Muscovite Schist Research Articles

Evolução tectônica da Zona de Cisalhamento Caxambu, MG

The NE-SW trending Caxambu Shear Zone (CSZ) is an example of a group of similar shear zones that crop out in southern Minas Gerais and adjacent Rio de Janeiro states. They represent the final stage of the Neoproterozoic tectonic activity related to the Brasiliano Orogeny in this region. It is a dextral shear zone with predominantly ductile behavior. The CSZ affects metaigneous and metasedimentary rocks of a Paleoproterozoic basement with its Neoproterozoic cover denominated Andrelândia Megasequence (AMS). Six mapping units were recognized: orthogneiss (basement), banded biotite gneiss (AMS), quartzite (AMS), biotite schist (AMS), garnet muscovite schist (AMS) and a plug of alkaline intrusive rocks of probable Cretaceous age. Two groups of structures were recognized in the area. The first group, attributed to deformation phase D B , related to the Brasilia Belt, includes the main foliation and associated lineation and extensive nappe structures with tectonic transport top to the east. The second group, related to the Ribeira Belt and labeled D R , includes the Caxambu Shear Zone. The CSZ is more then 100 km long and about 2.5 km wide; its dextral horizontal displacement in the studied area is estimated at about 14.5 km. Including published estimates from adjacent areas an average value of 17.6 km is obtained, resulting in na approximate average value for γ of simple shear of seven. Associated to the CSZ mylonitic rocks were formed containing the following shear sense indicators: mica fish and other minerals with similar shape and asymmetry, oblique foliation, S-C structures, mantled porphyroclasts and asymmetric microfolds. The metamorphic conditions during mylonitization are estimated at lower amphibolite facies.

Lithologic and mineral information extraction for gold exploration using ASTER data in the south Chocolate Mountains (California)

An ASTER image covering the south Chocolate Mountains area, California, U.S.A. was evaluated for gold-related lithologic mapping and alteration mineral detection. A supervised classifier was first applied to the 14-channel ASTER radiance data to map lithologies related to gold deposits. Subsequently, four alteration indices were extracted from the six SWIR channels and transformed to delineate alteration zones using a PCA transformed mineralogic indices approach. Finally, a subpixel unmixing algorithm, the constrained energy minimization (CEM) technique was used to detect significant alteration minerals using the ASTER VNIR and SWIR surface reflectance data and reference spectra from the ASTER spectral library. The classification results show that the ASTER data were capable of mapping flood basalt, quartz-biotite gneiss, muscovite schist, granitic, volcanic, and metasedimentary rock units. The ASTER-derived rock units show excellent correlation with those on the reference geologic map. The overall classification accuracy is 82% and the Kappa coefficient 0.76. A group of gneisses, locally the most favorable host rocks of gold deposits, were mapped with the ASTER data with a Producer's accuracy of 86%, and more importantly were also mapped in some areas that were not shown on the field geologic map. Four alteration minerals: alunite, kaolinite, muscovite and montmorillonite were detected by subpixel unmixing analysis of the ASTER reflectance data. This study compared different methods for extracting mineralogic information from ASTER data, compared the remotely derived maps to the mapped field geology, and used the ASTER data to map minerals and lithologies related to gold exploration.

Yukon–Tanana terrane in the southern Campbell Range, Finlayson Lake belt, southeastern Yukon: the geological setting of retrogressed eclogite of the Klatsa metamorphic complex

Yukon–Tanana terrane in the southern Campbell Range is composed of rocks that have different metamorphic, exhumation, and structural histories, and that have formed in disparate parts of the Paleozoic Yukon–Tanana volcanic arc. The geological relationships in the southern Campbell Range reveal the tectonic and structural history of the Klatsa metamorphic complex, which represents the remnants of an Early Mississippian subduction zone beneath the Yukon–Tanana arc. The Klatsa metamorphic complex is composed of foliated to massive serpentinite, leucogabbro, amphibolite, and retrogressed eclogitic quartz–muscovite schist with lenses of metabasite. It was structurally juxtaposed on Upper Mississippian to Lower Permian metasedimentary rocks of the White Lake, King Arctic, and Money Creek formations. Regional and local structural and stratigraphic relationships suggest that the Klatsa metamorphic complex is part of the Cleaver Lake thrust sheet, the structurally highest thrust sheet in a north- to northeast-vergent thrust belt that deformed the Yukon–Tanana terrane during the Early Permian. Restoration of the displacement on the Cleaver Lake and underlying thrust faults places the Klatsa metamorphic complex on the western margin of Yukon–Tanana terrane. Late Devonian to Early Mississippian subduction is thought to have occurred along this margin based on previous paleogeographic reconstructions. Generally north- to northeast-vergent D1 to D3 folds deformed the Klatsa metamorphic complex and adjacent metasedimentary rocks. Jurassic(?) D4 imbricate thrust faulting has, in part, reactivated the Cleaver Lake thrust fault contacts and imbricated the Klatsa metamorphic complex with metasedimentary rocks in fault panels that are repeated at a scale of 10 to hundreds of metres.

