Showing papers on "Petrography published in 2013"

Ore Genesis and Hydrothermal Evolution of the Baiyinnuo’er Zinc-Lead Skarn Deposit, Northeast China: Evidence from Isotopes (S, Pb) and Fluid Inclusions

Abstract: The Baiyinnuo’er zinc-lead deposit (32.74 Mt at 5.44% Zn, 2.02% Pb, and 31.36 g/t Ag), located in the south segment of the Great Xing’an Range, is the largest Zn-Pb deposit in northern China. Skarn and orebodies mainly occur between the different units of the Huanggangliang Formation, or within the contact zone between the intrusions and Permian marble. Several phases of igneous rocks exposed within the mining areas, and among them the Yanshanian plutonic rocks, which intruded into limestone of the early Permian Huanggangliang Formation, are interpreted to be the source of ore, since their Pb isotope compositions (206Pb/204Pb = 18.25–18.35, 207Pb/204Pb = 15.50–15.56, and 208Pb/204Pb = 38.14–38.32) are highly consistent with the sulfides, including sphalerite, galena, and chalcopyrite (206Pb/204Pb = 18.23–18.37, 207Pb/204Pb = 15.47–15.62, and 208Pb/204Pb = 37.93–38.44). Sulfur isotope values of the sulfides give a narrow δ 34S interval of −6.1 to −4.6‰ (mean = −5.4‰, n = 15), suggesting the ore-forming fluid is of magmatic origin. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observation, which are the preore stage (garnet-clinpyroxene-wollastonite-magnetite ± sulfides), the synore stage (sulfides-epidote-quartz-calcite ± garnet), and the postore stage (calcite-chlorite-quartz-fluorite). Several fluid evolution episodes can be inferred from microthermometric results at the Baiyinnuo’er Zn-Pb deposit: 1. Immiscibility: Preore-stage coexistence of halite-bearing brine inclusions (S1-type, ~44 wt % NaCl equiv) and vapor-rich fluid inclusions (V-type) sharing the same homogenization temperatures (~470°C) confirms that fluid unmixing occurred under lithostatic pressures of ~400 bars (~1.5 km), and the brine is considered to account for most prograde skarn minerals (e.g., clinopyroxene). 2. Overpressure trapping: Preore-stage brine inclusions homogenized by halite dissolution (S2-type) postdated the immiscible assemblages. This type of inclusions is characterized by high but variable (minimum) trapping pressures (150–3,000 bars) relative to S1-type inclusions and can be explained as a result of entrapment under overpressuring condition. 3. Boiling: The presence of both vapor and liquid inclusions (i.e., V- and L-type) in the same assemblages within synore-stage quartz, calcite, and sphalerite indicates the occurrence of fluid boiling (~350°C), at hydrostatic pressures of ~150 bars, and depth of ~1.5 km), which resulted in large-scale mineralization in the Baiyinnuo’er Zn-Pb deposit. 4. Mixing with external fluids: Fluid inclusions scattered within postore-stage calcite or secondary trails in synore-stage minerals show low homogenization temperatures (<260°C) and both decreasing (for L-type) and increasing (for CaCl2-bearing inclusions, i.e., Lc-type) trends for salinities as homogenization temperatures decrease, implying addition of both meteoric water (low-temperature, low-salinity) and basinal brines (low-temperature, Ca-rich), respectively. Systematic fluid inclusion studies also indicate that the mineralization-related fluid is of magmatic origin. Prograde minerals formed during the preore-stage fluid immiscibility while sulfides deposition occurred during the synore-stage fluid boiling. Mixing with external fluids began as the hydrothermal system cooled to <300°C, when the main metal precipitation process had ended.

