Extensional tectonic setting of the Triassic metallogenic event in the Xiaoqinling goldfield: Evidence from <sup>40</sup>Ar-<sup>39</sup>Ar chronology of the tectonite

Abstract

小秦岭金矿田位于华北克拉通南缘，同时也是秦岭复合型造山带的北缘组成部分。这里是我国第二大黄金产地，大规模金的成矿作用形成于早白垩世岩石圈大规模减薄的区域伸展构造背景。此外，越来越多的同位素年代学数据显示区内还存在三叠纪的成矿事件，发育钼、铅、铀、铌等与岩浆热液活动密切相关的多金属矿床，主要分布在小秦岭的南北边缘。目前，对于三叠纪多金属成矿作用的构造背景研究尚属薄弱，还没有直接对有关的构造变形开展过专题研究。为此，我们对小秦岭金矿田东端边缘的构造带进行构造观察和测量，并采用⁴⁰Ar-³⁹Ar年代学方法厘定变形时限，揭示三叠纪成矿事件的构造背景，为深入认识区内三叠纪多金属成矿作用特征和规律提供构造证据。研究表明，小秦岭金矿田三叠纪成矿事件发生于晚三叠世的伸展构造背景，可以划分为两个阶段，早期（221~216Ma）是在碰撞向后碰撞转折阶段的重力滑脱构造环境中，发育火成碳酸岩型钼铅、铀铌铅多金属矿床；晚期（214~203Ma）是在后碰撞阶段由俯冲板片断离或岩石圈拆沉作用等深部过程导致的区域伸展构造环境中，发育石英脉型和斑岩型钼矿床。小秦岭中生代晚三叠世和早白垩世两期成矿事件都是在伸展构造背景中的构造-岩浆-流体活动的产物。;The Xiaoqinling goldfield is located in the southern margin of the North China craton, which is also a marginal part of the Qinling complex orogeny. This goldfield is the second largest gold producer in China, where the large-scale gold mineralization developed in the extensional tectonic setting due to regional lithospheric thinning in Early Cretaceous. In addition, a growing number of isotope chronological data indicates that there also developed metallogenic events in Triassic here, which resulted in the formation of polymetallic deposits, such as molybdenum, lead, uranium, and niobium, etc. The Triassic deposits are closely related to magmatic hydrothermal activities, which are mainly distributed in northern and southern margins of the Xiaoqinling goldfield. However, there is little study on the tectonic setting of the Triassic polymetallic mineralization so far, especially the work on tectonic deformation is almost absent. Therefore, we have carried out the observation and measurement on the structure developed within the tectonic belt occurred in the eastern end of Xiaoqinling goldfield, and then employed the ⁴⁰Ar-³⁹Ar chronology method to determine the time limit of deformation, to reveal the tectonic setting of Triassic metallogenic event, providing tectonic evidence for further understanding characteristics and patterns of Triassic polymetallic mineralization in the Xiaoqinling goldfield. Our study shows that the metallogenic event occurred in the extensional tectonic setting in Late Triassic, which can be divided further into two stages. The early stage (221~216Ma) is of the transition stage from collision to post-collision, developing gravitational decollement tectonics, and carbonatite-type molybdenum, lead, uranium, and niobium polymetallic deposits are well developed in this extensional tectonic setting. The late stage (214~203Ma) is of post-collision, developing regional extensional tectonics caused by the break off of subducting slab or lithospheric delamination, and quartz vein-type and porphyry molybdenum deposits are well developed. The two metallogenic events in Late Triassic and Early Cretaceous in the Xiaoqinling goldfield are the products of the tectonic deformation-magmatic activity-fluid flow in extensional tectonic setting.