Studies On The Fluid Inclusions Of The Taochong Iron Deposit In Fanchang Area, Anhui Province

The Taochong iron deposit is located in the northeast of the Tongling-Fanchang Fe-Cu district of the Middle-Lower Yangtze River metallogenic belt. The most pronounced feature of the iron deposit is that skarns associated with iron orebodies occur as tabular bodies along the bedding-slip fault between the carbonate strata of the Lower Permian Qixia and Middle Carboniferous Huanglong Formations, with occurrence of evident boundaries between the skarn bodies and strata of two sides and no outcrops of pluton. A detailed study has been carried out on the fluid inclusions in the Taochong iron deposit, with a focus on the origin of the deposit.The following are major achievements obtained in this thesis.1. The petrographical study shows that there are many kinds of fluid inclusions, but the main types are liquid-rich and daughter mineral-bearing ones.2. The geochemical study of fluid inclusions indicates that the ore-forming fluids mostlybelong to NaCl-H₂O system, with H₂O as a major component of gas facies, and Cl^- and Na⁺ as main anion and cation of liquid facies, respectively. The homogenization temperatures, densities, lgfO₂ and pH values of the ore-forming fluids are defined mainly in the ranges of 260～460℃, 0.92～1.11 g/cm3, -19.3～-10.3, and 3.0～6.4, respectively, indicating formation of the Taochong iron deposit mainly in the high temperature, weak acidic and strong oxidizing environment.3. From early to late, increasing in number of liquid-rich inclusions with decreasing in homogenization temperature, salinity, lgfO₂, increasing in pH, and no evident varying in density and pressure occurs, associated with increasing in Na⁺ and Cl^- and decreasing in Ca²⁺ and SO₄^2- in the liquid facies and no evident changing in component of the gas facies. In contrast, from deep to shallow, no evident variation in most of parameters occurs except little increasing in number of gas and liquid-rich inclusions and value of lgfO₂. The data mentioned above suggest mineralization in the Taochong iron deposit might be mainly affected by temperature, salinity, pH, and Cl-concentration. The ore-forming fluid with salinity ranging from 34～48 wt.% NaCl equiv at the stages of skarnization and early mineralization is simple in composition, indicating its origin of magmatic evolution. In contrast with this, The ore-forming fluid having two ends with high salinity (NaCl equiv > 30%) and low salinity (NaCl equiv≤23.8%) at the stage of late mineralization is complex, indicating its mixing origin of magmatic water with meteoric water. The hydrogen and oxygen isotopic analyses of the ore-forming fluids also support involving the magmatic fluid at the stages of skarnization and early mineralization and the magmatic–meteoric mixed fluid predominated by magmatic fluid at the stage of late mineralization. Evidently, the ratio of magmatic water to meteoric water in the ore-forming fluids is gradually reduced from the early stage to the later stage.4. The petrographic, microthermometric and geochimecal data of fluid inclusions presented in this thesis indicate an origin of the skarn in the Taochong iron deposit by injection and crystallization of skarn magma, and a genesis of the associated iron orebodies possibly by precipitation of the mixed fluids of magmatic water with meteoric water. Fe in the fluids was transported mainly in the form of chloride complexes and precipitated owing to the mixing of magmatic water with meteoric water.