Graphitic Schists Research Articles

Electrical Conductance Map for the Kachchh Rift Basin: Constraint on Tectonic Evolution and Seismotectonic Implications

Geomagnetic field variations recorded by an array of magnetometers spread across the Kachchh Rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations. A non-uniform thin-sheet electrical conductance model is developed to account for the salient induction patterns. It indicates that the imaged conductivity anomalies can be related to the sediment-filled structural lows in between the fault bounded uplifts. It is suggested that sagging structural lows preserved the marine sediments deposited during the Mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with Late Eocene transgression. Depth integrated electrical conductance helped in mapping two depo-centres: along the ENE-WSW trending Banni half-Graben bounded by the Kachchh Main fault on the south and, second, along the Vinjan depression formed in response to the subsidence between the Vigodi fault and westward extension of the Katrol Hill fault together with the westward bending of the Median High. Presence of metamorphosed graphite schist clasts in shale dominated Mesozoic sequence and/or thin films of carbon resulting from the thermal influence of Deccan activity on Carbonate-rich formations can account for the high electrical conductivity anomalies seen in the depo-centres of thick Mesozoic and Tertiary sediments. Additionally two high conductivity zones are imaged encompassing a block defined by the 2001 Bhuj earthquake and its aftershocks. In agreement with gravity, magnetic and seismic velocity signatures, aqueous fluids released by recrystallizing magmatic bodies intruded in association with Deccan trap activity account for mapped high conductivity zones. High fluid pressure in such a fractured domain, surrounding the intruded magmatic plugs, perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 Bhuj earthquake.

The initial garnet‐in reaction involving siderite–rhodochrosite, garnet re‐equilibration and P–T–t paths of graphitic schists in the Black Hills orogen, South Dakota, USA

AbstractIt is generally thought that garnet in metapelites is produced by continuous reactions involving chlorite or chloritoid. Recent publications have suggested that the equilibrium temperatures of garnet‐in reactions may be significantly overstepped in regionally metamorphosed terranes. The growth of small spessartine–almandine garnet crystals on Mn‐siderite at the garnet isograd in graphitic metapelites in the Proterozoic Black Hills orogen, South Dakota, demonstrates that Mn‐siderite was the principal reactant that produced the initial garnet in the schists. Moreover, the positions of garnet compositions in isobaric, T–(C/H) pseudosections for the schists show that the temperature of the garnet‐in reaction from Mn‐siderite was overstepped minimally at the most. In the Black Hills, garnet was initially produced during regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons, and was subsequently partially or fully re‐equilibrated at more elevated temperatures and pressures during intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet occurs in graphitic schists in garnet, staurolite and sillimanite zones, the latter being a product of contact metamorphism by HPG. During metamorphism, coexisting fluid contained both CO2 and CH4. In the garnet zone, garnet crystals contain petrographically distinct cores with inclusions of quartz, graphite and other minerals. Centres of the cores have distinctly elevated Y concentrations that mark the positions of garnet nucleation. The elevated Y is thought to have come from the Mn‐siderite onto which Y was probably absorbed during precipitation in an ocean. In the upper garnet and staurolite zones, the cores were overgrown by inclusion‐poor mantles. Mantles are highly zoned and have more elevated Fe and Mg and lower Mn and Ca than cores. The growth of mantles is attributed to late‐orogenic heating by leucogranite magmas and attendant influx of H2O that caused consumption of graphite in rock matrices. A portion of the Proterozoic terrane that includes the HPG is surrounded by four large faults. In this ‘HPG block’, garnet is inclusion‐poor and its composition does not preserve its early growth history. This garnet appears to have re‐equilibrated by internal diffusion of its major components and/or recrystallization of an earlier inclusion‐rich garnet. It has equilibrated within the kyanite stability range, and together with remnant kyanite in the high‐strain aureole of the HPG, indicates that the HPG block had a ≥6 kbar history. The HPG block has undergone decompression during emplacement of the HPG. The decompression is evident in occurrences of retrograde andalusite and cordierite in the thermal aureole of the HPG. The data support a polybaric metamorphic history of the Black Hills orogen with different segments of the orogen having their own clockwise P–T–t paths.

