In-situ texture and geochemistry of apatite from the Jinling and Zhangjiawa iron skarn deposits, eastern North China Craton: Implications for ore-forming processes and formation of high-grade ores

Abstract

Apatite commonly contains abundant halogens and trace elements, which can occur in both the magmatic and hydrothermal stages, becoming favorable for recording the magmatic-hydrothermal properties and processes. Skarn-type iron deposits associated with high-Mg diorites are widespread in the Luxi Block, eastern North China Craton. Their ore-forming processes, especially how the different wallrocks (limestones and dolomites) controlled the formation of high-grade iron ores, have been poorly constrained. To unravel the mechanisms, in this contribution, in-situ textural, geochronological (U-Pb dating) and geochemical (halogens and trace elements) analyses by SEM, EPMA and LA-ICP-MS were conducted on apatite from the representative Fe skarn deposits (Jinling and Zhangjiawa deposits) in the Luxi Block. Three generations of apatite were identified in the Jinling deposit, which occur in the feldspathization (Ap1), garnet–pyroxene skarns (Ap2) and massive magnetite ores (Ap3), respectively. One generation of apatite occurring in the massive magnetite ores (Ap-3) was identified in the Zhangjiawa deposit. The Ap2 and Ap-3 apatite grains show patchy textures composed by residual bright zones (Ap2a and Ap-3a) and newly-formed grey to dark zones (Ap2b and Ap-3b), which indicate fluid metasomatism. LA-ICP-MS U-Pb dating shows that the Ap1, Ap2a and Ap3 apatite in the Jinling deposit formed at 130.4 ± 0.9 Ma (2σ), 128.5 ± 2.4 Ma and 128.3 ± 9.2 Ma, respectively, whereas the Ap-3a in the Zhangjiawa deposit formed at 128.1 ± 4.2 Ma. These ages are well consistent with the intrusive ages of the spatially associated high-Mg diorites within errors, corroborating the ore-forming fluids originating from the high-Mg rocks. All the studied apatite grains show F/Cl ratios higher than 1, especially in the early-stage apatite (e.g., 5–19 in Ap1), indicating that the originally exsolved fluids were enriched in F. The high F contents probably played a significant role in leaching Fe from the high-Mg rocks by enhancing rock porosities. The Eu/Eu* ratios of apatite increase from Ap2a to Ap3 while the Ce/Ce* ratios decrease. This suggests an increase of oxygen fugacity. The Sr concentrations in Ap3 are much higher than those in Ap-3a, correlating well with the wallrocks that limestones are developed in the former while dolomites in the latter. The above features indicate that fluid-rock interaction likely led to the increase of oxygen fugacity, which controlled the massive Fe deposition. In the Jinling deposit, the altered apatite grains (Ap2b) show much lower REE + Y but higher F contents than those of the unaltered (Ap2a), not only indicating the REEs being easily mobilized during metasomatism, but also suggesting that the high F contents likely contributed to the formation of high-grade ores by leaching additional Fe from the previously formed Fe-rich skarns. In the Zhangjiawa deposit, the altered apatite grains (Ap-3b) shows lower Cl contents than those of the unaltered (Ap-3a). This probably indicates the mixing of low-salinity fluids (e.g., recycling meteoric water), leading to the loss of Fe and thus lowering the Fe grade. The above results indicate that metasomatism is common in the skarn deposits, which can either elevate or lower the metal grade. The processes can be well recorded by apatite.