Showing papers on "Petrography published in 2001"

Almandine garnet in calc-alkaline volcanic rocks of the northern Pannonian Basin (eastern-central Europe): Geochemistry, petrogenesis and geodynamic implications

Abstract: of the Pannonian Basin and the tensional stress field may have Almandine garnet-bearing andesites and dacites occur frequently in enhanced their fast ascent from lower-crustal depths, allowing the Neogene calc-alkaline volcanic series of the northern Pannonian preservation of early-formed almandine phenocrysts. Basin (Hungary and Slovakia). They were erupted during the early stage of volcanism and occur along major tectonic lineaments. On the basis of petrographic and geochemical characteristics, garnets from these rock types are classified into (1) primary phases,

Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 1. Determination of mineralogy, chemistry, and classification strategies

Abstract: We have examined and applied existing classification schemes for volcanic rocks and developed new schemes using thermal emission spectra of terrestrial volcanic rocks. Laboratory thermal infrared spectra (5–25 μm, at 2 cm−1 spectral sampling), deconvolved modal mineralogies, and derived mineral and bulk rock chemistries were used to distinguish basalt, basaltic andesite, andesite, and dacite. Modal mineralogies derived from linear deconvolution of terrestrial volcanic rocks were compared to modes measured by an electron microprobe phase mapping technique to determine the accuracy of linear deconvolution in modeling specific mineral abundances. One σ standard deviations of the absolute differences between modeled and measured mineral abundances range from 2.4 to 12.2 vol %, with an average standard deviation of 4.8 vol % being in agreement with average uncertainties calculated in previous studies. Weighted average compositions of feldspars in the deconvolution generally overlap the measured ranges of plagioclase compositions and the presence of low-calcium and high-calcium pyroxenes was correctly identified. Bulk chemistries of volcanic rocks were derived with a relatively high degree of accuracy (1σ standard deviations ranging from 0.4 to 2.6 vol %) by combining the compositions of spectrally modeled phases in proportion to their relative abundances in a particular sample. These data were collectively used to examine existing and develop new volcanic rock classification schemes. However, no single classification scheme was effective in accurately classifying all samples. Multiple steps of classification were required to distinguish volcanic rocks, reflecting the mineralogic diversity and continuum of compositions that exists in volcanic rock types. In a companion paper [Hamilton et al., this issue] these schemes are applied to the classification of Martian surface compositions.

Weathering indices and their applicability for crystalline rocks

Abstract: In the recent past, several weathering indices have been proposed to characterize the extent of weathering and weatherability depending upon the nature and requirement of the study. The weathering index provides a quantitative measure of the extent of weathering of rock; hence it can provide input to the prediction models to assess the strength and deformational properties of rocks and classifications of weathered rock material. In the present study some of the important weathering indices, broadly categorized as chemical, (micro) petrographical and engineering weathering indices, are reviewed and studied experimentally for three common rocks of India, namely granite of Malanjkhand, basalt of Nagpur and quartzite of Delhi, along with results of other rocks reported by other researchers. The study reveals that none of the existing chemical weathering indices is valid for genetically different common rock types and useful for engineering purposes. However, loss on ignition (LOI) may provide an approximate estimation of altered minerals (clays and hydroxides) in tested rocks. It has also shown good correlation with petrographic indices and engineering index properties. Among the petrographic indices, crack density (ρcr) and unsound constituent (an input to the micropetrographic index) indices provide good correlation with engineering index properties. To quantify the extent of weathering in terms of strength degradation due to weathering in rock, an index is suggested – strength ratio (Rs) – which is the percentage of uniaxial compressive strength (σc) of weathered rock with respect to σc of fresh rock. Its significance is shown statistically through the relationships with other indices for several rock types including sedimentary and metamorphic rocks.

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Abstract: Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian‐ Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole-rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque‐Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole-rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well-preserved belemnite shells (non-luminescent to slightly luminescent) typically show stable isotope values of +4AE3& to ‐0AE7& d 13 C, +0AE7& to ‐3AE2& d 18 O, and trace element contents of 950 l gg ‐1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low-Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non-luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their d 13 C and d 18 O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or highfrequency secular variations in sea-water chemistry superimposed on the longterm isotopic trend. A pronounced positive carbon-isotope excursion (up to +4AE3&) is documented in the early Toarcian serpentinus biozone, which correlates with the Toarcian d 13 C maximum reported in other European and Tethyan regions.

