Showing papers on "Petrography published in 2010"

Igneous Rocks and Processes: A Practical Guide

Abstract: Preface. Acknowledgements. 1. An introduction to magmas and magmatic rocks. 2. Basalts and related rocks. 3. Magma differentiation. 4. Gabbroic rocks. 5. Ultramafi c and ultrabasic rocks. 6. Andesite, dacite and rhyolite. 7. How magmas erupt - an introduction to pyroclastic processes and products. 8. Granitic rocks. 9. Alkali rocks. Appendix A - Mineral identifi cation using a polarizing microscope. Appendix B - Petrographic calculations. Appendix C - Symbols, units and constants used in this book. Glossary. Answers to exercises. Bibliography. Index. Colour plate section. Companion website for this book: wiley.com/go/gill/igneous.

Illite: Origins, Evolution and Metamorphism

Abstract: This monograph discusses the major problems relating to the surface environment by using clay mineralogy as a unifying theme. Since the stability and development of illite, the most abundant and most common clay mineral, is the key to surface mineral dynamics, understanding its structure and transformation is an absolute prerequisite to understanding the problems of environmental change. Using illite as the frame, the authors describe problems in soil chemistry, clay stability and clay kinetics in sedimentary rocks. This book is valuable for graduate students, scientists and researchers in the fields of sedimentology, sediment petrography and soil sciences.

Paleoenvironmental controls on the texture and chemical composition of pyrite from non-conglomeratic sedimentary rocks of the mesoarchean witwatersrand supergroup, south africa

Abstract: A petrographic and mineral chemical study was conducted on pyrite from non-conglomeratic sedimentary rocks of the 3.0 to 2.8 Ga Witwatersrand Supergroup. Detailed petrographic analyses revealed the presence of three paragenetic associations, namely detrital, diagenetic and epigenetic pyrite. Detrital pyrite is commonly observed in sandstone and diamictite from proximal shelf deposits. The mineral chemistry of large (0.1 to 0.5 mm in size) and small (<0.05 mm in size) detrital pyrite grains, based on Co, Ni and As contents, suggests a metamorphic and diagenetic origin, respectively. Significantly, a variety of early diagenetic pyrite forms has survived greenschist facies metamorphism; these occur exclusively in carbonaceous mudstones intercalated with fluvial inner shelf sandstones. The high Ni content of early diagenetic pyrite and the occurrence of early diagenetic Cu-Pb sulfides highlight the importance of organic matter and host rock composition. However, in distal to starved outer shelf depositional settings, detrital pyrite is absent and diagenetic pyrite is scarce. Nevertheless, the composition of diagenetic pyrite in distal magnetite-rich lithologies is similarly affected by host rock composition and possibly Fe-Mn reduction (elevated Co and As concentrations). Epigenetic pyrite, pyrrhotite and base-metal sulfides are prevalent in sandstone intercalations near diabase sills and fault zones. The composition of epigenetic pyrite is influenced by the temperature of the fluids and the interaction of the fluids with the host rock, as well as with precursor and associated iron sulfides. This study finds that local depositional and tectonic settings are instrumental in dictating the distribution, morphology and composition of diagenetic pyrite. In this regard, the formation of metal-rich early diagenetic pyrite is favored in carbonaceous mudstones associated with fluvial regimes in the upper Hospital Hill, Government, Jeppestown, Johannesburg and Turffontein Subgroups, where continental runoff would have supplied nutrients and sulfur species to the shallow water environment. In contrast, the scarcity of diagenetic pyrite in marine-dominated environments of the lower Hospital Hill Subgroup (of which some are carbonaceous) and distal portions of the Government and Jeppestown Subgroups, is consistent with a lack of sulfate in the Archean ocean.