Genesis of the hydrothermal gold deposits in the Canan area, Lepaguare District, Honduras

The Canan area (Honduras) is characterized by a gold-bearing ore deposit that is associated with quartz-veined shear zones. Gold mineralization occurs in low-to medium-grade metamorphic host-rocks (graphitic and sericitic schists). Hydrothermal fluids, which are associated with the emplacement of Cretaceous-Tertiary granodioritic intrusions, are responsible for the formation of quartz veins and the hydrothermal alteration of wall-rocks. Three main altered zones have been detected in the wall-rocks as far as 150 cm from the quartz veins. The distal zone (up to 50-cm thick) contains quartz, chlorite and illite. The intermediate zone is the thickest (up to 80 cm) and is marked by quartz, muscovite, sulphides, kaolinite and native elements such as Au and Ag. The proximal zone, which is close to the quartz veins, is rather thin (up to 25 cm) and contains clay minerals, Al-oxides-hydroxides and sulphides. The transition from the distal to the proximal zone is accompanied by the enrichment of SiO2 and the depletion of all other major elements, except for Fe2O3(tot). Precious metals occur in the highest concentrations in the intermediate zone (Au up to 7.6 ppm and Ag up to 11 ppm). We suggest that gold was transported as a reduced sulphur complex and was precipitated from the hydrothermal solution by the reaction of the sulphur complexes with Fe2+ from the alteration of the mafic minerals of the host-rock. Fluid–wall-rock interactions seem to be the main cause of gold mineralization. Genetic relationships with a strike-slip fault system, hydrothermal alteration zones within the metamorphic wall-rocks, and an entire set of geochemical anomalies are consistent with orogenic-type gold deposits of the epizonal class.

A UNIDADE METASEDIMENTAR DO OFIOLITO DE RIBEIRÃO DA FOLHA, ORÓGENO ARAÇUAÍ, MINAS GERAIS: PETROGRAFIA, GEOTERMOBAROMETRIA E CALCOGRAFIA.

The Neoproterozoic oceanic remnants of the Araçuaí Orogen occur as tectonic dismembered sectionsin the region between 42°-43°W and 17°-21°S, southeastern Brazil. The Ribeirão da Folha ophioliteis one of these oceanic remnants, located in the Minas Novas region, northeastern Minas GeraisState. This paper presents field and microscopic studies, as well as quantitative geothermobarometricresults, for the ophiolitic metasedimentary unit mapped in the Baixa Quente-Ribeirão da Folha area.This metasedimentary unit consists of several varieties of quartz-mica schist, peraluminous micaschist,calc-silicate rock (metamarl), graphite-kyanite schist, graphite schist, metachert, diopsidite andbanded iron formation. Such well-developed, ophiolitic sedimentary pile represents pelagic pelitesand chemical-exhalative sediments, as well as mixtures of chemical, argillaceous and/or organiccomponents. These sediments were metamorphosed in the Brasiliano Orogeny (ca. 580 Ma) and itsregional schistosity (Sn) reached the kyanite zone of the amphibolite facies (530-600 °C and 4,9-5,5kbar), in the focused area. An older foliation (Sn-1), developed in greenschist facies, is locallypreserved. The sulfide-bearing rocks are different types of metachert and diopsidite, with massivesulfide bodies, sulfide-type banded iron formation and graphite schist. Sulfate efflorescence, formedby chemical weathering, is a common feature of sulfide-rich outcrops. Pyrrotite, pyrite, botrioidalpyrite, chalcopyrite and spharelite are the main sulfide phases found in metachert and diopsiditetypes. Covellite, marcasite and arsenopyrite are rare. We suggest three stages of sulfide crystallization:i) precipitation from hydrothermal fluids in oceanic environment; ii) recrystallization during theregional metamorphism, mainly along the Sn foliation; and iii) post-Sn hydrothermal alteration.

Exploration of new gold occurrences in the alteration zones at the Barramiya District, Central Eastern Desert of Egypt using ASTER data and geological studies

