Showing papers on "Petrography published in 2000"

Zircon ages for high pressure granulites from South Bohemia, Czech Republic, and their connection to Carboniferous high temperature processes

Abstract: Petrological and isotopic investigations were undertaken on high pressure granulites of granitic to mafic composition from the Prachatice and Blanský les granulite complexes of southern Bohemia, Czech Republic. The predominant felsic granulites are quartz + ternary feldspar (now mesoperthite)-rich rocks containing minor garnet, kyanite and rutile, and most show a characteristic mylonitic fabric formed during retrogression along the exhumation path. Three high temperature reaction stages at distinctly different pressures are recognized. Rare layers of intermediate to mafic composition, containing clinopyroxene, best record a primary high pressure–high temperature stage (>15 kbar, >900 °C), and a well-defined overprint at medium pressure granulite facies conditions (6–8 kbar, 700–800 °C) during which orthopyroxene (+plagioclase) formed from garnet and clinopyroxene. A further high temperature overprint at lower pressure (ca. 4 kbar) is reflected in the development of cordierite- and/or andalusite-bearing partial-melt patches in some felsic granulites. Conventionally separated zircons from the granulites were measured on a SHRIMP II ion microprobe. Near-spherical, multifaceted grains interpreted to be metamorphic, and short prismatic grains from the cordierite-bearing melt patch, are all concordant and yielded indistinguishable results producing an average age, for 83 individual grain spots, of 339.8 ± 2.6 Ma (2σ). Metamorphic grains from a meta-granodiorite associated with the granulites gave the same age (339.6 ± 3.1 Ma, mean of 9), whereas inherited magmatic grains of the same sample yielded 367.8 ± 1.4 Ma. A mean age of 469.3 ± 3.8 Ma was obtained for two short prismatic concordant grains in one of the granulites, whereas several of the rounded grains with ca. 340 Ma metamorphic zircon overgrowths had much older (207Pb/206Pb minimum ages up to 1771 Ma) discordant cores. In addition to analysis of conventionally separated grains, ion-microprobe measurements were also made on zircons extracted from thin sections (drilled-out, mounted and repolished) such that a direct relationship between the dated zircons and petrographic position could be made. Identical results were obtained from both preparation methods, thus showing that the considerable advantage in petrological control is not offset by any appreciable lack of precision when compared to conventionally prepared ion-microprobe samples. All these isotopic results are identical to those previously obtained by conventional multigrain and single-grain evaporation techniques, but rather than allowing a greater resolution of the age of the petrographically obvious different metamorphic stages the results document, for the first time, the apparent short time scale for high, medium and low pressure metamorphism in the granulites. The short time period between the 340 Ma age for the high pressure granulites, as derived here and from studies of similar rocks elsewhere in the European Variscides, and the 320–330 Ma ages for regional low pressure–high temperature metamorphism, migmatization and granite magmatism, strongly suggests an important link between these two high temperature processes.

Carbonate Cementation in a Sequence-Stratigraphic Framework: Upper Cretaceous Sandstones, Book Cliffs, Utah-Colorado

Abstract: Three macroscopic diagenetic features can be recognized in the sandstones of the Upper Cretaceous Desert Member of the Blackhawk Formation and Castlegate Sandstone of the Mesaverde Group exposed in the Book Cliffs, Utah, each of which have distinctive form, geometry, and stratigraphic distribution. Diagenetic alterations are: (1) leached zones ("whitecaps"), up to 10 m thick, beneath coal beds; (2) large (up to 8 m) concretionary carbonate-cemented bodies in amalgamated shoreface and thin fluvial sandstones; and (3) thin (up to 2 m), laterally extensive carbonate-cemented horizons beneath major marine flooding surfaces. Each feature has distinct petrographic and geochemical signatures, and formed through discrete diagenetic processes. Large isolated carbonate-cemented bodies are composed of ferroan dolomite, most of which precipitated during early diagenesis. Field and petrographic data, coupled with stable-isotope data (early cements, 13C = -2.5 to +3.4o/oo VPDB; 18O = -7.8 to -12.0o/oo VPDB; 87Sr/86Sr = 0.7078; later cements, 13C = -3.1 to -5.7o/oo VPDB; 18O = -12.0 to -15.1o/oo VPDB; 87Sr/86Sr = 0.7093) suggest precipitation from meteoric fluids, input into sediments during times of relative sea-level fall. The source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids. Carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events (relative sea-level rise). Cement precipitation in these horizons continued through sediment burial as a result of organic-matter oxidation reactions in overlying organic-rich mudstones. The results of this study show a link between sedimentation (related to changes in relative sea level) and diagenesis, leading to the potential for the development of process-based, predictive models of early diagenesis in depositional successions.

