Agate mineralization in spilitized Permian volcanics from “Borówno” quarry (Lower Silesia, Poland) – microtextural, mineralogical, and geochemical constraints

Powolny, Tomasz; Dumańska-Słowik, Magdalena; Sikorska-Jaworowska, Magdalena; Wójcik‐Bania, Monika

doi:10.1016/j.oregeorev.2019.103130

Cited by 19 publications

(17 citation statements)

References 75 publications

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“…According to Moxon and Ríos [8], moganite tends to recrystallize into more stable and water-poor α-quartz after silica deposition within particular cavities. Hence, this silica polymorph is nearly absent in agates found within relatively old volcanos (i.e., of Early-Permian age; see [59]), but could still be present in relatively young host basaltoids, such as those found in the vicinity of Asni and Aguim, but also Sidi Rahal and Kerrouchen.…”

Section: Discussionmentioning

confidence: 99%

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

et al. 2020

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Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The first two indicate that the formation of agates was likely marked by recrystallization of metastable silica phases (i.e., opaline silica or massive chalcedony). The presence of lattice-bladed (after barite and calcite) quartz may be, in turn, ascribed to the boiling-related conditions that could have triggered the formation of abundant copper and iron sulfides found within silica matrix. Additionally, the local occurrence of growth lines (so-called Bambauer quartz) and intergrowth of length-slow and length-fast chalcedony are linked to the variations of physico-chemical conditions during rock formation (alkaline-acidic). According to Raman spectroscopy, silica matrix of the agates is made of α-quartz with a local admixture of moganite (from 0.0 up to 78 wt.%), but also contains numerous solid inclusions of hematite, celadonite, as well as poorly-organized carbonaceous material and rutile. These phases were likely emplaced during low-temperature hydrothermal activity of SiO2-bearing fluids that originated from post-magmatic hydrothermal activity developed within host rocks and/or meteoric waters.

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Section: Discussionmentioning

confidence: 99%

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

et al. 2020

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“…Moreover, elevated concentrations of halogens (up to 380 ppm F and up to 154 ppm Cl; [63]) indicate that these two elements play an important role in fluids of the alteration and transport processes. In contrast, Sc, Nb, Ta, Th, Ti and other immobile elements are present in very low concentrations, often below the detection limit of trace-element analysis; only Powolny et al [55] reported significant amounts of Zr in agates from Borówno, Poland. Chemical analyses of separate agate zones or element profiles perpendicular to the banding provided more detailed information concerning the trace-element distribution within the microstructure of the agates [1,26,27,30,31,36,37,58,68,69,126].…”

Section: Trace Elementsmentioning

confidence: 98%

“…Other micro-textures (feathery/flamboyant, mosaic/jigsaw-puzzle) are significant indications for their generation during recrystallization of metastable silica phases such as amorphous silica precursors or chalcedony [46,49,55,60,102]. Replacement textures (lattice-bladed, pseudo-acicular) result from the secondary replacement of soluble phases (e.g., calcite, barite) by quartz.…”

Section: Microstructure Of Agates and Agate Bandingmentioning

confidence: 99%

“…For instance, primary growth textures such as colloform or botryoidal textures indicate precipitation of silica gel in free space, whereas comb-like, crustiform or zonal textures in quartz crystals point to a direct crystallisation from hydrothermal fluids [102]. Growth lines in euhedral quartz crystals, so called "Bambauer lamellae", are preferentially formed under strongly varying physico-chemical conditions during agate formation [46,52,55,60]. Most of these features can be related to epithermal conditions of formation and frequently occur in hydrothermal vein agates.…”

Section: Point Defects In Agatementioning

confidence: 99%

“…First results were also published for rare earth elements (REE) in agates [30,37,55], which are preferentially bound to fluid inclusions and therefore, reflect the composition of the agate mineralizing fluids. The chondrite normalized REE patterns of different agate zones in Figure 16 are characterized by pronounced negative Eu anomalies and a slight slope of the light REE (La-Sm), whereas the shape of the heavy REE patterns (Gd-Yb) is increasing.…”

Section: Trace Elementsmentioning

confidence: 99%

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Mineralogy, Geochemistry and Genesis of Agate—A Review

2020

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Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups. Most agate occurrences worldwide are related to SiO2-rich (rhyolites, rhyodacites) and SiO2-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (>90 ppm), B (>40 ppm) and U (>20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors.

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Hydrothermal genesis and growth of the banded agates from the Allumiere-Tolfa volcanic district (Latium, Italy)

et al. 2022

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In this work, we studied the hydrothermal agates from the Neogene–Quaternary volcanic district of Allumiere-Tolfa, north-west of Rome (Latium, Italy) using a combination of micro-textural, spectroscopic, and geochemical data. The examined sample consists of (1) an outer cristobalite layer deposited during the early stages of growth, (2) a sequence of chalcedonic bands (including i.e., length-fast, zebraic, and minor length-slow chalcedony) with variable moganite content (up to ca. 48 wt%), (3) an inner layer of terminated hyaline quartz crystals. The textures of the various SiO2 phases and their trace element content (Al, Li, B, Ti, Ga, Ge, As), as well as the presence of mineral inclusions (i.e., Fe-oxides and sulfates), is the result of physicochemical fluctuations of SiO2-bearing fluids. Positive correlation between Al and Li, low Al/Li ratio, and low Ti in hyaline quartz points to low-temperature hydrothermal environment. Local enrichment of B and As in chalcedony-rich layers are attributed to pH fluctuations. Analysis of the FT-IR spectra in the principal OH-stretching region (2750–3750 cm−1) shows that the silanol and molecular water signals are directly proportional. Strikingly, combined Raman and FT-IR spectroscopy on the chalcedonic bands reveals an anticorrelation between the moganite content and total water (SiOH + molH2O) signal. The moganite content is compatible with magmatic-hydrothermal sulfate/alkaline fluids at a temperature of 100–200 °C, whereas the boron-rich chalcedony can be favored by neutral/acidic conditions. The final Bambauer quartz growth lamellae testifies diluted SiO2-bearing solutions at lower temperature. These findings suggest a genetic scenario dominated by pH fluctuations in the circulating hydrothermal fluid.

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Agate mineralization in spilitized Permian volcanics from “Borówno” quarry (Lower Silesia, Poland) – microtextural, mineralogical, and geochemical constraints

Cited by 19 publications

References 75 publications

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

Mineralogy, Geochemistry and Genesis of Agate—A Review

Hydrothermal genesis and growth of the banded agates from the Allumiere-Tolfa volcanic district (Latium, Italy)

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