西昆仑赞坎铁矿床地质特征、形成时代及高品位矿石的成因

新疆赞坎铁矿床位于西昆仑塔什库尔干地块西段,是近年新发现的一个大型沉积变质型磁铁矿床。赋矿岩系布伦阔勒群主要由黑云母石英片岩、斜长角闪片岩、变粒岩、硅质岩及磁铁石英岩等组成。目前探明工业矿体4条,单个矿体长度大于2.5km,矿体厚10~70m;局部见高品位铁矿段(mFe50%),长度达900m,厚度40m左右。矿石类型主要为2种,一种为原生的条纹-条带状磁铁矿(为主);另一种为热液改造形成的块状(高品位铁矿石)及浸染状磁铁矿。矿石稀土元素配分(PAAS)表明,原生条纹-条带状铁矿石Ce和Y元素异常不明显(~1.15、~0.94),Eu具正异常(~1.69),Y/Ho平均值为25,稀土配分模式与沉积变质型铁矿相似。而受改造的矿石中,浸染状矿石具有较高的稀土总量,明显富集轻稀土,La和Ce显示正异常(~1.46、~1.17),Y显示负异常(=0.66~0.72),Eu表现为强烈的正异常(~4.37),稀土配分模式明显不同于原生条纹-条带状铁矿石。矿体围岩斜长角闪片岩(变沉积岩)中的碎屑锆石U-Pb年龄为591±1Ma,结合前人对矿区内侵入体的年代学研究(霏细斑岩,533Ma),大致反映沉积铁矿的形成时代为新元古代至早寒武世。电子探针显示,条带状磁铁矿中的TiO_2、AL_2O_3、MgO、MnO含量较低,标型组分含量与沉积变质型磁铁矿颇为接近,在磁铁矿单矿物成因图解中,条带状磁铁矿整体显示磁铁矿为沉积变质型铁矿;浸染状矿石和块状矿石的组成与典型沉积变质型铁矿的偏离反映了后期岩浆-构造热事件对条带状铁矿石的改造;上述结果显示赞坎铁矿整体属于沉积变质型铁矿(BIF)。调查发现赞坎高品位铁矿体与早寒武世侵入的霏细斑岩联系密切,高品位矿石及其围岩发育一定程度的矽卡岩化,如阳起石化、碳酸盐化和黄铁矿化。本文推测高品位铁矿石的成因可能为霏细斑岩的岩浆热液溶解并运移早期沉积变质铁矿中的含铁物质,在构造发育处充填交代形成块状磁铁富矿石。在早寒武世侵入到矿区中部的霏细斑岩体中,同时发育有角砾状磁铁矿和脉状磁铁矿,因此,岩浆热液改造原生条带状铁矿石形成高品位铁矿石的时代应为早寒武世。

Geological characteristics, formation age and high-grade ore genesis of Zankan banded iron deposit in the West Kunlun Mountains

The large-scale Zankan magnetite-rich Fe deposit (belt) in the Taxkorgan ancient landmasses (West Kunlun Mountains), Xinjiang, northwestern China, contains 4 proven Fe commercial ore bodies with more than 2.5km long and 10~70 meters thick. High-grade ore bodies (mFe>50%) are 900 meters long and 40 meters thick. The deposit is hosted in the previously defined Paleoproterozoic Bulunkuole Group, a suite of sedimentary rocks experience greenshist-facies metamorphism. The Fe orebodies are interlayered with plagioclase-amphibole schist, biotite-quartz schist, granulite and siliceous rocks. Most Fe ores display metasomatic textures with banded, disseminated or massive structures. Based on geology and geochemical analyses we recognized primary ores and reformed ores. Primary ores are mostly banded. Reformed ores are massive (high-grade ore) and disseminated structures. Ce and Y elements had no clear anomaly (Ce/Ce^*=0.92~1.15, Y/Y^*=0.86~0.94) but positive Eu anomaly (Eu/Eu^*=1.29~1.69). The REE pattern in accord with sedimentary metamorphic iron; Reformed ores are mainly disseminated ores and massive ores. Disseminated ores are enriched in REE and enriched in LREE. La and Ce have positive anomaly (La/La^*=1.12~1.46, Ce/Ce^*=1.03~1.17). Y has no clear anomaly but strong positive anomaly in Eu (Y/Y^*=0.66~0.72, Eu/Eu^*=3.16~4.37). The U-Pb analyses on the detrital zircons from plagioclase-amphibole schist (metasediments) yielded a youngest age of 591±1Ma, coupled with the previous geochronological study to the intrusions in Zankan, further suggested that the depositional age of the Zankan deposit is from 591Ma to 533Ma. Electron microprobe analysis shows the composition such as TiO₂, Al₂O₃, MgO, MnO of magnetite is very few. Magnetite typomorphic studies indicate the Zankan deposit is attached to metamorphism sedimentary iron deposit (BIF) but influenced by the magmatic activation. The phenomenon of skarnization, antinolitization, carbonatization and pyritization in high-grade orebodies shows that primary ore dissolved by magmatic hydrothermalism, filling and metasomatic in fracture zone lead to the formation of high-grade massive ores. The felsophyre which carries Fe ore xenoliths and Fe veins yielded an age of Early Paleozoic, represent the age of the hydrothermal modification.