Quartz-chlorite Schist Research Articles

Origin, tectonic environment and age of the Bibole banded iron formations, northwestern Congo Craton, Cameroon: geochemical and geochronological constraints

AbstractThe newly discovered Bibole banded iron formations are located within the Nyong Group at the northwest of the Congo Craton in Cameroon. The Bibole banded iron formations comprise oxide (quartz-magnetite) and mixed oxide-silicate (chlorite-magnetite) facies banded iron formations, which are interbedded with felsic gneiss, phyllite and quartz-chlorite schist. Geochemical studies of the quartz-magnetite banded iron formations and chlorite-magnetite banded iron formations reveal that they are composed of >95 wt % Fe2O3 plus SiO2 and have low concentrations of Al2O3, TiO2 and high field strength elements. This indicates that the Bibole banded iron formations were not significantly contaminated by detrital materials. Post-Archaean Australian Shale–normalized rare earth element and yttrium patterns are characterized by positive La and Y anomalies, a relative depletion of light rare earth elements compared to heavy rare earth elements and positive Eu anomalies (average of 1.86 and 1.15 for the quartz-magnetite banded iron formations and chlorite-magnetite banded iron formations, respectively), suggesting the influence of low-temperature hydrothermal fluids and seawater. The quartz-magnetite banded iron formations display true negative Ce anomalies, while the chlorite-magnetite banded iron formations lack Ce anomalies. Combined with their distinct Eu anomalies consistent with Algoma- and Superior-type banded iron formations, we suggest that the Bibole banded iron formations were deposited under oxic to suboxic conditions in an extensional basin. SIMS U–Pb data indicate that the Bibole banded iron formations were deposited at 2466 Ma and experienced metamorphism and metasomatism at 2078 Ma during the Eburnean/Trans-Amazonian orogeny. Overall, these findings suggest that the studied banded iron formations probably marked the onset of the rise of atmospheric oxygen, also known as the Great Oxidation Event in the Congo Craton.

Dehydroxylation of Minerals of Gadisunkapur Area with Special Reference to FTIR and Thermal Study

This paper attempts to explain the presence of OH group for micas along with some sulfate, chlorite, quartz and feldspar minerals. These minerals are found to be associated with barite vein, metasediments (quartz-chlorite schist), metarhyolite and varieties of granitic rocks of Gadisunkapur area of Hungund-Kushtagi Schist belt. The samples were collected in and around Gadisunkapur village of Bagalkote district, Karnataka and further processed to determine OH group anion in different minerals present based on FTIR and Thermal studies such as Thermogravimetric analysis (TGA) and Differential Thermal analysis (DTA). The high temperature dehydroxylation is investigated using this analysis. Furthermore, present study indicates the presence of different extra species (CO2, CO32- and OH-). The results show the incorporation of CO2 into the structures due to heating or enhanced by the dehydroxylation process.

Dehydroxylation of Minerals of Gadisunkapur Area with Special Reference to FTIR and Thermal Study

This paper attempts to explain the presence of OH group for micas along with some sulfate, chlorite, quartz and feldspar minerals. These minerals are found to be associated with barite vein, metasediments (quartz-chlorite schist), metarhyolite and varieties of granitic rocks of Gadisunkapur area of Hungund-Kushtagi Schist belt. The samples were collected in and around Gadisunkapur village of Bagalkote district, Karnataka and further processed to determine OH group anion in different minerals present based on FTIR and Thermal studies such as Thermogravimetric analysis (TGA) and Differential Thermal analysis (DTA). The high temperature dehydroxylation is investigated using this analysis. Furthermore, present study indicates the presence of different extra species (CO2, CO32- and OH-). The results show the incorporation of CO2 into the structures due to heating or enhanced by the dehydroxylation process.

