Sedimentary facies and mineralogy of the late Pleistocene Umukuri silica sinter, Taupo Volcanic Zone, New Zealand

The Umukuri silica sinter is a large, late Pleistocene hot spring deposit exposed along the eastern upthrown block of the Umukuri Fault, one kilometer southwest of the active Orakei Korako geothermal area, Taupo Volcanic Zone, New Zealand. Uplift along the fault has frozen sinter maturation to produce a characteristic silica-phase stratigraphy revealed by X-ray diffraction. Paracrystalline opal-CT dominates upper layers; opal-C occurs throughout middle and lower horizons; and recrystallized fabrics of microcrystalline quartz constitute lowermost exposed layers. Original and secondary matrix fabrics in the sinter include: fine-grained, porous, friable; dense, vitreous; and massive-mottled, diffusely layered. Original fabrics combine with silicified plant matter, peloids, pisoids, sinter fragments, detrital grains, diatoms, ostracodes, and filamentous to tufted microbial remains, to form nine depositional microfacies. Thinly laminated, plant-rich, and palisade types dominate. Sinter breccia and wavy-laminated varieties also are common, whereas curved laminae with lenticular voids ( = bubble mats), clotted, peloidal, and pisolitic microfacies are minor. Umukuri microfacies represent silicification in mid to low temperature waters on sinter apron terrace and slope areas, and in distal, marshy settings. Closely spaced, lateral and vertical intercalation of various microfacies in outcrop implies changing local flow and temperature conditions. No facies typical of high-temperature, proximal vent areas have been identified. Comparison with modern thermal spring analogs suggests that original Umukuri sinter fabrics recorded varying degrees of polymerization vs. monomerization of juvenile opal-A. Mineralogical and textural modification of matrix fabrics reflects a microscale, incremental continuum, following granular or fibrous habits along solution-precipitation pathways. Late-stage quartz rims and infills pores throughout the sinter.