赣中古塘地区锡石LA-ICP-MS U-Pb年代学及微量元素地球化学特征

LA-ICP-MS U-Pb chronology and trace element geochemistry of cassiterite in the Gutang area,central Jiangxi Province

江西中部相山矿田及其周边铀多金属成矿作用复杂。近些年,在其南部古塘地区地表新发现了一处锡石矿化带,赋存于加里东期花岗岩中。通过对该矿点锡石开展原位LA-ICP-MS U-Pb定年和微量元素分析,获得其207Pb/206Pb-238U/206Pb谐和年龄为150.44±2.78 Ma、下交点年龄为150.64±2.78 Ma,两者相近,可以代表该锡石矿的成矿年龄,与矿化带周边的燕山期山心单元花岗岩成岩年龄(152.4±1.1 Ma)相近。微量元素中,高场强元素Ti、Nb、Ta富集明显。与Sn容易发生类质同象的Mn、Nb、Hf、Ta等4种元素彼此正相关性明显,Fe和Mo呈正相关关系,却与Nb、Mn、Hf、Ta均呈现负相关性。在W-Fe图解中,处于花岗岩岩浆热液型锡矿床的右侧,说明研究区成矿物质应该主要来自岩浆热液,而且成矿温度较高。古塘地区锡石矿与山心单元花岗岩虽然具有相近的成矿、成岩年龄,但由于后者具有极高的氧逸度,不利于Sn元素的富集,而与Mo成矿有关;与山心单元花岗岩中可见辉钼矿矿囊而未发现锡矿化的地质现象保持一致。

Uranium polymetallic mineralization is complicated in Xiangshan orefield and its surrounding area in central Jiangxi Province.In recent years,a new cassillite mineralization zone has been discovered on the surface of Gutang area in the south of the Xiangshan orefield.Through in-situ LA-ICP-MS U-Pb dating and trace element analysis of the cassiterite,the harmonic age of 207Pb/206Pb-238U/206Pb is 150.44±2.78 Ma,and the lower intersection age is 150.64±2.78 Ma,which are close to each other.It can represent the metallogenic age of the cassillite deposit,which is similar to the diagenetic age of Shanxin unit granite(152.4±1.1 Ma)of Yanshanian.Among the trace elements,the high field-strength elements Ti,Nb and Ta are significantly enriched.Mn,Nb,Hf,and Ta,which are isomorphic with Sn,are positively correlated with each other.Fe and Mo are positively correlated with each other,but negatively correlated with Nb,Mn,Hf,and Ta.In the W vs.Fe diagram,it is on the right side of the granite magmatic hydrothermal tin deposit,indicating that the ore-forming materials in the study area should mainly come from magmatic hydrothermal,and the metallogenic temperature is high.Although the cassiterite and the granite in Gutang area have similar ore-forming and diagenesis ages,the latter is not conducive to the enrichment of Sn element due to its high oxygen fugescape,which is related to Mo mineralization.This also explains the reason why molybdenum ore sacs can be found in the Shanxin unit granite but no tin mineralization can be found.