Mineral chemistry and geothermometry of chlorites in relation to physico-chemical conditions of uranium mineralization in the central part of the Singhbhum Shear Zone, eastern India

The Singhbhum Shear Zone (SSZ) in eastern India is one of the major uranium repositories of the country. Some of the important uranium deposits viz., Bagjata, Jaduguda, Bhatin, Narwapahar and Banadungri-Singridungri are juxtaposed in the central part of SSZ. The uranium mineralization in central SSZ is mainly hosted by tourmaline-apatite-magnetite bearing sericite quartz chlorite schist with uraninite being the main radioactive mineral phase. A close spatial relationship exists between uraniferous minerals and chlorite occurrences in central SSZ. Three different types of chlorite assemblages were recognized and their relationship with uranium mineralization was studied in detail to unravel the physico-chemical conditions for precipitation of uranium vis-a-vis chlorite geothermometry and constrains on the chlorite chemistry.Detailed petrographic studies and mineral chemical studies of the host rocks indicate three distinct chlorite type viz. (1) metamorphic chlorite, forming a typical metamorphic assemblage with quartz, muscovite, sericite, biotite and K-feldspar, (2) hydrothermal alteration product of biotite and (3) hydrothermal vein chlorite. The three chlorite types were characterized based on: (1) the range of XFe and AlIV, (2) correlation between Si (apfu) and XFe and (3) compositional diagram between XFe vs. AlIV. Based on the mineral chemistry and thermometry of chlorite 3, the co-genetic uranium mineralization has been suggested to be precipitated from a hydrothermal fluid rich in Fe, Mg and Ti in a reducing environment within a temperature window of about 312 °C–355 °C. A fluid mixing model for the remobilization and reconstitution of this hydrothermal uranium mineralization vis-a-vis chlorite occurrence is postulated and its geothermometry suggest an epithermal mineralization in the study area. A comparative study of chlorites from four important uranium provinces of the world i.e. Jabiluka of Australia, Rozna of Czech Republic, Beaverlodge of Canada and central SSZ of India reveals a similarity in the mineral chemistry of the chlorites associated with uranium mineralization. Thus, textural study of chlorites corroborated with their mineral chemistry and geothermometry is a useful reconnoitre tool for uranium exploration.

Spacial analysis of Lithology and forest in Doddahalla watershed, Chikmagalur district, Vedavati river sub-basin, Karnataka, India

The Lithology are a significant geographical component of this watershed and are from the stratigraphy of topographical opportunity scale goes under Archean to Lower Proterozoic. The various kinds of geography in the investigation region which incorporates Basal Polymict Conglomerate, Granite(Shimoga), Meta basalt and Tuff Migmatites And Granodiorite - Tonalitic Gneiss, Quartz Chlorite Schist With Orthoquartzite and Ultra Mafic Schist. In the investigation zone, Migmatites And Granodiorite - Tonalitic Gneiss are predominant (453.4 km), Quartz Chlorite Schist With Orthoquartzite happened (89.9 sq km), Granite (Shimoga) (46.9 sq km), Ultra Mafic Schist is (2.1 sq km), Basal Polymict Conglomerate and Metabasalt and Tuff (0.9 sq km). By land use land cover information increment in agriculture ranch 12 to 27 %, forest 5 to 6 % and scrub 1 to 11 % all information from 1991 to 2018. Keywords: Granite (Shimoga), Lithology, Land Use Land Cover and Forest

Neoproterozoic and Early Paleozoic magmatic records from the Chalukou ore district, northern Great Xing’an Range, NE China: Implications for tectonic evolution and Mesozoic Mo mineralization

To constrain the pre-mineralization tectonic evolutionary history and the nature of the magma source of the Chalukou Mo deposit, which is located on the western margin of the Xing’an block, we recognized and studied a period of Neoproterozoic magmatism and a suite of Early Paleozoic volcanic rocks in this ore district. The magmatic zircons of quartz chlorite schist collected from the Wolegen Group yielded a concordant 206Pb/238U age of 685.1 ± 4.5 Ma (MSWD = 0.21), and they show very high εHf(t) values (+15.5 to +17.3) and young Hf modal ages (573–664 Ma) close to the formation age, indicating that the protolith was generated during the Neoproterozoic, and was most likely derived from a depleted mantle region. This Neoproterozoic magmatism might represent an important crustal growth event within the Xing’an Block. The host wallrocks in the Chalukou deposit mainly consist of rhyolite, dacite, andesite and volcanic tuff which have undergone slight deformation and metamorphism. The results of zircon LA-ICP-MS U-Pb dating indicated that they were generated during 472–475 Ma. The results of whole-rock geochemical analysis indicate that they have arc-affinity features characterized by LREE enrichment with moderate to strong fractionation between LREE and HREE (LaN/YbN = 7.7–13.6), intense negative Eu anomalies (δEu = 0.36–0.71), pronounced positive Ba, Th, U, Pb and La anomalies and negative Nb-Ta, P and Ti anomalies. Hf isotopes show various εHf (t) values ranging from −2.0 to +5.9 and slightly old two-stage model ages ranging from 1070 Ma to 1573 Ma. Combined with discrimination diagrams, these data indicate that they were derived from the partial melting of a slightly older lower crust with the addition of a juvenile crust component in an active continental margin setting related to the subduction of the Paleo-Asian Ocean between the Xing’an and Songliao blocks. The existence of Neoproterozoic magmatic activity and Mesoproterozoic lower crustal materials obtained from the Hf model ages of the Paleozoic volcanic rocks in the Chalukou ore district demonstrated that Precambrian basement exists in the Xing’an Block, but its distribution area may be limited. As the largest porphyry Mo deposit in China, the ore-forming materials might have multiple sources, considering the long-term and multiple magmatic events that occurred under different tectonic settings in the ore district. The previous modification and refertilization of the lower crust and/or lithospheric mantle by the subduction of the Paleo-Asian Ocean might be important in the generation of the fertile lower crust, which is the direct source of the ore-related porphyries.