The use of ASTER data and fieldwork supported by mineralogical and geochemical analyses enabled exploration of new gold occurrences in the alteration zones in the ultramafic–mafic successions at the Barramiya district. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) band ratios (4/8, 4/2 and 8/9 in R, G and B channels, respectively) helped in recognising of two listwaenite alteration zones (areas 1 and 2) promising for gold in the north-east and south-east of the Barramiya gold mine. Mineralogical studies and X-ray diffraction analysis revealed that areas 1 and 2 are characterized by variable concentrations of talc, ankerite, magnesite, quartz and calcite. Ore microscope studies revealed the sulphide minerals carry gold within these alteration zones; moreover, goethite and malachite are also observed. Fire assay results show Au contents in the range of 5.04 ppm in the graphite schist, 4.02 ppm in the quartz veins and 3.76 ppm in the listwaenite alteration. The atomic absorption analysis (AAS) of samples from area 1 yields an average Au content in the quartz-veins of 2.4 ppm, Ag content is 8.0 ppm and Cu content is 2.4 wt%. The listwaenite alteration gives an average Au content of 4.4 ppm and a Cu content of 2.8 wt%. In area 2, the AAS of the quartz-veins revealed an average Au content of 2.6 ppm, 6.2 ppm Ag and 1.9 wt% Cu. The listwaenite alterations of area 2 grade 3.5 ppm Au and 2.4 wt% of Cu. The Barramiya District is made up of ophiolitic ultramafic belts of serpentinites, talc carbonates and talc graphite schists, mainly thrust over the metavolcanic sequences. They include highly strained and tectonised parts enriched in sulphides, iron oxides and carbonates, with developed listwaenite alterations along the thrust contacts. Gabbro and granitic intrusions were intruded in the ultramafics and metavolcanic rocks. ASTER data are an accurate and helpful tool for detecting and mapping alteration zones for gold exploration.

Integrated application of heliborne and ground electromagnetic surveys for mapping EM conductor for uranium exploration and its subsurface validation, North Delhi Fold Belt, Rajasthan, India: A case study

A fracture-controlled uranium deposit was identified in Proterozoic Ajabgarh metasediments of the North Delhi Fold Belt within the Khetri subbasin at Rohil, Sikar district, Rajasthan, India. Uranium mineralization in the area is associated with geologic structures, albitization, and pyroxenization of metasediments and conductors such as metallic sulfides and carbonaceous phyllites/graphitic schists. To locate uranium mineralization akin to Rohil in nearby thick soil covered areas, this association was targeted through heliborne geophysical surveys. High-resolution heliborne magnetic and time domain electromagnetic (TEM) surveys were conducted around Rohil. The survey delineated several targets with favorable geologic structures and conductors such as graphitic schist for further uranium exploration. One favorable target near Chappar village was taken up for follow-up exploration work. The EM conductor mapped from heliborne survey was subsequently validated through ground time-domain electromagnetic surveys and subsurface exploration. Modeling of heliborne and ground-based electromagnetic data revealed the presence of subsurface conducting bodies with comparable model parameters. Drilling established the presence of a subsurface conductor up to a depth of 300 m, which was attributed to the presence of graphite and sulfides (pyrrhotite) along foliation plane of carbon phyllite/graphitic schist/quartz-biotite schist and calc-silicate rock. Further detailed laboratory investigations (petrology/X-ray diffraction) of selected core samples from the conductive zones confirmed the presence of pyrrhotite and graphite responsible for EM signature. This study, carried out by using multiparameter data sets, proved the efficacy of heliborne surveys in locating favorable targets for uranium exploration in Ajabgarh group of rocks.

Metabasic rocks in the Varied Group of the Moldanubian Zone, southern Bohemia - their petrology, geochemical character and possible petrogenesis

Metabasic rocks form an important constituent of the Chýnov and Ceský Krumlov units belonging to the Varied Group (Moldanubian Zone, south Bohemia). The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase. Hornblendes show compositional variation with Si ~ 6.5 apfu, Mg/(Mg + Fe) ~ 0.5 and (Na + K)A ~ 0.5 apfu. Garnet with clinopyroxene are subordinate and occur in a few samples only. No relics of previous greenschistor granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history. The petrology indicates rather close correlation of the Chýnov and Ceský Krumlov units. The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks. However, there is a major difference in the oxidation state. Most Chýnov amphibolites have Fe2O3/FeO = 0.70–1.00 and their protolith probably experienced an early incipient oxidation. Great deal of the parental basalts thus could have been effusive. The Ceský Krumlov amphibolites have Fe2O3/FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units. The dataset is dominated by EMORB-like tholeiite basalts interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (e50 Nd 0 = +8.6 to +9.4; TD Nd M = 0.43–0.50 Ga). This argues stoutly against Precambrian age of the Varied Group in south Bohemia. The composition of the remaining samples reflects contamination by upper continental crust (e50 Nd 0 = +3.1 to +1.3, progressive enrichment in Th, development of a significant negative Nb, and lesser P and Ti anomalies on the NMORB-normalized spiderplots). A much smaller group of amphibolites is characterised by steep REE patterns (LaN/YbN = 5.5–11) and high contents of HFSE (Nb, Ta, Zr and P). It is of a clear OIB affinity, with parental alkali basalt (Nb/Y = 0.7–1.6) generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric mantle source (e50 Nd 0 = +4.5 to +6.1; TD Nd M = 0.75–0.83 Ga). Metamorphosed doleritic/gabbroic dykes cutting the Palaeoproterozoic Světlik orthogneiss show rather unradiogenic Nd isotopic composition (e50 Nd 0 = +0.1 and –3.6; TD Nd M = 1.34 and 2.03 Ga). This precludes closed-system crystallization from depleted mantle derived melts in Phanerozoic times. The exact age and nature of their parental magma remain enigmatic but any genetic link with the amphibolites in the structurally overlying Ceský Krumlov Varied Unit seems ruled out. Overall, the most likely tectonic setting of the magmatism was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin. The minor OIB component preserved as alkali basalts as well as some contribution to the EMOR-like basaltic magmas was probably added by a rising mantle plume.

