北祁连西段卡瓦铁矿的地球化学特征及其地质意义

Geochemical Features of Kawa Iron Ore Deposit in the Western Section of the North Qilian Mountains and Their Geological Significance

卡瓦铁矿位于北祁连造山带西段，是新近发现的沉积变质型铁建造。矿石全铁（Fe2O3T，平均45．28％）含量最高，其次为SiO2含量（平均40．50％），并具有较高的亲生物化合物P2O5（平均0．73％）及陆缘碎屑组分Al2O3（平均4．69％）、TiO2（平均0．31％）；Ba以及对氧化还原敏感的元素U、Th相对富集，Nb、Ta、Sr、Zr、Hf相对亏损；稀土元素总量较高，LREE亏损、HREE富集，弱的La正异常，无Eu、Ce、Y异常，Sr／Ba、Y／Ho比值与陆源沉积物相近。综合分析认为卡瓦铁矿是中元古代早期海洋化学沉积的产物，在形成过程中伴有较多陆缘碎屑物质的加入。此外，卡瓦铁矿缺乏Ce负异常，U、Th的相对富集．表明铁矿沉积于低氧的环境。

The Kawa iron ore deposit is located in the west section of the North Qilian orogenic belt, which is a newly discovered sedimentary metamorphic iron formation. In the average major compositions of its iron ore, total Fe2 03 T is the highest（44.22% on average）, next is SiO2 （40.50%）, with rela- tive enrichment of P2 05 （ 0.73 % ）, A12 03 （4.69%） and TiO2 （0.31% ）. Trace elements show that Ba and redox-sensitive trace elements （ U ,Th） are relatively enrlched,while Ta, Nb, St, Zr and Hf are relatively depleted. The total rare earth element contents are high, and the PAAS-normalized REE patterns display depletion of light REE relative to heavy REE;La positive anomalies,non-significant positive anomalies of Eu, Ce,Y and low Sr/Ba, Y/Ho values. These characteristics demonstrate that the iron source of the Kawa iron ore deposit are the product of Early Proterozoic ocean chemical deposition, with much addition of continental margin clastic material during its formation. In addition, the Kawa iron ore deposit lacka Ce negative anomalies and has relative enrichment of U and Th,implying the oxygen fugacity was relatively low in its depositional environment.