弓长岭铁矿床物质来源及沉积古地理环境研究

弓长岭沉积变质型铁矿床是中国鞍山式铁矿床的典型代表.研究表明,该铁矿床赋存于反S型褶皱中,局部矿体出现褶曲现象.矿体产于主要由磁铁石英岩、斜长角闪岩、黑云母变粒岩和“蚀变岩”组成的含铁岩系中.文章通过研究该地区铁矿石的构造、结构和矿物组合,结合常量、微量及稀土元素研究,揭示了该地区形成铁矿床的物质来源与海底热液活动密切相关,在其形成过程中,很少或者没有含铝碎屑物质的加入,是由海洋化学沉积形成的,形成于还原环境.通过研究与铁矿床空间关系密切的斜长角闪岩、黑云母变粒岩和“蚀变岩”的构造和结构等,并利用常量、微量及稀土元素的测试结果,对斜长角闪岩、黑云母变粒岩和“蚀变岩”进行了原岩恢复研究,研究结果显示,该区斜长角闪岩的原岩应为洋中脊拉斑玄武岩,黑云母变粒岩的原岩应为大陆岛弧的杂砂岩,而“蚀变岩”的形成则与磁铁石英岩有着密切的关系.最后,提出了该地区铁矿床沉积的古地理环境为洋壳边缘、大陆岛弧附近,其形成环境应相当于弧后盆地.

Material sources and sedimentary environment of Gongchangling iron deposit

The Gongchangling iron deposit is a typical sedimentary-metamorphic iron deposit in Anshan-Benxi area; nevertheless, researches on the formation of the iron deposit have been very insufficient. Studies show that the iron deposit lies in a reverse S-shaped fold strata composed of plagioclase amphibolite, biotite leptynite, magnetite-quartzite and "altered rock". According to the investigation of structure and mineral assemblages of iron ore in combination with characteristics of major elements, REE and trace elements of iron ore, the authors have arrived at the following conclusions: the formation of iron deposit was closely related to mantle plumes; there was very little or no aluminum-bearing detrital material added to the iron deposit during its formation; the iron deposit was formed by marine chemical deposition and in a reducing environment. According to mineral assemblages of plagioclase-amphibolite, biotite leptynite and "altered rocks" in combination with features of major elements, REE and trace elements of plagioclase-amphibolite, biotite leptynite and "atlered rocks", the authors made an analysis of the protolith, and the results show that plagioclase-amphibolite came from mid-ocean ridge basalt, biotite-leptynite came from greywacke near continental island arc, and the formation of "altered wall" was closely related to magnetite-quartzite. It is held that the depositional environment for the iron deposit was the edge of the oceanic crust near the continental island arc.