The high-grade Fe skarn deposit of Jinling, North China Craton: Insights into hydrothermal iron mineralization

Skarn-type Fe deposits constitute a major source of iron in the world, and 57% of the high-grade iron ores in China are hosted by skarn deposits. Most researchers agree that skarn-type Fe deposits are genetically linked with the associated magmas and precipitate from hydrothermal fluids, however, whether these ore-forming fluids are mainly derived from magmas or some other external sources is controversial. Besides, the detailed process of iron concentration in the high-grade ores in skarn Fe deposits has not been well constrained. To better understand these issues, we investigated the petrological, mineral chemical, geochronological, fluid inclusion and oxygen isotope characteristics of the Jinling Fe deposit, which is a typical high-grade skarn-type Fe deposit in the North China Craton. Zircon grains in these skarns were demonstrated to result from completely modification of magmatic zircons by the later hydrothermal fluids and they yield a weighted mean Pb-206/U-238 age of 126.7 +/- 2.0 Ma (2 sigma) constraining the timing of the skarn iron formation, which is consistent with that of the ore-hosting Jinling complex. The similarity in chondrite-normalized REE patterns of the zircons in skarns and magmatic zircons suggests that the ore-forming hydrothermal fluids might mainly be derived from magmas. Additionally, the oxygen isotopes of the hydrothermal fluids in various stages as estimated based on delta O-18 values of representative hydrothermal minerals show relatively constant values during the evolution of hydrothermal system, suggesting that there was no significant involvement of external fluids. Three types of fluid inclusions, namely, high-salinity Type 1 with daughter minerals (homogenizing into liquid phase, with salinity of 42-71 wt% NaCl equivalent), vapor-rich Type 2 (homogenizing into vapor, with salinity <= 23 wt% NaCl equivalent), and relatively low-salinity Type 3 (homogenizing into liquid, with salinity <= 23 wt% NaCl equivalent) were identified in the various skarn minerals (garnet, clinopyroxene, epidote, and anhydrite). Among these, only Type 3 fluid inclusions occur in the clinopyroxene within the dominant massive ores, implying that the relatively low-salinity fluids were the predominant ore-forming hydrothermal fluids from which the magnetite ores precipitated. The high-salinity Type 1 fluids which might have resulted from phase separation (or boiling) of the primary magmatic hydrothermal fluids due to pressure drop, were believed to be responsible for the Fe-rich rims of zoned clinopyroxenes (with Fe-poor cores) in the thin magnetite-clinopyroxene interbedding layers between skarns and the massive magnetite orebodies. Thus, this zoned texture also implies an Fe re-enrichment event after formation of magnetite ores. The process of multiple Fe enrichment may significantly contribute to the formation of high-grade skarn-type Fe deposits.