Clastic Texture Research Articles

Effect of the Textures and Particle Sizes of Limestone on the Quicklime Reaction Activity

Quicklime is not only an important raw material for the steel and nano-calcium carbonate industries but also a key carrier for capturing carbon dioxide in the fight against global warming, and its reaction activity plays a vital role in these processes. Recent studies have found that quicklime produced from limestones with similar chemical compositions under the same production process has significantly different reaction activities, which indicates that something other than the chemical composition of limestone affects quicklime reaction activity. To explore the factors affecting quicklime reaction activity, this study analyzed the textures and calcite particle size of limestone collected from different areas of Guangxi, China, and measures the quicklime reaction activity with different calcination times. It has been found that: (1) limestone with a clastic texture is preferred to that with a crystalline texture (including transition type) in yielding quicklime with higher reaction activity; and (2) for limestone with a clastic texture, fine-grained limestone tends to produce the same or higher quicklime reaction activity with lower energy consumption.

Compositional and spectroscopic investigation of three ungrouped carbonaceous chondrites

AbstractUngrouped carbonaceous chondrites are not easily classified into one of the well‐established groups due to compositional/petrological differences and geochemical anomalies. Type 2 ungrouped carbonaceous chondrites represent a very small fraction of all carbonaceous chondrites. They can potentially represent different aspects of asteroids and their regolith material. By conducting a multitechnique investigation, we show that Queen Alexandra Range (QUE) 99038 and Elephant Moraine (EET) 83226 do not resemble type 2 carbonaceous chondrites. QUE 99038 exhibits coarse‐grained matrix, Fe‐rich rims on olivines, and an apparent lack of tochilinite, suggesting that QUE 99038 has been metamorphosed. Its polyaromatic organic matter structures closely resemble oxidized CV3 chondrites. EET 83226 exhibits a clastic texture with high porosity and shows similarities to CO3 chondrites. It consists of numerous large chondrules with fine‐grained rims that are often fragmented and discontinuous and set within matrix, suggesting a formation mechanism for the rims in a regolith environment. The kind of processes that can result in such chemical compositions as in QUE 99038 and EET 83226 is currently not fully known and clearly presents a conundrum. Tarda is a highly friable carbonaceous chondrite with close resemblance to Tagish Lake (ungrouped C2 chondrite). It comprises different types of chondrules (some with Fe‐rich rims), framboid magnetite, sulfides, carbonates, and phyllosilicate‐ and carbon‐rich matrix, and is consistent with being an ungrouped C2 chondrite.

Genesis of the Carboniferous karstic bauxites in Qingzhen region, central Guizhou, southwest China

In addition to the major source of aluminum, karstic bauxite also contains high concentrations of trace elements such as Li, Ga, V, Sc, Co, Nb, Zr, Ni, U and REE. At present, more than 33.9 million tons of early Carboniferous bauxite overlies the karstic surface of the Cambrian carbonate rocks in Qingzhen region, central Guizhou, southwest China. Here, new mineralogical and geochemical evidence of the Lindai bauxite in Qingzhen region have been provided, aiming to identify parental affinity, discuss the provenance differences between the Qingzhen and Xiuwen areas, illustrate the mineral phase transformation, analyze the factors affecting the distribution of critical elements, and reveal the ore-forming process. Mineralogically, the bauxite ores are dominated by diaspore, boehmite, illite, kaolinite and anatase, with small amount of zircon, hematite, pyrite, cassiterite and medium-low temperature hydrothermal minerals such as barite, chalcopyrite, galena, arsenopyrite. Textural characteristics are dominated by matrix-supported clastic texture, followed by pseudo-breccias, recrystallizational and ooidal textures. Clastic grains with various sizes and shapes are often mixed together without obvious sorting and directional structures, indicating that bauxite ores have undergone short-distance transportation in the form of gravity flow. Provenance analysis indicates that the protoliths of the Lindai bauxite were a mixed lithology including underlying Cambrian Shilengshui Formation dolomite, hydromica claystone and purple-red mudstone rather than single dolomite considered by previous studies. Particularly, the presence of hydromica claystone and purple-red mudstone has a significant contribution to high concentrations of trace elements and REE in the Jiujialu Formation. Studies of mineral genesis have shown that the formation of diaspore includes supergene crystallization and metamorphism. R-mode factor analysis revealed that (i) these elements including Al, Ti, Ga and HFSE are relatively stable and immobile during weathering process, and tend to be enriched in the karstic bauxite, (ii) the distributions of HREE are controlled by the REE minerals and Fe oxide-hydroxides, (iii) vanadate minerals have crucial roles in the distribution of LREE in addition to the phosphate minerals, and (iv) the enrichment of Ni is related to the adsorption of hydroxo complex of uranium. The studies reported in this paper provide new insights into the provenance and mineral phase transformation of the Carboniferous karstic bauxite in central Guizhou area.

