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，如蒙大拿州布特的斑岩铜矿床，压力从岩石静压到流体静压的急剧下降可能是产生石英脉的一个过程。在这类石英脉中几乎普遍存在辉钼矿(MoS2)，这可能是由压降形成的，而在其他脉型中辉钼矿的稀有表明，辉钼矿与石英一样，是通过压力降低而不是温度下降来沉淀的。以前的辉钼矿溶解实验没有讨论压力的作用，因此我们不知道一种化学机制会导致辉钼矿在压力下降时析出。对于石英，我们知道它的溶解度是压力的函数，但我们不知道将压降脉状石英与其他类型的石英区分开来的石英矿物和微量元素的来源。这项研究在三个协调的子项目中将压力降低作为岩浆热液系统中的一个关键过程：(1)在Wood实验室(美国爱达荷州)进行实验，以探索辉钼矿的溶解度随压力和温度的变化；(2)在Reed实验室(美国俄勒冈州)，测定通过压力下降和其他过程形成的天然脉状石英中的扫描电子显微镜-阴极发光结构和相关的微量元素组成、18O/16O比值和流体包裹体性质；以及(3)应用数值模拟方法(程序CHILLER)探索降压在岩浆热液系统中的作用。研究本身的好处包括：a)研究生和两名本科生将接受现代地球化学研究涉及的分析、实验和计算方法方面的培训；b)里德-伍德的研究协作将结合实验、分析和数值模拟地球化学方面的优势，从而有助于未来的地球化学发展；c)更深入地了解岩浆热液过程对于矿产勘探和未来从高温(400摄氏度)、高产地热系统中提取能源非常重要，例如冰岛现在正在钻探的地热系统。

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.