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

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.