Organic geochemical characteristics of the Paleocene–Eocene oil shales in the Nallıhan Region, Ankara, Turkey

Units of Paleozoic, Mesozoic and Cenozoic age are exposed in the study area which extends along the Sakarya River by the Sarýcakaya town of Eskiþehir at west and Nallýhan district of Ankara at East. Paleozoic is represented with metamorphites consisting of glaucophane schist, muscovite schist and marble alternations and granites cutting them. The Mesozoic composing of Jurassic–Cretaceous Soğukçam limestones at the northern part of the area is in tectonic contact with other units. The Tertiary is characterized by the Paleocene–Eocene Kızılçay Group which consists of the Beyköy, Çamalan and Laçin Formations. The Çamalan Formation is composed of sandstone, limestone, oil shale and locally coal bands. In the area, as a result of intense tectonic activity by the late Eocene, sedimentary sequence that compacted within the Paleozoic metamorphic rocks were folded and fractured. The Paleocene–Eocene shales have adequate organic material content to form potential source rocks. In order to determine hydrocarbon potential of the unit, various organic geochemical analysis techniques were performed. According to pyrolysis results (TOC, S 1 and S 2), the Paleocene–Eocene shales are perfect source rocks for oil. On the basis of Rock-Eval pyrolysis analyses, kerogens are Type I and Type II kerogens. Spore Color Index and T max evaluations indicate that maturity of the unit is between diagenesis and early mature oil zone and at the beginning of the oil-producing stage. Organic facies of the Paleocene–Eocene shales are A, AB and B.

Low-rhenium molybdenite by metamorphism in northern Sweden: Recognition, genesis, and global implications

Re–Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore deposits using the model age or isochron approach. But, molybdenite has a new niche in the greenschist- to granulite-facies metamorphic environment. Re–Os ages for metamorphic molybdenite may be used to construct regional metamorphic histories. Age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single, petrographically-characterized molybdenite occurrences. In this study, twelve geologically distinct molybdenite-bearing samples from two small Mo districts in northern Sweden trace a 150 m.y. Paleoproterozoic Svecofennian metamorphic history from ∼1900 to 1750 Ma. These data reveal a little-known, widespread and protracted, Late Svecofennian anatexis in northern Sweden. The Kåtaberget Mo–(Cu, F) deposit is located in the Moskosel granite batholith north of the economically-renown Skellefte district. Four different molybdenite samples from outcrop at Kåtaberget indicate an intrusion age of 1895 ± 6 Ma with the formation of later pegmatite–aplite at 1875 ± 6 Ma. The Allebuoda (Björntjärn) and Munka Mo–(W) deposits in the Rappen district are represented by three outcrop and five drill core samples of molybdenite-bearing aplite–pegmatite–granite. These two deposits were previously described as intrusion-related Climax-type Mo mineralization. Re–Os ages for molybdenites from these deposits range from 1865 to 1750 Ma and, significantly, Re concentrations are markedly low, extending to the sub-ppm level. Age agreement within the deposits is conspicuously lacking, whereas, with one exception, age agreement within any single sample (geologic occurrence), as established by analysis of additional molybdenite separates, is very good. These data, together with fundamental geologic observations discussed in this paper, suggest that Mo–(W) mineralization in northern Sweden is not intrusion-related, but the local product of episodic melting of Archean–Paleoproterozoic supracrustal gneisses related to the Svecofennian orogeny. Petrographic traverses across the boundary between widespread, foliation-parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation. Dehydration breakdown of zircon-rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine, post-deformational biotite plus sulfides, oxides, hydrothermal zircon and fluorite, all associated with microcline-dominant leucosomes. This process has profound implication for the traditional leucogranite, intrusion-related genesis attributed to the broad classification of Mo–W–Sn–base and precious metal mineralization (e.g., South Mountain Batholith, Nova Scotia; Okiep, Namaqualand, South Africa; Mactung, Yukon; Pogo–Liese, Tintina, Alaska; Carajás and Goiás–Rio Tocantins, Brazil; New England Batholith, NSW, Australia; Bergslagen, Sweden; Nevoria, Western Australia; Alpeinerscharte, Austria; Erzgebirge, Germany; Sardinia–Corsica Batholith). In addition to biotite, metallogenic contributions (e.g., Mo, W, Sn, U, Bi, Cu, Pb, Zn, Fe, Ni, Co, Au, Ag, Te, As, Sb, REE) in various combinations may also be controlled by breakdown of amphibole. In effect, the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite. In the absence of focusing structures (e.g., shear zones, sheeted vein development), this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades. Low Re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss (Mo–W) or muscovite schist (Sn–W).