Texture, Microstructure and Geochemistry of Magnetite from the Banduhurang Uranium Mine, Singhbhum Shear Zone, India – Implications for Physico-chemical Evolution of Magnetite Mineralization

Abstract The Singhbhum Shear Zone in eastern India is one of the largest repositories of uranium and copper in India. Besides uranium and copper, apatite-magnetite mineralization is widespread in this shear zone. This study aims at deciphering the physico-chemical evolution of magnetite mineralization in relation to progressive shearing integrating field relations, micro-textures, structures and compositions of magnetite in the Banduhurang uranium mine. Apatite-magnetite ores occur as discrete patches, tongues, and veins in the strongly deformed, fine grained quartz-chlorite schist. Textures and microstructures of magnetite indicate at least three stages of magnetite formation. Coarse-grained magnetite (magnetite-1) with long, rotational, and complex strain fringes, defined by fibrous and elongate quartz, is assigned to a stage of pre-/early-shearing magnetite formation. Medium grained magnetite (magnetite-2), characterized by single non-rotational strain fringe equivalent to the youngest fringe of magnetite-1, grew likely at the mid-/late-stage of shearing. Fine grained magnetite (magnetite-3) is generally devoid of any pressure shadow. This indicates even a much later stage of formation of this magnetite, presumably towards the closing stage of shearing. Some of the magnetite-1 grains are optically heterogeneous with a dark, pitted Cr-Ti-bearing core overgrown by lighter, fresh rim locally containing pyrite, chalcopyrite, and chlorite inclusions. The cores are also locally characterized by high Al and Si content. Homogeneous magnetite-1 is optically and compositionally similar to the overgrowth of heterogeneous magnetite-1. This homogeneous magnetite-1 that grew as separate phase is contemporaneous with the overgrowth on pitted core of heterogeneous magnetite-1. Magnetite-2 is compositionally very similar to homogeneous magnetite-1, but is devoid of sulfide inclusion. Magnetite-3 is generally devoid of any silicate or sulfide inclusion and is most pure with least concentrations of trace/minor elements. The high Al and Si content in some magnetite can be explained by coupled substitution that involves substitution of Si4+ for Fe3+ in the tetrahedral sites and Fe2+ for Fe3+ in the octahedral sites, with a simple substitution of Al3+ for Fe3+ in the octahedral sites. The mode of occurrences of apatite-magnetite ores indicates a predominantly hydrothermal origin of most magnetite. However, the Cr-Ti-bearing magnetite-1 cores and inferred mafic nature of the original protolith indicates that some magnetite was inherited from the original igneous rock. We propose that the pre-/early-shearing hydrothermal event of magnetite formation was associated with sulfide mineralization and alteration of existing magmatic magnetite. The second stage of magnetite formation at the mid-/late-stage of shearing was not associated with sulfide formation. Finally, fine-grained compositionally pure magnetite formed at the closing stage of shearing likely due to metamorphism of Fe-rich protolith.