Petrogenesis of Apatite-Rich Rocks (Nelsonites and Oxide-Apatite Gabbronorites) Associated with Massif Anorthosites

Abstract: We report field, petrographic, mineralogical, and geochemical data for two P-rich rock types of the anorthosite suite—Fe-Ti oxide-apatite rock (or nelsonite) and Fe-Ti oxide-apatite gabbronorite (or oxide-apatite gabbronorite). These rocks are take from the Château-Richer, Labrieville, and St. Urbain massifs in Quebec; from the Roseland massif in Virginia; and from the Carthage massif in New York. Our data set includes X-ray fluorescence (XRF) and instrumental neutron activation (INA) analyses of whole-rock samples, as well as electron-microprobe (EMP) and INA analyses of separated apatites. At St. Urbain and Labrieville, nelsonite forms a small capping on massive hemoilmenite deposits fully contained within anorthosite, whereas at Roseland, nelsonite occurs as dikelike bodies within country rock. All three nelsonite occurrences consist almost entirely of ilmenite + apatite (but do not have a 2:1 oxide/apatite ratio), with only trace amounts of other minerals. All oxide-apatite gabbronorites form sill-like layers within or near the margins of anorthosite massifs and contain ilmenite ± magnetite, apatite, orthopyroxene + clinopyroxene, and plagioclase. All apatites are F rich and Cl poor, consistent with high-temperature magmatic crystallization. Concentrations of apatite rare earth elements (REE) are highly variable (e.g., LaN ≈ 700–2,000× chondrites), but patterns show a consistent fractionation (e.g., LaN/LuN ≈ 10–20) with small to moderate negative Eu anomalies. Interestingly, the Labrieville nelsonite and its apatite lack an Eu anomaly, consistent with crystallization in a highly oxidizing environment. Apatites in both rock types are enriched in REE relative to whole-rock samples (by factors of 3 to 10), but REE patterns of apatites and whole rocks are parallel in each case. Mass-balance calculations indicate that for nelsonite, 100 percent of the REE is contained in apatite; for oxide-apatite gabbronorites, ~100 percent of the light REE, ~90 percent of the middle to heavy REE, and ~75 percent of Eu are in apatite. These results can be explained satisfactorily by assuming that the rocks are cumulate in origin. If, on the other hand, nelsonites and oxide-apatite gabbronorites represent liquids, apatite DREE values must vary by more than a factor of 3. Our field, mineralogical, and geochemical observations indicate that most nelsonites and oxide-apatite gabbronorites are not related directly. Indeed, apatites from oxide-apatite gabbronorites contain lower Sr and MgO but higher REE than those in nelsonites, indicating that nelsonites are the more primitive rock type. Nelsonites probably formed as cumulates primarily within larger deposits of Fe-Ti oxide (locally mobilized as dikelike bodies). oxide-apatite gabbronorites also represent cumulates, but these are related spatially and genetically to associated jotunites and mangerites and appear to form at several stages in the evolution of a particular anorthosite complex.

Petrology of kamafugites and kimberlites from the Alto Paranaíba Alkaline Province, Minas Gerais, Brazil

Abstract: Mafic rocks representative of the alkaline magmatism of the Alto Paranaiba Province in southwestern Minas Gerais, Brazil were studied by means of petrography, mineral, whole-rock and isotope geochemistry with the objective of better understanding this Cretaceous magmatism and the characteristics of the magma sources. Because of the variety and complexity of lithotypes examined in this research and the paucity of detailed studies of these Brazilian rocks in the literature, this study also attempts to establish parameters that allow for a clear distinction between kimberlite and kamafugite. Fifty-two occurrences are described and classified as kimberlite or kamafugite. Among the kamafugites, both ugandite (characterized by the presence of leucite) and mafurite (with kalsilite) end members have been characterized. Mineral compositions were found to be efficient in distinguishing between kimberlites, mafurites and ugandites in the province, primarily on the basis of phlogopite composition. The Re-Os isotope systematics permitted a better understanding of the relation of the sublithospheric mantle source to the magmatism in the region. Kimberlites, mafurites and ugandites have different 187Os/188Os ratios (0.117 to 0.129, 0.127 to 0.145 and 0.142 to 0.147, respectively). The Rb-Sr and Sm-Nd isotope systematics failed to indicate first-order differences between kamafugites and kimberlites, whilst 206Pb/204Pb ratios for the kimberlites are higher than those for the other rock types. Kimberlite and kamafugite isotopic compositions appear to be related to the mixture of at least two dominant mantle components: one with an isotopic signature similar to that of lithospheric peridotites, i.e., with 187Os/188Os ratios of the order of 0.118, similar to those observed in mantle-derived xenoliths entrained in kimberlites intruded in the Kaapvaal, Wyoming, and Siberian cratons; another with higher 187Os/188Os ratios of the order of 0.135, within the range of ratios reported for pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting depths of heterogeneous lithospheric sources by a mantle plume are suggested to explain the isotopic characteristics of the Alto Paranaiba Alkaline Province alkaline rocks.

