Constraints of alkaline magmatic evolution on the enrichment and mineralisation of rare-earth elements in the eastern part of the northern margin of the North China Craton: a case study of the Baerzhe and Saima deposits

The eastern part of the northern margin of the North China Craton is a key metal metallogenic province in China, hosting many important rare metal and rare earth element (REE) deposits and having great resource potential. This study investigated the Saima deposit, a niobium (Nb) and tantalum (Ta) deposit and the Baerzhe deposit hosting zirconium (Zr), Nb and REEs. Based on previous LA-ICP-MS zircon U–Pb dating results obtained by the authors’ team, this study compared the whole-rock geochemistry and whole-rock Sr–Nd–Pb isotopes of an orebody with its surrounding rocks. The geochemical characteristics of both deposits are used to systematically review the evolution of the alkaline magma and the differences in the physical and chemical conditions for enrichment and mineralisation of late-stage rare elements and REEs such as Nb, Ta and Zr. Comparing the mineralisation of the Saima and Baerzhe deposits, the alkaline magmatic evolution plays a crucial role in the enrichment of rare-earth elements, which are generally formed in the late stage of alkaline magmatic evolution. The Saima deposit, which has a metallogenic age of 224.40 ± 6.10 Ma, formed in nepheline syenites with high alkali (Na₂O + K₂O) and low MgO and CaO contents, with high TFeO/MgO ratios. The A-type granites formed in an extensional tectonic setting after the closure of the Paleo-Asian Ocean. The Baerzhe deposit, which has a metallogenic age of 125.2–122.15 Ma, formed in A-type alkali granites (orebody and surrounding rocks) that have high alkali (Na₂O + K₂O) and low MgO and CaO contents with high TFeO/MgO ratios, strong uranium (U) depletion and typical V-shaped distribution of REE. The mineralisation is controlled by melt–fluid interactions in the later stage of highly evolved magmas. The enrichment and mineralisation of rare elements and REEs in the eastern part of the north margin of the North China Craton are mainly associated with the high degree of evolution of alkaline magmas.

华北克拉通（North China Craton）北缘东段是我国关键金属成矿省，赋存众多重要的稀有金属与稀土元素（REE）矿床，资源潜力巨大。本研究以铌（Nb）、钽（Ta）型矿床赛马（Saima）矿床，以及赋存锆（Zr）、Nb与稀土元素的巴尔哲（Baerzhe）矿床为研究对象。基于研究团队前期获得的LA-ICP-MS锆石U-Pb定年结果，本研究对比了矿体及其围岩的全岩地球化学特征与全岩Sr-Nd-Pb同位素组成。结合两处矿床的地球化学特征，本研究系统探讨了碱性岩浆的演化过程，以及Nb、Ta、Zr等晚期稀有元素与稀土元素的富集成矿物理化学条件差异。通过对比赛马与巴尔哲矿床的成矿作用可知，碱性岩浆演化对稀土元素的富集起到关键控制作用，稀土元素通常形成于碱性岩浆演化的晚期阶段。成矿年龄为224.40±6.10 Ma的赛马矿床，形成于富碱（Na₂O+K₂O）、低MgO与CaO且TFeO/MgO比值较高的霞石正长岩中，其对应的A型花岗岩（A-type granites）形成于古亚洲洋（Paleo-Asian Ocean）闭合后的伸展构造背景下。成矿年龄为125.2~122.15 Ma的巴尔哲矿床，形成于富碱（Na₂O+K₂O）、低MgO与CaO且TFeO/MgO比值较高、具强烈铀（U）亏损以及典型稀土元素V型分布的A型碱长花岗岩（矿体与围岩）中，其成矿作用受高度演化岩浆晚期的熔体-流体相互作用控制。华北克拉通北缘东段稀有元素与稀土元素的富集成矿，主要与碱性岩浆的高演化程度密切相关。