贵州大坪电气石岩的发现及其找矿意义

The discovery of hyalotourmalite in Daping,Guizhou Province,and its ore-prospecting significance

在贵州从江大坪多金属矿区东部地质填图过程中发现电气石岩层,对其进行了岩矿鉴定和粉晶衍射(XRD)、电子探针(EPMA)、电感耦合等离子体质谱(ICP-MS)等分析,结果表明电气石岩的矿物组成为电气石40%～60%、石英约35%～55%,其他约5%为各低含量组分(绿泥石、稀土矿物、锆石等)之和;电气石化学组成中FeO/(FeO+MgO)(Fe#)和MgO/(FeO+MgO)(Mg#)分别为0.64～0.67和0.33～0.36,表明该电气石属于黑电气石-镁电气石固溶体系列;电气石岩的稀土元素特征为稀土元素总量低(ΣREE=21.43×10-6～26.82×10-6),轻稀土元素(LREE)富集,中稀土元素(MREE)亏损,并具有弱的Eu、Ce异常;成矿元素W、Sn、Zn、As、Bi等较高程度富集,指示W、Sn、Zn等具有很大的成矿潜力。电气石岩的发现对该区寻找W、Sn多金属矿床具有重要的指示意义。

Located in the transitional zone between the Yangtze craton and the South China fold system, Congjiang area lies in southwest Jiangnan orogenic belt. The main strata are Wentong Formation of Mesoproterozoic Sibo Group and Jialu Formation （Qbj）, Wuye Formation （Qbw）, Fanzhao Formation （Qbf） and Gongdong Formation （Qbg） of Qingbaikou System in Neoproterozoic Xiajiang Group. These strata are composed of metamorphosed sedimentary rocks, mainly sericite slate and sandy slate. Magmatic rocks and tectonics are well developed in this area. There are ultrabasic rocks, basic rocks and acid intrusive rocks, with the Motianling granite being the largest intrusive body. There exist three groups of faults, trending respectively NS, NWW and NNE, with the Zaibian fault being a discordogenic fault. Haylotourmalite was found by geological mapping in the east of the Daping polymetallic ore district in southeast Guizhou Province. Analyses by means of determinative mineralogy, X-Ray Diffraction （XRD）, Electron Probe Micro-Analyzer （EPMA） and Inductively Coupled Plasma Mass Spectrometry （ICP-MS） show that haylotourmalite is black to dark gray in color and has ribbon structure, with an alternate arrangement of dark and light colored stripes that shows a rhythmic varia-tion.The haylotourmalite is composed of tourmaline （40%～60%）, quartz （35%～55%）, and minor amounts of chlorite, rare-earth minerals and zircon. FeO/ （FeO＋MgO） （Fe#） and MgO/（FeO＋MgO） （Mg#） ratios of the tourmaline are from 0.64 to 0.67 and from 0.33 to 0.36, respectively, implying that the tourmaline belongs to the aphrizite-dravite solid-solution series. The hyalotourmalite has low ΣREE values （21.43 μg/g～26.82 μg/g） and is enriched with LREE and depleted in MREE, with （La/Yb）N ratios from 2.55 to 4.76 and （La/Sm）N and （Gd/Yb）N ratios from 5.84 to 6.17 and from 0.69 to 0.98 respectively, implying that the differentiation of the light rare earth elements and heavy rare earth elements is not very obvious; however, the differentiation of the light rare earth elements is stronger than that of the heavy rare earth elements. The haylotourmalite samples show indistinct Eu and Ce anomalies. The hyalotourmalite is also enriched with metallogenic elements such as W, Sn, Zn, As, Bi and B, which indicates a great potential in search for ore deposits of these elements. The discovery of hyalotourmalite has a great significance for the exploration of massive sulfide deposits.