Macrocrystalline Quartz Research Articles

The role of compaction in the diagenetic evolution of Pre-Salt Aptian deposits of Santos Basin, Brazil

The discovery of huge oil accumulations in the South Atlantic Pre-Salt carbonate lacustrine deposits has attracted much exploration and research interest on these challenging reservoirs, but no work has focused on the compaction processes. A petrological and geochemical study was therefore performed on two cored wells in Santos Basin to identify the main syngenetic and diagenetic constituents, and to identify the compaction features that affected these deposits. They are mainly composed of Mg-clays, calcite spherulites and fascicular shrubs, which have undergone a heterogeneous diagenetic evolution. Mechanical compaction occurred during shallow burial, promoting a closer packing of the constituents, reorientation and fracturing of calcite spherulites and bioclasts, and deformation of the clay matrix. Pressure dissolution occurred under increasing pressure and temperature during burial, developing inter-aggregate sutured contacts, dissolution seams and stylolites, as consequence of overburden. Macrocrystalline quartz and calcite, drusy quartz, and saddle dolomite precipitated during burial diagenesis, related to basinal fluids and probably to pressure dissolution, as source for the late carbonate phases. Understanding the role of compaction is essential to evaluate trends of porosity destruction, thickness modification and mass transfer, which directly impacted the quality and the dynamic evolution of fluids in the deep Pre-Salt reservoirs.

Facets of Variation: Macrocrystalline quartz reduction at Moolarben, central NSW, Australia

Macrocrystalline quartz is an abundant raw material used widely for artefact manufacture. In Australia its procurement, reduction and use are now considered more complex than previously thought. The analysis of a mid-late Holocene assemblage from Moolarben in eastern Australia demonstrates how geoarchaeological variability influences the manufacture of macrocrystalline quartz artefacts. The different flaking proprieties of the three available macrocrystalline quartz varieties (opaque milky, semi-transparent milky and crystal), along with the type of source, influenced core reduction as well as the resulting flake form and its use. Moreover, to counter the costs of flaking problems of macrocrystalline quartz, different reduction strategies were used for cores obtained from primary reef/vein outcrops and those from secondary sources (water-rolled alluvial pebbles). The systematic reduction strategies used to prepare cores and remove the elongated flakes suitable for backed artefacts represent an adaption of already established flaking strategies, and suggest that the disappearance of backed artefacts in Late Holocene assemblages from eastern Australia (associated with an increase in the use of macrocrystalline quartz in some areas), did not represent a dramatic change in technology, but rather an effective and significant innovation which ultimately meant that backing was no longer required. This technological shift is also reflected in the benefits of flaking a locally abundant material which retains a strong and sharp working edge. Measuring geoarchaeological variability adds greater insights into the costs and benefits of manufacturing macrocrystalline quartz artefacts, and these insights are applicable to studies worldwide.

Geochemistry and formation of agate-bearing lithophysae in Lower Permian volcanics of the NW-Saxonian Basin (Germany)

Geochemical and mineralogical investigations of the Lower Permian Kemmlitz rhyolite within the NW-Saxonian Basin (Germany) and associated lithophysae (high-temperature crystallization domains) as well as agates were carried out to constrain the genesis and characteristics of these volcanic rocks and the origin of the agate-bearing lithophysae. The volcanic rocks of rhyolitic composition are dominated by quartz, sanidine, and orthoclase and most likely derive from lava flows. Agate-bearing lithophysae were exclusively formed in a glassy facies (pitchstone) of the rhyolites, which was afterwards altered to illite-smectite mixed-layer clays. The results of this study show that agate formation can be related to the alteration of the volcanic rocks accompanied by the infill of mobilized silica into cavities of lithophysae. Fluid inclusion studies point to temperatures of agate formation above 150 °C, indicating that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activities. Remarkable high concentrations of B (29 ppm), Ge (> 18 ppm), and U (> 19 ppm) as well as chondrite-normalized rare earth element (REE) distribution patterns of the agates with pronounced negative Eu-anomalies, slightly positive Ce-anomalies and enriched heavy rare earth elements (HREE) indicate interactions of the host rocks and transport of SiO2 with magmatic volatiles (F/Cl, CO2) and heated meteoric water. Characteristic yellow cathodoluminescence (CL), heterogeneous internal textures as well as high defect density of micro- and macrocrystalline quartz detected by electron paramagnetic resonance (EPR) spectroscopy point to crystallization processes via an amorphous silica precursor under non-equilibrium conditions.

