Garnet is ubiquitous in skarn deposits. It is a sensitive recorder of physiochemical conditions and also a powerful geochronometer. In this study, we use garnet trace element geochemistry and U-Pb geochronology as well as sulfur isotopes of sulfides to constrain the age and mineralization process of the Lazhushan Fe skarn deposit. Garnets in the Lazhushan Fe skarn belong to the andradite-grossular solution series and yield a compositional range from Adr47.2Gro50.7 to Adr97.4Gro0.9. There are strong correlations between LaN/YbN ratios and andradite contents in garnet, indicating that garnet major element composition exerts a strong control on the REE fractionation. The decrease of HFSEs (Nb, Ta, Zr, Hf, and Ti) and Co, V, and Mn concentrations from prograde to retrograde garnets may reflect increasing water/rock ratio and decreasing temperature during skarn evolution, respectively. Co-precipitation with magnetite may be responsible for the low concentration of V in the retrograde garnets. The Th/U ratio increase from endoskarn garnet to exoskarn counterparts, probably reflecting an increase of fluid fO2 due to interactions with evaporate sulfate from the wall rocks. The sulfides in this deposit are enriched in δ34S (up to 13 ‰), conforming the involvement of evaporate sulfate during Fe skarn mineralization. The incursion of evaporate sulfate may have played an important role in magnetite mineralization for the Lazhushan deposit by increasing fluid fO2 and facilitating the oxidation of Fe2+ to precipitate magnetite. LA-ICP-MS U-Pb dating of garnets from 4 garnet-bearing samples yielded ages of 150.8 ± 6.8 Ma (2σ), 150.9 ± 5.3 Ma (2σ), 150.2 ± 7.9 Ma (2σ), and 150 ± 11 Ma (2σ), which are consistent with zircon weighted mean U–Pb ages of 150.1 ± 2.0 Ma (2σ) and 150.5 ± 1.0 Ma (2σ) of the ore-related intrusion. Combined with previously published metallogenetic ages of skarn deposits in the Edong district, we suggest that the Lazhushan deposit may represent the initiation of Fe skarn mineralization for the Edong district during the Late Mesozoic.
The Handan-Xingtai district, situated in the central part of the North China craton, is one of the most important concentrations of Fe skarns in China. Baijian is the largest Fe skarn deposit in this district with significant Fe reserves being newly identified. This deposit is spatially related with a monzodiorite stock intruding the Middle Ordovician evaporate-bearing marine carbonates, with Fe mineralization occurring in the contact zone or within carbonate wall rocks. This paper conducts a comprehensive investigation encompassing geological, mineralogical, geochronological, and stable isotope analyses of the Baijian deposit. The goal is to provide insights into its formation and mineralization processes and offer a broader understanding of regional Fe metallogeny. The skarn mineralogy in the Baijian deposit is predominantly characterized by Mg-rich minerals such as diopside, tremolite, serpentine, and phlogopite. Magnetite is the dominant metallic mineral, featuring low Ti contents (<0.11 wt%) and high Fe concentrations (>66.59 wt%), indicative of a hydrothermal origin. The majority of the magnetite trace element data are plotted in the skarn field on the Al + Mn versus Ti + V diagram. Pyrite, a notable component in ores, exhibits considerable variations in Co and Ni concentrations, with Co/Ni ratio generally higher than unity. Phlogopite 40Ar–39Ar dating constrains the formation of the Baijian Fe skarn deposit at ca. 128 Ma, aligning with zircon U-Pb ages (128.8 ± 0.9 Ma) of the associated monzodiorite. This temporal congruence suggests a genetic relationship between the magmatism and skarn mineralization. Combined with previous published geochronological data, this study identifies an increasing trend in Fe mineralization intensity within the Handan-Xingtai district, spanning from ca. 137 to 128 Ma. Geological and oxygen isotopic evidence advocates for a magmatic origin of the ore-forming fluids at the Baijian deposit. The δ18O values of these fluids experience elevation through interaction with carbonate wall rocks. The pronouncedly high δ34S values of pyrite (>16.1 ‰) in the Baijian magnetite ores underscore a substantial sulfur contribution from sulfate in evaporate beds. Drawing on geological, mineralogical, and isotopic evidence, the study suggests that the interaction between magmatic fluids and evaporate-bearing carbonate rocks plays an important role in magnetite precipitation at the Baijian deposit. This interaction serves to reduce fluid acidity and facilitate the oxidation of ferrous iron (Fe2+). The Fe skarn deposits in Handan-Xingtai district are mostly hosted in middle Ordovician evaporite-bearing carbonate strata with ore-related sulfides exhibiting strong 34S enrichment (δ34S > 10 ‰). The interaction of magmatic fluids with evaporate-bearing carbonates is likely a common process responsible for magnetite deposition in the Fe skarn deposits.
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