Hatay Mozaiklerinde Kullanılan Taş Tesseraların Türü ve Kökenin Tespiti, Müze Otel Örneği

(Museum Hotel) mosaics in Hatay province. Archaeometric analysis was performed on 60 tessera samples belonging to 6 different mosaics. In the scope of the study, color analysis, P-XRF and petrographic thin section optical microscopy analysis were performed on tessera samples. As a result of the analyzes, the color components of the tesserae were documented. According to the results of P-XRF analysis; main, artifact, transition and presence of rare earth elements were determined. At the same time, because the majority of tessera is composed of limestone, it is determined that there are high Ca elements in their structures and these results support the results obtained by petrography analysis.
 According to petrography analysis; the majority of the tessera are limestone, siltstone, clay stone and radiolarite rock species and just one rock type is not determined. It is concluded that these rock types are found in Antakya and surrounding of the region. When the tissue characteristics of tesserae samples were evaluated, it was seen that the tesserae belonging to the limestone species had micritic and sparitic texture and the other rock types had crystalline and clastic texture. When the hardness levels of tesserae samples were examined, it was found that the hardest tesserae was tesserae of the radiolith rock type (4,5- 5 mohs) and the others were generally (2- 3 mohs).

Application of Nanoindentation in the Characterization of a Porous Material with a Clastic Texture.

In materials science and engineering, a significant amount of research has been carried out using indentation techniques in order to characterize the mechanical properties and microstructure of a broad range of natural and engineered materials. However, there are many unresearched or partly researched areas, such as, for example, the investigation of the shape of the indentation load–displacement curve, the associated mechanism in porous materials with clastic texture, and the influence of the texture on the constitutive behavior of the materials. In the present study, nanoindentation is employed in the analysis of the mechanical behavior of a benchmark material composed of plaster of Paris, which represents a brand of highly porous-clastic materials with a complex structure; such materials may find many applications in medicine, production industry, and energy sectors. The focus of the study is directed at the examination of the influence of the porous structure on the load–displacement response in loading and unloading phases based on nanoindentation experiments, as well as the variation with repeating the indentation in already indented locations. Events such as pop-in in the loading phase and bowing out and elbowing in the unloading phase of a given nanoindentation test are studied. Modulus, hardness, and the elastic stiffness values were additionally examined. The repeated indentation tests provided validations of various mechanisms in the loading and unloading phases of the indentation tests. The results from this study provide some fundamental insights into the interpretation of the nanoindentation behavior and the viscoelastic nature of porous-clastic materials. Some insights on the influence of indentation spacing to depth ratio were also obtained, providing scope for further studies.

Mechanical behavior and damage-induced permeability evolution of mudstone and gypsum caprocks

Both mudstone and gypsum caprocks play a vital role in the accumulation and preservation of petroleum because of their low porosity and permeability. The transport property of caprocks changes due to the tectonic compression or uplift. The permeability evolution behavior may differ significantly between the two caprocks, however, they are not well-understood. Therefore, in this work, mudstone and gypsum samples were collected and a series of experiments were performed to investigate the permeability evolution at different stress conditions. The results show that the permeability of the original gypsum sample is one to two orders of magnitude lower than that of the original mudstone due to different texture of the rocks. The mudstone has bedding and clastic texture while the gypsum is crystalized. The permeability evolution in triaxial compression test after the sample failure increases up to two orders of magnitude for mudstone, while the permeability for gypsum does not increase and the final permeability is even lower than the initial permeability. Permeability evolution under unloading confining stress tests shows that it increases up to three orders of magnitude for mudstone and about one to two orders of magnitude for gypsum. The differences are mainly due to the different failure modes, brittle deformation for mudstone and ductile deformation for gypsum. Moreover, the generated fractures cross the mudstone samples while the generated shear bands do not cross the gypsum samples. A model was developed to describe the permeability behavior for both loading and unloading processes based on the change of volumetric strain.

