湘东北正冲金矿床成因:年代学和硫同位素制约

Genesis of the Zhengchong Gold Deposit in Northeast Hunan—Constraints from Geochronology and In-situ Sulfur Isotope

正冲金矿床是湘东北地区典型的金矿床,其矿体受NNE和NW向2组断裂控制,赋存于低变质程度的冷家溪群中,矿床成矿时代和成矿物质来源尚存在争议。借助LA-(MC)-ICP-MS分析方法,对正冲金矿床中含金石英脉中的绢云母进行原位Rb-Sr定年和黄铁矿原位S同位素分析,以确定其成矿时代和成矿物质来源,进而明确矿床成因。绢云母原位Rb-Sr定年结果显示:正冲金矿床的年龄在(219.1±6.0)~(215.8±7.7)Ma之间,黄铁矿的δ^(34)S值(-3.85‰~1.95‰)接近于0,且具有由负值向正值移动的趋势,表明深部岩浆(或幔源物质)可能是正冲金矿床成矿物质的主要来源,有少量地层物质参与成矿。结合矿床地质特征、成矿年代学和硫同位素地球化学特征,认为正冲金矿床形成于印支期陆内造山环境下,属于与深部岩浆活动有关的金矿床。

The Zhengchong gold deposit is a typical gold deposit in northeast Hunan.Its orebodies are controlled by NNE and NW trending faults and occur in Lengjiaxi Group with low metamorphism.The oreforming age and the source of ore-forming materials are still controversial.In this study,by using the LA-(MC)-ICP-MS analysis method,in situ Rb-Sr dating of sericite in gold-bearing quartz vein and in situ S isotope analysis of pyrite were carried out to determine the ore-forming age and source of ore-forming materials,so as to clarify the genesis of the deposit.According to the ore-vein interpenetrating relationship of ore-veins and mineral paragenesis,the mineralization can be divided into three ore-forming stages,namely quartz-pyrite-gold stage,quartz-pyrite-arsenopyrite-polymetallic sulfide stage,quartz-calcite stage.Microscopic observation show that sericite in stage 1 and stage 2 had a close intergrow with pyrite/arsenopyrite.In situ Rb-Sr dating of sericite show that the ore-forming ages of stage 1 and stage 2 were(219.1±6.0)Ma and(215.8±7.7)Ma,respectively.Three kinds of pyrite were identified under the backscattering(BSE)image.The pyrite(Py1)in stage 1 is mainly eudedral and medium-coarse-grained,withδ^(34)S ranging from−3.86‰to 1.03‰.The pyrite(Py2)in stage 2 has an euhedral,mesocoarse-grained structure and an obvious core-rim zonation. The core (Py2a) isuniform in the BSE image withδ^(34)S ranging from −1.43‰ to 1.51‰,while the edge (Py2b) has an obviousoscillation zone and other sulfide (galena) inclusions,with δ^(34)S ranging from −3.21‰ to 1.95‰.The δ^(34)S values(−3.85‰~1.95‰) of the three types of pyrite are close to 0 and the negative value moves to positive value,indicating that deep magmatic (or mantle-derived) materials may be the main source of ore-forming materialsin Zhengchong gold deposit,and a small amount of stratigraphic material is involved in the mineralization.Based on the geological characteristics,metallogenic chronology and sulfur isotope geochemistry,We believethat the Zhengchong gold deposit was formed in the Indosinian intracontinental orogenic environment andrelated to deep magmatic activity.