The Role of Petrography in the Study of Archaeological Ceramics

Abstract: Petrographic microscopy is a venerable geological technique that has been used in the service of archaeology at least since the 1930s (eg, Shepard, 1936, 1939) Compared to newer, sophisticated, “high-tech” approaches to the study of the compositional analysis of ceramics (eg, neutron activation or acid extraction), petrography surely rates the appellation “old fashioned”The goal of this chapter is to describe and evaluate critically the current status and potential of ceramic petrography as an approach to the compositional analysis of archaeological ceramics, especially in light of the increasingly widespread and successful application of newer technologies for determining the elemental composition of ceramics that might be seen as rendering petrography obsolete (useful earlier reviews of ceramic petrography may be found in Freestone, 1991 and 1995; Peacock, 1970; Williams, 1983)

Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil)

Abstract: The Araxa complex (16 km 2 ) comprises carbonatites forming a central core and a complex network of concentric and radial dykes as well as small veins; additionally, it includes mica-rich rocks, phoscorites and lamprophyres. Fenites also occur and are represented by Proterozoic quartzites and schists of the Araxa Group. The petrographic study of 130 borehole samples indicates that the complex is basically made up by two rock-types, carbonatites and mica-rich rocks, and subordinately by a third unit of hybrid composition. Carbonatites range chemically in composition, the most abundant type being magnesiocarbonatites. Dolomite and calcite correspond to the chief constituents, but other carbonate phases, including the Ce-group RE minerals, are also recognized. Phosphates and oxides are widespread accessories whereas silicate minerals consist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cumulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rocks mainly contain phlogopite and tetraferriphlogopite as cumulus and intercumulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE and have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-rich rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenites. Similar REE patterns for carbonatites and mica-rich rocks seem to suggest that they are related to a single

Weathering of gneiss in Calabria, Southern Italy

Abstract: The effects of weathering in a Mediterranean climate on the mineralogy and microfabric of Paleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observations show highly weathered rock forms a residual soil. Micromorphological and mineralogical properties of bedrock and saprolite show that the weathering process is characterized by at least two major stages, having two distinct rock microfabrics. In the first stage, the morphological features of the original rock are preserved and weathering is manifested mainly by microfracturing, and large portions of the rock remain unaltered. The second stage of weathering involves further development of microcracks and progressive chemical attack on the minerals. This latter stage occurs along both compositional and microstructural discontinuities, with etch pitting of feldspar, and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, and iron-bearing garnet. The determination of quantitative petrographic indices provides a measure of the various stages of weathering.

Topaz–albite granites and rare-metal mineralization in the Limu District, Guangxi Province, southeast China

Abstract: The topaz–albite granites of the Limu district are ultra-acidic, peraluminous, Li–F–Na-rich and Sn–Ta–Nb-mineralized. A distinct vertical zonation is developed in the granite stocks. There is an upward, systematic transition from leucocratic microcline-albite granite, through albite-microcline granite, topaz–albite granite, pegmatite stockscheider and layered pegmatite–aplite dikes, to K-feldspar–quartz veins and lepidolite–fluorite stringers in the country rocks. Snow-ball textures, homogeneous distribution of rock-forming and accessory minerals, disseminated mineralization, and melt inclusions in quartz, topaz, and albite are typical features indicative of their crystallization from the late stage Li–F–Na-rich and Sn–Ta–Nb-bearing residual granitic melts at a higher intrusion level. A comparison with rare-metal-bearing pegmatite, ongonite, topaz rhyolite and obsidian glass from other regions shows the worldwide existence of these specialized residual melts. Their emplacement and crystallization in a variety of geological environments result in the formation of a series of chemically similar rocks with different petrographic textures and mineral associations. The topaz–albite granites and associated mineralization in the Limu district provide a good example of highly evolved magmatic fractionation in the F-rich granite system and fluid/melt partitioning behavior of rare-metal elements during magmatic-hydrothermal evolution.

