Numerical Modelling of Thermal and Exhumation Histories of Magmatic Ore Deposits
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摘要: 通过对岩浆冷却过程的数字模拟研究, 揭示出岩浆在冷却成矿过程中的温度分布和变化规律及影响因素.在此基础上, 进一步应用高精度的温龄计组合来限定岩浆成矿体系的热演化和剥露历史, 精确地计算出岩浆的初始侵位时间和深度、矿物结晶时间、冷却速率、冷却和暴露地表时间, 以及剥露和剥蚀速率等重要参数, 并将模拟结果应用于斑岩铜矿床的成矿研究中.研究表明, 将精确的年龄测试手段与计算机模拟技术相结合, 可为定量研究岩浆矿床的热演化和剥露史、深入了解矿床的成因机制提供一种有效方法.Abstract: The purpose of this paper is to quantify the thermal and exhumation histories of magmatic ore deposits by combining U-Th-He thermochronometrical data with computer modelling techniques.The numerical modelling of magmatic cooling has been first attempted and then integrated with the exhumation cooling to produce a digitized cooling curve which is further constrained by U-Th-He thermochronometer.The modelling results indicate that the magmatic cooling of igneous bodies is complicated.The cooling history of an igneous body can be divided into two distinct stages.In the first stage, the igneous body cools rapidly while the ambient country rock is heated simultaneously.In the second stage, the temperature of the igneous body is slightly higher than or close to that of the country rock, but the geothermal gradient in the vicinity is still higher than the initial thermal conditions, and thus both the igneous and country rocks cool slowly until both reach a final thermal equilibration under the normal thermal conditions.The cooling of the igneous body is affected by many factors, among which the size and the emplacement depth are the principal factors controlling the cooling rates and the durations of the two cooling stages.The complete thermal history requires an understanding of the exhumation history and this is achieved by the combined modelling of thermal and exhumation cooling resulting in a temperature-age curve constrained by the apatite U-Th-He, zircon U-Th-He, and zircon U-Pb age data.The validity of this curve was successfully tested against data obtained from porphyry copper deposits in Iran.The digitized temperature-age curve defines the time and depth of emplacement, crystallization age of economic minerals, cooling rate, cooled and exposure ages, and exhumation/erosion rates for the porphyry copper deposit.Therefore, the combination of highly precise age dating and computer modelling techniques can not only quantify the thermal and exhumation histories of ore systems, but also provide an insight into the genesis of the ore deposits.
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Key words:
- numerical modelling /
- magmatic cooling /
- exhumation history /
- magmatic ore deposits
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图 1 圆柱状岩浆体在传导冷却过程中的垂向温度分布及随时间的变化
Fig. 1. Vertical variations of isotherms with time during the conductive cooling of the cylindrical igneous body
图 2 岩浆体核部的温度随时间的变化关系
Fig. 2. Changes in temperatures with time at the core of the cylindrical igneous body
图 3 剥蚀速率计算方法
Fig. 3. Algorithm for calculating the exhumation and erosion rates of an intrusive igneous body
图 4 模拟计算方案及温度-年龄模拟曲线
Fig. 4. Modelling strategy and resultant temperature-age curve with important ages indicated
图 6 300~500 ℃温度区和推导出的SC铜壳体的分布
Fig. 6. The distributions of the isotherm zones between 300 ℃ and 500 ℃ and the copper shell deduced from the modelling of the cooling of SC porphyry stock
表 1 主要参数及其初始值
Table 1. Main parameters and their initial values used in the modelling
表 2 SC斑岩的放射性年龄数据
Table 2. Sar Cheshmeh porphyry radiometric age data
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