Showing papers on "Petrography published in 2019"

Heavy Minerals for Junior Woodchucks

Abstract: In the last two centuries, since the dawn of modern geology, heavy minerals have been used to investigate sediment provenance and for many other scientific or practical applications. Not always, however, with the correct approach. Difficulties are diverse, not just technical and related to the identification of tiny grains, but also procedural and conceptual. Even the definition of “heavy minerals” is elusive, and possibly impossible. Sampling is critical. In many environments (e.g., beaches), both absolute and relative heavy mineral abundances invariably increase or decrease locally to different degrees owing to hydraulic-sorting processes, so that samples close to "neutral composition" are hard to obtain. Several widely shared opinions are misleading. Choosing a narrow size-window for analysis leads to increased bias, not to increased accuracy or precision. Only point-counting provides real volume percentages, whereas grain-counting distorts results in favor of smaller minerals. This paper also briefly reviews the heavy mineral associations typically found in diverse plate-tectonic settings. A mineralogical assemblage, however, only reproduces the mineralogy of source rocks, which does not correlate univocally with the geodynamic setting in which those source rocks were formed and assembled. Moreover, it is affected by environmental bias, and by diagenetic bias on top in the case of ancient sandstones. One fruitful way to extract information on both provenance and sedimentological processes is to look for anomalies in mineralogical–textural relationships (e.g., denser minerals bigger than lower-density minerals; harder minerals better rounded than softer minerals; less durable minerals increasing with stratal age and stratigraphic depth). To minimize mistakes, it is necessary to invariably combine heavy mineral investigations with the petrographic analysis of bulk sand. Analysis of thin sections allows us to see also those source rocks that do not shed significant amounts of heavy minerals, such as limestone or granite, and helps us to assess heavy mineral concentration, the “outer” message carrying the key to decipher the “inner message” contained in the heavy mineral suite. The task becomes thorny indeed when dealing with samples with strong diagenetic overprint, which is, unfortunately, the case of most ancient sandstones. Diagenesis is the Moloch that devours all grains that are not chemically resistant, leaving a meager residue difficult or even impossible to interpret when diagenetic effects accumulate through multiple sedimentary cycles. We have conceived this friendly little handbook to help the student facing these problems, hoping that it may serve the purpose.

Comparison of fluid processes in coexisting wolframite and quartz from a giant vein-type tungsten deposit, South China: Insights from detailed petrography and LA-ICP-MS analysis of fluid inclusions

Abstract: Granite-related wolframite-quartz veins are the world's most important tungsten mineralization and production resource. Recent progress in revealing their hydrothermal processes has been greatly facilitated by the use of infrared microscopy and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of both quartz- and wolframite-hosted fluid inclusions. However, owing to the paucity of detailed petrography, previous fluid inclusion studies on coexisting wolframite and quartz are associated with a certain degree of ambiguity. To better understand the fluid processes forming these two minerals, free-grown crystals of intergrown wolframite and quartz from the giant Yaogangxian W deposit in South China were studied using integrated in situ analytical methods including cathodoluminescence (CL) imaging, infrared microthermometry, Raman microspectroscopy, and fluid inclusion LA-ICP-MS analysis. Detailed crystal-scale petrography with critical help from CL imaging shows repetition of quartz, wolframite, and muscovite in the depositional sequence, which comprises a paragenesis far more complex than previous comparable studies. The reconstruction of fluid history in coexisting wolframite and quartz recognizes at least four successive fluid inclusion generations, two of which were entrapped concurrently with wolframite deposition. Fluctuations of fluid temperature and salinity during precipitation of coexisting wolframite and quartz are reflected by our microthermometry results, according to which wolframite-hosted fluid inclusions do not display higher homogenization temperature or salinity than those in quartz. However, LA-ICP-MS analysis shows that both primary fluid inclusions in wolframite and quartz-hosted fluid inclusions associated intimately with wolframite deposition are characterized by strong enrichment in Sr and depletion in B and As compared to quartz-hosted fluid inclusions that are not associated with wolframite deposition. The chemical similarity between the two fluid inclusion generations associated with wolframite deposition implies episodic tungsten mineralization derived from fluids exhibiting distinct chemical signatures. Multiple chemical criteria including incompatible elements and Br/Cl ratios of fluid inclusions in both minerals suggest a magmatic-sourced fluid with the possible addition of sedimentary and meteoric water. Combined with microthermometry and Raman results, fluid chemical evolution in terms of B, As, S, Sr, W, Mn, Fe, and carbonic volatiles collectively imply fluid phase separation and mixing with sedimentary fluid may have played important roles in wolframite deposition, whereas fluid cooling and addition of Fe and Mn do not appear to be the major driving factor. This study also shows that fluid inclusions in both wolframite and coexisting quartz may contain a substantial amount of carbonic volatiles (CO2 ± CH4) and H3BO3. Ignoring the occurrence of these components can result in significant overestimation of apparent salinity and miscalculation of LA-ICP-MS elemental concentrations. We suggest that these effects should be considered critically to avoid misinterpretation of fluid inclusion data, especially for granite-related tungsten-tin deposits.