Chemical variation of the large Apollo 15 olivine-normative mare basalt rock samples

Abstract: Most chemical analyses of Apollo 15 olivine-normative mare basalts have been conducted on subsamples of 4 g) to obtain greater whole-rock representivity. These subsamples were individually ground and homogenized, and splits were taken for analysis. Furthermore, we used both X-ray fluorescence and neutron activation techniques to analyze for a comprehensive set of elements suitable for petrogenetic interpretation. The analyses show that the samples form a single coherent suite with almost all of the variation corresponding with olivine control (15% range). A few of the coarser rocks are not quite represented even at this sampling size. The analyses show that the rocks are individually distinct and that analyses are not merely of unrepresentative pieces of a single rock, undifferentiated rock unit, or rocks differing only by short-range unmixing of residual fluids. The petrographic features, including the low abundance of olivine and its small size, and the vesicularity of even some of the coarser samples, show that the olivine that controlled the chemical variation is not accumulated in any of the rocks. The Apollo 15 olivine-normative mare basalts were extruded as a series of magmas from a shallow but not locally surficial, olivine-accumulating magma system and formed a sequence of thin flows. A greater understanding of the relationships within and among other mare basalt sequences would be obtained by obtaining comprehensive chemical analyses on splits taken from subsamples of 5 g of all rocks large enough to obtain such subsamples.

Partial Melting of High-P-T Metapelites from the Tshenukutish Terrane (Grenville Province): Petrography and U-Pb Geochronology

Abstract: Kyanite-bearing metapelites from the Tshenukutish terrane INTRODUCTION (Manicouagan Imbricate zone, Grenville Province) display evidence Partial melting of metapelites has been extensively inof anatexis by means of dehydration melting of micas. These vestigated at lowto mid-P granulite-facies conditions, in rocks were metamorphosed during a Grenvillian high-P–T crustal the stability field of sillimanite, but studies of these rocks thickening event with monazite ages ranging from 1040 to at higher pressures are relatively few (Barbey et al., 1990; 1017 Ma. Samples that best preserve textural differences between Godard et al., 1996). However, high-P granulites occur former melt and restitic domains provide evidence for dehydration in several continental collision belts (e.g. European Vamelting of white mica at>1400–1600 MPa, followed by extensive riscides: Vielzeuf & Pin, 1989; Carswell & O’Brien, to complete dehydration melting of biotite up to temperatures in 1993; O’Brien et al., 1997; Willner et al., 1997; Grenville excess of 850°C, and subsequent crystallization of the melt at lower Province: Indares, 1995; Indares et al., 1998; eastern pressures (>1100 MPa) during cooling to>750°C. Dehydration Himalayas: Liu & Jong, 1997) suggesting that crustal melting of biotite was accompanied by growth of garnet with rocks involved in orogenesis may attain metamorphic distinctive Ca, Y and Cr patterns, locally around subsolidus garnet. temperatures well in excess of 800°C at a wide range of In addition, garnet in one sample displays evidence of partial pressures within the kyanite field. Under these conditions, consumption before the latest stage of growth. This is consistent metapelites are expected to undergo extensive dewith dehydration melting of phengite instead of muscovite, according hydration melting of micas, and study of such rocks has to a theoretically defined reaction: Phe + Grt + Qtz = Bt + the potential to provide new constraints on partial melting Ky + Kfs + L. In all samples, melt crystallization was in deeply buried crustal segments. accompanied by growth of retrograde biotite and was completed at A P–T framework of melting reactions involving mustemperatures above the stability field of white mica. In samples that covite and biotite in the kyanite field can be established achieved textural equilibrium during or after melt crystallization by using available experimental data (LeBreton & only the composition of garnet provides some hints about the partial Thompson, 1988; Vielzeuf & Holloway, 1988; Carmelting history. rington & Harley, 1995) extrapolated to high pressures

Alteration associated with gold deposition at the Getchell, Carlin-type gold deposit, north-central Nevada

