山东昌邑-安丘铁成矿带BIF铁矿地球化学及矿床成因

Geochemistry and genesis of BIF iron ore in Changyi-Anqiu iron metallogenic belt,Shandong

昌邑-安丘地区位于华北板块东缘的胶北隆起西南部,BIF铁矿赋存于古元古代粉子山群变质岩中。以昌邑-安丘地区BIF铁矿为关注点,通过矿床地质、地球化学研究,揭示昌邑-安丘地区古元古代BIF铁矿的地质特征,探讨其矿床成因和成矿物质来源。该区矿体呈透镜状、似层状,以角闪磁铁石英岩为主要矿石类型,变质程度为绿片岩相-低角闪岩相。铁矿石富含SiO_(2)(36.27%~46.83%)和TFe_(2)O_(3)(38.92%~55.35%),SiO_(2)+TFe_(2)O_(3)变化于83.57%~95.60%(平均91.34%),含较高Al2O3(平均2.13%)、TiO2(平均0.12%);富含Th、Hf等高场强元素。根据矿石及围岩含较多铁铝榴石等富铝矿物以及ACFM图解的特征,可以判断原岩成分为胶体化学沉积但有较多碎屑或泥质物质加入。与PAAS相比,轻稀土元素亏损、高Y/Ho值以及La和Y正异常表明铁矿沉淀于海相环境,而高的SiO_(2)/Al2O3、Ti/V值,高Cr、Co、Ni和Zn含量以及Eu正异常等显著特征,均表明成矿物质来源于火山热液和海水的混合溶液。CaO/(CaO+MgO)值(平均0.49)较低、Eu/Eu∗值(平均1.74)<1.8,比Algoma型铁矿小,推测昌邑-安丘地区BIF铁矿可能形成于远离扩张洋中脊或者火山喷口的位置,并且在形成过程中的高温热液对成矿物质的贡献较少,可能属于Superior型铁矿。与华北克拉通太古宙BIF相比,昌邑-安丘地区铁矿是有较多陆源碎屑物质参与的条带状铁矿床。

The Changyi-Anqiu area is located in the southwest of Jiaobei uplift on the eastern margin of the North China plate. BIF iron ore occurs in the metamorphic rock of the Paleoproterozoic fenzishan group. Focusing on BIF iron ore in Changyi-Anqiu area, the geological characteristics of Paleoproterozoic BIF iron ore in Changyi-Anqiu area are revealed through geological and geochemical study of the deposit.The genesis of the deposit and the source of ore-forming materials are discussed. The ore body in this area is lenticular and stratoid, with hornblende magnetite quartzite as the main ore type.The metamorphic grade is from greenschist facies to low amphibolite facies. Iron ore is rich in SiO2(36.27%-46.83%) and TFe2O3(38.92%-55.35%). SiO2 + TFe2O3 varies from 83.57% to 95.60%(average 91.34%), showing high Al2O3(average 2.13%) and TiO2(average 0.12%) values, and rich in Th and Hf. The characteristics of ore and wall rock with more Al-containing minerals, such as almandine and ACFM diagram, indicate that the original rock is composed of colloidal chemical deposition, but with more clastic or argillaceous materials. Compared with PAAS, LREE depletion, higher Y/Ho values and positive anomalies of La and Y all indicate that the iron ore is precipitated in a marine environment.The high SiO2/Al2O3, Ti/V values, high Cr, Co, Ni and Zn contents and positive anomalies of Eu confirm that the ore-forming materials are derived from the mixed solution of volcanic hydrothermal solution and sea water. Compared to Algoma type iron ore, the low value of CaO/(CaO + MgO)(0.49 on average), low Eu/Eu* value(1.74 on average, less than1.8), can be speculated that BIF iron ore in Changyi-Anqiu area may be formed far away from the spreading ocean ridge or volcanic vent.The high temperature hydrothermal solution in the formation process has less contribution to the ore-forming material, which may belong to superior type iron ore. Compared with the Archean BIF of the North China Craton, the Changyi-Anqiu iron ore deposit is a banded iron deposit with more terrigenous clastic materials.