Showing papers on "Petrography published in 1992"

Petrography, Mineral Chemistry, and Phase Relations of the Basement Boninite Series of Site 786, Izu-Bonin Forearc

Abstract: The 720 m of igneous basement that was penetrated at Site 786 of Ocean Drilling Program Leg 125 consists of boninite-series volcanics. Bronzite andesites dominate the lithology and primitive magmas of high-Ca, intermediate-Ca, and low-Ca boninite are present in subordinate amounts. Sparsely phyric boninites typically contain olivine and orthopyroxene phenocrysts with Mg numbers [= Mg/(Mg + Fe) in moles] between 86% and 87%. Their high whole-rock Mg numbers, and the absence of zonation in the phenocrysts, imply equilibration at temperatures probably between 1200° and 1250°C, and 20° to 50°C below their liquidus. Equilibrium olivine and orthopyroxene have identical Mg numbers, and Mg/Fe partitioning between these minerals and the melt thus can be described with a single Kd. The invariably phenocryst-rich bronzite andesites contain Plagioclase that has spectacular zoning and mafic phases that can be as magnesian as those of the boninite parent. The most evolved melts are rhyolites with hypersthene, Plagioclase (An50), and magnetite. Eruption temperatures for the rhyolites are estimated at about 1000°C. Some magmas contain ferroactinolite in the groundmass, which is most likely a secondary, low-temperature phase. The locally large contrasts in degree of alteration are consistent with multiple episodes of magmatic activity. However, all igneous events produced boninite volcanics. Only the first, the edifice-building episode, gave rise to differentiated magmas. Differentiation of parental boninites took place by limited fractional crystallization, producing bronzite andesites. The erupted andesites, dacites and rhyolites are filter pressed extracts from these bronzite andesite magmas, which, as a result, have accumulated crystals. Subsequent younger igneous events produced high-Ca and intermediate-Ca boninites which intruded as dikes and sills throughout the basement sequence. The mineralogy of the dikes and sills reflects variable degrees of subliquidus cooling of the magma before emplacement.

Volcanic rocks from the Bolivian Altiplano: Insights into crustal structure, contamination, and magma genesis in the central Andes

Abstract: Lavas from Quaternary minor volcanic centers behind the Central Andean volcanic arc in Bolivia form a compositional continuum with rocks from the arc front, extending to more primitive compositions. The lavas show petrographic and geochemical evidence for deep crustal contamination during differentiation. Sr, Nd, and Pb isotopic data from centers within 30 km of each other encompass the entire range found in the Central Volcanic zone of the Andes. The most mafic rocks are largely typical of arc magmas elsewhere, and may represent parental magmas to the porphyritic andesites and dacites erupted from stratovolcanoes along the arc to the west. Andean arc magmas therefore undergo considerable contamination during ascent through the thick crust. The only basalts lacking petrographic evidence for contamination are picrobasalts from Chiar Kkollu that form the most primitive end member of the compositional spectrum. These rocks resemble ocean-island basalts in some respects, indicating that variations in crustal contamination across the strike of the arc may be superimposed on variations in the mantle source composition.

A combined petrographical-geochemical provenance study of the Newland Formation, mid-Proterozoic of Montana

Abstract: A provenance study was conducted on the Mid-Proterozoic Newland Formation, in which petrographical features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation. Sandstones of the Newland Formation are typically arkosic sands and arkoses with very-well-rounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments. The predominant feldspar types are K-spar and microcline, feldspar grains are smaller than quartz grains, and feldspars show little alteration due to weathering. Detrital modes of Newland sandstones (QFL diagrams) indicate that they were derived from a stable cratonic source. These petrographical features imply a source area dominated by granites and granitoid gneisses, semi-arid to arid climate, tectonic quiescence, and overall peneplain conditions. Shales of the Newland Formation are dominated by illite, quartz silt, and fine crystalline dolomite. They have small La/Th rations, relatively large Hf contents, and small contents of Cr, Co, and Ni, all indicative of derivation from crust of granitic composition. Small Tio 2 /Al 2 O 3 ratios also suggest source rocks of granitic composition. The average chemical index of alteration (CIA) for Newland shales is 71.8, which in light of the probable granitoid source indicates modest amounts of chemical weathering. Relatively large SiO 2 contents and large K 2 O/Na 2 O ratios reflect derivation from stable cratonic areas and tectonic quiescence. Thus, in general, the petrography of sandstones and geochemistry of shales provides the same provenance clues for the Newland Formation. One notable discrepancy between the two approaches is that the sandstones indicate an arid to semi-arid climate with very minor chemical weathering, whereas the CIA of the shales indicates at least modest amounts of chemical weathering. This indicates on one hand the need to better calibrate the CIA with a large variety of muds from modern climatic settings, and on the other hand the possibility that this discrepancy is due to transport segregation.