Abstract: Wall-rock alteration at the Getchell underground deposit was examined to determine the effects of Au-bear-ing fluids on host lithologies and the relationship between K-bearing alteration minerals and Au deposition. The major, minor, and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the Getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike. Each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high-grade ore to moderately altered rock or waste rock. Analyses were obtained for 39 elements, 10 oxides, and loss on ignition, using multiple techniques. Petrographic studies were integrated with geochemistry and X-ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes. Geochemical analyses indicate that Ti, Al, Zr, and Th behave as immobile elements. Immobility isocon diagrams show that significant Hg, Sb, Se, Te, Tl, and Cs are typically added to the wall rocks with Au. Fe is im-mobile in some transects, added to wall rocks in others, and removed from rhyodacite host rocks. Minor S and W are usually added to the rock, whereas minor Cu and Mo are variably added or removed. SiO 2 appears to be either added during alteration or remain constant. Elements that are consistently removed during alteration include Ca, Mn, Sr, and Sc and minor Mg, Ba, and K. Na was significantly removed from the rhyodacite. Au correlates positively with Ag, Hg, Sb, Se, Si, Te, and Tl and negatively with Ca, Mn, and Sc. Element fluxes correspond with observations of mineral abundances and show that sulfidation and decar-bonitization are spatially associated with gold mineralization. Au, Hg, Sb, Tl, Cu, and possibly Te and W were incorporated in trace element-rich pyrite as host-rock Fe was sulfidized. Fe was added to mineralized rocks in some transects, indicating that pyritization occurred in addition to sulfidation. Cs is a component of galkhaite, a trace mineral that is relatively common in some parts of the Getchell system. Decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of Ca, Mn, Sr, and Sc. Although SiO 2 , As, and S are significantly added to Carlin-type systems, they do not appear as major added components in the isocon diagrams. This is because the halos for these components are much larger than the areas examined by the sample transects. X-ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals, including illite and montmorillonite. Kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks. Montmorillonite does not correlate with gold and textures are consistent with pore-filling deposition of montmorillonite following Au deposition. Illite abundance exhibits a positive correlation with Au ( r 2 = 0.80). Intergrowths of illite with ore-stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the Au-related hydrothermal event. Mineral abundances and textures suggest that illite formed by alteration of K feldspar during Au deposition. Results show that dating of K-bearing phases in sedimentary rocks, other than illite, would provide ages unrelated to gold deposition.

Compositional zoning of fluid inclusions in the Archaean Junction gold deposit, Western Australia: A process of fluid ‐ wall‐rock interaction?

Abstract: The Junction gold deposit, in Western Australia, is an orogenic gold deposit hosted by a differentiated, iron‐rich, tholeiitic dolerite sill. Petrographic, microthermometric and laser Raman microprobe analyses of fluid inclusions from the Junction deposit indicate that three different vein systems formed at three distinct periods of geological time, and host four fluid‐inclusion populations with a wide range of compositions in the H2O–CO2–CH4–NaCl ± CaCl2 system. Pre‐shearing, pre‐gold, molybdenite‐bearing quartz veins host fluid inclusions that are characterised by relatively consistent phase ratios comprising H2O–CO2–CH4 ± halite. Microthermometry suggests that these veins precipitated when a highly saline, >340°C fluid mixed with a less saline ≥150°C fluid. The syn‐gold mineralisation event is hosted within the Junction shear zone and is associated with extensive quartz‐calcite ± albite ± chlorite ± pyrrhotite veining. Fluid‐inclusion analyses indicate that gold deposition occurred during the unmixing ...

Depositional environments of Quaternary lacustrine travertines and stromatolites from high- altitude Andean lakes, northwestern Argentina

Abstract: Four distinctive depositional subenvironments of fossil travertines and stromatolites are identified in three high-altitude (3500-4000 m above sea level) lacustrine basins: El Peinado, San Francisco (Las Coladas Salar subbasin), and Las Peladas (southern Andean Altiplano, northwestern Argentina). These late Quaternary occurrences are character- ized using geomorphological, sedimentological, petrographic, and stable isotopic data. Stromatolites of cyanobacterial origin only develop in shallow lacustrine margins of El Peinado basin. In the same basin, macrophytic travertines occur both near thermal spring seepage areas along the lake margin as in situ facies and in littoral lacustrine environments up to water depths of several metres as phytoclastic travertine facies. The stromatolites and macrophytic travertines have relatively heavy δ 18 O compositions, suggesting initial 16 O-depleted waters and (or) evaporation effects through time. Their high δ 13 C compositions are interpreted as a reflection of intense CO2 evasion from the thermal groundwaters feeding the lakes. Similar laminated travertine facies, with no petrographic evidence for biotic origin, occur in both Las Coladas and Las Peladas basins. Neither petrographic nor isotopic data alone can differentiate between these two cases. Besides, diagenetic overprint in Las Peladas facies precludes the use of isotopic values as original isotopic signatures. However, the depositional environmental conditions defined by the geomorphological and sedimentological features are different. Laminated aragonitic crusts in Las Coladas basin formed in a shallow, saline lake and are associated with shoreline and terrace deposits cemented by aragonite. These travertine crusts represent periods of spring, 16 O-rich dis- charge to the lake, as suggested by the lighter oxygen isotopic compositions. In contrast, travertines from Las Peladas occur as laminated calcitic and aragonitic units intercalated at the top of fining-upward sequences composed of con- glomerates, sandstones, and intraclastic limestones. Sedimentological data suggest that these travertines originated in fluvial-influenced lake margins during low lake-level episodes. 1283

