Graphitic Schists Research Articles

Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil

The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW-SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and post- metamorphic. Fluid inclusion studies identified early CO2 (CH4-N2) and CO2 (CH4-N2)-H2O-NaCl inclusions that show highly variable phase ratios, CO2 homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less asso- ciated with postentrapment modifications. The ore-bearing fluid typically has 18-33mol% of CO2, up to 4mol% of N2, and less than 2mol% of CH4 and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ 18 OH2O and δDH2O (310-335°C) values of +6.2 to +8.4‰

Metamorphism and gold mineralization of the Kenticha–Katawicha area: Adola belt, southern Ethiopia

I present geological and mineral chemistry results aiming at understanding the relationship between metamorphism and gold mineralization in the N-trending Neoproterozoic Kenticha–Katawicha area in the Adola belt of southern Ethiopia. The Kenticha–Katawicha area comprises low-grade metamorphic rocks including pelitic schists, marble, graphite schists and Fe–Mn quartzites (Kenticha marine metasedimentary rocks) and serpentinites and talc–tremolite–chlorite schists with podiform chromites (Kenticha ophiolite). Lenses of high-grade amphibolite schist are also locally present within the Kenticha–Katawicha area. These rocks are sandwiched between high-grade para- and ortho-gneisses, migmatites, schists and amphibolites. Post-orogenic granites intrude the metamorphic sequences. Mineral assemblages and textural data obtained from the high-grade rocks indicate that the dominant metamorphic condition is of upper amphibolite facies. Garnet–biotite (garnet-core–matrix-biotite) and amphibole–plagioclase geothermometry gave estimates for peak metamorphic temperatures of 630–650 °C and pressure of 7 kbar for the high-grade rocks. On the other hand, the low-grade rocks recrystallized in the greenschist facies conditions. The estimated peak T– P values for the high-grade rocks suggest a burial to ∼25 km depth and a clockwise P– T path is deduced from mineral thermometry and the textural and paragenetic relations. This combined with the geochemistry of the magmatic rocks and clockwise P– T is consistent with a collision setting in which rocks of the calc-alkaline, volcanic-arc, and oceanic fragments are assembled. In the study area, gold occurs in quartz veins and veinlets in lenses of biotite schists within ultramafic rocks that are confined to shear zones. Fluid inclusion studies of gold quartz veins and veinlets indicate a H 2O- and CO 2-rich fluid with low salinity (<5 wt% NaCl equivalent). The gold mineralization in the area is post-peak regional metamorphism and is associated with the low-grade wall-rock alteration.

Archaeogeological implication of lithic artifacts from the Unjeonri Bronze Age site, Cheonan, Republic of Korea

The Bronze Age lithic artifacts from the Unjeonri sites in the Republic of Korea were studied on the basis of petrological, geochemical and archaeogeological interpretations. The use and rock names of the artifacts are a stone-grinding pestle (medium-grained two-mica granite), two stone-grinding plates (fine-grained two-mica granite and fine-grained quartz diorite), two stone axes (coarse-grained syenitic rock and graphitic schist), an unknown use artifact (biotite gneiss) and a stone knife (black slate). Values of magnetic susceptibility (MS) of the lithic artifacts are mainly less than 0.5 (10 −3 SI unit); however, the syenitic rock (stone axes) ranges from 2.0 to 2.7. In the rocks around the relic sites, the MS values discovered were lower than 0.5. However, the syenitic rock stone axe, the graphitic schist stone axe, and the black slate stone knife showed different values from some of the measured rocks of an outcrop. Various kinds of rocks observed around the Unjeonri sites along the stream could be used for the stone-grinding pestle, the stone-grinding plate, and one of the stone axes. Each artifact was coupled with raw material samples rocks, and analyzed geochemically. The geochemical pattern of both lithic artifacts and sampled rock were identical when normalization was used to identify the behavior, enrichment, compatibility, and incompatibility of the elements. Therefore, it is possible that the source rocks of the interpreted lithic artifacts are distributed in the vicinity of the relic sites with the exception of one a stone axe (graphitic schist), the stone knife (black slate), and a stone arrowhead (gray shale); they did not conform to typical occurrences within a 10 km radius distribution area of the site. Consequently, in the Unjeonri sites, the artifacts suggest that the two types of materials coexist as domestic and foreign lithic artifacts. Using raw materials in situ, the domestic rock types (grinding pestle, plate, and axe) are self-sufficient. The foreign rock types (knife, arrowhead, and axe) are presumed to have been introduced from a different area. The foreign types should be examined for various introduction possibilities such as tribal migration, diffusion, plundering, or trade.

