Detrital Biotite Research Articles

Potential critical sources of error in the analysis of microcharcoal in Greenlandic palynological samples – an initial study

AbstractIt is common practice to count charcoal particles alongside pollen and non‐pollen palynomorphs in palynological samples. The classification of material as charcoal is dependent upon a number of morphological criteria which, in certain geological zones, could be misclassified. In a preliminary study we tested the effects of different preparation techniques on the count result of charcoal from the same samples of Greenland lake sediments. We demonstrate that the use of hydrofluoric acid has a major impact on the number of recorded charcoal particles. We show that detrital biotite is responsible for a large fraction of the particles misclassified as charcoal and suggest that by using a neodymium magnet any inaccuracy can be minimised. This protocol was then applied to further samples which removed the biotite fraction from the total charcoal count.

Development of Deformation Bands and Deformation Induced Weathering in a Forearc Coal-Bearing Paleogene Fold Belt, Northern Japan

We conducted microstructural and microchemical analyses of deformation bands in a forearc fold belt consisting of the Eocene Urahoro Group located in northern Japan. In the study area, there was one flexure (or monocline) developed where deformation bands pervasively occurred in arkosic sandstone intercalated with mudstone and coal layers. Deformation bands formed at the maximum burial depth of c. 1.5–2.5 km; this was inferred from both the thickness of the overlying strata and vitrinite reflectance values (%RO) of the coal layers (c. 0.5). These bands were inferred to have originated as phyllosilicate bands, which developed into cataclastic bands with increasing strain on sandstones with up to c. 10% volume of phyllosilicate. In the cataclastic bands, the detrital grains in the host parts were crushed into sizes less than one-half to one-fifth of the original ones, and the long axis of the fractured grains tended to align parallel to the deformation bands. It was found that the deformation bands became a site of intense weathering at later stages, where not only detrital biotite grains were altered to vermiculite and kaolinite, but also authigenic clay minerals such as smectite grew in pore spaces created by the fracturing of detrital grains.

The Bigrlyi Tabular Sandstone-Hosted Uranium–Vanadium Deposit, Ngalia Basin, Central Australia

The Bigrlyi deposit is a tabular, sandstone-hosted, uranium–vanadium deposit of Carboniferous age located in the Ngalia Basin of central Australia. The deposit is similar to the continental, fluvial Saltwash-type of sandstone-hosted U-V deposits which are well known from the Colorado Plateau, USA. Most mineralization at Bigrlyi occurs as thin, multiple-stacked, stratiform lenses at the base of fluvial channels near the contact between a grey sandstone succession and a hematitic, purple–red sandstone succession. A larger halo of lower grade vanadium mineralization extends beyond the main U-V-mineralized zone. The host is an immature, feldspathic sandstone, grading into arkose and lithic-rich variants. Lithic ‘rip-up’ clasts of clay-rich sediments are common in the basal parts of fluvial channels, and are frequently the focus of, and have acted as sites for, U-V mineralization. Coffinite and uraninite are the main uranium minerals, with the former dominant. Vanadium is mainly hosted by Fe-V-bearing clays and chlorite, including roscoelite, grading into vanadian illite, the interlayer mineral corrensite, and altered detrital biotite. The V-Fe–oxyhydroxide minerals montroseite, haggite and doloresite, and altered detrital Fe-Ti oxides, are minor V-hosts. Mineralized zones correlate with enrichments in Se, Li, Ba, Be, Mo, Mg and Fe, and elevated Se/S ratios are characteristic of U-mineralized zones. Petrographic studies show that a heterogeneous mixture of variably mineralized lithic clasts is present; in the same rock, some clasts are Fe-rich and only weakly U-V-mineralized, while other clasts are strongly V- and/or U-mineralized. These observations point to mineralization processes that did not take place in-situ in the host sandstone at the site of deposition as required by conventional groundwater models. Lead isotope results provide evidence of the open-system mobility of radiogenic elements in parts of the deposit. In V-bearing zones, radiogenic Pb contents were found to be unsupported by current U levels, suggesting that over time U has been mobilized from these zones and redistributed, resulting in U-enrichment in other parts of the deposit. Mobility pathways were likely open over time from early in the history of the Bigrlyi deposit. A hybrid mineralization model, involving an interplay between solution-precipitation processes, detrital transport and post-depositional U remobilization, is proposed for Bigrlyi. Ferrous-ion-bearing clay minerals and pyrite are considered to be the most likely primary reductants/adsorbents, while the deposit is lacking carbonaceous matter.

