The mechanisms for the formation of the alteration halos in tungsten deposits of Nanling Mountains, South China
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摘要:
中国地质工作者在20世纪80年代已发现南岭地区许多石英脉型钨矿床的蚀变晕宽度随深度递减，然而这一蚀变特征的形成机制至今仍未得到较好的解释。通过模拟热液运移和硅从裂隙带向邻近围岩的扩散过程，发现流体温度和围岩孔隙度是影响石英脉型钨矿床蚀变特征的重要变量。高温和高孔隙度会加速硅从裂隙向邻近围岩扩散，从而形成较宽的蚀变。在围岩孔隙度均一分布的情况下，由于深部温度高于浅部，深部围岩蚀变宽于浅部蚀变。围岩孔隙度随深度递减会抵消温度对硅扩散速率的影响，使深部围岩形成较窄的蚀变。围岩孔隙度随深度递减可能是形成石英脉型钨矿床蚀变宽度随深度减小的有效机制。前人将钨矿蚀变特征归因于岩浆热液过渡性流体不均一的物理性质，该研究为这一科学问题提供新的解释。
Abstract:Geologists discovered in the 1980s that alteration halos decrease with increasing depth in many tungsten deposits of the Nanling Mountains. However, the mechanism for the formation of the alteration characteristics remains poorly understood. In this paper, the authors investigated hydrothermal flow and silica diffusion from fractures to adjacent wallrock at these tungsten deposits by using finite element based numerical experiments. The authors have found that fluid temperature and wallrock porosity exert a strong influence on silica diffusion from fractures to adjacent wallrock. Both high temperature and high porosity favor silica diffusion from fractures to adjacent wallrock and form wide alteration halos. Constant-porosity wallrock forms wider alteration halos at deeper levels, which is inconsistent with alteration characteristics of the tungsten deposits in the Nanling Mountains. Wallrock porosity that decreases with increasing depth forms alteration halos like those in those the tungsten deposits. The wall rock lithology and fracture distribution those tungsten deposits favor the formation of depth-dependent porosity and permeability. Evaluation of these two factors may help the exploration. Aqueous NaCl solutions were used in the numerical experiments. It is therefore concluded that inhomogeneous magmatic hydrothermal fluids are unnecessary in explaining the alteration characteristics at these tungsten deposits.
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Keywords:
- tungsten deposits /
- alteration halos /
- fluid flow /
- porosity /
- diffusion coefficient /
- Nanling
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致谢: 本文数值实验在澳大利亚昆士兰大学超级计算机Savanna上完成，审稿专家提出了详细的修改意见，在此一并致谢。
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图 1 南岭成矿带钨矿分布示意图[29]
Figure 1. Distribution of tungsten deposits in the Nanling Mountains, South China
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图 2 南岭地区石英脉型钨矿典型矿脉
a—江西大吉山钨矿深部脉体，标高417m，脉体走向为近东西向，陡倾，向上拍摄；b—江西漂塘钨矿深部大脉，标高268m，脉体走向为近东西向，陡倾，向上拍摄。Qtz—石英；Wol—黑钨矿
Figure 2. Typical veins at deeper levels of tungsten deposits in the Nanling Mountains
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图 3 石英脉型钨矿床热液运移数值模型
Figure 3. Numerical model of hydrothermal flow at the tungsten deposits in the Nangling Mountains
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图 4 数值实验1温度(a)和Péclet数(b)的分布
Figure 4. The temperature (a) and Péclet number (b) in the first numerical experiment
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图 5 数值实验1热液运移1a后(a)和2a后(b)硅有效扩散系数在裂隙带附近的分布
Figure 5. The effective diffusion coefficients of silica in the wallrock adjacent to fracture zones after one year (a) and two years (b) in the first numerical experiment
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图 6 数值实验2温度(a)和Péclet数(b)的分布
Figure 6. The temperature (a) and Péclet number (b) in the second numerical experiment
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