Electrical anisotropy in the Main Central Thrust Zone of the Sikkim Himalaya: Inference from anomalous MT phase

The Sikkim Himalaya in the eastern sector of the Himalayan collision belt shows some distinct features, such as the occurrence of the Main Central Thrust as a several-km-wide zone (MCTZ) of intense ductile deformation and its deviation from the general east–west strike of the Himalayan thrusts, suggesting that the tectonics of this region could be different from the regional Himalayan tectonics. We have carried out a MT survey across MCTZ to understand the tectonics of the region. The broadband MT data of the site at the northern boundary of the zone reveals anomalous phase exceeding 90° for frequencies smaller than 1Hz whereas other sites further north of this zone show normal behavior. Such anomalous phase behavior has been explained earlier in terms of anisotropy in the medium and special configuration of local conductors. We have analyzed the observed anomalous phase behavior in terms of anisotropy based on geological consideration that alternating bands of graphitic schists are present within high-grade crystalline gneisses at the northern boundary of MCTZ. Among various models analyzed, a block model of the crustal structure with strong anisotropy contrast between the block and the underlying anisotropic layer provides good fit to the observed data. We, thus, infer that the presence of a localized strongly anisotropic block at the northern boundary of MCTZ could be a viable explanation for the observed anomalous phase behavior.

U–Pb zircon ages of orthogneisses and supracrustal rocks of the Cariris Velhos belt: Onset of Neoproterozoic rifting in the Borborema Province, NE Brazil

New geochronological and geochemical constraints on Precambrian sedimentary and volcanic successions exposed in the western part of the Central Domain of the Borborema Province, NE Brazil, indicate the presence of two distinct tectono-stratigraphic complexes: Riacho Gravatá and São Caetano. Both complexes and associated orthogneisses are referred in the literature as the Cariris Velhos belt, having depositional, extrusive, or intrusive ages within the interval 985–913 Ma. The Riacho Gravatá complex consists of bimodal (but mostly felsic) volcanic and volcanoclastic rocks, muscovite ± graphite schists, quartzites, and marble with local occurrences of banded-iron-formation. The São Caetano complex mainly consists of metagreywackes, marbles, calc-silicate rocks, and rare meta-mafic rocks. Meta-mafic rocks from both complexes have geochemical signatures similar to those of continental flood basalts, with ɛNd (1.0 Ga) values ranging from −1.0 to −2.8. Felsic volcanic rocks from the Riacho Gravatá complex show ɛNd (1.0 Ga) values ranging from −1.0 to −7.4 and geochemical signatures similar to A 2-type granitoids. New SHRIMP U–Pb zircon data from felsic volcanic rocks within the Riacho Gravatá complex yielded ages of 1091 ± 13 Ma and 996 ± 13 Ma. In contrast, meta-graywackes from the São Caetano complex show a maximum deposition age of ca. 806 Ma in the northern part and ca. 862 Ma in the southern part of the outcrop area. The orthogneisses show ɛNd (1.0 Ga) values ranging from 1.0 to −4.2 with U/Pb TIMS and SHRIMP ages ranging from 960 to 926 Ma and geochemical signatures of A 2-type granitoids. The data reported in this paper suggest at least two periods of extension within the Central Domain of the Borborema Province, the first starts ca. 1091 Ma with magmatism and deposition, creating the Riacho Gravatá basin and continued intrusion of A-type granites to 920 Ma. A second rift event, which reactivated old faults, generated a basin with a maximum deposition age of ca. 806 Ma. Furthermore, the oldest granitoids cutting these metasedimentary rocks have crystallization ages of ca. 600 Ma. This suggests that the second rift event could be early Brasiliano in age. The resulting São Caetano basin received detritus from a variety of sources, although detritus from the Riacho Gravatá complex dominated. Deposition ages of the Riacho Gravatá and the São Caetano complexes are coeval with deposits in other basins of the Borborema Province (Riacho do Tigre in the Central Domain; Macururé and Marancó in the Sergipano Belt of the Southern domain). The Macaúbas Group from SE Brazil and its counterparts in Africa, the Zadanian and Mayumbian Groups, in the western edge of the Congo Craton are also coeval. Closure of the Riacho Gravatá and São Caetano basins occurred during the Brasiliano convergence (705–600 Ma). During the last stage of convergence, ca. 612 Ma, pull-apart basins were created and filled; final basin closure took place 605–592 Ma, after deposition ceased.