Low-Crystallinity Index Chalcedony from Longhua, China: Characteristics and Formation

Abstract A low-crystallinity index chalcedony was found in the rhyolitic ignimbrite of the Late Jurassic Zhangjiakou Formation, located in Longhua County, Hebei Province, China. This chalcedony occurs as fillings along the fragile fractures of the host rock and is distinct from any other chalcedony deposits, such as the known basalt and carbonate-related types. The host rock is rhyolitic ignimbrite, comprising sanidine (50–70 vol.%), plagioclase (10–15 vol.%), quartz (8–10 vol.%), magnesian biotite (3–5 vol.%), and accessory minerals. The chalcedony appears as long lenticular veins and irregular-shaped bodies, occasionally containing small fragments of the surrounding rock at the boundary. It is colored in yellow, red, and/or white/colorless, with physical properties of specific gravity 2.55–2.56, reflection index of 1.54, Mohs hardness of 6.07–6.34, and weight loss of 1.97%–2.32% by heating. From the boundary to the inner center, its growth structure changes from comb-like macrocrystalline quartz to thin fiber crystallites and then to a relatively uniform cryptocrystalline phase, indicating precipitation from a crystalline to the cryptocrystalline sequence. Electron probe and Raman spectroscopy analyses reveal that the component minerals of the chalcedony are α-quartz and moganite and that the red inclusions are hematite. Quartz in chalcedony exhibits platelet shapes with tiny pores, which are cemented by nanograins, and such a structure is closer to that of opal. It’s crystallinity indexes (CIs) range ~1–3, as indicated by the X-ray diffraction patterns. This low CI and structural features, together with its occurrence, suggest a low temperature of 40°C–80°C during its formation. All these properties show a distinction from those of the most reported chalcedonies. This chalcedony is interpreted as an intermediate transitional type from normal chalcedony to opal, shedding new light on understanding microcrystalline silica mineral aggregate and exploration for a similar gem deposit.

Mineralogy of Agates with Amethyst from the Tevinskoye Deposit (Northern Kamchatka, Russia)

The Tevinskoye agate deposit is located in the North of the Kamchatka peninsula (Russia) and represented by agate-bearing Eocene basaltic and andesitic rocks of the Kinkilsk complex. Agate mineralization occurs in lavas and tuffs as amygdales, geodes, lenses and veins, which are the main sources of the resupply of coastal agate placers. The present study aimed to perform a comprehensive mineralogical, geochemical, and O-isotope investigation of amethyst-bearing agates, and to evaluate data concerning the origin of mineralization and the conditions for amethyst formation. Agates exhibit spectacular textures, with variation in the sequence of silica filling of amygdales and geodes. The mineral composition of the agates is mainly represented by micro- and macro-crystalline quartz, amethyst, length-fast and zebraic chalcedony, moganite, goethite, and clinoptilolite. Carbonate forms individual bands in the outer zones of some agates. The presence of small amounts of native copper, covellite, chalcopyrite and pyrite is a feature of these agates. Copper and iron mineralization are probably typomorphic features related to the host rock composition. The measured values of crystallite size (525–560 Å) and the high moganite content (up to 50%) of agate with amethyst are evidenced by the young age (~45 Ma) of agate-hosting rocks. Agate formation temperatures (21–229 °C) were calculated from the O-isotope composition of chalcedony (+19.6 to +25.5‰), quartz (+18.1 to +22.3‰), and amethyst (+18.2 to +21.5‰). The cold-water monophase fluid inclusions revealed in amethyst crystals suggest that the mineralizing fluids have low temperatures (<100 °C) and low salinity. Magnetite grains in host rock, together with goethite inclusions identified within the amethyst crystals, point to a change in redox conditions and the presence of iron in the agate-forming fluids, which entered the quartz lattice during crystallization and influenced the formation of the violet color.

