Using tourmaline to trace Li mineralization in the Mufushan granitic batholith, South China

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2024-11-12 DOI:10.1016/j.chemgeo.2024.122485

Leguang Li, Lianxun Wang, Rolf L. Romer, Changqian Ma, Liang Cao, Yang Tian

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Abstract

Granites and rare metal pegmatites of the Mufushan granitic batholith form a continuous magmatic sequence linked by fractional crystallization. Tourmaline is present in muscovite leucogranites and all types of pegmatites, including highly evolved Li-rich pegmatites. We utilized major element, trace element and in-situ B isotope analyses of tourmaline to investigate the effects of magmatic fractional crystallization and magmatic volatile phase (MVP) exsolution on Li migration and exceptional Li enrichment. Eight types of tourmaline are identified across three rock units: (i) Isolated (Tur Ia) and nodular (Tur Ib) tourmaline within muscovite leucogranites; (ii) black tourmaline in veins and/or clusters (Tur IIa), as isolated crystals (Tur IIb) and in tourmaline-quartz segregations (Tur IIc) within Li-poor pegmatites; and (iii) tourmaline as isolated pink crystals with zoning patterns (Tur IIIa), as isolated pink crystals and/or radiating clusters (Tur IIIb), and as isolated crystals enclosed in quartz block (Tur IIIc) within Li-rich pegmatites. Tourmaline in Mufushan muscovite leucogranites and Li-poor pegmatites belongs to the alkali-group and schorl series with Mg/(Mg + Fe) ratios of 0.10–0.31 and 0.12–0.48, respectively, containing almost no Li* and F (apfu, based on X + Y + Z = 15). In contrast, tourmaline in Li-rich pegmatites exhibits schorl-elbaite and elbaite-rossmanite compositions with low Mg/(Mg + Fe) ratio (avg. = 0.01), and evolved Li* (0.01–0.90 apfu, avg. = 0.41 apfu) and F (0.00–0.91 apfu, avg. = 0.36 apfu) contents. A pronounced increase in YAl, (Li* + Mn) contents, and Y[Al/(Al + Fe)] ratio is observed across the transition from Li-poor to Li-rich pegmatites, consistent with the anticipated pattern of fractional crystallization. The concentration of Li exhibits a sharp increase in Li-rich pegmatites (avg. Li = 6786 ppm) compared to Li-poor pegmatites (avg. Li = 114 ppm) and leucogranites (avg. Li = 469 ppm). Lithium contents increase and reach a peak during the crystallization of Tur IIIb (6686–11,667 ppm), and have lower peak contents during the precipitation of Tur IIIc (8261–9160 ppm), indicating that the incorporation of Li is influenced by MVP accumulation and exsolution. MVP exsolution significantly reduces the solubility of Nb, Ta, and Be in the residual melt, promoting the precipitation of beryl and columbite group minerals and facilitating the migration of fluid-mobile elements such as Li, Rb, Cs, and Ga to form lepidolite. The B isotope compositions of tourmaline range from −14.8 ‰ ∼ −12.6 ‰ in Li-poor pegmatites to −17.1 ‰ ∼ −14.0 ‰ in Li-rich pegmatites. Rayleigh fractionation modeling reveals that MVP saturation occurs after approximately 60 % B was removed from the pegmatite melt. The compositional variation of tourmaline demonstrates that Li enrichment is not only governed by continuous fractional crystallization, but also by MVP-related accumulation and exsolution mechanism.

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利用电气石追踪华南幕阜山花岗岩浴积层中的锂矿化现象

幕阜山花岗岩浴成岩的花岗岩和稀有金属伟晶岩形成了一个连续的岩浆序列，并通过分块结晶连接起来。电气石存在于白云母花岗岩和所有类型的伟晶岩中，包括高度演化的富锂辉石。我们利用电气石的主要元素、微量元素和原位硼同位素分析，研究岩浆分块结晶和岩浆挥发相（MVP）外溶对锂迁移和特殊锂富集的影响。在三个岩石单元中发现了八种类型的电气石：(i)在白雲母白榴石中的孤立電氣石（Tur Ia）和節狀電氣石（Tur Ib）；(ii)在貧鋰偉晶岩中的脈狀和/或簇狀（Tur IIa）、孤立晶體（Tur IIb）和電氣石-石英分離（Tur IIc）的黑色電氣石；(iii) 在富鋰偉晶岩中，以獨立粉紅色晶體及帶有分帶圖案的電氣石 (Tur IIIa) 、獨立粉紅色晶體及／或放射狀晶體簇 (Tur IIIb) ，以及被石英塊包圍的獨立晶體 (Tur IIIc) 形態出現的電氣石。在幕阜山白雲母白榴石和貧鋰偉晶岩中的電氣石屬於鹼群和蛭石系列，鎂/（鎂+鐵）比率分別為0.10-0.31和0.12-0.48，幾乎不含鋰*和鐵（apfu，以X + Y + Z = 15計算）。相比之下，富含鋰的偉晶岩中的電氣石則呈現錳/（鎂+鐵）比率較低（平均值=0.01）的矽卡岩-白雲母和埃爾白雲母-紅雲母成分，以及逐漸增加的鋰*（0.01-0.90apfu，平均值=0.41apfu）和鈣（0.00-0.91apfu，平均值=0.36apfu）含量。在从贫锂辉石向富锂辉石过渡的过程中，观察到 YAl、（Li* + Mn）含量和 Y[Al/（Al + Fe）]比率明显增加，这与预期的分块结晶模式一致。与贫锂伟晶岩（平均 Li = 114 ppm）和白云母（平均 Li = 469 ppm）相比，富锂伟晶岩（平均 Li = 6786 ppm）中的锂含量急剧增加。锂含量在 Tur IIIb 的结晶过程中增加并达到峰值（6686-11667 ppm），而在 Tur IIIc 的沉淀过程中峰值含量较低（8261-9160 ppm），这表明锂的掺入受到 MVP 累积和外溶的影响。MVP的外溶显著降低了残余熔体中Nb、Ta和Be的溶解度，促进了绿柱石和铌铁矿族矿物的沉淀，并促进了Li、Rb、Cs和Ga等流体移动元素的迁移，形成鳞片岩。電氣石的硼同位素組合介乎貧鋰偉晶岩的-14.8‰∼-12.6‰至富鋰偉晶岩的-17.1‰∼-14.0‰。瑞利分馏模型显示，MVP饱和是在伟晶岩熔体中约60%的B被去除之后发生的。电气石的成分变化表明，锂的富集不仅受制于连续的分馏结晶，还受制于与MVP有关的累积和外溶机制。

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来源期刊

Chemical Geology 地学-地球化学与地球物理

CiteScore

7.20

自引率

10.30%

发文量

374

审稿时长

3.6 months

期刊介绍： Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.