Waste disposal on karstic terrain: a case study from the ancient marble quarries in Iznik (Nicaea), Turkey

This study is conducted in Iznik, a city situated in the Marmara region of Turkey. There are number of ancient marble pits in the area, some of which are used as sewage and solid waste dump sites. This study is aimed at investigating the hydrogeological and hydrogeochemical properties of Iznik plain with special emphasis on these waste sites. In terms of geological features, Derekoy metamorphics are basement rocks that consist of graphite schist, muscovite-quartz schist and marble lenses. Since Iznik marble was used as a natural construction material in the area since in the Roman era, 20 ancient marble quarries in different sizes existed in and around the study area. Wastewater from the city of Iznik and surrounding residential areas was disposed in three of these ancient marble quarries. In terms of hydrogeological features, the Iznik marble and alluvium units constitute the main aquifer system in the study area. Because of the highly fractured structure of the Iznik marble, the wastewater in the quarries was able to move rapidly within the rock. Analyses of groundwater samples revealed that the wastewater flowing in the marble unit discharges eventually into the alluvial unit and thereby affecting groundwater quality around the antique quarries. Based on chemical analyses results, the wastewater was Na-Cl water type and contained high concentrations of Na (3260 mg/l), K (903 mg/l) and Cl (10396 mg/l). Samples from wells down gradient of the wastewater source had comparatively higher Na, K and Cl concentrations.

Nature and origin of the protolith succession to the Paleoproterozoic Serra do Navio manganese deposit, Amapa Province, Brazil

Until its closure in 1997, the Serra do Navio deposit, located in Amapá Province, Brazil, was one of the most important sources of high-grade manganese ore to the North American market. The high-grade manganese oxide ores were derived by lateritic weathering from metasedimentary manganese protoliths of the Serra do Navio Formation. The local geological context and nature of this protolith succession are not well understood, due to poor surface outcrop conditions, and intense deformation. However, based on similar age, regional tectonic setting and lithology the Paleoproterozoic volcanosedimentary succession that includes the Serra do Navio Formation is widely believed to be similar in origin and laterally equivalent to the Birimian Supergroup in West Africa. For the present investigation several diamond drill cores intersecting the protolith succession were studied. Detailed petrographic and whole rock geochemical studies permit distinction of two fundamental lithological groups comprising of a total of five lithotypes. Biotite schist and graphitic schist lithotypes are interpreted as former metapelites. A greywacke or pyroclastic protolith cannot be excluded for the biotite schist, whereas the graphitic schist certainly originated as a sulfide-rich carbonaceous mudstone. Rhodochrosite marble, Mn-calcite marble and Mn-silicate rock are grouped together as manganiferous carbonate rocks. Manganese lutite constitutes the most probable protolith for rhodochrosite marble, whereas Mn-calcite marble was derived from Mn-rich marl and Mn-silicate rock from variable mixtures of Mn-rich marl and chert.The sedimentary succession at the Serra do Navio deposit is similar to that encountered at many other black shale and chert-hosted Mn carbonate deposits. A metallogenetic model is proposed, predicting deposition of manganese and closely associated chert in intra-arc basins, in environments that were bypassed by distal siliciclastic (carbonaceous mud) and proximal pyroclastic/siliciclastic detritus. Positive Ce anomalies and δ13CVPDB values of −4.3 to −9.4 per mill suggest that manganiferous carbonates derived during suboxic diagenesis from sedimentary Mn4+ oxyhydroxide precipitates. Metamorphic alteration of manganese carbonate–chert assemblages resulted in the formation of Mn-silicates, most importantly rhodonite and tephroite; porphyroblastic spessartine formed where Mn-carbonate reacted with aluminous clay minerals. Microthermometric studies of fluid inclusions in spessartine porphyroblasts suggests that peak metamorphic conditions reached the upper greenschist facies (1–2kbars and 400–500°C). Retrograde metamorphism is marked by partial re-carbonation, expressed by the formation of small volumes of rhodochrosite, and Mn-calcite that are closely associated with quartz, chlorite and minor amounts of sulfides related to post-metamorphic veinlets. It is this metamorphosed succession that sourced the high-grade manganese oxide ores during prolonged lateritic weathering.

