黔北务正道地区铝土矿鲕状矿石中鲕粒的微区元素地球化学特征及其成矿意义

Microscopic Geochemical Characteristics of Oolite in Oolitic Bauxite Ores from Wuchuan-Zheng′an-Daozhen Area in the Northern Guizhou Province and Their Metallogenic Significance

铝土矿矿床中普遍含有不同比例的鲕状矿石,但目前对这些鲕粒的成因及其与成矿作用相互关系的了解相对有限。对贵州务正道地区铝土矿矿床中典型鲕状矿石中鲕粒进行了微区原位主量元素电子探针分析(EPMA)和激光剥蚀等离子体质谱(LA-ICP-MS)主量和微量元素分析,并与共存基质的元素组成进行了对比。研究表明,鲕粒的形成时间晚于基质,地表化学风化作用本身并不能导致铝土矿的形成;鲕粒的形成发生于强氧化和高酸度的环境,其成因被解释为在海进过程中在地层中一定的深度条件下形成了以黄铁矿为主的硫化物矿物,随后的海退作用过程导致硫化物蚀变并形成强酸环境,使得成矿元素Al相对于杂质元素(Si、Fe)发生相对富集,且该过程具有多期次振荡重复的特点。铝土矿形成是热带湿热气候条件下周期性海进-海退循环与长期的大陆古隆起区平坦海岸地形相互耦合的结果。

Varied proportions of pisolitic-oolitic ore are recognized in most bauxite deposits, but the genesis of this texture and its relationship with Al-mineralization remain controversial. This paper reports a com- prehensive geochemical study of typical oolitic ores of the bauxite deposits from the.Wuchuan-Zheng^an- Daozhen area, northern Guizhou Province and uses methods of in situ major element analysis by EPMA and major- and trace element analyses by laser ablation plasma mass spectrometry. It shows that the ooids were epigenetic relative to their matrix, and it is highly unlikely that the bauxitization was solely resulted from surface chemical weathering of laterized sedimentary profiles. By contrast, geochemistry of oolitic textures infers that the ooids are formed in strongly oxidizing and highly acidic environment. Coupled with documented whole rock data and field observation, a genetic model is then suggested. A large amount of pyrite was formed during seawater transgression upon laterized profiles and distributed at a broad coastal plain. The pyrites were constrained to have formed at a middle depth range of the profiles due to variation of oxygen fugacity. Oxidation and decomposition of pyrite during regression resulted in strong sulfuric acid aqueous solution, which carried out a large quantity of matrix elements （e. g. Si and Fe） and advanced bauxitization. This process is featured by multiple recycling. In fact, the bauxitization is a coupling result of periodically transgression-regression recycling of seawater and laterized profiles locating at broad coastal plain of long-term uplifted continental margin under tropical humid condition.