Rare earth element geochemical characteristics of the late Permian Badamlu karst bauxite deposit, NW Iran

The Badamlu karst bauxite deposit, located 55 km southeast of Miandoab city in northwestern Iran occurs in karstic depressions of carbonates of the late Permian Nessen Formation. The studied bauxitic ores have a dominant pelitomorphic texture, and locally developed ooidic, pisoidic, and clastic textures in a pelitomorphic matrix. Mineralogically, the bauxite ores are composed mostly of diaspore, hematite, and kaolinite, as well as lesser amounts of rutile, chlorite, goethite, and illite. In addition, REE minerals, such as churchite and cerianite were detected. Cerianite occurs in the core of ooids, and churchite is surrounded by a kaolinite-rich matrix. Churchite and cerianite formed in acidic and oxidizing conditions in the uppermost part of the weathered profile, respectively. Total rare earth elements La-Lu contents in the bauxite ores range widely from 29.1 to 2138.2 ppm (average = 746.5 ppm, std = 722.3 ppm), with Ce being the dominant REE (1.3–1825.0 ppm, average = 606.7 ppm, std = 645.1 ppm). The ΣREE contents, (LREE/HREE)ch ratio, and Ce anomaly in the bauxite ores increase upward through the profile, in which LREE and HREE are La–Eu and Gd–Lu, respectively. The Ce anomaly in the bauxite ores has a bimodal distribution with higher Ce anomalies in the upper 7.5–m-thick succession (2.7–20.8, average 10.5), compared to the lower and middle 4.5–m-thick succession (0.1–0.3, average 0.1). The downward decrease in the Ce anomaly may be due to the lack of cerianite and a decrease in the oxidation state. The REE distribution patterns normalized to chondrite exhibit a variable depletion/enrichment of LREE and HREE, with a negative Eu anomaly (0.3–0.7).

Geochemical and mineralogical features of karst bauxite deposits from the Alborz zone (Northern Iran): Implications for conditions of formation, behavior of trace and rare earth elements and parental affinity

The present study focuses on three outcrops of Upper Permian to Lower Jurassic karst bauxite deposits in the Alborz zone, north of Iran. The deposits occur as sharp semi-continuous lenses within the Upper Permian Ruteh Formation (Gheshlagh) and the Upper Triassic Elika Formation (Siahrudbar and Separdeh) carbonate rocks. Textural characteristics such as pelitomorphic matrix, local ooidic-pisoidic, fluidal, and clastic textures indicate authigenic-allothigenic origins. Mineralogically, kaolinite, boehmite, goethite, and hematite in the Gheshlagh, diaspore, hematite, and kaolinite in the Siahrudbar, and kaolinite, hematite, goethite, diaspore, siderite, and chamosite in the Separdeh deposits are the main phases. The distribution patterns of major elements normalized to the upper continental crust (UCC) depict enrichments of Al, Fe, and Ti in all three deposits. The three deposits are variably enriched in transition trace elements (Cr, Ni, Co, V), U, Th, and high field strength elements (Ga, Hf, Zr, Nb, Y) relative to UCC. The studied bauxites have average concentrations of rare earth elements (ΣREE, La-Lu) between 305.97 and 373.69 ppm. Geochemical data reveal progressive enrichment of REE and an intense (LREE/HREE)N fractionation toward the lower parts of the profile in the Separdeh deposit due to the alkaline conditions of the carbonate bedrock, and irregular distribution patterns at the Gheshlagh and Siahrudbar deposits due to water level and pH fluctuations. The positive Ce anomalies at Separdeh and Siahrudbar suggest oxidizing conditions in the upper parts of the profiles. At Gheshlagh, the negative to nil Ce anomalies reflect low intensity of oxidation. Based on previous geochemical analysis data, the Upper Permian basalts in the Gheshlagh, and the Upper Triassic basalts in the Separdeh and Siahrudbar areas were the most likely source materials. The local uplift of the Permian-Upper Triassic series led to the formation of karstic topography filled with materials derived from weathering of the basaltic rocks under humid tropical paleoclimate. The initial weathering converted the source rocks to kaolinitic soils characterized by the enrichment of Al, Fe, and Ti. Then, they were transferred to the karstic depressions and sinkholes and underwent in situ leaching and desilicification bringing about a relatively higher ore grade. The overall field and petrographic evidence demonstrate that these deposits were formed in shallow and swampy environments. In the eastern part of the Alborz zone (at Siahrudbar and Gheshlagh), better drainage conditions have enhanced the quality of ores.

