论姑山铁矿床的形成条件

ON THE CONDITIONS OF FORMATION OF GUSHAN IRON DEPOSIT

On the basis of geological characteristics the Gushan iron deposit should be assigned to volcano-hydruthermal type with hematite+qusrtz as its principal mineral assemblage.Iron concentration of the ore-forming fluid has been estimated from the ratio of hematite to quartz in the ores. By using experimental and thermodynamic data the soinbilities of iron minerals at elevated temperatures and pressures are calculated in the system FeO-Fe2O3-NaCl-HCl-H2O. The effect of T, P, pH, fe2 and total concentration of chlorine on the solubilities of iron minerals are discussed. Thermodynamic calculations based on presumed physicochemical conditions for the ore-forming solutions are in good agreement with geological observation. The calculation shows that iron minerals were deposited at log fo2=-21--25, log(mKCl+mbl^+) =-2,5--3, P=1-0.75 (or 0.5) Kb,T=400-350℃. It is believed that the original fluid was an acid NaCl-bearing solution of magmatic derivation. However, iron in the solution was enriched with falling temperature by dissolving pre-exist iron minerals in the consolidated rocks rather than extracted directly from the magma. Either decreasing temperature (below 400℃) or pressure is capable of depositing iron minerals from the solution, but the ratio of Fe to Si in the ore is dependent mainly upon the pH. The widespread silica vein at later stages is a reflection of decreasing acidity of the solution. Increase in fo2 will also favor the deposition of iron minerals. The hypabyssal occurrence and the existance of the Huangmaqing shale contribute greatly to the formation of hematite.

Based on experimental data on the system K_2O-Al_2O_3-SiO_2-H_2O, a topological phase diagram has been established for this multisystem. It is suggested that constructing the phase diagram of a multisystem froma unary to a multicomponent system is an effective method. When the chemical potential of K_2O is low, i.e., corresponding to the phase relation in the system Al_2O_3-SiO_2-H_2O stable assemblages, such as pyrophyllite-andalusite, pyrophyllite-diaspore and pyrophyllite-kaolinite will be produced under the condition of P_(H_2O)=P_(totol). In the case of P_(H_2O)<P_(total), andalusite may coexist with kaolinite stably. This idea has been confirmed by geological evidence in the field.The application of this topological phase diagram in petrology is also described in the present paper.