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，如蒙大拿州Butte的斑岩铜矿存款，压力从岩石静力学到流体静力学的急剧下降是产生石英脉的可能过程。 在这种可能由压力下降形成的石英脉中几乎普遍存在的硅铍石（MoS 2）和其他脉类型中硅铍石的稀缺性表明硅铍石像石英一样通过压力降低而沉淀，而不是温度降低。 以前的钙铝石溶解度实验没有解决压力的作用，因此我们不知道一个化学机制，将导致钙铝石沉淀后，在压力下降。 对于石英，我们知道它的溶解度作为压力的函数，但我们不知道石英矿物的来源和微量元素组成，区分降压脉石英从其他类型的石英。本研究以岩浆热液系统为研究对象，分三个子项目进行：（1）在伍德实验室（U。爱达荷州），以探索作为压力和温度的函数，（2）在里德实验室（美国。俄勒冈州），测定了天然脉石英在降压等过程中的SEM阴极发光结构及相关微量元素组成、18 O/16 O比值和流体包裹体性质;（3）应用数值模拟方法（程序CHILLER）探讨了降压在岩浆热液系统中的作用。 研究本身以外的好处包括：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）里德-伍德研究合作将在实验、分析和数值模拟地球化学方面形成互补优势，这将有助于地球化学的未来发展; c）更深入地了解岩浆热液过程对于矿物勘探和未来从高温（400摄氏度），高产地热系统中提取能源非常重要，例如现在正在冰岛钻探的地热系统。

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.