Detrital zircon geochronology and geochemistry of metasediments from the Vorontsovka terrane: implications for microcontinent tectonics

ABSTRACTThe Vorontsovka terrane (VT) is an important component of the East Sarmatian Orogen (ESO) which divides the Precambrian cores of the Sarmatian and Volgo-Uralia segments of the East European Craton (EEC). The tectonic framework of the VT remains controversial due to poor constraints from geochemical and geochronological studies. In this article we present detrital zircon U–Pb ages and geochemical features of the Precambrian meta-sedimentary rocks from the VT, which occur interlayered with calc-silicate rocks and metabasites. Most of the zircons from metasediments possess oscillatory zoning and high Th/U ratios (>0.2), indicating magmatic provenance. Their 207Pb/206Pb ages cluster around 2093 ± 7, 2126 ± 7, 2158 ± 12, 2189 ± 16, and 2210 ± 31 Ma, correlating with the ages of magmatic zircon cores from the surrounding igneous suites, and reflecting a single tectono-magmatic cycle (~2200–2100 Ma) in the source area. Age of the youngest detrital zircon grain from the metasedimentary rocks and the cores of zircon grains from igneous suites show 207Pb/206Pb ages at 2094 and 2106 Ma, respectively. Together with the largest age clusters of 2126 ± 7 and 2158 ± 12 Ma of the magmatic cores of the detrital zircons, the timing of sedimentation is inferred as ~ 2100–2170 Ma.The metapelites display strong rare earth element fractionation with variable Eu anomalies ((La/Yb)N = 7.0–14.5, Eu/Eu* = 0.49–1.23). In contrast, the calc-silicate rocks and metabasites lack Eu anomalies ((La/Yb)N = 5.2–11.5, Eu/Eu* = 0.87–1.00). The large-ion lithophile (LILE) and high field strength element (HFSE) concentrations of most samples are comparable with those of the upper continental crust (UCC). The rocks possess negative anomalies of Th, Nb, Sr, and Zr relative to UCC. Their high Index of Compositional Variability (0.85–1.32, up to 1.8 in metabasites) and relatively low Chemical Index of Alteration (46.1–70.4) indicate that the metapelitic sediments were immature to weakly immature and probably underwent minor chemical weathering. The protoliths of the metabasites are interpreted as interlayered volcano-sedimentary and pyroclastic material. Relict clastic textures of the VT rocks, their geochemical features, and the grain morphology of detrital zircons suggest that the sediments were derived from intermediate and felsic provenances, which were most likely deposited in an environment with active volcanism. We envisage an active continental margin setting in the southwestern part of the Volgo-Uralia segment of the EEC related to the assembly of the Palaeoproterozoic Columbia supercontinent. Combined with recent data from surrounding terranes of the ESO, our results suggest that the VT represents an accretionary prism along a continental arc within the Sarmatia and Volgo-Uralia oceanic realm in the Palaeoproterozoic.

Geochemical monitoring of clays for diagenetic evolution of the Paleozoic–Lower Mesozoic sequence in the northern Arabian plate: Hazro and Amanos regions, Southeastern Turkey