Sedimentary iron cycling in the Benguela upwelling system off Namibia

Iron (Fe) is an essential micronutrient and its availability controls primary production in large parts of the ocean. Further, the variability of oxygen minimum zones (OMZ) exerts a major control on the availability of Fe, concentrations of which are elevated off Namibia. Therefore, to understand the sources and processes involved in the cycling of Fe off Namibia, we determined major and trace element contents (Fe, Al, Si, K, P, organic carbon, Mn, Co, Zr) and the isotopic composition of Fe (δ56Fe) in seven sediment cores along a transect from the shelf to the abyssal plain at ∼5 km water depth (Meteor cruise M76-1) as well as in the local lithogenic background (Namibian river bed sediments and dust).The depletion of Mn vs. the local lithogenic background (LLB) reflects the reducing character of sediments from the shelf to the lower slope. While depletion of Mn and enrichment of Co indicates the anoxic–sulfidic environment on the shelf, the enrichment of both elements in abyssal sediments documents highly oxidizing conditions there.Iron/Al ratios are elevated (0.6 to 1.6) and therefore equal to, or higher than the LLB. The highest Fe enrichment is seen on the shelf (core 1) and the shelf break (core 2) with a clear decrease towards the abyss. The bulk δ56Fe values range from +0.30 to −0.05‰ in the marine sediments, with an average of ∼0.08‰, which is slightly lighter than the δ56Fe of the LLB (0.16 to 0.21‰). Further, combined enrichments of K and Fe suggest the presence of detrital biotite (core 1) and authigenic glauconite (core 2), both of which are perfectly correlated with Zr/Al ratios reflecting the highly dynamic conditions of the shelf environment (0-300 m water depth).The absent large sedimentary Fe isotope fractionation in the sediments of this study compared to Peru sediments is likely due to a combination of the following: (i) a highly variable OMZ leading to random re-oxidation of any sediment-sourced Fe which prevents the export of larger amounts of dissolved Fe to the interior of the ocean, and which also prevents focused enrichments of Fe, and (ii) an enhanced dust flux, which frequently resupplies terrestrial Fe dampening any Fe isotope fractionation.This study is a snapshot on sedimentary Fe cycling in one area from the vast Namibian margin. Compared to the Peruvian margin, the data suggest little sedimentary Fe release, transport and redeposition. However, previous water column data suggest substantial Fe release. Hence, for a more subtle investigation of the Fe loss and re-supply a higher sampling resolution of multiple cores across the shelf and slope will be required.

Diagenetic mobilization of Ti and formation of brookite/anatase in early Cambrian black shales, South China

Titanium (Ti) is typically hosted in detrital minerals in marine sediments and has long been considered to be immobile during diagenesis. In this study, the authigenic titania minerals, brookite and anatase, are observed in early Cambrian carbonaceous shales from the Meishucun and Zhajin sections of South China, respectively. Black shales in the Meishucun section have total organic carbon (TOC) contents from 1.6 to 3.9 wt% and HI (hydrogen index) values from 3.8 to 20 mg HC/g TOC, whereas black shales in the Zhajin section have much higher TOC (7.1–15.6 wt%) but lower HI (<2.0 mg HC/g TOC) and contain abundant bitumen (~3 vol%). Brookite in black shales from the Meishucun section crystallized invariably along cleavages of detrital biotite. This intimate association suggests that the Ti required to form titania minerals was derived from detrital biotite and that Ti was mobilized only on a nano- to micro-meter scale. In contrast, anatase aggregates in black shales from the Zhajin section are intergrown with bitumen. It is proposed that Ti in these shales was mobilized in low pH organic-rich fluids and, subsequently, preferentially precipitated as anatase with increasing pH. The mobilization of inert Ti in black shales is indicative of a possible diagenetic or post-diagenetic elemental redistribution in carbonaceous rocks. Our study further demonstrates that proxies based on geochemical component of black shales can be employed to estimate redox state of ancient oceans, but must be applied with caution.

