Journal of South American Earth Sciences ( IF 1.704 ) Pub Date : 2021-01-13 , DOI: 10.1016/j.jsames.2021.103164 Leonardo Boiadeiro Ayres Negrão; Marcondes Lima da Costa
A bauxite-bearing lateritic profile, developed along a borehole within the supergenic Fe-deposits of the Carajás Mineral Province in Brazil, is investigated after its textural aspects, mineralogy, and geochemistry. Petrographic studies, XRD mineralogical analysis, SEM-EDS, and whole-rock chemistry were used to study selected samples. The 156 m depth borehole is composed of basaltic andesite at the substratum and an overlying lateritic profile well-structured in distinct horizons, from base to top composed of: a saprolite, with rock relicts within a clayey matrix of mainly vermiculite and kaolinite; a clayey horizon dominated by kaolinite; a bauxitic horizon with gibbsite as the main phase; a ferruginous crust mainly composed of hematite and goethite. XRD mineralogical quantifications indirectly confirmed the chemistry of Si, Al, Fe, and Ti, as the most abundant elements in the lateritic horizons. Statistical factor analysis of the whole chemistry and mass balance calculations showed groups of elements arranged according to their enrichment or depletion character in the distinct horizons. Si, Ca, Na, and K are leached out to the top of the profile following the destabilization of their carrier mineral phases feldspar, hornblende, and clinochlore, whereas Al, Fe, and Ti are enriched to the upper horizons, respectively in the structure of gibbsite, hematite/goethite, and anatase. The basaltic andesite at the substratum has its parental affinity confirmed after textural changes, REE normalized patterns, and Zr/Ti ratios throughout the profile, nevertheless, parent rock variations are expected. The studied lateritic profile is comparable to most of the bauxite-bearing laterites in the Amazon region, except for its unparalleled 22 m thick Fe-rich bauxitic horizon and absence of a truncating clay cover on the top of the profile. The lateritic sequence was probably formed during the Paleocene-Eocene bauxitization event that reached the Amazon region and survived as a complete unaltered fossil lateritic profile through the time.
经过结构，矿物学和地球化学方面的研究，研究了沿巴西卡拉加斯矿产省的超铁矿床中的一个钻孔发育的含铝土红土剖面。岩石学研究，XRD矿物学分析，SEM-EDS和全岩石化学研究了选定的样品。156 m深的钻孔由基底的玄武质安山岩和上覆的红土剖面组成，在不同的层段中构造良好，从底部到顶部，由以下组成：腐泥土，在主要由ver石和高岭石组成的粘土质基体中有岩浆。以高岭石为主的粘土层；以三水铝铁矿为主要相的铝土质层；主要由赤铁矿和针铁矿组成的铁质外壳。XRD矿物学定量分析间接证实了Si，Al，Fe和Ti的化学性质，是红土层中最丰富的元素。整个化学反应的统计因素分析和质量平衡计算表明，在不同的视野中，元素按照其富集或耗尽特性排列。硅，钙，钠和钾在其运载体矿物相长石，角闪石和斜绿石失稳后被浸出到剖面的顶部，而铝，铁和钛分别富集到上部结构中三水铝石，赤铁矿/针铁矿和锐钛矿。在整个剖面中，质地变化，REE归一化模式和Zr / Ti比值确定后，基底上的玄武质安山岩的亲代亲和力得到了确认。所研究的红土剖面可与亚马逊地区的大多数含铝土矿的红土媲美，除了其无与伦比的22 m厚的富铁铝土层，并且剖面顶部没有截断的粘土覆盖。红土序列可能是在古新世—始新世的铝土化事件中形成的，该事件到达了亚马逊地区，并一直作为完整的未改变的化石红土剖面而幸存下来。