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四川雪宝顶W-Sn-Be矿床矿物学特征和形成机制
Mineralogical characteristics and genetic mechanism of the Xuebaoding deposit in northwestern Sichuan Province
投稿时间:2015-12-29  修订日期:2017-06-06
中文关键词:成因矿物学  矿床形成机制  雪宝顶钨-锡-铍矿床  四川西北
英文关键词:genetic mineralogy  ore-forming mechanism  Xuebaoding W-Sn-Be deposit  northwestern Sichuan
基金项目:国家自然基金资助项目(41222023,41102039);中国地质调查局地质调查项目(1212011020000150011-03)
作者单位
刘琰 中国地质科学院 地质研究所, 北京 100037 
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中文摘要:
      四川雪宝顶W-Sn-Be矿床位于龙门山西北缘,主要赋存在盘口和浦口岭花岗岩之间的大理岩张性裂隙中。雪宝顶矿床中出现的矿物晶体颗粒巨大,且矿脉中矿物分带明显。矿脉在花岗岩中主要由绿柱石、锡石、白云母和钾长石(fd1、fd2和fd3)组成,在大理岩围岩中则由绿柱石、白钨矿、锡石、萤石、方解石、石英、钠长石晶体(Ab4和Ab5)以及针状电气石和细粒磷灰石组成。3种不同形态的钾长石和2种不同形态的钠长石贯穿了整个矿脉的演化。随着围岩从花岗岩到大理岩的转换,晶体颗粒从小于1 cm的绿柱石、锡石演化至可达20 cm的绿柱石、锡石、萤石和白钨矿。采用EPMA、XRF、ICP-MS对单矿物颗粒进行全岩测试分析,结果显示:雪宝顶板状绿柱石介于Na-Li绿柱石和Li-Cs绿柱石之间,白钨矿中富集ΣREE+Y(>350×10-6),白云母属于含Li白云母,磷灰石属于氟磷灰石,钾长石和钠长石比较纯净。结合前人研究资料推测矿床形成机制为:在花岗岩演化的晚期,富F流体的脱熔作用大量富集了Li、Rb、Cs、W、Sn、Be、P等元素。这些来自于熔体的元素以不同的化合物形式(如 SnF2)在分离结晶过程中富集,通过成矿流体运移然后在花岗岩裂隙中小规模沉淀。花岗岩体的冷却引发的体积缩小导致了大理岩围岩中出现了放射状的张性裂隙。张性裂隙是控制成矿流体输运的主要通道,并引发了流体不混溶(相分离)。这个过程还伴随着包裹体均一温度不断下降和含矿络合物与围岩之间不断发生反应导致络合物不断分解。此时,成矿围岩从花岗岩变成大理岩,含F络合物大量被破坏造成成矿物质W-Sn-Be等元素大量沉淀,形成颗粒巨大的矿物晶体。选取与大颗粒绿柱石晶体共生的云母样品进行Ar-Ar定年并获得反等时线年龄195.7±2.5 Ma,代表了雪宝顶矿床形成的主成矿期年龄。
英文摘要:
      The Xuebaoding deposit is located in northern Longmen Mountain of Sichuan Province. The deposit lies mainly in the contact marble between Pankou and Pukouling granites. The deposit is characterized not only by the coarse-grained beryl, scheelite, cassiterite and fluorite but also by mineral zones in ore veins. The typical ore veins can be grouped into three parts. In the first part, granite is the main host rock, and beryl, cassiterite, muscovite with diameters less than 1 cm and K-feldspar with diameters up to 20 cm (fd1, fd2 and fd3) can be found. Part Ⅲ is the most important part of the ore veins with coarse-grained fluorite, beryl, scheelite, cassiterite, albite (Ab4 and Ab5), muscovite and apatite up to 20 cm. All the K-feldspar and albite with various kinds of morphology are distributed throughout these veins. EPMA, XRF and ICP-MS were employed to examine the chemical composition of minerals. Tabular beryls in the Xuebaoding deposit are rich in Li, Rb, Cs, belonging to a type of beryl between Na-Li and Li-Cs beryls. Scheelites are rich in REE+Y (>350×10-6). Muscovite is enriched in Li, Rb, Cs, suggesting a kind of Li-bearing muscovite. Apatite is a type of F-bearing fluorapatite. F-feldspar and albite are very pure with chemical composition of fd1 (Or 95.34~93.96), fd2 (Or 96.28~97.88), fd3 (Or 95.74~98.39), Ab4 (Ab 99.19~100) and Ab5 (Ab 99.58~100). According to the morphology, and locations, K-feldspar and albite samples were named fd1 to fd5 separately: fd1, fd2 and fd3, Ab4 (in marble host rock) and Ab5 (in marble host rock Li, Rb, Cs, W, Sn, Be were highly concentrated in the hydrothermal fluids during the late stage of the evolution and crystallization differentiation as various types of complex such as SnF2 from the melt. With the increasing temperatures of emplaced magma, the reduced volume of magma after its emplacement led to the formation of fissures in marbles, which also resulted in the fluid immiscibility. Fluid immiscibility might have been induced by the pressure released by extension joints, thereby facilitating the mineralization found in Part III. To constrain the formation age of Xuebaoding deposit, the author selected muscovite with coarse-grained beryl from Part III for Ar-Ar dating and obtained the age of 195.7±2.5 Ma.
刘琰,2017,四川雪宝顶W-Sn-Be矿床矿物学特征和形成机制[J].岩石矿物学杂志,36(4):549~563.
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