Quantitative Mineralogical Characterization of Karst Bauxite Deposits in the Southern Apennines, Italy

Abstract: The southern Italian karst bauxite deposits occur at a Late Cretaceous hiatus on a Bahamian-type carbonate platform succession cropping out in the southern Apennines, and are mainly located in the Abruzzi Mountains, in Apulia (Gargano peninsula and Murge area), and in Campania. The bauxites of southern Italy are presently uneconomic, but can be considered as a model analogue for economic karst bauxite ores. The texture of the bauxite deposits is mainly oolitic to pisolitic but can also be arenitic-conglomeratic, suggesting reworking of evolved lateritic soils. The minerals boehmite and hematite are more abundant in the ooids, whereas kaolinite is found at high levels in the groundmass, and titanium oxides are also present. Furthermore, small amounts of rare earth elements have been detected in most of the bauxite occurrences. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and geochemical analyses on several of the deposits confirmed the diagenetic texture of the bauxite, and provided an estimate of the mineralogy of the different components and their chemical composition. The mineralogical and petrographic characteristics of bauxite are crucial for the Bayer process and can influence its economic evaluation: both the particle size distribution in the ore and the presence of reactive silica phases associated with Ti and Fe oxides can reduce the efficiency of processing. Quantitative analyses combining different methods (X-ray diffraction [XRD]-Rietveld quantitative phase analysis [QPA] and QEMSCAN ® ) have been carried out on a limited number of samples from two of the bauxite districts of the Campania region: the Matese Mountains and the Caserta province. QEMSCAN ® analysis allowed a more rapid quantification of mineralogy (including trace detrital phases) and assessment of the individual textural characteristics of the bauxite lithotypes, showing a detailed image of the distribution of economic and noneconomic minerals and their intergrowths. This methodology can augment or replace other time-consuming quantitative phase analyses for mineralogical studies of bauxites, provided that the species identification protocol (SIP) database has been carefully validated by preliminary use of XRD and SEM-EDS. Both XRD (QPA) and QEMSCAN ® analytical techniques can be complementary for bauxite ore evaluation, and a very powerful tool for exploitation and mineral processing.

13 SEM Observations on Ion-milled Samples of Devonian Black Shales from Indiana and New York: The Petrographic Context of Multiple Pore Types

Abstract: Middle Devonian Marcellus Shale and late Devonian New Albany Shale samples were argon-ion milled and studied by scanning electron microscope for petrographic features and pore development. Petrographic observations revealed that a finite number of pore types exist in spite of considerable variability in composition, depositional setting, and compaction history. The four major pore types are framework pores, framework shelter pores, dissolution pores within inorganic grains, and organic matter pores. Framework pores occur in open spaces of the grain fabric. The most common framework pores are defined by phyllosilicate (clays, micas) and carbonate grains (biogenic, diagenetic) and also occur in areas with abundant diagenetic silica. Framework pores associated with carbonate grains can be well developed where abundant skeletal debris provides shelter porosity. Phyllosilicate framework pores increase in abundance with increasing clay content, but more critically, their abundance in compacted shales hinges on the presence of pressure shadows generated adjacent to mechanically competent grains (quartz, feldspar, dolomite, calcite, pyrite) that resist compaction. Dissolution pores in inorganic grains were predominantly encountered in association with calcite and dolomite grains. In places, other minerals, such as pyrite, also can show dissolution effects, but do not contribute significantly to overall porosity. Dissolution pores probably reflect decreased pH associated with the formation of carboxylic and phenolic acids at elevated temperatures (about 80 to 120° C). At low carbonate contents (a few percent), dissolution pores constitute only isolated porosity, but in shale intervals that contain abundant carbonate, or where carbonate grains were concentrated into laminae, this pore type may be an important facilitator of gas storage and transmission. Pores within organic matter are maturity dependent and restricted to amorphous organic matter (bituminite/amorphinite) in thermally mature samples (>0.6%Ro). Grain and fabric shrinkage during core storage and sample processing produced pore artifacts as well (false pores), and their correct identification is critical for accurate petrographic pore assessment.