Re-evaluation of the petrogenesis of the Proterozoic Jabiluka unconformity-related uranium deposit, Northern Territory, Australia

The world class Jabiluka unconformity-related uranium deposit in the Alligator Rivers Uranium Field, Australia, contains >163,000 tons of contained U3O8. Mineralization is hosted by shallow-to-steeply dipping basement rocks comprising graphitic units of chlorite–biotite–muscovite schist. These rocks are overlain by flat-lying coarse-grained sandstones belonging to the Kombolgie Subgroup. The deposit was discovered in 1971, but has never been mined. The construction of an 1,150 m decline into the upper eastern sector of the Jabiluka II deposit combined with closely spaced underground drilling in 1998 and 1999 allowed mapping and sampling from underground for the first time. Structural mapping, drill core logging and petrographic studies on polished thin sections established a detailed paragenesis that provided the framework for subsequent electron microprobe and X-ray diffraction, fluid inclusion, and O–H, U–Pb and 40Ar/39Ar isotope analysis. Uranium mineralization is structurally controlled within semi-brittle shears that are sub-conformable to the basement stratigraphy, and breccias that are developed within the hinge zone of fault-related folds adjacent to the shears. Uraninite is intimately associated with chlorite, sericite, hematite ± quartz. Electron microprobe and X-ray diffraction analysis of syn-ore illite and chlorite indicates a mineralization temperature of 200°C. Pre- and syn-ore minerals extracted from the Kombolgie Subgroup overlying the deposit and syn-ore alteration minerals in the Cahill Formation have δ18Ofluid and δDfluid values of 4.0±3.7 and −27±17‰, respectively. These values are indistinguishable from illite separates extracted from diagenetic aquifers in the Kombolgie Subgroup up to 70 km to the south and east of the deposit and believed to be the source of the uraniferous fluid. New fluid inclusion microthermometry data reveal that the mineralising brine was saline, but not saturated. U–Pb and 207Pb/206Pb ratios of uraninite by laser-ablation ICP-MS suggest that massive uraninite first precipitated at ca. 1,680 Ma, which is coincident with the timing of brine migration out from the Kombolgie Subgroup as indicated by 40Ar/39Ar ages of 1,683±11 Ma from sandstone-hosted illite. Unmineralized breccias cemeted by chlorite, quartz and sericite cross-cut the mineralized breccias and are in turn cut by straight-sided, high-angle veins of drusy quartz, sulphide and dolomite. U–Pb and 207Pb/206Pb ratios combined with fluid inclusion and stable isotope data indicate that these post-ore minerals formed when mixing between two fluids occurred sometime between ca. 1,450 and 550 Ma. Distinct 207Pb/206Pb age populations occur at ca. 1,302±37, 1,191±27 and 802±57 Ma, which respectively correlate with the intrusion of the Maningkorrirr/Mudginberri phonolitic dykes and the Derim Derim Dolerite between 1,370 and 1,316 Ma, the amalgamation of Australia and Laurentia during the Grenville Orogen at ca. 1,140 Ma, and the break-up of Rodinia between 1,000 and 750 Ma.

Tourmaline-bearing rocks in the Singhbhum shear zone, eastern India: Evidence of boron infiltration during regional metamorphism

Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about fluid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic to psammopelitic and quartzofeldspasthic rocks, including meta-granite. The rocks were affected by two sets of folding (F1 and F2) and ductile shearing associated with F1 during ca. 1.6–1.8 Ga tectonic activity in this belt. Quantitative geothermobarometry and the stability relations of the metamorphic assemblages developed in the pelitic rocks establish the presence of two episodes of metamorphism; a prograde M1 event that culminated at 480 ± 40 °C, 6.4 ± 0.4 kbar, and an M2 event that caused retrogression of the M1 assemblages. Field and petrographic observations in combination with data on chemical composition of tourmaline indicate three stages of tourmaline growth. The earliest stage is represented by pretectonic (pre−metamorphic) greenish-yellow tourmaline cores in the tourmalinite from the Surda area (Association A). The textural relations demonstrate that in the second stage, greenish-yellow cores of oscillatory zoned tourmaline in the metagranite (albite + quartz + biotite + chlorite ± muscovite ± apatite ± magnetite, Association C), very fine tourmaline grains in muscovite schist (muscovite ± quartz, Association B) and greenish yellow cores of tourmaline in biotite-muscovite schist (biotite + muscovite + magnetite + chlorite + apatite+ quartz, Association D) crystallized during the F1-M1 events. Post-tectonic, greenish-blue tourmaline, belonging to the third stage, replaces the earlier tourmaline grains along their margins and cracks. Compositionally, tourmalines of all four associations fall in the alkali group. Most of the data fall in the dravite field with a few analyses straddling the boundary between schorl and dravite. An abrupt compositional change was noted across the different color zones in the tourmaline grains. This change is explained by one or more of the coupled substitutions, □Al(NaR)−1, CaR(NaAl) −1, and Mg(Fe) −1, where R = Mg + Fe2+ + Mn. Integrating the field, petrographic, and phase-compositional data, it has been demonstrated that tourmaline growth in the second and third stages was induced by infiltration-driven, B metasomatism during prograde metamorphism of the rocks. Deep-seated magma was the source of the B-bearing hydrothermal fluid.

A Landslide Hazard Assessment and Vulnerability Appraisal Procedure: Vunguvungu/Banga Catchment, Northern Malawi

Landslides are a common phenomenon in parts of Malawi. A number of historical landslides have been documented, and are summarised here. This paper examines the occurrence of landslides in the Rumphi district of northern Malawi, concentrating on the catchment of the Vunguvungu and Banga rivers. This is an area of deeply weathered biotitic gneiss and muscovite schist, with deep, sandy soils, comparatively steep slopes and a rainfall in excess of 1,500 mm per annum. These factors, in association with changing land use patterns, have contributed to the landslide vulnerability of the area. The investigation focuses, as a case study, on the Banga landslide of 1997 for which data are available, and which occurred after unseasonal rain. A unique combination of natural and human induced factors is proposed in explaining the occurrence of landslides. The paper concludes by proposing an elementary vulnerability appraisal procedure for the catchment and by discussing the potential risk of landslides in this area.