Preferential embrittlement of graphitic schists during extensional unroofing in the Alps: the effect of fluid composition on rheology in low‐permeability rocks

AbstractInterlayered graphitic and non‐graphitic schists from the Tauern Window, Eastern Alps, record contrasting mechanical behaviour during extensional exhumation. Graphitic schists contain mesoscale extension fractures, pervasive microcracks in garnet, and abundant secondary fluid inclusion planes; all three types of structures are oriented perpendicular to the stretching lineation. Crack spacings in garnet from graphitic samples are tightly clustered around a mean of 180 μm. Non‐graphitic schists have fewer and more randomly oriented microcracks and fluid inclusion planes and maintained strain compatibility via crystal plasticity. The presence or absence of graphite appears to have exerted a fundamental control on rheology during unroofing. Calculations for a model graphitic rock at 500 °C and fO2 = 10−24 MPa show that the equilibrium metamorphic fluid evolves from XCO2 = 0.07 to 0.38 during decompression from 700 to 400 MPa, in agreement with microcrack fluid inclusion data that show a change from XCO2 &lt; 0.1 to 0.45 in graphitic samples over the same pressure interval. This compositional shift results in &gt;60% expansion of the pore fluid during decompression. H2O‐rich fluid in non‐graphitic rocks expands &lt;15% over the same pressure interval. The greater pore fluid expansion in low‐permeability graphitic horizons likely promoted tensile failure during unroofing. These results suggest that microcracking should be an inevitable consequence of decompression in many graphitic schists, whereas rocks that lack graphite are less likely to undergo microcracking. Microseismicity is predicted to be more common in graphitic than non‐graphitic rocks during unroofing of mountain belts.

Introduction: linked processes of metamorphism and deformation

The interaction of deformation and metamorphism within the crust results in textures, structures and mineral growth that provide a record of tectonic activity as well as the progressive metamorphic and structural development of the crust. Analysis of these textures and structures is central to deciphering and evaluating the progressive nature of their development. Such rocks are commonly interpreted from the viewpoint of either structural or metamorphic processes; however, integration of these two perspectives has proved to be a powerful tool in enhancing our understanding of the structural development of metamorphic rocks at all scales. In recent decades, research into the linkages between metamorphism and deformation has increasingly involved the integration of textural, chemical and structural data. The present volume arose from a topical session titled Understanding Coupled Metamorphic and Deformational Processes: Advances in Integrated Textural, Chemical and Microstructural Analysis held at the 2003 annual meeting of the Geological Society of America, Seattle, USA. We were pleased that the session attracted 21 contributions from 48 geologists representing seven countries. The breadth of topics covered by the presentations, and the interest expressed by both participants and attendees encouraged us to assemble a set of thematic papers related to the topical session. The result is the present special issue of the Journal of Metamorphic Geology. Whitmeyer & Wintsch document the linkages between stress, metamorphic reactions, grain shapepreferred orientation (SPO) and reactivation. They describe the reactivation of an amphibolite facies ultramylonite along discrete greenschist facies mylonitic shear bands. A random SPO within the ultramylonite created a texturally hardened rock that displaced strain into adjacent weaker foliated gneiss. Later replacement reactions within the ultramylonite were concentrated at areas of high stress and resulted in a chemical and mechanical softening of the rock via the development of a SPO of muscovite, chlorite and quartz. Reactivation of the ultramylonite was localized along discrete mylonitic shear bands at the sites of these reactions and accompanying SPO development. The relationship between strain and SPO development is also explored by Stallard et al. who measured the 3D shape, size and orientation of quartz grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand. Quartz SPO was initiated by dissolution–precipitation creep at sub-greenschist facies conditions, and was triggered by strain accompanying fold development. Detrital grains became blade-shaped, and were an important component of the metamorphic foliation. A second fold event, accompanied by crenulation cleavage development and resulting segregation of the rock mass, initiated a change in both the texture of the quartz SPO and deformation mechanisms. Previously discrete and aligned detrital quartz grains were replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and alignment. Pressure solution then affected the margins of quartz-rich layers rather than individual grains. The authors emphasize the contribution of quartz SPO to foliation development and the significance of fold events as a control on SPO formation and development. In a companion paper that analyses the 3D shape, size and orientation of white mica grains from the same samples, Stallard & Shelley describe the initiation and development of white mica SPO with increasing metamorphic grade. The character of white mica SPO is determined early in the deformation and metamorphic history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic micas in favour of numerous new smaller metamorphic grains of contrasting composition, higher aspect ratio and a strong SPO. The SPO results from the progressive competitive anisotropic growth of blade-shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher-grade samples. Selverstone describes how changes in fluid composition correlate with changes in mechanical behaviour of graphitic schists during decompression. Interlayered graphitic and non-graphitic schists from the Tauern Window, Eastern Alps, record different mechanical behaviour during extensional exhumation. Graphitic schists contain brittle extensional structures oriented J. metamorphic Geol., 2005, 23, 409–410 doi:10.1111/j.1525-1314.2005.00593.x