Nd-Sr isotopic, geochemical, and petrographic stratigraphy and paleotectonic analysis: Mesozoic Great Valley forearc sedimentary rocks of California

Abstract: Measurements of Nd-Sr isotopes, major and trace elements, and model mineralogy were made on Upper Jurassic and Cretaceous Great Valley forearc sedimentary rocks to test models for the temporal and spatial evolution of Sierra Nevada arc sources. Isotopes and major and trace elements are sensitive provenance indicators because of the large west-east isotopic, geochemical, and age gradients in the plutonic rocks of the Sierra Nevada batholith, and because petrographic models indicate that source areas moved east during the Cretaceous. Isotopic and chemical variations are correlated in the forearc sandstone; as ϵNd decreases, Th, U, La, Nb, Zr, Hf, Pb, Rb, SiO2, and K2O concentrations increase, and FeO, MgO, TiO2, Ni, and Cr concentrations decrease. This relation is the same as that observed in the plutonic rocks and indicates that the arc was the primary source of sediment and that the sandstone chemistry was not disturbed by sedimentary processes. The ϵNd-ϵSr relation of San Joaquin Valley sandstone is the same as the plutonic rocks, but Sacramento Valley sandstone is elevated in ϵSr because of seawater exchange, weathering, and diagenesis. Whole-rock sandstone decreases in ϵNd from +7 to -5 and increases in 87Sr/86Sr from 0.7045 to 0.7073 with decreasing stratigraphic age. The Nd-Sr isotopic composition is correlative with the plagioclase to feldspar ratio and indicates that source areas moved inland during the Cretaceous. Upper Cretaceous San Joaquin Valley shale is similar in ϵNd to the sandstone, indicating that sandstone and shale were derived from the same source and that the Nd isotopic composition is independent of grain size. The shale is higher in 87Sr/86Sr than the sandstone, possibly due to concentration of biotite in the fine fraction during transport and subsequent Rb loss during diagenesis. Nd-Sr isotopes were used to construct models to locate source areas. Parameters include lithology, drainage basin geometry, and erosion rate. The age and isotopic compositions of the calculated igneous component of the sandstone correspond to the age and isotopic compositions of the plutonic rocks of the batholith; this correspondence indicates that (1) the isotopic composition of the plutonic rocks and the coeval volcanic cover were similar, (2) the volcanic front was denuded within a few million years, and (3) the sediment was derived from the head of the drainage basin, located at the migrating volcanic front.

Origin of petroleum and mercury in the Culver-Baer cinnabar deposit, Mayacmas District, California

Abstract: The Culver-Baer mercury deposit (Mayacmas district) is located within the strike-slip Mercuryville fault zone at the margin of the Geysers geothermal area where serpentine + magnetite protolith is altered to silica-carbonate rock. Silica-carbonate rock is silica rich at the center of the deposit and consists of the assemblage quartz + magnesite + pyrite. This assemblage is cut by numerous quartz-chalcedony veins and breccia veins composed of quartz + chalcedony + or - cinnabar + or - petroleum + or - millerite + or - barite. The cinnabar, which is chemically pure, and the petroleum, which consists predominantly of polycyclic aromatic hydrocarbons, occur preferentially along the hanging walls of the breccia veins. Outward from the deposit center there is a zonation to carbonate-rich silica-carbonate rock which consists of magnesite + or - quartz + or - pyrite. At the deposit margin itself, magnesite + or - magnetite is the dominant stable assemblage. All rock samples reveal a consistent temporal evolution from magnesite veinlets to silica veinlets.Geochemical modeling indicates that the mineralogical zonation from serpentinite to silica-rich silica-carbonate rock in the center of the deposit can arise from the interaction of serpentinite with a CO 2 -rich, H 2 O-CO 2 fluid (PCO 2 > or =30 bars at 200 degrees C). Petrographic and analytical data suggest that other than CO 2 , the only components added to the system during alteration and mineralization were H 2 S, Hg, and petroleum. Isotopic and biological marker data support the hypothesis that all of these components may have been derived from local sedimentary rocks in response to heating and transported to the site of deposition in a gas phase. The association of petroleum and cinnabar, particularly along the hanging walls of breccias, is most likely due to a common source and transport in a relatively buoyant fluid rather than due to a direct chemical link. Because there is no evidence for the introduction of any other components, these data suggest that the Culver-Baer deposit may be a fossil condensate zone akin to the model of White et al. (1971).