RELATIVE TIMING OF ALBITIZATION AND CHLORINE ENRICHMENT IN BIOTITE IN PROTEROZOIC SCHISTS, SNAKE CREEK ANTICLINE, MOUNT ISA INLIER, NORTHEASTERN AUSTRALIA

In the Snake Creek area, near Cloncurry, Mount Isa Inlier, in northeastern Australia, multiple episodes of deformation, lowpressure metamorphism, intrusion of abundant mafic and granitic bodies, and extensive metasomatism occurred throughout the Mesoproterozoic Isan Orogeny. Albitites and associated metasomatic biotite-rich schists are abundant in the core of the Snake Creek Anticline, and are concentrated in five areas. Albitization peaked during D1 and was commonly localized along shear zones. Muscovite schists adjacent to albitites were altered to albite-bearing biotite-rich schists. Porphyroblast growth in metasomatic rocks commenced with cordierite and andalusite from late syn-D1, but episodic growth continued to post-D5, with the peak of metamorphism occurring late syn-D3 to early syn-D4. The timing of albitization was determined from the presence or absence of albite in various stages of andalusite growth, and similarly the Mg# [Mg/(Mg + Fe)] and Cl contents of biotite were examined over the span of porphyroblast-growth history. Differential mass-transport of Mg and Fe took place from the albitites to the adjacent schists predominantly during D1, and the Mg# for biotite in metasomatic rocks was largely fixed during D1. The Cl content of the matrix biotite varies considerably in the different areas of albitite development, from below detection to a maximum of 1.5 wt%. This could indicate that the Cl enrichment event was not related to albitization, but there is nevertheless an overall spatial relationship between them. In any particular sample, the Cl content in biotite is generally higher in the matrix than for inclusions in late andalusite, with inclusions in early andalusite being the poorest in Cl. Enrichment of Cl in biotite is a function of f(HCl)/f(H2O) rather than salinity, and so could be related to the replacement of muscovite by albite, as this involves release of H + . The ratio f(HCl)/f(H2O) would have been low during periods of high flux of fluid that produced the albitites, but increased during low flux, in the waning stages of albitization, resulting in maximum enrichment in Cl relatively late in the metasomatic history.

Ubendian basement and its late Mesoproterozoic and early Neoproterozoic structural and metamorphic overprint in northeastern Zambia

The Palaeoproterozoic basement in the Muyombe and Luwumbu River areas of northeastern Zambia comprises a WNW–ESE (to E–W) trending cordierite–garnet–sillimanite granulite unit with numerous enderbite bodies and an amphibolite-facies migmatite unit. Zircons from a biotite metatonalite intruding the granulites were dated at 1960.7 ± 0.4 Ma, and this is time-equivalent with the Nyika granite in adjacent Malawi. Mesoproterozoic intrusions into this basement are represented by a nepheline syenite at Mivula Hill (zircon age: 1360.1 ± 0.8 Ma) and the porphyritic Ntendele biotite metagranite (zircon age: 1329.1 ± 0.6 Ma). The Ntendele granite attains plutonic dimensions north of Muyombe. The Mesoproterozoic Mafinga Group occurs in two major belts imbricated in the basement, i.e., the Makutu Range belt and Nyika escarpment belt, both trending NE–SW, nearly at right angles to the Ubendian structures in the basement. Numerous smaller tectonic slices of the Mafinga Group, in the Ubendian basement gneisses farther south of the two major belts, document a significant areal extent of this unit. Structures and metamorphic effects (greenschist-facies, biotite ± garnet zone) imprinted in the Mafinga Group rocks correlate with those in the Irumide belt farther west near Isoka and to the north, in the Mafinga Hills. A mylonitic foliation and newly-formed mineral assemblages (biotite zone) in the Ntendele granite correspond to deformation and metamorphism of the Mafinga Group. Mylonitization affecting the basement complex near the Mafinga Group slices resulted in strongly but heterogeneously sheared domains with a corresponding Irumide-age structural and metamorphic overprint. 40Ar/ 39Ar dating of muscovite from mylonitic gneisses derived from basement rocks and from a Mafinga Group phyllite point to closure of the K–Ar system in the interval 860–890 Ma. These early Neoproterozoic ages correlate with published muscovite and biotite K–Ar ages of biotite gneisses and muscovite schists from other parts of the Irumide belt in NE Zambia. They probably indicate slow cooling rates after main Irumide tectonism and associated magmatism around 1015 Ma. The 40Ar/ 39Ar data show that the Muyombe area was not affected by Pan-African thermal overprinting on a regional scale. In view of contrasting relations between the area studied and published results from northern Malawi, where both structural and thermal Pan-African overprints are documented, it is suggested that the NW–SE trending Mugesse shear zone in northern Malawi and along the border with NE Zambia is an important structural boundary separating contrasting crustal domains, i.e. the Ubendian belt, Irumide belt, and East African Orogen.

Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia

Tourmaline is a commonly found gangue mineral in Archean greenstone belt lode-gold deposits worldwide. In this paper, we report major, trace, and rare-earth element (REE) and boron isotopic compositions of tourmaline from two major amphibolite-grade Archean gold deposits (Big Bell and Mount Gibson; Murchison Province, Yilgarn Craton, Western Australia), in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization. Tourmalines in these two major deposits all belong to the dravite–schorl series, and tend to be Mg-rich where closely associated with gold and sulfide ores. The ore-related tourmalines also contain high concentrations of ore metals such as Au, Ag, Pb, and Cu. In the Big Bell deposit, tourmalines occur along the foliation in lode schists, and show low Fe/(Fe+Mg) ratios of 0.11–0.30, TiO 2 contents of 0.15–0.73 wt.%, and negligible MnO (<0.05 wt.%). This type of tourmaline has δ 11B values of −17.7‰ to −15.0‰. In the Mount Gibson deposit, the tourmalines closely associated with the sulfide ores are the most Mg-rich, with low Fe/(Fe+Mg) ratios of 0.19–0.22, while tourmalines from garnet-bearing biotite–muscovite schists that overly the lode-gold orebody are more Fe-rich (Fe/(Fe+Mg)=0.30–0.63). Tourmalines from an amphibolite schist are Mg-rich, with Fe/(Fe+Mg) ratios (0.27–0.34) that lie between the above two groups of tourmalines. The compositional trend from Fe-rich tourmalines in the sulfide-poor schists to Mg-rich in the sulfide ores at Mount Gibson is similar to that observed in massive sulfide ore deposits worldwide. Alternatively, the Mg-rich nature of the sulfide-related tourmalines may be due to metamorphic reactions that are accompanied by Fe incorporation into sulfides, leaving coexisting silicates more Mg-rich. The REE geochemistry of the tourmalines from the Mount Gibson ore zone schists displays strong HREE enrichments and positive Eu anomalies with Eu/Eu* ratios up to 2.4, which are similar to tourmalines associated with massive sulfide deposits. The ore-related tourmalines disseminated in the host schists show δ 11B values of −18.6‰ to −17.0‰, whereas one coarse-grained tourmaline vein has a lower δ 11B value of −21.9‰ to −21.4‰. These data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal-rich, hydrothermally altered, seafloor horizon that is typical of VMS-style mineralization, with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation.

Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden

The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The volcanic pile is intruded by a synvolcanic Jorn-type granitoid (Viterliden intrusive complex) and is overlain by a thick sequence of metasedimentary rocks (Vargfors Group). Mineralization occurs at two main stratigraphic levels, at the base of the felsic volcanic succession and at the contact with the metasedimentary rocks of the Vargfors Group. The Kristineberg Cu–Zn mine is the largest deposit (approximately 21 Mt) and occurs at the base of the volcanic pile, close to the contact with the Viterliden intrusive complex. Four smaller deposits (Ravliden, Ravlidmyran, Horntrask and Nyliden) occur along the upper ore horizon. These deposits are thought to be related to a late intrusive phase of the Viterliden complex which cuts the altered volcanic rocks at the Kristineberg deposit. Within an area of about 50 km2 surrounding the Kristineberg deposit, felsic volcanic rocks between the two ore horizons are affected by extensive albite-destructive alteration (sodium depletion) and development of chlorite and muscovite (strong co-enrichment in magnesium and potassium). The Kristineberg deposit is enveloped by a large and partly transposed quartz–chlorite alteration zone, approximately 2 km in diameter, and a distal but coherent pyrite–quartz–muscovite alteration zone extending as far as 4 km from the deposit. Chlorite(±talc) in the mine area is notably magnesium-rich and contains anomalous F, Ba, Zn and Mn. High fluorine is also present in coexisting muscovite and phlogopite. The magnesium-rich chlorite alteration contrasts sharply with the iron enrichment observed in many other felsic, volcanic-hosted Precambrian massive sulfide deposits. This may indicate fixation of iron by large amounts of pyrite in the section or entrainment of large amounts of seawater in the hydrothermal upflow zones. Kyanite is developed locally in the chlorite-rich pipe at Kristineberg in response to regional thermal metamorphism of highly aluminous alteration in the immediate foot-wall rocks. Spectacular, andalusite-bearing quartz–muscovite schists and quartz–biotite–cordierite schists also occur where the altered felsic volcanic rocks are intruded by the late Revsund granite. However, similar metamorphic mineral growth is not observed where the volcanic rocks at the contact are less altered. Deposits near the top of the felsic volcanic succession are characterized by magnesium-rich chlorite alteration in the foot wall and proximal calc-silicate assemblages (dolomite, calcite, tremolite, ±garnet, ±margarite) where the host sedimentary rocks are carbonate-rich. In general, the calc-silicate alteration is restricted to the immediate hanging wall and zones lateral to the deposits and does not represent a regionally extensive exploration target. The two main ore horizons in the Kristineberg area are not linked by any obvious discordant structures or alteration zones. However, mineral-chemical studies highlight several possible fluid flow pathways leading from the Kristineberg deposit to the Ravliden ore horizon, more than 2 km upsection. Overprinting regional metamorphic minerals have inherited the hydrothermal signature of the ore-related alteration. Electronic Supplementary Material is available at http://dx.doi.org/10.1007/s00126-002-0299-y. On that page (frame on the left side), a link takes you directly to the supplementary material.