Chapter 41 Glaciogenic and related strata of the Neoproterozoic Kingston Peak Formation in the Panamint Range, Death Valley region, California

Abstract Glaciogenic deposits in the Death Valley region occur within the Neoproterozoic Kingston Peak Formation (Fm.). In the Panamint Range, immediately west of Death Valley, the formation is as much as 1000 m thick and is continuously exposed for nearly 100 km along the strike of the range. Although the strata are variably metamorphosed and locally exhibit pronounced ductile strain, original sedimentary textures are well preserved in many places. Diamictite occurs in two distinct intervals, a lower one comprising the Limekiln Spring and Surprise members, and an upper one, the Wildrose Sub-member of the South Park Member. Lonestones, bullet-shaped and striated clasts, and rare dropstones within these members, along with the impressive lateral continuity of diamictic units, support a glacial origin. Both diamictic intervals are succeeded by well-defined carbonates, the oldest is the Sourdough Member of the Kingston Peak Fm. and the younger one is the Sentinel Peak Member of the overlying Noonday Dolomite. The stratigraphic succession between the Sourdough Member and the Wildrose Sub-member (i.e. the Middle Park, Mountain Girl and Thorndike sub-members of the South Park Member) is c. 300 m thick and includes lithologies recording deposition in braided fluvial to platform carbonate settings. Lithostratigraphic and chemostratigraphic profiles of δ 13 C for the Sourdough (–3‰ to +2‰, increasing upward) and Sentinel Peak (–3‰ ±1‰) members suggest correlation with, respectively, the older Cryogenian (commonly referred to as ‘Sturtian’ in previous literature) and younger Cryogenian (commonly referred to as ‘Marinoan’ in previous literature) cap-carbonate sequences recognized worldwide. Potentially economic uranium deposits (secondary brannerite) occur in graphitic schist of the Limekiln Spring Member and sub-economic uranium and thorium (hosted by detrital monazite) occur within quartz-pebble conglomerate in the South Park Member. The Kingston Peak Fm. strata in the Panamint Range contain no fossils, radiometric age control or primary magnetizations.

Characterization of gold mineralization in Garin Hawal area, Kebbi State, NW Nigeria, using remote sensing

Garin Hawal area, Kebbi State, NW Nigeria is part of the Neoproterozoic to Early Phanerozoic terrane separating the west African and Congo Cratons. Three main gold-bearing shear zones were detected in the study area from the processed Landsat ETM+ images and extensive ground investigation. Field and petrographical studies indicate that the Neoproterozoic rocks are represented by a highly folded and faulted belt constituted of hornblende, muscovite and graphite schist. They are intruded by granondiorites and late to post granitic dykes. Extensive alteration zones were identified using high resolution QuickBird image along Garin Hawal shear zone. The alteration zones and associated quartz veins are generally concordant with the main NE–SW regional structural trend and are dipping to the NW. Geochemical studies indicate that the gold content reaches 8 g/t in the alteration zones, while it reaches up to 35 g/t in the quartz veins. Mineralogical studies indicate that the alterations are strongly potassium-enriched. Pyrophyllite, kaolinite, illite, gypsum and quartz also occur. The main ore minerals are gold, chalcopyrite, arsenopyrite, pyrite, galena and iron oxides. This study indicates that the alteration zones and the associated quartz veins in the muscovite schist are promising and need more detailed exploration for Au and Ag mineralization to evaluate their potential.

Fine gold and platinum group metals in the graphite schists Bureya array - a new type of precious-metals mineralization in the Far East of Russia.

Fine gold and platinum group metals in the graphite schists Bureya array - a new type of precious-metals mineralization in the Far East of Russia.