Description and correlation of Eocene rocks in stratigraphic reference sections for the Green River and Washakie basins, Southwest Wyoming

Abstract: Stratigraphic reference sections of the Wasatch, Green River, and Bridger (Washakie) Formations were measured on outcrops in the Green River and Washakie basins adjacent to the Rock Springs uplift in southwest Wyoming. The Washakie basin reference section is 7,939 feet thick and consists of 708 beds that were measured, described, and sampled to evaluate the origin, composition, and paleontology of the rocks. The reference section in the Green River basin is 6,587 feet thick and consists of 624 beds that were measured and described but were not sampled. Columnar sections that have been prepared combine information on the stratigraphic nomenclature, age, depositional environments, lithologies, and fossils of each bed in the reference sections. Eocene strata in the Green River and Washakie basins have been correlated biostratigraphically, chronostratigraphically, and lithostratigraphically. The time boundaries of the lower, middle, and upper Eocene rocks in the reference sections are located partly from biostratigraphic investigations and partly from chronostratigraphic investigations. The time boundaries agree with North American land mammal ages. Major stratigraphic units and key marker beds correlated between the reference sections appeared similar in thickness and lithology, which suggests that most depositional events were contemporaneous in both basins. Rocks sampled in the Washakie basinmore » reference section were examined petrographically and were analyzed using heavy mineral separations, X-ray techniques, and assays. The mineralogy suggests that source rocks in the lower part of the Eocene were mostly of plutonic origin and that source rocks in the upper part of the Eocene were mostly of volcanic origin. Economically significant beds of oil shale and zeolite were identified by the analyses. 51 refs., 31 figs., 5 tabs.« less

Rodingites from the Southern Appalachian Piedmont, South Carolina, USA

Abstract: Rodingites composed predominantly of grossular-andradite and epidote-group minerals have been identified at one locality in the ophiolitic (?) Hammett Grove Meta-igneous Suite in the southern Appalachian Piedmont, South Carolina. The rodingites occur at or near the contact between altered (steatitized) ultramafic rocks and gneissic country rocks of the Inner Piedmont belt. The rodingites formed contemporaneously with antigorite in the host rock as the last of several cooling assemblages following the middle or early Paleozoic metamorphic thermal peak at middle- or upper-amphibolite facies conditions. Petrographic and geochemical evidence suggests that the rodingites formed by calcium metasomatism of gabbroic and immature graywacke protoliths, and that static conditions prevailed during metasomatism. Fluid inclusion work indicates that the serpentinizing fluid was relatively saline; it contained 5.3 ± 1.0 equiv. wt.% NaCl and the crystallization temperature is estimated to have been between 350-500°C

Dolomitization of Quaternary reef limestone, Aitutaki, Cook Islands

Abstract: Six holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void-filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ18O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO3 and broad and weak ordering X-ray reflections at 2·41 and 2·54 A. The fine-scale replacement of skeletal grains and freshwater void-filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ18O=+2·6‰0 PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater-seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ13C=2·7‰) and magnesium were derived from seawater. Low-temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section. The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.

Ancient egyptian limestone quarries: a petrological survey*

Abstract: Ancient Egyptian limestone quarries in the Nile Valley occur in six geological formations of Palaeogene age. Samples were collected from 23 of the 48 known quarries, and analysed by thin-section petrography and X-ray fluorescence spectrometry. Results of the analyses show that the geological formations can be identified from rock texture and allochem types, and a plot of SiO2/Al2O3 versus CaO/[CaO + MgO]. The application of these petrographic and geochemical parameters make it possible to determine the geographic provenance of limestone used in ancient Egyptian sculptures and monuments.