Petrogenesis of a Hybrid Cluster of Evolved Kimberlites and Ultramafic Lamprophyres in the Kuusamo Area, Finland

Abstract: Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types, yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies [Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi] distributed along a 60 km NE–SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g. Mg-rich ilmenite, Al–Ba-rich, Ti–Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb ¼ 0·8 6 0·1; Th/ Nb ¼ 0·07 6 0·01; Nb/U ¼ 66 6 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which, combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggests derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba–Al-rich cores) to those more typical of orangeites–lamproites (increasing Si–Fe, decreasing Al–Ti–Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe–Ti-rich) and spinel [high Fe2þ/(Fe2þ þ Mg)] compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb–Sr dating of phlogopite in Dike 15 yields an age of 1178·8 6 4·1 Ma (2r), which is considerably older than the ~750 Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira–Kuhmo–Kostomuksha olivine lamproites emplaced ~100 km to the SE. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is probably the result of exploitation of similar trans-lithospheric corridors.

Advanced mineral characterization and petrographic analysis by μ-EDXRF, LIBS, HSI and hyperspectral data merging

Abstract: Petrography and mineralogy are fundamental for understanding processes in various geoscientific fields. Plutonic rock nomenclature is based on mineralogical composition. Therefore, identifying and quantifying minerals is a key for plutonic rock classification. To accomplish this, novel advancements in instrumentations, such as energy dispersive x-ray fluorescence mapping (μ-EDXRF) or hyperspectral imaging (HSI) provide fast and non-destructively spatially resolved and large-scale chemical information. This work describes a comprehensive approach to combine chemical, mineralogical and textural information from μ-EDXRF, Laser Induced Breakdown Spectroscopy (LIBS) and HSI for petrographic analysis of plutonic rocks. Using supervised classification of spectral information, mineral distribution maps are obtained for image analysis including geometrical data of each grain, such as grain size, grain orientation and grain location for subsequent targeted analysis and the modal mineralogy for plutonic rock classification in a QAPF-diagram for 20 rock slabs. The combination of the three mapping techniques can provide valuable information within the limit of each technique such as of spatial resolution or element sensitivity, but with little time needed for sample preparation and measurement. In general, it is an objective, repeatable and quantifiable way for modal mineralogy and petrographic image analysis.

Mineralogy, petrography and geochemistry of an early Eocene weathering profile on basement granodiorite of Qaidam basin, northern Tibet: Tectonic and paleoclimatic implications

Abstract: Weathering, as an important process in the earth surface system, can be significantly influenced by tectonics and climates over long time scales. Here, we use mineralogical, petrographic and geochemical data of a paleoweathering profile developed on basement granodioritic rocks of northern Qaidam basin, northern Tibet, to reconstruct early Eocene weathering conditions and to discuss how paleoclimates and tectonics dominated the weathering process. The results indicate that neoformed mineral phases in weathering products are dominated by smectite, and the profile has overwhelmingly low chemical index of alteration values (ca. 51–59) and significantly decreasing micropetrographic index values (from 25.0 to 0.2) from bottom to top. These findings suggest that the basement rocks experienced mild chemical weathering but relatively intensive physical weathering. We favor that non-steady-state weathering, in which mechanical erosion rates compare favorably with rates of chemical weathering, prevailed in northern Tibet during the early Eocene. The weathering conditions were likely an integrated response to active tectonism and dry climates at that time. Furthermore, chemical element mobility evaluation demonstrates that most of large ion lithophile elements and light rare earth elements (LREEs) of granodioritic rocks are quite active during weathering and can be easily leached even under mild chemical weathering conditions. Significant mass loss of Al and LREEs in upper weathered samples probably reflects acidic weathering conditions, which were likely due to extremely high atmospheric CO2 level during the early Eocene. This study, from the unique perspective of weathering process, suggests that intensive deformation and rapid tectonic erosion occurred in northern Tibet during the early Eocene, as a far-field response to the India-Eurasia collision. It also agrees with warm and relatively dry climates, which were likely attributed to the global greenhouse climates and the Paleogene planetary-wind-dominant climate system in Asia, respectively.