Late Archean Aragonite Precipitation: Petrography, Facies Associations, and Environmental Significance

Abstract: Large crystal pseudomorphs composed of limestone and dolomite that radiate upward to form centimeter to meter tall fans are known from every well preserved Late Archean carbonate platform on earth In many cases these crystal fans are an important facies constituting as much as 50 of the observed volume of carbonate rock Texturally the fans are composed of elongate blades consisting of amosaicof crystals with randomly oriented optic axes In some pseudomorphs trains of inclusions define thefibrous characterof theprecursor mineral and the blades exhibit blunt terminations when draped by micritic sediment Some of the pseudomorphs contain strontium concentrations of up to 3700 ppm Associated facies include strongly elongate giant stromatolites hummocky cross stratified sandstones ooid intraclast packstone to grainstone small domal stromatolites and several thinly laminated micritic facies that may display desiccation cracks Previously some of these crystal fans have been interpreted as calcite and dolomite replaced pseudomorphs after gypsum formed under restricted conditions resulting from evaporative concentration of seawater However replacement textures and elevated strontium concentrations suggest that the crystal fans are more likely the result of neomorphism of large botryoids of aragonite that formed thick crusts directly on thesea floor Furthermore occurrence of the crystal fans in direct association with strongly elongate giant stromatolites and hummocky cross stratified sediments suggests precipitation of the fans in open marine wave and current swept environments Although evaporation of seawater may have contributed to the growth of fans in some peritidal environments most occurrences are not associated with any other indicators of evaporitic conditions such as halite orgypsum pseudomorphs The reinterpretation of most reported occurrences of Late Archean gypsum pseudomorphs as aragonite pseudomorphs indicates that calcium sulfate precipitation from Late Archean seawater was rare and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth history Rapid aragonite precipitation rates and the paucity of calcium sulfate precipitation can be accounted for in a model for Late Archean seawater featuring relative to present day seawater higher supersaturation with respect to calcium carbonate and high HCO concentrations

Brittle Deformation in Sandstone Diagenesis as Revealed by Scanned Cathodoluminescence Imaging with Application to Characterization of Fractured Reservoirs

Abstract: The stable observing conditions, high magnifications, and sensitive light detection that are characteristic of scanning electron microscope-based cathodoluminescence (scanned-CL) imaging overcome several of the disadvantages of conventional light microscope-based CL systems, allowing more routine application of this petrographic method for description of micron-scale textural relationships between detrital grains, cements, and fractures in sandstones. Scanned-CL imaging has great utility for documenting the interrelation between deformation and diagenesis at the micrometer scale in siliciclastic rocks. A survey of sandstone units of widely varying age, location, and burial history suggests that quartz-lined and quartz-filled microfractures are nearly ubiquitous in lithified quartzose sandstones. Because fractures formed in association with quartz precipitation are prevalent in quartz-cemented siliciclastic hydrocarbon reservoir rocks, scanned-CL imaging of microfractures can potentially yield important information on subsurface fracture populations that have economic significance.

Heavy Bombardment on the Earth at ~3.85 Ga: The Search for Petrographic and Geochemical Evidence

Abstract: The Moon experienced an interval of intense bombardment peaking at -3.85 ± 0.05 Ga; subsequent mare plains as old as 3.7 or 3.8 Ga are preserved. It can be assumed that the early Earth must have been subjected to an even more intense impact flux resulting from its larger size and because of its proximity to the Moon. Siderophile-element analyses (e.g., Ir abun­ dance) of the oldest sediments on Earth could be used to indicate past escalated influxes of extraterrestrial material. In addition, shocked minerals may also be present in the oldest extant rocks of sedimentary origin as detrital minerals. and remnants of impact ejecta might exist in early Archean formations. Searches for impact signatures have been initiated in the oldest sedi­ ments on the Earth, from the early Archean (>3.7 Ga) terrane of West Greenland; some of these rocks have been interpreted to be at least 3.8 Ga in age. So far, unequivocal evidence of a late heavy bombardment on the early Earth remains elusive. We conclude that either the sedimen­ tation rate of the studied sediments was too fast and therefore too diluting to record an obvi­ ous signal, or the ancient bolide flux has been overestimated, or the bombardment declined so rapidly that the Greenland sediments, some even at -3.85 Ga in age, do not overlap in time with it.

