云南大坪金矿白钨矿微量元素、稀土元素和Sr-Nd同位素组成特征及其意义
Trace elements,rare earth elements(REE)and Nd-Sr isotopic compositions in scheelites and their implications for the mineralization in Daping gold mine in Yunnan province,China.
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摘要: 利用ICP-MS和同位素质谱分析了大坪金矿含金石英脉中白钨矿的微量元素、稀土元素和Sm-Nd、Rb-Sr同位素组成,结果显示大坪白钨矿中富Sr、Ba,而亏损Mo、Bi、Sn、Nb、Ta等,指示原始成矿流体与岩浆的结晶分异作用无关,并非前人普遍认为的岩浆水和大气降水的混合流体;样品的REE球粒陨石标准化配分曲线为高度一致的右倾和MREE富集型,Eu出现正异常,表明白钨矿与流体之间REE元素发生了强烈分异,白钨矿中REE的配分行为主要表现为REE^3＋与Na^＋成化合价补偿形式替代Ca^2＋选择性进入白钨矿晶格中,成矿流体是相对封闭的高温、富Na^＋的还原性热液体系;Sr-Nd同位素组成显示本区原始成矿流体主要来自下地壳,但不排除有幔源物质加入.原始成矿流体的形成与区域性剪切带的活动有关,韧性剪切作用导致下地壳富CO2流体上升,并与闪长岩发生强烈的水岩反应,而剪切带中脆性断裂的形成是成矿流体迁移、集中、沸腾和矿质沉淀的触发因素.Abstract: Scheelite is a widely-distributed accessory mineral in auriferous quartz veins in Daping gold deposit,China.Trace elements,rare earth elements(REE)and Nd-Sr isotopic compositions of the scheelite were determined respectively by ICP-MS and mass spectrograph in this work.The results show that the scheelite is enriched with Sr,Ba and depleted with Mo,Bi,Sn,Nb and Ta, indicating that primitive ore-forming fluid of the Daping mine probably didn't derive from magmatic crystallization and differentiation, and was not a mixed fluid of magmatic and meteoric water proposed by many previous researchers.The scheelites picked from different samples possess very similar MREE-enriched and dextral-inclined chondrite-normalied patterns and positive Eu anomaly,suggesting that intensive REE differentiation occurred between the scheelites and the ore-forming fluid,REE were substituted in Ca~(2+) site of the scheelites by charge compensation mechanism(REE~(3+)+Na~+=2Ca~(2+)),and the primitive ore-forming fluid was a relative high temperature,Na~+ enriched and reducing hydrothermal system.High correlation of trace elements and REE compositions in scheelites and diorite around rocks and their alterations suggests that relative intensive fluid-rock interaction occurred before the scheelites precipitation.Sr-Nd isotopic composition of the scheelites indicates the primitive mineralizing fluid derived predominantly from the lower crust,but interfusion of the mantle-derived fluid could not be excluded.The primitive mineralizing fluid had close relationship with the regional ductile deformation in the mining district,the ductile shear zone penetrated the crust,induced CO_2 enriched fluid ascended from the lower crust,and gave birth to intensive fluid-rock interaction with the diorite rock.Formation of the brittle fractures in the shear zone triggered transportation,concentration and boiling of the ore-forming fluid and precipitation of the ore minerals.
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Key words:
- Scheelite /
- Trace elements /
- REE /
- Nd-Sr isotopic compositions /
- Ore-forming fluid /
- Diorite /
- Daping gold mine
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