Origins of authigenic minerals and their impacts on reservoir quality of tight sandstones: Upper Triassic Chang-7 Member, Yanchang Formation, Ordos Basin, China

ABSTRACTReservoir quality is critical for sweet-spot evaluation in tight sandstone plays, but few studies have focused on the origin of authigenic minerals in tight sandstones and their impact on reservoir quality. This study integrates petrographic analysis, carbon and oxygen isotopic data and mercury injection capillary pressure data of the Upper Triassic Chang-7 tight sand reservoir samples in the Ordos Basin, China to understand the origin of authigenic minerals and the impact of authigenic minerals on reservoir quality. Carbonate minerals, including calcite, ferroan calcite and dolomite, and kaolinite, are the major authigenic minerals in the Chang-7 sandstone. They were derived from the chemical diagenetic alteration of detrital feldspar and biotite, with the involvement of ions that are believed to be primarily from the connate water in interstitial pore space of the interbedded mudstones. Meteoric water and organic fluids from hydrocarbon generation may also have been involved in the alteration processes of the minerals in the Chang-7 tight sandstone. The origin of kaolinite indicates that the porosity was increased by dissolution of detrital grains, offsetting the porosity loss from burial compaction. Authigenic minerals appear to have a weak correlation with pore structure, suggesting that the pore structure of the Chang-7 tight sandstone is not only controlled by major authigenic minerals but also affected by other geological factors.

δ18O and chemical composition of Libyan Desert Glass, country rocks, and sands: New considerations on target material

– Oxygen isotope and chemical measurements were carried out on 25 samples of Libyan Desert Glass (LDG), 21 samples of sandstone, and 3 of sand from the same area. The δ18O of LDG samples range from 9.0‰ to 11.9‰ (Vienna Standard Mean Ocean Water [VSMOW]); some correlations between isotope data and typological features of the LDG samples are pointed out. The initial δ18O of a bulk parent material may be slightly increased by fusion due to the loss of isotopically light pore water with no isotope exchange with oxygen containing minerals. Accordingly, the δ18O of the bulk parent material of LDG may have been about 9.0 ± 1‰ (VSMOW). The measured bulk sandstone and sand samples have δ18O values ranging from 12.6‰ to 19.5‰ and are consequently ruled out as parent materials, matching the results of previous studies. However, separated quartz fractions have δ18O values compatible with the LDG values suggesting that the modern surface sand inherited quartz from the target material. This hypothesis fits previous findings of lechatelierite and baddeleyite in these materials. As the age of the parent material reported in previous studies is Pan-African, we measured the δ18O values of bulk rock and quartz from intrusives of Pan-African age and the results obtained were compatible with the LDG values. The main element abundances (Fe, Mg, Ca, K, Na) in our LDG samples conform to previous estimates; Fe, Mg, and K tend to be higher in heterogeneous samples with dark layers. The hypothesis of a low-altitude airburst involving silica-rich surface materials deriving from weathered intrusives of Pan-African age, partially melted and blown over a huge surface by supersonic winds matches the results obtained.