A new method for quantitative petrography based on image processing of chemical element maps: Part I. Mineral mapping applied to compacted bentonites

Abstract: Most natural rocks or engineered materials display a multi-scale heterogeneity ranging from the nanometer to the centimeter. Their spatial textural heterogeneity can be approached from chemical element maps acquired using various techniques (SEM, EPMA, SXAM, synchrotron μ-XRF, TEM), depending on the chosen magnification. Chemical map processing that yields quantitative petrographic information is improved here according to newly developed mineral thresholding methods that accommodate mixtures and solid solutions. The complex case of an MX80 compacted bentonite is used as a test case. The 14 major chemical elements of this sample were mapped using an electron probe microanalyzer, and chemical map processing yielded a quantitative map of the 18 mineral species of bentonite with a spatial resolution of a few micrometers. The textural heterogeneity of the solid part of the sample is thus visualized and quantified on an area ranging between 0.1–1 cm2. The method also provides a complete modal analysis of the sample. The methodology is expected to have broad applications in Earth and materials sciences.

Petrography and Geochemistry of the Jajarm Bauxite ore Deposit, Northeast Iran: Implications for Source Rock Material and Ore Genesis

Abstract: The Jajarm bauxite deposit, northeast Iran, is the largest such deposit in Iran. The deposit is sandwiched between the Triassic Elika formation and the Jurassic Shemshak formation, housed within karstic features developed within the former unit. The deposit generally shows an internal layering defined by the following four distinct horizons (from bottom to top): (a) a lower argillaceous horizon, approximately 50-80 cm thick, is mainly composed of clay minerals that directly overlies the carbonate footwall (Elika formation); (b) a bauxitic clay layer approximately 2-3 m thick that consists mainly of hematite, kaolinite, anatase, and diaspore; (c) a red bauxite layer (the main high-grade ore), about 5 m thick and composed of diaspore, kaolinite, anatase, and hematite; and (d) an upper kaolinitic layer that is 20-50 cm thick, composed mainly of kaolinite, and overlain by the Shemshak formation. Detailed petrographic studies reveal diagenetic alteration of the bauxitic protolith. The main observed bauxite textures are microgranular, oolitic, pisolitic, fluidal-collomorphic, and microclastic. Microgranular and microclastic textures associated with the residual fractured and corroded quartz grains, as well as feldspar grains are almost completely replaced by platy diaspore. Geochemical analyses of the red bauxite reveal enrichment of less mobile elements (Nb, Th, Zr, Mo, Ga, and Cr) and depletion of mobile elements (Rb, K, Na, Sr, La, Mg, and Pb); the opposite result is obtained for the bauxitic clay. Chondrite-normalized REE (Rare Earth Element) patterns for the upper kaolinite layer are similar to those for the underlying red bauxite, and the patterns obtained for the lower argillaceous layer are similar to those for the overlying argillaceous bauxite horizon. Ce shows a positive anomaly in the red bauxite and a negative anomaly in the bauxitic clay. The correlation coefficients calculated between REE and other elements demonstrate that the likely REE-bearing minerals are oxides of Ti and Nb, clay minerals, and zircon. In contrast to the present diasporic mineralogical composition of the Jajarm bauxite, the geochemical and mineralogical data indicate an original gibbsitic composition. Finally, the observed mineralogical and textural evidence, combined with the evidence provided by variation diagrams and REE patterns, indicates a mixed origin for the Jajarm bauxite from both basic igneous and sedimentary rocks. In fact, bauxitization was initiated on basic source rocks and continued during reworking and replacement within the karstic features.

Biogenically enhanced permeability: A petrographic analysis of Macaronichnus segregatus in the Lower Cretaceous Bluesky Formation, Alberta, Canada

