大厂锡矿床硅、氧同位素地球化学

SILICON AND OXYGEN STABLE ISOTOPIC GEOCHEMISTRY OF DACHANG TIN DEPOSIT

本文对大厂锡矿床不同类型石英和岩石进行系统的硅、氧同位素研究表明，各类矿化体中石英的硅同位素组成几乎相同，而且它们又与直接容矿硅质岩的δ30Si值十分接近，但完全不同于矿区侵入岩的有关值。这说明层状矿体中的石英与直接窑矿硅质岩有相同的硅质来源，后期各类脉状矿化过程中的硅质来自容矿硅质岩，而不是花岗岩。层状矿体中荣带状岩石和含冰长石混合样的δ30Si值为-0．3‰±0．2‰，这组负值可能是由冰长石引起的。但冰长石中的硅是由海底热水活动系统还是火山灰尘提供尚待研究。对比发现，本矿区硅质岩δ30Si值与部分深海（洋壳区）、半深海（陆壳区）放射虫硅质岩有关资料相似，但其δ18O值低于全球各类硅质岩，它们可能是在海底热水活动与生物化学作用共同参与下形成的。

Systematic analyses of silicon and oxygen isotopic compositions have been made of adjacent host rocks, of quartz and mixtured quartz+sericite +adularia from different types of mineralizations, and of whole rocks of intrusions in the Dachang tin mine. The results show that the silicon isotopic compositions of quartz from different types of mineralizations are almost identical, and very simi1ar to those of adjacent host rocks, but quite different from those of intrusions in the ore field. This indicates that there exists the same source of silicon for quartz in strati form orebodies and adjacent siliceous host rocks, and late vein mineralizations of various types derived their silicon from the siliceous host rock other than from granitic intrusions- Low δ30Si values averaging -0. 3‰±0.2‰ have been observed for adularia-bearing mixtured samples from banded rocks and ores in strati form orebodies, which may have resulted from the presence of adularia. But whether silicon in adularia was provided by submarine hydrothermal systems or volcanic ashes is yet to be further investigated. Comparative studies show that the δ30Si values of siliceous rocks from the Dachang tin deposit are similar to those of a part of abyssal (oceanic crust), bathyal (continental crust) radiolarian cherts, but the lower δ18O values (averaging 15. 4‰±2. 5‰) of siliceous rocks in Dachang relative to those of most cherts throughout the world may be jointly caused by submarine hydrothermal activities and biochemical processes.