华南大瑶山地区加里东期钨矿床

Caledonian tungsten deposits in Dayaoshan area of South China

早期研究认为华南加里东期岩浆岩一般不成矿,但自20世纪80年代以来相继发现了一些矿床的事实打破了此观点,特别是近年来大瑶山地区与加里东期岩浆岩有关的钨多金属矿床的大量发现,进一步表明加里东期岩浆岩也能形成矿集区规模的矿产地。大瑶山地区目前已发现9处加里东期的钨矿床(点),包括社垌和玉坡2个大型矿床,已查明WO3资源量接近20万吨,矿床类型包括矽卡岩型、石英脉型和斑岩型。成矿时间集中在432~466 Ma。成矿元素以W为主,部分矿床Mo、Cu、Bi、Ag、Pb、Zn、Au也达到综合利用价值。矿石矿物以白钨矿为主。除与磁黄铁矿化、绢云母化、绿帘石化和透辉石化的岩浆岩时空密切相关外,斑岩型、石英脉型矿床还与一组近东西向或北西向的陡立小断层和节理带有关;矽卡岩型矿床则与寒武系中的灰岩夹层密切有关,并具有元素分带性。矿床地球化学表明主成矿阶段均一温度集中在225~370℃,盐度w(NaCleq)集中在5%~25%,H-O-S同位素示踪显示成矿流体和成矿物质主要来源于岩浆岩。从早到晚,温度和盐度降低、pH值升高以及流体混合是造成矿石沉淀的主要机制。与成矿有关的花岗闪长岩属于低分异还原性的“I”型花岗岩,单体呈小的岩株和岩枝产出,但常成群成带分布,成岩时间跨度较长(432~479.6 Ma)。大瑶山地区与钨矿有关的岩体缺乏伟晶岩,未发现富铝或富含挥发分矿物,但含角闪石。地球化学判别属于钙碱性到高钾钙碱性系列,弱过铝质花岗岩;具有低分异指数,低Li、B、La/Yb和Rb/Sr,高CaO/(Na2O+K2O)、Mg/(Mg+Fe)和高固结指数、高镁铁质矿物含量和弱的Ba、Sr、P和Ti亏损;稀土元素配分曲线为明显右倾斜,弱的负Eu异常;全岩和锆石的Zr/Hf比值较高,说明与钨成矿有关的岩体为低分异岩体,与传统上认为含钨岩体为高分异花岗岩明显不同。升高的锆石O同位素、弱的负εHf(t)值以及1.5~1.6 Ga的TDM2年龄表明花岗闪长岩来源于中元古代地壳物质的部分熔融,并混有一定量的地幔物质。大瑶山地区强烈的加里东期岩浆活动及成矿作用与“华南裂谷”最终关闭导致的陆内碰撞有关。该区加里东期岩浆活动的强度、范围和成矿作用可与该区燕山期的媲美,具有较大的找矿潜力。大瑶山地区新发现钨矿具有矿集区规模的事实,进一步证实华南加里东期花岗岩也能成大矿,这一结论对深入认识华南地区加里东期构造岩浆演化和成矿作用具有重要的科学意义。

Caledonian magmatic rocks in South China were believed to be barren,but since the 1980 s,the fact that some deposits have been found broke this view.Especially in recent years,many tungsten polymetallic deposits related to Caledonian magmatic rocks have been discovered in Dayaoshan area,which further indicates that Caledonian magmatic rocks can also be mineralized.In the Dayaoshan area,nine Caledonian tungsten deposits(occurrence)have been discovered,including two large deposits,Shedong and Yupo.The WO3 resource has been reported to be close to 200000 tons.The types of deposits include skarn-,quartz vein-and porphyry-types.The metallogenic time concentrated between 432 and 466 Ma.The metallogenic elements is W,with Mo,Cu,Bi,Ag,Pb,Zn,and Au in selective deposits.The main ore mineral is scheelite.In addition to being closely related to the spacetime of magmatic rocks of pyrrhotite,sericite,epidote and diopside,porphyry-type and quartz vein-type deposits are also associated with a set of steep small faults and joints in the EW and NW directions.The skarn deposit is closely related to the limestone interlayer in the Cambrian and has elemental zones.The geochemistry of the deposit indicates that the homogenization temperature in the main metallogenic stage concentrated between 225~370℃,and the salinity ranged from 5%to 25%w(NaCleq).H-O-S isotope tracing shows that the ore-forming fluids mainly come from magmatic rocks.The variation of temperature,salinity,pH,and fluid mixing are the factors intriguing the ore precipitation.The granodiorites related to mineralization belong to the I-type granite with low level of differentiation and reduction.These rocks are outcropped as stocks and apophysis,and often distributed in groups and belts.The concentrated rocks generated between 432~480 Ma.The granodiorites lack pegmatite,contain hornblende without aluminum-rich or volatile minerals found.Geochemical discrimination indicates that these magmatic rocks can be classified to the calcium alkaline to high potassium calcium alkaline series,weak over-aluminum granite with low differentiation index,low Li,B,La/Yb and Rb/Sr,high CaO/(Na2O+K2O),Mg/(Mg+Fe)and high consolidation index,high mafic mineral content,and weak Ba,Sr,P,and Ti losses,and the higher Zr/Hf ratio from both whole rock and zircon.The rare earth element distribution curve is obviously rightsloping with weak negative Eu anomaly.All these indicate that the rock mass related to tungsten mineralization is low-differentiated rock mass,which is obviously different from the traditionally considered tungsten-bearing rock mass as high-differentiated granite.Elevated zircon O isotope,weak negativeεHf(t)value,and TDM2 age of 1.5~1.6 Ga indicate that granodiorite originated from the partial melting of Mesoproterozoic crustal material and mixed with a certain amount of mantle material.The extensive Caledonian magmatism and mineralization in the Dayaoshan area could have been related to the collision caused by the final closure of the"South China Rift".The intensity,scope and mineralization of the Caledonian magmatic activity in this area are comparable to those of the Yanshanian period,which has great potential for prospecting.The fact that the recently discovered tungsten deposits forms an ore concentration area in the Dayaoshan area updated the previous understanding that the granite of the Caledonian in South China is generally barren,and is important for further understanding the Caledonian tectono-magmatic evolution and mineralization in South China.