Abstract: A detailed petrographic analysis was conducted on several core samples from the Cretaceous-aged Bluesky Formation in the La Glace area in Alberta, Canada. Within the gas-producing zone of these upper shoreface sediments, a fine- to mid-medium-grained chert-rich litharenite is intensely bioturbated with Macaronichnus segregatus. Petrographic analysis showed that the burrow mantle is generally lined with dark-colored, iron-rich (mainly chert, shale clasts, and organic grains) fragments, whereas the burrow fill contained mainly quartz and light-colored chert fragments. The reason for the dark-colored grain segregation of the tracemaker is unclear, but in the Bluesky Formation presented in this study, grain segregation has improved the reservoir quality by effectively resorting compaction- and cement-resistant chert and quartz into the burrow fill. Primary reservoir quality can be preserved in the presence of chert as pore-occluding quartz overgrowths do not form on chert fragments as they do on monocrystalline quartz, thus leaving open, well-connected primary pores and hence elevated permeability. Chert fragments are resistant to the effects of mechanical compaction and are not easily squeezed into adjacent pore spaces as are ductile rock fragments. Further research is needed to test the chemical constituents of the grains that modern tracemakers ingest. This may lead to a better understanding of why sands burrowed with Macaronichnus and similar burrowed sediment can have elevated reservoir quality.

Hydrothermal alteration of plagioclase in granitic rocks from Proterozoic basement of SE Sweden

Abstract: Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel the mechanisms and conditions of plagioclase alterations. The ...

Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China

Abstract: SUMMARY Magnetic polarity stratigraphy has proved to be useful in dating of both marine and terrestrial sedimentary sequences over the world. However, the reliability of magnetostratigraphic results of red soil sequences in subtropical southern China has been found to be variable in different regions. To probe into the capabilities of recording magnetic polarity stratigraphy in the red soils, three red soil sequences have been selected for detailed mineral magnetic, petrographic and/or palaeomagnetic analyses. These include the Xuancheng and Qiliting sequences, which are located in the lower reaches of the Yangtze River and the Damei sequence in the Bose Basin near the Tropic of Cancer. Palaeomagnetic results indicate that the Xuancheng sequence has recorded the Brunhes Chron and the late Matuyama Chron, including a short interval probably representing the Santa Rosa geomagnetic event. The Xuancheng and Qiliting sequences have faithfully recorded the palaeogeomagnetic field behaviour while the Damei sequence has failed. Detailed mineral magnetic and petrographic measurements suggest that four magnetic minerals (magnetite, maghemite, haematite and goethite) are contained in the three studied sequences, but the characteristic remanent magnetization (ChRM) carriers are different. For the Xuancheng and Qiliting samples, the ChRM carriers are magnetite and haematite, which are all of detrital origin. However, the ChRMs of Damei samples are carried by pedogenic haematite with an unblocking temperature (TB) of about 630–640 °C, which has overprinted the primary remanence of the Damei red soil deposits. This pedogenic haematite is mainly produced by chemical weathering, which is commonly intensified by the climate of high temperature and rainfall. It further suggests that climatic conditions have a great effect on the nature of palaeomagnetic signals of the red soil sequences. In addition, the red pigment components in the red soils may be used to indicate the degree of remagnetization.

Fault-related hydrothermal dolomites in Cretaceous carbonates (Cantabria, northern Spain): Results of petrographic, geochemical and petrophysical studies

Abstract: The present contribution documents NW-SE oriented fault and fracture related dolomites in Aptian-Albian carbonates (Karrantza area; northern Spain). Field observations revealed two main dolomite types, namely massive and zebra dolomite. Texturally, these dolomite types are mostly planar and nonplanar and variably reworked by subsequent alterations, which resulted in neomorphism and recrystallization, cataclastic deformation and calcite filling of dolostones. Petrographic and geochemical studies demonstrate the superposition of different diagenetic events, which were involved in multiphase dolomitization. Several phases of hydrothermal calcite cement pre- and post-date the dolomitisation events. Massive dolomites show overlapping stable isotopic ratios ranging from -16.9 to -8.9{per thousand} ({delta}18 O V-PDB), and -2.6 to +3.1{per thousand} ({delta}13 C V-PDB). Zebra dolomite shows more depleted values of {delta}18O and {delta}13C as compared to massive dolomites ({delta}18O: -18.1 to -15.2{per thousand}V-PDB and {delta}13C: -8.1 to +1.6{per thousand} V-PDB). Fluid inclusion analyses show homogenization temperature (Th) values from 120 to 200°C and estimated salinities range between 10 and 24 eq. wt.% NaCl. Both dolomite types are nearly stoichiometric, with CaCO3 values between 50 and 52 mole% Limestones close to the dolomites show depleted {delta}l8O values (similar to those of the dolomites), implying isotopic resetting during dolomitization. Recrystallization appears to have decreased the bulk porosity values in the interlocking nonplanar dolomite (with negligible porosity), while late-stage calcite cements occlude most of the remaining porosity, and make petrophysical measurements difficult to interpret. The possible source of dolomitizing fluids can be deeply buried Triassic evaporitic strata in the intra-platform basin, Keuper salt diapirs and/or Mg-bearing igneous rocks

