湘西西安钨矿床白钨矿的地球化学特征及对矿床成因的指示
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本文为国家自然科学基金资助项目(编号:41972090、41073036)和湖南省自然科学基金资助项目(编号:2020JJ7058)的成果


Geochemistry of scheelite in the Xi’an tungsten deposit, western Hunan, and its implications for its ore genesis
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    摘要:

    赋存于变质岩中的层控(层状)型钨矿床的成因在国际矿床学界一直存在争议,白钨矿的元素和同位素地球化学研究是解决其成因争议的有效手段。位于雪峰弧形构造带中的西安钨矿床是该类矿床的典型代表,其矿床成因也存在着很大争议。本文利用EMPA、ICP- MS、ID- MS等多种分析手段,对该矿白钨矿的主量元素、微量元素、及Sm- Nd和Sr同位素进行了研究。研究表明,该区白钨矿中CaO含量小于其理论值,这可能与热液流体中的REE、Sr、Pb、Ba等元素与白钨矿中的Ca发生类质同象置换有关;该区白钨矿微量元素组成具有富Sr、而亏损Mo、Bi、Sn、Nb、Ta等特征,类似于西澳绿岩带型金矿、云南大坪和湘西沃溪等造山型金矿床中的白钨矿,其Mo含量和Sr/Mo值均显示其具变质热液成因特征,而与岩浆热液无关;该区白钨矿的稀土元素配分模式相当一致,均表现为LREE亏损、HREE相对富集、MREE最为富集的特征,其分配行为主要表现为REE3+与Na+以电价补偿形式替代Ca2+而选择性进入白钨矿晶格中;该区白钨矿n(87Sr)/n(86Sr)测定值为0. 75412~0. 78231,表明形成该区白钨矿的成矿流体并非来自海水或岩浆热液,其矿床成因也并非前人所认为的“海底热泉(卤水)沉积成因”、“海底同生喷流沉积成因”或“岩浆期后热液成因”。在LREE—MREE—HREE三角判别图解中,西安矿区白钨矿样品点均落入加拿大、澳大利亚和湘西造山型金矿区域,明显不同于与岩浆活动有关的钨矿床,该矿是属于造山型钨矿;在Sr—Nd同位素组成图解中,该区白钨矿与湘西一带出露的新元古代地层明显分布在不同区域,但部分样品点与华南最老地层—太古宇崆岭群重叠,暗示该矿成矿物质可能来自深部的太古宇崆岭群,而非来自赋矿的新元古界变质岩。

    Abstract:

    The ore genesis of stratabound (stratiform) tungsten deposits hosted in metamorphic rocks has remained controversial during the past several decades. Elemental and isotopic geochemistry of scheelite is an effective tool to solve this above dispute. The Xi’an tungsten deposit in Western Hunan, located at the Xuefeng arc structural zone, is a typical deposit, its ore origin is still disputed. Using different analytical methods including EMPA, ICP- MS and ID- MS, the geochemical compositions of the scheelite samples collected from the Xi’an tungsten deposit have been determined in order to reveal their elemental and isotopic signatures. It is revealed that the contents of CaO in scheelite are less than the corresponding theoretical value, which can be ascribed to the substitution of REE, Sr, Pb, Ba and other trace elements into Ca site in the scheelite lattice; The studied scheelite is relatively rich in Sr and depleted in Mo, Bi, Sn, Nb and Ta, which is similar to those scheelite in the gold/tungsten deposits in Western Australia, Daping and Woxi in China. The Mo concentrations and Sr/Mo ratios for scheelite samples in this study reveal that, scheelite in the Xi’an deposit is of metamorphic—hydrothermal origin rather than of magmatic—hydrothermal origin. All scheelite samples are characterized by the LREE- depleted, MREE- and HREE- enriched pattern, the REE substitution into Ca2+ site of the scheelites was implemented by the charge compensation mechanism (REE3++Na+=2Ca2+); The n(87Sr)/ n(86Sr) values of the studied scheelite samples vary in the range of 0. 75412~0. 78231, significantly higher than the Sr isotopic composition of Phanerozoic seawater or of granitic rocks in central Hunan, which implies that the mineralizing fluid isn’t derived from magmatic water or paleo- seawater, and that the deposit shouldn’t be classified into the “magmatic hydrothermal origin” or “seafloor hot spring (brine) sedimentary origin or seafloor syngenetic sedex origin” as previously expected. Combined with the LREE—MREE—HREE diagram, it can be concluded that the Xi’an tungsten deposit is a typical orogenic deposit. It is revealed that the studied scheelite and the Neoproterozoic strata outcropped in western Hunan are obviously distributed in different areas in the diagram of Sr—Nd isotopic composition, but a few of scheelite samples overlap with the Archean Kongling Group in South China, therefore, the tungsten mineralization in this area is probably associated with the underlying Archean strata rather than the hosted Neoproterozoic strata.

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文静,胡阿香,彭建堂.2022.湘西西安钨矿床白钨矿的地球化学特征及对矿床成因的指示[J].地质论评,68(5):1772-1788,[DOI].
WEN Jing, HU A’xiang, PENG Jiantang.2022. Geochemistry of scheelite in the Xi’an tungsten deposit, western Hunan, and its implications for its ore genesis[J]. Geological Review,68(5):1772-1788.

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  • 收稿日期:2022-04-28
  • 最后修改日期:2022-08-31
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  • 在线发布日期: 2022-09-19
  • 出版日期: 2022-09-15