西南三江地区镇沅金矿载金黄铁矿稀土与微量元素特征

Rare Earth Element and trace element features of auriferous pyrite in the Zhenyuan gold deposit,Sanjiang region,Yunnan Province,China

镇沅金矿（也称老王寨金矿）是西南三江地区哀牢山金矿带超大型金矿床之一。前人研究表明黄铁矿的稀土与微量元素含量可以反映成矿流体与成矿物质来源的特征,本次采用电感耦合等离子质谱仪（ICP-MS）技术对镇沅金矿主要载金矿物黄铁矿进行研究。通过野外实地调查及室内显微镜下观察,将镇沅金矿载金黄铁矿分为四个阶段：Ⅰ阶段脉状黄铁矿;Ⅱ阶段团块状黄铁矿;Ⅲ阶段与辉锑矿等硫化物共生浸染状黄铁矿;Ⅳ阶段破碎状黄铁矿。矿区内多种地层均有矿化现象,矿石类型包括变砂岩型、炭质板岩型、蚀变大理岩型、蛇绿岩套型、蚀变花岗岩型、煌斑岩型等。ICP-MS测定结果显示Ⅰ阶段黄铁矿富集高场强元素Th、U、Ta、Nb、Zr、Hf,亏损大离子亲石元素;其后三阶段黄铁矿该特征则不明显。黄铁矿总体稀土元素含量较低,∑REE含量为4.72×10-6~48.96×10-6,平均为24.93×10-6。稀土配分曲线显示黄铁矿总体呈较平缓的右倾型轻稀土元素富集配分模式;存在负δEu异常,基本无δCe异常。各阶段黄铁矿稀土配分模式存在微弱差异,Ⅰ阶段黄铁矿总体REE含量较高,部分具有LREE/HREE比值较低的特征,与超基性岩REE配分模式相近;Ⅱ阶段黄铁矿REE含量明显偏低,LREE/HREE比值较高;Ⅲ阶段黄铁矿∑REE含量较低,但较Ⅱ阶段要高;Ⅳ阶段黄铁矿∑REE含量低,有与Ⅱ阶段黄铁矿REE相近特征。黄铁矿中Au与As伴生现象明显。Co/Ni值反映黄铁矿具有沉积与热液黄铁矿成因特征,另外黄铁矿的稀土微量元素特征还反映出成矿流体为含F-的还原性流体。研究认为镇沅金矿至少存在两期金成矿作用,早期载金黄铁矿呈脉状产出,并可能与印支期古特提斯洋闭合中含超基性岩的增生楔形成有关;晚期载金黄铁矿与辉锑矿、黄铜矿等硫化物共生,与新生代区域范围内红河剪切带活动及扬子板块的俯冲导致的广泛变质作用有关。

The Zhenyuan gold deposit,also named Laowangzhai gold deposit,is one of the super-large Au deposits in the Ailaoshan gold belt of the Sanjiang region,SW China. Previous studies had verified that the characteristics of ore-forming fluid can be reflected by REE and trace elements patterns contained in pyrite minerals. Pyrites＇ REE and trace element compositions from the Zhenyuan gold deposit was measured by ICP-MS in our study. Four stages of pyrites were identified,including pyrite veins（ stageⅠ）,massive pyrite（ stageⅡ）,disseminated pyrite（ stage Ⅲ） accompanied by stibnite and chalcopyrite and crushed massive pyrite（ stage Ⅳ）. Diverse types of rocks in the deposit were mineralized and ore types include altered sandstone,carbonaceous slate,marbles,ophiolite,altered granite and lamprophyre. ICP-MS test result shows compared to the other three stages pyrite,pyrite in stageⅠ has enrichment of HFSE such as Th,U,Ta,Nb,Zr and Hf,and deficit of LILE. ∑REE content of pyrite in Zhenyuan gold deposit is low. ∑REE ranges from4. 72 × 10- 6to 48. 96 × 10- 6,with an average of 24. 93 × 10- 6. REE distribution reveals a light REE（ LREE） enrichment pattern,with negative Eu anomalies and absence of Ce anomalies. ∑REE content in stage Ⅰ is relative high,and the LREE / HREE ratios are low,showing similarity with those in the ultrabasic rocks. Pyrite from stage Ⅱbears a much lower ∑REE content than that in StageⅢ,while the LREE/HREE ratios are higher. ∑REE contents in stage Ⅳ pyrite are also very low. Pyrites from the deposit have a combined genesis of sedimentary and hydrothermal in the Co / Ni diagram. Pyrite elemental compositions also indicate ore-forming fluid in the Zhenyuan gold deposit might be a reductive fluid containing F-. Due to this study,two periods for Au mineralization were identified in the Zhenyuan gold deposit. Pyrite of stageⅠrepresents the early Au mineralization related to the formation of accretionary wedge containing ultramafic rocks during the closure of Paleotethyan Ailaoshan Ocean. A later Au mineralization period expressed by the stage Ⅲ pyrite accompanied with stibnite and chalcopyrite was triggered by the metamorphism due to the underthrust of the South China block and the shearing along the Ailaoshan-Honghe belt.