Oxidation of CH 4 to CO 2 and H 2O by chloritization of detrital biotite at 270 ± 5 °C in the external part of the Central Alps, Switzerland

The upper end of the CH 4-zone in the External part of the Central Alps of Switzerland results from an oxidation reaction of methane into water and carbon dioxide at 270 ± 5 °C [Tarantola, A., Mullis, J., Vennemann, T., Dubessy, J., de Capitani, C., 2007. Oxidation of methane at the CH 4 / H 2O–(CO 2) transition zone in the external part of the Central Alps, Switzerland: Evidences from stable isotope investigations. Chemical Geology 237, 329–357.]. Petrographic investigations of the North-Helvetic Flysch revealed the presence of detrital biotite, muscovite and chlorite in the CH 4-zone. At the transition from the methane to the water zone, the sudden increase of CO 2 in the fluid is associated with the formation of chlorite and muscovite in replacement of detrital biotite. In the H 2O-zone, biotite is entirely replaced by chlorite-muscovite interlayered stacks. Mössbauer spectroscopy showed a minimum decrease of XFe 3+ from 0.18 to 0.09 within the sheet silicate fraction at the transition from the CH 4- to the H 2O–(CO 2) dominated fluid zone. Detrital biotite of the investigated host rocks incorporates at the same conditions larger amounts of Fe 3+ than chlorite in its structure and XFe 3+ (biotite) > XFe 3+ (new-chlorite) > XFe 3+ (detrital chlorite). The reaction of chloritization of detrital biotite is interpreted as a fluid controlled mineralogical isograde at 270 ± 5 °C, marking the transition from the low to the high grade anchizone, and allowing the oxidation of up to 36 moles of CH 4 per cubic meter of rock.

Oxidation of CH4 to CO2 and H2O by chloritization of detrital biotite at 270 + 5°C in the external part of the Central Alps, Switzerland.

Oxidation of CH4 to CO2 and H2O by chloritization of detrital biotite at 270 + 5°C in the external part of the Central Alps, Switzerland.

Provenance of late Palaeozoic metasediments of the Patagonian proto-Pacific margin (southernmost Chile and Argentina)

In this provenance study of late Palaeozoic metasediments of the Eastern Andean Metamorphic Complex (EAMC) along the south Patagonian proto-Pacific margin of Gondwana, the palaeogeological setting of the continental margin in Devonian–Carboniferous and Permian times is reconstructed. The study is based on detrital heavy mineral contents, chemical compositions of tourmaline grains, and whole rock element and Nd-Sr isotopic compositions. Element and isotopic compositions reveal that Devonian–Carboniferous metaturbidites deposited before the development of a Late Carboniferous–Permian magmatic arc along the margin were mainly fed from felsic, recycled, old continental rocks. The last recycling phase involved erosion of metasediments that were exposed in Patagonia. Feeder systems to the basin cut either through epidote-rich or garnet-rich metasediments. In Permian time, EAMC metaturbidites were deposited next to the evolving magmatic arc and were derived from felsic, crustal rocks. Two provenance domains are recognised. The metasediments of the northern one are chemically similar to those of the Devonian–Carboniferous metasediments. This domain was fed from the metasedimentary host rocks of the magmatic arc. The southern domain probably was fed from the arc proper, as indicated mainly by the dominance of metaplutonic lithic fragments, abundant detrital biotite, and the major element composition of the metasediments.

Rb-Sr Isotopic and elemental studies of the origin of glauconite in the Permian northern Karoo Coal Fields, South Africa: Evidence for a thermal Mid-Jurassic influence