Distribution of silicification intervals throughout the Barra Velha and Itapema formations: Host rock controls and chronology of silica precipitation (Pre-Salt, Santos Basin, Brazil)

The Barra Velha and Itapema formations, Aptian, Santos Basin, constitute large hydrocarbon reservoirs, mainly in carbonate rocks. These rocks host up to centimeter-thick silica bodies with a variety of textures and geometries along different carbonate facies, suggesting syngenetic, diagenetic, and hydrothermal processes in their formation. The mineralogical and textural changes related to these processes can directly influence the porosity and permeability of the reservoir rocks, turning the study of the silicified intervals important in the exploration and production of these deposits. We present here a systematic macroscopic and microscopic description of the silica bodies and its distribution through the carbonate facies the of Barra Velha and Itapema formations from the Sapinhoá and Búzios fields. Data allow us to identify four geometric types of silica bodies: tabular, irregular, oval and disseminated, which are composed of chalcedony, microcrystalline and macrocrystalline quartz. Tabular, oval, and irregular silica bodies are cross-cutted by mechanical compaction structures such as vertical fractures, boudins and faults. In addition, these bodies include relicts of the host rock lamination, which in turn deflects around them. Thus, silica precipitation must have been penecontemporaneous or in the early stages of diagenesis, before the main mechanical compaction stage. These features were not observed in the disseminated silica type, in the contrary, this type involves and overlaps oval and irregular bodies, indicating that this silica precipitation probably occurred in latter stages in relation to other body types. The genesis of disseminated bodies can then be related to diagenetic or hydrothermal processes during diagenesis. Therefore, the geometry of silica bodies and their spatial distribution are influenced by the host carbonatic petrofabrics and the chronology of silica precipitation within the sedimentary to diagenetic evolution of the sedimentary package. Finally, we present a systematics that can be easily applied to recognize these silica types in other exploration fields of pre-salt interval. The results presented here have great potential to be applied in the research on pre-salt intervals from other Brazilian and African basins since the host carbonate rocks are very similar and favor the development of similar silicification processes and results.

Agates from Mesoproterozoic Volcanics (Pasha–Ladoga Basin, NW Russia): Characteristics and Proposed Origin

Agate gemstones occurring in the Mesoproterozoic volcanic rocks of the Priozersk Formation (PrF) within the Pasha–Ladoga Basin (Fennoscandian Shield, NW Russia) were investigated to characterize the mineral and geochemical composition of the agates and provide new information concerning their origin. Optical and scanning electron microscopy, EDS microanalysis, X-ray powder diffraction, X-ray fluorescence spectrometry, Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and C-O isotope analysis were used for the study. Agate mineralization appears mostly as an infill of fissures, cavities, gas vesicles in massive and vesicular basalts, lava-breccias. The mineral composition of agates is dominated by alpha-quartz (fibrous chalcedony, microcrystalline and macrocrystalline quartz), but it also displays abundances of calcite. The characteristic red-brownish agate’s coloration is caused by multiple hematite inclusions distributed in an agate matrix. The study revealed the two phases of agate formation in the PrF volcanics, which are most likely controlled by two distinctly different fluids and/or their mixture. At first, agates appeared due to post-magmatic iron-rich fluids. The late hydrothermal activity was probably triggered by intrusion of gabbro-dolerite sill and resulted in the second phase of agate formation. We suggest that the late hydrothermal fluids remobilized the iron compounds from the crust of weathering underlying the PrF volcanics, which led to additional formation of vein agates and filling of gas vesicles with hematite-rich calcite/silica matter.

