Metallogenic differences revealed by magnetite texture and trace element geochemistry: A case study of the Huanggangliang FeSn deposit in the southern Great Xing'an Range, NE China

Tin mineralization is typically associated with polymetallic systems, and the scarcity of economic magnetite–cassiterite deposits highlights the unique hydrothermal and physicochemical constraints governing their formation. The Huanggangliang deposit (135 ± 1 Ma, 180 Mt. Fe @ 38.29 %, and 0.456 Mt. Sn @ 0.29 %) is a granite-related skarn deposit and is the largest FeSn polymetallic deposit north of the Yangtze River. Five main mining areas (SK-I–SK-V) are distributed in a SW to NE direction. The contents of cassiterite and metal sulfides gradually increase from SW to NE. However, the controlling factors remain unknown. The granular magnetite in granite (Mt-G) is disseminated, has a uniform texture and is locally oxidized to hematite. The medium- to fine-grained magnetite replaced skarn minerals such as garnets in SK-I (Mt-Ia) and SK-II (Mt-IIa), and the coarse-grained magnetite in SK-I developed more carbonate dissolution holes (Mt-Ib), along which fine-grained cassiterite grew, but all of them were virtually free of metal sulfides. In SK-V, fluorite, arsenopyrite and pyrite surround granular magnetite, and a large amount of quartz replaces granular magnetite (Mt-Vb). Fine-grained cassiterite is present in the magnetite dissolution voids, and some of the magnetite (Mt-Vc) is syngenetic with sphalerite. Large amounts of cassiterite, pyrite, chalcopyrite, arsenopyrite, and sphalerite replace massive (Mt-IIIb) or acicular magnetite (Mt-IIIc) in SK-III, and more alteration minerals (e.g., epidote and chlorite) have developed. The Ti and V contents decrease sequentially from granite to SK-I, SK-II, SK-III and SK-V, whereas the Sn content sequentially increases. The differences in the Al + Mn vs. Ti + V contents of the Huanggangliang magnetite indicate that the formation temperature of magnetite significantly varies between mining areas (higher in granite and lower in SK-V). The high Mg + Al + Si content of magnetite in SK-III and the extensive development of wall–rock alteration suggests that the SK-III mining area may have experienced the strongest fluid–rock interactions, which may be important mechanisms for the precipitation of cassiterite and metal sulfides in the SK-III mining area. The magnetite in the Huanggangliang deposit extensively replaced skarn and was later replaced by polymetallic sulfides. The texture and trace element composition of magnetite in the layered ore body (Ti + V vs. Ca + Al + Mn, Ti + V vs. Ni / (Cr + Mn)) are similar to those of typical skarn-type deposits worldwide, supporting a magmatic–hydrothermal origin. SW–NE zonation is controlled by temperature, fO₂, and host rock reactivity. Andesite-hosted SK-I retained high fO₂, inhibiting sulfides, whereas marble-hosted SK-III/V enabled sulfide–cassiterite deposition. Multistage Sn recycling from skarn to hydrothermal cassiterite highlights fluid chemistry and alteration as key drivers of Sn redistribution. We emphasize that Sn-anomalous magnetite may serve as a key tool for prospecting concealed tin deposits, especially in altered terrains of zoned mineralization.