Research Article| December 01, 2004 Rb-Sr Isotopic and elemental studies of the origin of glauconite in the Permian northern Karoo Coal Fields, South Africa: Evidence for a thermal Mid-Jurassic influence J. M. Barton, Jr.; J. M. Barton, Jr. Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: jmb@na.rau.ac.za; bc@na.rau.ac.za Search for other works by this author on: GSW Google Scholar B. Cairncross; B. Cairncross Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: jmb@na.rau.ac.za; bc@na.rau.ac.za Search for other works by this author on: GSW Google Scholar I. McLachlan I. McLachlan P.O. Box 307, Parow 7499, South Africa, e-mail: mclachlai@petroleumagencysa.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2004) 107 (4): 499–504. https://doi.org/10.2113/gssajg.107.4.499 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation J. M. Barton, B. Cairncross, I. McLachlan; Rb-Sr Isotopic and elemental studies of the origin of glauconite in the Permian northern Karoo Coal Fields, South Africa: Evidence for a thermal Mid-Jurassic influence. South African Journal of Geology 2004;; 107 (4): 499–504. doi: https://doi.org/10.2113/gssajg.107.4.499 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Rb-Sr isotopic analyses of glauconite from Permian sedimentary rocks associated with coal seams in the northern Karoo basin yield ages ranging from ~181 to ~178 Ma. These ages, being significantly younger than times of deposition of the host rocks, are interpreted to reflect the time of recrystallization and glauconite maturation during the Middle Jurassic. These ages are very similar to the time of widespread volcanism and intrusion of dolerite dykes and sills into Karoo sedimentary strata at ~180 Ma. Some link, whether just heating or hydrothermal activity leading to maturation of glauconite is implied. Rb-Sr ages between ~400 and ~750 Ma for detrital biotite and muscovite from these same sedimentary rocks limit the maximum temperature of the thermal activity to <~250°C, and vitrinite reflectance and the presence of oil in the sediments indicate a maximum temperature of <120°C, well below the ~230°C block temperature of glauconite in the Rb-Sr system. Therefore the second possibility seems more probable. You do not currently have access to this article.

TEM study of the mechanism of transformation of detrital kaolinite and muscovite to illite/smectite in sediments of the Salton Sea Geothermal Field

The reaction by which kaolinite transforms to I-S has been studied by SEM and TEM using core samples from the Salton Sea Scientific Drilling Project. Kaolinite is abundant from the surface to a depth of ∼300 m (∼115°C), decreasing in abundance and becoming undetectable at a depth of ∼500 m (∼160°C), with a concomitant increase in abundance of I-S. In a 256-m deep sample, kaolinite occurs as large (∼100 μm) detrital grains, in the fine-grained (< 1 μm) matrix, and as packets forming stacks in detrital biotite and muscovite. In sharp contrast, no kaolinite was detected in the sample from a depth of 477 m but illite and minor I-S occur in the matrix. Authigenic chlorite occurs as subhedral crystals in the matrix with subparallel illite crystals, and as muscovite-chlorite stacks. I-S occurs in three ways: (1) Interlayered within kaolinite as a few, curved layers with 10- and 20-A periodicities with the characteristics of collapsed, dehydrated I-S. Along-layer transitions with change in (001) d-value from 7 to 10 A occur. (2) As coalescent, randomly oriented packets filling pore space in the matrix. (3) Interlayered with detrital muscovite. These observations show that R1 T-S in Salton Sea-area sediments forms primarily through alteration of kaolinite and muscovite of detrital origin over a temperature interval of ∼100 to < 200°C, with I-S serving as a precursor to illite. Reaction occurs both by direct layer-by-layer replacement of kaolinite and muscovite, and by dissolution, ion transport and crystallization in pore space. Both processes involve dissolution and crystallization, with only the scale of transport varying from that of along-layer interfaces to the dimensions of pore space. The pH value and activity of K+ in the hydrothermal brines play important roles in the alteration process and control the simultaneous transformation of both kaolinite and muscovite to I-S.

Testing for the presence of a terrane boundary within Neoproterozoic (Dalradian) to Cambrian siliceous turbidites at Callander, Perthshire, Scotland

The Southern Highland Group (Dalradian) and Keltie Water Grit Formation, which includes the Lower Cambrian Leny Limestone, form an inverted, 1.4 km thick, largely arenaceous, sequence at Callander. The grits have the same detrital mineralogy throughout, mainly quartz, plagioclase (An 1–3 ), muscovite, and biotite. Chlorite formed from detrital biotite during low-grade regional metamorphism ( T &lt;270°C). There are some vertical changes in major element (but not trace element) chemistry of the grits, and detrital muscovites have a wide, but comparable, range in composition throughout, apart from an influx of Na-rich micas in the Keltie Water Grits. 40 Ar/ 39 Ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600–1800 Ma in the Dalradian rocks; similar old ages occur in the Keltie Water Grits but are diluted by ages of 507–886 Ma. We interpret these new data as showing that the rocks were most likely deposited as a single sequence, possibly with a disconformity, in Neoproterozoic to Early Cambrian times, before the onset of Grampian orogenesis in the Early Palaeozoic. No major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes.