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

Abstract: Darasun, Teremkyn, and Talatui are neighboring deposits in the Darasun gold district of Transbaikalia (Russia) that are hosted by Middle-Late Jurassic subduction-related porphyry intrusions and dikes. Darasun consists of vertically extensive, steeply dipping Au mineralized veins and zones, spatially related to a K-rich granodiorite-porphyry intrusion. Within the deposit are also pipelike bodies of tourmaline sulfide hydrothermal breccias. Teremkyn consists of gently and steeply dipping veins and zones, always associated with felsic dikes comagmatic with the Darasun intrusion. Talatui consists of variably shaped mineralized zones systematically associated with dikes of different compositions and with the host Middle-Late Jurassic granite porphyry. A large proportion of both veins and mineralized zones in these deposits consists of sulfide and oxide minerals, with pyrite and arsenopyrite mostly present at Darasun and Teremkyn and pyrite, magnetite, and hematite present at Talaui. The three deposits host also a large number of sulfosalt, bismuth, oxide, and telluride minerals. Gold of variable purity occurs as free nuggets or hosted by opaque phases. The gangue mineralogy is similar in the three deposits and includes commonly quartz, tourmaline, and calcite. Microthermometry and microchemical analyses of fluid inclusions with petrographic and compositional data on ore minerals constrain the stages of ore deposition in the district. At room temperature, the entrapped fluids are of four compositional types: (1) multiphase, with a vapor bubble, liquid, and one or more solids; (2) liquid-vapor aqueous; (3a) vapor-rich with a small proportion of liquid and rare solids, and also (3b) vapor-rich with a small proportion of aqueous liquid and carbonic liquid. Types (1) and (3) or (2) and (3) coexist within the same assemblages and are present mostly in paragenetically early quartz, whereas the type (2) fluid within later quartz, sphalerite, and calcite shows consistent phase proportions. At Darasun and Teremkyn, type (2) fluid is the main ore fluid, but all fluid types are associated with gold deposition at Talatui. Microthermometric measurements from all fluid types show a range of phase transitions that vary systematically at the deposit scale. At Darasun and Teremkyn, coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid states, respectively, within the same 290° to 460°C range. The corresponding bulk salinities vary between 0.7 and 44.8 wt percent NaCl equiv. In contrast, at Talatui coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid phases and show a distribution of T h(total) and bulk salinities between about 300° and 610°C and 0.9 and 48 wt % NaCl equiv, respectively. Mass spectrometric and chromatographic data from bulk samples show that the main-stage ore fluid in the three deposits varies compositionally according to location within the district. In detail, Cl, Na, K, Cs, Br, Sr, Rb, Mn, and Tl are relatively enriched in the Talatui fluid, whereas CO 2 , CH 4 , HCO, Cu, Pb, Sb, Fe, Hg, and REE are enriched at Darasun and Teremkyn or show no systematic variations. Au concentrations vary within the 0.01-to 3-g/t range at Darasun and Teremkyn, whereas at Talatui values do not exceed 0.4 g/t. The salinities and homogenization temperatures of the main-stage ore fluids types (1), (2), and (3) are typical of the intrusion-related class of gold deposits. This suggests a dominant magmatic component of the ore fluid during the first stages of mineral precipitation and a magmatic derivation of gold itself in the district. Phase separation was active during the early stages of ore deposition and took place between about 600° to 400°C at Talatui and 450° to 300°C at Teremkyn and Darasun at pressures of 10 to 160 MPa (i.e., 1- to 2.5-km depth). This explains the systematic distribution of ore minerals and fluid chemistry within and around the orebodies. Gold precipitation took place within the three deposits during both early-stage phase separation (Talatui and Darasun) and main-stage ore formation. Microchemical data of fluid inclusions show that the ore fluid was characterized by Au/Cu ratios between 15 and 35 (units: g/t vs. wt %). This range is five orders of magnitude higher than that determined for porphyry Cu deposits (~1·10 −4 , Ulrich et al., 1999) and compares well with the range of Au/Cu ratios measured in the most fractionated melt hosting the Timbarra intrusion-related deposit of Australia (20–200, Mustard et al., 2006). Hence, similar to porphyry Cu systems, the Au/Cu ratios of the ore fluid at Darasun could have been controlled by the fundamental Au/Cu ratio of the magmatic source rock. The petrographic evidence for an association between gold and galena and the evidence for consistently high concentrations of Pb, Zn, and Fe in the fluid inclusions suggest that a fraction of the fluid that deposited the orebodies did not always reach saturation in gold and other ore minerals during the early magmatic-hydrothermal stage. We speculate that a fraction of the ore fluid was transported through the orebodies and dispersed at lower temperatures into the outer fringes of the deposits, where they formed the documented galena-sphalerite halo around the Darasun stock.

