Tracking crystallinity in siliceous hot-spring deposits

Abstract

Siliceous hot spring deposits (sinters) entrap paleoenvironmentally significant components and are used as extreme-environment analogs in the search for early Earth and extraterrestrial life. However, sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals. For ancient hydrothermal settings including those close to the dawn of life, these transformations have long since occurred, so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change. Three young sinters preserve the entire diagenetic sequence of silica phases, from opal-A to quartz. The 6000 to ϳ 11,500 years BP ؎ 70 years sinter at Steamboat Springs, Nevada, the ϳ 1600 -1900 ؎ 160 years BP Opal Mound sinter at Roosevelt Hot Springs, Utah, and the ϳ 456 ؎ 35 years BP deposit at Sinter Island, Taupo Volcanic Zone, New Zealand, provide an opportunity to track crystallographic, mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches. Worldwide, sinter forms from cooling, alkali chloride waters as noncrystalline opal-A, transforming first into noncrystalline opal-A/CT, then paracrystalline opal-CT ؎ moganite, paracrystalline opal-C, and eventually to microcrystalline quartz. In this study, these changes were identified by the novel and combined application of electron backscatter diffraction, X-ray powder diffraction, and scanning electron and optical microscopy techniques. We show that mineralogical changes precede morphological and accompanied crystallographic transformations. During this modification, silica particles grow and shrink several times from the micron-to nano-meter scales via dissolution, reprecipitation and recrystallization, and diagenesis follows the Ostwald Step rule. All deposits followed nearly identical diagenetic pathways, with time as the only variable in the march toward physicochemically stable quartz crystals. Diagenesis alters original environmental signatures trapped within sinters. After five silica phase changes, filamentous microfossils are modified but still remain recognizable within sinter from the Opal Mound and Steamboat Springs deposits, and during the opal-A to opal-CT silica phase transformations at Sinter Island. Therefore, delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot-spring deposits.

Silica hydrate preserved with δ18O-rich quartz in high-temperature hydrothermal quartz in the high sulfidation copper-gold deposit at El Indio, Chile

“…10). This process has been well documented in low-temperature siliceous sinters (Lynne et al, 2007;Day and Jones, 2008). We interpret the presence of residual silica hydrates (opal and moganite) as evidence that the euhedral quartz microcrystals at El Indio evolved from a hydrous, metastable precursor.…”

“…1F-H; doubly terminated crystals and clusters of euhedral quartz microcrystals) are similar to those of quartz evolved from metastable silica hydrate in silica sinters from hot spring environments at ~ 100 °C (e.g. Lynne et al, 2007). The euhedral El Indio quartz microcrystals exhibit distinct α-quartz morphology, indicating that the final stage of crystallization of euhedral quartz from precursor material occurred at <573°C.…”

Textural Characteristics of Noncrystalline Silica in Sinters and Quartz Veins: Implications for the Formation of Bonanza Veins in Low-Sulfidation Epithermal Deposits

“…Previous studies on silica sinters have shown that the thermodynamically unstable opal-A matures and transforms over time, forming thermodynamically more stable paracrystalline [47] opal-CT, then opal-C, and ultimately blocky, microcrystalline quartz [51,54,55,[59][60][61][62][63]. In the case of the Taupo Volcanic Zone in New Zealand, microcrystalline quartz becomes a common phase in silica sinters older than~20,000 years [51].…”

“…The transformation from opal-A to opal-CT is typically accompanied by textural changes [51,54,55,58,59], although the mineralogical changes may outpace textural maturation [63,65]. Opal-CT commonly forms lepispheres that are similar in size to the opal-A microspheres [51,[54][55][56]58,59].…”

“…In addition to lepispheres, opal-CT sometimes also forms complex three-dimensional spindle and barrel frameworks [58]. The transition from opal-A to opal-CT most likely occurs through a dissolution-reprecipitation process [58,62,63].…”