| 云南大丫口祖母绿矿床中电气石的地球化学特征及其对成矿的指示意义 |
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| 引用本文： | 龙政宇,余晓艳,郑育宇,郭碧君.云南大丫口祖母绿矿床中电气石的地球化学特征及其对成矿的指示意义[J].地学前缘,2021,28(2):333-347. |
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| 作者姓名： | 龙政宇 余晓艳 郑育宇 郭碧君 |
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| 作者单位： | 中国地质大学(北京)珠宝学院,北京100083 |
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| 基金项目： | 中国地质调查局中国矿产地质志项目(DD20190379-88) |
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| 摘 要： | 祖母绿是由微量Cr和/或V致色的绿色绿柱石。位于云南省麻栗坡县的大丫口祖母绿矿床是中国重要的祖母绿矿床,近年来取得了一系列的研究进展,但与祖母绿相关的电气石的研究工作还未展开。本文以大丫口矿床含祖母绿矿脉和非矿脉中的电气石为研究对象,在详细的野外调查和岩相学研究基础上,对电气石进行成分测试,旨在探讨电气石成因、查明物质来源和流体演化过程,进一步探究大丫口祖母绿矿床的成矿机制。结果显示:含矿脉电气石单位分子中Na含量为0.62~0.79 apfu,Al含量为5.36~6.17 apfu,Fe/(Fe+Mg)值为0.31~0.41;非矿脉电气石单位分子中Na含量为0.64~0.76 apfu,Al含量为5.66~6.38 apfu,Fe/(Fe+Mg)值为0.14~0.34。大丫口电气石具有富Mg、Y位(Y-site)上呈低Al或无Al的特征,属于碱族镁电气石,但是含矿脉电气石则显示更高的Fe/(Fe+Mg)值。电气石成分的差异可能主要与形成环境有关,电气石的成分差异具有指示祖母绿是否富集的潜力。大丫口电气石具有成分分带且V2O3含量为0.65%~4.76%,其形成与持续的热液流体交代围岩有关。大丫口矿床是一个岩浆起源的动态热液体系,流体通过碱交代作用参与水岩反应萃取围岩中的成矿物质。早期流体的物质组成以源于花岗质熔体的Si、Al、Be、F、P为主,而随着演化的进行,Ca、V等来自地层的成分逐渐增加。研究表明,铍的氟化物或氟铍络合物是大丫口成矿流体中Be的一种重要的迁移方式。萤石、氟磷灰石等含氟矿物的结晶促使铍的氟化物或氟铍络合物分解,流体中氟元素的减少可能是大丫口祖母绿成矿的重要机制之一。
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| 关 键 词： | 云南大丫口 祖母绿矿床 电气石 Be元素迁移 成矿机制 |
| 收稿时间： | 2020-02-29 |
Geochemical characteristics of tourmalines from the Dayakou emerald deposit in Yunnan Province: implications for emerald mineralization |
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| LONG Zhengyu,YU Xiaoyan,ZHENG Yuyu,GUO Bijun.Geochemical characteristics of tourmalines from the Dayakou emerald deposit in Yunnan Province: implications for emerald mineralization[J].Earth Science Frontiers,2021,28(2):333-347. |
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| Authors: | LONG Zhengyu YU Xiaoyan ZHENG Yuyu GUO Bijun |
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| Institution: | School of Gemology, China University of Geosciences (Beijing), Beijing 100083, China |
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| Abstract: | Emerald is the green variety of beryl and its color is predominantly associated with trace chromium and/or vanadium. The Dayakou emerald deposit in Malipo County, Yunnan Province is a significant emerald deposit in China. A series of researches have been done in recent years on a variety of minerals in the deposit, but the emerald-associated tourmaline has not been investigated. Here, we studied the tourmalines from emerald-bearing and barren veins in the Dayakou deposit. Based on detailed field survey, petrographic investigation and component analysis, we aimed to investigate the origin of tourmaline to ascertain its fluid source and evolution in order to further understand the metallogenic mechanism of the Dayakou emerald deposit. The results show that, in tourmalines from emerald-bearing and barren veins, the measured Na contents were 0.62-0.79 and 0.64-0.76 apfu, respectively; the Al contents were 5.36-6.17 and 5.66-6.38 apfu, respectively; and the Fe/(Fe+Mg) ratios were 0.31-0.41 and 0.14-0.34, respectively. The Dayakou tourmaline belongs to alkline dravite, rich in Mg, with low or zero Al content in the Y site, whereas tourmaline from emerald-bearing veins has a comparably high Fe/(Fe+Mg) ratio. These chemical compositional differences in tourmalines may be related mainly to the ore-forming condition, which makes tourmaline a potential indicator of emerald mineralization. The Dayakou tourmaline contains 0.65%-4.76% V2O3 in component zonation, formed by metasomatism during sustained fluid-rock interactions. The early fluids of the Dayakou deposit were granite-derived fluids consisting of Si, Al, Be, F, and P. As hydrothermal alkali-metasomatism progressed, the Ca and V (extracted from wall rock) contents gradually increased. We believe beryllium fluoride (BeF) or Be-F complex is a significant Be migration mode in the Dayakou ore-forming fluids. The crystallization of F-containing minerals such as fluorite and fluorapatite resulted in the decomposition of BeF or Be-F complex, and the reduction of fluorine in the ore-forming fluid may be one of the important emerald mineralization mechanism of the Dayakou emerald deposit. |
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| Keywords: | Yunnan Dayakou emerald deposit tourmaline beryllium migration metallogenic mechanism |
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