Diagenesis of 1900-year-old siliceous sinter (opal-A to quartz) at Opal Mound, Roosevelt Hot Springs, Utah, U.S.A.

White, vitreous, siliceous sinter dated by 14C at ∼1900 years BP is located along the crest of the Opal Mound deposit at Roosevelt Hot Springs, Utah, U.S.A. Deposition at vents resulted from the Opal Mound fault opening conduits to the surface, allowing thermal fluids to discharge. Fault movement subsequently fractured the sinter and initiated a second period of fluid discharge. Colorful silica was deposited in this later period and formed the distal slope sinter, dated at ∼1600 years BP. The sinter preserves both end-members in the mineralogical maturation of silica, from initial opal-A to mature quartz, and also records incremental intermediate steps, as revealed by X-ray powder diffractrometry and by scanning electron, optical and Raman laser microscopy. Textural and mineralogical changes from one silica phase to another transpired gradually and gradationally. During diagenesis, incremental morphological changes alternated three times through nano- to micro-particle size transitions. The three corresponding mineralogical steps comprise: (1) opal-A to opal-CT, (2) opal-CT to opal-C, and (3) opal-C to quartz. Mineralogical changes preceded all morphological changes. Each step was initiated when silica nanostructures reached a critical diameter of 200 nm. The initial starting point of fresh (modern), polymeric, siliceous sinter deposition worldwide is nano-spherical particles of colloidal opal-A. Opaline sinter at Opal Mound is slightly more mature, reflecting the beginning of the aging process, and comprises botryoidal clusters of silica microspheres (< 8 μm diameter). The conversion from these agglomerated microspheres to opal-CT bladed lepispheres at Opal Mound is marked by the reorganization of the microspheres into randomly oriented rows (∼800 nm in length) of aligned nanospheres, up to 200 nm diameter. Each row merges into joined beads, which sharpen into classic opal-CT morphology of bladed lepispheres (∼4 μm diameter). During this first diagenetic step, the X-ray powder diffraction trace undergoes an early shift in peak position, from ∼4.0 to 4.09 Å, lining itself up in the position of the next more advanced mineralogical state, prior to emergence of the classic opal-CT blades. In the second step, the bladed lepispheres subsequently change into rectangular nano-rods (∼50 × 100 nm long segments) during early opal-C development. During the third step of early quartz formation, the opal-C nano-rods recrystallize into groups of blocky nanostructures, each up to 200 × 300 nm. Quartz crystals grow at the expense of and pseudomorph opal-C, shown by remnant bands of blocky nanostructures at the base of the quartz crystals. Two generations of quartz crystals occur at Opal Mound: (1) diagenetic, which developed as opal-C nanostructures recrystallize to quartz; and (2) hydrothermal, formed by the injection of silica-infused thermal fluids into fractures and veins within the sinter deposit. Several silica phases commonly co-exist within individual samples. Moganite, a metastable silica phase, occurs with opal-CT, opal-C and diagenetic quartz, but is most abundant with opal-CT. Hence, the complete diagenetic continuum from opal-A to quartz is preserved within this sinter deposit. It formed in the short time span of ≤ 1900 years.