某区磁铁矿床中钠质交代作用的特征及其找矿意义

河北省某区接触交代型磁铁矿床与燕山期角闪闪长岩-闪长岩和闪长岩-二长岩两个岩石系列的中性侵入岩有成因联系。磁铁矿体主要赋存在这些侵入岩与中奥陶统碳酸盐岩的接触带中。与铁矿床的形成有密切联系的蚀变现象主要是钠长石化和矽卡岩化。本文仅就钠质交代作用予以较详尽论述,对矽卡岩化等顺便提到,不作赘述。

THE CHARACTERISTICS OF SODIUM METASOMATISM IN MAGNETITE DEPOSITS OF A CERTAIN REGION AND ITS PROSPECTING SIGNIFICANCE

The magnetite deposits of contact-metasomatic type in a region of Hebei province occur largely in the contact zone between the dioritic porphyrite intrusions of Yan-shanian age and the carbonate rocks of middle Ordovician. The sodium metasomatism well developed in the rocks close to the iron ore offered an important indicator for prospecting. The principal characters of this kind of metasomatic rocks such as colour, mineral and chemical compositions, texture, volume weight, magnetization and so on, are described in this paper. The relationship between the sodium metasomatism and the genesis of iron ore deposits is also discussed. The replacement concerned took place after crystallization of the sodium replaced magmatic rocks, just before the formation of skarn and magnetite deposits, and closely associated in spatial distribution with the iron ore body. The genera trend of the replacement is weaker toward the ore body, and its scale is nearly in direct proportion to the size of the ore body. It is suggested that the ferric material separated from the sodium metasomatism is an essential source of iron which formed magnetite deposits. The migration took place probably in form of a fluoric (chloric) complex of Fe-K.Finally, the authors point out that the ore-bearing part and the ore-free part in covered contact zone may be distinguished according to the characteristics of sodium replaced rocks, and the relative size of the blind ore-body may be also estimated in accordance with the extension of the sodic altered rocks.