A palaeo Tibet–Myanmar connection? Reconstructing the Late Eocene drainage system of central Myanmar using a multi-proxy approach

Abstract: Strain resulting from the collision of India with Asia has caused fundamental changes to Asian drainage patterns, but the timing and nature of these changes are poorly understood. One frequently proposed hypothesis involves the connection of the palaeo Tsangpo drainage to a precursor to the Irrawaddy River of central Myanmar in the Palaeogene. To test this hypothesis, we studied the provenance of Palaeogene fluvio-clastic sedimentary rocks that crop out in central Myanmar, namely the Late Middle Eocene–Early Oligocene Pondaung and Yaw Formations. Isotopic analysis on bulk-rock and petrographic data indicate a primary magmatic arc source, and a secondary source composed of recycled, metamorphosed basement material. Although the exact location of both sources is hardly distinguishable because Burmese and Tibetan provinces share common lithological features, the presence of low-grade metamorphic fragments, the heterogeneity in Sr–Nd isotopic values of bulk sediments and westward-directed palaeoflow orientations indicate a proximal source area located on the eastern Asian margin. Central Myanmar was the locus of westward-prograding deltas opening into the Indian Ocean, supplied by the unroofing of an Andean-type cordillera that extended along the Burmese margin. We found no evidence to support a palaeo Tsangpo–Irrawaddy River, at least during the Late Eocene. Supplementary material: Data locations, and isotopic and petrographic results are available at www.geolsoc.org.uk/SUP18655.

The influence of the characteristics of quartz and mineral deterioration on the strength of granitic dimensional stones

Abstract: It is important to have a thorough knowledge of the petrographic characteristics of rocks to evaluate and understand their mechanical behavior. This paper deals with the influence of the texture and the mineral characteristics of nine Portuguese granites employed as dimensional stones. This study evaluates different mineral characteristics, such as mineral deterioration, grain size, quartz fissuration and the contacts between quartz and others mineral groups. The main differences in the studied granites were identified, clarifying the relationship between the petrographic characteristics. The consequences on their respective mechanical behavior were subsequently evaluated. The physical and textural characteristics of quartz in the studied granites have proven to be very important for understanding the behavior of granitic rocks under compressive stress. Uniaxial compressive strength values are not proportional to the quartz–feldspar ratio or quartz content and there is a tendency for strength to decrease with the increase in quartz. This could be related to the increase in quartz-quartz contacts and the decrease in the rock capacity for accommodating the deformation.

Petrology and geochemistry of igneous inclusions in recent Merapi deposits: a window into the sub-volcanic plumbing system

Abstract: Recent basaltic-andesite lavas from Merapi volcano contain abundant and varied igneous inclusions suggesting a complex sub-volcanic magmatic system for Merapi volcano. In order to better understand the processes occurring beneath Merapi, we have studied this suite of inclusions by petrography, geochemistry and geobarometric calculations. The inclusions may be classified into four main suites: (1) highly crystalline basaltic-andesite inclusions, (2) co-magmatic enclaves, (3) plutonic crystalline inclusions and (4) amphibole megacrysts. Highly crystalline basaltic-andesite inclusions and co-magmatic enclaves typically display liquid–liquid relationships with their host rocks, indicating mixing and mingling of distinct magmas. Co-magmatic enclaves are basaltic in composition and occasionally display chilled margins, whereas highly crystalline basaltic-andesite inclusions usually lack chilling. Plutonic inclusions have variable grain sizes and occasionally possess crystal layering with a spectrum of compositions spanning from gabbro to diorite. Plagioclase, pyroxene and amphibole are the dominant phases present in both the inclusions and the host lavas. Mineral compositions of the inclusions largely overlap with compositions of minerals in recent and historic basaltic-andesites and the enclaves they contain, indicating a cognate or ‘antelithic’ nature for most of the plutonic inclusions. Many of the plutonic inclusions plot together with the host basaltic-andesites along fractional crystallisation trends from parental basalt to andesite compositions. Results for mineral geobarometry on the inclusions suggest a crystallisation history for the plutonic inclusions and the recent and historic Merapi magmas that spans the full depth of the crust, indicating a multi-chamber magma system with high amounts of semi-molten crystalline mush. There, crystallisation, crystal accumulation, magma mixing and mafic recharge take place. Comparison of the barometric results with whole rock Sr, Nd, and Pb isotope data for the inclusions suggests input of crustal material as magma ascends from depth, with a significant late addition of sedimentary material from the uppermost crust. The type of multi-chamber plumbing system envisaged contains large portions of crystal mush and provides ample opportunity to recycle the magmatic crystalline roots as well as interact with the surrounding host lithologies.