Geology, lithogeochemistry and paleotectonic setting of the host sequence to the Kangasjärvi Zn-Cu deposit, central Finland: Implications for volcanogenic massive sulphide exploration in the Vihanti-Pyhäsalmi district

The Kangasjarvi Zn-Cu deposit is a highly deformed and metamorphosed Paleoproterozoic volcanogenic massive sulphide (VMS) deposit located in the Vihanti-Pyhasalmi base metal mining district of central Finland. The host sequence to the deposit, referred to as the Inner Volcanic Sequence (IVS), is comprised of a bimodal suite of metavolcanic rocks and a regionally extensive tonalite-trondhjemite gneiss (sub-volcanic intrusions?). A separate and perhaps younger sequence of mafi c volcanic rocks, with irregular intervals of undifferentiated intermediate to felsic schists and metalimestones, referred to as the Outer Volcanic Sequence (OVS), are separated from the IVS sequence by intervals of metagreywacke and U-P-bearing graphitic schists. A stratigraphic scheme for rocks within the IVS is proposed based on outcrop observations, locally preserved volcanic textures, aspects of seafl oor-related hydrothermal alteration and lithogeochemistry. In this scheme, rare andesites form the lowermost volcanic stratigraphy and are overlain by typical island-arc basalts that were erupted in a subaqueous setting. Tonalite-trondhjemite subvolcanic intrusions were locally emplaced within andesites and coeval rhyolites were extruded on the basaltic substrate. The extrusion of rhyolites, including high-silica rhyolites, was coeval with regional-scale, pre-metamorphic seafloor hydrothermal alteration and local sulphide mineralization. Extensively altered rhyolites envelope massive sulphides and are underlain by altered basalts. The latter rocks are now characterized by a variety of low-variance metamorphic mineral assemblages (e.g. orthoamphibole-cordierite rocks) and define a domain of intense pre-metamorphic chlorite ± sericite alteration in the stratigraphic footwall of the deposit. The altered nature of these rocks is attributed to reaction with seawater-related hydrothermal fluids within a zone of upflow at or near the seafloor. The fundamental controls on convective hydrothermal circulation and subsequent alteration and massive sulphide mineralization at Kangasjarvi, and possibly elsewhere in the district, share many characteristics with other well-described, ancient VMS deposits (e.g. massive sulphide deposits in the Flin Flon Belt, Manitoba, Canada). These characteristics include: 1) an association with bimodal volcanism developed in extensional settings; 2) a close spatial association with regionally extensive felsic subvolcanic intrusions; and 3) petrogenesis of ore-associated volcanic rocks (e.g. high-silica rhyolites, felsic subvolcanic intrusions) indicative of substantial heat transfer from the mantle to the upper crust and the development of anomalous thermal corridors. These features translate into geochemically distinctive rock types that, when combined with aspects of stratigraphy and pre-metamorphic alteration, may be used to develop regional exploration strategies in the Vihanti-Pyhasalmi district.

Modeling outflow from the Ernest Henry Fe oxide Cu–Au deposit: implications for ore genesis and exploration

