川南沐川地区宣威组底部铌-稀土多金属富集层富集规律、沉积环境与成矿模式

Enrichment regularity, sedimentary environment and metallogenic model ofniobium-rare earth polymetallic enrichment layer at the bottom of the XuanweiFormation in Muchuan area, south Sichuan

云南、贵州地区对晚二叠世宣威组底部Nb-REE多金属富集层的研究取得了重要进展,但其成因还有较大争议。为厘清沐川地区宣威组底部Nb-REE多金属富集层成因机制及富集规律,本研究开展了野外实地调查、矿物学、岩相古地理与岩石地球化学等系统性研究,探讨其物源、富集规律,建立了成矿模式。结果表明,川南沐川地区宣威组底部Nb-REE多金属富集层产出于宣威组底部,厚度介于5.09~15.33 m之间,平均厚度10.10 m,铌氧化物(Nb;O;)含量为37.0~909μg/g,平均256μg/g,稀土总量为200~15500μg/g,平均1200μg/g;独立的稀土矿物极少,稀土元素的赋存状态可能是以类质同象替代的形式赋存于黏土矿物中为主,未发现独立的铌矿物,铌元素的富集可能主要与锐钛矿有关;下部紫红色铁质泥岩属于残积平原相,其上覆的灰白色、浅灰绿色铝质泥岩属于较平静的滨湖亚相环境,上部的灰色泥岩、深灰色碳质泥岩属于较平静的浅湖亚相环境;下部紫红色铁质泥岩Nb-REE多金属富集层的物源主要来源于峨眉山玄武岩,上部另外峨眉山地幔柱消亡阶段喷发的碱性火山灰也可能提供了部分物源;Nb-REE多金属富集层中的稀土总量及铌元素含量总体上呈"先增加、再降低"的变化规律,稀土元素在紫红色铁质泥岩中的富集程度比铌元素高,铌、稀土元素含量变化曲线在上部的灰白色、浅灰绿色、灰色、深灰色泥岩中表现出"同增同减"的规律;沐川地区Nb-REE多金属富集层的成矿模式可分为峨眉山玄武岩喷溢阶段、风化富集阶段、沉积富集阶段和盖层沉积阶段等4个阶段,成矿作用以风化淋滤和沉积作用为主;以上成果对西南地区关键金属找矿和补充完善峨眉山大火成岩成矿系统具有重要意义。

Though significant research progress has been made on REE in the Nb-REE polymetallic enrichment layer in Yunnan, in the Guizhou region of Late Permian Xuanwei Formation, several larger questions remain to be addressed. This study focusses the bottom of the Xuanwei Formation Nb-REE polymetallic enrichmentin the Muchuan area to investigate its formation mechanism and enrichment regularity. Field investigations, systemic mineralogical, lithofacies, paleogeographic, and rock geochemistry studies have been undertaken to discuss the content source and enrichment regularity to establish a metallogenic model. The results showed that the Nb-REE polymetallic enrichment layer at the bottom of the Xuanwei Formation in the Muchuan area, southern Sichuan has a thickness of 5.09~15.33 m and an average thickness of 10.10 m. The content of niobium oxide(Nb;O;) was 37.0~909 μg/g, with an average of 256 μg/g. The total amount of rare earth was 200~15500 μg/g, with an average of 1200 μg/g. The provenance of Nb-REE polymetallic enrichment layer mainly comes from the basalt of Mount Emei. In addition, the alkaline volcanic ash ejected at the mantle plume stage of Mount Emei may also provide some provenance. Independent rare earth minerals are rare, and the occurrence state of rare earth elements may be mainly in clay minerals in the form of similar-image substitution. No independent niobium minerals have been found, and the enrichment of niobium may be mainly related to anatase. The lower purple red iron mudstone belongs to the eluvial plain facies, the overlying gray white and light gray green aluminum mudstone belongs to the relatively calm lakeside subfacies environment, while the upper gray mudstone and dark gray carbonaceous mudstone belong to the relatively calm shallow lake subfacies environment. The enrichment degree of rare earth elements in purplish red ferritic mudstone is higher than that of niobium.The variation curve of niobium and rare earth element content in the upper gray white, light gray green, gray and dark gray mudstone shows the rule of "increase with the same decrease".The metallogenic model of Nb-REE polymetallic enrichment layer in Muchuan area can be divided into four stages: Emeishan basalt gusher stage, weathering enrichment stage, sedimentary accumulation stage and capping deposition stage. The mineralization is dominated by weathering leaching and deposition. The above results are of great significance to the key metal prospecting and the improvement of the Emeishan igneous mineralization system in Southwest China.