Geology and geodynamic setting of Archaean silicic metavolcaniclastic rocks of the Bien Venue Formation, Fig Tree Group, northeast Barberton greenstone belt, South Africa

Mapping of the northeastern part of the Barberton greenstone belt (BGB) has delineated a new lithostratigraphic unit, the Bien Venue Formation, within the Fig Tree Group, Swaziland Supergroup, South Africa. The formation is composed mainly of quartz–muscovite (±andalusite±chlorite±chloritoid±pyrophyllite) schists derived from a sequence of quartz-phyric, dacitic-to-rhyodacitic volcaniclastic protoliths dated at 3256±1 and 3259±5 Ma. Subordinate rock types include banded chert, phyllite, biotite–plagioclase (±chlorite±carbonate) schists derived from plagioclase-phyric dacitic volcaniclastic assemblages, and chlorite (±carbonate±amphibole) schists representing altered basalt or basalt-andesite. These rocks were deposited on deep-water shales, turbiditic greywackes, banded iron-formations (BIFs) and silicic metavolcaniclastic assemblages correlated with the Belvue Road Formation, and are, in turn, overlain by alluvial to shallow-marine sandstones and conglomerates of the Moodies Group along a regional unconformity. In places, the quartz–muscovite schists host subeconomic massive sulphide (Zn–Cu–Pb–Ag) and barite deposits of probable volcanogenic origin. The sedimentary structures, lithologies, stratigraphic associations, and mineral deposits suggest that the Bien Venue Formation represents a proximal volcaniclastic succession formed in a largely subaqueous depositional environment that may have varied from deep (>1 km depth) to shallow water. The protoliths of the Bien Venue schists were geochemically similar to calc-alkaline and transitional tholeiitic-calc-alkaline suites formed at Phanerozoic mature active continental margins. Variably fractionated rare earth element (REE) profiles, moderate abundances of Yb and Y, and relative depletions at Ta–Nb, P and Ti on normalised immobile multi-element diagrams are consistent with hydrous fusion of upper mantle material above steeply subducted oceanic crust undergoing dehydration. The features suggest that the Bien Venue Formation records Early Archaean continental arc volcanism along the north-east flank of the BGB. The lithochemical signatures of the Bien Venue rocks are distinctly different from those of silicic volcanic suites hosting major syngenetic polymetallic massive sulphide deposits in Canada. The host rocks of the Bien Venue massive sulphide deposit do not appear to have undergone significant fractionation in a high-level magma repository which could have supplied heat for the development of a major mineralising hydrothermal fluid system. These observations suggest low potential for the discovery of economic base-metal massive sulphide deposits in the rocks of the Bien Venue Formation. The Bien Venue rocks were previously included in the Theespruit Formation of the Onverwacht Group, however, the isotopic age data indicate that they are some 290 million years younger than the oldest assemblages within the lower Onverwacht. The schists are also 30 million years older than plagioclase-phyric dacitic rocks within the Schoongezicht Formation (ca. 3226 Ma), uppermost formation of the Fig Tree Group along the northwest flank of the BGB, precluding direct correlation with this unit. Published ages indicate that the Bien Venue Formation is a temporal correlative of proximal volcaniclastic rocks making up the Auber Villiers Formation (3256 Ma), and possibly also some distal dacitic tuffaceous sequences within the Mapepe Formation (3258–3227 Ma), both of which form part of the southern Fig Tree facies.

MORB-DERIVED VARISCAN AMPHIBOLITES IN THE NORTHERN APENNINE BASEMENT: THE CERRETO METAMORPHIC SLICES (TUSCAN-EMILIAN APENNINE, NW ITALY)

In the Cerreto area metamorphic rocks belonging to the basement of the Northern Apennine are exposed in association with Triassic evaporites and quartzites. The Cerreto basement rocks consist of lenses and plurimetric bodies of amphibolites included in a sequence of metapsammites (biotite and/or muscovite schists). Both metasediments and amphibolites underwent a retrograde evolution starting from an initial medium-pressure amphibolite stage (M1: P ~ 8 kbar, T ~ 650°C) down to greenschist condition. The amphibolites show a well preserved earlier foliation associated with development of green amphibole (aluminotschermakite) + plagioclase + quartz + ilmenite assemblage followed by partial retrograde recrystallization in epidote-amphibolite facies under static conditions and along shear zones (M2: ~ P 4-5 kbar, T ~ 530°C); pre-kinematic garnet porphyroblasts rimmed by amphibole + plagioclase coronas are widespread. Metasediments show a pervasive mylonitic deformation associated with amphibolite to greenschist facies assemblages and rare relics of an earlier fabric, testified by inclusions of quartz + biotite in plagioclase and garnet porphyroclasts. The latest stages of deformation are accompanied in both metasediments and amphibolites by retrograde development of greenschist-facies assemblages in millimeter-scale shear zones crosscutting the former structures. Major and trace element compositions of the amphibolites suggest that most protoliths are cogenetic and could be related through fractional crystallization of plagioclase + olivine and/or pyroxene from variably evolved tholeiitic liquids. Geochemical data also indicate that most of the amphibolites have protolith features resembling Enriched-type MORB. 40Ar/39Ar dating performed on two hornblende samples (312 and 328 Ma) indicate a Variscan age for the amphibolite-facies metamorphism implying that the Cerreto metamorphic rocks belong to a unit deformed and metamorphosed at deep structural levels during the main deformational event (i.e. collisional stage) of the Southern Europe Variscan belt. The present tectonic setting of the Cerreto rocks, i.e. rootless slices within cover unit, could be related to the Apenninic thrusting. The Apenninic tectonic history likely occurred at very high structural levels without development of pervasive syn-metamorphic structures.