A Comparison of Carlin-type Gold Deposits：Guizhou Province,Golden Triangle,Southwest China,and Northern Nevada,USA

Abstract: Several Au deposits in Guizhou Province,southwest China,described as being similar to the highly productive Carlin-type gold deposits in northern Nevada.USA,were examined to identify similarities and differences between the two districts.Samples were collected along transects from lowto high-grade rock,where possible,and fram stockpiles at the Shuiyindong,Zimudang,Taipingdong,Yata and Jinfeng(formerly Lannigou)deposits.Methods used to examine ore and alteration minerals included hand-sample description:reflectance spectroscopy using an ASD Terraspec spectrometer;analyses of hand samples by carbonate staining with Alizaren red and potassium ferricyanide;transmitted and reflected light petrography;chemical analyses,mineral identification,and imaging using a JEOL.JSM-5610 scanning electron microscope:and quantitative chemical analyses using a JEOL JXA-8900 electron probe microanalyzer.Geochemical analyses of hand samples for 52 elements were done by ALS Chemex.Results indicate both similarities and differences between the two districts.Both districts have similar geologic histories.and deposits at both locations appear to have formed as a result of similar tectonic events.though the district in southwest China lacks evidence of eoeval felsic igneous activity;however,the ore-stage minerals and the fluids that produced the minerals and deposits have some significant differences.The Nevada deposits were dominated by fluid-rock reaction in which host rock Fe was sulfidized to form Au-bearing pyrite.Although ore fluids sulfidized host rock Fe in the Cuizhou deposits.the timing of Fe metasomatism is unknown,so whether the deposits formed in response to sulfidation or pyritization is unclear.Fluid-rock reaction between an acidic,aqueous fluid and highly reactive calcareous rocks in Nevada caused extensive decarbonatization of host rocks,jasperoid replacemerit of carbonate minerals,and alteration of silty rock components to illite and kaolinite.In Guizhou,CO2-bearing ore fluids with temperatures and pressures approaching 10

Geochronology of Metasomatic Events

Abstract: In order to date any geological event, suitable mineral geochronometers that record that and only that event must be identified and analyzed. In the case of metasomatism, recrystallisation is a key process that controls both the petrology and the isotopic record of minerals. It can occur both in the form of complete neocrystallisation (e.g. in a vein) and in the form of pseudomorphism, whereby dissolution/reprecipitation at the submicroscopic scale plays a central role. Recrystallisation may be complete or not, raising the possibility that relicts of a pre-metasomatic assemblage may be preserved. Because recrystallisation is energetically less costly at almost any temperature than diffusion, and because radiogenic isotopes (except 4He) never diffuse faster than major elements forming the mineral structure, there is a strong causal link between petrographic relicts and isotopic inheritance (as demonstrated for zircon, monazite, titanite, amphibole, K-feldspar, biotite, and muscovite). Metasomatic assemblages commonly contain such mixtures between relicts and newly formed phases, whose geochronology is slightly more complex than that of simple, ideal systems, but can be managed by techniques that have become routine in the last decade and which are described in this chapter. Because of its crucial role in controlling the isotope systematics, the petrogenesis of a mineral needs to be understood in extreme detail, especially using microchemical analyses and micro-imaging techniques, before mineral ages can be correctly interpreted. As the occurrence of recrystallization is limited by the availability of water, minerals act as “geohygrometers” that allow constraints to be placed on the nature and age of fluid circulation episodes, especially metasomatic events.