Mineralogy of Miocene Petrified Wood from Central Washington State, USA

Silicified wood occurs abundantly in Middle Miocene flows and sedimentary interbeds of the Columbia River Basalt Group (CRBG) in central Washington State, USA. These fossil localities are well-dated based on radiometric ages determined for the host lava. Paleoenvironments include wood transported by lahars (Ginkgo Petrified Forest State Park), fluvial and palludal environments (Saddle Mountain and Yakima Canyon fossil localities), and standing forests engulfed by advancing lava (Yakima Ridge fossil forest). At all of these localities, the mineralogy of fossil wood is diverse, with silica minerals that include opal-A, opal-CT, chalcedony, and macrocrystalline quartz. Some specimens are composed of only a single form of silica; more commonly, specimens contain multiple phases. Opal-A and Opal-CT often coexist. Some woods are mineralized only with chalcedony; however, chalcedony and macrocrystalline quartz are common as minor constituents in opal wood. In these specimens, crystalline silica filling fractures, rot pockets, and cell lumen may occur. These occurrences are evidence that silicification occurred as a sequential process, where changes in the geochemical environment or anatomical structures affected the precipitation of silica. Fossilization typically began with precipitation of amorphous silica within cell walls, leaving cell lumen and conductive vessels open. Diagenetic transformation of opal-A to opal-CT in fossil wood has long been a widely accepted hypothesis; however, in opaline CRBG specimens, the two silica polymorphs usually appear to have formed independently, e.g., woods in which cell walls are mineralized with opal-A but in which lumen contain opal-CT. Similarly, opal-CT has been inferred to sometimes transform to chalcedony; however, in CRBG, these mixed assemblages commonly resulted from multiple mineralization episodes.

Facets of Variation: Macrocrystalline Quartz Reduction in Australia

Facets of Variation: Macrocrystalline Quartz Reduction in Australia

Black Agates from Paleoproterozoic Pillow Lavas (Onega Basin, Karelian Craton, NW Russia): Mineralogy and Proposed Origin

The present study provides the first detailed investigation of black agates occurring in volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield). Three characteristic texture types of black agates were identified: monocentric concentrically zoning agates, polycentric spherulitic agates, and moss agates. The silica matrix of black agates is only composed of length-fast and zebraic chalcedony, micro- and macro-crystalline quartz, and quartzine. In addition to silica minerals, calcite, chlorite, feldspar, sulphides, and carbonaceous matter were also recognised. The black colour of agates is related to the presence of disseminated carbonaceous matter (CM) with a bulk content of less than 1 wt.%. Raman spectroscopy revealed that CM from black agates might be attributed to poorly ordered CM. The metamorphic temperature for CM from moss and spherulitic agates was determined to be close to 330 °C, whereas CM from concentrically zoning agates is characterised by a lower temperature, 264 °C. The potential source of CM in moss and spherulitic agates is associated with the hydrothermal fluids enriched in CM incorporated from underlaying carbon-bearing shungite rocks. The concentrically zoning agates contained heterogeneous CM originated both from the inter-pillow matrix and/or hydrothermal fluids.

Mineralogy, Geochemistry and Genesis of Agate—A Review

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.

