中国铝土矿成矿作用的物质来源与深时环境因素:进展与讨论

Provenance and deep-time environmental factors for bauxitization in China:Progress and discussion

铝土矿是工业上可供利用的铝矿石总称,可视为陆表化学风化作用的终极产物。作为一种战略性大宗矿产,当前我国铝土矿使用总量的60%以上依赖于进口铝土矿石,铝土矿及其伴生的其他金属资源对国家资源安全的重要性突出。铝土矿化作用、红土化作用、热带成壤作用是热带风化作用的不同表现形式,主控因素包括适宜的气候(温度、降水)、侵蚀速率、母岩、时间、植被、地形地貌等。三者之间存在联系,但区别也较为显著,主要体现在时间尺度和沉积作用复杂程度两方面。红土化作用或热带成壤作用可能包含在铝土矿化作用的某一阶段。中国铝土矿资源主要集中在晚古生代,在不同地区存在不同的物质来源,可概括为:稳定的板块内部原地-准原地物源、板块边缘异地火山活动物源、板块边缘造山带异地物源3种主要类型。物源成分则可能来自于下伏沉积岩基底或板块边缘火山弧物质,或者两者的混合供给。不同成矿物质来源的铝土矿中保存了特征明显的碎屑锆石年代学分布规律,且在一定程度上决定了不同地区铝土矿床的地球化学组成特征,其中TiO_(2)含量变化显著,可视为潜在的物源指标。中国晚古生代铝土矿化作用除特殊的大地构造背景外,古气候、海平面变化、古水文条件等深时环境因素也需加以考量。在古气候条件类似的情况下,通过使用逻辑回归模型计算,本文认为区域构造活动性是中国铝土矿形成的关键因素,决定了铝土矿成矿物质来源和构造抬升强度等成矿条件。海平面变化为次级因素,相对较低的海平面条件能够促使成矿母质更强烈的暴露和淋滤,形成优质的铝土矿资源。

Bauxite is the general term for industrially available aluminum ores,which can be regarded as the ultimate product of continental chemical weathering.As a kind of strategic major mineral resources,bauxite and its associated minerals are important for national resource security.At present,more than 60%of the total use of bauxite in China depends on imported bauxite ore,and the challenge of selfguaranteeing of resources is evident.Bauxitization,lateritization and tropical pedigenesis are different manifestations of the tropical weathering mechanism,and their main controlling factors are similar,including suitable climate(temperature and precipitation),erosion rate,parent rock,time,vegetation,and topography.There are connections among the three,but the differences are also significant,mainly reflected in the time scale and the complexity of sedimentation.Lateritization or tropical pedigenesis may be included in a certain stage of bauxitization.The important bauxite resources in China concentrated in Late Paleozoic,they have different provenances in different regions,which can be summarized as:stable autochthonous and quasi-autochthonous sources in the plate interior,allochthonous volcanic ash sources in the plate margin and allochthonous provenance from the orogenic belt on the plate margin.The provenance components may come from the underlying sedimentary rock basement or plate margin volcanic arc material,or a mixture of both.The bauxites from different parent materials preserve the traces of detrital zircon chronological distribution.Different provenances also determine the geochemical compositions of bauxite deposits in different regions.Among them,the TiO2 content changes significantly,which can be considered as a potential provenance identification indicator for bauxite deposits.In addition to the special tectonic background,deep-time environmental factors such as paleoclimate,sea-level change,and paleohydrological conditions also need to be considered in the Late Paleozoic bauxitization in China.In the case of stable paleoclimate conditions,this paper constrains the regional tectonic activity as the decisive factor for the formation of bauxite in China by using logistic regression model calculations.This factor determines the key bauxitization conditions such as the source of weathering minerals and the intensity of structural uplift.Sea-level change is the secondary factor.Relatively low sea-level conditions can promote more intense exposure and leaching of ore-forming parent materials,forming high-quality bauxite ores.