西南三江甭哥与富碱侵入岩有关金矿床成矿热史的磷灰石裂变径迹年代学证据

The metallogenic thermal evolution of the Bengge gold deposit related to alkaline-rich intrusive rocks in the Sanjiang region, SW China: Evidence from apatite fission-track (AFT) thermochronology

甭哥金矿位于哀牢山-金沙江富碱斑岩成矿带的北段,属于西南三江地区与富碱侵入岩有关的典型金矿床,发生于喜马拉雅期的印-亚板块碰撞造山事件使其具有多期复合成矿的特点,矿床的成矿年代学研究较为薄弱,制约着成矿机制的研究。磷灰石裂变径迹热年代学技术是近年来研究矿床热史演化的有效手段之一,本文通过甭哥含矿岩体的磷灰石裂变径迹年龄测试,获得三组裂变径迹年龄峰值:30Ma、25Ma及15Ma。利用裂变径迹热史模拟分析,揭示了甭哥金矿自喜马拉雅期以来主要经历的成矿活动热史:42~32Ma温度降幅20~30℃,冷却速率为3~3.8℃/Ma;32~16Ma温度降幅20~10℃,冷却速率为1.5~0.71℃/Ma;16~6Ma发生了快速降温活动,温度降幅50~45℃,冷却速率为11~6.25℃/Ma;前两个时期为缓慢降温阶段说明处于挤压缩短的环境,最后一个时期是快速冷却阶段,暗示这个阶段是以拉张的构造活动为主。裂变径迹热史模拟反映出甭哥金矿区经历了两个阶段的成矿过程,第一个阶段处于挤压缩短的环境与当时西南三江地区处于压扭性的构造背景相吻合,裂变径迹年龄30Ma代表了主要金矿体的形成时间;第二个阶段反映了拉张的构造环境与西南三江地区处于伸展张裂的演化机制相同,代表了锑-金矿体的形成过程,15Ma是成矿作用的具体时限;表明成矿构造活动经历了由挤压到拉伸的演化机制。通过磷灰石裂变径迹数据的模拟分析,构建了甭哥金矿在喜马拉雅期的成矿热事件过程,结果表明甭哥金矿的演化机制与西南三江地区富碱岩浆带内的成矿事件大致相同,显示甭哥金矿属于该成矿系统的一部分,两者之间具有相似的地球动力学机制。

The Bengge gold deposit, which is located in the northern region of the Ailaoshan-Jinshajiang alkali-rich porphyry metallogenic belt, is a hydrothermal type gold-polymetallic deposit related to alkaline rocks in the Sanjiang area of southwest China. The collisional orogen of India-Asia plates occurring in the Himalayan made it multi-stage and composite mineralizations, but research on its metallogenic chronology of the deposit is still poor and restricts the understanding of metallogenic mechanism. Apatite fission-track(AFT) thermochronology technique is one of the most effective approaches to study the thermal evolution of mineralization in recent years. This study carried out the apatite fission track testing, and three groups of fission track ages of 30 Ma, 25 Ma and 15 Ma have been obtained. Simulation analysis reveals the main metallogenic thermal history of the Bengge gold deposit since the Himalayan epoch. The period of 42 Ma^32 Ma had a decreasing amplitude of 20~30℃, with a cooling rate of 3~3.8℃/Ma;the period of 32 Ma and 16 Ma had a temperature decreasing of 20~10℃ with a cooling rate of 1.5~0.71℃/Ma;the period of 16 Ma^6 Ma witnessed a temperature decreasing of obviously 50~45℃, with cooling rate of 11~6.25℃/Ma. The former two phases were in a slow decreasing of temperature, indicating a setting of squeezing and shorting. While the last was rapid cooling phase, indicating a extension-dominated tectonic activity. Fission track thermal history simulation reflects that the Bengge gold deposit has undergone two stages of metallogenic process. The first stage was in the environment of extrusion and shortening, which coincides with the tectonic background of compression and torsion in the Sanjiang area at that time;while the fission track age of 30 Ma represents the formation time of major gold mineralization. The second stage reflects that the environment of tectonic extension is the same as the extensional setting in which the Sanjiang area was located. 15 Ma is the specific time limit for mineralization, indicating that the metallogenic tectonic activity has undergone an evolutionary mechanism from compression to extension. Based on the fission track data for apatite and metallogenic simulation, this paper established the ore-forming thermal events of the Bengge gold deposit in the Himalaya period. The results in this study demonstrates that evolution mechanism of the Bengge gold deposit is similar to the main metallogenic event occured for the alkaline magmatic belt in Sanjiang area, and also shows that the Bengge gold deposit was part of the composite metallogenic system in the Sanjiang area in southwest China, suggesting that the two have similar geodynamic mechanism.