铝土矿及伴生关键金属矿床的岩石(矿石)组成、成矿序列及成矿机理^*

Lithologic(ore)types, formation sequence and metallogenic mechanism of bauxite and associated critical metal deposits

摘要

大陆风化是地球表层系统中的关键过程,涉及多种能量形式(太阳能、风能、化学能和重力势能)和物理化学过程,是地球化学元素在地球不同层面(如岩石圈、水圈、生物圈和大气圈)之间迁移和重新分配的基础。在此过程中,异常的元素富集可形成工业级的风化淋滤矿床。风化淋滤矿床可进一步区分出风化矿床和沉积—淋滤型矿床2种类型。传统的矿床学和沉积学研究方法在理解风化淋滤矿床形成过程中存在局限性,特别是在解析成矿阶段和动态成矿过程方面仍不甚明晰,导致该类型矿床在基础研究和勘查方面长期存在亟待解决的一批问题。本研究在厘清大陆风化作用、风化壳(古风化壳)、土壤(古土壤)等概念的基础之上,对现代风化淋滤剖面(如广西贵港三水铝石矿床)和古老风化淋滤型矿床(如中国华南地区早石炭世—晚二叠世铝土矿床及沉积淋滤型稀土矿床)开展了系统的沉积学分析与对比工作。研究发现,风化淋滤矿床的结构组分主要包括碎屑、团粒与包粒、块状黏土等结构类型,分别对应碎屑状、包粒状、致密状矿石的形成。成岩过程主要依赖于氧化物矿物及黏土质的胶结与填隙作用。基于上述认识,提出了风化淋滤矿床的淋滤序列和剖面结构划分方案。该方案以风化剖面中渗流带与潜流带作为基本的划分依据,进一步区分出风化矿床序列与沉积—淋滤型矿床序列,两者主要以是否出现沉积单位作为主要识别标志。以古气候为主线分析了风化淋滤矿床的成因机理,并结合近年来质量平衡计算相关成果,总结了淋滤过程中元素迁移规律,将风化淋滤型矿床的成矿过程概括为成矿母质形成、成矿物质风化和后期改造3个阶段。

Abstract

Continental weathering represents a pivotal mechanism in the Earth’s surface system,encompassing a diverse range of energy forms,including solar,wind,chemical,and gravitational potential energy,alongside a series of intricate physical and chemical processes. This multifaceted process facilitates migration and redistribution of geochemical elements across different Earth subsystems(e.g.,lithosphere,hydrosphere,biosphere,and atmosphere). During this transformation,anomalous elemental enrichment can lead to the formation of industrial-scale weathering-leaching deposits,which can be categorized into two types: weathering deposits and sedimentary-leaching deposits. However,traditional approaches in mineral deposit studies and sedimentology face limitations in understanding the stages and dynamics of mineralization,leading to unresolved issues in both fundamental research and exploration. To address these exigent issues,this study delineates the fundamentals of continental weathering,as well as the concepts of weathering crust(including paleoweathering crust)and soil(extending to paleosol),providing a comprehensive analysis of modern and ancient weathering-leaching profiles. We conducts a meticulous sedimentological analysis and comparison of profiles examing modern weathering and leaching profile found in the gibbsite deposit in Guigang,Guangxi,in contrast with ancient deposits l from the Early Carboniferous-Late Permian bauxite deposits and sedimentary leaching Rare Earth Element(REE)deposits in Southwest China. Our findings underscore that weathering-leaching deposits primarily consist of structural components such as clastic,aggregate and coating grains,massive clay. These correlate to the genesis of clastic,pelletizing,and massive ores within weathering-leaching deposits. Diagenetic processes are largely driven by the cementation and infilling activities of oxide minerals and clays. Drawing upon these conclusions,the study proposes a classification framework for weathering-leaching deposits,where the vadose and phreatic zones within the weathering profile serve as foundational criteria for division. This framework further discriminates between the weathering ore sequence and sedimentary-leaching ore sequence,primarily based on the presence or absence of sedimentary units. By integrating paleoclimatic analysis and recent advances in mass balance calculations of element migration during the leaching process,this study elucidates the mechanisms of weathering-leaching deposit formation,summarizing the process into three phases: (1)the formation of ore-forming parent material,(2)subsequent weathering of these metallogenic substances,and(3)later stages transformations. This in-depth exploration promotes a nuanced understanding of the intricate processes underlining continental weathering,paving the way for future research avenues in this pivotal facet of Earth’s surface system.

关键词

大陆风化 / 铝土矿 / 关键金属 / 成矿序列 / 成矿机制

Key words

continental weathering / bauxite / critical metal / metallogenic sequence / metallogenic mechanism

基金资助

*国家重点研发计划项目(编号: 2022YFF0800200),国家自然科学基金项目(编号: U1812402),贵州省科技厅重大科技工程项目(编号: 黔科合找矿战略[2022]ZD004)联合资助