Abstract

The Berners Bay district at the north end of the Juneau gold belt, southeast Alaska, contains a series of structurally controlled mesothermal gold-bearing quartz veins. The 106 Ma Jualin Diorite, which intrudes metabasalt of the Wrangellia terrane, hosts most of the > 2 Moz of gold resources. Wrangellia is one of a sequence of lithotectonic terranes that were accreted along the western margin of the Coast Mountains during Cretaceous time. Following a period of mid-Cretaceous deformation, igneous activity, and regional metamorphism, a belt of tonalite bodies 5 km east of the district were intruded at 71 to 61 Ma. Eocene granodioritic bodies of the Coast Mountains batholith were emplaced at shallow levels 10 km east of the district, coevally with gold vein emplacement. 40 Ar/ 39 Ar age spectra for hydrothermal sericite from variably oriented structures in the Berners Bay district indicate that hydrothermal activity occurred between 56.5 and 53.2 Ma. Gold occurs in quartz carbonate veins containing pyrite and tellurides and with distinctive ankeritic alteration zones. Gold-bearing veins were deposited at temperatures of about 200 degrees to 235 degrees C and a depth of 3 km. The ore fluids were composed of 93 to 97 mole percent H 2 O, 2 to 5 mole percent CO 2 , 1 to 2 mole percent NaCl, <0.5 mole percent N 2 , and traces of CH 4 , H 2 S, and SO 2 . Isotopic study of silicate gangue indicates a delta 18 O fluid value of 6 per mil and a delta D fluid value of approximately -25 per mil. These data suggest that prograde devolatilization reactions in rocks of the Wrangellia terrane, perhaps triggered by heat from magmatic-arc emplacement, are the most likely fluid source. Measured delta 34 S values of -2.5 to +1.6 per mil for sulfide minerals are consistent with ore-fluid sulfur being leached either from metabasalt of the Wrangellia terrane or from the Jualin Diorite. Structural elements important for ore control include premineralization, chlorite-bearing shear zones that strike northwest and dip steeply northeast and southwest. Heterogeneous blocks of homogeneous diorite bounded by chloritic shear zones were favorable locations for gold-bearing vein formation. Most of the vein deposits are controlled by north-to northwest-striking and steeply east-dipping...

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