Clay minerals in the diagenetic/very low-grade metamorphic–sedimentary series from southeastern Anatolia in Turkey were analyzed to determine their mineralogical and chemical compositions. In the Amanos region, the lowermost unit is composed of metaclastics with primary clastic textures, as well as slaty cleavages and chlorite-mica stacks including volcanic rock intercalations. The Lower Cambrian is composed of mainly very low-grade metamorphic clastic rocks, while the Ordovician units have siliciclastic and carbonate rocks. In the Hazro region, the Late Silurian–Lower Triassic units are represented by highly diagenetic carbonate and clastic rocks. All of the rock units include illite. In addition, chlorite, mixed-layered illite–chlorite and chlorite–vermiculite are present in the Amanos region, while calcite, dolomite, kaolinite, mixed-layered illite–smectite (I–S) and glauconite occur in the Hazro region. The illites are characterized by the dominance of 2M1 polytype in the Amanos samples; and 1Md+2M1 in the Hazro samples. The I–S, glauconite and kaolin have R1 and R3, 1M and kaolinite polytypes, respectively. The illites have greater tetrahedral and lower octahedral substitutions than the I–S. Total trace element contents, elemental substitutions and chondrite-normalized trace element and REE values decrease toward illite–I–S–kaolinite. There are obvious fractionations for some major – trace and rare earth elements with respect to each other and clear enrichment with respect to the chondrite, with strong anomalies of positive for Gd and negative for P, K and Eu in the clay minerals. The textural, morphological and geochemical data indicate that kaolinite and I–S in the Hazro area occur in supergene conditions with due to a full neoformation mechanism, whereas illites in the Amanos region represent the hypogene origin. In brief, the K2O contents, ratios of Eu/Eu* and LaN/LuN and δ18O and δD values of I–S and illite exhibit notable relationships with increasing diagenetic/metamorphic grade.

Characteristics and alteration origins of matrix minerals in volcaniclastic kimberlite of the Muskox pipe (Nunavut, Canada)

The matrix of volcaniclastic kimberlite (VK) from the Muskox pipe (Northern Slave Province, Nunavut, Canada) is interpreted to represent an overprint of an original clastic matrix. Muskox VK is subdivided into three different matrix mineral assemblages that reflect differences in the proportions of original primary matrix constituents, temperature of formation and nature of the altering fluids. Using whole rock X-ray fluorescence (XRF), whole rock X-ray diffraction (XRD), microprobe analyses, back-scatter electron (BSE) imaging, petrography and core logging, we find that most matrix minerals (serpentine, phlogopite, chlorite, saponite, monticellite, Fe–Ti oxides and calcite) lack either primary igneous or primary clastic textures. The mineralogy and textures are most consistent with formation through alteration overprinting of an original clastic matrix that form by retrograde reactions as the deposit cools, or, in the case of calcite, by precipitation from Ca-bearing fluids into a secondary porosity. The first mineral assemblage consists largely of serpentine, phlogopite, calcite, Fe–Ti oxides and monticellite and occurs in VK with relatively fresh framework clasts. Alteration reactions, driven by deuteric fluids derived from the juvenile constituents, promote the crystallisation of minerals that indicate relatively high temperatures of formation (> 400 °C). Lower-temperature minerals are not present because permeability was occluded before the deposit cooled to low temperatures, thus shielding the facies from further interaction with fluids. The other two matrix mineral assemblages consist largely of serpentine, phlogopite, calcite, +/− diopside, and +/− chlorite. They form in VK that contains more country rock, which may have caused the deposit to be cooler upon emplacement. Most framework components are completely altered, suggesting that larger volumes of fluids drove the alteration reactions. These fluids were likely of meteoric provenance and became heated by the volcaniclastic debris when they percolated into the VK infill. Most alteration reactions ceased at temperatures > 200 °C, as indicated by the absence or paucity of lower-temperature phases in most samples, such as saponite. Recognition that Muskox VK contains an original clastic matrix is a necessary first step for evaluating the textural configuration, which is important for reconstructing the physical processes responsible for the formation of the deposit.

Carbon, oxygen and boron isotopic studies of Huangbaishuwan witherite deposit at Ziyang and Wenyuhe witherite deposit at Zhushan