Multi-method marble provenance determinations: the carrara marbles as a case study for the combined use of isotopic, electron spin resonance and petrographic data*

Abstract: A multi-method approach, based on petrography, stable isotope data and electron spin resonance spectroscopy for assigning the provenance of white marbles, is presented. A total of 11 experimental variables has been used, including the quantitative measurement of the sample colour and its homogeneity. On this basis we attempt to distinguish the three main quarrying basins of the Carrara district, that is, Torano, Miseglia and Colonnata. Such discrimination, extremely uncertain on the basis of the single techniques, may be performed with acceptable confidence’ using a properly selected subset of six petrographic, isotopic and spectroscopic variables. The final resubstitution and jackknifed re-assignment rates are, respectively, 85% and 81%. Inclusion of the remaining five variables into the classification rule does not improve the results, but simply increases data overinterpretation. The practical possibility of using such an approach for more general provenancing problems is considered.

The origin of fluids and nature of fluid–rock interaction in mid-crustal auriferous mylonites of the Renco mine, southern Zimbabwe

Abstract: Geothermometric constraints on auriferous shear zones of the Renco mine in the Northern Marginal Zone of the late-Archaean, granulite-facies Limpopo Belt in southern Zimbabwe indicate that deformation and associated mineralization occurred at temperatures of at least 600 °C up to more likely 700 °C. Mid- to upper-amphibolite facies conditions during mineralization correspond to the regional-scale retrogression of granulite facies wall rocks during the late-Archaean thrusting of high-grade metamorphic rocks of the Northern Marginal Zone onto low- to medium-grade granite-greenstone terrains of the Zimbabwe craton. Mineral assemblages indicate that the ore fluid was moderately oxidized with log fO2 values between 10−17 and 10−18 bars with high H2S activities of 0.25–0.75. Elements enriched in the shear zones include Au, S, Fe, Cu, Mo, Bi, Te, Ni, Co, and H2O, Au and Cu being the most enriched. Geochemically, Au correlates with Cu but not with S, which, together with the fact that gold is only rarely intergrown or in direct contact with sulfides, possibly indicates a transport of gold as a chloride complex. The siting of gold along fractures or within implosion breccias suggests that gold was precipitated due to fluid immiscibility induced by catastrophic fluid pressure drops during seismic slip events. Fluid inclusions are predominantly CO2 (±CH4 ± N2)-rich, but petrographic work indicates that fluid inclusions have undergone extensive post-entrapment modifications due to the pervasive recrystallization of mineral textures in the high-temperature shear zones. The mineralized shear zones are enriched in 18O compared to wall-rock enderbites, which is interpreted to represent an influx of externally derived fluids of probably metamorphic origin. Based on temporal and spatial relationships between mineralization, late-Archaean overthrusting of the Northern Marginal Zone onto the Zimbabwe craton, and coeval amphibolite-facies hydration of granulites, we suggest that the Renco mineralization formed in a mid-crustal environment from metamorphic fluids that were generated from dehydration of subcreted greenstone terrains of the Zimbabwe craton.

Isotopic signature of burial diagenesis and primary lithological contrasts in periplatform carbonates (Miocene, Great Bahama Bank)