Genesis of Dioctahedral Phyllosilicates During Hydrothermal Alteration of Volcanic Rocks: I. The Golden Cross Epithermal Ore Deposit, New Zealand

Abstract: To characterize the evolution of dioctahedral interstratified clay minerals in the Golden Cross epithermal deposit, New Zealand, hydrothermally altered volcanic rocks containing the sequence smectite through illite-smectite (I-S) to muscovite were examined by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopies (TEM/AEM). XRD analyses of 30 oriented clay samples show a broad deposit-wide trend of increasing illite content in I-S with increasing depth and proximity to the central vein system. Six representative samples were selected for SEM/TEM study on the basis of petrographic observations and XRD estimates of I-S interstratification. Ca and Na are the dominant interlayer cations in smectite, but as the proportion of illite layers in I-S increases, so does the K content and (IVAl + VIAl)/Si ratio. Layers and packets tend to flatten and form larger arrays, reducing the amount of pore space. Smectite coexists with (R = 1) I-S, rather than being (R = 0) I-S where R is the Reichweite parameter. The highest alteration rank samples contain discrete packets of mica to ∼300 A thick, but a limited chemical and structural gap exists between illite, which is intermediate in composition between common illite and muscovite, and illite-rich I-S. Selected-area electron diffraction (SAED) patterns of mica show that the 1M polytype dominates, rather than the common 2M1 polytype. Petrographic, SEM, and TEM data imply that all phyllosilicates formed via neoformation directly from fluids. Relatively mature I-S and micas form simultaneously, without progressing through the series of transformations that are commonly assumed to characterize diagenetic sequences during burial metamorphism in mud-dominated basins. Although the overall distribution of clay minerals is consistent with temperature as a controlling variable, local heterogeneities in the distribution of clay minerals were controlled by water/rock ratio, which varied widely owing to different rock types and fracture control.

Zeolites in Burial Diagenesis and Low-grade Metamorphic rocks

Abstract: Many zeolite assemblages originated during burial and subsequent low-grade metamorphism of rocks, particularly those of volcaniclastic origin. Eskola (1939) first suggested this type of zeolitization and applied the term “zeolite facies” to assemblages formed under such low-temperature and low-pressure conditions. Although several definitions of the “zeolite facies” have been proposed over the years (e.g. Fyfe et al. 1958), the phrase as used here includes both low-grade metamorphic assemblages and zeolite assemblages formed successively during burial. The regional occurrence of zeolites in sedimentary rocks was first recognized by Coombs (1954), who made detailed petrographic studies of Triassic tuffaceous greywackes and tuffs of andesitic to rhyolitic in composition in the Taringatura area of New Zealand. Coombs recognized a zonal arrangement of heulandite and analcime near the top of the sequence and heulandite and laumontite near the bottom. He also recognized a similar progressive zeolitization with depth in clastic sediments in other orogenic belts. The zeolite assemblages described by Coombs represent a rather advanced stage of alteration, as opposed to an earlier stage of zeolitization that was subsequently described in Neogene silicic volcaniclastic rocks from Japan. In the latter rocks, zones (from top to bottom) of fresh glass, clinoptilolite + mordenite, analcime + heulandite, laumontite, and albite were recognized (Utada 1965, 1970). Progressive zeolitization has also been reported as a result of the thermal effects of intrusive masses (see Seki et al. 1969) and around volcanic calderas (Utada and Ito 1989). Recently, Boles (1991) described the low-temperature formation of laumontite and stilbite in fractures and faults related to post-Jurassic folding and uplift of the Southland Syncline, New Zealand. The physico-chemical conditions of zeolite crystallization in burial diagenesis and low-grade metamorphic environments can vary widely. Both borehole data (see Iijima 1995) and experimental results (Liou 1970; 1971a,b,c …