Being stratiform or stratoid, the Huangbaishuwan witherite deposit at Ziyang and the Wenyuhe witherite-barite deposit at Zhushan occur in the lower Lower Cambrian siliceous rocks and the orebodies are remarkably controlled by lithological character and petrography. Boron, carbon and oxygen isotopic studies of witherite, barytocalcite and calcite have shown that the car-bon, involved in the formation of these minerals, was derived mainly from hydrocarbons and bio-genetic gases resulting from degradation, polycondensation and dehydroxylation of bio-organic matter in sediments at the early stage of diagenesis; the boron was a mixture of boron in pore wa-ter and that released in the process of degradation of organic matter, with a minor amount of boron from cycling brines in the deep interior of the basin. Boron, carbon and oxygen isotopic studies unanimously demonstrated that witherite was precipitated in this sort of organic carbon-rich pore water medium during the early stage of diagenesis. Extensive occurrence of biodetritus and clastic texture in witherite ores strongly evidenced that Ba 2+ was concentrated and settled down in the form of bio-barite on the seafloor as a result of biological processes, thereafter forming the initially enriched orebodies of barium deposits. Biological processes in seawater and early diagenesis in sediments are the major ore-forming mechanisms of witherite deposits in the region studied.

Amalgamation of disparate crustal fragments in the Walcott Bay‐Foster Glacier area, South Victoria Land, Antarctica

Five structurally distinct crustal sub‐blocks are recognised in the Walcott Bay‐Foster Glacier area of South Victoria Land. The Radian sub‐block consists of variably deformed greenschist and amphibolite facies metasediments, some of which have original clastic textures preserved. The Riicker sub‐block is made up almost entirely of amphibolite facies marble folded into a broad synform on the 10 km scale. The Glee sub‐block is made up of three parallel belts of metasediments with northeast strike. The Renegar sub‐block consists of metasediments deformed around kilometre‐scale plutons. The Kemp sub‐block is composite, including exotic quartzofeldspathic metasediments and orthogneiss extensively altered by metamorphic muscovite. The five sub‐blocks have been amalgamated into the Walcott crustal block along amphibolite facies ductile shear zones which had dominantly strike‐slip motion. The Walcott Block has a northeast‐striking structural grain, and has been intruded by Middle Cambrian alkaline intrusions. The numerous structural discontinuities identified in the Walcott Bay‐Foster Glacier area preclude mapping of lithostratigraphy and metamorphic isograds.

Evaporitic Subtidal Stromatolites Produced by in situ Precipitation: Textures, Facies Associations, and Temporal Significance

ABSTRACT The transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites. Stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters, to isopachous and continuously laminated on a scale of a centimeter to a few meters. The isopachous lamination texture may be composed of either: (1) micritic or radial-fibrous calcite, or (2) dolomite. Isopachous stromatolitic laminae are remarkably uniform, varying little in thickness over a given lateral distance, in contrast to stromatolites formed of peloidal laminae, which show marked variation in thickness over an equivalent lateral distance. These isopachous textures are uncommon on most open-marine carbonate platforms and apparently developed in transitional carbonate-to-evaporite settings because of increasing temperature, salinity, and anoxia related to water stratification, which would have created ecologic restriction and an opportunity for stromatolite growth. Stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea-floor-encrusting calcite and possibly dolomite, whereas the stromatolites composed of peloidal, discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats. While the presence of microbes in almost all near-surface environments nullifies use of the term abiotic to describe most precipitated minerals, we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the absence of microbial mats, and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats. These transitional stromatolite facies are best developed atop Proterozoic and Paleozoic carbonate platforms that underlie major evaporite successions. However, inspection of Jurassic and younger evaporite basins, such as the Messinian of the Mediterranean region, shows that stromatolites with thin, isopachous lamination and radial-fibrous textures, though present, are rare. Instead, these facies may have been replaced by stromatolites with peloidal, clastic textures and by low-diversity diatomaceous and coccolith mudstones. Accumulation of the mudstones would have imposed two important effects: (1) Production of coccoliths would have helped extract calcium carbonate from seawater, thus lowering the growth potential for precipitation of sea-floor-encrusting stromatolites. (2) Settling of both coccoliths and diatoms would have created a sediment flux to the sea floor, which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals.

