白云鄂博地区好沁-尖山北逆冲推覆构造及其对碳酸岩深部延伸的制约

Haoqin-North Jianshan Thrust Fault and its constraints on the extension of ore-bearing carbonatite in the Bayan Obo area

白云鄂博是世界上最大的稀土矿,其资源储量影响着全球稀土资源配置的格局。稀土矿赋存于碳酸岩中,因钻孔深度所限,碳酸岩深部的延伸情况存在争议,是可以延伸较深的岩浆通道,还是根据三维剪切波速度结构研究结果显示的有限深度(约2.5km)。本文在地表展开矿区及周边岩性-构造填图和所卷入岩石的变形特征研究,将剖面上与平面上的构造样式进行对比,将有利于解决上述争议问题。本次研究在好沁、尖山北部、西矿北部一带发现大型逆冲断层,并将其命名为“好沁-尖山北逆冲断层”。该逆冲断层下盘为互层状的(粗)砂岩、灰岩和泥岩,粗砂岩的碎屑磷灰石下交点最年轻群组为1284±104Ma,砂岩的碎屑锆石最年轻组为1179±8Ma,均比白云鄂博碳酸岩的年龄(约1.3Ga)年轻;结合寒武-奥陶纪化石,厘定下盘地层为下古生界。断层上盘为变砾岩、变石英砂岩、板岩和赋矿碳酸岩,变石英砂岩的碎屑锆石存在1.94Ga和2.55Ga的2个峰值,认为其沉积于古元古代。好沁-尖山北逆冲断层将元古代变沉积岩及赋矿碳酸岩推覆于古生代地层之上,断层面上广泛发育断层泥和断层角砾,厚度自20cm至7m不等,沿断层面有含铁质流体灌入而形成赤铁矿,断层面倾角变化大,从水平到直立产状,可能是受后期褶皱影响的结果;结合卷入断层活动的最年轻地质体为二叠纪花岗岩、以及区域内白垩纪不整合沉积在断层及下伏地层之上,限定好沁-尖山北逆冲断层的活动时代发生于二叠纪之后、白垩纪之前。水源头出露有变石英砂岩飞来峰,白云鄂博矿区与南部约30km处发育相似的、4条NE-SW走向的雁列式排列的航磁异常带,它们限定逆冲推覆距离为8~30km。白云鄂博碳酸岩属于推覆体上盘地质体,好沁-尖山北逆冲断层将其与根部错开,白云鄂博地区的碳酸岩未延伸到断层下盘,下盘岩石可能是古生代岩石,因此,矿区深部的剪切波速度突变可能与该断层的发育相关。

The Bayan Obo deposit is the largest Rare Earth Element(REE)mine in the world,and its reservation affects the global REE resources.The REE resources are preserved in carbonatite,and due to limited drilling depth,the depth of the carbonatite body has been debated as a theoretical deep-extended magmatic vent or limited extension(ca.2.5km)revealed by a 3-dimensional S-wave velocity structure.To resolve this debate,we did a lithological-structural mapping of the Bayan Obo deposit and its surrounding areas to constrain map-view structures that can correlate structures in-depth.In this study,a megathrust has been found along Haoqin,North Jianshan,and Northwest Orebody,and it is termed as“Haoqin-North Jianshan Thrust Fault”.The footwall rocks of the deposit consists of interlayered sandstones,limestones and mudstones.The detrital apatite of the youngest group in sandstone yields a lower intersection age at 1284±104Ma,and the detrital zircon at 1179±8Ma,both of which are younger than the age of the Bayan Obo carbonatite bodies(ca.1.3Ga).Combined with the occurrence of Cambrian-Ordovician fossils in limestone and mudstone/chert,sediments in the footwall are constrained to be the Early Paleozoic.The hanging wall comprises of mylonitic conglomerate and quartz sandstone,slate and ore-bearing carbonatite.Among them,the quartz sandstone has two peaks of detrital zircons at 1.94Ga and 2.55Ga,and is intruded by ca.1.3Ga carbonatite,demonstrating that they were deposited in the Paleoproterozoic era.The Haoqin-North Jianshan Thrust Fault transports Proterozoic metasediments and ore-bearing carbonatite over the Paleozoic rocks.On the fault surface,fault gouge and breccia widely occur with thicknesses ranging from 20cm to 7m.Hematite is formed by iron-bearing fluid injected along the fault plane,and the dipping angle of the fault plane varies greatly from horizontal to vertical,which may be the result of the influence of late folding.In the Bayan Obo area,the thrust truncates Permian granite as the youngest geological body,and both have been unconformably covered by Cretaceous conglomerate and sandstone,constraining the age of the Haoqin-North Jianshan Thrust Fault to the Permian-Cretaceous period.According to the northmost Shuiyuantou klippe of meta-quartz sandstone,and similar four NE-SW-striking en-echelon aeromagnetic anomaly belts between the Bayan Obo deposit and that at about 30km to the south,the displacement of this thrust is constrained to be 8~30km.The Bayan Obo orebody-bearing carbonatite is located in the hanging wall of the fault,and is far transported from its root.It further demonstrates that the abrupt change of shear wave velocity in the Bayan Obo deposit may represent different rock units that have been juxtaposed by this thrust fault.