Collaborative Research: Pressure Decrease as a Cause of Quartz and Molybdenite Vein Mineral Precipitation in Magmatic-Hydrothermal Systems

在高温地热系统和岩浆热液系统中，如蒙大拿Butte的斑岩铜矿床，压力从静压向静压的急剧下降可能是石英脉产生的一个过程。辉钼矿（MoS2）几乎普遍存在于这种可能由压降形成的石英脉中，而辉钼矿在其他类型的石英脉中缺乏，这表明辉钼矿与石英一样，是通过压力降低而不是温度降低而沉淀的。以前的辉钼矿溶解度实验没有解决压力的作用，因此我们不知道导致辉钼矿在压力下降时沉淀的化学机制。对于石英，我们知道它的溶解度是压力的函数，但我们不知道石英矿物的来源和微量元素组成，以区分压降脉石英和其他类型的石英。本研究将压力下降作为岩浆热液系统的关键过程，通过三个协调的子项目进行研究：(1)在爱达荷州Wood实验室研究辉钼矿溶解度随压力和温度的变化规律；(2)在俄勒冈州Reed实验室研究由压降等过程形成的天然脉状石英的sem -阴极发光结构及相关微量元素组成、18O/16O比值和流体包裹体性质；(3)应用数值模拟方法（程序CHILLER）探讨压力降低在岩浆热液系统中的作用。除了研究本身的好处还包括：a)研究生和两名本科生将接受现代地球化学研究中涉及的分析、实验和计算方法的培训；b) Reed-Wood研究合作在实验、分析和数值模拟地球化学方面具有互补优势，有助于地球化学的未来发展；c)对岩浆热液过程的更深入了解对于矿产勘探和未来从高温（400摄氏度）、高产地热系统（如目前正在冰岛钻探的地热系统）中提取能源非常重要。

In high temperature geothermal systems, and magmatic hydrothermal systems such as the porphyry copper deposit at Butte, Montana, sharp decreases in pressure from lithostatic toward hydrostatic are a likely process for producing quartz veins. The nearly universal occurrence of molybdenite (MoS2) in such quartz veins that likely form by pressure drop and the scarcity of molybdenite in other vein types suggest that molybdenite, like quartz, precipitates by pressure reduction, as opposed to a decrease in temperature. Previous molybdenite solubility experiments have not addressed the role of pressure, thus we do not know a chemical mechanism that would cause molybdenite to precipitate upon a drop in pressure. For quartz, we know its solubility as a function of pressure, but we do not know the origin of quartz mineral and trace element compositions that distinguish pressure-drop vein quartz from other types of quartz. This research addresses pressure decrease as a key process in magmatic hydrothermal systems in three coordinated sub-projects: (1) experiments in the Wood laboratory (U. Idaho) to explore molybdenite solubility as a function of pressure and temperature, (2) in the Reed laboratory (U. Oregon), determine the SEM-cathodoluminescence textures and related trace element compositions, 18O/16O ratios and fluid inclusion properties in natural vein quartz formed by pressure drop and other processes; and (3) apply numerical modeling methods (program CHILLER) to explore the role of pressure-decrease in magmatic hydrothermal systems. Benefits beyond the research itself include the following: a) graduate students and two undergraduates would be trained in the analytical, experimental, and computational methods involved in modern geochemical research; b) the Reed-Wood research collaboration joins complementary strengths in experimental, analytical and numerical modeling geochemistry that would aid future developments in geochemistry; c) a deeper understanding of magmatic hydrothermal processes is important for mineral exploration and for future energy extraction from high-temperature (400 degrees C), high-yield geothermal systems, such as that now being drilled in Iceland.