Facies architecture of the felsic lava‐dominated host sequence to the Thalanga massive sulfide deposit, Lower Ordovician, northern Queensland

The Thalanga volcanic‐hosted massive sulfide deposit occurs in the Cambro‐Ordovician Mt Windsor Subprovince in northern Queensland. The orebody comprises steeply dipping, stratiform, sheet‐like, polymetallic massive sulfide lenses. Overall, the volcanic facies architecture at Thalanga is dominated by quartz‐ and/or feldspar‐phyric lavas and synvolcanic intrusions that comprise coherent facies and in situ and resedimented autoclastic facies. Systematic phenocryst logging (mineralogy, abundance, size) has been used to discriminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and dacite types can also be distinguished using immobile‐element geochemistry. Rhyolitic lavas and intrusions in the footwall are weakly to strongly altered. Apparent clastic textures resulting from hydrothermal alteration and metamorphism are widely developed in the coherent facies. Genuine clastic textures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow‐laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clasts and matrix, and normal grading. Mass‐flow‐emplaced, rhyolitic breccia units delineate palaeo‐sea‐floor positions in the footwall that are potentially prospective for exhalative massive sulfide mineralisation. A comparison of the distribution of clastic and coherent facies with the geometry of strongly altered zones in the footwall indicates that intense hydrothermal fluid flow was independent of the facies arrangement. The massive sulfide lenses conformably overly altered footwall rhyolite and occur in a distinctive facies association which includes coarse quartz‐phenocryst‐rich rhyolitic sills with peperitic contacts and crystal‐rich polymictic breccia. The hangingwall to the orebody consists of largely unaltered dacitic lavas and synvolcanic intrusions and minor dacitic pumice breccia, dacitic breccia and polymictic volcanic breccia. The facies architecture shows that the Thalanga massive sulfide deposit formed in a below‐storm‐wave‐base depositional environment on top of an elevated, lava‐dominated, rhyolitic volcanic centre. A modern analogue for the setting of the Thalanga massive sulfide is the PACMANUS hydrothermal field on the crest of the dacite lava‐dominated Pual Ridge in the eastern Manus backarc basin (Papua New Guinea).

The Origin and Features of Late Sinian Phosphatic Stromatolites in Fuquan, Guizhou Province

AbstractThe phosphatic stromatolite discussed in this paper is a columnar type, comprised chiefly of collophane and subordinately of microcrystalline apatite. The column is composed of alternations of dark, cryptocrystalline laminae formed by the biological processes of algae and bacteria, and light, microcrystalline laminae formed by chemical precipitation. The intercolumnar filler, showing a concave anti‐stromatolitic structure, is composed of alternations of loose laminae formed by mechanical deposition, and dense, crypto‐, microcrystalline laminae formed by chemical precipitation and algal bacterial organic process.Under the SEM, the column differs clearly from the intercolumnar filler in ultramicro‐fabric. The three basic types of ultramicro‐laminae that can be distinguished in the column are: prismatic apatite laminae of chemical origin, clastic apatite laminae of mechanical origin and branched‐tubular apatite laminae of algal‐bacterial origin, of which the first and second categories constitute light laminae and the third, dark laminae. The dark and light laminae were formed during the alternation of flourishing and dormant periods of algae and bacteria.Under the SEM, the clastic texture is obvious in the loose laminae of the intercolumnar filler. And four types of ultramicro‐apatite assemblages can be distinguished in the dense laminae, i. e., the. framboidal and the tubular apatite of algal‐bacterial origin, the framboidal‐prismatic and the framboidal‐tubular‐prismatic apatite of chemical organic origin. The alternation of the loose and the dense laminae depends upon the changes of the energy in the water.In the stromatolites, no replacement phenomena have been found and there exists abundant prismatic apatite kept in the initial form of upward‐crystallization, and especially, in the forms of algae and bacteria (framboidal, tubular and branched). All these undoubtedly indicate that the stromatolites were originally formed by algae and bacteria.The ultramicro‐fabric features indicate that the collophane is not a simple mineral, but a disorderly arranged aggregate of ultramicro‐granular apatite.

Zoned epidote nodules from sedimentary rocks of the Eastport Formation, southwest New Brunswick

Zoned epidote-rich nodules occur in shallow-water sedimentary rocks of the Eastport Formation in southwestern New Brunswick. The host rocks of the nodules are sandstone and shale displaying well preserved clastic texture and minimal metamorphic effects, whereas the recrystallized nodule cores consist of epidote and quartz, with minor proportions of prehnite and actinolite. Whole rock analyses show depletion of sodium, potassium, and magnesium in the nodule relative to the host as well as relative enrichment in calcium and iron. Chemical, mineralogical, and textural similarities to calcareous concretions lead to the conclusion that the nodules represent metamorphosed equivalents of ironstone concretions. The St. George pluton is thought to have been the source of heat for metamorphism. Differences in degree of recrystallization between the nodules and their host rocks are attributed to differences in original bulk composition. Although the primary textures in the pelitic and quartzofeldspathic host rocks remained essentially unchanged during very low grade metamorphism, the nodule cores, richer in calcium and iron, recrystallized into metamorphic assemblages.

