Ore-forming fluid characteristic of Nihe iron deposit in Lu-Zong basin, Anhui Province and its significance to ore genesis

The Lu-Zong Mesozoic volcanic basin is located in the Lower Yangtze River fault-depression zone, where is the north margin of the Yangtze Block. Nihe deposit located in the northwest part of Lu-Zong basin, which is a new found large iron-sulfuranhydrite deposit. Anhydrite is a suitable mineral for fluid inclusion study as it is widely developed in each alteration-mineralization stage of Nihe deposit. Based on detail field study, the fluid inclusions in anhydrite of different stages were studied by fluid inclusions petrography, microthermometry and Raman spectrometry. Our results show that three types of fluid inclusions exist: I a type primary fluid inclusions (L + G), lb tpye secondary fluid inclusions (L + G) and II type fluid inclusions (L). The Laser-Raman microspectroscopy results show that the compositions of fluid inclusions are mainly H₂O with trace CO₂, N₂ and CH₂. Microthermometry results show that the average homogenization temperature of fuild inclusions in anhydrite-pyroxene-magnetite stage, anhydrite-pyritemagnetite stage, kaolinite-anhydrite-quartze-pyrite stage and barite-calcite-anhydrite stage are 460. 0 ~ 380. 0°C, 350. 0 ~ 270. 0°C, 250.0 ~ 190. 0°C and 190.0 ~ 150. 0°C, salinity are 14. 11 % NaCleqv, 10. 73% NaCleqv, 4. 03% NaCleqv and 3. 26% NaCleqv, respectively. The ore-forming fluids evolution is from medium-high temperature and medium salinity to low temperature and low salinity. The ore-forming process and metallogenetic model has been discussed in the paper.