摘要
笔者通过电子探针观察在湖南省柿竹园夕卡岩型钨—钼—铋—锡矿床中找到了大量微米粒级的锡石。这些锡石在空间上与磁铁矿共生,在成因上却与早期含锡夕卡岩的交代蚀变作用有关。本文根据矿物组合特征和锡石产出形式讨论了锡石的形成过程,并且提出了本区含锡磁铁矿夕卡岩中锡石的成因模式。
The Shizhuyuan W--Mo--Bi--Sn--bearing skarn is located in the contact zone ofthe Qianlishan two--mica granite and the Cpper Devonian carbonate rocks of theShetianqiao Formation. The tactites are mineralogically complex; a common min-eral assemblage consists of amphibole, pyroxene, garnet, fluorite, magnetite andminor amounts of ore minerals such as wolframite, scheelite, molybdenite, bismu--thinite and pyrite. The specimens containing cassiterite are principally composedof amphibole, pyroxene and andradite, of which andradite is usually replaced byamphibole, magnetite, fluorite and cassiterite, showing a typical relict texture. Subhedral magnetite grains range in size from 0.02 to 0.2 mm and constituteabout 5 to 10 percent by volume of the specimens studied. Examination of poli-shed sections reveals that cassiterite occurs on margins or in cracks of the magnet-ite, or in the interstices of the grains of amphibole and fluorite. This mode ofoccurrence is totally different from that of cassiterite as an exsolution product inmagnetite in the Lost River tin--bearing skarn described by G. A .Desborough and C.L.Sainsbury (1970). The grain size of the cassiterite ranges from 0. 001 to 0. 02 mm. Electron microprobe analysis of cassiterite, magnetite and other skarn mineralsfor Si, Al, Ti, Fe, Mg, Mn, Ca and Sn was conducted using standards of puremetals, oxides and silicates. Results of the analyses reveal that the andraditecarries about 0.4 wt％ SnO_2 while the magnetite has an average SnO_2 contentaround 0.02 wt％. Analysis of three grains of cassiterite gives an average compo- sition of about 98 wt％ SnO_2 and about 1 wt％ FeO impurity. According to the mineral assemblage characteristics and the association ofthe cassiterite with other minerals in the speciments it is suggested that the cassi-terite in the deposit may be precipitated together with magnetite, fluorite andamphibole at the expense of the tin--bearing andradite during the late hydro-thermal alteration stage. The reaction formula describing the process is as follows:Ca_3(Fe, Sn)_2Si_3O_(12) + 2HF + 5SiO_2 + 8FeOandradite --Ca_2Fe_5 (Si_8O_(22)) (OH)_2 + Fe_3O_4 + SnO_2 + CaF_2actinolite magnetite fluoritecassiterite
出处
《地质论评》
CAS
CSCD
北大核心
1992年第2期164-172,T001,共10页
Geological Review
关键词
锡石
产状
钨-钼-铋-锡
矿床
成因
Shizhuyuan in Hunan Province
W--Mo--Bi--Sn doposit
occurrence and origin of cacssiterite