Geological relationships and U-Pb zircon and 40 Ar/ 39Ar tourmaline geochronology of gneisses and tourmalinites from the Nevado–Filabride complex (western Sierra Nevada, Spain): Tectonic implications

The Nevado–Filabride complex, the lowest complex in the Betic hinterland, forms a stack of Alpine nappes. The tectonic units consist of metasedimentary sequences whose ages are not well constrained. Gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex. New radiometric data from U-Pb zircon dating on gneisses from the western Sierra Nevada confirm the presence of late Carboniferous intrusive rocks in the Betic hinterland. These results, combined with available data from the literature and a detailed structural analysis, suggest that the gneisses represent a single late Variscan magmatic event. Evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations, in conjunction with geochemical data, U-Pb zircon (314 ± 7; 304 ± 23 Ma) and 40Ar/ 39Ar tourmaline (319.85 ± 5.81; 317.85 ± 3.67 Ma) geochronology. A pre-late Carboniferous age for the basal formation of the Nevado–Filabride sequence can be inferred. The gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites, respectively. Superposed tectonic units rather than a continuous Palaeozoic sequence is supported by the occurrence of Palaeozoic rocks at the top of the Nevado–Filabride complex. The nappe tectonics, as evidenced from the contractional character of the unit boundaries and the superposition of higher-grade on lower-grade metamorphic rocks, is additionally supported by the superposition of older on younger rocks as revealed from radiometric data.

Structural Evolution of the Rock Creek Gold Deposit, Seward Peninsula, Alaska

Sheeted quartz veins at Rock Creek contain an indicated gold resource of 404,000 ounces (oz) at a grade of 1.31 grams per ton (g/t). The deposit, located on the Seward Peninsula, 12 km north of Nome, Alaska, occurs in variably graphitic greenschist-facies schist, fine-grained quartzite, and dirty marble of the lower Paleozoic to upper Proterozoic Nome Group. The veins at Rock Creek, as at many prospects in the southwestern Seward Peninsula, are hosted by the most carbonaceous part of the Nome Group. These strata underwent complex polyphase deformations that were progressively less penetrative through time. They include four distinct styles of folding, followed by an episode of thrust faulting. The Mid-Cretaceous dilational vein system formed during the onset of D4 deformation along the northeast-trending Albion fault. The overall shape of the vein system in relationship to controlling faults shows that a preexisting structural fabric may have influenced its formation; tension veins lie along but slightly oblique to the northeast-striking Albion fault in a left-stepping array. Younger “Albion” veins parallel and lie within the Albion fault zone. The highest ore tenor and greatest concentration of veins occur near the intersections of northeast and northwest structures. The veins diminish in frequency along strike across the northwest-trending structures. The vein system occurs primarily in the hanging wall of a curviplanar dextral thrust fault that splays from the high-angle Albion fault and becomes gently dipping in its upper reaches. Vein concentration diminishes abruptly below this curviplanar transpressional structure. Small stratigraphic offsets show that the vein system was not displaced by the structure but rather formed preferentially above it. The orientations and sequencing of structural elements show that the tension vein system formed during a period of highly focused dilation. The Albion fault zone subsequently propagated through the maximum zone of dilation as it evolved to a throughgoing dextral strike-slip fault system. Vein chemistry changed from primarily Au-As (tension veins) to Sb-Au-Ag-As-Pb (Albion-style veins) contemporaneously with a 10° counterclockwise migration of vein orientation. These changes mark a period of highly focused extension immediately prior to the onset of throughgoing transpression along the Albion fault, which resulted in a collapse of the hydrothermal system. The left-stepping geometry of tension veins and later evolution of the overall structural system to compression suggest that it formed within a restraining bend along the dextral strike-slip fault system. The general shape of the vein system, the positions and orientations of controlling structures, and their vectors and styles of displacement define a sidewall ripout that flanks the Albion fault zone.