Geochemical micro-behavior of natural U-series nuclides in granitic conglomerate from the Tono mine, central Japan.

In order to understand uranium migration behavior through geological media, the granitic conglomerate from the Tono uranium mine, Gifu prefecture, central Japan, was studied by grain size division and selective chemical leaching technique. The bulk contents of U and Th in fine-grained samples are larger than those of coarse-grained ones, and have positive correlations with the abundance of detrital biotite grains. The total amount of U eluted by the leaching reagents was more than half in all divided samples. In particular, a large amount of U was leached by sodium acetate/acetic acid (pH 5.0), and hydroxylamine hydrochloride/acetic acid solutions. It is inferred that carbonate and iron oxide minerals play an important role in U ore genesis. The adsorption density of fine-grained samples is larger than that of the coarse-grained one, which suggests that the fine U minerals are abundant in fine-grained sample and/or the U concentration around the complexing site of iron oxyhydroxides may have become high. The bulk 234U/238U activity ratio is 0.95 ± 0.05, and the bulk 230Th/234U activity ratio is 1.36 ± 0.06. For most of the sieved samples, 234U/238U activity ratios are 1, showing that U is partially removed from the sediments. The uranium nuclides in the matrix of conglomerate are inferred to be not in equilibrium within 300, 000 y. The ion exchangeable U, whose 234U/238U activity ratio was >1, is bound weakly and exchangeable with U in the surrounding water such as interstitial water and groundwater with 234U/238U activity ratios >1. The 234U/238U activity ratios of the residue fraction are less than 1 (finer sample) or more than 1 (coarser sample), indicating that the recoil ejection effect of U is advantageous for the fine-grained samples while injection effect is advantageous for the coarse-grained samples. The 234U/238U activity ratios of other fractions are less than 1, which is quite similar to the bulk ratio. The ratio decreases in the order of carbonate > iron oxide > organic/sulfide fractions. The 230Th/234U activity ratios is very low in ion exchangeable fraction and >1 in carbonate, iron oxide, and residue fractions. For the residue fraction, 234U/238U > 1 and 230Th/234U > 1 indicate the elution of U and enrichment of 234U relative to 238U by alpha recoil injection. The organic/sulfide complex fraction is depleted in 230Th compared to 238U and 234U. This fact suggests that U was secondarily enriched over Th. As U was enriched at the surface of pyrites, it is inferred that pyrites adsorbed a portion of the eluted U and helped the secondary fixation of U in modern times.

Heavy minerals in the Torlesse Terrane of the Wellington area, New Zealand

Heavy minerals from dated sandstones of the Torlesse Terrane in the Wellington area show several distinct assemblages. Samples from the Rakaia Subterrane (n = 19) have assemblages of two types: one dominated by secondary minerals (mainly in the Terawhiti area), and one by detrital biotite (mainly in the eastern part of the subterrane). Both types were present in the Kapiti Island area. Pahau (n = 3) and Esk Head samples of Pahau affinity (n = 3) are distinct from Rakaia samples in containing more than trace amounts of garnet, and suggest a different provenance (i.e. little reworking). The remaining two Esk Head samples resemble the biotite‐rich assemblage of the Rakaia but contain minor pyroxene and no epidote minerals. The distinction between epidote‐rich Pahau and biotite‐rich Rakaia assemblages has a parallel in North Canterbury. Semi‐opaque debris and pumpellyite indicate low‐grade metamorphism of Pahau rocks and of some Rakaia rocks from near Cape Terawhiti. However, the difference between the two Rakaia assemblages is unlikely to be due solely to this metamorphism, as zircon morphology and garnet compositions suggest different provenances. Many of the garnet compositions in Pahau and Esk Head samples are distinct from those in Rakaia samples, suggesting that the Rakaia Subterrane was not the provenance. Systematic distinctions such as these in Torlesse rocks offer potential for refining provenances of overlying Cretaceous‐Cenozoic sandstones.