Ore mineralization in the Miedzianka area (Karkonosze-Izera Massif, the Sudetes, Poland): new information

AbstractThe Miedzianka mining district has been known for ages as a site of polymetallic ore deposits with copper and, later, uranium as the main commodities. Although recently uneconomic and hardly accessible, the Miedzianka ores attract Earth scientists due to the interesting and still controversial details of their ore structure, mineralogy and origin. Our examination of the ore mineralization from the Miedzianka district was based exclusively on samples collected from old mining dumps located in the vicinity of Miedzianka and Ciechanowice, and on samples from the only available outcrop in Przybkowice. In samples from the Miedzianka field, chalcopyrite, pyrite, galena, bornite, chalcocite, digenite, arsenopyrite, magnetite, sphalerite, tetrahedrite-tennantite, bornite, hematite, martite, pyrrhotite, ilmenite, cassiterite and covellite are hosted in quartz-mica schists and in coarse-grained quartz with chlorite. In the Ciechanowice field, the ore mineralization occurs mainly in strongly chloritized amphibolites occasionally intergrown with quartz and, rarely, with carbonates. Other host-rocks are quartz-chlorite schist and quartzites. Microscopic examination revealed the presence of chalcopyrite, pyrite, sphalerite, galena, tetrahedrite-tennantite, bismuthinite, native Bi, arsenopyrite, löllingite, cassiterite, cobaltite, gersdorffite, chalcocite, cassiterite, bornite, covellite, marcasite and pyrrhotite. Moreover, mawsonite and wittichenite were identified for the first time in the district. In barite veins cross-cutting the greenstones and greenschists in Przybkowice, we found previously-known chalcopyrite, chalcocite and galena. The composition of the hydrothermal fluids is suggested to evolved through a series of consecutive systems characterized, in turn, by Ti-Fe-Sn, Fe- As-S, Fe-Co-As-S, Cu-Zn-S and, finally, Cu-Pb-Sb-As-Bi compositions.

Greenschist-facies sub-ophiolitic metamorphic rocks of Andaman Islands, Burma–Java subduction complex

Sub-ophiolitic greenschist facies metamorphic rocks occur at the sole of ophiolite slices and as blocks in the mélange zone beneath the Andaman ophiolite. These are represented by metabasics as actinolite schist to actinolite–chlorite schist and metasediments as garnetiferous quartzo-feldspathic mica–chlorite schist and piemontite quartzite to piemontite bearing quartz–muscovite–chlorite schist to muscovite–quartz-chlorite schist. Actinolite occurs along the schistosity and also as porphyroblasts. Syn to post-tectonic garnet shows no compositional zoning and represent almandine–spessartine solid solution (Alm 44–47, Sps 23–27, Gros 13–17, Pyr 9–10). The metabasics are enriched in LILE and depleted in Zr and Y compared to N-MORB. The lithological features suggest that residual heat was the main heat source for greenschist-facies metamorphism. Top part of the subducting slab and overlying trench sediments were metamorphosed and dislocated by the close spaced thrusts in an accretionary prism setting. The field association indicates that metamorphism and the uplift of metamorphic rocks along with ophiolite slices were bracketed in between Cretaceous and Oligocene period. These processes were later than the Pre-Cretaceous emplacement of the ophiolites of Sumatra and Java.

Structure and petrology of the Dadeldhura Group, far western Nepal, Himalaya

The granites, phyllites, schists, and gneisses of the Dadeldhura Group exhibit a significant Himalayan metamorphic imprint. The rocks of the group constitute a synform and the group is delimited by thrusts. The North Dadeldhura Thrust (NDT) zone is dominated by granitic mylonite with subordinate quartzite, quartz-chlorite schist, and amphibolite. In the quartz-chlorite schist from the NDT zone, relict kyanite is observed, which together with recrystallisation textures in the granitic mylonite indicate that low-temperature syn-tectonic retrogression has affected the thrust zone. Prograde garnets with spiral structures from a zone 2–4 km structurally above the base of the NDT are associated with mylonite-like rocks, and indicate distinct prograde shear zones in this area. This suggested that prograde thrust stacking has affected about 4 km wide north belt of the Dadeldhura Group. Rim thermobarometry from the garnet holding rocks shows that the minerals were re-equilibrated at 440–550 °C and 6.5–9.5 kbar. In the southern part of the Dadeldhura Group, textures in quartz and feldspar from the Saukhark Granite-Gneiss indicate that temperatures during recrystallisation were around 450–550 °C. These P–T estimates suggest that most of the now exposed rocks in the Dadeldhura Group experienced prograde epidoteamphibolite facies metamorphism during early orogenic build up and thrusting of the Dadeldhura Nappe. A subsequent syntectonic retrograde phase is observed in the NDT zone as low temperature recrystallisation of quartz and feldspar in mylonitic rocks, and chlorite and quartz in chlorite-grade rocks holding relict kyanite. Retrograde trusting was also focused along a distinct “back thrusting” zone within the northern part of the group.

