湖南沃溪金锑钨矿床中超纯自然金的成矿溶液特征及成因研究

Gold fineness is very important in studying on Au-Ag minerals and their deposits, showing the indication of ore-forming fluid and mineral genesis. However, there are many controlling factors of gold fineness, and theoretical studies on this filed are still being improved, resulting in a research status that the indication of gold fineness is hardly applied into a study of the gold deposit. The applicant proposed the thermodynamic calculation method based on previous researches, and obtained the solubility of Au-Ag minerals with different gold finenesses and the major controlling factors of the gold fineness. Since very high gold finenesses were found in Woxi Au-Sb-W deposit where native gold is called super-pure native gold, characteristics of ore-forming fluids and mineral genesis are of scientific significance. Therefore, questions of how to know characteristics of ore-forming fluids of super pure native gold and how to analyze the indication of the gold fineness based on experimental data are planned to be resolved in this program. High gold concentrations and Au/Ag concentration ratio have been determined in LA-ICP-MS analyses of some fluid inclusions in the Woxi deposit, which provided the important data and feasibility for this program. Mineral and fluid inclusions experiment are designed to understand characteristics of ore-forming fluids, and then provide more data of the thermodynamic calculation for analyses and demonstration, that is expected to show a new way of studying on gold deposits.

金成色值对金银矿物及所属矿床均具有重要的研究意义，对矿物的成矿溶液特征和成因可以起到指示作用。然而，由于金成色值的影响因素较多，而且相关理论研究一直在完善，造成了金成色值的指示作用很少在单个矿床的研究中被具体分析的现状。申请人根据前人的研究成果提出了新的热力学计算方法，获得了金银矿物的溶解度，并揭示了各种溶液条件对金成色值的影响程度。另外，湖南沃溪金锑钨矿床产出的自然金因金成色值较高被称作超纯自然金，其成矿溶液特征和成因具有较高的研究价值。因此，本项目拟解决如何验证超纯自然金的成矿溶液特征的问题，及如何根据实验结果计算分析金成色值的指示作用的问题。申请人通过LA-ICP-MS实验已测定该矿床部分流体包裹体含有较高的金元素浓度及金银比值，为项目的开展提供了必要的基础和可行性的保障。本项目计划进行针对性的矿物和流体包裹体实验获得成矿溶液特征，再结合理论计算进行分析和验证，为成因分析提供依据。

本项目以湖南沃溪金锑钨矿床为研究对象，聚焦自然金的成矿溶液特征，主要开展了矿床地质特征、矿物学及矿化特征、流体包裹体的显微测温、盐度、激光拉曼光谱和LA-ICP-MS测试、热力学模拟计算等方面的研究。通过实验结果与模拟计算相结合的研究方法取得了以下成果：①查明矿体的几何学特征及品位变化规律，证明多期次的热液成矿是沃溪矿床的主要成因；②EPMA测定自然金成色值范围在996.8-1000之间，属于典型的超纯自然金；③192个流体包裹体的均一化温度范围在125.8℃- 434.5℃之间，两个峰值分别出现在190℃和310℃附近，108个流体包裹体盐度范围在1.2-9.8wt%之间，峰值出现4-5wt%之间，白钨矿和闪锌矿中流体包裹体的温度和盐度相对较高；④激光拉曼光谱测定32个流体包裹体的气相成分为CO2和N2；⑤24个流体包裹体的LA-ICP-MS测试结果显示白钨矿和部分含矿石英中流体包裹体的金属元素含量相对较高，而其他矿物中流体包裹体的金属元素含量相对较低；⑥热力学模拟计算揭示超纯自然金的成矿溶液特征与白钨矿相似，以高温、中低盐度和多种金属元素富集为代表，其金成色值的主要影响因素为温度、金银离子浓度比、pH和硫含量。以上结果不仅合理地分析并解释了立项时提出的自然金成矿溶液特征和金成色值影响因素这两个科学问题，还补充和完善了沃溪矿床的成因及成矿规律的理论研究，为找矿预测提供了依据，达到了预期目标。