黔北旦坪铝土矿床胶体包粒成因及其形成时间估算^*

Genesis of colloidal pellets of Danping bauxite deposit in northern Guizhou Province and estimation of their mineralization duration

摘要

世界大多数铝土矿床中均发育有同心层状的颗粒结构,其生长过程记录了铝土矿成矿相关信息,具备重建铝土矿成矿环境的潜力。区别于碳酸盐岩鲕粒,对铝土矿床中这种类似鲕粒的多圈层包粒结构的研究还不够深入,其基本的定义、成因与生长时限均不明确,亟待进一步研究。针对这一问题,结合现代热带氧化土剖面中淀积黏化层产出包粒结构这一特点,对黔北务川—正安—道真(务正道)地区旦坪矿区下二叠统铝土矿开展了相关研究,提出将铝土矿同心层状结构命名为“胶体包粒结构”。本研究在对旦坪铝土矿床典型含矿岩系剖面地质特征研究基础上,重点对其中的胶体包粒开展了研究。旦坪铝土矿中的胶体包粒由不同类型的核心及周围交替出现的铝赤铁矿、勃姆石圈层组成。采用二分值法对包粒圆度进行了统计,分析结果显示其圆度以圆形、近圆形为主。归一化处理包粒圈层范围为11.2~319.5 μm。对胶体包粒圈层数目的统计发现,其层数以1~3层为主。基于以上统计数据,研究使用Fick第一定律及其拓展式对铝土矿胶体包粒生长时间进行估算,并尝试构建其扩散生长模型。胶体包粒生长时间计算结果表明: 胶体包粒内铝赤铁矿圈层的形成时间为0.1~96.9年,勃姆石圈层的形成时间为0.2~143.6年。微观尺度胶体包粒(粒径0.1~0.4 mm)的形成时间约为0.6~481.2年; 手标本尺度胶体包粒(粒径2.0~5.0 mm)的形成时间为1178~7364年。预估铝土矿中各尺度胶体包粒(粒径0.1~5.0 mm)的形成时间为0.6~7364年。以上研究结果表明,胶体包粒结构与铝土矿形成过程中淋滤作用之间存在较为复杂的联系: 互层状圈层的形成指示了旦坪地区含铝土矿地层中胶体包粒的周期性凝聚作用,从微观上刻画了周期性的成矿节律。胶体包粒形成过程中铝的地球化学行为可能是主导因素,其行为特征主要受到风化剖面中有机酸浓度、氧化还原条件、圈层矿物表面电荷属性等关键控制条件对羟基铝和交换铝的吸附富集过程的影响。

Abstract

In most bauxite deposits around the world,concentric colloidal pellet structures are commonly observed. The growth process of these structures records valuable information about the mineralization of bauxite,offering the potential to reconstruct the bauxite-forming environment. Unlike the more extensively studied oolitic structures in carbonate deposits,the research on similar multilayered pellet structures in bauxite deposits is still limited. The basic definition,formation mechanism,and time constraints of these structures remain unclear,and thus require further investigation. In view of this issue and the character of pelleted structure in the deposition-cementation layers of modern tropical laterite profiles,this research was carried out on the Lower Permian bauxite deposits in the Danping mining area of Wuchuan-Zheng’an-Daozhen region in northern Guizhou,China. The concentric layered structure of bauxite deposits is proposed here to be named as “colloidal pelleted structure”. On the basis of the geological characteristics of ore-bearing lithostratigraphic profiles in the Danping bauxite deposit,this study focuses on investigating the colloidal pellet structures. The colloidal pellets in the Danping bauxite deposit consist of alternating layers of different types of core and surrounding alternating Al-hematite and boehmite minerals. The sphericity of the colloidal pellets was statistically analyzed using the dichotomy,which showed that they are predominantly circular or nearly circular. The normalized pellet layers range from 11.2 to 319.5 μm in thickness. According to the statistics of the number of pellet layers,the majority are around 1-3 layers. Based on the above statistical data,Fick’s first law and its extended form were used to estimate the growth time of the colloidal pellets,and construction of a diffusion growth model was attempted. The results show that the formation time of the Al-hematite layers within the pellets ranges from 0.1 to 96.9 years,while the formation time of the boehmite layers ranges from 0.2 to 143.6 years. The formation time of the microscopic-scale colloidal pellets(0.1~0.4 mm)and hand-sized colloidal pallets(2.0~5.0 mm)is estimated to be around 0.6 to 481.2 years and 1178 to 7364 years,respectively. Furthermore,the estimated formation time of colloidal pellets of various sizes(0.1~5.0 mm)in bauxite deposits is 0.6 to 7364 years. These findings indicate that there is a complex relationship between colloidal pellet structures and leaching processes during bauxite formation. The formation of interlayered concentric structures indicates the periodic agglomeration of colloidal pellets within the aluminum-bearing strata in the Danping area,providing a microscopic depiction of the periodic ore-forming rhythm. The geochemical behavior of aluminum during the formation of colloidal pellets may be the dominant factor,and it is influenced by key control conditions such as the concentration of organic acids in weathering profiles,Redox Conditions and the surface charge properties of layered minerals affecting the adsorption and enrichment processes of hydroxyaluminum and exchangeable aluminum.

关键词

淋滤作用 / Fick第一定律 / 扩散系数 / 风化剖面

Key words

leaching process / Fick’s first law / diffusion coefficient / weathering profile

基金资助

*国家重点研发计划项目“植物登陆的环境资源效应”(编号: 2022YFF0800200)和国家自然科学基金项目“中国西南(贵州)喀斯特地区特色矿产成矿理论及综合利用”(编号: U1812402)联合资助