赣东北朱溪钨(铜)矿区高分异花岗岩的成因及与钨矿的关系

摘要

朱溪钨(铜)矿是江西东北部近年来发现的超大型钨矿,本文报道了与成矿相关岩体的主量元素、微量元素、Nd同位素成分和黑云母、斜长石电子探针数据,以探讨岩体成因以及F在岩浆演化及成矿过程中的作用.朱溪矿区花岗岩具有富硅铝、贫钙镁的特征,属于典型的S型花岗岩,而不是A型花岗岩.Nd同位素(εNd(t)=-10.1～-7.9)、岩石学、地球化学特征表明朱溪矿区花岗岩是在超高温麻粒岩相条件下形成于下地壳含富Mg/Ti黑云母的麻粒岩相残留体的再次部分熔融,并非双桥山群变质沉积岩的直接部分熔融以及九岭(和杨草尖)黑云母花岗岩的重融形成.三类岩石可能代表同一个母岩浆的分离结晶的产物,结晶的先后次序为二云母花岗岩-白云母花岗岩-白云母花岗斑岩.这些花岗岩具有明显的富F特征,F的加入降低了花岗岩岩浆的固相线温度(低锆饱和温度,621 ～ 779℃)和粘度,延长了岩浆演化过程,导致了强烈的分离结晶作用和熔体-流体作用,并形成偏离正常岩浆体系的微量元素比值(低K/Rb、Zr/Hf、La/Nb、La/Ta比值).朱溪矿区花岗岩较长的岩浆演化时间和较低的结晶温度使其可以长时间萃取双桥山群等围岩中的W元素.F与W具有明显的正相关性,W与F络合随流体迁移,巨量的W在合适的温压条件下沉淀.朱溪矿区高分异花岗岩对于钨矿的形成起了重要作用.%The Zhuxi tungsten (copper) deposit is a super-large tungsten found in the northeastern Jiangxi Province in recent years.In this study,we report major elements,trace elements,Nd isotopic data,plagioclase and biotite microprobe data for mineralized granite to explain its petrogenesis and the role of fluorine in magmatic evolution and ore-forming process.Zhuxi granite is rich in alumina and silica,poor in calcium and magnesium,indicating they belong to typical S-type granite rather than A-type granite.Evidence of Nd isotopic data (εNd (t) =-10.1 ～-7.9),petrology and geochemistry indicate that Zhuxi granite is the product of partial melting of residue containing Mg/Fe-rich bitotite in granulite-facies,the residue is equilibrated with the melt generating Jiuling and Yangcaojian biotite granite.Three kinds of granite belong to comagmatic evolution with order two-mica granite,muscovite granite and granite porphyry.Peculiarly,Zhuxi granite is enriched in fluorine which reduced the solidus temperature (low zirconium saturation temperature,621 ～779℃) and viscosity of the granitic magma,prolonged the magmatic evolution.As a consequence,fractional crystallization and melt-fluid interaction were strengthened;some elements' ratios were changed relative to normal igneous system (low K/Rb,Zr/Hf,La/Nb,La/Ta ratios).Longer magma evolution time and lower crystallization temperature of granite from the Zhuxi ore deposits have long extraction time of W element from Shuangqiaoshan Group.Meanwhile,F and W have apparently positive correlation,they form complex compound to migrate in the fluid.Massive tungsten will precipitate in appropriate temperature-pressure condition.Therefore,we conclude that Zhuxi highly differentiated granite has a decisive effect on tungsten ore formation.