Genesis of Garnets within pegmatites in the northwestern margin of the Mufushan granite (central Jiangnan Orogen),and its implication for rare metal mineralization: comparison from mineralized and unmineralized pegmatites
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摘要: 江南造山带中段幕阜山地区已成为我国重要的稀有金属资源基地之一。断峰山伟晶岩型铌钽矿床是幕阜山复式岩体西北缘唯一的大型矿床,其成因及物理化学条件不明确,尤其是对新发现的微斜长石伟晶岩型铌钽矿化研究甚少。本文以断峰山地区成矿与未成矿两种微斜长石伟晶岩脉中石榴子石为研究对象,进行了BSE和CL微观结构观察、电子探针(EPMA)和LA-ICP-MS原位微区主微量元素分析,探讨石榴子石成因及其对微斜长石伟晶岩成矿作用的指示。断峰山地区伟晶岩中石榴子石均属于铁铝榴石-锰铝榴石(Alm48.61-Sps48.21)固溶体系列,成矿伟晶岩脉中石榴子石以铁铝榴石为主(Sps42.56Alm54.63),而未成矿伟晶岩脉中石榴子石以锰铝榴石为主(Sps58.93Alm37.18)。两种石榴子石稀土元素均呈HREE富集、LREE亏损、明显的Eu负异常(Eu/Eu*=0-0.09)的特征。结果表明,该地区成矿和未成矿伟晶岩中石榴子石均为岩浆成因,可能由同时代不同岩浆演化形成。两者均形成于中高温、中低压力环境。成矿伟晶岩中石榴子石较低的Mn含量(12.07-19.00 wt.%)是由于铌钽矿物结晶的影响,稀有金属成矿与外接触带伟晶岩脉低的过冷程度有关。石榴子石Y/Gd和Y/Dy比值及电气石的出现均证实伟晶岩岩浆富含挥发分,有利于Nb和Ta等稀有金属元素的富集。但未成矿伟晶岩由于内接触带高的过冷程度导致岩相分带不发育。Abstract: The Mufushan area,in the central Jiangnan Orogen,South China,has become one of the most significant rare metals resource bases in China. The Duanfengshan pegmatite-type Nb-Ta deposit is the only large-sized deposit in the northwestern margin of the Mufushan granitic batholith. However,its genesis and physicochemical conditions are not well studied,in particular of the newly discovered microcline pegmatite-type Nb-Ta mineralization. This study focuses on garnets in the mineralized and unmineralized microcline pegmatites in the Duanfengshan area. Garnets were observed using cathodoluminescence (CL) and backscattered electron (BSE) imagery. The major and trace elements are determined by EPMA and LA-ICP-MS,which are used to discuss the genesis of garnet and indication for Nb-Ta mineralization of microcline pegmatites. Garnets in mineralized and unmineralized pegmatites in the Duanfengshan area both belong to the solid solution series of almandite-spssartite. Almandite (Sps42.56Alm54.63) dominantly occurs in the mineralized pegmatites,whereas spssartite (Sps58.93Alm37.18) mainly distributes in the unmineralized pegmatites. Garnets both are characterized by enrichment in heavy rare earth elements (HREE),and depletion in light rare earth elements (LREE) with Eu negative anomaly (Eu/Eu*=0-0.09). These mineralogical and geochemical features suggest a magmatic origin for garnets within microcline pegmatites in the Duanfengshan area. The two types of pegmatite would be derived from different magmatic sources in the same period. Both formed in medium-high temperatures and medium-low pressures. The low Mn contents (12.07-19.00 wt.%) of garnet in the mineralized pegmatites is caused by the crystallization of niobium-tantalum minerals. Rare metal mineralization is related to the low degree of liquidus undercooling of pegmatite veins in the outer contact zone. The Y/Gd and Y /Dy ratios of garnets,and the appearance of tourmalines in pegmatites confirm that the pegmatitic magmas are enriched in volatiles,which is beneficial to the enrichment of rare metal elements. However,the undeveloped zoning of unmineralized pegmatites is due to the high degree of liquidus undercooling in the inner contact zone.
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Key words:
- Mineralized and unmineralized pegmatites /
- garnet /
- magmatic origin /
- Rare metal deposit /
- Mufushan area
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