Alteration of organic matter in the Viburnum Trend lead-zinc district of southeastern Missouri

Abstract: Organic petrography and elemental analyses were used to study alteration of organic matter in the Bonneterre Formation dolostones. Reflectance measurements and petrographic characteristics of organic particles define an alteration zone about 20 km wide, centered on the Viburnum Trend ore deposits. Within this zone, bitumen has been generated from kerogen. In contrast, no evidence of local bitumen generation was observed in samples located more than 10 km from the Viburnum Trend. This could indicate a level of thermal maturity significantly greater than the regional background level.The organic matter in the Bonneterre Formation consists primarily of amorphous marine kerogen, algal remains (alginite), and solid bitumen. In the absence of vitrinite, relative levels of thermal maturity were determined from the reflectance, fluorescence, and state of preservation of fossil unicellular algae. Within the alteration zone the alginite has been impregnated with bitumen, and there is convincing petrographic evidence that the bitumen was locally generated. Outside of the alteration zone, the algae are well preserved and exhibit the bright yellow fluorescence characteristic of immaturity. There is also petrographic evidence of increased solid bitumen reflectance due to interaction with pore fluids, both within and outside the alteration zone. Elemental analyses suggest that organic matter has been oxidized throughout the study area.Although there is no direct evidence linking the organic maturation and alteration with ore deposition, the association of the alteration zone with a major Mississippi Valley-type deposit permits the hypothesis that the regional flow of hot brines associated with the deposit was the cause of the anomalous heating, and that the evolution of methane and other gases during this process played a role in the ore deposition.

Titanita-biotita granitos do baixo rio uaupés, província rio negro, amazonas. parte i: geologia, petrografia e geocronologia

Abstract: Titanite biotite granitoids and gneisses are dominant rocks in the Rio Negro Province (RNP), NW Amazonas State, Brazil. Batholithic bodies, composed of these rocks, outcrop in the Low Uaupes River area. In this region, little deformed titanite-biotite monzogranites (TBMG) are the major rock type. The granites of this area are here named Rio Uaupes Granite (RUG). Their principal petrographic characteristics are: relatively low modal quartz; significantly high anorthite contents of plagioclase (cores of calcic andesine); extremely high average modal percentages oftitanite + opaques (magnetite + ilmenite ± pyrite) + apatite (4.7% in the TBMG). Magnetic susceptibility measured on hand samples is systematically high in the TBMG (395.3 to 189.7 x 10 -4 SI), decreasing in the hololeucocratic fades (108.9 to 21.2 x 10 -4 SI). Therefore, the RUG are classified as magnetite series granites. Rb-Sr whole rock isochronic dating of RUG gives an age of 1459±32 Mawith 87 Sr/ 86 Sr) initial ratio of 0.70631 ± 0.00117, suggesting that the RUG magma formed directily from crustal rocks or was derived from mantle sources but suffered strong crustal contamination. Concerning the tectonic evolution of RNP, a continental collision model is coherent with the available information, but more geological and structural data are necessary to demonstrate this hypothesis.

Petrography, Mineralogy and Diagenesis of Overpressured Tertiary and Late Cretaceous Mudrocks from the East Shetland Basin

Abstract: The petrography, mineralogy and diagenesis of an overpressured Tertiary and Late Cretaceous mudrock sequence from a single well in the East Shetland Basin (North Sea) are described. The clay fraction is believed to be dominated by volcanic glass above 1400 m depth and by smectite below 1400 m. The smectite content increases with depth due to an increase in detrital smectite and to in situ alteration of volcanic glass and grains; this commenced almost immediately after deposition and is still not complete at burial depths >2300 m

Origin of Eocene-covered karst bauxites of the Transdanubian Central Range (Hungary); evidence for early Eocene volcanism