Rôle de la déformation métamorphique dans la cristallinité du graphite : l'exemple des schistes graphiteux de la vallée de la Lotru (Carpathes, Roumanie)

Role of metamorphic strain in the cristallinity of graphite: the example of the graphitic schists from the Lotru valley (Carpathians, Romania). The study of the schists from Lotru valley (Carpathians mountains, Romania) shows the role of stress in graphite crystallinity. The carbonaceous material was separated and studied by X-ray diffraction and Raman microspectrometry. There exists a clear contrast between the crystallization degree of graphite found in the quartz- muscovite schists (good) and that found in the quartzites (poor). This observation is in conformity with a clear differentiation of fabric. The graphite suffered an anisotropic deformation in the graphitic schists, whereas it was protected from deformation by quartz in the quartzites. In this last case, the development of graphite from the organic material took place with a volume decrease, protected by the surrounding quartz. To cite this article: S.-C. Barzoi, B. Guy, C. R. Geoscience 334 (2002) 89-95.

Sr, Nd isotopes and geochemistry of the Bayuda Desert high-grade metamorphic basement (Sudan): an early Pan-African oceanic convergent margin, not the edge of the East Saharan ghost craton?

The high-grade metamorphic basement of the Bayuda Desert is situated at the inferred transition between the juvenile Neoproterozoic mainly greenschist facies Arabian–Nubian Shield (ANS) and the pre-Neoproterozoic mainly amphibolite facies domain of the East Saharan Ghost Craton. New geochemical and Sr–Nd isotope data reveal that this basement in Bayuda constituted a Neoproterozoic oceanic convergent margin succession with limited and probably late input of old material. Within this series, garnet amphibolites and epidote–biotite gneisses have geochemical characteristics of HFSE-depleted tholeiitic basalts and low- to medium-K dacites and rhyodacites, indicating magmatism in an oceanic island arc or back-arc basin environment. This magmatism occurred at 806±19 Ma (Sm–Nd 11 WR isochron), similar in age to arc magmatism in the ANS. Leucocratic gneisses, muscovite schists and garnet–biotite schists form the dominant meta-sedimentary rocks of the study area. They were primarily derived from two different sources: volcanogenic sediments from a Neoproterozoic island arc (T DM Nd model ages between 790 and 900 Ma) and terrigeneous sediments from an older continental source (T DM Nd model ages up to 2100 Ma). The volcanosedimentary succession was metamorphosed under amphibolite facies conditions prior to 670 Ma, probably at approximately 700 Ma. The high-grade metamorphism is related to a frontal collisional event that also produced syn-collisional peraluminous granites preserved as muscovite–biotite gneisses. Meta-igneous rocks from eastern Bayuda have ε Nd values at 806 Ma of +5.2±0.4, indicating a less depleted mantle source (crustal contamination can be excluded) than the neighbouring Gabgaba–Gebeit terrane ( ε Nd, ca. +7) from the Arabian–Nubian shield. Less depleted mantle source is also known at Jebel Moya to the south, and inside the Arabian–Nubian Shield to the southeast of Bayuda. Lithological and structural similarities (dominantly northeast striking foliation) with the Bayuda Desert succession occur in many parts of central and western Sudan, suggesting a comparable geodynamic evolution. It is proposed that a Neoproterozoic oceanic convergent margin (Bayuda type succession) collided at approximately 700 Ma to the northwest with the East Saharan ghost craton, whose easternmost limit must lie below or more to the west than previously thought.

&lt;b&gt;Earth Sciences&lt;/b&gt;&lt;br&gt;&lt;br&gt;The geochemistry, tectonic setting and origin of the massive melanocratic amphibolite in the Ilesha schist belt, Southwestern Nigeria.

A massive melanocratic amphibolite, (MMA) occurs in Ilesha schist belt within a series of muscovite schists and amphibolite gneiss. Though metamorphosed, MMA shows no obvious textural deformation. Actinolite, tremolite, hornblende and boitite constitute the major minerals in MMA. Minor minerals in MMA include calcite, plagioclase and pyroxene while monazite, zircon and apatite form the accessory minerals. Chemical studies revealed that MMA contains low K2O and Na2O. Its Mg1, Cr and Ni contents are considerably lower than those of similar basalts derived from purely primitive mantle. Enrichment of LREE, negative Eu/Eu* anomaly and occurrence of monazite in its mineralogy are all indications that the precursor magma of MMA contains a sedimentary input. The plots of immobile trace (Zr, Ti, Nb, Y) and rare earth elements during the greenschist-amphibolite metamorphic grade for MMA show that it was derived from a low-K-Tholeiitic magma in a volcanic geotectonic setting (back arc basin). Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 85-90

