| 一个与前寒武纪火山碎屑沉积岩及超镁铁岩有成因联系的锡矿床 |
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| 引用本文：.1988.A tin deposit genetically related to submarine volcaniclastic rocks and ult ramafic rocks[J].Mineral Deposits,7(3):29~41 |
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| 中文摘要:经详细研究九毛锡矿各类地质体的含锡量，确定了锡在火山一沉积变质岩及变超镁铁岩中的初始富集。根据不同变质程度岩石含锡量变化趋势、微量元素含量的继承性关系、成因矿物等方面的研究，确定锡矿床与超镁铁岩及火山—沉积岩间有成因联系。本文强调了锡的多重性及锡在超镁铁岩中的初始富集，指出变质作用是古陆区锡成矿的重要地质营力。 |
| 中文关键词:古陆区 锡矿床 超镁铁岩 初始富集 变质成矿 广西九毛 |
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| A tin deposit genetically related to submarine volcaniclastic rocks and ult ramafic rocks |
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| Abstract:The Jiumao tin deposit occurs in schists of Upper Proterozoic Sibao Group, meta-ultramafic intrusion and their contact zone. The ore consists of aseries of metamorphic minerals, such as biotite, muscovite, albite, garnet, quartz, cassiterite and a little sulfides, with metaerystal and palimpsest texture and schistose structure. Cassiterites are commonly in orientational arrangement along cleavages of biotite and muscovite, with part of them inlaid with albite and quartz. The authors have reached the conclusion that the orebodies are genetically related to their country rocks-submarine volcaniclastic rocks and ultramarie rocks on the basis of the following evidence: (1) tin contents of these two sorts of rocks are very high, averaging 9.2 ppm and 33 ppm respectively, with the mineralization coefficient being 1.5 and 5.1; (2) the χsn values of the rocks get lower with the heightening of the metamorphism and increase remarkably near the orebodies, forming clear positive and negative anomalies; (3) trace element contents show inheritance between orebodies and country rocks; (4) mineralogical evidence of cassiterite, biotite and pyrrhotite points to the same conclusion; (5) the orebodies are consistent with the metamorphosed country rocks in such aspects as mineral association, texture and structure. It is suggested that there occurred primary concentration of tin in the submarine volcaniclastic rocks and ultramafio rocks. During the metamorphism of late Proterozoic, tin was activated and transferred by metamorphic solutions, and then precipitated to form valuable tin ore deposits in the process of retrogressive metamorphism accompanied by the potassium metasomatic process characterized by the formation of Mg2+-rich biotite; in metaultramatic rocks, the precipitation of cassiterite was accompanied by the filling and metasomatie action of metamorphic solutions which were mainly derived from ultramafic intrusions. The ore deposit should be classified as cassiterite-silicate formation superimposed by some sulfides. Cassiterites were deposited at 220-350℃, 400-600 bar, low Eh(about-0.33-0.57 V), low pH(about 4.2-5.4), and low fO2(about 10-26-10-36 bar). The siderophile nature of tin and the primary concentration of tin in ultramafic rocks are emphatically discussed in the paper, and the important role that the metamorphic solution played in the activation and migration of tin is also stressed. All the studies and discussions indicate the possibility of finding cassiterite-silicate formation in old land area. |
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