Formation of authigenic Fe2+‐bearing smectite‐vermiculite during terrestrial diagenesis, southern New Zealand

Abstract Late Cretaceous to Quaternary terrestrial sediments derived from fault scarps and actively growing folds in schist and greywacke basement are locally preserved in Otago and South Canterbury, New Zealand. Sediments are immature, sparsely carbonaceous, and generally poorly sorted and have undergone little oxidation during rapid erosion and deposition cycles. Mild oxidative alteration of sediment clasts in some horizons has resulted in pseudomorphous replacement of detrital biotite and metamorphic chlorite in greywacke clasts, and chlorite in schistose clasts, by a green, interlayered, ferrous iron‐bearing smectite‐vermiculite mineral of variable composition. Smectite‐vermiculite compositions form a regular trend from low Si and Al, high Fe and Mg to moderate Si and Al, to low Fe and Mg. The Fe2+ smectite‐vermiculite also occurs as pools in the matrix and small veinlets, and as a cement intimately intergrown with fine‐grained clastic matrix. The smectite‐vermiculite is a diagenetic product formed preferentially in sedimentary horizons which are reduced and moderately acidic (pH 3.5–4.5), neutral, or possibly alkaline. There is a close association between the Fe2+ smectite‐vermiculite mineral and authigenic gold deposition; the smectite‐vermiculite is occasionally coated and impregnated with fine‐grained (micrometre scale) gold. Gold precipitation probably occurs by electroless deposition from groundwaters that are undersaturated with respect to gold.

Formation of corrensite, chlorite and chlorite‐mica stacks by replacement of detrital biotite in low‐grade pelitic rocks

ABSTRACTTransmission and scanning electron microscopy were utilized to investigate the nature and mechanisms of alteration of abundant detrital biotite of volcanic origin and progressive modification of phyllosilicate aggregates in a prograde sequence of pelitic rocks (illite crystallinity index = 0.19–0.58d̀λ2θ) from the Gaspé Peninsula in Quebec.Detrital biotite has been diagenetically altered to form corrensite and chlorite through two mechanisms; (1) layer‐by‐layer replacement gave rise to interstratification of packets of layers and complex mixed layering via several kinds of layer transitions between biotite and chlorite, corrensite or smectite; (2) dissolution‐transport‐precipitation resulted in the formation of relatively coarse‐grained aggregates of randomly orientated, corrensite‐rich flakes and fine‐grained corrensite intergrown with chlorite and illite in the matrix.The data show that stacks consisting of alternating packets of trioctahedral and dioctahedral phyllosilicates originated during early diagenesis when lenticular fissures in strained altering biotite were filled by dioctahedral clays. Subsequent prograde evolution of dioctahedral clays occurred through deformation, dissolution and crystallization, and overgrowth. Illite evolved to muscovite, with K in part provided through biotite alteration, and corrensite/chlorite to homogeneous chlorite. The alteration of detrital biotite is closely related to the formation of titanite and magnetite in diagenetic rocks, and pyrite, calcite and anatase or rutile in the higher grade rocks.The observations demonstrate that detrital biotite of volcanic origin may be the principal precursor of chlorite in chlorite‐rich metapelites originating in marginal basins. The mineral parageneses suggest that the transitions from corrensite to chlorite and illite to muscovite may be a function of local chemistry and time.