Abstract: The stable isotope geochemistry of Miocene sediments from the leeward margin of the Great Bahama Bank was examined to investigate burial diagenetic processes in periplatform carbonates. Data indicate that, in addition to differences in bulk proportions of neritic and pelagic carbonate along the slope, rhythmic variation in primary carbonate content has controlled patterns of burial diagenesis and associated geochemical signatures throughout much of the succession examined. The present study focuses on Ocean Drilling Program Sites 1006 and 1007, the most distal of five sites drilled from marginal to deep basin environments during Leg 166. These Miocene sections are characterized by their cyclic appearance, manifest as decimeter- to meter-scale alternations between light-colored ooze/chalk/limestone and dark-colored marl/marlstone. The section at Site 1006 contains a high proportion of pelagic carbonate and is unlithified to a subbottom depth of ~675 m. Fluctuations in δ 18 O and δ 13 C values at this site are independent of lithological variation and reflect primary conditions. At Site 1007, located at the toe-of-slope and composed of a mixture of bank-derived and pelagic carbonate, limestones are densely cemented, show little evidence of compaction and have δ 18 O values up to 2‰ higher than coeval sediments at Site 1006. Marlstones at Site 1007 are poorly cemented, exhibit an increase in compaction-related features with depth and have lower and more variable δ 18 O values that are similar to those of coeval sediments at Site 1006. Isotopic and petrographic characteristics of limestone interbeds result from cement precipitation from cold sea water during the first ~100 m of burial. Higher proportions of insoluble materials and pelagic carbonate seem to have inhibited diagenetic alteration in adjacent marlstones; in spite of significant compaction and pressure solution during burial, original isoto pic compositions appear to be best preserved in these intervals at Site 1007. The documented contrasts in petrographic and isotopic patterns illustrate the role of primary sediment composition in controlling lithification processes in periplatform carbonates and stress the importance of consid ering such factors when interpreting geochemical data from ancient shelf and slope limestones.

Search for petrographic and geochemical evidence for the late heavy bombardment on earth in early archean rocks from Isua, Greenland

Abstract: The Moon was subjected to intense post-accretionary bombardment between about 4.5 and 3.9 billion years ago, and there is evidence for a short and intense late heavy bombardment period, around 3.85 ± 0.05 Ga. If a late heavy bombardment occurred on the Moon, the Earth must have been subjected to an impact flux at least as intense. The consequences for the Earth must have been devastating. In an attempt to investigate if any record of such a late heavy bombardment period on the Earth has been preserved, we performed a petrographic and geochemical study of some of the oldest rocks on Earth, from Isua in Greenland. We attempted to identify any remnant evidence of shock metamorphism in these rocks by petrographic studies, and used geochemical methods to detect the possible presence of an extraterrestrial component in these rocks. For the shock metamorphic study, we studied zircon, a highly refractive mineral that is resistant to alteration and metamorphism. Zircon crystals from old and eroded impact structures were found earlier to contain a range of shock-induced features at the optical and electron microscope level. Many of the studied zircon grains from Isua are strongly fractured, and single planar fractures do occur, but never as part of sets; none of the crystals studied shows any evidence of optically visible shock deformation. Several samples of Isua rocks were analyzed for their chemical composition, including the platinum group element (PGE) abundances, by neutron activation analysis and ICP-MS. Three samples showed somewhat elevated Ir contents (up to 0.2 ppb) compared to the detection limit, which is similar to the present-day crustal background content (≤0.03 ppb), but the chondrite-normalized siderophile element abundance patterns are non-chondritic, which could be a sign of either a small extraterrestrial component (if an indigenous component is subtracted), or terrestrial (re)mobilization mechanisms. In absence of any evidence for shock metamorphism, and with ambiguous geochemical signals, no unequivocal conclusions regarding the presence of extraterrestrial matter (as a result of possible late heavy bombardment) in these Isua rocks can be reached.

Field and Geochemical Studies of the Melilite-Bearing Arydzhangsky Suite, and an Overall Perspective on the Siberian Alkaline-Ultramafic Flood-Volcanic Rocks

Abstract: This paper presents the first comprehensive geologic, petrographic, geochemical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava suite, the last suite in the entire north Siberian, flood-volcanic sequence to require study by modern methods. Within this sequence, only the Arydzhangsky Suite includes melilite-bearing lavas; it is composed of melanephelinites to limburgites (both melilite-bearing and melilite-free), with rare melilitites and picrites. The lavas contain from 0 to 60 vol% melilite, with MgO contents ranging from 5.7 to 29.5 wt%. Nonetheless, these compositionally diverse lavas are all quite similar in incompatible-element geochemistry. They are distinct from all other Siberian alkaline-ultramafic lavas and, among these lavas, show the most resemblance to the Yakutian kimberlites. With this contribution, all of the north Siberian, alkaline-ultramafic lavas will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on t...

