STRESS, AQUEOUS FLUIDS, MAGMATISM, AND ORE DEPOSITS IN THE SUBDUCTION SETTING: BACKGROUND AND SESSION OVERVIEW

Stress, magmatism, hydrothermal fluid release, and ore formation are unstably integrated on an exceptionally large scale in the subduction setting. Recent work suggests the subduction shear zone precariously traps nearly lithostatically overpressured aqueous fluids, which are released during the slip events they facilitate, sometimes producing large gold districts such as the Mother Lode and other orogenic Au deposits found worldwide in accretionary settings. Arc magmatism is regular in time and space, but the intrusions which comprise it can release their aqueous magmatic volatiles very rapidly. Even large porphyry deposits like Butte appear to have formed from the very rapid (<1000 years) and high volume release of magmatic volatiles. More rapid, explosive, volatile release, such as occurred at Mt. Pinatubo, is common but expels sulfur and metals to the atmosphere rather rather than depositing them in the subsurface. The controlled volatile release that occurs over 100s to ~1000 years and forms porphyry deposits is typically repeated several times over an interval of a few million years. Basaltic magmatism is implicated in these ore-forming events, suggesting highly episodic crustal extension. High pore fluid pressures and mineral deposition homogenize stress during ore deposition, accommodating the strikingly symmetric ore-shell geometries of porphyry deposits, but subduction shearing de-homogenizes stress, encouraging vein deposition. Understanding the sea-saw interactions of stress, fluid release, chemical reaction, and ore formation in the subduction setting is the focus of the T103 session. This talk will provide background and context for what will be discussed, as well as selected specific perspectives of the authors.