Biosignatures in Subsurface Filamentous Fabrics (SFF) from the Deccan Volcanic Province, India

The morphology, chemical, and mineralogical composition of subsurface filamentous fabrics (SFF) from the Deccan Volcanic Province (DVP) were investigated to determine the origin of these spectacular aggregates. SFF occur in a wide variety of morphologies ranging from pseudo-stalactites to irregular fabrics and are classified as SFFIr (irregular) or SFFMa (matted). The SFF samples exhibit a thread-like (or filament-like) center from which mineral precipitation starts to form the final macroscopic morphologies. Detailed investigations revealed organic material (fungal chitin) in the innermost filamentous core, which may have acted as an initial nucleus for the mineralization processes. The morphometric characteristics of certain filamentous fabrics are very similar to those of microbial filaments and the fabrics formed from them but are clearly distinct from similar types of non-biological precipitates (fibrous minerals, speleothems, and “chemical gardens”). These features indicate that the filamentous cores might be products of microbial communities that were active in the basaltic cavities. The SFF cross-sections display similar concentric layers of the mineral succession and reach thicknesses of several centimeters with spectacular lengths up to 100 cm and constant diameters. The typical mineralization sequence points to temporal variation in the chemical composition of the mineralizing fluids from Fe(Mg)-rich (Fe-oxides/-hydroxides, Fe-rich sheet silicates such as celadonite and di-/tri-smectite) to Ca-dominated (Ca-rich zeolites) and finally pure SiO2 (opal-CT, chalcedony, and macro-crystalline quartz). Assuming biological activity at least during the early mineralization processes, circumneutral pH conditions and maximum temperatures of 100–120 °C were supposed. The formation of filamentous cores including Fe-bearing phyllosilicates probably occurred near the surface after cooling of the lava, where the elements necessary for mineral formation (i.e., Si, Mg, Al, Fe) were released during alteration of the volcanic host rocks by percolating fluids.

Investigation of Fluids in Macrocrystalline and Microcrystalline Quartz in Agate Using Thermogravimetry-Mass-Spectrometry

Gaseous and liquid fluids in agates (banded chalcedony—SiO2) of different localities were investigated systematically by thermogravimetry-mass-spectrometry within a temperature range from 25 to 1450 °C, for the first time. Chalcedony and macrocrystalline quartz from twelve agate samples were investigated, from Germany (Schlottwitz, St. Egidien, Chemnitz and Zwickau), Brazil (Rio Grande do Sul), Scotland (Ayrshire) and the USA (Montana). They originate from mafic and felsic volcanic rocks as well as hydrothermal and sedimentary environments. The results were evaluated regarding compounds of hydrogen with fluorine, chlorine, nitrogen, carbon and sulphur. Additionally, oxygen compounds were recognized with hydrogen, fluorine, nitrogen, sulphur and carbon. The nature of the compounds was identified based on their mass-charge-ratio and the intensity ratios of the associated fragments. Due to interferences of different compounds with the same mass-charge-ratio, only H2O, HF, NO, S, SO, CO3—as well as several hydrocarbon compounds (for example CO32− or CO)—could be properly identified. The main degassing temperatures were detected at around 500 and 1000 °C. Generally, a difference between quartz and chalcedony regarding the composition of their fluids could not be found. The results indicate a silica source for the agate formation from aqueous solutions but also a possible role of fluorine compounds. Additionally, CO2 and other fluids were involved in the alteration of volcanic rocks and the mobilization and transport of SiO2.

Origin and geochemistry of agates in Permian volcanic rocks of the Sub-Erzgebirge basin, Saxony (Germany)

