江西德兴金山金矿晋宁期花岗质超糜棱岩的发现及意义

Discovery and of Jinningian Granitic Ultramylonite in Jinshan Gold Deposit,Dexing Area,Jiangxi Province its Significance

江西金山金矿位于江西省东北部德兴市境内,区域上位于钦杭成矿带东段,属于受韧性剪切带控制的动力变质热液型金矿。本文在发现金山金矿的赋矿岩石为双桥山群花岗质超糜棱岩的基础上,通过系统的野外地质调查、室内岩矿相学研究,对花岗质超糜棱岩进行了主量元素、微量元素与稀土元素的测试以及LA-ICP-MS U-Pb法测年,探讨了花岗质超糜棱岩原岩形成的构造环境、年代学与区域成矿意义。其SiO2含量介于53.32%~69.86%之间,为中酸性岩。全碱指数(ALK)变化于3.02~6.00,属于亚碱性岩石。铝过饱和指数(A/CNK)变化于0.96~2.49之间,为准铝质和过铝质系列,具S型花岗岩特征。里特曼指数(σ)为0.55~1.62,K2O/Na2O变化于0.48~12.13,为钙碱性和高钾钙碱性系列。富集大离子亲石元素Rb、Th、K、U和La、Ce、Nd、Zr、Hf等元素,亏损Ba元素和Ta、Nb、Sr、P、Ti等高场强元素。稀土元素配分曲线为右倾型,重稀土元素部分的曲线形态较为平缓。∑REE=107.64×10-6~168.82×10-6,稀土元素总量偏低。LREE/HREE为5.18~6.44,(La/Yb)N为4.11~6.20,轻重稀土元素分异明显。δEu=0.55~1.00,平均为0.68,具有中等的Eu负异常。锆石中Th/U值介于0.57~2.04,为岩浆成因。原岩金山金矿花岗质超糜棱岩,岩性为花岗闪长岩,属于S型花岗岩,形成于岛弧环境。锆石的LA-ICP-MS定年结果表明其成岩与成矿时代一致,为新元古代(834.3±3.8 Ma),属晋宁期成矿。本文测试得到其最高金含量可达15.3×10-6,建议重视金山周边地区及整个江南造山带乃至华南其它地区与剪切带有关的新元古界双桥山群火山质变质岩中Au的找矿评价工作。

The Jinshan gold deposit in the Dexing city of northeast Jiangxi Province lies in the east of Qinzhou-Hangzhou metallogenic belt. It is a dynamic metamorphism hydrothermal gold deposit controlled by a ductile shear zone. It is of great significance to determine its metallogenic epoch for deposit genesis study and regional prospecting work. We first discovered that the Jinshan gold ore-bearing rock is the Shuangqiaoshan Formation granitic ultramylonite. Through systematic field investigations and petrographic and ore microscopic studies,analysis of major elements,trace elements and REE analysis and LA-ICP-MS dating on the granitic ultramylonite,we have studied tectonic setting,geochronology and regional metallogenic features of this rock. The SiO2 content of the granitic ultramylonite ranges from 53. 32% to 69. 86%,belonging to acid rock. Alkali index(ALK) falls in the range of 3. 02 ~ 6. 00,indicative of its sub-alkaline type. Aluminum saturation index(A /CNK) varies between 0. 96 and 2. 49,showing features of metaluminous and peraluminous series with the S-type granites characteristics. Rittmann index( σ) values are in the range of 0. 55 ~ 1. 62,while K2 O /Na2 O values are 0. 48 ~12. 13,which means the granitic ultramylonite is calc-alkaline rock series. In the granitic ultramylonite,large ion lithophile elements such as Rb,Th,K,U and La,Ce,Nd,Zr and Hf are enriched,while Ba and high field strength elements of Ta,Nb,Sr,P and Ti are depleted. The REE patterns are rightdeclining,and the curve shape of HREE is relatively flat. The total REEs are lower( ∑REE = 107. 64 × 10-6~ 168. 82 × 10-6),and light and heavy REEs have obviously fractionation with LREE /HREE 5. 18 ~ 6. 44 and(La /Yb)N4. 11 ~ 6. 20,respectively. They have medium negative Eu anomalies with δEu = 0. 55 ~ 1. 00(0. 68 on average). Zircons in the granitic ultramylonite are of magmatic origin,while the Th /U values of zircons are in the range of 0. 57 ~ 2. 04. The protolith of Jinshan granitic ultramylonitic are S-type granites,which formed in an island arc environment. LA-ICP-MS zircon dating shows that the diagenetic and metallogenic epochs of the Jinshan gold deposit are consistent. The rock formed at Neoproterozoic(834. 3 ± 3. 8 Ma),and the metallogenic epoch is the Jinningian. The gold content of the deposit can reach 15. 3 × 10-6. Much attention should be paid on the Neoproterozoic Shuangqiaoshan formation volcanic metamorphic rocks related to the shear zone in the Jinshan surrounding areas,the Jiangnan orogenic belt and other regions in south China in prospecting gold ore deposits.