Lithological Controls on Seismicity in Granitic Rocks

Abstract: Seismicity induced from a tunnel excavation through two lithological units, granite and granodiorite, at the Canadian Underground Research Laboratory (URL) is analyzed in an attempt to understand observed lithological differences in the damage-zone development. The results from seismicity recorded by the 16 triaxial accelerometer array found the damage zone around granite excavations to have more events occurring ahead of the tunnel face and a shorter overall seismic response time than the granodiorite. Petrographic analysis of the rock samples show stress relief microcracking predominantly in the larger quartz crystals, suggesting that these are the weakest mineral grains. We propose that initial in situ cracking occurs in the large quartz crystals, significantly reducing the strength and resulting in rapid formation of the excavation damage zone. The smaller-grained, more homogenous granodiorite shows less stress relief microcracking, probably due to the stresses being distributed over a larger number of grain boundaries. From the seismic and petrographic evidence we propose that the crack initiation stress is lower in the granite than the granodiorite. The events from the granite and granodiorite have a similar range in magnitude ( M w = –2.9 to –4.2) and source dimension ( r = 0.13–0.51 m). They have a mean P - to S -wave corner frequency ratio of 1.0, probably indicating relatively slow rupture velocities. About 25% of the events have an S - to P -wave energy ratio less than 10, agreeing with previous source mechanism studies, which find a number of events at this depth have significant isotropic components.

Evolution of gabbroic rocks of the Northern Apennine ophiolites (Italy): Comparison with the lower oceanic crust from modern slow-spreading ridges

Abstract: Field and petrographic relationships together with major and trace element mineral chemistry have allowed us to determine the igneous to high-temperature metamorphic evolution of the gabbroic rocks of the Northern Apennine ophiolites. Gabbroic rocks formed by the intrusion of liquids of normal mid-oceanic-ridge (N-MORB) type in a heterogeneous mantle section under low-pressure conditions. These liquids underwent an igneous-differentiation process controlled by fractional crystallization, most likely associated with late percolation in the gabbroic cumulate pile of a volatile-bearing igneous agent, possibly a trondhjemite-type liquid. Such an igneous-differentiation process produced highly evolved liquids that gave rise to Fe-rich rocks (mostly Fe-Ti oxide-bearing diorites). The gabbroic rocks underwent high-temperature (T 900 °C) recrystallization in ductile shear zones in the absence of seawater-derived fluids. The gabbroic rocks of the Northern Apennine ophiolites bear striking similarities to those recovered from modern slow-spreading ridges, such as the Southwest Indian Ridge, the Mid-Atlantic Ridge (at its intersection with the Kane Fracture Zone, i.e., the MARK area), and the Mid-Cayman Rise. The Northern Apennine ophiolites are interpreted to represent a fossil analogue of a magma-poor slow-spreading center that formed as a result of continental lithospheric extension.

Outcrop characterization of reservoir quality and interwell-scale cement distribution in a tide-influenced delta, Frontier Formation, Wyoming, USA

Abstract: Petrographic study of the Frewens sandstone, Upper Cretaceous Frontier Formation, documents reservoir-scale diagenetic heterogeneity. Iron-bearing calcite cement occurs as large concretions that generally follow bedding and are most common near the top of the sandstone. Median thickness of the concretions is 0.6 m, length 4.5 m, and width 5.7 m; median volume is 5.2 m 3 . Coneretions comprise 12% of the sandstone. The minus-cement porosity of concretion samples is low, indicating that the calcite precipitated near maximum burial depth. Isotopic and burial history data suggest that the calcite precipitated at ∼54°C from evolved meteoric water enriched in 18 O or from a mixed meteoric-marine pore-water. Shell-bearing transgressive shales above the Frewens sandstone are interpreted to be the source of calcium carbonate. Concretions of this size and distribution would influence fluid flow in a reservoir and would reduce the amount of hydrocarbons in place.