Deciphering a multistage history affecting U–Cu(–Fe) mineralization in the Singhbhum Shear Zone, eastern India, using pyrite textures and compositions in the Turamdih U–Cu(–Fe) deposit

The ∼200-km-long intensely deformed Singhbhum Shear Zone (SSZ) in eastern India hosts India’s largest U and Cu deposits and related Fe mineralization. The SSZ separates an Archaean cratonic nucleus to the south from a Mesoproterozoic fold belt in the North and has a complex geologic history that obscures the origin of the contained iron-oxide-rich mineral deposits. This study investigates aspects of the history of mineralization in the SSZ by utilizing new petrographic and electron microprobe observations of pyrite textures and zoning in the Turamdih U–Cu(–Fe) deposit. Mineralization at Turamdih is hosted in intensively deformed quartz–chlorite schist. Sulfides and oxides include, in inferred order of development: (a) magmatic Fe(–Ti–Cr) oxide and Fe–Cu(–Ni) sulfide minerals inferred to be magmatic (?) in origin; followed by (b) uranium, Fe-oxide, and Fe–Cu(–Co) sulfide minerals that predate most or all ductile deformation, and are inferred to be of hydrothermal origin; and (c) Fe–Cu sulfides that were generated during and postdating ductile deformation. These features are associated with the formation of three compositionally and texturally distinct pyrites. Pyrite (type-A), typically in globular–semiglobular composite inclusions of pyrite plus chalcopyrite in magnetite, is characterized by very high Ni content (up to 30,700 ppm) and low Co to Ni ratios (0.01–0.61). The textural and compositional characteristics of associated chalcopyrite and rare pyrrhotite suggest that this pyrite could be linked to the magmatic event via selective replacement of magmatic pyrrhotite. Alternatively, this pyrite and associated sulfide inclusions might be cogenetic with hydrothermal Fe-oxide. Type-B pyrite that forms elongate grains and irregular relics and cores of pyrite with high Co contents (up to 23,630 ppm) and high Co to Ni ratios (7.2–140.9) are interpreted to be related to hydrothermal mineralization predating ductile deformation. A third generation of pyrite (type C) with low Co, low Ni, and moderate Co to Ni ratios (0.19–13.93) formed during and postdating the ductile deformation stage overgrowing, replacing, and surrounding type-B pyrite. The textural evolution of pyrite parallels the tectonometamorphic evolution of the shear zone demonstrating grain elongation during progressive ductile deformation and prograde metamorphism, annealing at the peak metamorphic condition, porphyroblastic growth at the retrograde path and cataclasis following porphyroblastic growth. Compositional characteristics of hydrothermal pyrite and available geological information suggest that the U–Cu(–Fe) deposit at Turamdih might be a variant of the Fe oxide (–Cu–U–rare earth elements) family of deposits.

Laboratory simulation of the salt weathering of schist: II. Fragmentation of fine schist particles

AbstractRecent developments in long term landform evolution modelling have created a new demand for quantitative salt weathering data, and in particular data describing the size distribution of the weathered rock fragments. To enable future development of rock breakdown models for use in landscape evolution and soil production models, laboratory work was undertaken to extend existing schist/salt weathering fragmentation studies to include an examination of the breakdown of sub‐millimetre quartz chlorite schist particles in a seasonally wet tropical climate. Laser particle sizing was used to assess the impact of different experimental procedures on the resulting particle size distribution. The results reveal that salt weathering under a range of realistic simulated tropical wet season conditions produces a significant degree of schist particle breakdown. The fragmentation of the schist is characterized by splitting of the larger fragments into mid‐sized product with finer material produced, possibly from the breakdown of mid‐sized fragments when weathering is more advanced. Salinity, the salt addition method and temperature were all found to affect weathering rates. Subtle differences in mineralogy also produce variations in weathering patterns and rates. It is also shown that an increase in drying temperature leads to accelerated weathering rates, however, the geometry of the fracture process is not significantly affected. Copyright © 2006 John Wiley & Sons, Ltd.

