Experimental study on the effective measurement of the total homogenization temperature of high internal pressure CO<sub>2</sub>-rich fluid inclusions in orogenic gold deposits: A case study of the Yufeng gold deposit, Eastern Tianshan Orogen

Abstract

形成于中-深成条件下的造山型金矿的成矿流体属于H₂O-NaCl-CO₂体系，金矿化阶段的石英中发育大量的富CO₂流体包裹体。长期以来，用常规冷热台对其进行显微测温热力学研究时，较高的内压力会造成绝大多数CO₂充填度>0.4的包裹体在尚未达到完全均一状态前就发生爆裂或泄露，导致所测的完全均一温度值明显偏离真实值，甚至无法测得完全均一温度，这制约了对造山型金矿床成矿条件的认知。针对该问题，本文以东天山玉峰金矿床富金矿石中的H₂O-NaCl包裹体（A型）和H₂O-NaCl-CO₂包裹体（AC型）为研究对象，介绍了利用最新型热液金刚石压腔（HDAC-VT型号）测试富CO₂高内压的AC型包裹体完全均一温度的实验流程与结果，并将实验数据与Linkam THMSG600冷热台测得的A型、AC型包裹体的完全均一温度进行了对比分析。实验结果表明，热液金刚石压腔能够有效阻止富CO₂流体包裹体在升温过程中发生爆裂、泄露等非弹性体积改变现象的发生，从而获得有效的完全均一温度。同时，本文还提出了一个新的压力-温度拟合线两线相交法，对热液金刚石压腔所测富CO₂流体包裹体的完全均一温度数据进行校正，以最大程度上减少外压力造成的影响，获得更为接近真实成矿流体的完全均一温度。基于此，获得玉峰金矿床的成矿温度和成矿压力分别为312~343℃和181~268 MPa。本文的实验研究工作显示了热液金刚石压腔在中-深成造山型金矿富CO₂成矿流体的研究领域具有重要应用前景。;The ore-forming fluids of orogenic gold deposits normally belong to a H₂O-NaCl-CO₂ system, and there are usually lots of CO₂-rich fluid inclusions hosted in the quartz at the gold mineralization stage. The trapped fluid inclusions in this type of gold deposit usually have high internal pressures since they are formed at a mid-deep depth in the earth's crust. For a long time, almost all researchers carry out the homogeneous thermomechanical properties study on CO₂-rich inclusions (CO₂ phase filling ratio>0.4) under a 1atm pressure condition. However, when the internal pressure is greater than the external confining pressure, the fluid inclusion will be deformed, such as stretching, leakage and decrepitating, which eventually lead to the measured total homogenization temperature (THT) deviate significantly from the actual value, or even cannot be measured. In order to solve these problems, we choose H₂O-NaCl (A-type) and H₂O-NaCl-CO₂ (AC-type) inclusions in quartz minerals from Yufeng orogenic gold deposit in Eastern Tianshan Orogen as the research object to measure their THTs with high internal pressures by using the latest hydrothermal diamond-anvil cell (HDAC; HDAC-VT type). At the same time, this manuscript compares the THTs obtained by a HDAC-VT with those by a Linkam THMSG600 heating-freezing stage. Our experimental results show that the HDAC can effectively prevent the inelastic volume change of CO₂-rich fluid inclusions during heating process, and consequently obtain an effective THT. In order to minimize the influence of external pressure on the measured THTs and obtain more reliable and accurate metallogenic temperatures, we propose a new two pressure-temperature fitting lines intersection method to calibrate the THTs of CO₂-rich fluid inclusions measured by a HDAC. Based on these improvements, the ore-forming temperatures and pressures of the Yufeng gold deposit are estimated to be 312~343℃ and 181~268MPa, respectively. The experimental work in this paper suggests that the HDAC has an important application prospect in the research field of CO₂-rich fluid inclusions from orogenic gold deposits.