The Ernest Henry Fe oxide Cu–Au (IOCG) deposit (>ca. 1.51 Ga) is hosted by breccia produced during the waning stages of an evolving hydrothermal system that formed a number of tens of metres to a kilometre scale, pre- and syn-ore alteration halos, although no demonstrable patterns have been attributed to fluids expelled through the outflow zones. However, the recognition of a population of hypersaline fluid inclusions representing the ‘spent’ fluids after Cu–Au deposition at Ernest Henry provides the basis to model the geochemical characteristics of the deposit's outflow zones. Geochemical modeling at 300 °C was undertaken at both high and low fluid/rock ratios via FLUSH models involving three host rock types: (1) granite, (2) calc–silicate rock, and (3) graphitic schist. In models run at high fluid/rock ratios, all rock types are essentially fluid-buffered, and produce an albite–quartz–hematite–barite-rich assemblage, although in low fluid–rock environments, the pH, redox, and geochemical character of the host rock exerts a greater influence on the mineralogy of the alteration assemblages (e.g., andradite, Fe–chlorite, and magnetite). Significant sulphide mineralization was predicted in graphitic schist where sphalerite occurred in both low- and high-porosity models, which indicates the possibility of an association between high-temperature IOCG mineralization and lower temperature base metal mineralization. Cooling experiments (from 300 to 100 °C) using the ‘spent fluids’ predict early high- T (300–200 °C) Na-, Ca-, Fe-, and Mn-rich, magnetite-bearing hydrothermal associations, whereas with cooling to below 200 °C, and with progressive fluid–rock interaction, the system produces rhodochrosite-bearing, hematite–quartz–muscovite–barite-rich assemblages. These results show that the radical geochemical and mineralogical changes associated with cooling and progressive fluid influx are likely to be accompanied by major transformations in the geophysical expression (e.g., spectral and magnetic character) of the alteration in the outflow zone, and highlight the potential link between magnetite- and hematite-bearing IOCG hydrothermal systems.

群馬県藤岡市たたら沢に産するアンモニアを含むグラファイト層間化合物

Graphites in graphite schist of the Sanbagawa metamorphic belt, Tatarazawa, Fujioka, Gunma Prefecture, Japan, have been identified as ammonia-bearing graphite intercalation compounds (GIC). The ingestion of ammonia and sodium in GIC was confirmed by Nessler’s reagent, micro XRD (X-ray diffraction), EPMA (electron probe micro analysis) and micro Raman spectrometer.The EPMA examination clarified that ammonia and sodium were inhomogeneously distributed in graphite grains. The Raman spectra revealed the natural GIC to be consistent with an ammonia-containing graphite. Occurrence of the specimens implied that genesis of natural GIC would have undergone ammonia-bearing hydrothermal process in the past.A finding of graphite intercalation compounds proved that ammonia could be contained by graphite in Sanbagawa metamorphic belt. Likewise, a detection of sodium from graphite has an implication for the existence various graphite intercalation compounds awaiting discovery in such metamorphic rocks.

Ni?Cu?(PGE) magmatic sulfide deposits in the Yangliuping area, Permian Emeishan igneous province, SW China

The Ni–Cu–PGE sulfide deposits in the Yangliuping area, SW China, are hosted in mafic–ultramafic sills. The four mineralized sills are located in the Yangliuping tectonic dome and intrude Devonian carbonaceous marble, graphitic schist. The sills are 200–300 m thick and 1,000–2,000 m in strike length and now consist chiefly of serpentinite, talc schist, tremolite schist, and meta-gabbro. Disseminated Ni–Cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills. Massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite. Although the sulfide ores have been modified by hydrothermal activity, there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores. The Yangliuping Intrusions and the Dashibao Formation have similar primitive-mantle normalized trace element and platinum group element (PGE) patterns, indicating that they are derived from a common parental magma type. The positive correlation between Cu concentrations and Cu/Zr ratios of the Dashibao Formation basalts indicates that the chalcophile elements were removed before eruption. We propose that fractional crystallization of the Yangliuping magma accompanied by the introduction of S and CO2 from the wall rocks caused the magma to become S-saturated leading to the segregation of magmatic sulfides that became enriched in Ni–Cu–(PGE). The sills acted as conduits for the overlying Dashibao Formation basalts with the sulfide liquid, along with early crystallizing olivine and pyroxene, segregating from the magma as it passed through the conduits prior to eruption.

Geochemistry of a high-T hydrothermal dolostone from the Emirli (Ödemiş, western Turkey) Sb-Au deposit