Abstract: Fission track (FT) ages measured in detrital zircon crystals from Eocene-covered bauxites of the Transdanubian Central Range (TCR) appear to reflect the cooling age of the source rock(s). Thus, by means of FT-dating of zircons new information could be obtained to explain the problem of source rocks debated so far. Since the majority of the detrital zircon crystals proved to be euhedral, their typologic analysis permitted conclusions regarding the petrographic character of the source rocks as well. (1) In the light of the age results it is clear that the pyroclastics of a Pre-Middle Eocene volcanism contributed to the bauxite material which was deposited on the exposed karstified carbonate terrain. Andesitic tuffite layers intercalated in the upper part of the overlying Eocene sequence are traces of younger explosions of the same Paleogene volcanic event. Zircon grains found in the bauxite are morphometrically identical are morphometrically identical to those found in the younger Eocene pyroclastics of andesitic charater. (2) A euhedral zircon population of 210 to 260 million years years was also identified in some of the bauxite deposites studied. These crystals derive most probably from Middle Triassic trachyandesite tuffs of the Central Range, though anatectic granitic sources cannot be excluded. (3) Those zircon grains which are the oldest and fairly well rounded are throught to derive from the parametamorphic of Hercynian age of the surrounding areas. The bauxites of the TCR, though obviously of polygenetic origin, clearly received considerable amounts of pyroclasic contribution related to a Lower Eocene to Lower Lutetian volcanic event and this probably played an important role in the formation of the deposits

HYDROCARBON POTENTIAL OF NORTHERN IRELAND: Part III. Reservoir potential of the Permo-Triassic

Abstract: Thick Permo-Triassic sandstones occur in two basins in Northern Ireland. The sandstones are fluvio-deltaic, including an oolitic facies, and are interbedded with mudrocks. They exhibit petrographic features which are typical of red beds and are widely found in the Permo-Triassic elsewhere in the British Isles. The predominant cements are haematite, potassium feldspar, illite and carbonates. Gypsum, anhydrite and dolomite cements occur in the regions of deepest burial, which represent the basin centres. There was widespread leaching of carbonate/sulphate cements, but calcite is preserved in the oolitic facies and below the sub-Cretaceous (chalk) unconformity. The dissolution of cements and framework grains was due to leaching by meteoric fluids. Precipitation of kaolinite in secondary pores, and subsequent illitization, represent fluctuations in the meteoric hydraulic head. Deep boreholes exhibit sandstone porosities which indicate that there may be significant reservoir potential where adequate seals and traps are present.

Petrographic image analysis: an alternate method for determining petrophysical properties

Abstract: Image analysis uses two-dimensional (2-D) geometric parameters of a pore network cross section to derive estimates of three-dimensional (3-D) petrophysical properties. A purpose of petrographic image analysis (PIA) is to obtain estimates of permeability (k) and porosity (o) in cores that are difficult to analyze by conventional methods, e.g., sidewall cores, drill cuttings, and laminated and friable samples.

Petrographic and microthermometrical studies of emeralds in the ‘Garimpo’ of Capoeirana, Nova Era, Minas Gerais State, Brazil

Abstract: Petrographic investigations in the area of the Capoeirana emerald deposit, Minas Gerais State, revealed two main lithostructural units. The first unit is comprised of gneissic rocks of granitic composition belonging to the basement complex, and the second is composed of a highly weathered metasedimentary-metavolcanic sequence represented by metapelitic schists, amphibolites, schists derived from ultramafic rocks, and quartzites. Quartz and pegmatoid veins appear near the contacts between the gneissic rocks and the mineralization metasedimentary-metavolcanic sequence. The emerald mineralization is dominantly concentrated within the intercalations of meta-ultramafic schists near the contact of the pegmatoid veins. Microthermometric studies of the fluid inclusions of the emerald grains indicate that crystallization occurred in the pressure and temperature ranges of 2000 to 2750 bar and 450 to 650 °C, respectively. These data suggest that the mineralizing solutions have had a late hydrothermal-pneumatolytic origin characterized by low pressures, suggestive of the paragenesis talc + tremolite + carbonate + biotite-phlogopite + chlorite of the emerald wall rocks.

