豫西偃师龙门地区上石炭统本溪组含铝岩系矿物学特征及其原岩分析

Mineralogical Characteristics and Original Rock Analysis of Al-bearing Rock Series in Upper Carboniferous Benxi Formation in Longmen District,Yanshi,Western Henan

以华北陆块南部豫西偃师县龙门镇地区全取芯铝土矿钻孔ZK4704为主要研究对象,通过岩芯观察和垂向上连续取样,运用扫描电镜、能谱分析、X射线衍射分析、差热分析、红外光谱分析等手段对本溪组含铝岩系的矿物学特征及其垂向变化规律进行了分析。研究表明,偃龙地区本溪组含铝岩系的矿物均为自生矿物,除后期重结晶作用的影响外,主要以隐晶质或微晶存在,矿物成分在垂向上具有明显的变化规律:下部和上部泥岩以粘土矿物为主,但下部泥岩以伊利石为主,上部泥岩以高岭石为主。中部铝矿物含量较高,粘土矿物含量较少。在上述分析基础上,以化学风化作用的基本原理和产生的条件为联结本溪组含铝岩系矿物学特征和原岩的纽带,分析了铝土矿的原岩。认为,可以迅速水解的火山灰是最有可能的含铝岩系的原岩,而由弱碱性的海相环境逐渐转变为弱酸性沼泽环境的海退序列可以很好地解释水解过程中化学环境的变化,含铝岩系基底强烈的古岩溶作用,可以为硬水铝石的产生提供良好的泄水条件。偃龙地区本溪组铝土矿的火山灰物源可由华北陆块北缘和我国西部地区同时期强烈活动的火山作用提供。

Taking full coring ZK4704 as study sample, located in the southern margin of the North China Craton, Yanlong area, Western Henan, through core observation and vertical continuous sampling, the mineralogical characteristics and vertical varying pattern of bauxite mineral have been analyzed by rock thin section, SEM, EDS, X-ray diffraction, as well as differential thermal and infrared spectrum. The results show that all the minerals in Al-bearing rock series in Benxi Formation are authigenic minerals mainly existes in cryptocrystalline or microcrystalline except those affected by recrystallization during late stage. The mineral compositions and assemblages show significant variation in the vertical. Minerals in the lower and upper mudstone mainly are clay minerals. But the lower mudstone mainly consists of Illite, and the upper mudstone mainly consists of Kaolinite. Mierals in the middle between lower and upper mudstone mainly consist of aluminum minerals with a small number of clay minerals. On the basis of above, taking the basic principles and generation conditions of chemical weathering as the link of mineralogical characteristics of Al-bearing rock series and raw rock to analyze the original rock of bauxite, it is theorized with a high degree of probability that the Al-bearing rock series in Benxi Formation might be volcanic ash which could be rapidly hydrolyzed, the change of hydrolysed chemical environment can be explained by the transformation from weakly alkaline marine environment to weakly acid marsh environment, the strongly karstification of the basement under Al-bearing rock series provided well drainage condition for the formation of diaspore. Violent volcanic process in the northern margin of the North China Craton and China ＂s western region can provide ash provenance of Benxi Formation in Yanlong area.