AbstractA dolostone layer is found in spatial association with the Emirli epithermal Sb-Au deposit in western Turkey. It occurs within an argillic alteration zone adjacent to the major Emirli fault zone, which is the controlling structure for the deposit, and is composed of large closely packed subhedral to anhedral planar and nonplanar dolomites. Pyrite is the only accessory mineral in the layer and occurs as disseminations and veinlets up to 100 μm wide. Dolomite crystals are petrographically homogeneous and have consistent deep-red cathodoluminescence (CL) colour with no zoning which implies single-stage dolomitization of a calcitic precursor which is partly preserved as remnant patches of orange-coloured CL zones. Some crystal boundaries have dark (or no) CL colour. Electron microprobe line-scan analyses across these regions indicate intense enrichment of Fe and Mg depletion, revealing late-stage Fe-metasomatism (ankeritization) especially prevalent near pyrite veinlets and disseminations.Dolomite crystals are composed of 52.4–55.0 mol.% CaCO3, 29.9 –41.2 mol.% MgCO3, 1.8 – 14.4 mol.% FeCO3 and 0.75 –3.2 mol.% MnCO3 indicating ferroan dolomite. The relationship between Ca and Mg is not stoichiometric due to substitution of Mg2+ by Fe2+ after dolomitization, as demonstrated by a strong negative correlation between Fe and Mg. Whole-rock Fe contents of the dolostone layer increases toward the Emirli fault zone.The δ13C(PDB) compositions of the Emirli dolomite, calcitic marbles, and graphite-schists are in the ranges of –1.6 to 0.8‰, 1.5 to 1.5‰ and –6.6 to –23.5%, respectively, indicating that dolomite was formed due to interaction of light-carbon-enriched fluids with calcitic marbles; light-carbon may have been derived from decarboxylation of the graphitic schist layers. δ18O(PDB) values of dolomite and marble range –15.2 to – 11.2‰ and –2.4 to – 3.5‰, respectively. This large isotopic difference between dolomite and marble was probably inherited from oxygen isotope exchange between the dolomitizing fluid and the precursor calcites, as well as other minerals enriched in light-oxygen.Fluid inclusions in dolomite are two-phase, and homogenize into liquid within the temperature interval 242 – 362ºC with a mode of 290ºC, and have salinities of 1 – 3 wt.% NaCl equiv. Using this modal temperature, the average δ18O(PDB) composition of water in isotopic equilibrium with the Emirli dolostone was estimated to be –19.4±2.1‰, which is interpreted as an indication of modified surface-waters; this interpretation is also supported by low fluid salinity and Na and Sr contents. These fluids migrated along graben-related faults, penetrating deeper levels where they were transformed into hydrothermal fluids due to the high heat-flow of the Küçük Menderes graben system, and flowed-up mainly through the Emirli and Haliköy faults that control mineralization in local deposits of Sb-Au and Hg, respectively. Due to interaction with chlorite-bearing graphite-schists, the fluid may have dissolved Mg2+ from chlorite and been enriched in isotopically-light carbon due to decarboxylation of graphite. Dolomitization occurred as a result of the interaction of these fluids with a calcitic marble band adjacent to the Emirli fault zone. Subsequent introduction of Fe2+ caused ankeritization along dolomite crystal boundaries during first-stage Sb-Au mineralization.

Engineering geological investigations along the Ilıksu Tunnels, Alanya, southern Turkey

The main objective of this study is to assess the engineering geological characteristics of the rock mass and to suggest appropriate support recommendations along the two autoroad tunnel projects, named as Ilıksu 1 and Ilıksu 2, which are located along the Antalya–Alanya Highway. The study area consists of pelitic schist and calc schist overlain by a thick sequence of recrystallized limestone and an intercalation of pelitic schist, calc schist and graphite schist. The tunnel ground support types and categories were determined according to the Q-system, Rock Mass Rating (RMR) method and New Austrian Tunneling Method (NATM). Slope stability analyses were performed at the portal and cut slope sections. Kinematic and limit equilibrium analyses incorporating the effects of water pressure were performed for the failed rock slopes. Slope stability analyses of irregularly jointed, highly foliated lithologies were performed using soil slope stability software. The interactions of the tunnel support systems with the rock mass were analyzed through finite element analysis.

Fluid–rock interactions and the role of late Hercynian aplite intrusion in the genesis of the Castromil gold deposit, northern Portugal