Zoning of rock facies and chemical composition in the Toki granitic body, Central Japan

Abstract: The Toki granite, a single pluton, is located in the Cretaceous Sanyo Belt in the Inner Zone of Southwest Japan. 483 samples collected from 19 borehole sites in the Toki granite display spatial variations in mineral assemblage, mode, bulk chemical composition and mineral composition, indicative of a zoned pluton. Based on the mode and the mineral assemblages with or without hornblende, the Toki granite can be divided into three rock facies from the margin to the interior: muscovite-biotite granite (MBG), hornblende-biotite granite (HBG) and biotite granite (BG). The ASI values [molar Al2O3/(CaO+Na2O+K2O)], Al2O3, K2O and Na2O change systematically from MBG through HBG to BG, corresponding to a systematic variation from peraluminous in the margin to metaluminous granite in the interior. Fe3+/Fe2+ ratios gradually increase from MBG through HBG to BG, corresponding to a systematic change from ilmenite-series in the margin to magnetite-series in the interior. The sedimentary rocks of the Mino terrane (Kamiasou unit) were intruded and metamorphosed to hornfels by the Toki granitic magma. Na2O/K2O and Fe3+/Fe2+ ratios become closer to those of the hornfels towards the margin of the body, suggesting the assimilation of the crustal host rock by the Toki granitic magma. However, the same ranges in SiO2 among three rock facies leave the possibility that each rock facies derived from a different magma. Both petrography and bulk chemistry imply that 1) the chemical variation of the Toki granite is not solely due to fractional crystallization, and 2) assimilation of crustal host rocks or simultaneous intrusion of different magmas has played a significant role during the emplacement process.

European provenance of the Numidian Flysch in northern Tunisia

Abstract: Terra Nova, 00, 1–9, 2010 Abstract The ultimate source of the deepwater Numidian Flysch sediments has long been a matter of controversy. Did the sediments of this late Cenozoic orogenic belt in the western Mediterranean derive from a European or African source, or from a combination of the two? New data presented here strongly favour a European provenance. Zircon ages of 514 ± 19 Ma from Tunisia and 550 ± 28 Ma from Sicily can only have derived from rocks of European affinity. These zircons have been separated from quartz-rich sandstones with a distinctive, highly mature heavy mineral assemblage that is different from those of North African autochthonous formations. The mature petrography and dominance of euhedral prismatic zircon grains indicate a medium to high-grade metamorphic source. Most palaeocurrent data are indicative of flow from the N and NW. The original European provenance is most likely now represented by predominantly metamorphic rocks of the Kabylie belt in northern Algeria, as a result of microplate movement and thrust emplacement.