云南墨江金厂金矿是哀牢山金矿带内的一个大型金矿床,矿床受剪切带控制,矿体赋存于古生代浅变质岩中,其成矿物质来源一直是争论的焦点.为进一步精确约束赋矿地层的形成时代和大地构造背景,探讨其与金成矿物质来源和成矿作用之间的关系,本文对古生代浅变质岩中的凝灰岩开展了年代学、地球化学、黄铁矿成分和同位素特征研究.LA-ICP-MS锆石U-Pb定年结果(256.6±2.3Ma)表明墨江金矿区内存在晚二叠世火山碎屑岩.锆石稀土元素含量较高,Ce正异常(δCe=2.02～32.98)和 Eu 负异常明显(δEu=0.06～0.38).岩石具有低钛(TiO2=0.47％～0.71％)、较高铝(Al2O3=10.96％～16.33％)、富铁(Fe2OT3=3.21％～9.12％)和硫(S=0.95％～3.92％)等特征,属于英安质-流纹质凝灰岩;此外,所有样品轻重稀土元素分馏显著((La/Yb)N=7.50～15.7),Eu 异常明显(δEu=0.54～0.68),亏损 Nb、Ta、Ti、Sr、Ba,富集 K、Pb,显示出弧火山岩的地球化学特征.Nd同位素(εNd=-9.24～-9.61)显示富集特征,二阶段钕模式年龄为1.61～1.79Ga.凝灰岩中富含黄铁矿,黄铁矿属铁亏损型(S/Fe=2.00～2.09),存在Ni对Fe、As对S的类质同象置换;S主要来自地壳(δ34SV-CDT=-8.8‰～-9.7‰);Co、Ni含量高但没有相关性,指示其为火山成因,后期受到了富Ni流体交代;黄铁矿的Au含量偏高,但未发现独立金矿物,推测可能存在纳米金.全岩地球化学特征、Nd同位素特征和锆石微量元素特征指示墨江金矿区凝灰岩形成于陆缘弧构造背景,为古特提斯洋俯冲消减过程中玄武质岩浆底侵导致古元古代变质基底重熔的产物,岩浆形成过程中伴有地幔物质的加入,同时存在金的初始富集,形成含金黄铁矿,并在随后的区域变质作用及构造热事件中发生金的活化迁移,为金矿床的最终形成提供成矿物质.晚二叠世弧岩浆作用对于墨江金矿的形成具有重要意义.
Petrogenesis and mineralization implications of the tuff in Mojiang Au deposit,Yunnan Province
Jinchang large-scaled gold deposit in Ailaoshan gold belt is located in Mojiang County,Yunnan Province.Ore bodies hosted in Paleozoic epimetamorphic rocks in this deposit are constrained by shear zones,however,the origin of ore-forming materials is still under debate.In order to constrain the age and geodynamic setting of the ore-hosting strata,and reveal its implication on gold mineralization,geochronological and geochemical analyses on the tuff sandwiched in Paleozoic epimetamorphic rocks,and geochemical analyses of pyrites in the tuff were carried out.The tuff has LA-ICP-MS zircon U-Pb age of 256.6±2.3Ma,which makes it certain that there are Late Permian volcaniclastic rocks in Mojiang Au deposit.Zircons in the tuff are enriched in rare earth elements,displaying positive Ce anomalies and negative Eu anomalies(δCe=2.02～32.98,δEu=0.06～0.38),with crystallization temperatures of 682～1139℃.The rocks belong to dacite-rhyolite tuff,characterized by low TiO2(0.47％～0.71％),and relatively high Al2O3(10.96％～16.33％),total Fe2O3(3.21％～9.12％)and S(0.95％～3.92％)contents.The fractionation between light rare earth elements and high rare earth elements and Eu anomalies are remarkable((La/Yb)N=7.50～15.7,δEu=0.54～0.68).All rocks are significantly depleted in Nb,Ta,Ti,Sr and Ba,and enriched in K and Pb,displaying a geochemical affinity to arc volcanic rocks.Trace element ratios,such as Nb/Ta,Nb/U,Zr/Hf and Th/U,are within the ranges of 11.63～12.04,3.95～4.53,32.11～34.16 and 4.02～5.62,respectively,similarly to that of the continental crust.They have εNd values of-9.24～-9.61,and two-stage Nd model ages of 1.61～1.79Ga.Ubiquitous pyrite in tuff belongs to Fe-depleted-type one(S/Fe=2.00～2.09),and there were substitution of Ni to Fe and As to S.Sulfur in pyrite was mainly derived from crustal sedimentary(δ34SV-CDT=-8.8％o～-9.7％o).Cobalt and nickel contents are relatively high without any linear dependence,suggesting that pyrite was of volcanic origin and metasomated by Ni-rich fluid.The Au content(0.09％～0.21％)in pyrite is relatively high,while none of gold minerals had been founded in it,implicating that gold was hosted in pyrite in the form of nano-particles.Geochemical characteristics suggest that the tuff in Mojiang Au deposit was generated in continental margin arc during the Late Permian subduction of Paleo-Tethys Ocean and was generated by melting of Paleoproterozoic metamorphosed basement triggered by underplating of basaltic magma,while some mantle-derived magma took part in this process.Meanwhile,gold in Paleoproterozoic rocks and mantle was transported into magma chamber and hosted in pyrite during the diagenesis process.Dissolution-migration-reprecipitation of gold in pyrite might take place in the subsequent tectonic-magma-fluid processes,and provided part of gold resource for the deposit.The Late Permian continental arc magmatism was critical for the formation of Mojiang gold deposit.
TuffLate PermianContinental arc magmatismMojiang gold depositAilaoshan gold belt
万方数据
维普