Laboratory simulation of the salt weathering of schist: 1. Weathering of schist blocks in a seasonally wet tropical environment

AbstractData describing sediment generation focusing on the temporal evolution of size gradation are required for the prediction of long‐term landform evolution. This paper presents such data for the salt weathering of a quartz‐chlorite schist obtained from the Ranger Uranium Mine in northern Australia. Rock fragment samples are subjected to three different climate regimes: (1) a dry season climate; (2) a wet season climate (both based on observations at the Ranger site); and (3) an oven‐drying sequence designed to test the sensitivity of the weathering process by exposing the rocks to more extreme temperatures. Two MgSO4 salt solutions are applied, one being typical of wet season runoff and the other a more concentrated solution. Salt solution is applied daily in the wet season experiments and once only at the beginning of the dry season experiments. Results of the experiments reveal four stages of weathering. The kinetics of each stage are described and related to the formation of sediment of different sizes. Wet season climate conditions are shown to produce greater moisture variability and lead to faster weathering rates. However, salt concentrations in the wet season are typically lower and so when climate is combined with observed salt concentrations, the dry and wet season experiments weather at approximately equal rates. Finally, small variations in rock properties were shown to have a large impact on weathering rates, leading to the conclusion that rock weathering experiments need to be carefully designed if results are to be used to help predict weathering behaviour at the landscape scale. Copyright © 2006 John Wiley & Sons, Ltd.

The role of moisture cycling in the weathering of a quartz chlorite schist in a tropical environment: findings of a laboratory simulation

AbstractLong‐term weathering of a quartz chlorite schist via wetting and drying was studied under a simulated tropical climate. Cubic rock samples (15 mm × 15 mm × 15 mm) were cut from larger rocks and subjected to time‐compressed climatic conditions simulating the tropical wet season climate at the Ranger Uranium Mine in the Northern Territory, Australia. Fragmentation, moisture content and moisture uptake rate were monitored over 5000 cycles of wetting and drying. To determine the impact of climatic variables, five climatic regimes were simulated, varying water application, temperature and drying. One of the climatic regimes reproduced observed temperature and moisture variability at the Ranger Uranium Mine, but over a compressed time scale.It is shown that wetting and drying is capable of weathering quartz chlorite schist with changes expected over a real time period of decades. While wetting and drying alone does produce changes to rock morphology, the incorporation of temperature variation further enhances weathering rates. Although little fragmentation occurred in experiments, significant changes to internal pore structure were observed, which could potentially enhance other weathering mechanisms. Moisture variability is shown to lead to higher weathering rates than are observed when samples are subjected only to leaching. Finally, experiments were conducted on two rock samples from the same source having only subtle differences in mineralogy. The samples exhibited quite different weathering rates leading to the conclusion that our knowledge of the role of rock type and composition in weathering is insufficient for the accurate determination of weathering rates. Copyright © 2005 John Wiley & Sons, Ltd.

Middle Archaean gold deposits in the North Shaw–Coongan Greenstone Belt, Australia: the relative timing of events responsible for gold mineralisation

