川西冕宁-德昌稀土矿带正长岩-碳酸岩杂岩体中锆石矿物学特征与矿化过程

Mineral characteristics of zircons in the syenite-carbonatite complex in the Mianning-Dechang REE ore belt, Sichuan Province, SW China:Indicative of REE mineralization

川西冕宁-德昌稀土成矿带不同矿床中各种产状的正长岩是主要的赋矿围岩,运用各类显微图像、拉曼光谱、电子探针等测试方法分别选取其中新鲜(Ⅰ类)、重结晶(Ⅱ类)、受热液蚀变(Ⅲ类)和受成矿流体蚀变(Ⅳ类)的锆石进行矿物学研究。Ⅰ类和Ⅱ类锆石特征相似,透射图清晰,含岩浆子晶矿物包裹体,结晶程度好,拉曼光谱半高宽较小;Ⅲ类锆石透射图呈云雾状,晶体结构由于受到破坏而呈筛状,拉曼光谱半高宽较大,含少量钍石包裹体,低Zr,高U、Th、Hf、Ce含量;Ⅳ类锆石由于受到更强烈的蚀变,结构破坏最强烈,拉曼光谱半高宽最大,包裹体大量出现,锆石成分含有更多的U、Th、Hf以及La、Ce、Nd、Sm。各矿床正长岩-碳酸岩杂岩体中的锆石,Ⅰ类—Ⅲ类—Ⅳ类经历的蚀变程度逐渐加深、结构破坏增强、钍石包裹体和U、Th、Hf、Ce含量不断增加,反映了岩浆-伟晶岩-成矿过程中,锆石经历了不同的蚀变过程,同时也反映了稀土从岩浆到成矿不同阶段的蚀变矿化和富集。

All REE deposits in the Mianning-Dechang REE ore belt of western Sichuan Province are closely related to syenite-carbonatite complex in genesis. Despite the similar geological backgrounds, syenite-carbonatite complexes have various occurrences. Also, various types of zircons in these syenites rather than carbonatite exhibit the forms of fresh zircons （type Ⅰ）, recrystallization zircons （type Ⅱ）, hydrothermal fluid altered zircons （type Ⅲ）, and ore-forming fluids altered zircons （type Ⅳ）. All these types of zircons were examined carefully by BSE images, CL images, Raman spectra, EMPA, and some other means. Type Ⅰ and type Ⅱ zircons have nearly identical crystal structures, chemical composition and BSE images, with edges of Type Ⅱ zircons being recrystallized and having well-developed crystal structure because of their smaller full-widths at half-maximum FWHM; type Ⅲ zircons have the murky transmissive images, destroyed sieve texture and more increased FWHM, some thorite inclusions and decreased Zr and relatively high content of U, Th, Hf, Ce; type Ⅳ zircons have experienced more serious alteration by ore-forming fluids with many thorite inclusions, more destroyed sieve texture and the most increased FWHM, and high concentrations of U, Th, Hf as well as La, Ce, Nd, Sm. Different zircons show various morphologies, inclusions, degrees of destroyed texture and isomorphous substitutions between U, Th, Hf, Ce and Zr, which, in turn, suggests the evolution from magmatic to hydrothermal processes in the formation of REE deposits.