Mineralogical and geochemical investigations of agates from Permian volcanic rocks of the Sub-Erzgebirge basin (Saxony, Germany) were made to constrain the genesis and characteristics of these spectacular forms of silica in acidic volcanic host rocks. Samples from the main agate occurrences of Chemnitz, Hohenstein-Ernstthal, St. Egidien and Zwickau were selected for detailed analyses.The results of the study show that agate formation can be related to volcanic activity (Rochlitz ignimbrite) and the subsequent alteration of the volcanic rocks. Most agates originate from the infill of silica into cavities of lithophysae (high-temperature crystallization domains), which formed during cooling of welded ignimbrite. Agate formation temperatures of probably >150°C were calculated from fluid inclusion and oxygen isotope studies, which indicate that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activity.The trace-element composition of chalcedony and macrocrystalline quartz in agates is different from that of quartz from magmatic or metamorphic rocks and pegmatites. Elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) were released during the alteration processes and accumulated in the SiO2 matrix of the agates. Extraordinary high contents of Ge (>90ppm), B (46ppm) and U (>18ppm) were also detected, which can exceed the Clark concentration and sometimes the element concentration in the surrounding host rocks. In addition, chondrite-normalized REE distribution patterns of the agates show strong negative Eu-anomalies, enriched HREE as well as tetrad effects. These patterns are different from the REE patterns of the volcanic host rocks and point to fractionation processes during agate formation. The specific geochemical features indicate interactions of the host rocks with magmatic volatile fluids and heated meteoric water, and transport of SiO2 and other elements both in aqueous solution and via stable fluorine (and chlorine) compounds such as SiF4, BF3, GeF4, and UO2F2.The results of cathodoluminescence (CL) microscopy and spectroscopy revealed a microstructure of the agates that is similar for all occurrences. Characteristic features are irregular internal textures and sector zoning in quartz as well as luminescence colours and spectra, respectively, which are caused by a typical high defect density (oxygen vacancies, silanol groups). According to these results, the formation of agates can be explained by crystallization via an amorphous silica precursor under non-equilibrium conditions.

Uranium and uranyl luminescence in agate/chalcedony

AbstractA systematic investigation of agates from different occurrences in Europe, Northern and Southern America reveals that macrocrystalline quartz and chalcedony within them have an unusually high uranium content. Whereas agates may contain >70 ppm of U, quartz from magmatic and metamorphic rocks as well as pegmatite quartz commonly exhibit U concentrations at sub-ppm levels. Spatially resolved trace-element analyses by laser ablation inductively coupled plasma mass spectrometry show that the distribution of U within the agate samples is heterogeneous and coincides with the structural banding. The results indicate that U is incorporated into agate as uranyl ions. These ions, which are bound to the silica surface, are interpreted to originate from the parallel accumulation of Si and U by alteration processes of surrounding host rocks during agate formation.The uranyl ion is the cause of greenish photoluminescence (PL) in agate, which can only be excited by short wavelengths (<300 nm). The green PL is due to the electron transition from an excited to a ground state of the uranyl ion and is shown by a typical emission line at ∼500 nm accompanied by several equidistant lines. These are due to the harmonic vibration of oxygen atoms along the uranyl axis. Luminescence can be detected in samples with a U content down to the 1 ppm level.

Fluid inclusion analysis of silicified Palaeoarchaean oceanic crust – A record of Archaean seawater?

In recent years, the role of Archaean seawater and hydrothermal fluid in the extensive silicification of Palaeoarchaean volcano-sedimentary successions has been a matter of considerable debate. In an attempt to constrain the conditions of silica precipitation, and the sources and chemical composition of the interacting fluids, we used fluid inclusion microthermometry, bulk crush-leach and oxygen isotope analyses of chert and quartz veins in silicified komatiites and sediments from the Barberton greenstone belt, South Africa. Chert vein margins consist of microcrystalline quartz and carbonaceous matter, whereas the vein centres are often filled with macrocrystalline quartz that contains abundant fluid inclusions.Oxygen isotope ratios of vein chert and macrocrystalline quartz vary from 18 to 21‰, with the macrocrystalline quartz having slightly higher δ18O values (0.7±0.3‰). The data are consistent with silica precipitation during low-temperature (≤100°C) hydrothermal processes on the Archaean seafloor. Macrocrystalline quartz contains homogeneous 2-phase (L+V) inclusions at room temperature with a relatively constant vapour fraction. The inclusions have a salinity of 3–11wt.% NaCl equiv. and homogenisation temperatures (Th) of 150–200°C. Whereas some of the inclusions form intragranular fluid inclusion clusters that appear to be primary, other inclusions form transgranular fluid inclusion trails and are clearly secondary. Both types of inclusions share the same microthermometric characteristics, indicating that fluid entrapment occurred during a later metamorphic event and not near the seafloor. The event likely coincided with regional deformation and metamorphism at 3.23Ga. Chlorite thermometry from vein host rocks reveals peak conditions of ∼257±31°C. In conjunction with microthermometry, the data correspond to a crustal depth of 3–6km. A secondary origin of fluid inclusions is also consistent with the Cl/Br, Na/Cl and Na/K ratios of the macrocrystalline veins, which are similar to those found in metamorphic quartz veins. The fluid inclusions thus do not provide information on the conditions and temperatures during chert formation. While the fluids potentially carry the geochemical signature of modified Archaean seawater (i.e. hydrothermal fluid), characterised by low Cl/Br and Na/K ratios, and low Mg-contents, the fluid composition was likely modified during regional metamorphism.

