江西香炉山钨矿床矽卡岩矿物和成矿花岗岩地球化学特征及其指示意义

Geochemical characteristics of skarn minerals and causative granites of the Xianglushan tungsten skarn deposit,Jiangxi,South China

江西香炉山大型钨矿床位于长江中下游成矿带南侧新识别的钨矿带西段,为一矽卡岩型白钨矿床。矽卡岩矿体主要产于中寒武统杨柳岗组含炭质灰岩和香炉山花岗岩接触部位。香炉山花岗岩具有细粒至中粗粒结构,主要由石英、钾长石、斜长石和黑云母组成,副矿物以钛铁矿为主。香炉山花岗岩的SiO_(2)含量为66.37%~67.76%,并具有较高的Na_(2)O+K_(2)O(8.33%~8.57%)和A/CNK(1.31~1.35),因此属于强过铝质花岗岩。香炉山矽卡岩矿物主要为钙铁辉石为主,而石榴石以钙铝榴石为主并有少部分锰铝-铁铝榴石,同时主要的白钨矿分布于矽卡岩晚期网脉中呈他形粒状并具有贫Mo的特征。因此,香炉山钨矿具有还原型矽卡岩钨矿的特征。黄铜矿和磁黄铁矿的δ^(34)S_(CDT)值分别为1.0‰~4.2‰和3.0‰~5.2‰,均呈现出典型的岩浆硫同位素的特征,同时矽卡岩后期白钨矿稀土元素的流体模拟配分形式也为右倾,与香炉山花岗岩稀土元素特征一致,表明岩浆热液流体的参与贯穿整个矽卡岩成矿过程。香炉山花岗岩具有较低但变化范围较宽的Nb/Ta(6.7~11.4),同时具有高Rb/Ba(1.6~2.4)和Rb/Sr(5.9~7.2),暗示壳源富粘土物质为原始岩浆的物源,并可能伴有极少量的幔源物质加入。同时香炉山花岗岩高Rb(339×10^(-6)~406×10^(-6))和Y+Nb(49.2×10^(-6)~62.0×10^(-6))及高Nb(15.7×10^(-6)~24.1×10^(-6))和低Y(33.5×10^(-6)~43.4×10^(-6)),指示其为板内花岗岩。因此,香炉山钨矿形成于板内的拉张环境。结合区域研究资料,其可能与古太平洋板块中伊扎纳吉板片与太平洋板片间的洋脊俯冲,及太平洋板片回撤有关。

The Xianglushan tungsten deposit in the Middle-Lower Yangtze River Metallogenic Belt,is a typical skarn-type scheelite deposit.Skarn orebodies are hosted in the contact between the carbonaceous siliceous-argillaceous limestone and the Xianglushan granite.The Xianglushan granite is fine-to medium-grained,mainly composed of quartz,K-feldspar,plagioclase,and biotite.The granite has SiO_(2)contents of 66.37%~67.76%,and high Na_(2)O+K_(2)O contents(8.33%~8.57%)and A/CNK ratios(1.31~1.35),belonging to highly peraluminous granites.In the Xianglushan deposit,skarn minerals are dominated by hedenbergitic pyroxene and grossular garnet with subordinate late-stage spessartine-almandine garnet,and the Mo-poor scheelite is abundant in the late-stage veins,which is typical of the reduced tungsten skarns.Theδ^(34)S_(CDT) values of chalcopyrite and pyrrhotite are 1.0‰~4.2‰and 3.0‰~5.2‰,respectively,typical of magmatic sulfurs.Furthermore,the LSM-modeled chondrite-normalized REE patterns of the hydrothermal fluids resulting in the precipitation of scheelite are generally right-dipping,which is consistent with the chondrite-normalized REE patterns of the Xianglushan granite,indicating that the magmatic-hydrothermal fluids participated in the whole skarn mineralization.The Xianglushan granite has relatively low Nb/Ta(6.7~11.4)with high Rb/Ba(1.6~2.4)and Rb/Sr(5.9~7.2),suggesting a crustal clay-rich source,but the melts were probably mixed with minor amounts of mantle-derived materials.Additionally,the granite contains high Rb(339×10^(-6)~406×10^(-6)),Y+Nb(49.2×10^(-6)~62.0×10^(-6)),high Nb(15.7×10^(-6)~24.1×10^(-6)),and low Y(33.5×10^(-6)~43.4×10^(-6)),indicating that it is an intraplate granite.Overall,the formation of the Xianglushan tungsten deposit could be formed in an extensional setting,which might be related to the ridge subduction of the paleo-Pacific plate and the later roll-back of the Pacific slab.