Oxygen Isotopic Analysis of Allende Olivine by Ion Microprobe and Implications for Chondrule Origin

Abstract: lntroductiorl; Oxygen isotope systematics have shown that there are relationships within and among different classes of extraterrestrial samples. Most data have been obtained from bulk samples because of mg sample requirements to achieve a precision of about f0.2 %. These bulk measurements preclude analysis of most individual mineral grains and therefore density separates must be used. In addition, grains within thin section and profiles within petrographic units are not possible. Oxygen isotope measurements using SlMS (1,2) require extraction of 20pm samples and subsequent pressing into a conductive metal to eliminate charging with consequent loss of petrographic relations. Lorin et al. (3,4,5) have analyzed minerals in polished samples of deep sea particles, Greenland micrometeorites and Ornans using low energy electron charge neutralization with 20 errors of about k2%. Hewig et al. (6) have described a technique for oxygen isotope analysis of minerals in thin section which uses a highenergy electron flood gun to neutralize charge build-up on the sample. Interfering molecular ions are removed by collecting secondary ions ejected with >300 eV initial kinetic energy. Analysis of standards allows empirical corrections for instrumental mass fractionation as well as estimates of precision and accuracy. Precision is limited to about k2 %, due both to counting errors for 170 and instability of the secondary ion count rate beyond -1.5 hour analysis times. This technique and initial results are described below. instrumental techniaue and data analvsis: Mass fractionation occurs during SlMS analysis and is a function of element, sample and instrument. A series of olivines of known chemical and isotopic composition was analyzed to obtain an empirical correction factor for the compositional and instrumental mass fractionation effect on the known isotopic ratio. Five samples spanning an Mg/(Mg+Fe) ratio from 1.0 to 0.8 (atomic) were measured and the fractionation effects (per mil per amu): F17 = -18.3 0.2(Fo mop/,) and F18 = -29.2 0.4(Fo mol%). Over the range of olivine compositions, the compositional effect

The Lance Formation: Petrography and Stratigraphy, Powder River Basin and Nearby Basins, Wyoming and Montana

Abstract: From abstract: The uppermost Cretaceous Lance Formation was studied in the Powder River Basin and nearby basins of Wyoming and Montana. The Lance Formation is overlain by the continental Paleocene Fort Union Formation. This report is a petrography and stratigraphy study on these basins.

Analyse de la sédimentation post-phase sarde: les dépôts initiaux à placers du SE de la Sardaigne

Abstract: The Punta Serpeddi Formation (Caradoc-Ashgill) of Sardinia is composed of platform facies deposits accumulated in a sedimentary environment dominated by storm processes. Petrographic studies reveal that these sediments are rich in heavy minerals (up to 12% by volume; the clastic supply probably came from the erosion of four main source-areas: -pre-Caradocian volcanic units belonging to the «Porfidi grigi»; -Cambro-Ordovician deposits of the «Arenarie di San Vito» -a basic mantellic rock not identified; -a cratonic source-area situated in the NE of Sardinia

Evidence of temperature increase of CO 2 -bearing fluids from Aluto-Langano geothermal field (Ethiopia); a fluid inclusions study of deep wells LA-3 and LA-6

Abstract: The Aluto-Langano geothermal field is located within one of the lost active volcanic areas of the Main Ethiopian Rift system. At Aluto, exploratory drilling has revealed the existence of high temperature (ca. 335 o C) two-phase fluids. This study is focused on wells LA-3 and LA-6, which are located on the main upflow zone of the geothermal system. Evidence for the occurrence of high intensity water-rock interaction is given by the presence of minerals such as calcite, chlorite, hematite, illite, quartz, sphene, epidote, garnet, prehnite, biotite and actinolite. Seven samples have been selected from drillcores recovered at different depths for petrographic observation, microthermometry and crushing experiments. Primary and essentially secondary fluid inclusions are recognized in authigenic calcite and quartz crystals as well as in igneous quartz crystals of the reservoir rock. Most of the inclusions are two-phase liquid-rich (liquid fraction: 0.8-0.9), but single-phase vapour inclusions are also present, especially in the upper part of the system. A total of 333 temperatures on cooling and 392 temperatures on heating were measuring for liquid-rich (P and S) inclusions. The recognition of first melting indicates a fluid dominated by Na(K)Cl. Melting ice temperatures (T mi ) range from 0.0 to −1.9 o C. Clathrate was never observed. Homogenization temperatures (T h ) range from 240 to 350 o C and are generally lower than the measured inhole temperatures at each depth. Both T mi and T h values show bimodal frequency distributions suggesting that several generations of fluids have circulated early in the system. Crushing experiments and cooling demonstrate that important chemical changes have occurred from early hydrothermal fluid circulation to the present day, especially as regards P CO2 . The T mi -T h distribution defines a dilution trend between low salinity heated ground waters and a saline (about 3% NaCl eq.) high temperature fluid (350 oC ) rising from a deep source. The mineral association calcite, epidote, garnet and/or prehnite is consistent with the initial very low CO 2 content. The fluid inclusions study demonstrates heating up of the system from about 250 o C to the present-day temperatures of 320-335 o C. This heating could be explained by an intake of high enthalpy fluid in the upflow of the system