Magmatic anhydrite in granitic rocks: First occurrence and potential petrologic consequences

Abstract: The Cajon Pass Deep Scientific Drillhole penetrated a minimum of 1 km of Late Cretaceous (?) intermediate plutonic rocks containing magmatic anhydrite, which occurs as both matrix grains and relic microphenocrysts within poikilitic hornblende, plagioclase, and sphene. Phase equilibria of coexisting silicates, oxides, and apatite support petrographic evidence of hypersolidus anhydrite, indicating that intermediate rocks from a mafic-felsic calc-alkalic suite (or suites) crystallized from about 800 to 700 °C at P tot ~6 kb from hydrous and oxidized, sulfate-saturated andesitic to dacitic magma. The occurrence of anhydrite as inclusions within early crystallizing phases and its association with sulfate-enriched apatite indicates that, despite the potentially rapid destruction of matrix anhydrite by subareal weathering, petrologic evidence of the nature of such volatile-rich magma systems can be retrieved. If found to be widespread by future work, such evidence could be useful in understanding the mechanisms of volatile enrichment in explosive sulfur-rich volcanic systems and the potential relative roles of metasomatized lithospheric mantle sources and shallow-level mixing/assimilation processes in Cordilleran arc magma systems.

Quantitative Petrographic Technique for Concrete Damage Due to ASR: Experimental and Application

Abstract: An automatic petrographic test procedure for quantifying damage in concrete affected by alkali-silica reaction (ASR) is presented in this paper. A computer image analysis program was designed to quantify the degree of microcracking and the amount of silica gel resulting from ASR. The procedure involves the petrographic examination at a magnification of ×20 of polished concrete sections impregnated with fluorescent epoxy resin for cracking and uranyl acetate coated sections for the determination of their silica gel content. Also, the data were compared to the results obtained from a semi-quantitative petrographic method, i.e., the Damage Rating Index commonly used in Canada for evaluating the condition of concrete affected by ASR. The petrographic examination was first carried out on laboratory sections cut and polished from concrete prisms incorporating two different aggregate types, the Spratt limestone and the Potsdam sandstone. Both methods were also applied to specimens prepared from a core collected from a large dam affected by ASR. Good correlation was obtained between analytical parameters derived from the image analysis method and the expansion levels of the laboratory test prisms; however, no explicit relation was found to date between the amount of gel as measured in this study and the expansion level. This study shows that the quantitative petrographic method using the image analysis and the Damage Rating Index Method can be used to estimate the condition and current expansion of concrete specimens cored from concrete structures affected by ASR.

The Salobo iron oxide copper-gold deposit, Carajás, northern Brazil

Abstract: The Salobo iron oxide copper-gold deposit is located in the Carajas Mineral Province, northern Brazil. The copper-gold ore is hosted by the Archean Salobo-Pojuca Group, which is formed by a sequence of amphibolites, banded iron formations, metagraywackes and quartzites. These rocks were deposited in a trondhjemitic basement, where a continental rift basin, that has been further described as a pull apart basin, was developed. Principal ore assemblages are magnetite-bornite-chalcocite and magnetite-bornite-chalcopyrite, with magnetite dominant and variable amounts of copper sulphides. The iron oxide copper-gold ore shows elevated concentrations of Ag, U, Co, Mo, F and LREE. Differences in geochemistry and textures between magnetite of iron-rich rocks and magnetite of banded iron formation suggest a hydrothermal origin for the mineralization. Fluid inclusion data for quartz veins and apatite indicate the involvement of highly saline fluids in the deposit formation. Adominantly magmatic source of the sulphur is indicated by isotope ratios determined for chalcopyrite and bornite (834S between 0.2%o and 1.6%o). Petrographic evidence supported by preliminary geochronological data indicates that the mineralization post-dates the metamorphism. Hydrothermal alteration effects on host amphibolites have been also investigated. The studied amphibolites occur as lenses or layers close to the contact with the gneissic basement or included in metagraywackes of the Salobo-Pojuca Group. Trace element chemistry of these rocks indicates that they are subalkaline basalts with tholeiitic affinity. Based on the K.,0 content, three alteration groups have been defined and informally named "less altered", "medium altered" and "very altered" types. They characterize rocks affected by different degrees of alkali metasomatism, resulting in major compositional changes. "Less altered" rocks ( 3.5 wt%) values. The spatial association of "very altered" rocks with the main ore zone suggests a relationship between alkali metasomatism and mineralization. Similarities in the the hydrothermal alteration pattern combined with the ore mineralogy and chemistry indicate that the Salobo deposit belongs to the class of iron oxide (Cu-U-Au-REE) deposits.

