| 四川冕宁包子山稀土矿床富Sr碳酸岩的发现及意义 |
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| 引用本文： | 曲云伟,谢玉玲,尹淑苹,于超,夏加明,崔凯.四川冕宁包子山稀土矿床富Sr碳酸岩的发现及意义[J].岩石学报,2021,37(9):2861-2874. |
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| 作者姓名： | 曲云伟 谢玉玲 尹淑苹 于超 夏加明 崔凯 |
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| 作者单位： | 北京科技大学土木与资源工程学院, 北京 100083;中国地质科学院地质研究所, 北京 100037 |
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| 基金项目： | 本文受国家自然科学基金项目（41472072、41072066）资助. |
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| 摘 要： | 全球范围内出露的碳酸岩大多为钙质、镁质、铁质碳酸岩,少量为钠质和硅质碳酸岩,极少有富Sr碳酸岩的报道,其岩石成因、资源意义及对碳酸岩岩浆演化的指示意义尚不清楚。本次在四川省牦牛坪稀土矿区南部包子山稀土矿床的露天采坑中发现了超级富Sr的碳酸岩,其呈不规则的脉状侵入到构造角砾岩中。岩石呈紫色-淡紫色,微晶-斑状结构,斑晶主要为萤石,基质主要为菱锶矿、方解石、氟碳铈矿、氟碳钙铈矿、金云母、重晶石并含少量的金属硫化物和氧化物。全岩的微量元素分析表明,其稀土元素总量(∑REE)达3.5%～6.1%,Sr含量达19.0%～27.7%,已超过稀土矿床和锶矿床的工业品位要求。岩石中的中、重稀土元素含量占稀土元素总量的1.14%～1.77%,一些高价值稀土元素含量较高,如Pr(939×10~(-6)～1399×10~(-6))、Nd(2783×10~(-6)～3937×10~(-6))、Gd(237×10~(-6)～320×10~(-6)),因此除轻稀土元素外,中、重稀土和锶元素也具有重要的资源意义。岩石强烈富集REE、Sr、Ba,而明显亏损P、Nb、Ta、Zr、Hf元素,可能与岩浆演化过程中锆石和其它基性矿物的结晶分离有关。全岩的Sr-Nd同位素组成与牦牛坪、里庄稀土矿床的碳酸岩相似,表明它们为同源岩浆产物。笔者认为,富Sr的碳酸岩代表了碳酸岩岩浆演化晚期的产物,REE、Sr、Ba、F和S元素均在岩浆演化晚期的碳酸岩中高度富集。碳酸岩岩浆超浅成侵位至构造角砾岩中,并与下渗的大气水相遇导致岩浆的淬冷和微晶-斑状结构的形成。早期基性矿物(如霓辉石、黑云母)及碳酸盐矿物(如方解石、白云石等)的结晶分离是造成晚期碳酸岩中稀土元素富集的重要原因。富Sr碳酸岩中石英斑晶的发现和其较低的SiO_2含量表明碳酸岩岩浆演化晚期可能是硅饱和的,且这种岩浆具有很低的SiO_2溶解能力。以菱锶矿(体积分数 50%)为主要碳酸盐矿物的稀土碳酸岩可能代表了一种新的碳酸岩类型,明显不同于已知的钙质、镁质、铁质和钠质碳酸岩。
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| 关 键 词： | 富Sr碳酸岩 碳酸岩岩浆演化 资源意义 包子山稀土矿床 四川冕宁 |
| 收稿时间： | 2021/6/1 0:00:00 |
| 修稿时间： | 2021/8/25 0:00:00 |
The discovery of Sr-rich carbonatite and its significance in Baozishan REE deposit, Mianning, Sichuan Province |
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| QU YunWei,XIE YuLing,YIN ShuPing,YU Chao,XIA JiaMing,CUI Kai.The discovery of Sr-rich carbonatite and its significance in Baozishan REE deposit, Mianning, Sichuan Province[J].Acta Petrologica Sinica,2021,37(9):2861-2874. |
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| Authors: | QU YunWei XIE YuLing YIN ShuPing YU Chao XIA JiaMing CUI Kai |
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| Institution: | School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China;Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China |
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| Abstract: | Carbonatites reported in the world are mainly of calcio-, magnesio- and ferro-ones, and subordinately of sodium- and silico-carbonatites, while only very a few belong to Sr-rich carbonatites. Therefore, the petrogenesis, the resource and indicative significances and the magmatic evolution of the Sr-rich carbonatite remain unclear. Here we report a special type of Sr-rich carbonatites with microcrystalline to porphyritic textures in Mianning County, Sichuan Province. The samples were collected in the open pit of the Baozishan REE deposit, about 6km southern from the center of Maoniuping REE deposit. Based on field observation, petrographic, SEM/EDS and Sr-Nd isotopic results, the genesis of the Sr-rich microcrystalline carbonatite and its implication for the magma evolution are discussed. The Sr-rich carbonatites occur as small dikes intruded into a structure breccia. The rock is light purple to purple in color, and with microcrystalline to porphyritic textures. The mineral assemblage includes strontianite, fluorite, phlogopite, calcite, barite, with minor quartz, bastnaesite, parasite, sulfide, ilmenite and rutile. The petrochemical analysis results of the samples show that the carbonatite has a total REE of 3.5%~6.1%, among which the MREE and HREE take up 1.14%~1.77%. It has high Pr (939×10-6~1399×10-6), Nd (2783×10-6~3937×10-6), Gd (237×10-6~320×10-6) contents, which are much higher than the values of La and Ce. Thus, the Sr-rich carbonatites themselves comprise high quality REE ores. In the chondrite-normalized partition pattern, the Sr-rich carbonatites show LREE enriched characteristics, without any Ce or Eu abnormalities. Trace element analysis results show that the Sr-rich carbonatite is enriched in REE, Sr, Ba, and depleted in P, Nb, Ta, Zr and Hf, implying that a mafic mineral fraction crystallization probably occurred during the magmatic evolution. The Sr-Nd isotopic compositions are similar to the rock assemblages in the Maoniuping and Lizhuang deposits, implying they have a similar mantle origin. The chilling texture of the rock may be originated from the shallow emplacement of the carbonite and encounter with infiltrated meteoric water through the fracture zone. The Sr-rich carbonatite represent a late-stage carbonatite magma. The mafic minerals, calcite or dolomite fractional crystallization may lead to REE, Sr, Ba enrichment in the later stage of the carbonatite magma. The quartz phenocryst and low SiO2 content in the Sr-rich carbonatite imply that the carbonatite magma of the late stage is SiO2 saturated and has a very low SiO2 solubility. The strontianite (>50%) dominated carbonatite may present a new type carbonatite, the Sr-rich carbonatite, which is obviously different from the known calcio-, magnesio-, ferro-, sodium- and silico-carbonatites. |
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| Keywords: | Sr-rich carbonatite Carbonatite magmatic evolution Resource significance Baozishan REE deposit Mianning of Sichuan Province |
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