Gold deposits of the Mid-Archaean East Pilbara Granitoid–Greenstone Terrane, Australia, are unique because they characterise metallogenic evolution of the Early Earth. In the North Shaw–Coongan Greenstone Belt, quartz–sulphide lodes within shear and fault zones are host to gold mineralisation. Detailed field mapping of crosscutting structures at the Sharks Well Mining Centre and Blue Bar Mine in the Coongan Greenstone Belt, and at the North Shaw Mining Centre in the North Shaw Greenstone Belt, have established a relative chronology for gold deposits. The results suggest that the accepted classification of regional gold mineralisation, in terms of two events, may need modification. The Sharks Well Mining Centre is hosted by meta-basalt to andesite and carbonatised quartz–chlorite schist of the Mt. Ada Basalt (Warrawoona Group, ca. 3.49–3.31 Ga). The gold-bearing quartz–sulphide lodes are principally situated in the Shark Gully Shear Zone. The lodes and veins formed early in D1 (local) and prior to development of cleavage and folds. Gold occurs in NW-trending fractures that cut across the quartz lodes and veins, suggesting syn- to post-D1 gold mineralisation. The maximum relative age of gold mineralisation at the mining centre is constrained by the age of the Duffer Formation, which is stratigraphically younger than the Mt. Ada Basalt and dated at 3471–3463 Ma. The Blue Bar Mine is hosted by quartz–fuchsite schist of the Central Coongan Shear Zone, which trends northerly and dips steeply eastward. The mine consists of a single, brecciated, gold-bearing quartz lode that is unconformably overlain by scree breccia, and in turn, by the Mount Roe Basalt of the Fortescue Group (ca. 2.7 Ga). The maximum relative age of gold mineralisation at the mine is constrained by the age of the Euro Basalt at 3395–3325 Ma, while the minimum relative age is constrained by the Mount Roe Basalt at ca. 2.7 Ga. The North Shaw Mining Centre consists of several gold-bearing quartz lodes (including the Auroria and Eldorado lodes), as well as a number of historic gold workings (alluvial and shaft mining) on the plains adjacent to the Shaw River. The mining centre is located on the contact margin of the North Shaw Suite of the Shaw Granitoid Complex. The host rocks consist of contact metamorphosed basalt–andesite and hornblende amphibolites. Five stages of (local) deformation are recognised in the rocks of the mining centre: D1—macroscopic folding of the North Shaw Greenstone Belt about a northerly trending fold axis; D2—NNE-trending foliation and folds; D3—development of a NE-trending disjunctive cleavage, folds and conjugate kinks; D4—development of faults and master joints during emplacement of dykes of the Black Range Suite; D5—formation of the WNW-trending Callina Fault Zone. The fault zone crosscuts mafic dykes of the Black Range Suite, which places constraints on the minimum relative age of gold mineralisation in the fault zone. Based on crosscutting structural relationships, the relative and absolute chronology of the gold deposits in the North Shaw–Coongan Greenstone Belt indicates they are not all of the same age. The crosscutting structural relationships justify a review of the currently accepted ‘Event 1 or 2’ classification of Huston et al. [Econ. Geol. 97 (2002) 691]. Two new events, one at 3325–2774 Ma, and one younger than 2.7 Ga, also affected the mafic and ultramafic rocks of the Warrawoona Group.

Behaviour of Lead in Uranium Ore Deposit; Application of Isotope Analysis by Secondary Ion Mass Spectrometry (SIMS)

Lead is a multi-isotope element, and each isotope has different primeval and/or radiogenic origin. The isotope composition of lead in rocks and minerals reflects their age and the migration behaviour of lead. Especially, in the vicinity of uranium deposits, lead isotope composition of minerals can provide us detailed information on the migration behaviour of uranium and lead quantitatively. When a geological event that migrates lead from a uranium ore body to surrounding formation occurs, radiogenic lead produced in the ore body commonly forms lead minerals like galena. The assemblage of uraninite and galena often occurs in uranium ore deposits. Since galena close to uraninite consists almost only radiogenic lead produced in uraninite, the formation age cannot be estimated by the Pb-Pb dating method. Secondary Ion Mass Spectrometer (SIMS) is a powerful instrument for analysing isotope compositions of geological materials in micronmeter scale. Appropriate combination of isotope analysis by SIMS with mineralogical observation including chemical analysis by EPMA can reveal not only chronological information but also migration behaviour of lead in a uranium ore body. In this study, lead isotope analysis of the samples from the Koongarra uranium deposit, Australia was carried out by SIMS. The main ore body of the Koongarra deposit occurs in a quartz chlorite schist (Snelling, 1980). We discuss the ages of the original uraninite and the event that formed current uraninite and released lead from the ore body. The fraction of lead release is also estimated. were Au coated petrologic thin sections from the drill cores of the Koongarra deposit. Analysed minerals were uraninite in the primary ore body (DDH58-129) and sulphide minerals including galena (PbS), pyrite (FeS2) and chalcopyrite (CuFeS2) in a sample just above the primary ore body (DDH58-126.6). Pyrite grains from a barren rock (DDH60-125) at the border of alteration halo 30m in distance from DDH58-129 and 126.6 are also analysed to establish the primeval lead fraction at Koongarra.

Lower miocene larger foraminifera and petroleum potential of the Tai Formation, Mergui Group, Andaman Sea

Tertiary larger foraminifera are recorded for the first time from Thailand. The fauna studied is restricted mainly to the reefal carbonate of the Tai Formation, which rests unconformably on the pre-Late Eocene quartz-chlorite schist basement in the Central High region of the Mergui Basin. The formation is broadly correlatable with the Peutu Formation of the North Sumatra Basin. The Tai Formation can be divided into three units at the type locality; a basal unit of interbedded anhydrite, dolomite, shale and sandstone; a middle unit of coral/algal reefal limestones, and an upper unit of calcarenites interbedded with silty shales and sandstones. The middle and upper units have yielded a fauna comprising; Lepidocyclina (Nephrolepidina) japonica, Spiroclypeus yabeii, Cycloclypeus eidae, Cycloclypeus sp. A, Heterostegina sp. A, Lepidocyclina (N) sp. A, Miogypsina sp. A, and Miogypsinoides sp. L. (N) japonica and Miogypsinoides sp. can range up into the Middle Miocene (Lower Tf) whilst C. eidae can range down into the Upper Oligocene (Lower Te). The fauna is typical of the Indo-West Pacific Miocene faunal province of Adams (1970) and shows closer affinities to the faunas of Indonesia than those of India-Pakistan.

