鞍山、本溪地区鞍山群变质岩岩石化学研究及条带状铁矿的成矿条件

通过对鞍山群变质岩石及条带状铁矿的岩石化学研究,探讨了鞍本地区太古宙老变质岩(正变质岩为斜长角闪岩及中酸性变粒岩等;副变质岩主要为泥质—泥灰质岩及硅铁质胶体化学沉积岩石)岩石学特征,并恢复变质前之原岩建造。太古宙条带状铁矿石的化学成分较单一;SiO_2、Fe_2O_3、FeO之和为85—95％,是一种成分很纯的磁铁矿石。作者从矿体的产状呈层;与围岩之间基本整合;条带状构造所显示的原生层理;铁矿石成分简单以及在世界上的分布情况表明它是海底火山沉积矿床。本文基于铁矿成因和与原岩建造的关系,进而讨论了铁矿的赋存

THE PETROCHEMICAL STUDY OF THE ARCHEAN BANDED IRON DEPOSIT IN ANSHAN-BENXI DISTRICT, LIAONING PROVINCE

The Archean Anshan group of the Anshan-Benxi district, Liaoning province, is composed of ortho-, para-metamorphic rocks and migmatite. Their original rocks are igneous (such as tholeiite, intermediateacidic volcanic rocks and tuffs), sedimentary rocks (such as . argillo-calcareous rocks) and banded iron deposits, etc. These rocks, which are of the Anshan group, constitute the Archean crust of this district. Among these rocks, the migmatites are the most widespread. Other metamorphic rocks appear like little boats floating in the "sea" of migmatites. But the other metamorphic rocks are mostly of basic and intermediate-acidic volcanic composition, rarely of seidmentary origin. This shows that igneous rocks instead of sedimenatry rocks are dominant in the oldest crust formed at the Archaeozoic age. Compared the metamorphosed sedimentary rocks with the Phanerozoic shale, the former are not argi-lloceous rocks, but argillocalcareous types, coarser clastic rocks being absent. The petrochemical characters of these rocks indicate that the surficial weathering-sedimentation of the Arch can was rather undeveloped.The chemical compostion of the banded iron deposit is quite unique, the sum of SiO2 and F2O3 + FeO reaching as high as 85-95%, much higherthan that of other types of iron deposits (40-80%).The magnetite in the bonded iron deposit is a very pure mineral, with the Fe2O3+FeO content averaging 98.12%. Accordingly its average Fe content is up to 70.20%, approaching the theoretical content of the magnetite. Such pure magnetite generally is considered as the product of complete sedimentation differentation.Based on the facts mentional above, the banded iron deposit should be a volcanic-sedimentary one.The model of the mineralizations the ore-forming Si-Fe material derived its source from the activity of the oceanic volcano. The source material existed in the strongly acidic sea water near the centre of volcanic activities. Far away from the centre the sea water under the influence of the terrestrial water became weakly acidic to intermediate, with a high Eh value. As the Si-Fe material migrated away from the centre of the marine-volcanic activities to the regions of strong terrestrial influence, it finally deposited in the weakly acidic to intermediate aquatic environment. In the long duration of mineralization, the change of the ore-forming conditions has led to the deposition of sporadic small iron lenses in the basic volcanic formation near the centre at the bottom of the sea, whereas in the areas remote from the centre were deposited thick banded iron deposits in the sedimentary-interme-diate-acidid volcanic formation. In between the two, multistratoid banded iron deposits were formed.