安徽凤阳和张八岭地区含金石英脉的^(40)Ar/^(39)Ar年龄及其地质意义

^(40)Ar/^(39)Ar Dating of Gold-bearing Quartz Veins in Fengyang and Zhangbaling Areas, Anhui Province, and Its Geological Significance

用快中子活化法测定了安徽凤阳和张八岭地区朱顶、毛山和上成 3个金矿床第一阶段晚期和第二阶段的含金石英脉 ,石英的阶段加热40 Ar/ 3 9Ar坪年龄值域为 (116 .1± 0 .6～ 118.3± 0 .5 )Ma,分别与其最小视年龄和等时线年龄接近。坪年龄、最小视年龄和等时线年龄 3种年龄值域为 (113.4± 0 .4～ 118.3± 0 .5 )Ma ,可以作为石英的形成年龄域。根据含金石英脉和围岩的空间关系 ,该年龄值域作为石英脉金矿的形成年龄是合理可靠的。金矿床形成于早白垩世阿普特期 。

Tectonically Fengyang and Zhangbaling areas belong respectively to the North China craton and the Dabie orogene, with Neo_Archean gneiss and amphibolite and metamorphosed marine facies sodic volcanic rocks being the main outcrops respectively. The Zhangbaling terrane strike_skipped along the Tan_Lu fault zone in Mesozoic and Cenozoic and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur extensively in the two areas. Gold_bearing quartz veins occur in metamorphic rocks in Fengyang area and in granodiorite and metamorphosed marine facies sodic volcanic rocks in Zhangbaling area. Generally, three stages of quartz veins can be recognized, i.e., gold_deficient sulfide quartz veins, gold_rich quartz sulfide veins and gold_deficient barite and/or carbonate veins. The 40 Ar/ 39 Ar step_heating plateau ages of the late first_stage and the second_stage quartz aggregates from the Zhuding, Maoshan and Shangcheng gold deposits range between 116.1～0.6 Ma and 118.3 ～0.5 Ma and are quite close to their least apparent ages and isochronal ages respectively. All plateau, least apparent and isochronal ages range from 113.4～0.4 Ma to 118.3～0.5 Ma, considered to be the formation age range of the quartz. It is reasonable and reliable to take the 40 Ar/ 39 Ar age range of the quartz as the formation age range of gold_bearing quartz veins on the basis of spatial relationship between gold_bearing quartz veins and their country rocks. The gold deposits in the two areas were formed in Aptian Stage of Cretaceous, when the Ta_Lu fault zone moved as a normal fault with slightly right_lateral strike_skip, extended and had very strong magmatic activity. It is shown that the magmatic hydrothermal fluid is a very important component part in the ore_forming hydrothermal fluid in Fengyang and Zhangbaling areas. The gold ore deposits in Fengyang and Zhangbaling areas genetically related to the extensional movement of the Ta_Lu fault zone and the magmatic activity were formed under the extensional dynamic condition in Late Cretaceous. Therefore, the extensional movement of the Ta_Lu fault zone provided energy and space for magmatic and gold ore_forming processes.