Aragonitegrossular intergrowths in eclogite-facies marble, Alpine Corsica

The occurrence and preservation of aragonite in eclogite-facies rocks of north-eastern Corsica is linked to an uncommon microtexture. Aragonite exclusively occurs as oriented fibres in garnet crystals of a graphitic, more or less siliceous marble that immediately overlies a serpentinite body of the meta-ophiolitic unit. The arrangement of the fibres is grossly radial, but more clearly sectoral in subhedral garnet, the fibres growing perpendicular to the garnet/matrix interface. Raman mapping reveals that the carbonate is calcite in the matrix and in poikiloblastic garnet cores, and that the fibres in the garnet mantle are aragonite alone in the case of a carbonate matrix, and both aragonite and quartz (in distinct fibres) in a quartz–carbonate matrix. These features are interpreted as prograde intergrowths, the result of garnet nucleation and growth in a calcite and then aragonite matrix (±quartz). Upon further heating and/or decompression, the aragonite matrix transformed back to calcite while the carbonates included in garnet retained their original structure, in spite of the relatively high temperature attained ( ca. 500 °C). These aragonite relics are one more example of the preservation of a high-pressure polymorph through mechanical shielding of inclusions in a rigid host. The aragonite–garnet intergrowths are similar to quartz–garnet intergrowths described in amphibolite-facies graphitic schists. They are evidence that oriented inclusions in garnet are not necessarily precipitates (‘exsolutions’). Unlike precipitates, their orientation is controlled more by the shape of the garnet growth front than by symmetry constraints.

Geoquímica das rochas meta-vulcânicas máficas do greenstone belt de Riacho de Santana, Bahia, Brasil

The Riacho de Santana greenstone belt (GBRS) of Archaean age, located in the southwestern of Bahia State, Brazil, occurs as an unit in the regional crystalline basement rocks known as Gaviao Block unit. Three distinct lithostratigraphic units are identified in GBRS: (i) Lower Unit, comprising of komatiitic ultramafic rocks with thin intercalations of metabasalts, metagabbros, quartzites and metacherts, calc-silicates and aluminous schists; (ii) Intermediate Unit, comprising of fine-grained metabasalts and meta gabbros, intermediate to acid meta tuffs, aluminous schists, graphite schists; and (iii) Upper Unit, comprising of carbonate-silicate sequence. In this study, we report major, trace and earth rare element composition of the basic metavolcanic rocks belonging to the Intermediate Unit, and infer the likely source (s) for these metavolcanics. All of the samples may be classified as high-iron tholeiites. Flat HREE patterns and smaller LREE/HREE fractionation in these rocks suggest that these melts were generated at a shallow depth in the absence of garnet in the source mineralogy. Primitive-mantle normalized REE patterns (and Ba/Zr, Ba/Nb, Ti/Y, Nb/Y, Nb/Th, and Zr/Y ratios) suggest that the primary magmas of these metavolcanics were derived from an enriched mantle reservoir or a mixture of enriched and depleted mantle reservoir. The magma composition was significantly affected by either due to assimilation of granitic crustal material, or due to the presence of subducted crustal sediments and/or thin Archaean subcontinental lithosphere in the mantle source region. Future work on Sr, Pb, and Nd isotope systematics may reveal the effect of crustal assimilation

Microtextural constraints on the interplay between fluid–rock reactions and deformation

Schists from two mylonitic localities in the footwall of a low-angle normal fault in the eastern Alps record different degrees of embrittlement during exhumation, depending on the extent to which fluid–rock reactions proceeded. At one site, graphitic schists preserve textural evidence for two metamorphic reactions that modified \( {\rm X}_{\mathop {\rm CO}\nolimits_2 } \) and/or fluid volume: (1) reaction between graphite and aqueous fluid that increased \( {\rm X}_{\mathop {\rm CO}\nolimits_2 } \) without changing the molar amount of fluid, and (2) replacement of titanite by rutile, calcite, and quartz. The latter reaction involved net consumption of increasingly CO2-rich fluid. Areas where the first reaction proceeded are associated with abundant Mode I microcracks. Fluid inclusion arrays within the microcracks show that \( {\rm X}_{\mathop {\rm CO}\nolimits_2 } \) increased from ∼0.1 to 0.6 during decompression from 4.75 to 2 kbar at a reference temperature of 500°C. Titanite consumption is most pronounced within transgranular Mode I microcracks, but microcracks do not crosscut products of this reaction; fluid consumption during reaction was coeval with the end of microcracking, at least on a local scale. At the other site, graphitic schists lack small-scale Mode I cracks as well as evidence for graphite consumption during decompression. SEM imaging shows that graphite is anhedral and pitted at the first site, but occurs in clusters of euhedral grains at the second site. Mass balance calculations demonstrate that rocks with partially consumed graphite were infiltrated by an externally derived, H2O-rich fluid that drove subsequent graphite-fluid reaction. Evidence for similar fluid infiltration is absent at the second site. Variations in the degree of reaction progress indicate that fluid pathways and deformation style were heterogeneous on the scale of millimeters to kilometers during exhumation from mid-crustal depths.

Preliminary Results from the Yala-Xiangbo Leucogranite Dome, SE Tibet

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.91