Heterogeneous Packing and Quartz Cementation of the Eureka Quartzarenite (Middle Ordovician), Utah and Nevada, U.S.A

Abstract The Eureka Sandstone is a quartzarenite 200 m thick that was deposited on the eastern shelf of the Cordilleran foreland basin from Canada to California. It has many characteristics of other lower Paleozoic quartzarenites in terms of its sheet-like geometry, mineralogical and textural maturity (99.5% detrital quartz, almost free of detrital clay), shallow-marine environment of deposition, and abundance of quartz cement (10–26%). However, it displays wide variations in compaction as measured by intergranular volume (IGV range of 16%) that are not all related to differences in burial depth plus IGV values as low as 10%. Sutured grain contacts and fractured grains cannot account for such abnormally low IGV values, which suggests that dissolution of the edges of grains where they overlapped their neighbors (the Fuchtbauer effect), leaving minimal evidence, was a major process of compaction. The normal evolution of microquartz overgrowths ( 10 mm) with time was retarded; consequently microquartz and mesoquartz cement (80%) dominate over macroquartz (20%) and are the only cements in the poorest-cemented beds and laminations. Illite co-precipitated with micro- and mesoquartz and impeded quartz cementation by coating quartz crystal faces. Typical macrocrystalline quartz was the final cement only in illite-poor samples. Heterogeneous illite distribution may reflect heterogeneous abundance of K-feldspar or detrital illite that possibly served as seed crystals for authigenic illite. In spite of reaching temperatures > 135° C for ~ 100 My, many Eureka sandstones, especially in Utah, remain incompletely and unevenly cemented. Consequently, beds range from weakly friable to pervasively quartz-cemented “quartzite” in the same outcrop. Porosity was generally reduced to less than 2% except in rare laminations where it reaches 22%. Grain size had some influence on the amount of quartz cement locally, but the chief cause of cement heterogeneity appears to have been illite abundance. The most likely major source of silica for quartz cement was quartz grain dissolution at shale beds and clay drapes, although documentation is absent. Intergranular grain dissolution was a minor source of silica; overlap quartz averages less than 4%. Oxygen isotopic data are compatible with the precipitation of quartz either from diagenetically evolved meteoric water that invaded the sandstones from tectonic highlands to the west or evaporative brines.

Characteristics and origin of agates in sedimentary rocks from the Dryhead area, Montana, USA

