鲁东昌邑古元古代BIF铁矿矿床地球化学特征及矿床成因讨论

Geological and geochemical characteristics of Paleoproterozoic Changyi banded iron formation deposit,Jiaodong Peninsula of eastern China

昌邑铁矿位于华北克拉通东部的胶北地体,为赋存于古元古代粉子山群变质岩中的条带状铁建造(BIF)铁矿。矿体主要呈透镜状、似层状,以(含)角闪石英磁铁岩为主要矿石,经历了温度高达636℃的角闪岩相变质作用。铁矿石富SiO2和Fe2O3T(SiO2+Fe2O3T=82.5%～97.7%),含少量Al2O3、MgO和CaO等,显示主要为化学沉积但有少量碎屑或泥质加入的特征。与PAAS相比轻稀土元素亏损、高的Y/Ho比值以及La和Y正异常表明铁矿沉淀于海相环境,而高的Ti/V比值、高Cr、Co和Ni含量以及Eu的正异常表明火山热液的参与,成矿物质来源于火山活动。无明显的Ce负异常表明当时可能存在一个缺氧的大气环境。昌邑铁矿与华北克拉通太古宙BIF相比,总体上没有显著差别,但Al2O3、CaO、MgO和K2O含量相对较高,Eu正异常相对较弱,表明其可能形成于具有更多碎屑物质和更少热液参与的浅水环境。

Changyi iron deposit, located at Jiaobei terrane, eastern North China Craton （NCC）, is a banded iron formation （BIF） deposit kept in Paleoproterozoic Fenzishan Group. Ore bodies of the deposit are lens- or layer-like and mainly composed of amphibolite-bearing magnetite quartzite, which have expericed 636℃ amphibolite facies metamorphism. Major contents of SiO2 and Fe2O3T （SiO2＋Fe2O3T=82.5%～97.7%） coupled with minor Al2O3, MgO and CaO suggest that the ores originated from chemical deposition contaminated by clasitic or pelitic materials. LREE depletion, high Y/Ho ratios and positive La and Y anomalies indicate a marine environment. In addition, high Ti/V ratios, high Cr, Co and Ni concentrations as well as positive Eu anomalies suggest involvement of high-temperature hydrothermal fluids, implying ore-forming elements were related to submarine volcanic activities. The lack of negative Ce anomalies also suggests an anoxic environment. Compared with Archean BIFs of NCC, the higher Al2O3, MgO, CaO and K2O contents as well as weaker positive Eu anomalies indicate that the Paleoproterozoic Changyi BIF deposit obtained more clastic or pelitic materials and less hydrothermal fluids in a shallower marine environment.