Correspondence and discriminant analyses of sand and sand temper compositions, tonto basin, arizona*

Abstract: Geologists use petrographic modal analysis to relate fluvial sand composition to source rock composition, thus establishing provenance. Archaeologists seeking to establish provenance of sand temper in pottery can use similar petrographic methods, but their finer scale of investigation requires more precise statistical tools than those employed by geologists. A quantitative method for performing that task is presented. It utilizes correspondence analysis and discriminant analysis of logratio transformed point-count data to define petrofacies, or sand temper resource procurement zones. The procedure is illustrated with sand and sand-tempered sherd samples collected from the Tonto Basin, central Arizona; temporal trends in utilitarian ceramic production c. AD 100–1350 are reviewed.

Repeated tabular injection of high-level alkaline granites in the eastern Bushveld, South Africa

Abstract: Field observations, petrography and magnetic susceptibility fabrics are used to characterize the structure and evolution of the Nebo granite, forming part of the Bushveld Complex, in South Africa. The Nebo granite is the dominant component of the Lebowa granite suite, the largest known anorogenic granitoid complex. It is a composite tabular intrusion, comprising alkali feldspar granites and syenogranites. There are seven main petrographic units, defined by differences in modal content, crystallization sequence, magmatic water content, magnetic susceptibility and magnetic fabric. Each unit was emplaced independently and fed by vertical dykes. Anisotropy of magnetic susceptibility used to map magmatic fabrics within the granite shows that they are dominantly controlled by alignment of early-crystallizing magnetite and are similar in outcrop and pluton scale. Magnetic foliations and lineations are horizontal, reflecting vertical host-rock compression and horizontal magma flow during emplacement. Space was created by magma pressure which resulted in roof uplift and floor depression: displacements were accommodated by normal and reverse faulting and flexural-slip along bedding planes in host-rock metasediments.

Petrographic Characteristics of Neogene Lignites from the Ptolemais and Servia Basins, Northern Greece

Abstract: In this study the petrographic analysis results of Ptolemais and Servia Neogene lignites, Northern Greece, are presented. The geological position, lithotype, and maceral content of these lignites, as well as some chemical parameters and changes in the reflection of different macerals, are described. The interdependence between maceral content and environmental conditions in the process of peat formation is traced. Petrographic analysis shows that coals are represented by different kinds of lignites from dark brown to black, semibright lignites, consisting mainly of the coal lithotype humoclarain, rare strips and bands of bright humovitrain, and dull humodurain. The sample from the Lava deposit in the Servia Basin is composed of grayish to black, dark humoclarain. The predominance of humodetrinite macerals ( ) densinite and attrinite and the low contents of inertinite show that the lignites in the Ptolemais and Servia Basins are formed in limno-telmatic conditions and in different mire types, being similar to herbaceous swamp, fen, or marsh.

Significance of trace element composition of quartz cement as a key to reveal the origin of silica in sandstones: an example from the Cretaceous of the Barrow Sub-basin, Western Australia

Abstract: Trace elements have been used to constrain the origin of silica in quartz cement: one of the most abundant authigenic minerals in marine sandstones of the Lower Cretaceous Barrow Group, Barrow Sub-basin, Australia. Petrographic, fluid inclusion, electron microprobe and cathodoluminescence data from quartz cements in these sandstones indicate multiple stages of cementation at different temperatures. Three quartz cementation phases were detected by using the cathodoluminescence imaging. Homogenization temperature calculations from fluid inclusions indicate precipitation temperature in a range of 60–140°C; phase I between 60 and 90°C; phase II between 90 and 105°C; phase III between 105 and 140°C. Oil stains and bitumen coat the second phase of quartz overgrowths and were trapped between the middle and the late phases, implying that the late phase formed after oil migration commenced. Trace element concentrations and mass balance calculations suggest two predominant sources for silica: feldspar alteration and pressure dissolution. Aluminium varies consistently between each cement phase with an average 245 p.p.m. for phase I,805 p.p.m. for phase II and 89 p.p.m. for phase III. Textural analysis and depth-related patterns suggest that silica in the two first phases was probably a by-product of feldspar alteration reactions whilst the last phase was probably a result of pressure dissolution. The lack of correlation between aluminium content and temperature implies that the primary source of the silica is the main control on the aluminium content of quartz cement.