黔西北上二叠统峨眉山玄武岩风化壳中铌富集机制初探

近年越来越多的研究显示,贵州西北一带的峨眉山玄武岩风化壳上存在一套富铌、锆、钛、镓、稀土的多金属矿层,主要与风化壳中的黏土岩有关,不过,其中有关铌的赋存状态和富集机制仍有待研究.文章对黏土岩物质来源做出厘定,认为风化壳黏土岩层主要来源于高钛的峨眉山玄武岩,铌、锆、稀土等元素也继承自玄武岩.铌主要赋存于铁铝质黏土岩中(w(Nb)100×10-6～250×10-6),含矿岩层中上部铌含量高,下部低,具体表现为w(Nb)自下而上,升高—(略微)降低—(再)升高的规律,尤以铁铝质黏土岩中铌含量最高.在峨眉山玄武岩喷发事件中,铌一方面随玄武岩喷出地表,在平缓低洼的盆地地带冷凝,另一方面酸性火山灰进入大气,形成酸雨并随之降落汇聚在先期成岩的低洼地区,形成酸性环境使原岩内的铌被侵蚀残留在原地.随着火山作用停止,酸雨不再产生,沉积环境中的水体因热水循环、海侵海退和大气降水与外界物质交换,酸性逐渐被中和乃至偏碱性,铌随黏土质沉积成岩,最终在玄武岩风化壳内富集.

Preliminary study on the enrichment mechanism of niobium in clay layer of weathering crust of Upper Permian basalt in north western Guizhou

Recently, studies have shown that there is a set of Nb, Zr, Ti, Ga and REE polymetallic ore beds in the weathering crust of Permian Emeishan basalt in western Guizhou and eastern Yunnan, which mainly occur in the clay rock layers in the weathering crust. The research on rare earth is becoming more and more perfect, but little is known about the occurrence and enrichment mechanism of other rare metals such as NB. In this paper, the source of clay rock is determined by means of trace rare earth element geochemistry. It is believed that the clay rock in weathering crust mainly comes from Emeishan basalt with high titanium content, and niobium, zirconium and rare earth elements also inherit from Emeishan basalt. The content of niobium is high in the upper part of clay rock, but low in the area close to basalt in the lower part, which shows the tendency of increasing-(slightly) decreasing-(further) increasing from bottom to top. The content of niobium is the highest in ferro-aluminous clay rock. It is deduced that during the eruption of Emeishan basaltic magma, on the one hand, niobium-bearing magma erupted on the surface and formed a large number of basaltic rocks in the gentle and low-lying basin area; on the other hand, niobium rich volcanic ash entered the atmosphere and formed acid rain, which then fell and gathered in the low-lying area of early basalts, forming an acidic environment; niobium in the protolith was eroded and remained in place. With the cessation of volcanism, acid rain no longer occurs, and the water body in the sedimentary environment exchanges within the hydrothermal circulation, marine transgression-regression and atmospheric precipitation cycle. The acid environment is neutralized or even become alkaline, and niobium begins to precipitate and be absorbed in the clay layers of weathering products of igneous rock minerals,, niobium gradually accumulates in the clay layer over time.