新疆西准噶尔苏云河斑岩型钼矿的剥露和保存条件:来自裂变径迹和(U-Th)/He热年代学的约束

Exhumation and preservationconditions of Suyunhe porphyry molybdenum deposit in the West Junggar, Xinjiang (NW China): Constraints fromthe fission track and (U-Th)/He thermochronology

斑岩型矿床多形成于汇聚型板块边界。由于其较浅的就位深度,大部分古老的斑岩型矿床很容易受到后期的剥蚀而消失殆尽。研究斑岩型矿床成矿后的埋藏和去顶过程对于深入理解矿床的保存条件和区域找矿前景至关重要。新疆西准噶尔西部的苏云河斑岩型钼矿床形成于晚石炭世,是一处保存良好的斑岩型矿床,为我们研究前中生代斑岩成矿系统的保存条件提供了天然的实验室。本文首次针对该矿床开展了锆石和磷灰石裂变径迹与锆石(U-Th)/He低温热年代学分析,结合热历史反演模拟以及前人的年代学数据显示,苏云河斑岩钼矿的蚀变过程至少持续了55Myr。在早二叠世到中三叠世,矿区接受5.2~8.1km厚的沉积物覆盖。中三叠世至早白垩世(240~120Ma),矿床经历了快速剥露作用,剥露速率为49.0~56.7m/Myr,去顶量为7.4~9.2km。早白垩世(120Ma)至今为缓慢剥露阶段,剥露速率为6.7~21.7m/Myr,去顶量为0.8~2.6km。中三叠世至早白垩世的快速冷却事件可能并不是特定构造事件(比如:南部羌塘和昆仑-柴达木碰撞或者羌塘和拉萨碰撞)远程效应的产物,而与区域内走滑断层的活化密切相关。而矿床早期沉积的巨厚盖层以及早白垩世以来干旱气候和缓慢剥露,为石炭-二叠纪斑岩型钼矿得以保存提供了条件。

Porphyry deposit is a kind of epithermal deposit,which is mostly formed in the convergent plate boundary.However,ancient porphyry deposit was often destroyed and disappeared due to later tectonic or exhumation.Therefore,it is vital to reveal the exhumation and uplift process after mineralization for further understanding the preservation conditions and regional prospecting prospects of the deposits.Suyunhe porphyry molybdenum deposit in the west part of the West Junggar,Xinjiang was formed in Late Carboniferous.It is a wellpreserved porphyry deposit,which provides a natural laboratory for us to study the preservation conditions of ancient porphyry metallogenic system.In this paper,we present new zircon and apatite fission track(ZFT and AFT)and zircon(U-Th)/He(ZHe)data for the Suyunhe porphyry molybdenum deposit.Combined with thermal modeling inversion,we suggest that the hydrothermal alteration process of the Suyunhe porphyry molybdenum deposit lasted at least 55Myr.It underwent a thick burial(5.2~8.1km)sediments from Early Permian to Middle Triassic.The deposit experienced a rapid exhumation stage from Middle Triassic to Early Cretaceous(240~120Ma).The exhumation rate was 49.0~56.7m/Myr,and the corresponding denudation amount was 7.4~9.2km.After the Cretaceous(120Ma),it entered a slow exhumation stage with an exhumation rate of 6.7~21.7m/Myr and exhumation amount of 0.8~2.6km.Among them,the Middle Triassic to Early Cretaceous rapid exhumation was not the product of far-field effects(e.g.,the collision between Qiangtang and Kunlun-Qaidam or the collision between Qiangtang and Lhasa),but related to the activation of strike slip faults in the region.Therefore,early thick burial and later dry climate and slow exhumation provided key conditions for the preservation of the Carboniferous Suyunhe porphyry molybdenum deposit.