湘南芙蓉锡矿床首次发现斧石:一个重要的含B矿物及其指示意义

The first discovery of axinite in the Furong tin deposit,southern Hunan:An important boron-bearing mineral and its implications for tin mineralization

世界上绝大部分锡矿床均与富F、Cl、Li、B等挥发份的高分异花岗岩有关,前人对F、Cl、Li等组份在花岗质岩浆演化及锡成矿过程中的作用已有较为深入的认识,但对挥发份B关注较少。南岭中段的芙蓉超大型锡矿床是我国重要锡矿产地,我们首次在该矿床发现了硼硅酸盐矿物中罕见的斧石,为研究挥发份B对锡成矿的作用提供了机会。通过详细的岩石学、矿物学和地球化学研究,揭示了斧石的产出特征、晶体结构和化学成分特征,进而探讨了挥发份B与该区花岗岩成岩成矿的关系。芙蓉锡矿床新发现的斧石为丁香褐色、块状构造、硬度较大,显微镜下呈无色、浅黄色等,主要为自形尖劈状的楔形、长条状等自形结构、正高突起、干涉色较低,与符山石、萤石、石榴子石、榍石等矿物共生。电子探针分析表明,芙蓉矿床的斧石属铁斧石,其平均化学式为Ca_(2.22)(Fe_(0.52),Mn_(0.41),Mg_(0.13))1.06Al_(2.11)B_(0.72)Si_(3.98)O_(15.5)。结合前人研究成果,芙蓉矿床斧石、电气石、硼镁铁矿等富B矿物的广泛产出,指示了骑田岭花岗岩富含挥发份B;硅酸盐熔体中挥发份B的存在,能降低岩浆粘度,增加岩浆的结晶分异程度,延长岩浆寿命,进而有利于Sn等不相容元素在岩浆演化过程中向残余熔体富集。B与锡成矿的关系密切,岩浆中的B能提高源区Sn的萃取效率,增加原始岩浆中Sn的含量;B能形成锡的络合物,有利于锡的长距离运移;B与Sn具有相似的地球化学行为,在岩浆演化过程中,富集于残余熔体中,进一步演化后富集于成矿流体中。此外,在芙蓉锡矿床,随着含B矿物的不断被发现,表明该矿床可能具有寻找伴生硼矿的潜力。

Most of the Sn mineralization worldwide is commonly associated with highly-fractionated granites,especially with those granites rich in volatile components such as F,Cl,Li and B.However,previous researches paid little attention to the volatile B.The giant Furong tin deposit,located in the middle part of Nanling Region,is of great significance in China.Axinite was found in skarn in this deposit for the first time,which provides an ideal opportunity to explore the role of volatile B during the formation of tin deposit.By means of detailed mineralogical,petrological and geochemical studies,the petrographic characteristics,crystal structures and chemical compositions of axinites from Furong were depicted,followed by a discussion on the relationship between the volatile B and granite emplacement,as well as tin mineralization in this area.The axinite from Furong deposit is lilac brown in color,massive in texture,hard,colorless and light yellow under microscope,with self-shaped wedge shape,high protrusion and low interference color,and intergrown with vesuvianite,fluorite,garnet and sphene.It was revealed that the axinite belongs to axinite-(Fe),with a general chemical formula of Ca 2.22(Fe 0.52,Mn 0.41,Mg 0.13)1.06 Al 2.11 B 0.72 Si 3.98 O 15.5.There are various B-rich minerals including axinite,tourmaline and ludwigite in the Furong deposit,indicating that the Qitianling granitic pluton is rich in boron.The enrichment of the volatile B in granite can reduce the magma viscosity,and increase the degree of magma differentiation,which helps the enrichment of incompatible elements like Sn in the residual melt during the magma evolution.Boron is also closely related to tin mineralization,since the B enrichment can facilitate the extraction of Sn from source rocks,improving the Sn concentrations in the primary magma.Like F and Cl,boron can form a complex with Sn,which is beneficial to the long-distance migration for tin;and they can be enriched in residual melt during magma evolution because of similar geochemical behaviors of B and Sn during the fractional crystallization,and these magmas are considered to be the sources of the mineralizing fluid responsible for tin mineralization.In addition,with the continuous discovery of B-bearing minerals in the Furong tin deposit,it indicates that there is a good potential for the prospecting of associated boron ores in the studied area.