| 沉积变质中交代岩和有关铁矿形成机理的实验研究 |
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| THE EXPERIMENTAL STUDY ON THE MECHANISM FOR THE FORMATION OF METASOMATIC ROCKS AND THEIR IRON ORE DEPOSITS IN SEDIMENTATION AND METAMORPHISM |
| High-grade iron deposits of Shilu, Hainan province and Menjiagang, Hei-longjiang province belong to the well-known metamorphosed sedimentary type of iron ores of China. The ores occur in the Sinian and Archaean interbedded mica-quartz-schists and metamorphosed carbonate rocks respectively as intercalated beds or lenticular layers. The host rocks in direct contact with the ores are metasomatites composed chiefly of tremolite,diopside,garnet,quartz, etc. The chief ore mineral of the Shilu diposit is hematite, rarely associated with magnetite, and that of the Mengjiagang region is magnetite. These are intermediate to acid intrusions around both deposits, but the authors haven't found any genetic relationship between the intrusive rocks and the metasomatites as well as the iron ores.Samples collected for experimentation are sericite-quartz schist and dolomite from Shilu and biolite-quartz-schist and dolomitic marble from Mengjiagang. With due consideration of the actual geological conditions of the two deposits, the experimentation was performed in the Bridgman sealed pressure vessel,the same apparatus was used by the senior author for similar experiments on metasomatites and related iron deposits and the laboratory procedures were almost the same (Liang et al. , 1986, Bull. Geol. Int. , GAGS, Nol5; Liang et al. , 1987, mineral deposits, VI, No. 2). The temperatures for the experiments were 300-700℃, pressures, 30-200MPa and durations of the exper- iments, 120-160 hours. Experimental products were chemically analysed and studied under the microscope and also by X-ray as well as infrared spe-ctrographic methods. The paragenetic association of the minerals formed in experiments was engaed in a series of calculations of the phase equilibrium and the .study of the thermodynamic reaction. The mutual relationship among the T, P. fo2, pH and Eh conditions related to the formation of metas-omatites, including the Fe-rich types (being provisionally called "low-grade iron ore"), in the experimental products. The results of experiments show that:(1) The mineral association of the metasomatites (including the iron-rich type) from the Shilu samples was formed by the metasomatism of the dolomite by the metamorphic hydrothermol solution which contains 0.5M FeCl3+0.2M KCl + 0.3M NaCl with some SiO2. That of the Mengjiagang samples was formed by the metasomatism of the biotite-quartz-schist by the solution containing 1M NaHCO3 + 0.2M KF with some MgO, CaO and Fe2O3.(2) The temperatures for the formation of the Shilu metasomatites were 400-650℃, pressures, 50-140MPa; those for the Mengjiagang metasomatites were 375-650℃ and 30-140MPa.(3) The oxygen fugacity for the formation of the Shilu metasomatiteswas 1g fo2= -21.763-11.066; and that for the Mengjiagang productslgfo2=-38.637-16.514.(4) Shilu metasomatites were formed in the acid medium of pH=2.0-6.0 and the oxidizing-weak oxidizing conditions of Eh = 1.09-0.13V; and the Mengjiagang metasomatites, in the neutral-soda medium of pH=7.5-13.0 andthe weakoxidizing-reducing environment of Eh = - 0.27-1.15V.(5) Pco2 for the formation of the Shilu metasomatites was 5-70MPa; and that for the Mengjiagang, 0.06-30MPa.The above-stated were very probably the actual conditions and mechanism of the formation of metasomatites and iron ores of Shilu and Mengjiagang. The iron source for the rocks and ores were very probably derived from the iron-rich silty and argillaceous rocks of these districts, being transported and enriched by metamorphic hydrothermal solution, and not by solutions of pyrometasomatic nature related to the granitoids. |
| LIANG Xiang-ji,QIAO Li.1988.THE EXPERIMENTAL STUDY ON THE MECHANISM FOR THE FORMATION OF METASOMATIC ROCKS AND THEIR IRON ORE DEPOSITS IN SEDIMENTATION AND METAMORPHISM[J].Acta Geoscientica Sinica,10(1):159-186. |
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