Impacts of the petrophysical and diagenetic aspects on the geomechanical properties of the dolomitic sequence of Gebel El-Halal, Sinai, Egypt

Abstract: The aim of this paper is to demonstrate the controlling effect of the mineral composition, the diagenetic history and the petrophysical properties on the geomechanical properties and durability of the dolomitic El-Halal Formation in its type section in North Sinai as well as its economic potential for construction purposes. The petrographic studies include descriptions of both the mineral composition and diagenetic processes, while the petrophysical studies measure the density, porosity, permeability and true formation resistivity factor. In addition, some geomechanical laboratory tests were conducted, including the petrographic description of the mineral composition and diagenetic processes, as well as the Schmidt Hammer number (SHV), point load index (IS50), uniaxial compressive strength (UCS), and ultrasonic longitudinal wave velocity measurements. Scanning electron microscopy was used to help in pore-type description (intergranular, vuggy, etc.). Based on lithologic changes and mineral composition, the El-Halal Formation can be subdivided into three informal members: (1) lower dolomitic limestone, (2) middle sandy dolostone and (3) upper dolostone member. Petrographically, the sequence consists of three dominant microfacies: (1) dolomitic mudstone microfacies (dolomitic micrite to dolomicrite), (2) dolowackestone (clayey to sandy dolomicrite), and (3) dolomudstone microfacies (dolosparite). The most diagnostic diagenetic processes are dolomitization, cementation by calcite, aggrading neomorphism and the creation of authigenic illite. In the study area, dolomitization has affected almost all the Cenomanian succession. The SHV, IS50, and UCS values of the samples indicate high-strength rocks. The present study indicates the dependence of the geomechanical properties on the petrophysical properties and the mineral composition of the studied rocks. Modeling the properties indicates a reliable correlation between the different parameters which can be applied for predicting and characterizing the dolomitic El-Halal Formation elsewhere. The results of the present investigation are useful for studying the geomechanical and petrophysical properties of similar dolomitic sequences and in ranking its potential as construction materials.

Petrographic and geochemical characteristics of organic-rich shale and tuff of the Upper Triassic Yanchang Formation, Ordos Basin, China: implications for lacustrine fertilization by volcanic ash

Abstract: The Upper Triassic Chang 7 Member lacustrine organic-rich shale of the Yanchang Formation of the Ordos Basin is a significant hydrocarbon source rock containing abundant of tuff intervals ranging f

Multiscale petrographic heterogeneity and their implications for the nanoporous system of the Wufeng-Longmaxi shales in Jiaoshiba area, Southeast China: Response to depositional-diagenetic process

Abstract: The organic matter-rich shales in Wufeng-Longmaxi Formation, Jiaoshiba area, Southeast China, are showing a notable petrographic heterogeneity characteristic within the isochronous stratigraphic framework, which lead to vast differences in the mineral composition and organic matter abundance in the adjacent sections of the shale reservoir. The studied shale has been divided into three systems tracts: a transgressive systems tract (TST), an early highstand systems tract (EHST), and a late highstand systems tract (LHST). Multiple-scale petrographic observation and detailed mineralogical and geochemical analyses were combined to investigate the manifestation, origin, and the ways by which the shale heterogeneity is affected. The results indicate that polytropic depositional environments lead to different components in sediment. Subsequently, these differences among shale sections become more apparent through different diagenetic pathways. During the deposition of the section TST, the Hirnantian glaciation and regional volcanism played a crucial role, contributing to the abundant accumulation of fine-grained intrabasinal silica and organic matter. In diagenesis stage, authigenic quartz aggregates derived from siliceous organisms are formed. They filled in primary interparticle pores, forming a rigid particle-bracing structure that provide effective resistivity against the compaction and spaces for organic matter migration and occlusion. Finally, the migrated organic matter left plenty of newly created pore spaces that constituted a great portion of the total porosity of shale reservoir. The depositional process of section EHST is strongly influenced by contour current, which brings about more extrabasinal influx and impoverishes organic matter. In diagenesis stage, the rigid particle-bracing structure could only be preserved in limited areas, since insufficient siliceous supply could not produce enough authigenic quartz. Primary interparticle pores are significantly reduced owing to compaction, leaving less space for later organic matter migration and occlusion. As a result, the total porosity of shale reservoir declines in this section. In a rapid tectonic-uplifting background, the deposition of section LHST is associated with a rapid increase in terrigenous clay minerals, which further dilutes organic matter. Ductile clay experienced strong compaction and then occupies most of the primary interparticle space. Rigid particles are wrapped by a large number of clays, which has destroyed the particle-bracing structure. As a result, the nanoporous system in the shale could not be well preserved.

