Tectonic evolution and structural control of auriferous veins in the Juneau gold belt, southeastern Alaska.

Abstract

A portion of northern southeastern Alaska known as the Juneau gold belt is composed of a disparate assemblage of lithotectonic terranes which range in age from Paleozoic and perhaps older to Cretaceous. Five progressive deformational events associated with contractional tectonism began in the mid-Cretaceous time and continued well into the Tertiary. Widespread plutonism occurred in the region from Cretaceous through Tertiary time. Gold-bearing quartz vein systems in the Juneau gold belt formed within a 160-km-long by 5- to 8-km-wide zone along the western margin of the Coast Mountains, Alaska. The vein systems are localized in second and third order shear zones spatially associated with terrane-bounding, mid-Cretaceous thrust faults. Mesoscopic structures integrated with ⁴⁰Ar/ ³⁹Ar ages from vein sericite are interpreted to indicate that a fluid cycling event along the entire belt occurred between 56.5 and >52.8 Ma. Structural analysis of the vein orientations and geometries are interpreted to indicate that mineralization developed under a near-field stress regime of subhorizontal contraction along a west-southwest to east-northeast trending axis. The axis of extension plunged steeply to the southeast. Slight variations in the interpreted stress axes may have been the result of variations in fluid pressure. Post-mineralization deformation was associated with a dextral transpressive regime along the Denali-Chatham Strait fault system. Gold vein mineralization occurred during the latter stages of orogenesis. Fluid flow and subsequent vein development was temporally associated with changes in plate motion during Eocene time. Veining was also synchronous with exhumation and voluminous plutonism immediately inboard of the gold belt. These interacting tectonic events likely facilitated fault-valve action and vein development along now exhumed shear zones.