新疆“帕米尔式”铁矿床成因与成矿背景分析

Study on metallogenic genesis and metallogenic settings of Xinjiang “Pamir-type” iron deposit

“帕米尔式”铁矿床是新疆西昆仑地区重要的磁铁矿床类型之一，其矿床地质特征、形成时代明显有别于典型的BIFs类型磁铁矿床。本文以塔什库尔干地区翁吉勒磁铁矿床和孜勒依磁铁矿床作为研究对象，通过研究其赋矿围岩和岩体的岩石地球化学特征、矿石矿相学特征、磁铁矿微量元素地球化学特征和原位Pb—Pb同位素特征，探讨了“帕米尔式”磁铁矿床的矿床成因和成矿背景。岩石地球化学研究表明翁吉勒黑云母二长花岗岩具有高SiO2、Al2O3、K2O和Na2O3低MgO、CaO和TiO2，而Na2O／K2O值变化范围很小。铝饱和指数为（1．44～1．51），显示其具有为弱过铝质岩石的特征，稀土总量富集，具有强的负Eu异常。在微量元素蛛网图上显示其富集大离子亲石元素（LILE），如K、Rb、Sr、Ba及轻稀土元素（LREE），亏损Nb、Ta，P、Ti等高场强元素。而黑云母石英片岩稀土含量亦较富集，具有中等强度的负Eu异常。低的Rb／Sr比，明显的Th、Zr、Hf、Nb、Ta，P、Ti负异常，明显的Ba正异常，显示岩石具有古大陆边缘环境的富铝沉积岩特征。翁吉勒黑云母二长花岗岩和布伦阔勒岩群黑云母石英片岩地球化学性质非常相似，表明翁吉勒黑云母二长花岗岩的形成与布伦阔勒岩群黑云母石英片岩的重熔有关。锆石LAICP—MS定年结果表明翁吉勒黑云母二长花岗岩形成于15．0±0．3Ma和15．4±0．2Ma。磁铁矿LAICP—MS微量元素地球化学特征表明翁吉勒铁矿床中的磁矿床中主要Si、Mn、Zn、Sn、Hf和Pb相对较高，其中Mn和Ga变化范围较大，而孜勒依铁矿床中的磁铁矿具有相对较高的亲石元素，如Mg、A1、Ti、V、Co、Ni、Ga和Sc等，其Mn和zn含量较低且变化范围较大，其它元素变化范围均相对较窄。二者相比，翁吉勒铁矿床中的磁铁矿中微量元素变化范围相对较大，而孜勒依铁矿床中的磁铁矿变化范围较小。两个矿床的磁铁矿的原位Pb—Pb同位素存在明显的不同，两类岩石全岩Pb同位素与两个矿床中磁铁矿Pb．Pb同位素研究表明翁吉勒铁矿床中磁铁矿可能与黑云母二长花岗岩有关，为热液成因。综上所述，在塔什库尔干地区不仅存在与沉积作用有关的铁矿床，还可能存在一部分与年轻岩浆岩有关的热液型磁铁矿床。

