(1. 中南大学 地球科学与信息物理学院 有色金属成矿预测教育部重点实验室,长沙 410083;
2. 中国科学院 地球化学研究所 矿床地球化学国家重点实验室,贵阳 550002;
3. 湖南省有色地质勘查局二总队,湘潭 411102)
摘 要: 包金山金矿床是近年在湘中地区探获的以金为主的多金属矿床,其成矿与印支晚期岩浆活动关系密切。利用高精度SIMS锆石U-Pb定年方法得到矿区两条花岗闪长斑岩年龄分别为(225.1±1.5) Ma和(223.3±1.4) Ma,在误差范围内一致,属印支晚期大规模岩浆活动的产物;矿区两条花岗闪长斑岩的176Hf/177Hf值均集中分布在0.2825附近,对应的εHf(t)值介于-5.9~3之间,平均为-3.98,两阶段模式年龄TDM2为0.96~1.45 Ga,平均为1.35 Ga;δ18O值为0.824%~0.973%;Hf-O同位素研究表明花岗闪长斑岩主要为中元古代下地壳岩石重熔形成,且有部分幔源物质参与成岩作用;与紫云山花岗岩体对比分析,表明两者为同源岩浆演化的产物,且间接指示包金山金矿成矿时间为227~223 Ma;花岗闪长斑岩形成于多板块汇聚的动力学背景,为印支地块向北挤压和太平洋板块向西俯冲共同作用的结果。
关键字: SIMS 锆石U-Pb年龄;Hf-O同位素示踪;花岗闪长斑岩;包金山矿床;湘中
(1. Key Laboratory of Non-ferrous Metals Metallogenic Prediction, Ministry of Education, School of Geosciences and Info-physics, Central South University, Changsha 410083, China;
2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences,Guiyang 550002, China;
3. The Second Corps of Nonferrous Geological Prospecting Bureau in Hunan Province, Geological Exploration Institute, Xiangtan 411102, China)
Abstract:Baojinshan gold deposit is Au-dominated polymetallic deposit discovered recently in central Hunan, its mineralization is considered to be closely related to the magmatic activities in late Indosinian. High accuracy SIMS zircon U-Pb age determination was used in this study to obtain the ages of two granodiorite-porphyry dykes at (225.1±1.5) Ma and (223.3±1.4) Ma, indicating that this deposit was the product of extensive magma activity in late Indosinian. The 176Hf/177Hf values of two granodiorite-porphyry dykes intensively distributed around 0.2825, with the corresponding εHf(t) ranged from -5.9 to 3, averaging -3.98. The TDM2 model age was estimated from 0.96 GPa to 1.45 Ga, averaging 1.35 Ga; the δ18O value of granodiorite-porphyry ranged from 0.824% to 0.973%. Hf-O isotope analysis shows that the granodiorite-porphyry is mainly formed by the rock remelting of Mesoproterozoic lower crust, and mantle-sourced materials are considered to participate the diagenesis. Contrastive analysis on the granodiorite-porphyry in Baojinshan with Ziyunshan granite suggests that both of them are the products of comagmatic evolution, which indirectly indicates that the metallogenetic time of Baojinshan gold deposit ranges in 227-223 Ma. The granodiorite-porphyry is formed under the multi-plates convergent dynamics background, and affected by the northward squeeze of Indochina Block and westward subduction of Pacific Plate.
Key words: SIMS U-Pb zircon dating; Hf-O isotopes tracing; granodiorite-porphyry; Baojinshan gold deposit; central Hunan Province