KINEMATIC ANALYSIS AND AGE CONSTRAINTS OF THE KENSINGTON GOLD DEPOSIT

The Kensington gold deposit at the northern end of the Juneau gold belt, southeast Alaska, is a classic Cordilleran-style orogenic gold deposit. Previous studies demonstrate that gold mineralization is hosted by 56-53 Ma quartz-carbonate veins emplaced within the ~105 Ma Jualin Diorite during a northeast-southwest contractional regime (Miller et al., 1995, Economic Geology). Additional structural and age data from the deposit expand on this scenario.

Most faults within the Jualin diorite strike ~330º and dip steeply to moderately east, parallel with regional foliation. Kinematic data associated with shallowly north plunging striae consistently indicate a latest, right lateral-reverse sense of motion. A second fault set strikes ~80º, dips moderately south, and shows latest, left lateral-reverse motion. Ore zones at the Kensington deposit are hosted by quartz-carbonate±pyrite veins that are proximal to the major faults. Vein types include fault-fill veins and extensional veins and arrays. Fault-fill veins are hosted within northwest-striking, steeply northeast-dipping chloritic shear zones. Some fault-fill veins terminate into sigmoidal extensional vein arrays consistent with measured sense of motion of the host shear. Extensional veins typically strike north-northeast, dip moderately east or west and often show mutually crosscutting relationships with each other and foliation in adjacent shear zones. Kinematic and vein orientation data indicate mineralization occurred in a dominantly right lateral strike-slip regime. U-Pb SHRIMP-RG analyses (collected at the USGS-Stanford facility) of zircon from a felsic dike that cuts the Jualin diorite yield a 207Pb-corrected, weighted mean 206Pb/238U age of 102.8 ± 1.2 Ma (MSWD = 1.7). Zircons from quartz vein samples yield ages mostly ranging from 240 to 90 Ma. These are interpreted as igneous grains incorporated from the adjacent wall rock at depth. One sample yielded a single oscillatory-zoned zircon that gave a 206Pb/238U age of 47.8 ± 1.2 Ma (2σ) and represents either a xenocrystic or hydrothermal zircon in the host quartz vein. This result indicates that quartz vein formation was a longer event than previously believed. Additional evidence is now required to establish the timing of gold precipitation within the longer-lived quartz vein system.