Siliceous Tuff Clasts in the Upper Paleogene of Southern California

Pebbles, cobbles, and boulders of rhyolitic and other siliceous tuffs are abundant in upper Paleogene conglomerates that crop out along and near the California coast between Los Angeles and San Diego. Typical clasts are made up of feldspar and quartz crystals in a fine-grained groundmass that is nonporous and in large part crystalline, although relics of the clastic texture are usually present, and some examples contain ghosts of pumice fragments. Possible source rocks of similar lithology and more or less appropriate age include the Sidewinder Volcanics of the Mojave Desert, northeast of Los Angeles and 70 to 140 miles north of the conglomerate outcrops, and the Imuris Volcanics of northern Sonora, 400 miles southeast of San Diego. Petrographic and chemical differences between the conglomerate clasts and the Sidewinder Volcanics show that few, if any, of the clasts can have been derived from the Mojave Desert formation. The Imuris Volcanics are more similar but perhaps too young; moreover, they seem to require 140 to 165 miles of fluviatile transport, as well as 300 miles of strike-slip movement on the San Andreas and related faults, with separation of a hypothetical Peninsular-Sonoran batholith into a very large California-Baja California portion and a very small Sonora portion. Possibly the conglomerate clasts were derived from much nearer but now unavailable sources, either higher levels of the Peninsular Ranges of southern California (above the present erosion surface) or rocks down-faulted beneath the Salton Trough east of these ranges. Exceptional tuff-clast conglomerates at and near the base of the Rose Canyon Formation (middle Eocene), in the La Jolla section of San Diego, were probably fed from very close, perhaps southern, sources.

PRINCIPLES OF CLASSIFICATION AND NOMENCLATURE OF THE ANCIENT VOLCANIC CLASTIC ROCKS

Volcanic clastic rocks comprise various rocks of clastic texture, formed as a result of volcanic activity. Classification of these rocks should be based, in the first place, on petrographical characteristics, which are possible to determine macro- and microscopically. In the suggested classification volcanic clastic rocks are subdivided into four groups, according to the composition of cementing mass and to the relative contents of pyroclastic and sedimentary material, namely: lava breccias, pyroclastic rocks (welded tuffs, tuffs, volcanic breccias), essentially pyroclastic rocks (tuffites) and pyroclastic-sedimentary (tuffogenic-sedimentary) rocks. Further subdivision of the rocks depends on their granulometric composition, chemico-petrographical composition and also on homogeneity or heterogeneity of the clastic material. In some cases there are also taken into account the relative contents of vitric, crystal and lithic pyroclastic material and the degree of abrasion and sorting of the fragments. Resear...

Petrology and Paleogeography of Greenbrier Formation

This paper describes the petrology and paleogeography of the Mississippian Greenbrier formation as determined from study of well samples, insoluble residues, heavy minerals, and thin sections. In the Greenbrier formation, clastic limestone beds composed of lime sand, oolites, and quartz sand alternate with beds of fine-grained limestone in which clastic texture is indistinct or can not be recognized. The clastic limestones appear to be nearshore sediments and in part are probably ancient bar, beach, channel, and dune deposits and have the shapes and trends characteristic of such deposits. Approximately half of the production of oil and gas from the Greenbrier formation is from clastic limestones; half from dolomite and dolomitic limestone, which are largely confined to th basal 20-30 feet of the Greenbrier formation. This basal zone appears to transgress both time units and structure. The dolomite clearly replaces limestone. Four methods by which magnesium-bearing waters could be introduced into the formation are suggested and the probable pattern of dolomitization resulting from each method is considered. The quartz sand in the Greenbrier was derived from two or more different sources on the north. Neither earlier Mississippian nor Upper Devonian sands of West Virginia and adjacent parts of Ohio and Pennsylvania were important sources of the quartz sand.