Resources and Hazards of Hydrothermal Systems in Cascades Volcanoes

Staff

Tasks

View of the southwest side of Mt. Adams showing hydrothermal alteration on the edifice and young (>200 ybp) Salt Creek lahar that contains abundant blocks of hydrothermally altered rocks derived from near the summit of Mt. Adams

Project summary:

Hydrothermally-altered rocks are present on many active volcanoes in the High Cascades and are widespread in the Oligocene-Pliocene volcanic and plutonic rocks of the Western Cascades in the northwestern United States. Hydrothermal alteration is a manifestation of ore-forming processes and is characteristic of mineral deposits formed in continental-margin arcs. Hydrothermal alteration can result in severely weakened volcanic edifices that may be susceptible to failure and catastrophic landslides. Hydrothermal alteration also can result in rocks highly enriched in sulfur and potentially toxic metals that can be leached into surface and ground waters.

Hydrothermal alteration occurs when rocks interact with hot fluids resulting in mass transfer between rocks and fluids and crystallization of hydrothermal minerals in the rocks. In active volcanoes, these fluids originate from many sources, including degassing of magmas, convecting heated meteoric ground waters, precipitation, or mixtures from all of these sources. Hydrothermal alteration resulting from interaction with extremely acidic fluids produces rocks highly enriched in sulfur and potentially toxic metals and metalloids, such as As, Hg, Se, Tl, and Zn. Extreme acid alteration (acid leaching) results in rocks that have no acid-buffering capacity, and oxidation of hydrothermal sulfide minerals in these rocks can form natural acidic waters highly enriched in potentially toxic metals. Hydrothermal alteration of stratovolcanoes is characteristic of the upper parts of magmatic-hydrothermal systems that form important mineral deposits, including major world sources of copper, gold, and silver. Many large mineral deposits are associated with the Oligocene-Pliocene Cascade arc in Washington, Oregon, California, and Nevada. In contrast, little mineralization is known in the Quaternary arc. Hydrothermal alteration of active volcanoes also can result in weakened volcanic edifices which may be susceptible to failure and catastrophic debris flows. Clay-rich hydrothermally-altered rocks can produce cohesive debris flows that travel much farther than debris flows that lack clay minerals. Hydrothermally-altered rocks are present in many of the Quaternary Cascades volcanoes, but with the exception of Mt. Rainier, the distribution, composition, and origin of hydrothermal alteration products generally are poorly known.

This project aims at understanding the processes that form hydrothermal alteration in active volcanoes, including depth, duration, temperature, and sources of fluid components. It also addresses the metallogeny of the Oligocene-Pliocene Cascade arc (Western Cascades) and compares it to the Quaternary arc. Results will be used to develop genetic models of ore deposition, the degassing of magmas, the hydrologic structure of volcanoes, and the contribution of rock-alteration effects to volcanic hazards, and to gain a better understanding of the regional controls on metallogenesis of the Oligoene-Quaternary Cascades arcs.