Abstract Agates from the Bighorn district in Montana (USA), the so-called Dryhead area, and their adjacent host rocks have been examined in the present study. Analyses by XRD, polarizing microscopy, LA-ICP-MS, cathodoluminescence (CL), SEM and of oxygen isotopes were performed to obtain information surrounding the genesis of this agate type. Investigations of the agate microstructure by polarizing microscopy and CL showed that chalcedony layers and macrocrystalline quartz crystals may have formed by crystallization from the same silica source by a process of self-organization. High defect densities and internal structures (e.g. sector zoning) of quartz indicate that crystallization went rapidly under non-equilibrium conditions. Most trace-element contents in macrocrystalline quartz are less than in chalcedony due to a process of ‘selfpurification’, which also caused the formation of Fe oxide inclusions and spherules. Although the agates formed in sedimentary host rocks, analytical data indicate participation of hydrothermal fluids during agate formation. Trace elements (REE distribution patterns, U contents up to 70 ppm) and CL features of agate (transient blue CL), as well as associated minerals (fluorite, REE carbonates) point to the influence of hydrothermal processes on the genesis of the Dryhead agates. However, formation temperatures <120°C were calculated from O-isotope compositions between 28.9‰ (quartz) and 32.2‰ (chalcedony).

GREENISH QUARTZ FROM THE THUNDER BAY AMETHYST MINE PANORAMA, THUNDER BAY, ONTARIO, CANADA

Naturally occurring greenish quartz found within the context of amethyst-bearing deposits is not simply the result of the exposure of amethyst to thermal bleaching or exposure to the sun. Rather, it can represent a set of distinct color-varieties resulting from the changing chemical and thermal nature of the precipitating solution. Greenish quartz occurs at the Thunder Bay Amethyst Mine Panorama (TBAMP), Thunder Bay, Ontario, Canada, in several distinct varieties. Yellowish green quartz and dark green quartz with purple hues occur as loose detritus, and pale greenish gray quartz occurs as part of a color-gradational sequence of mineralization involving macrocrystalline quartz of other colors and chalcedony. The TBAMP system contains a number of color varieties of quartz including greenish, amethyst, colorless, and smoky. Spectroscopic, irradiation and controlled heating studies show that changes in salinity and temperature of the hydrothermal system that produced the TBAMP deposit are reflected in the changing coloration of the quartz. The greenish quartz, especially the greenish gray variety, has increased turbidity and fluid inclusions in comparison with the adjacent amethyst. Analysis of different colors on major (r = {101 1}) and minor (z = {011 1}) rhombohedral sectors within the quartz indicates that changes in the growth rate also have influenced color development. As the system evolved, two factors contributed to the color changes. A minor ferric component appears to change position from interstitial to substitutional within specific growth-sectors, and the trace-element composition of the quartz evolved. The samples from the TBAMP deposit are compared to isolated samples of greenish quartz collected from three other amethyst-bearing localities: Farm Kos and Farm Rooisand (Namibia), Kalomo-Mapatiqya (Zambia), and southern Bahia (Brazil). All included similar greenish hues with the exception of the yellowish green variety. Colors within the quartz are consistently correlated with the speciation of hydrous components. Darker green samples incorporate larger amounts of molecular H_2O than either pale greenish gray samples, colorless samples, or amethyst. The appearance of strong hydroxyl peaks in the infrared spectra is limited to amethyst and colorless varieties.

Agate recrystallisation: Evidence from samples found in Archaean and Proterozoic host rocks, Western Australia

The crystallite size and density of agates from three Western Australian host rocks aged 1.84, 2.72 and 3.48 Ga have been determined. Infrared and Raman spectra from these ancient agates have respectively shown minimal water and zero moganite content. Data comparison of agates from Western Australian and younger hosts showed an advanced crystallisation in the Western Australian samples with, in some instances, the crystallite size comparable to macrocrystalline quartz. Low-grade metamorphism is present in the host rocks and localised heating is one possible cause of the increased crystallite growth. Micrographs of one agate sample from the Warrawoona Group (3.48 Ga) show a breakdown in the fibrous chalcedony; regional variations within the agate were supported by data from infrared and X-ray diffraction. It has been shown that part of the sample had undergone a transformation of the fibrous chalcedony to granular microcrystalline quartz. The Crystallinity Index for microcrystalline quartz, based upon the degree of resolution of the (212) quartz reflection, offers a rapid method to identify abnormal crystallite growth that could serve as a palaeoindicator of host-rock metamorphism.