Whole Rock, Mineral Chemistry during Skarn Mineralization-Case Study from Tongshan Cu-Mo Skarn Profile

Studying the activation, migration and precipitation processes of ore-forming elements is essential for understanding the genesis and mechanisms of skarn deposits. A typical skarn profile formed by the intrusion of Yanshanian granodiorite into the Carboniferous carbonate strata was studied. The profile is highly consistent with the classic skarn profile, ranging from the intrusion, weak alteration belt, skarn belt (inner and outer skarn belt) and mineralization belt (mainly characterized by Cu mineralization) to the surrounding marble without being affected by late-stage low-temperature or supergene weathering alteration. Whole-rock data show that the major and trace elements exhibit relatively small changes in the granodiorite and inner skarn, but huge variation in the boundary between the inner and outer skarn; Na, Al, Ti and Sr show significant decreases, while Fe, Mg, Zn, V and Ni show significant increases. The elemental content in the outer skarn is 10–100 times or more higher than that in the marble, but the elements such as Ca, Sr and Cs diluted from the marble. During the migration process from the inner skarn to the outer skarn, some elements (such as K, Rb and Ba) were depleted in the inner, but not enriched in the outer, indicating that they may migrate to farther locations. Grossularite developed in the inner skarn, with light rare earth element (LREE) depletion and heavy REE enrichment, as well as positive and negative anomalies of Eu (δEu = 0.42–3.95). Andradite developed in the outer skarn, with zonation development, light REE enrichment, and heavy REE depletion and a positive Eu anomaly (δEu = 0.36–46.83). Some negative Eu anomalies appear at the edges of garnets in the outer skarn, indicating fluctuations in fO2 during the late skarn process. A positive correlation between Fe3+ and REE3+ in the garnets from the inner skarn, as well as between Al3+ and REE3+ from the outer skarn indicated that there are different YAG substitution mechanisms of REE between the inner and outer skarn. Low garnet REE contents and highly variable Y/Ho ratios in outer skarn suggest that the significant fluctuations in REEs may be primarily controlled by water-rock interactions. Considering the whole-rock major and trace element contents, as well as the trace element features of garnet, we found that whole-rock Na, Al, Ti and Sr elements, garnet Ti, Zr and Nb elements exhibit significant differences between the inner and outer skarn. These characteristics can be used to distinguish the boundary between the rock body and carbonate during the skarnification process.