Moisture expansion associated to secondary porosity: an example of the Loseros Tuff of Guanajuato, Mexico

Abstract: The old mining city of Guanajuato in middle Mexico preserves one of the most important historical legacies in colonial buildings, the UNESCO declared the city World Heritage Site in 1988. Practically all the colonial constructions were built with natural stones from the neighbourhood, of which stands a greenish to reddish vulcanite, called Loseros Tuff. Although the Loseros Tuff is widely used in historical buildings in the city. It shows significant deterioration and weathering effects, principally in the parts where the tuff shows a coarse grain size. The petrographic, petrophysical, mineralogical and geochemical properties of the Loseros Tuff were analysed in order to determine the causes, effects, behaviour and response to deterioration of this volcanic rock. The results of the investigations suggest that in addition to the parameters like the grain size and the porosity properties, the pore radii distribution is decisive for the effectiveness of porosity and the water transport into the rock. It is recognized that once the liquid water invades the rock the dissolution of the matrix occurs, which is accompanied by a sudden moisture expansion favoured by the newly formed secondary porosity and the high content of expandable clay minerals.

Diagenetic Implications of Stylolitization in Pelagic Carbonates, Canterbury Basin, Offshore New Zealand

Abstract: Stylolites are irregular discontinuity surfaces that are thought to result from localized stress-induced dissolution during burial or tectonic compression. The genesis of stylolites and the controls on stylolitization are still debated, and the interplay between stylolitization, generation of carbonate-rich fluids, diagenetic fluid flow within fractures and matrix, cementation, and porosity modifications is complex. All of these processes have important diagenetic effects potentially altering the intrinsic properties of the host rock, with implications for hydrocarbon exploration and water resources in aquifers. We investigate the process of stylolitization by a macroscopic, petrographic, and geochemical study of pressure-solution features in Eocene to early Oligocene limestones in cores from the Integrated Ocean Drilling Program (IODP) Hole 317-U1352C (Canterbury Basin). The results indicate correlations among stylolite amplitude, stylolite density, and siliciclastic content of the host rock. These relationships are interpreted to suggest that siliciclastic content in carbonate rocks increases heterogeneity, which in turn impacts stylolite nucleation. Moreover, the geochemical data support that clay along the stylolite is not authigenic, but a relict from impurities in the limestone host rock. The statistical approach of stylolite spacing used in this study, which is different than previous studies, reinforces the model of random occurrence of stylolites in (carbonate) lithologic units. This study shows that local stresses in tectonically passive areas may allow the formation of rare oblique stylolites with peaks perpendicular to the stylolite plane. Estimation of the amount of limestone dissolved during stylolitization (minimum 7% to 12% of the depositional limestone) and the volume of sparite in the host rock (up to 1.6% of the compacted limestone) suggests that the pressure-solution fluids cemented the micropores, reducing porosity to about 10% (from a common porosity of about 40% in mechanically compacted chalk). This study thus highlights the importance of microporosity as a sink for burial cements, an observation difficult to make in thin-section. Analysis of stylolite infills at Site U1352 shows no evidence that the stylolites acted as conduits for diagenetic fluids, unless if the fluid was host-rock buffered and thus its chemistry indistinguishable from that of the host rock. The link between siliciclastic content in carbonate rocks and the spacing and amplitude of stylolites improves understanding of burial compaction processes. Predicting the morphology and spacing of stylolites in the subsurface can be used to identify baffles to cross-stylolite fluid flow or cementation (of microporosity) influencing heterogeneity of petrophysical properties in (subsurface reservoir) host rocks.

Petrography and geochemistry of impactites and volcanic bedrock in the ICDP drill core D1c from Lake El'gygytgyn, NE Russia