＂Pamir-type＂ iron deposit is one of the most important categorized magnetite deposits in West Kunlun, Xinjiang, and its metallogenesis is still unclear. Previous studies on geological characteristics, ore-forming ages suggested that these magnetite deposits were obviously distinguished from those typical BIFs magnetite deposits globally. Wengjile magnetite deposit and Zileyi magnetite deposit, two representative magnetite deposits in this region, were selected in this study. Litho geochemistry of admellite and wall rocks, ore-forming minerals characteristics, trace element geochemistry and in situ Pb-Pb isotopic composition of magnetite were discussed in this study to explore the metal]ognesis and ore-forming background of ＂Pamir-type＂ iron deposit. Wengjile magnetite deposit lies in the contact zone between Wengjile biotite-admellite and Paleoproterozoic Bulunkuole Group biotite quartz-schist. The deposit is closely associated with Wengjile biotite-admellite in spatial and temporal. Geochemical characteristic of Wengjile biotite- adamellite is higher in SiO2 （ 69. 10%- 74. 51% ）, A12 03 （ 14. 24% -16. 13% ）, K20 （4.46% - 4. 91% ） and Na20 （ 3.82% - 4.29%）, and lower in MgO （0.25% -0.69%）, CaO （1.1% -1.99%） and TIO2（0.139% -0.398）,and has a very narrow variation of Na20/K20 （0. 80 -0. 87 ） and unanimous Rittmann Index （2. 20 - 3.24 ）. Aluminum saturation index （ 1.44 - 1.51 ） suggested it belonged to a weak peraluminous rock. Total Rare Earth Element is higher （ 170.4 × 10 -6 - 400. 5 ×10 -6 ） , and 8Eu （0. 44- 0. 87） is a strong negative Eu anomaly. The rocks contain variable contents of trace element in the trace element spider diagram, but generally have a similar variation trend such as systematic enrichment of LILE such as K, Rb, Sr, Ba （where Sr 〉 421 × 10-6 ,Ba 〉853 × 10-6） and REE but depletion of HFSE such as Nb, Ta, P and Ti. The content of the major oxides in Bulunkuole biotite quartz schist are SiO2 （57. 07% - 60. 99% ）, and A1203 （ 14. 46% - 8. 61%, MgO （2. 22% - 3.60% ）, CaO （ 1.12% - 2. 86% ）. ,TiO2 （0. 75%- 1.12% ）, and are rich in REE （ 146. 0 × 10-6 - 200. 5× 10-6 ）, and has a median Eu anomaly （SEu is 0. 65-0. 74）. Trace element analyses suggested there is a lower Rb/Sr ratio （0. 16 - 0. 89） and obvious depletion of Th, Zr, Hf, Nb, Ta, P and Ti but apparent positive anomaly of barium, which may suggest that the rock has the characteristics of aluminum-rich sedimentary formed in the environment of margin of ancient continental. The geochemical similarity between Wengjile biotite-admellite and Bulunkuole biotite schist demonstrated there is a genetic relationship between them. Zircon LA ICP-MS U-Pb dating of Wengjile biotite-admellite suggested the rocks formed around 15.0 ±0. 3Ma and 15.4 ± 0. 2Ma. Trace element geochemistry of magnetite using LA ICP-MS indicated that magnetite from Wengjile magnetite deposit has higher Si, Mn, Zn, Pb, Hf, Sn and variable Mn and Ga, and magnetite from Zileyi magnetite deposit has relatively higher LILE such as Mg, Al, Ti, V, Co, Ni, Se and lower variable Mn and Zn. In contrast, the range of variation of trace elements in magnetite from the Wengjile magnetite deposit is relatively larger, while the range of magnetite in the Zileyi magnetite deposit is relatively small. The Pb-Pb isotopes of the magnetite from the two deposits are distinctly different. The 206pb/204Pb of the magnetite from Wengjile magnetite deposit varied from 9. 0699 to 41. 4030 with an average value of 20. 4808, and 207pb/204pb varied between 11. 0454 and 34. 1993 with an average of 17. 5573 ; and 208pb/204Pb varied between 28. 6800 and 84. 1533 with an average value of 44. 8547. The 206pb/204pb, 207Pb/204pb and 208pb/204pb of the magnetite from Zileyi magnetite deposit are lower. The calculated isotope ratios of 206pb/207pb varied from 0. 67 to 1.37 with an average of 1.16; 205Pb/206pb varied from 1.85 to 3.61 with an averige of 2. 24, and Z08pb/204pb between 2. 36 and 3.04, with an average of 2.56. Pb-Pb isotope results indicated that the magnetite from the Wengjile magnetite deposit may be related to biotite-adamellite and is hydrothermal but magnetite from Zileyi was not. In summary, there are not only deposits-related to sediment process, but also some hydrothermal magnetite deposits, which related to magrnatie hydrothermal activity in the Tashkurgan region.