Precipitation mechanism, REE characteristics of anhydrite in Nihe deposit and its relation to mineralization

Nihe is a major magnetite-apatite deposit located in the northwestern the Lu-Zong volcanic basin in the Middle-Lower Yangtze River Valley Metallogenic Belt. Anhydrite is extensively developed in the deposit. Based on the betailed field geological work and petrographic observation, the anhydrite can be divided into three types on the basis of the mineral assemblage and stage: Type I, tabular violet crystals coexisting with diopside, magnetite, pyrite and apatite; Type II tabular white crystals coexisting with pyrite; and Type III, equant pink or white crystals in carbonate-anhydrite veins. We present in situ LA-ICP-MS geochemical data for the three different types of anhydrite in Nihe deposit. LA-ICP-MS data indicate that type I anhydrite contains much more REE than the other two types, and the REE pattern and Eu anomaly is also quite different ((La/Yb)_N = 1.1 ~466, 𝛿Eu = 0.3 ~ 2.4). Type II and Type III have similar REE pattern. Crystallography has little influence on anhydrite REE content in the Nihe iron deposit, temperature, fluid evolution and complex type is main factor controlling the REE pattern. In the early high-temperature hydrothermal solution, REE mainly migrates in the form of Cl complex. After precipitating the REE-rich minerals, the REE content greatly decreased in the fluid, resulting in a lack of REE in types II and III, and their REE patterns are flat. In addition, decreasing fluid salinity caused lower Cl_-/SO₄^2- ratios in later stages. Furthermore, some type III anhydrite has elevated LREEs, suggesting some late-stage dissolution and reprecipitation of anhydrite due to fluctuating fluid compositions. Addition of abundant anhydrite to the ore forming fluid increases f_O2, allowing iron to be transported far away and helpful to the formation of large-scale iron ore bodies.