| 福建某稀有金属伟晶岩矿田地质地球化学特征 |
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| 引用本文：.1983.Geological and geochemical characteristics of a certain pegmatite ore field of area metals in Fujian Province[J].Mineral Deposits,2(2):49~58 |
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| Geological and geochemical characteristics of a certain pegmatite ore field of area metals in Fujian Province |
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| Abstract:The ore field in question occurs in early Sinian metamorphic rocks, assuming intrusive contact with the adjoining rocks. On its east and west sides two intrusions-granitic bodies A and B-crop out, which contain in average one to five times as many Nb and Ta as acidite. The pegmatites are mostly found distributed in metamorphic rocks as lenticular or stratoid veins, only a few of them are observed in granites. Pegmatites in this ore field might be divided into five types: l. biotite-microcline-albiclase pegmatite; ll. muscovite-albiclase-microcline pegmatite; lll. weakly albitized muscovite-microcline pegmatite; IV. albitized spodumene-micro cline pegmatite; V. intensly albitized pegmatite. In spatial distribution, pegmatites alter regularly with the increasing clistance from the granitic bodies: l) types of pegmatites change from simple to complex; 2) metasomatism grows from weak to strong; 3) mineralization of rare elements varies in the order of REE→∑Y, Nb (Ta) Be→Ta (Nb) Li→Ta; 4) the ore-forming temperatures fall gradually. The pegmatite-forming process might be divided into two periods-original crystallization and replacement. During the former period, as a result of magmatic crystallization differentiation, the melt-solution brought out massive mi'crocline and quartz, and also gave birth to tabular columbite, variscite, beryl, etc. The alkali metals evolved in order of Ca, Na→K→Li→K (Li). Due to the accumulation of volatiles and H2O, the earlier crystallized minerals tended to undergo aLbitization, greisenization and sericitization by replacement, With the intenSification of albitization, Nb and Ta contents increased, and columbite, tantalite and microlite were separated out. Dtuing the replacement stage, alkali metals evolved in order of Na→K(Si)→Li(Na)→K(Rb). Following is a summary of the geochemical characteristics of rare elements in pegmatites of this field. Nb, Ta: From type I to V and from early to late stage, Nb, Ta contents as well as Ta2O5/Nb2O5 ratio increase. Ta-rich pegmatites are generally distributed far from the parent intrusion, where elevations are relatively high. Be: Be content increases from type I to III with more beryl separated out, and decreases to some extent in types IV and V. It increases with the intensification of albitization in various replacement assemblages. Li: Li2O content increases from type Ito IV with spodumene and anblygonite being separated out in type IV, then decreases in type V. Rb. Cs: Rb and Cs vary in a similar way, which has much to do with K. The ratio of Cs/Rb decreases progressively ,from type I to V (from 13 to 3.4). The homogeneous temperatures of inclusions in pegmatitic minerals fallfrom type I to IV (400-180℃) and Ta-abundant pegmatites were formed at lower temperatures (260-180℃). CO2 content in inclusions increases or decreases in direct proportion to the contents of Nb and Ta. The formation of pegmatites is evidently related to the intrusion of granites on both east and west sides. Following this intrusion, melt-solution were formed at the late stage of crystallization differentiation, which migrated and subsequently produced pegmatites under relatively closed conditions. Late stage-hydrothermal replacement further concentrated rare elements, forming Nb, Ta-rich pegmatites. Pegmatites were formed in an unstable structural enivironment subjected to various degrees of stress. That accounts for their characteristic-banded structures. Such factors as the intensity of albitization, mineral-formiry; temperatures and characteristics of spatial distribution serve as indicators in search for Ta-rich pegmatites. |
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