巴西 AM 省 Morro dos Seis Lagos 红土结壳与碳酸盐岩之间的地球化学联系

IF 3.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Ana Carolina de Faria Duarte , Claudio Gerheim Porto , Artur Cezar Bastos Neto , Reiner Neumann , Lucy Takehara , João Pedro Proença Bento
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引用次数: 0

摘要

塞斯拉各斯碳酸盐岩群(SLCC)中的碳酸盐岩属于菱铁矿碳酸盐岩,是覆盖该碳酸盐岩群的红土地壳中蕴藏的榍石-黑云母矿化物的来源。这项研究基于巴西地质调查局(CPRM)在 20 世纪 80 年代进行的 6 个钻孔的地球化学数据,以及对这些岩芯进行的矿物学描述和分析。对地球化学数据的质量进行了评估,以便选择最可靠的元素进行统计处理。鉴于地球化学数据的成分特点,这些数据被转换为中心对数比(CLR),并提交给主成分分析。通过这些数据,确定了 4 类碳酸盐岩的地球化学关联:(i) C1,菱铁矿碳酸盐岩;(ii) C2,易碎菱铁矿碳酸盐岩;(iii) C3,轻菱铁矿碳酸盐岩;(iv) B1,碳酸盐岩角砾岩。B1 的磷化程度较高,富含 Al、P、La、Ce、Ba、Sr、Zr、S、Be、U 和 REE 矿物,如 bastnaesite-(Ce)、monazite-(Ce),以及在较晚的碳氢热液阶段形成的属于铅云母类的矿物(如 florencite-(Ce))。由于 C1 之后的热液蚀变作用,C3 也富含同类金属。从 C1 到易碎等同物 C2 的过渡更为突然,这表明风化作用在深度上有明确的界限,并可能受到结构的限制。C2 的特征是 Th、Zn、Sn、Co 和 Sb 的富集,同时受到 Mn 和 Fe 的一些影响,以及 Ce、Ba、La 和 Sr 的浸出。红土结壳可分为五种类型:(i) L1,破碎的洞穴状结壳;(ii) L2,红褐色结壳;(iii) L3,锰结壳;(iv) L4,灰色结壳;(v) L5,紧密的灰白色结壳。L1 结壳富含 Al、P、Zr、La 和 U,这是由于其不动元素的残余富集以及次生铝磷酸盐(主要是萤石(Ce))的形成。L3 锰结壳的特征是由 Mn、Ba、Mo、Co 和 Ce 组成的一整套金属。上部和下部结壳之间的成分对比更可能是由于菱铁矿碳酸盐岩的成分差异造成的,而不是反映了风化的深度和强度。铌钛矿化可能与哪种特定类型的结壳有关,尚未确定。对于较深的结壳,如 L4 和 L5,没有发现特定的地球化学关联;但是,这些结壳中出现了富含铈的辉绿岩、铈镧矿和碳酸盐,而上部结壳中却没有,这表明上述结壳的演化程度较低,因此更接近碳酸盐岩原岩。我们的研究结果表明,尽管风化作用强烈,但原生菱铁矿碳酸盐岩的成分对红土结壳的成分有影响。不过,我们无法确定结壳与菱铁矿碳酸盐岩之间直接的、取决于岩性的关系。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Geochemical associations between the lateritic crusts and carbonatitic rocks of the Carbonatitic Complex at Morro dos Seis Lagos, AM, Brazil

The carbonatite rocks present at the Seis Lagos Carbonatite Complex (SLCC) are siderite carbonatites, and constitute the source of the titano-niobian mineralization hosted in the lateritic crusts that cover the complex. This study is based on the geochemical data from 6 drill holes conducted by the Geological Survey of Brazil (CPRM) in the 1980s plus the mineralogical descriptions and analyzes performed on these cores. The quality of the geochemical data was evaluated in order to select the most reliable elements for statistical treatment. Given the compositional character of the geochemical data, these were transformed into Centered Log Ratio (CLR) and submitted to principal component analysis. These data enabled definition of geochemical association of 4 types of carbonatite rock: (i) C1, siderite carbonatite; (ii) C2, friable siderite carbonatite; (iii) C3, light siderite carbonatite; and (iv) B1, carbonatite breccia. B1 is more phosphatic and enriched in Al, P, La, Ce, Ba, Sr, Zr, S, Be, U and REE minerals like bastnaesite-(Ce), monazite-(Ce), as well as those belonging to the plumbogummite group (such as florencite-(Ce)) — which were formed during a later carbohydrothermal stage. C3 is also enriched in the same suite of metals as a result of hydrothermal alteration after C1. The transition from C1 to its friable equivalent, C2, is more abrupt, suggesting that the weathering took place with well-defined limits in depth and was possibly delimited by structures. C2 is characterized by its enrichment in Th, Zn, Sn, Co, and Sb, with some influence from both Mn and Fe, and the leaching of Ce, Ba, La, and Sr. C2 contains goethite, rutile, brookite and gibbsite, evidencing the impact of weathering. Lateritic crusts are distinguished into 5 types: (i) L1, a fragmented cavernous crust; (ii) L2, a reddish-brown crust; (iii) L3, a manganese crust; (iv) L4, a gray crust; and (v) L5, a compact, grayish crust. The L1 crusts are richer in Al, P, Zr, La and U due to the residual enrichment of their immobile elements and to the formation of secondary aluminum phosphates, mainly florencite-(Ce). The L3 manganese crusts are characterized by a suite of metals, composed of Mn, Ba, Mo, Co and Ce. The contrasting compositions between segments of the upper and lower crusts it is more likely due to compositional differences in the siderite carbonatite, rather than reflecting the depth and hence intensity of weathering. A specific type of crust to which the NbTi mineralization might be associated with was not identified. No specific geochemical association was identified for the deeper crusts, such as L4 and L5; however, the occurrence of Ce-rich pyrochlore, cerianite and carbonates in them, coupled with their absence in the upper crusts, suggests that the aforementioned ones are less evolved and therefore, closer to the carbonatite protolith. Our results indicate that despite intense weathering the composition of the primary siderite carbonatite exerts an influence on the composition of the lateritic crusts. However, it was not possible to establish a direct, lithodependent relationship between the crusts and the siderite carbonatites.

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来源期刊
Journal of Geochemical Exploration
Journal of Geochemical Exploration 地学-地球化学与地球物理
CiteScore
7.40
自引率
7.70%
发文量
148
审稿时长
8.1 months
期刊介绍: Journal of Geochemical Exploration is mostly dedicated to publication of original studies in exploration and environmental geochemistry and related topics. Contributions considered of prevalent interest for the journal include researches based on the application of innovative methods to: define the genesis and the evolution of mineral deposits including transfer of elements in large-scale mineralized areas. analyze complex systems at the boundaries between bio-geochemistry, metal transport and mineral accumulation. evaluate effects of historical mining activities on the surface environment. trace pollutant sources and define their fate and transport models in the near-surface and surface environments involving solid, fluid and aerial matrices. assess and quantify natural and technogenic radioactivity in the environment. determine geochemical anomalies and set baseline reference values using compositional data analysis, multivariate statistics and geo-spatial analysis. assess the impacts of anthropogenic contamination on ecosystems and human health at local and regional scale to prioritize and classify risks through deterministic and stochastic approaches. Papers dedicated to the presentation of newly developed methods in analytical geochemistry to be applied in the field or in laboratory are also within the topics of interest for the journal.
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