Tento přispěvek přinasi informace o geneticky kompletni radě
ložisek prvků ze skupiny vzacných zemin (REE) v Mongolsku;
konkretně vazaných na karbonatity (ložiska Mushgai Khudag,
Bayan Khushu, Khotgor, Cogt Obo, Ulugei Khid, Lugiin Gol, Omnot
Olgii, Khurimt Khad Tolgod) a peralkalicke granity se Zr-Nb-REE
mineralizaci (Khaldzan Buregtei a Khan Bogd) a metasomatity s
nejasnou genezi (Bomin Khara, Gzarta Hudag).
关于现今板块构造体制何时启动是目前地球科学研究的焦点问题。本文在原报道的古元古代丰镇火成碳酸岩中发现的榴辉岩捕虏体基础上,开展了详细的岩石学研究。该榴辉岩捕虏体分为两种类型:即相对富石榴石的Fz-2和贫石榴石的Fz-16,它们产于同一地点,且具有相同的矿物成分和结构构造特征。Xu et al.(2018)的研究表明该捕虏体具有1839±26Ma和1766±7Ma的独居石U-Th-Pb年龄且具有大洋辉长岩原岩的全岩成分特征。本文通过进一步的岩相学研究发现该榴辉岩至少经历了两期变质阶段:M1,角闪石/绿帘石-榴辉岩阶段;M2,硬柱石-榴辉岩阶段。具有放射状裂纹包裹特征的柯英石假象在石榴石变斑晶和基质绿辉石中以包体形式出现。以蓝晶石与黝帘石共存为特征的柱状硬柱石假象,也偶尔以包体形式存在于石榴石中。变斑晶石榴石分为富含包体的核部和比较干净的边部。石榴石从中心到边部具有明显的镁铝榴石含量增加和钙铝榴石含量降低的环带特征,通过相平衡模拟和等值线投图得到其温压范围为2.6~3.7GPa和655~670℃,记录了从M1到M2的近等温增压的进变质过程。通过石榴石边部-绿辉石-蓝晶石-石英的地质温压计计算得到温压条件为3.0GPa、734℃。金红石中的锆含量温度计也给出了相似的温度条件,即在2.6~3.7GPa压力时为601~685℃。石榴石边部的柯石英假象和硬柱石假象支持了M2硬柱石-榴辉岩阶段的存在,这表明丰镇古元古代榴辉岩可能是目前发现的世界上最古老的低温超高压变质岩。同时,我们得到该榴辉岩代表的进变质过程中的地温梯度为216±35℃/GPa,证明至少在~1.8Ga以来代表现今板块构造体制的板块冷俯冲作用就开始启动了。
The placer type ilmenite and ion-adsorption type rare earth elements (iREEs) deposits are two important weathering-related mineral resources in South China, a global supplier of Ti and REEs critical metals. It was believed that these two deposits are not linked to each other because of their different ore-forming conditions and mechanisms. However, many studies have reported that parts of placer ilmenite deposits, especially for those developed by weathering of intermediate igneous rocks, also contain abundant iREEs resources that have not been noticed before. To elucidate the origin of this interesting co-mineralization phenomenon that is of both scientific and economic significance, we conducted a comparative study of three deposits, including Qinghu (QH), Madi (MD), and Tangcun (TC) from Guangxi Province in the western segment of South China. Investigations showed that although the ore-forming parent rocks from the QH, MD, and TC deposits are intermediate igneous rocks, their corresponding regolith profiles are significantly different in their mineralization characteristics. The diorite-derived QH and tonalite-derived MD deposits showed co-mineralization of ilmenite with iREEs, whereas the pyroxene diorite-derived TC deposit only showed independent mineralization of ilmenite. Evidences from systematic mineralogical and geochemical examinations indicated that the occurrence of ilmenite and iREEs co-mineralization, besides the impact of weathering, may principally regulated by the nature of accessory mineral assemblage in the parent rocks. Specifically, both Ti-bearing (ilmenite, anatase) and REEs-bearing (apatite, titanite) accessory minerals were largely identified in the QH diorite and MD tonalite. Analysis of degree of saprolitization (DOS) and weathering index of granite (WIG) revealed that they have experienced strong weathering, which gave rise to residual accumulation of Ti-bearing minerals (mainly ilmenite) at upper horizons of the regolith profiles, and simultaneously, secondary enrichment of iREEs by eluviation-illuviation process at lower horizons. It explained the ore horizons of placer ilmenite and iREEs developed symbiotically but show somewhat dislocation within the regolith. In contrast, the TC pyroxene diorite was found to be rich only in Ti-bearing minerals but poor in REEs-bearing minerals. It is conductive to develop weathering mineralization of ilmenite rather than iREEs in terms of ore source. Also, there is rich iron oxides and hydroxides in the pyroxene diorite-derived regolith, which might has inhibited REEs eluviation-illuviation and act as the other reason for absence of iREEs mineralization in the TC profile. This study not only provides a genetic model to understand the co-mineralization of placer ilmenite and iREEs ores developed by weathering of intermediate igneous rocks, but also highlights the importance of a comprehensive evaluation in future prior to exploration and resource utilization.
Bastnäsite is the end member of a large group of carbonate–fluoride minerals with the common formula (REE) CO 3 F·CaCO 3 . This group is generally widespread and, despite never occurring in large quantities, represents the major economic light rare earth element (LREE) mineral in deposits related to carbonatite and alkaline intrusions. Since bastnäsite is easily altered and commonly contains inclusions of earlier‐crystallised minerals, in situ analysis is considered the most suitable method to measure its U‐Th‐Pb and Sr‐Nd isotopic compositions. Electron probe microanalysis and laser ablation (multi‐collector) inductively coupled plasma‐mass spectrometry of forty‐six bastnäsite samples from LREE deposits in China, Pakistan, Sweden, Mongolia, USA, Malawi and Madagascar indicate that this mineral typically has high Th and LREE and moderate U and Sr contents. Analysis of an in‐house bastnäsite reference material (K‐9) demonstrated that precise and accurate U‐Th‐Pb ages could be obtained after common Pb correction. Moreover, the Th‐Pb age with its high precision is preferable to the U‐Pb age because most bastnäsites have relatively high Th rather than U contents. These results will have significant implications for understanding the genesis of endogenous ore deposits and formation processes related to metallogenic geochronology research.
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