NOVAS EVIDÊNCIAS DE REMANESCENTES OCEÂNICOS NA FAIXA ARAÇUAÍ: AS ROCHAS META-ULTRAMÁFICAS DE SÃO JOSÉ DA SAFIRA

Neoproterozoic oceanic slivers have been characterized in the Araçuaí Belt, eastern Brazil. Close to the SãoJosé da Safira town, Minas Gerais State, meta-ultramafic rocks consist of serpentinite with preservedperidotite cores, talc-anthophyllite schists, a diopside-rich rock, and tremolite schist. They are closelyassociated with banded iron formation (magnetic oxide, sulfide and silicate types), muscovite schist andgraphite schist. This rock assemblage is interpreted as an ophiolitic mélange emplaced by thrusts betweentwo different units. One of them, correlated to the Salinas Formation, consists of quartz-mica schist (withgarnet, staurolite, kyanite, and/or sillimanite), sedimentary-derived calc-silicate rock, and sparseorthoamphibolite. The unit correlated to the Capelinha Formation consists of orthoquartzite, hematite andmica quartzites. All described rocks were transported from NE to SW in response to a dextral, obliquethrustsystem. Geochemical data from metaultramafic rocks of São José da Safira indicate an ophioliticorigin. The comparison with data from other Neoproterozoic oceanic remnants (Abadiânia and Fuchuan)corroborates this interpretation.

Experimental Constraints on Himalayan Anatexis

We have melted metapelitic rocks from the High Himalayan Crystalline Sequence that are likely sources of leucogranite magmas. Starting materials were a muscovite schist and a tourmaline-bearing muscovite–biotite schist. Both are kyanite-zone rocks from the hanging wall of the Main Central Thrust. Experiments were conducted at 6, 8 and 10 kbar and 700–900°C, both without added H 2 O (dehydration–melting) and with 1–4 wt % added H 2 O. Dehydration-melting begins at 750–800°C, and produces melts that are virtually identical in composition to the Himalayan leucogranites. Adding H 2 O lowers the solidus by promoting plagioclase + quartz melting. Melts produced from these starting materials at T ≤ 750°C by H 2 O-fluxing are trondhjemitic, and different in composition from most Himalayan leucogranites. Leucogranite magmas in the Himalaya formed by dehydration-melting of metapelites during adiabatic decompression, at 6–8 kbar and 750–770°C. The dehydration-melting solidus for muscovite schist has a smaller dP/dT slope than that for biotite schist. In consequence, muscovite schist undergoes decompression-melting more readily than does biotite schist. The two solidi probably cross over at ∼10 kbar, so that muscovite may be a more important deep crustal H 2 O reservoir than biotite.

Tectonic history of the metamorphic basement rocks of the Sierra del Carmen, Coahuila, Mexico

The Ouachita interior zone, arching through central Texas into the Big Bend region of the Rio Grande, has been defined chiefly on the basis of well samples and its associated gravity high. The only known exposure of rocks of the interior zone lies at the base of the Sierra del Carmen normal fault escarpment under approximately 1100 m of Lower Cretaceous carbonates in northern Coahuila, Mexico. These lower greenschist facies metamorphic rocks are thinly interlayered graphitic muscovite schist, graphitic marble, and minor quartzite with abundant quartz veins. The dominant structures shown by the metamorphic rocks are isoclinal folds (F2) that have a well-defined axial planar foliation (S2). Earlier (F1) folds refolded by F2 are locally preserved. At the microscopic scale, S2 is a crenulation cleavage of the S1 foliation that has been enhanced by pressure solution. Feldspar porphyroblasts overgrow a folded graphitic S1 foliation. Throughout the area a second crenulation cleavage (S3) is present on the S2 foliation surface as an intersection lineation (L3). Subsequent Laramide regionalto outcrop-scale folds and associated thrusts overprint these ductile structures. Lower Cretaceous carbonates form a large, east-verging overturned to recumbent anticline-syncline fold pair with a northwest-trending fold axis. The S2 metamorphic foliations define an antiform that is nearly coincident in trend to that of the fold in the overlying carbonates, indicating that the metamorphic rocks were also folded by this regional structure. A Basin and Range normal fault is interpreted to have reactivated the Laramide thrust fault responsible for the contractional deformation. One to two normal faults cut the carbonates and metamorphic rocks and truncate the anticline-syncline fold pair. The maximum offset along the normal fault is 1600 m. Rb/Sr data from the muscovite schist gives a six-point, mineralÐwhole-rock isochron age of 277 ± 10 Ma, indicating that polyphase deformation and metamorphism occurred during the late Paleozoic Ouachita orogeny. An initial Sr ratio of 0.721 suggests an ancient source for these metasediments. Common Pb ratios of the metamorphic whole rock and galena from the Puerto Rico Mine are isotopically distinct from those of the North American craton, suggesting that they were derived from a nonÐNorth American source and thus may be equivalent to unmetamorphosed Ouachita flysch sediments. This indicates that the metamorphic rocks of the Ouachita interior zone were derived from an ancient allochthonous source that was deformed and metamorphosed during the late Paleozoic Ouachita orogeny.