小秦岭金矿田成因的锶同位素约束

本文报道了前人对小秦岭金矿田部分金矿床流体包裹体、赋矿围岩(太华群)、文峪花岗岩的锶同位素结果以及作者新获得的中-新元古界地层(官道口群-栾川群)的缌同位素测试结果。鉴于小秦岭金矿田的金矿床和文峪花岗岩均形成于130Ma 左右,作者将所有~(87)Sr/~(86)比值返算为130Ma 时的初始值,即 I_(Sr-130)比值。文峪花岗岩的8件 I_(Sr-130)=0.7077～0.7084, 平均0.7082;太华群的12件 l_(Sr-130)=0.7073～0.8090,平均0.7557;中-新元古界地层的15件 I_(Sr-130)=0.7119～0.8373,平均0.7623;赋存于太华群中的金矿床流体包裹体的22件 I_(Sr-130)值变化于0.7334～0.7519,平均0.7440:赋存于文峪花岗岩中的矿脉流体包裹体7件 I_(Sr-130)=0.7135～0.71 85,平均0.7162。由此可见,赋存于太华群的金矿床 I_(Sr-130)值远高于文峪岩体,而落入太华群和中-新元古界地层的 I_(Sr-130)值范围,表明成矿流体不可能源于文峪花岗岩岩浆分异,而源于太华群和/或中-新元古界地层的变质脱水;赋存于文峪岩体中的含矿石英脉 I_(Sr-130)值低于太华群和/或中-新元古界地层,也低于太华群中的金矿床,但显著高于文峪岩体,应是高 I_(Sr-130)的成矿流体与低 I_(Sr-130)值的文峪花岗岩围岩相互作用的结果如此,碰撞造山成岩成矿与流体作用模式可较好阐释小秦岭金矿田的成因,即,中生代扬子与华北板块碰撞过程中,由中-新元古界地层及下伏太华群组成的陆壳板片沿小河断裂向北 A 型俯冲到小秦岭金矿田之下,俯冲板片经变质脱水作用为小秦岭金矿田形成提供了成矿流体,成矿流体与赋矿围岩的相互作用造成太华群中的矿脉与文峪岩体中的矿脉具有明显不同的 I_(Sr-130)值。

Sr-isotope constraint on the origin of the Xiaoqinling gold field.

This paper discusses the Sr isotope data obtained from previous studies on the ore-host rocks (Taihua Group),the Wenvu granite and the fluid inclusions of gold-bearing quartz veins in the Xiaoqinling gold field as well as new Sr isotope data recently obtained by the authors for the Neo-Mesoproterozoic strata (Guandaokou and Luanchuan Groups).All the ~(87)Sr/~(86) Sr ratios are calculated into initial ~(87)Sr/~(86)Sr ratios at 130 Ma,i.e.I_(Sr-130),considering that both the Wenyu granite and the gold deposits were formed at about 130 Ma.Eight I_(Sr-130) ratios of the Wenyu granite range from 0.7077 to 0.7084 with an average of 0.7082;twelve I_(Sr-130) ratios of the Taihua Group range from 0.7073 to 0.8090 with an average of 0.7557;fifteen I_(Sr-130) ratios of the Neo-Mesoproterozoic strata range from 0.7119 to 0.8373 with an average of 0.7623;twenty-two I_(Sr-130) ratios of the Taihua Group-hosted quartz veins range from 0.7334to 0.75 I9 with an average of 0.7440;and seven I_(Sr-130) ratios of the Wenyu granite-hosted quartz veins range from 0.7135 to 0.7185 with an average of 0.7162.Obviously,the I_(Sr-130) ratios of the Taihua Group-hosted quartz veins are far higher than those of the Wenyu granite,but accord with the I_(Sr-130) range of the Taihua Group and/or the Neo-Mesoproterozoie strata,suggesting that the ore fluids might be sourced from the metamorphic devolatilization of the Taihua Group and/or the Neo-Mesoproterozoic strata,instead of derivation from the Wenyu granite magma.The Wenyu granite-hosted quartz veins,whose I_(Sr-130) values are lower than those of the Taihua Group,Neo-Mesoproterozoie strata and the Taihua Group-hosted quartz veins,and higher than those of the Wenyu granite, should be the resuhs of interaction between the high I_(Sr-130) fluids and the low I_(Sr-130) Wenyu granite.Hence a tectonic model that combines eollisional orogeny,metallogeny and hydrothermal fluid flow is proposed to interpret the origin of the Xiaoqinling gold field.During the Mesozoie collision between the Yangtze and North China plates,a crustal slab containing the Neo-Mesoproterozoie strata and the Taihua Group was intracontineutally (A-type) subducted northwards beneath the Xiaoqinling gold field along the Xiaohe fault.Metamorphic devolatilization of the underthrust slab provided the ore fluids to develop the Xiaoqinlin gold field.Fluid-wallrock interaction resuhed in the significant differences in I_(Sr-130)value between the Wenyu granite- and Taihu Group-hosted quartz veins.