Constraints on the mineralization processes of the Makeng iron deposit, eastern China: Fluid inclusion, H O isotope and magnetite trace element analysis
Abstract The Makeng iron deposit is located in the Yong'an Meizhou depression belt in Fujian Province, eastern China. Both skarn alteration and iron mineralization are mainly hosted within middle Carboniferous lower Permian limestone. Five paragenetic stages of skarn formation and ore deposition have been recognized: Stage 1, early skarn (andradite grossular assemblage) Stage 2, magnetite mineralization (diopside magnetite assemblage) Stage 3, late skarn (amphibole chlorite epidote johannsenite hedenbergite magnetite assemblage) Stage 4, sulfide mineralization (quartz calcite fluorite chlorite pyrite galena sphalerite assemblage) and Stage 5, carbonate (quartz calcite assemblage) Fluid inclusion studies were carried out on inclusions in diopside from Stage 2 and in quartz, calcite, and fluorite from Stage 4. Halite bearing (Type 1) and coexisting two phase vapor rich aqueous (Type 3) inclusions in the magnetite stage display homogenization temperatures of 448 564 degC and 501 594 degC, respectively. Salinities range from 26.5 to 48.4 and 2.4 to 6.9 wt% NaCl equivalent, respectively. Two phase liquid rich aqueous (Type 2b) inclusions in the sulfide stage yield homogenization temperatures and salinities of 182 343 degC and 1.9 20.1 wt% NaCl equivalent. These fluid inclusion data indicate that fluid boiling occurred during the magnetite stage and that fluid mixing took place during the sulfide stage. The former triggered the precipitation of magnetite, and the latter resulted in the deposition of Pb, Zn, and Fe sulfides. The fluids related to magnetite mineralization have d18Ofluid VSMOW of 6.7 9.6%0 and dD of 96 to 128%0, which are interpreted to indicate residual magmatic water from magma degassing. In contrast, the fluids related to the sulfide mineralization show d18Ofluid VSMOW of 0.85 to 1.04%0 and dD of 110 to 124%0, indicating that they were generated by the mixing of magmatic water with meteoric water. Magnetite grains from Stage 2 exhibit oscillatory zoning with compositional variations in major elements (e.g. SiO2, Al2O3, CaO, MgO, and MnO) from core to rim, which is interpreted as a self organizing process rather than a dissolution reprecipitation process. Magnetite from Stage 3 replaces or crosscuts early magnetite, suggesting that later hydrothermal fluid overprinted and caused dissolution and reprecipitation of Stage 2 magnetite. Trace element data (e.g. Ti, V, Ca, Al, and Mn) of magnetite from Stages 2 and 3 indicate a typical skarn origin.