Primary gold mineralization in the Pre-Cambrian and lower Paleozoic rocks, Lungri Khola region, Rolpa district

Primary gold mineralization occurring in the Lungri Khola region is observed in the Pre­ Cambrian green schist facies and Lower Paleozoic micaceous marble and limestone. The Pre-Cambrian green schist facies comprising mainly of sericite and chloritic quartzite, chlorite schist, quartz-chlorite schist, chlorite phyllite and schistose pebble beds include a discontinuous auriferous zone which persists laterally for about 30 km extension from the Gajul khola in the west to the upper reaches of Gam khola in the east. The auriferous host rocks are found confined close to the contact with the overlying Lower Paleozoic carbonates. Eight distinct auriferous hosts are encountered in the Pre-Cambrian green schists facies. Occurrence of auriferous host in the Lower Paleozoic limestone and marble is rather irregular and insignificant So far minor quartz-carbonate veins have indicated the presence of gold. Finely disseminated gold with minor amount of pyrite, chalcopyrite and rarely arsenopyrite occurs in the above hosts. Visible gold ranging from 0.08 to 2 mm dimension has been recovered. Gold content of upto 6.7 gm/ton has been recorded but in general they show 0.1 gm/ton gold which is significantly 30 to 60 times higher in magnitude than the background value. Depending upon the type of the hosts, three possible models, namely (1) volcanogenic (2) hydrothermal and (3) syn-sedimentary, could be suggested for the origin of the primary gold mineralization which has later undergone remobilisation after the initial deposition.

Migration Behavior of Uranium Series Nuclides in Altered Quartz-Chlorite Schist

ABSTRACTThe migration behavior of uranium series nuclides in an altered quartz-chlorite schist has been studied by utilizing data on the activity distributions of uranium series nuclides at Koongarra in the Northern Territory of Australia. The variation of 230Th/234U activity ratios (ARs) with distance along surface (2 m depth), intermediate (16 m) and deep (31 m) layers show different trends in the three layers parallel to the water flow. The relationship between the 234U/238U and 230Th/238U ARs reveals that the mobility of the uranium series nuclides is in order 238U, 234U and 230Th, and that the retardation factors oft 234U are greater than those of 238U by a factor of 1.1, 1.9 and 1.0 in the surface, intermediate and deep layers, respectively. X-ray diffraction patterns show different mineral assemblages, which are the alteration products of chlorite at the three layers. These results imply that the migration behavior of uranium series nuclides at each depth would be related to the alteration of chlorite.

GEOLOGIA DA RAIZ DE UM GREENSTONE BELT NA REGIÃO DE FORTALEZA DE MINAS. MINAS GERAIS

The aim of this study is to define the volcano-sedimentary sequence of Morro do Ferro, located in Fortaleza de Minas, as a part of a larger greenstone belt. The volcano-sedimentary sequence, in contact with orthogneisses, migmatites, cataclastic rocks of the sialic basement is unconformably overlied by the phillytes and marbles of the Araxa Group. The basal unit of the sequence, Morro do Niquel Unit, is composed of ultramafic flows with spinifex texture intercalated with chemical sediments and aluminous tuffs. Over it is the middle unit called Corrego Salvador, composed of basaltic rocks associated with acid tuffs and chemical sediments. The upper unit, Morro do Ferro, is exclusively constituted of sedimentary rocks, phillytes, marbles, iron formation, and muscovite-chlorite schist and quartz-chlorite schist which may represent reworked volcanic rocks. The volcano-sedimentary belt is intensively tectonized having tight vertical isoclinal N-W folds. There are indications of three fases of deformation. The regional metamorphism is of greenschist facies with choritoid and chlorite-albite-epidote-tremolite parageneses in basic rocks. Potentially the ultramafic flows are the most favorable rocks to have mineralization, Ni-Cu massive sulfides.