The Nkonko Ni-Cr bearing regolith in DRC: a study of petrography and geochemistry to understand serpentinization and weathering processes of mantle rock

IF 1 4区 地球科学 Q4 GEOSCIENCES, MULTIDISCIPLINARY Geosciences Journal Pub Date : 2024-01-18 DOI:10.1007/s12303-023-0037-2
Douxdoux Kumakele Makutu, Jung Hun Seo
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Abstract

Regolith-hosted Ni-Cr prospects in the Nkonko serpentinized massif are located near Kananga city in the Democratic Republic of Congo (DRC). This serpentinized massif displays an underground regolith profile divided into three main layers: (1) unweathered serpentinite bedrock, (2) saprolite serpentinites, and (3) the pedozone top layer. The bedrock serpentinites consist of serpentine (including lizardite and antigorite “garnierite”) as a major phase, with Cr-spinel and magnetite as a subsidiary phase. The rock contains rare pseudo-olivine relicts. Saprolite serpentinites contain serpentines (e.g., lizardite, chrysotile, antigorite “garnierite”) and subordinate ferrochromite and Cr-magnetite. These saprolite serpentinites are crosscut by veins of “garnierite” phase, talc, brucite, magnetite, magnesite, and chlorite. The pedozone primarily consists of lateritic soils with red and yellow limonite. Saprolite serpentinites exhibit relatively higher Al2O3, FeO, Fe2O3, and volatiles (LOI: loss on ignition) compared to bedrock serpentinites, while maintaining similar SiO2 contents. For trace elements, saprolite serpentinites contain elevated Cr (up to 5.4 wt%) and Ni (up to 2.0 wt%) compared to bedrock samples (Cr up to 0.9 and Ni up to 0.3 wt%). The REE concentrations are low (total REE of about 2.0 ppm) and remain unchanged between the saprolites and bedrocks. Spinel is identified as the main host mineral for Cr, while secondary serpentine “garnierite” is the primary host phase for Ni. Plots of Al2O3/SiO2 versus MgO/SiO2 for bedrock and saprolite serpentinites, along with the REE patterns, suggest that the serpentine regolith originated from the alteration and weathering of harzburgite.

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刚果民主共和国 Nkonko 含镍铬的碎屑岩:通过岩石学和地球化学研究了解地幔岩石的蛇纹石化和风化过程
位于刚果民主共和国卡南加市附近的 Nkonko 蛇化岩群中的辉绿岩型镍铬矿区。该蛇绿岩化地块的地下风化层主要分为三层:(1) 未风化蛇绿岩基岩,(2) 边岩蛇绿岩,(3) 块状顶层。基岩蛇纹石以蛇纹石(包括蜥蜴石和反闪长岩 "加尼耶石")为主相,Cr-闪长岩和磁铁矿为副相。岩石中含有罕见的假橄榄石遗迹。钠长石蛇纹岩含有蛇纹石(如蜥蜴石、温石棉、锑长石 "石榴石")以及从属的铁铬铁矿和铬磁铁矿。这些钠长蛇纹岩被 "嘎尼尔岩 "相、滑石、青金石、磁铁矿、菱镁矿和绿泥石脉交叉切割。岩层主要由红土与红色和黄色褐铁矿组成。与基岩蛇纹岩相比,边坡岩蛇纹岩的 Al2O3、FeO、Fe2O3 和挥发物(LOI:点火损失)含量相对较高,而 SiO2 含量保持相似。在微量元素方面,与基岩样品(铬含量最高为 0.9%,镍含量最高为 0.3%)相比,蛇纹石中的铬(最高为 5.4%)和镍(最高为 2.0%)含量较高。REE浓度较低(REE总量约为百万分之2.0),并且在边长岩和基岩之间保持不变。尖晶石被确定为铬的主要寄主矿物,而次生蛇纹石 "加尼耶石 "则是镍的主要寄主相。基岩和边长岩蛇纹石的 Al2O3/SiO2 与 MgO/SiO2 的对比图以及 REE 模式表明,蛇纹石雷公岩源于哈兹堡垒岩的蚀变和风化。
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来源期刊
Geosciences Journal
Geosciences Journal 地学-地球科学综合
CiteScore
2.70
自引率
8.30%
发文量
33
审稿时长
6 months
期刊介绍: Geosciences Journal opens a new era for the publication of geoscientific research articles in English, covering geology, geophysics, geochemistry, paleontology, structural geology, mineralogy, petrology, stratigraphy, sedimentology, environmental geology, economic geology, petroleum geology, hydrogeology, remote sensing and planetary geology.
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