霍邱矿田李老庄铁矿地质与地球化学特征及对沉积环境的指示

Geological and geochemical characteristics of the Lee Laozhuang iron mine in Huoqiu iron deposit: Implications for sedimentary environment

李老庄铁矿位于华北克拉通南缘安徽霍邱铁矿田中部。铁矿体赋存于角闪斜长片岩(片麻岩)和大理岩中,其原岩为细碎屑岩和富镁碳酸盐岩。铁矿石主要以条带状、浸染状和块状构造为主。本文主要对李老庄铁矿进行了岩石学、矿物学研究,对矿区铁建造和围岩的主量和微量元素进行了分析。结果显示李老庄铁矿石主要由Fe2O3、SiO2和MgO组成,具有较低的Al2O3和TiO2含量,微量元素含量和稀土总量均较低,矿石(La/Yb)PAAS=0.2～0.5,显示重稀土富集,具有明显的La(La/La*=2.07～4.03)、Eu(Eu/Eu*=1.72～3.60)、Y(Y/Y*=1.50～1.87)的正异常,较高Y/Ho比值(Y/Ho=38.4～47.31,平均42.12)和Sr/Ba比值(Sr/Ba=4.92～28.90,平均13.68)。这些特征表明李老庄条带状铁矿成矿物质主要来源于海底热液与海水混合的贡献。条带状铁矿无明显的Ce负异常(Ce/Ce*=0.67～0.81),且矿石Fe2O3T/FeO值偏低(Fe2O3T/FeO=2.60～3.19,平均2.95),表明矿石类型为原生矿。此外地球化学分析表明,作为李老庄铁矿层夹层的片岩、片麻岩的原岩可能为形成于大陆边缘环境的粘土或砂质沉积岩,其FeO/Fe2O3T比值为0.72～0.77,暗示了铁矿沉淀时的海水为低氧逸度的还原环境,球粒陨石标准化稀土配分型式呈现明显的轻稀土亏损,重稀土富集的特征,(La/Yb)N=8.46～25.78,平均19.48,暗示其沉积物质可能来源于陆源供给。综合分析认为,李老庄BIF铁矿为形成于活动大陆边缘局限盆地且受海底热液喷流作用影响的BIF铁矿。

The Lee Laozhuang iron mine lies in the central parts of Anhui Huoqiu iron deposit which is tectonically located in the southern margin of the North China Craton. The iron ore body is hosted in biotite quartz hornblende plagioclase gneiss （ schist） and marble,both of their protoliths are fine-grained clastic rocks and magnesium-rich carbonate rocks,respectively. The iron ore body is mainly occurred as banded,disseminated and massive. Petrology and mineralogy study are conducted on samples from the Lee Laozhuang iron mine,while major and trace element compositions were analyzed on the iron ores and surrounding rocks. The results have shown that Lee Laozhuang iron ores are mainly composed of Fe 2 O 3 ,SiO 2 ,MgO with low content of Al 2 O 3 and TiO 2 . The trace element compositions and total contents of rare earth element are low. The iron ore samples have （ La / Yb） PAAS of 0. 2 ～ 0. 5,significant positive anomaly in La （ La / La ＊ = 2. 07 ～ 4. 03） 、Eu （ Eu / Eu ＊ = 1. 72 ～ 3. 60） and Y （ Y / Y ＊ = 1. 50 ～ 1. 87） . They have high Y / Ho （ Y/Ho =38. 4 ～47. 31 with an average of 42. 12） and Sr/Ba （ Sr/Ba =4. 92 ～28. 90 with an average of 13. 68） . These characteristics demonstrated that metallogenic materials of the Lee Laozhuang iron mine are mainly sourced from the mixture of hydrothermal and seawater. The banded iron formations have no significant negative anomaly of Ce （ Ce / Ce ＊ = 0. 67 ～ 0. 81） ,in combined with low Fe 2 O 3 T / FeO （ Fe 2 O 3 T / FeO = 2. 60 ～ 3. 19 with an average of 2. 95） ,indicating it is the primary ore. In addition,the geochemical analysis have shown that the protoliths of schist and gneiss within iron ore in the Lee Laozhuang iron mine may be formed from clays or sandy sedimentary rocks of the continental margin,suggesting that the seawater was reduced with low oxygen fugacity during the iron ore precipitation. The normalized REE pattern have shown strongly LREE depletion and HREE enrichment with a wide range （ La / Yb） N （ 8. 46 ～ 25. 78 with an average of 19. 48） ,indicating deposited materials are possibly sourced from terrigenous contribution. In summary,the Lee Laozhuang BIF iron mine was formed in confined basins of active continental margin,which was influenced by the submarine volcanic hydrothermal activities.