Siliceous Sinter Diagenesis at the Opal Mound, Roosevelt Hot Springs, Utah, USA

Abstract

Siliceous sinter from the Opal Mound at Roosevelt Hot Springs preserves a complete diagenetic continuum from opal-A to quartz. Sinters from vent, near-vent and mid- and distal-apron environments were studied. 14C AMS dating yielded ages of 1900 years BP for a sample of quartz mineralogy and 1600 years BP for a distal slope sample with opal-A/CT mineralogy. The transformation from opal-A to quartz in the near-vent sample is more rapid than in sinter deposits of the Taupo Volcanic Zone, New Zealand, which take as long as 40,000 years to transform to diagenetic quartz. Two generations of quartz occur at Opal Mound, diagenetic and hydrothermal quartz. Diagenetic quartz is identified by the presence of moganite, and equant, blocky nanocrystals of opal-C incorporated into quartz crystals growing parallel to the sinter surface. Hydrothermal vein quartz infills fractures with crystals that grow perpendicular to the sinter surface and do not contain moganite. Conductive heat transfer from the injection of thermal fluids that deposited vein quartz in fractures may be responsible for accelerating diagenesis at Opal Mound.