胶东夏甸金矿床构造-热历史: 成岩-成矿年代学与磷灰石裂变径迹热年代学综合约束

胶东是全球唯一已知前寒武纪变质地体内的晚中生代巨型金成矿系统, 其成矿机制独具特色, 属胶东型金矿床成因新类型; 深入剖析该成矿系统的形成、变化与保存过程对丰富和完善金成矿理论具有重要意义。夏甸超大型金矿床位于胶东招平金矿带中南段, 赋存于晚侏罗世玲珑花岗岩和早白垩世郭家岭花岗岩中, 是一典型的胶东型金矿床; 该矿床成岩-成矿时代已有精确限定, 但其精细演化过程与成矿后变化-保存尚缺乏有效约束, 是开展矿床形成与隆升剥蚀研究的理想选择。因此, 本文通过成岩-成矿年代学与磷灰石裂变径迹热年代学综合约束该矿床的构造-热历史, 揭示其主控因素, 提高对该成矿系统的认识, 为深部找矿潜力评价提供支撑。本次获取的锆石U-Pb定年结果显示玲珑花岗岩和郭家岭花岗岩分别于151.7±2.1Ma(2σ)和125.8±1.6Ma(2σ)侵位。区域已有黑云母和白云母⁴⁰Ar/³⁹Ar年龄表明玲珑岩体侵位后缓慢冷却, 并发生韧性变形; 郭家岭花岗岩则经历了同岩浆韧性变形和快速冷却; 约于124Ma两者均冷却至350±50℃, 区域发生韧脆性变形转换。夏甸金矿床与载金矿物共生的热液绢云母⁴⁰Ar/³⁹Ar和热液独居石U-Pb年龄表明, 该区于约120Ma发生大规模脆性构造活动和金成矿作用。本文利用采自夏甸金矿床-652m中段532巷道中包括主控矿断层上、下盘经历不同变形程度的7件岩/矿石样品进行磷灰石裂变径迹(AFT)分析。除遭受异常热事件影响的主断裂上盘次级断层断层泥样品外, 其余6件样品AFT中值年龄在32.5±1.7Ma(1σ)至23.6±2.9Ma(1σ)之间, 其径迹长度平均值低于14μm(范围为12.0±0.3μm~13.8±0.3μm)、为单峰分布、并略具负偏斜特征, 表明该矿床大致在32~23Ma单调缓慢通过磷灰石裂变径迹部分退火带(125~60℃)。综合区域已有热年代学资料与上述锆石U-Pb、AFT数据, 重建了夏甸金矿床自赋矿玲珑和郭家岭花岗岩侵位以来的构造-热演化历史, 计算出自120Ma金成矿至今该矿床平均冷却和剥蚀速率分别为1.8℃/Myr和0.059km/Myr, 总剥蚀量约为7km, 对比前人对胶西北地区整体剥蚀程度与深部成矿潜力的定量评估以及目前~1.5km的钻孔最大见矿深度, 推断其深部找矿潜力良好。

Apatite fission-track thermochronology and magmatism-mineralization geochronology constraints on the tectono-thermal history of the Xiadian gold deposit,Jiaodong Peninsula,eastern China

Jiaodong Peninsula is the only known Late Mesozoic giant gold metallogenic system with unique metallogenic mechanism within the Precambrian metamorphic terrane in the world, it belongs to a new genetic type of Jiaodong-type gold deposits. The formation, change and preservation process of this metallogenic system is of great significance to enrich and perfect the theory of gold mineralization. Xiadian is a super-large gold deposit that located in the middle-south section of Zhaoping gold belt in Jiaodong Peninsula. It is a typical Jiaodong-type gold deposit, the main ore-hosting rocks are Late Jurassic Linglong granite and a small amount of hidden Early Cretaceous Guojialing granite. The magmatism and mineralization ages of this deposit have been precisely defined, but its fine evolution process and post-ore change and preservation are not effectively constrained. Therefore, the tectonothermal history of the deposit is comprehensively constrained by using magmatism-mineralization geochronology and apatite fission track thermochronology, and its controlling factors are revealed, so as to improve the understanding of this metallogenic system and provide support for the evaluation of deep prospecting potential. Zircon U-Pb dating results show that the Linglong and Guojialing granites are emplaced at 151.7±2.1Ma (2σ) and 125.8±1.6Ma (2σ), respectively. The 40Ar/39Ar ages of biotite and muscovite in the region indicate that the Linglong intrusion cooled slowly after emplacement and underwent ductile deformation. The Guojialing granitoid underwent syn-magmatic ductile deformation, followed by rapid cooling. Both intrusions cooled to 350±50℃ at ca.124Ma, and the transition from ductile deformation to brittle occurs in the region later. The 40Ar/39Ar age of hydrothermal sericite and the U-Pb age of hydrothermal monazite in the Xiadian gold deposit indicate that intense brittle tectonic events and gold mineralization occurred at ca.120Ma. In this paper, apatite fission track (AFT) analysis was carried out using 7 rock/ore samples collected from the 532 tunnel at the -652m level of the Xiadian gold deposit, including the hanging wall and footwall of Zhaoping detachment fault that underwent different degrees of deformation. Except for the secondary fault gouge samples from the hanging wall affected by abnormal thermal events, the median AFT age of the other six samples ranged from 32.5±1.7Ma (1σ) to 23.6±2.9Ma (1σ), and the AFT length are unimodal with slightly negative deviation characteristics. The average AFT length is lower than 14μm (range from 12.0±0.3μm to 13.8±0.3μm), indicating that the deposit monotonically slowly passed through the apatite partial annealing zone (125~60℃) at approximately 32~23Ma and underwent a similar thermal history. Combining the existing thermochronology data and the zircon U-Pb and AFT data in this study, the tectonothermal history of the Xiadian gold deposit since the emplacement of the host rocks (Linglong and Guojialing intrusions) was reconstructed. The average cooling and denudation rate of the Xiadian gold deposit since ca.120Ma gold mineralization to nowadays is 1.8℃/Myr and 0.059km/Myr, respectively. Thus, the total amount of denudation is 7km. Compared with the quantitative evaluation of the overall denudation degree and the deep mineralization potential of the northwest Jiaodong area and the maximum ore depth of the borehole ~1.5km, it is concluded that the deep prospecting potential of the Xiadian gold deposit is promising.