A supergene-hydrothermal origin of the itabirite-hosted high-grade iron ores in the Mbarga prospect, Mbalam iron ore district, southern Cameroon, Congo Craton

IF 3.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
George Lemewihbwen Ngiamte , Eleanor C.R. Green , Olugbenga Akindeji Okunlola , Roland Maas , Alan Greig , Cheo Emmanuel Suh
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Abstract

The Mbarga itabirite deposit in the Mbalam iron district on the northwest edge of the Congo Craton (CC) hosts two main types of iron ore enrichments: supergene and specularite ores. This study presents mineralogical, geochemical, and isotopic datasets on these ores to determine their genesis.

Ore microscopic studies indicate that the itabirites are of the oxide facies type, with magnetite showing partial to extensive alteration to hematite-martite. The supergene ores consist of hematite + martite + goethite ± gibbsite ± magnetite ± quartz, while the specularite ores are mainly composed of hematite + martite ± quartz. Magnetite microchemistry suggests formation under low-T hydrothermal conditions (~200–300 °C) with high fO2. Geochemical analyses show that the supergene and specularite ores have higher Fe2O3 (88.27 to ~100 wt%) and lower SiO2 (<0.01 to 0.18 wt%) contents than the itabirites (31.95 wt% Fe2O3, 67.16 wt% SiO2). The enrichment of Fe in the supergene ores is attributed to the depletion of major oxides and trace elements due to weathering and supergene enrichment, while the high Fe content in the specularite ores stems from the precipitation of iron-rich, but trace- and rare earth elements (REE)-deficient hydrothermal fluids. The slightly higher Al2O3 content and positive Ce anomalies in the supergene ores suggest the retention of Al-bearing minerals (gibbsite) and reveal highly oxidative conditions during martitization. Stable isotope analyses reveal that the supergene and specularite ores have δ18O values of −2.5 to −0.3 ‰ and − 2.0 to −3.4 ‰, and δ2H values of −75 to −123 ‰ and − 70 to −119 ‰, respectively, suggesting the involvement of isotopically light-evolved meteoric water in their formation. In contrast, the itabirites exhibit heavier δ18O (8.5 to 10.2 ‰) and δ2H (−85 to −91 ‰) values, suggesting formation from mixed magmatic and metamorphic fluid sources. A “polygenic-supergene-hydrothermal” model is suggested for the formation of the Mbarga itabirite-hosted iron ores.

刚果克拉通喀麦隆南部姆巴拉姆铁矿区姆巴加探矿区伊塔比里特矿床高品位铁矿石的超生热液成因
刚果克拉通(CC)西北边缘姆巴拉姆(Mbalam)铁矿区的姆巴加(Mbarga)itabirite矿床富含两种主要类型的铁矿石:超生铁矿石和镜铁矿石。矿石显微镜研究表明,itabirites 属于氧化物面类型,磁铁矿部分至大面积蚀变为赤铁矿-方铁矿。超级基因矿石由赤铁矿 + 马氏体 + 戈氏体 ± 赤铁矿 ± 磁铁矿 ± 石英组成,而镜长石矿石主要由赤铁矿 + 马氏体 ± 石英组成。磁铁矿的微观化学成分表明是在高 fO2 的低 T 热液条件下(约 200-300 ℃)形成的。地球化学分析表明,与itabirites(Fe2O3 含量为 31.95 wt%,SiO2 含量为 67.16 wt%)相比,超级基因矿石和镜铁矿石的 Fe2O3 含量较高(88.27 至 ~100 wt%),SiO2 含量较低(<0.01 至 0.18 wt%)。超生矿石中铁的富集是由于风化和超生富集造成的主要氧化物和微量元素的损耗,而镜长石矿石中铁的高含量是由于富含铁但微量元素和稀土元素(REE)缺乏的热液的沉淀。超生矿石中的 Al2O3 含量略高,Ce 异常值为正,这表明含铝矿物(吉比特)被保留下来,并揭示了马氏体化过程中的高度氧化条件。稳定同位素分析表明,超基性矿石和镜铁矿石的δ18O值分别为-2.5至-0.3‰和-2.0至-3.4‰,δ2H值分别为-75至-123‰和-70至-119‰,表明它们的形成与同位素轻演化的陨石水有关。相比之下,迭部岩的δ18O(8.5至10.2‰)和δ2H(-85至-91‰)值较重,表明它们是由岩浆流体和变质流体混合形成的。建议采用 "多源-超源-热液 "模式来形成姆巴加伊塔比里特矿床铁矿石。
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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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