Ore Types and Genesis of Weathered Deposits in Mianning-Dechang REE Ore Belt, Western Sichuan Province, Southwestern China
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摘要: 川西冕宁-德昌REE矿带中的风化型矿石研究相对缺乏.根据近几年的野外地质调查，采用电子探针、X光粉晶衍射、全岩分析等对大陆槽、牦牛坪和木落寨矿床中疑似风化矿的样品进行类型和成因分析.结果表明：大陆槽风化型矿石中大量粘土矿物（达40%）和矿石矿物（达60%）发育，风化程度高；相反，牦牛坪和木落寨疑似风化型矿石中几乎没有粘土矿物，风化程度低.通过对牦牛坪和木落寨疑似风化型矿石进行地球化学特征、矿物组成分析并与围岩、矿脉的组成和产状对比，表明疑似风化型矿石主要是由矿脉和围岩在强烈的构造作用下发生机械风化形成.频繁的角砾构造使大陆槽矿石发生强烈的机械风化作用，并加速了矿石发生一定的水岩反应，促进矿石发生中等强度的化学风化作用.目前整个矿带中未发现明显的生物风化作用.整个稀土矿带的风化型矿石比南岭离子吸附型稀土矿石的风化程度低，形成时间较晚（11 Ma），氟碳铈矿大量保存，还没有形成离子吸附型矿床.Abstract: The study of weathered ores is relatively limited in the Mianning-Dechang REE ore belt, western Sichuan. According to the geological survey in recent years, ore types and genesis of suspected weathered ore from Dalucao, Maoniuping and Muluozhai deposits were studied through EPMA, XRD and whole rock analysis. The results show that a large number of clay minerals (up to 40%) and ore minerals (up to 60%) are developed in the weathered ore of the Dalucao deposit, and the weathering degree is relatively high. On the contrary, suspected weathered ore types almost have not developed clay minerals from Maoniuping and Muluozhai deposits, and the weathering degree is relatively low. Through the geochemical characteristics and mineral composition analysis of the suspected weathered ore in Maoniuping and Muluozhai deposits, combining the compositions and occurrences of the wall rock and veins, it is obvious that the suspected weathed ore has been formed by mainly mechanically weathering from the nearby veins and wall rock under the strong tectonic action. Frequent breccia activities promote the strong mechanically weathering of weathered ores, and accelerate the water-rock reaction in Dalucao deposit. At present, there is no obvious biological weathering in the ore belt. The weathered ores in the belt have lower weathering degree than that of the Nanling ion adsorption type rare earth elements ores, and the age of the formation of the belt is younger (11 Ma) and a large amount of bastnaesite preservation, so it has not formed the ion adsorption type deposit.
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Key words:
- Mianning-Dechang /
- REE deposit /
- weathered deposit /
- ore type /
- ore genesis /
- geochemistry
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图 1 川西冕宁-德昌稀土成矿带地质构造简图
Fig. 1. Sketch of Mianning-Dechang REE ore belt, Sichuan Province
图 2 川西冕宁-德昌稀土成矿带各矿床地质简图
图a, b据袁忠信等(1995)；图c据杨光明等(1998)著四川省德昌县大陆槽稀土矿床成矿条件研究.a.牦牛坪矿床；b.郑家梁子矿床；c.大陆槽矿床
Fig. 2. Sketches of deposits in Mianning-Dechang REE ore belt, Sichuan Province
图 3 牦牛坪疑似风化型矿石的野外照片
Fig. 3. Field photos of suspected weathered ore from Maoniuping deposit
图 4 木落寨矿床二叠系地层和岩浆岩
据中国地质科学院矿产综合利用研究所(2008)编四川省冕宁县南河乡阴山村方家堡稀土矿区普查报告
Fig. 4. Schematic stratigraphic and magmatic of the Permian in the Muluozhai deposit
图 5 冕宁-德昌REE矿带风化型矿石的背散射照片
a, b.牦牛坪黑色风化型矿石：a.氟碳铈矿沿裂隙分布，钾长石周围分布着一些Fe-Mn氧化物和矿物碎屑；b.块状及长条状的氟碳铈矿.c, d.牦牛坪黄色风化型矿石：c.氟碳铈矿、Fe-Mn氧化物及其他脉石矿物的分布状态；d.氟碳铈矿的分布状态；e~h.大陆槽风化矿：e.长条状的独居石叠加到钠长石中；f.赤铁矿与氟碳铈矿和伊利石共生，表明矿石发生风化作用.g.针状的氟碳铈矿叠加到萤石碎屑中，构成碎屑支撑结构；h.块状的氟碳钙铈矿叠加到萤石碎屑中，构成碎屑支撑结构.Bsn.氟碳铈矿；Par.氟碳钙铈矿；Mnz.独居石；Kfs.钾长石；Chl.绿泥石；Mt.磁铁矿；Phl.金云母；Qtz.石英；Ab.钠长石；Hem.赤铁矿；Ⅲ.伊利石；Fl.萤石.e~g据Liu et al.(2015c)
Fig. 5. BSE images of weathered ores in Mianning-Dechang REE ore belt
图 6 冕宁-德昌REE矿带疑似风化型矿石和主要赋矿围岩(正长岩)的球粒陨石标准化配分模式图
据Sun and McDonough(1989).a.牦牛坪矿床两种疑似风化型矿石的微量元素蛛网图；b.牦牛坪矿床两种疑似风化型矿石的REE配分曲线；c.木落寨矿床疑似风化型矿石的微量元素蛛网图；d.木落寨矿床疑似风化型矿石的REE配分曲线；e.牦牛坪主要赋矿围岩正长岩的微量元素蛛网图；f.牦牛坪主要赋矿围岩正长岩的REE配分曲线；e, f来自Hou et al.(2006)
Fig. 6. Chondrite-normalized of suspected weathered ore from Mianning-Dechang REE ore belt
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