雅鲁藏布江缝合带造山型金矿床成矿流体特征:来自He-Ar同位素的约束

Ore-forming fluid characteristics of orogenic gold deposits in the Indus-Yurlung Zangbo suture zone:constraints from He-Ar isotope geochemistry

雅鲁藏布江缝合带造山型金成矿带是近些年来在青藏高原新确立的碰撞造山型金成矿带,成矿流体是一套CO_(2)-H_(2)O-NaCl-CH_(4)-N_(2)体系,以变质水为主。为探讨地幔组分对成矿的影响,本文选取雅鲁藏布江缝合带内邦布矿床和念扎矿床两个大型的造山型金矿为研究对象,对与成矿相关的黄铁矿及胶黄铁矿进行He-Ar同位素分析。结果显示,邦布矿床含金石英脉中黄铁矿中流体包裹体的He和Ar的浓度变化范围较窄,^(3)He/^(4)He为0.59~0.70 Ra,40Ar/36Ar为365.1~423.1;2件胶黄铁矿样品的^(3)He/^(4)He为0.11 Ra及0.19 Ra,^(40)Ar/^(36)Ar为302.1和305.9;念扎矿床石英-多金属硫化物脉中黄铁矿的^(3)He/^(4)He为0.27~0.60 Ra,^(40)Ar/^(36)Ar为370.2~1777.9。雅鲁藏布江造山型金成矿带中造山型金矿成矿流体为壳幔源混合流体,地壳端元主要为变质水,地幔组分在邦布金矿和念扎金矿中的贡献率分别为12.8%~15.2%和5.7%~13.0%;这与藏南拆离系中的金锑成矿作用明显不用,藏南拆离系中的金锑矿床成矿流体主要表现为改造型饱和大气水的特征,未有地幔组分的贡献。

The Yurlung Zangbo orogenic gold metallogenic belt is a newly established collisional orogenic gold metallogenic belt in the Tibetan Plateau.The metallogenic fluid is a set of CO_(2)-H_(2)O-NaCl-CH_(4)-N_(2) system that is mainly composed of metamorphic water.In order to explore the influence of mantle component on mineralization,two large-scale orogenic gold deposits,the Bangbu deposit and the Nianzha deposit in the Indus-Yurlung Zangbo suture zone(IYS)are selected as the research objects for He-Ar isotopic analyses.The results show that the concentration ranges of He and Ar isotopes in fluid inclusions of pyrite from the gold bearing quartz vein of the Bangbu deposit are narrow with ^(3)He/^(4)He=0.59~0.70 Ra,40Ar/36Ar=365.1~423.1 respectively.The ^(3)He/^(4)He and ^(40)Ar/^(36)Ar values of the two melnikovite samples are 0.11 Ra,0.19 Ra and 302.1,305.9.The ^(3)He/^(4)He and ^(40)Ar/^(36)Ar of pyrite in quartz polymetallic sulfide vein of the Nianzha deposit are 0.27~0.60 Ra and 370.2~1777.9 respectively.The ore-forming fluid of the orogenic gold deposits in the IYS is a mixture of crustal and mantle fluid whose crustal end member is mainly metamorphic water.The contribution of mantle components in the Bangbu gold deposit and Nianzha gold deposit are 12.8%~15.2%and 5.7%~13.0%respectively,which is obviously different from the Au-Sb mineralization in the southern Tibetan detachment system.The ore-forming fluid of Au-Sb deposit in the detachment system of southern Tibet is mainly characterized by the modified saturated atmospheric water,without the contribution of mantle component.