temperature hydrothermal fluid. The epsilon(Nd)(t) values of magnetite from massive iron ores (-7.7 similar to-6.1) are more negative than those of banded. iron ore (-6.9-1.5), indicating the hydrothermal fluid sourced from shallow crust. The lead isotope a values of magnetites from banded iron ores range from 9. 36 to 10.75, showing geochemical characteristics of crust - mantle material mixing, While the high mu values of magnetites from massive iron ores (9.58 similar to 13.14) indicate reformation of crustal-derived fluid at the later stage. The sulfur isotope composition of pyrite from massive iron ores (1.88 parts per thousand similar to 3.36 parts per thousand with average of 2. 39 parts per thousand) is heavier than that of handed iron ores (-2.08 parts per thousand similar to -0.66 parts per thousand with average of 1.52 parts per thousand), suggesting metasomatism of fluid with heavy sulfur isotope. Consequently, the ore-forming fluid with heavy sulfur isotope and crustal-derived feature should be the mixture of metamorphic hydrothermal fluid and meteoric water. The massive iron were formed by mixing hydrothermal fluid reformation on banded iron ores in the late Paleoproterozoic, causing desilication to form high-grade iron ores.