石英脉型金矿床中热液含铀-钍副矿物的成因识别及U-Th-Pb定年：以小秦岭矿集区西矿带为例

Two contrasting models have been proposed, to explain the large-scale gold mineralization in the Xiaoqinling gold district, including the Triassic continental collision of the Qinling Orogen and the early Cretaceous lithospheric thinning of North China Craton. It is very powerful and useful tool to understanding the ore genesis and tectonic setting through accurate and reliable geochronological study. Thus, in this proposal, we selecte three representative gold deposits from the western metallogenetic belt, Xiaoqinling district, including Dongtongyu, Xitongyu and Chen'er lode gold deposits, where lack of high-quality data of radiometric age. After detailed field investigation and petrographic study, scanning electron microscopy combined with energy dispersive spectrum (SEM-EDS), electron probe microprobe (EMP), laser raman microprobe (LRM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis were utilized to present the occurrences, distribution, crystal morphology, internal texture, fluid/mineral inclusions and trace-element composition of U-Th enriched accessory minerals (eg, monazite, xenotime, zircon and apatite, etc.) in auriferous quartz veins from lode gold deposits. An attempt is made to identify hydrothermal origin of accessory minerals accurately which associated with the gold mineralization, and determined the precise ore-forming age of Dongtongyu, Xitongyu and Chen'er gold deposits by LA-(MC)-ICPMS and SHRIMP in situ accessory minerals U-Th-Pb dating. These results, combination with previously published data for gold deposits from other metallogenetic belt in Xiaoqinling, can discuss the geochronology, genesis and geodynamic setting of the large-scale gold mineralization in Xiaoqinling district.

目前关于小秦岭矿集区大规模的金成矿作用主要有两种截然不同的观点，即与秦岭造山带造山作用有关的早中生代成矿和与中国东部构造体制转折有关的晚中生代成矿。精确可靠的成矿年代学研究是正确理解该区矿床成因和成矿构造背景的关键。因此，拟选取成矿年代学研究十分薄弱的小秦岭西矿带中三个规模较大的脉状金矿床(东桐峪、西潼峪及陈耳)作为对象，在详细的野外地质观察基础上，通过扫描电镜-能谱分析、电子探针、激光拉曼和激光剥蚀等离子体耦合质谱仪等研究，查明含金石英脉中含铀-钍副矿物(如独居石、磷钇矿和锆石等)的产状分布、形貌、内部结构、矿物共生关系、流体/矿物包裹体及元素地球化学等特征，从中识别出热液成因副矿物；进而利用LA-(MC)-ICPMS和SHRIMP对这些热液副矿物进行原位U-Th-Pb定年，准确限定西矿带金矿床的成矿年龄，并结合其他矿带的研究成果，探讨小秦岭地区金成矿作用的时代、成因及成矿动力学背景。

小秦岭地区是我国最重要的金矿产区之一，目前对该区金矿成矿作用存在两种不同的观点，即与秦岭造山带造山作用有关的早中生代成矿和与华北克拉通破坏相关的晚中生代成矿。精确激光阶段加热40Ar/39Ar定年结果显示，小秦岭西矿带陈耳、东桐峪金矿床热液绢云母和黑云母40Ar/39Ar坪年龄分别为130.9 ~ 126.5 Ma和143.5 ~ 118.9 Ma。对秦南金矿热液独居石进行U-Th-Pb定年，岩相学研究表明独居石与黄铁矿等载金矿物近于同时形成。13个独居石颗粒的206Pb/238U和208Pb/233Th加权平均年龄分别为120.9 ± 0.9 Ma和122.6 ± 1.9 Ma，在误差范围内完全一致，一致表明小秦岭地区绝大多数金矿床形成于早白垩世。小秦岭东桐峪金矿床不同世代黄铁矿LA-ICPMS分析结果表明，金的富集与As无关，第三世代黄铁矿中Au、Ag及Te存在显著富集且稳定的线性正相关关系，指示Te而不是As在金的迁移、富集沉淀等过程具有重要作用，暗示金矿床的成矿物质可能来自深部岩浆的脱挥发分或地幔脱气作用，而与区域变质作用关系不大。结合小秦岭邻区胶东界河金矿床绢云母40Ar/39Ar坪年龄119.7 ± 0.8 ~ 118.8 ± 0.7 Ma，表明界河金矿床形成于早白垩世，而不是前人认为的始新世（46.5 ± 2.3 Ma）。精确年代学一致证明华北克拉通大规模金成矿作用集中发生于晚中生代，与华北克拉通破坏关系密切相关。