Abstract: The 3.6 Ma old and 18 km diameter El'gygytgyn impact structure in NE Siberia was drilled in 2008/09 by ICDP (International Continental Scientific Drilling Program). A 517 m long core hole (D1c) was drilled into the outer flank of the central uplift structure, with an overall core recovery of approximately 63%. Thereby, approximately 315 m lake sediments and approximately 202 m impactites were recovered. Here, we present a detailed petrographic and geochemical assessment of the impact breccia and bedrock sections in this core. The 97 m long lower bedrock unit (517–420 m below lake floor [blf]) consists of an ignimbrite. In the overlying upper bedrock unit (420–390 mblf), the core recovered a sequence of similar ignimbrite and several decimeters of mafic rocks. We interpret these units as rocks that are located close to their former, preimpact position, but have been somewhat rotated due to collapse of the central uplift (i.e., it represents parautochthonous basement). From about 390 to 328 mblf occurs a suevite package with an impact melt poor, clast-dominated matrix, and lithic and mineral clasts that cover the entire range of volcanic target rocks known from the El'gygytgyn region. All stages of shock metamorphism (unshocked to melted) were observed in clasts, and in microclasts of the matrix, of suevite from different depths. Immediately below this package, at the contact to the underlying bedrock, occurs a 1 m wide sheared zone within vitrophyric ignimbrite, which we consider the actual crater floor. The uppermost approximately 12 m, from 328–316 mblf depth, seem to comprise reworked suevite, consisting of a mixture of sediments and suevite with more and, on average, stronger shocked minerals than found in the main suevite unit. This includes a small component of glassy spherules and impact melt fragments. Toward the top of this unit, lake sediments progressively become the dominant material in this section. We assume that this unit contains a fallback component from the ejecta plume that was mixed with the first sediments of the postimpact crater lake, and possibly some rocks that slumped off the inner crater wall—similar to a thin layer at the base of the sediment section of borehole LB-5A recovered in Lake Bosumtwi (Ghana).

Tinderet volcano, Kenya: an altered natrocarbonatite locality?

Abstract: The Tinderet volcano (19.9 to 5.5 Ma), located within the Kavirondo rift in Kenya, contains blocks of carbonatite lavas with calcite, minor apatite, fluorite, spinel-group minerals, accessory perovskite and ‘plumbopyrochlore’; nyerereite is present as inclusions in the perovskite. At least four types of calcite are present in the carbonatite lavas; they differ in morphology, composition and origin. The dominant variety is secondary type-II calcite, which is enriched in sodium (up to 1.1 wt.% Na2O) and strontium (up to 1.3 wt.% SrO). The spinel-group minerals are manganese-bearing and include Mn-rich magnetite, magnesioferrite and jacobsite. Oxygen isotope data for bulk carbonatite samples (δ18O = +16.2‰ to +22.6‰ VSMOW) support a low crystallization temperature for the secondary calcite. Petrographic, mineralogical and isotopic data indicate that the Tinderet carbonatites are similar to natrocarbonatites from the Oldoinyo Lengai and Kerimasi volcanoes that have altered and recrystallized to form calcite carbonatites. These data support the hypothesis that some of the Tinderet carbonatites were originally alkali-rich rocks which contained primary nyerereite.

Linking process, dimension, texture, and geochemistry in dolomite geobodies: A case study from Wadi Mistal (northern Oman)

Abstract: Understanding the distribution and geometry of reservoir geobodies is crucial for net-to-gross estimates and to model subsurface flow. This article focuses on the process of dolomitization and resulting geometry of diagenetic geobodies in an outcrop of Jurassic host rocks from northern Oman. Field and petrographic data show that a first phase of stratabound dolomite is crosscut by a second phase of fault-related dolomite. The stratabound dolomite geobodies are laterally continuous for at least several hundreds of meters (1000 ft) and probably regionally and are one-half meter (1.6 ft) thick. Based on petrography and geochemistry, a process of seepage reflux of mesosaline or hypersaline fluids during the early stages of burial diagenesis is proposed for the formation of the stratabound dolomite. In contrast, the fault-related dolomite geobodies are trending along a fault that can be followed for at least 100 m (328 ft) and vary in width from a few tens of centimeters to as much as 10 m (1–33 ft). Petrography, geochemistry, and high homogenization temperature of fluid inclusions all point to the formation of the dolomite along a normal fault under deep burial conditions during the Middle to Late Cretaceous. The high 87Sr/86Sr ratio in the dolomite and the high salinity measured in fluid inclusions indicate that the dolomitizing fluids are deep basinal brines that interacted with crystalline basement. The dolomitization styles have an impact on the dimension, texture, and geochemistry of the different dolomite geobodies, and a modified classification scheme (compared to the one from Jung and Aigner, 2012) is proposed to incorporate diagenetic geobodies in future reservoir modeling.