Gold metallogeny in the Sino-Korean platform : examples from Hebei Province, NE China

Abstract: 1 Introduction.- 1.1 Regional Tectonic Division of China.- 1.2 The Gold Provinces of Northeastern China.- 1.3 Gold in Eastern Hebei Province.- 2 Geologic Framework of the Sino-Korean Platform.- 2.1 Major Orogenic Events.- 2.1.1 Archean: Qianxi Orogeny.- 2.1.2 Early to Middle Proterozoic: Fuping and Wutai Orogenies.- 2.1.3 Middle to Late Proterozoic: Zhongtiao and Yangtze Orogenies.- 2.1.4 Paleozoic: Caledonian and Variscan Orogenies.- 2.1.5 Mesozoic: Indosinian and Yanshanian Orogenies.- 2.1.6 Cenozoic: Pacific Margin and the Himalayan Orogeny.- 2.2 The Precambrian Basement.- 2.2.1 Archean Rocks.- 2.2.2 Proterozoic Rocks.- 2.3 Structural Geology.- 2.3.1 Precambrian Structures.- 2.3.2 Regional Faults and Lineaments.- 2.3.3 Structural Features of the Major Fault Zones.- 2.4 Yanshanian Granites.- 2.4.1 Regional Characteristics and Plate Tectonic Setting.- 2.4.2 Yanshanian Magmatism in Eastern China.- 2.4.3 Yanshanian Granites in Eastern Hebei Province.- 3 Description of Selected Gold Deposits.- 3.1 Niuxinshan District.- 3.1.1 Host Rock Lithologies.- 3.1.2 Host Rock Structures.- 3.1.3 Gold Mineralization.- 3.1.4 Wall Rock Alteration.- 3.1.5 Age of Mineralization.- 3.1.6 Fluid Inclusions.- 3.1.7 Stable Isotopic Data.- 3.2 Sanjia District.- 3.2.1 Host Rock Lithology.- 3.2.2 Host Rock Structures.- 3.2.3 Gold Mineralization.- 3.2.4 Wall Rock Alteration.- 3.2.5 Age of Mineralization.- 3.2.6 Fluid Inclusions.- 3.2.7 Stable Isotopic Data.- 3.3 Yuerya District.- 3.3.1 Host Rock Lithology.- 3.3.2 Host Rock Structures.- 3.3.3 Gold Mineralization.- 3.3.4 Wall Rock Alteration.- 3.3.5 Age of Mineralization.- 3.3.6 Fluid Inclusions.- 3.4 Jinchangyu District.- 3.4.1 Host Rock Lithology.- 3.4.2 Host Rock Structures.- 3.4.3 Gold Mineralization.- 3.4.4 Wall Rock Alteration.- 3.4.5 Age of Mineralization.- 3.4.6 Fluid Inclusions.- 3.5 Banbishan District.- 3.5.1 Host Rock Lithology.- 3.5.2 Host Rock Structures.- 3.5.3 Gold Mineralization.- 3.5.4 Wall Rock Alteration.- 3.5.5 Age of Mineralization.- 4 Aspects of Metallogenesis.- 4.1 The Age of Mineralization.- 4.1.1 Field Relations.- 4.1.2 Isotopic Age Dates.- 4.2 The Source of Gold.- 4.2.1 Gold in the Qianxi Group: The "Source Bed" Concept.- 4.2.2 Gold in Yanshanian Granites.- 4.2.3 Isotopic Contraints.- 4.2.4 The Mantle Connection.- 4.2.5 Conclusions.- 4.3 The Role of Iron-Rich Host Rocks.- 4.3.1 Banded Iron Formations in Northeastern China.- 4.3.2 Petrography and Composition of Magnetite Quartzites.- 4.3.3 Sulfidization and Gold Mineralization.- 4.3.4 Conclusions.- 4.4 The Role of Yanshanian Granites.- 4.4.1 Granite Compositions.- 4.4.2 Heat Production.- 4.4.3 Conclusions.- 4.5 Structural Controls.- 4.5.1 Regional Structures.- 4.5.2 Local Structures.- 4.5.3 Regional Tectonic Interpretation.- 4.6 Fluid Composition and P-T Conditions of Mineralization.- 4.6.1 Evidence from Fluid Inclusions.- 4.6.2 Stable Isotopic Data.- 4.6.3 Gold Transport and Precipitation.- 5 Towards a Metallogenetic Model.- 5.1 Previous Models.- 5.2 The Preferred Model for Eastern Hebei Province.- 5.3 Open Questions.- 6 Comparison with Other Archean-Hosted Gold Provinces.- Appendix 1.- Appendix 2.- References.

