辽东地区沉积变质硼矿床及硼同位素研究

辽东地区元古界硼矿床属于沉积变质硼矿。根据现代盐湖沉积 ,认为硼矿物沉积通常为硼砂 ,然后在区域变质作用过程中 ,硼砂矿物转变为硼镁矿物或硼铁矿等硼酸盐矿物 ,但辽东硼矿床中不同硼矿物的硼同位素有一定差别。电气石富10 B ,δ11B值较低或呈负值 ;而硼镁石与硼镁铁矿类矿物明显富集11B ,δ11B值较高 ,为 +2 3‰～ +17 4‰。根据水岩作用过程中硼同位素分馏特征的研究 ,11B与10 B比较有下列 4个特征 :( 1) 11B属于极不相容元素 ,优先进入水相 ,因此在变质残余矿物相中形成低的δ11B值 ;( 2 )在蚀变和交代变质反应中硅优先替代11B进入矿物晶格 ,因此在硅化交代中可以降低δ11B值 ,而脱硅反应中可以提高δ11B值 ;水化作用中 ,OH-带入11B ,使富水矿物具有较高的δ11B值 ;( 3)热水沉积及热液交代成因电气石均具有较低的δ11B值或者为负值 ;( 4)通过最近的研究表明 ,在热蒸馏过程中 ,硼同位素会发生明显的分馏 ,11B倾向于进入蒸汽相 ,而使残余相或后蒸馏相亏损11B ,形成较低的δ11B值。因此可以认为辽东地区元古界硼矿床属于热水沉积电气石岩在后期区域变质或热变质过程中分解出硼酸气水溶液交代镁碳酸盐形成硼镁石或硼镁铁矿矿物 ,而非蒸发沉积变质矿床。

SEDIMENTARY-METAMORPHIC BORON DEPOSITS AND THEIR BORON ISOTOPIC COMPOSITIONS IN EASTERN LIAONING PROVINCE

Proterozoic boron ore deposits in the east part of Liaoning Province belong to sedimentary-metamorphic type. According to the study on sedimentations in modern salt lakes, boron minerals in sedimentation are usually borax. Borax turned into boron-magnesium minerals and boron-iron minerals etc. in regional metalmorphic processes. Boron isotopes are different in different boron minerals in eastern Liaoning boron deposits.δ 11B value of tourmalines being rich in 10B are rather low or are negative value. While δ 11B value of boromagnesites and ludwigites being rich in 11B are quite high, up to +2.3‰—+17.4‰ .The study on fractionating feature of boron isotope in water and rock interaction, compared with 11B and 10B, shows four characteristics. (1) 11B is an incompatible element and is apt to enter liquid phase. So there formed a low value of δ 11B in residuals of metamorphic phase. (2)During the alteration and metamorphic metasomatism 11B is replaced by silicon in the mineral lattice. So δ 11B value decreases in silicification and increases in the desilicification. (3)Because 11B is brought in by (OH)-, the value of δ 11B is high in water-rich minerals. Hydrothermal depositional tourmaline and hydrothermal metasomatic tourmaline have low or negative 11B values. (4) Our recent research shows that there is obvious fraction in distillation process and 11B inclined to enter gas phase. This made residual phase and later phase short of 11B to form a low value of δ 11B. Based on the above analyses, the authors have proposed that boron deposits in eastern Liaoning Province is the result of metasomatism of magnesian by boric acid solutions, which have generated from regional metamorphic process of hydrothermal sedimentary tourmaline rocks. So it is not evaporate-depositional metamorphic ore deposits.