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Abstract

Microthermometry, laser Raman spectroscopy, and mass spectrometry were used to study fluid inclusions in gold-bearing quartz veins from the mines of the Juneau gold belt, Unmixing of a CO2-rich parent fluid led to the contemporaneous trapping of H2O-dominant and CO2-dominant inclusions during gold deposition at the Alaska-Juneau, Reagan, and Ibex mines. Ore fluids at all other mines were trapped as homogeneous, H2O-dominant fluids, with less than 10 mole percent CO2. Both N2 and CH4 are present at the percent level within the volatile phases in all deposits; H2S makes up one-third of the volatile phase and 2 mole percent of the total ore fluid at the Sumdum Chief mine. The ore fluids contained less than 5 equiv wt percent NaCl. Gold deposition occurred at temperatures above 250°C and at depths of at least 5 km. The gold-forming fluids are believed to have been derived from devolatilization reactions associated with prograde metamorphism of dominantly pelitic, subducted crust.

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