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

AbstractA prograde sequence of corrensite and chlorite in pelitic rocks of the diagenetic zone, anchizone, and epizone (illite crystallinity indices = 0.17–0.58°Δ2θ) of the Gaspé Peninsula, Quebec, was studied by analytical and transmission electron microscopy (AEM and TEM). The data collectively suggest that diagenesis/metamorphism of chlorite and corrensite follows a sequence of phase transitions, compositional homogenization, and recrystallization, approaching a state of equilibrium for which chlorite is the stable phase.Corrensite occurs as coalescing, wavy packets of layers intergrown with chlorite and illite in the diagenetic and low-grade anchizonal rocks. Intergrowths of discrete chlorite and corrensite crystals, interstratified packets of chlorite and corrensite layers, terminations of smectite-like layers by chlorite layers, and 2–3 repeats of R2- and R3-ordered chlorite-smectite mixed layers occur. These materials are alteration products of detrital biotite or other precursor phases like trioctahedral smectite. The crystal size and proportion of corrensite decrease significantly from the diagenetic zone to the anchizone. Deformed corrensite is crosscut by straight packets of chlorite and corrensite in the diagenetic sample. Some chlorite occurs as discrete, euhedral to subhedral crystals intergrown with or enclosed by other phases in the absence of corrensite. The crystal size of chlorite and definition of crystal boundaries increase whereas density of crystal imperfections and randomness in orientation decrease with increase in grade of diagenesis/metamorphism. Crystals that are kinked or bent, or display gliding along (001) form low-angle boundaries with relatively defect-free crystals, implying deformation during crystal growth. Abundant well-defined low-angle boundaries associated with dislocations are observed in the higher grade rocks, consistent with a stage of readjustment of crystal boundaries during crystal growth. The AEM analyses show that the corrensite has lower Fe/(Mg + Fe) and Al/(Si + Al) than the coexisting chlorite in the diagenetic sample, and that the ranges of composition of chlorite of different grades overlap and become smaller with increasing grade, implying prograde homogenization.The data imply that corrensite is a unique phase that is metastable relative to chlorite: its conversion to chlorite occurred at a grade as low as that of the high-grade diagenetic zone. The textural relations suggest that the metamorphic crystallization and recrystallization were coeval with deformation processes due to tectonism, partially modified by subsequent contact metamorphism. The data, combined with those of previous reports, suggest that the Gaspé Ordovician rocks constitute a part of a regional distribution of trioctahedral phyllosilicate-rich rocks in the northern Appalachians. The regional occurrence of abundant chloritic minerals is thus directly related to a specific tectonic regime with precursor sediments largely derived from an andesitic arc system(s).

Relation between U-series nuclide migration and microstructural properties of sedimentary rocks

Natural radionuclide migration in Miocene sedimentary rocks has been studied at the Tono U deposit in Japan, which is a potentially useful analogue of radioactive waste isolation in geological environments. Uranium series disequilibrium studies have revealed that natural radionuclides have been redistributed during the past 3.5 × 10 5 a in the U-mineralized zone. Permeability tests using core samples of the U-mineralized zone have shown that the microfabrics of sedimentary rocks, such as the connectivity of pores which control the groundwater movement, directly influence the redistribution of nuclides. Detailed observations using cathodoluminescence (CL) and electron probe microanalysis (EPMA) were carried out after a dye-impregnation test in order to improve the understanding of the importance of microfabrics in relation to nuclide migration. Results of the observations show that U has migrated within detrital grains, such as biotite and quartz, as well as between these grains. Uranium-series disequilibrium studies of each detrital quartz and biotite grain have been shown that these detrital minerals behave differently in respect of the migration of U, owing to their different textural properties. The detrital biotite flakes, especially along the (001)-cleavage planes, appear to have fixed U for a long period of time, whereas the U has remained mobile within the microfractures in detrital quartz grains.

Flow-Path Structure in Relation to Nuclide Migration in Sedimentary Rocks

The migration of natural uranium in geosphere has been studied at Tono uranium deposit, located in the Gifu Prefecture, central Japan. Geological and geochemical investigations have been carried out, as well as laboratory experiments, in order to characterize those structural and geochemical factors governing nuclide migration and to develop a conceptual nuclide migration model in sedimentary rocks. The results can be summarized as: 1. Micro-fractures within quartz grains, cleavages of detrital biotite flakes, and pores between detrital grains play an important role in nuclide migration, both as pathways and retention sites. Natural feature of uranium concentration in minerals and hydrogeological estimations suggest that diffusion may also exert a significant role on nuclide migration.2. Flow-path structures, e.g. connectivity of pores, influence the sorption capacity of the host formation. Connectivity of pores probably change the number of accessible sorption sites along the flow-path in the sedimentary rocks.