The Esquel low-sulfidation epithermal vein deposit in Chubut province, Argentina, is hosted by a Late Jurassic bimodal arc assemblage. Shortwave infrared (SWIR) reflectance spectroscopy of vein samples using a hand-held field spectrometer identified ammonium-bearing illite, jarosite, and adularia (buddingtonite) in banded quartz displaying colloform and crustiform textures. The SWIR technique proved to be a rapid and accurate means of detecting ammonium-bearing species, particularly buddingtonite, which provides evidence for paleoboiling zones and thus of precious metal mineralization.

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