江西乐华铅锌矿花岗斑岩锆石年代学、微量元素特征及其地质意义

U-Pb Ages and Trace Element Characteristics of Zircon from Granite Porphyry in Lehua Lead-zinc Deposit, Jiangxi Province and their Geological Significances

乐华铅锌矿位于江西乐-德成矿带SW段,矿区花岗斑岩LA-ICP-MS锆石U-Pb年龄集中于170 Ma(162～174 Ma)和860 Ma(857～862 Ma)两个年龄段,另有1063 Ma、2010 Ma锆石的出现。170 Ma年龄段的锆石为原生锆石, U-Pb谐和年龄为170.4±2 Ma,代表成岩年龄,其余为继承锆石,其中860 Ma年龄段锆石U-Pb谐和年龄为861±8 Ma。结合区域地质背景和170 Ma年龄段锆石微量元素特征,指示其形成于燕山期大陆伸展构造背景, 860 Ma年龄段锆石形成于晚晋宁期华南古洋壳俯冲消减之后华夏板块与扬子板块碰撞阶段。花岗斑岩富集轻稀土,亏损重稀土,具轻微的Eu负异常(Eu/Eu*=0.75～0.97),与江西乐-德成矿带NE段德兴、银山花岗岩类的稀土元素特征一致,而且两者的成岩年龄在误差范围内基本一致,区域构造背景相似,因此认为乐华花岗斑岩与德兴、银山的花岗质成矿斑岩体极可能具同源性,也属壳幔混合源,是同系列花岗质岩浆活动产物,乐华矿区花岗斑岩也可能具有形成类似多金属矿的潜力。

The Lehua lead-zinc deposit is located in the southwest section of the Lehua-Dexing Metallogenic Belt. LA-ICP-MS analytical results of zircon from the granite porphyry in the deposit showed that the porphyry was crystallized at 170.4±2 Ma, and the rock also hosts inherited zircon grains with ages of 857 Ma to 2010 Ma. The magmatic zircon grains have trace element compositions similar to those formed in continental extensional tectonic setting, and low crystallization temperatures of 556 ℃ to 811 ℃, with a mean value of 694 ℃. On the contrary, inherited zircon grains with ages of ca. 860 Ma were formed in a collision setting, which is related to the collision between the Cathaysian and Yangtze plates. These inherited zircon grains have crystallization temperatures of 705 ℃to 770 ℃, with a mean value of 740 ℃. The granite porphyry has total REE concentrations of 117×10–6 – 212×10–6 and showing chondrite-normalized rare earth element patterns similar to those from the Dexing and Yinshan deposits which are located in the North-East section of the Lehua-Dexing Metallogenic Belt, which are characterized by LREE enrichment, HREE depletion and slight negative Eu anomalies(Eu/Eu* = 0.75 – 0.97). The ore-bearing granite porphyries in the Lehua, Dexing, and Yinshan deposits have identical age of crystallization, highly consistent chondrite-normalized REE patterns, and same regional geological background. It is believed that the granite porphyries from the Lehua, Dexing and Yinshan deposits are likely comagmatic. Therefore, it is reasonable to deduce that the granite porphyry in the Lehua deposit may have ore potential similar to the other two porphyries and can be a promising target for ore prospecting.