Mineralogy and Petrography of the Shallow - depth Emplaced Granitic Rocks Distributed in the Southern Part of the Kyeongsang Basin , Korea

Abstract: The granitic rocks in the southern part of the Kyeongsang basin, Korea, are divided into seven masses based on the petrographical features. They are generally characterized by shallow-depth intrusions. The chemical compositions of amphibole and biotite from the granitic rocks show systematic variations in Fe/(Fe+Mg) ratios as well as Mn and F contents. Hornblende geobarometer indicates low pressure condition (less than 2.3 kbar) at the time of final crystallization of hornblende crystals. The composition of biotite indicates that oxygen fugacity has been almost buffered during crystallization regardless of rock types. The oxygen fugacity is presumably close to the value of Ni-NiO buffer. From the Fe-Ti oxide geothermometry, it seems likely that most of the Fe-Ti oxides have been re-equilibrated during cooling and have suffered oxidation under subsolidus condition.

Multistage dolomitization in the Society Cliffs Formation, northern Baffin Island, Northwest Territories, Canada

Abstract: Dolostones of the Upper Proterozoic Society Cliffs Formation on northern Baffin Island are host to the Nanisivik lead–zinc sulfide deposits. Two distinct stages of dolomitization have occurred, distinguished by their petrographic, cathodoluminescence, and isotopic characteristics: (1) massive dolomitization of precursor carbonates, and (2) late-stage cementation. Sabkha facies of the lower member of the formation and laminated algal stromatolitic to massive dolostones of the upper member are isotopically similar: δ18O ranges from −1.5 to −6.5‰; δ13C from +1.7 to +3.1‰ PDB; and 87Sr/86Sr averages 0.70761. Petrographic and geochemical evidence indicates that these dolostones formed chiefly by seawater thermal convection beneath shale of the overlying Victor Bay Formation, although mixing of meteoric and marine water also could have occurred. Later fracture-filling dolomitic cements have consistently lower δ18O, but δ13C is similar to that in the host dolostones. Microtraverses of single fractures filled wit...

Hydrothermally altered and fractured granite as an HDR reservoir in the EPS-1 borehole, Alsace,

Abstract: As part of the European Hot Dry Rocks Project, a second exploration borehole, EPS-1, has been cored to a depth of 2227 m at Soultz-sous-Forets (France). The target was a granite beginning at 1417 m depth, overlain by post-Paleozoic sedimentary cover. Structural analysis and petrographic examination of the 800-m porphyritic granite core, have shown that this rock has undergone several periods of hydrothermal alteration and fracturing. More than 3000 natural structures were recorded, whose distribution pattern shows clusters where low-density fracture zones (less than 1 per meter) alternate with zones of high fracture density (more than 20 per meter). Vein alteration, ascribed to paleohydrothermal systems, developed within the hydrothermally altered and highly fractured zones, transforming primary biotite and plagioclase into clay minerals. One of these zones at 2.2 km depth produced a hot-water outflow during coring, indicating the existence of a hydrothermal reservoir. Its permeability is provided by the fracture network and by secondary porosity of the granitic matrix resulting from vein alteration. This dual porosity in the HDR granite reservoir must be taken into account in the design of the heat exchanger, both for modeling the water-rock interactions and for hydraulic testing.