Microfacies and diagenetic evolution of the limestones of the upper part of the Crato Formation, Araripe Basin, northeastern Brazil

Abstract: This paper presents the results of a petrographic and diagenetic study of the laminated limestones of the upper part of the Aptian to Albian Crato Formation, northeast of Brazil. The applied techniques were optical microscopy, cathodoluminescence and scanning electron microscopy (SEM) coupled to a wavelength-dispersive spectrometer (WDS). Petrographic analysis has revealed that most of the laminated limestones are calcilutites with a dominance of a micritic matrix, indicating a low-energy depositional environment. Microstructures such as microfaults, microfractures, microslumps, and loop bedding were observed. Based on textural, structural and paleontological features, seven microfacies were recognized: massive limestone, limestone with parallel laminations, limestone with undulated laminations, limestone with slumps, limestone with loop bedding, limestone with ostracods and limestone with peloids. In addition, the processes of cementation, dissolution, replacement, recrystallization and compaction, which are related to different diagenetic stages, were also recognized. The diagenetic constituents found in the sections include calcite, pyrite, silica and sulfates. We can conclude that a large part of the microstructures (microfaults, microfractures, microslumps and loop bedding) can be related to local seismicity, probably due to the reactivation of the Patos Shear Zone. The diagenetic constituents indicate an early to late diagenesis (eogenetic, mesogenetic and telogenetic stage).

Relationships between the petrographic, physical and mechanical characteristics of sedimentary rocks in Jurassic weakly cemented strata

Abstract: Physical and petrographic properties of sedimentary rocks have great influence on their mechanical behavior. Numerous laboratory tests were conducted on intact sandstones and mudstones obtained from Jurassic weakly cemented coal-bearing strata. Several physical and mechanical parameters, particularly the rock material constant mi and the brittleness coefficient BRIT−T, were determined. The microfabric of these sedimentary rocks was also analyzed to quantify their mineral compositions and to determine quantitative relationships among mineral compositions, physical and mechanical parameters using the regression analysis. Overall, both bulk density (ρ) and P-wave velocity (Vp) exhibit positive power relationships with uniaxial compressive strength (UCS) and Young’s modulus (E), respectively. The UCS of sandstones increases with increasing quartz content and decreasing feldspar content, and the brittleness of mudstones increases with the increase of quartz and feldspar contents. In addition, the rock material constant mi presents a logarithmic correlation with the ratio of feldspar-to-quartz content for sandstones and a polynomial correlation with the ratio of clay minerals to the quartz and feldspar contents for mudstones.

Geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities: A case study on tight oil sandstones of the Upper Triassic Yanchang Formation, southwestern Ordos Basin, China

Abstract: Calcite cementation has been identified as an active process in the Upper Triassic Yanchang Formation throughout its burial history and as a major diagenetic factor causing strong reservoir heterogeneities. The origins of calcite cements and their relevance to reservoir heterogeneities were investigated using a suite of petrographic and geochemical methods, including optical microscopy with fluorescence and cathodoluminescence, scanning and backscattered electron microscopy with energy-dispersive spectrometry, x-ray diffraction, x-ray fluorescence, electron probe microanalysis, quantitative evaluation of minerals by scanning electron microscopy, fluid inclusion analysis, and carbon and oxygen stable isotope analyses. The sandstones are compositionally immature with relatively high amounts of volcanic rock fragments. The two generations of calcite cements are Ca-I and Ca-II. The Ca-I calcites are distributed along the interface of sandstone and mudstone units and were formed during the Late Triassic to Early Jurassic at formation temperatures of approximately 90°C. The Ca-II calcite mainly developed in the lower part of the fining-upward sandstone units and was formed in the Late Jurassic at higher temperatures of approximately 110°C. The origins of calcite cements were constrained by geochemical and isotope measurements, fluid inclusion homogenization temperature, and in situ element analysis. The Ca-I calcite cement originated from dissolution of the lacustrine depositional carbonates in the interbedded mudstones and reprecipitation in the adjacent sandstones. The Ca-II calcite was mainly related to organic matter decarboxylation, with Ca2+ having been provided internally by volcanic fragment alteration and plagioclase dissolution. Calcite cementation had caused strong reservoir heterogeneities in the Yanchang Formation tight sandstones. The Ca-I calcite cementation destroyed reservoir properties along the interface of sandstones and mudstones. The lower parts of the fining-upward sandstone units were tightly cemented by Ca-II calcite, although they originally had high porosity and permeability. The middle–upper parts of the fining-upward sandstone units contain less calcite cements and thus have better preserved reservoir pores because of oil emplacement inhibiting the calcite cementation processes.