五台山区五台群变质——沉积铁矿形成环境的分析

GENESIS ANALYSIS OF THE METAMORPHIC-SEDIMENTA RYIRONDEPOSITS IN WUTAI GROUP,WUTAI DISTRICT,SHANXI

本文讨论的五台群是指位于台怀不整合之上,滹沱群之下的一套变质程度达绿片岩相——次绿片岩的变质岩系。在这套变质岩系中条带状磁铁石英岩十分丰富;主要赋存在下亚群的变质火山岩系中。

The wutai group unconformablly overlying on the Archaen rocks was formed in an early Froterozoicgeosyncline.It can be divided into two subgroupst: a lower subgroup, consisting of volcanic rocks metamorphosed to green-schistfacies, and an upper sub group, consisting of clastic sediments metamorphosed to subgreen-schist-facies.In the lower subgroup the metamorphic volcanic rocks are mainly composed of tholei ite with the iron bands originally situated at its lower horizons. In the upper part of the lower subgroup that cousisting calc-alkaline volcanic rocks, no banded iron depo sits has been found.The character and the depositional environment of the banded iron deposits varied with time regularly. Three subformations of the iron bands may be recognized. Among them, the middle one is the richest in iron content and then is the uppermost. No commercial valuable iron deposits have been found in the lowest subformation.It is noted that the iron formations were merely deposited in submarine volcanic depressions by the landside which were separated from the seaward ones by middly rises, and to these latter depressions no commercial valuable deposits were found.Obviously, the origination of the banded iron deposits was related to volcanism. The volcano not only served as the source of iron, butaalso controlled its deposition.In addition marine environments where far from the volcanos but rich in carbonate may be suitable for the formation of banded iron deposits.The green-schist-facies metamorphism is believed profitable to the formation of mag onetotaconite. Because higher degree of metamorphism would couse deminish or even disappearence of magnetotaconite. Repetition of the iron deposits caused by folding made mining easier.