Castromil (northern Portugal) is one of several important orogenic gold deposits located within the “Central Iberian” geotectonic zone of northwest Iberia. The deposit occurs at the margin of a Variscan, syn- to late-D3 biotite granite, and is spatially associated with a small tourmaline aplite body that intrudes the granite at its contact with a secondary anticline of Palaeozoic arenaceous and argillaceous metasediments of the Valongo Belt. Identification of the ore fluids and their pathways, and the reconstruction of the P– T– X conditions during mineralisation were obtained by combining the geometric characteristics of veins and microstructures together with a detailed study of the inclusion fluids. Several stages of fluid percolation following contact metamorphism can be recognised. At each stage, the contact zone, characterised by intrusive aplites, related faults and fractures, appears to have focused the hydrothermal flow and acted as a structural conduit for deeper-sourced hydrothermal fluids. The earliest fluid stage (Stage I) is characterised by aqueous-carbonic fluids dominated by CO 2 and CH 4 that were probably generated by high-temperature fluid–rock interaction (400–500 °C) with graphitic schists interbedded with the metasediments. These fluids were responsible for significant alteration (greisenisation) of the aplite and its host granite, and the formation of silicified, flat lying structures that can be traced along the strike length of the deposit. At temperatures between 400 and 500 °C, fluid pressure ranges from 230 to 300 MPa, which is equivalent to a depth of 10±1.5 km. The second stage of mineralisation (Stage II: As-ore stage) is also characterised by aqueous-carbonic fluids and represents the main phase of quartz–arsenopyrite–pyrite deposition. The third stage of mineralisation (Stage III: Au-ore stage) was accompanied by intense microfracturing of the preexisting quartz veins and the preferential deposition of gold along microfractures in the sulphides. The introduction of gold corresponds to the percolation and mixing of two distinctive aqueous fluids of contrasting salinity at relatively low temperatures (150–275 °C). Based on compositional and temperature data, it is suggested that during the main phase of uplift, shallow waters penetrated deep into the basement, allowing gold to be leached from potential source rocks (most probably the Palaeozoic metasediments) and deposited in structural and geochemical traps formed during earlier stages of the hydrothermal system. The decrease in pressure during the As-ore stage corresponds to a significant tectonic uplift (around 5–6 km), and probably marks the transition from lithostatic to hydrostatic pressure conditions. Furthermore, if uplift had already been initiated during aplite emplacement, the prevailing sub-isothermal high-temperature conditions provide an explanation for the presence of decrepitated aqueous-carbonic inclusions in metamorphic quartz lenses and veins in the surrounding metasediments. To conclude, localised heat flows linked to late Hercynian magmatism at deeper structural levels appears to be the main cause of fluid circulation at Castromil. Evidence suggests that contact zones related to faulting along a secondary anticline of the Valongo Belt controlled both aplite intrusion and subsequent long-lived hydrothermal fluid circulation. The proposed genetic model differs from orogenic gold deposit models in emphasising the role of late stage aqueous fluids in the development of economic grade (10–15 g/t) gold ores.

Stratigraphic profiling with ground‐penetrating radar in permafrost: A review of possible analogs for Mars

We review our past and ongoing use of ground‐penetrating radar to investigate permafrost in Alaska and in the Dry Valleys of Antarctica. The results may be relevant to radar efforts on Mars because of arid conditions and the presence of ice. The pulses were centered at 50, 100, and 400 MHz. We interpret profiles from two sites in the eastern Taylor Valley to show glaciolacustrine and glaciofluvial stratigraphy. The maximum depth of stratigraphy profiled there was about 33 m. Near Fairbanks, Alaska, the depth of penetration at 50 MHz was near 80 m in marginally frozen and stratified alluvial sands. At the Fairbanks sites, supplementary drilling was required to differentiate between reflections from conductive bedrock, a graphitic schist, and those from the water table at depths of 20–25 m. At a site on the North Slope of Alaska, we profiled present and remnant freezing fronts in an alluvial floodplain. The relative permittivity at most sites ranged between about 4 and 5.5, which is consistent with dry conditions, the mineralogy, and low ice content. Weak interface reflectivity or the lack of further interfaces may have limited the interpretation of maximum penetration where no water table was present because signal absorption should have been low and scarce diffractions imply that scattering was weak. The interface reflectivities beneath Taylor Valley may be a function of only density contrasts, since free water, and possibly ice, is absent.

Evidence of a contact metamorphic aureole with high-temperature metasomatism in the deepest part of the active geothermal field of Larderello, Italy

The deep part (1.5–4.5 km) of the Larderello geothermal field (Tuscany, Italy) consists of rocks that were metamorphosed by the Hercynian and Alpine orogenies, and which were thermally metamorphosed in the same place during the emplacement of granite intrusions of 3.8–1.0 Ma age. These rocks are potential deep-seated reservoirs and could be the target of future exploitation. The petrographic, geochemical and fluid inclusion data indicate that thermally metamorphosed phyllite, micaschist, gneiss, amphibolite and carbonates underwent a recrystallisation at temperatures of 425–670 °C, under a lithostatic pressure regime of 95-130 MPa. Li–Na-rich fluids of magmatic origin, and aqueous-carbonic fluids with varying proportions of H 2O and CO 2 that formed during the contact metamorphism, were present during this stage. The fluids present during the contact metamorphic event were responsible for a widespread B-metasomatism and local F-metasomatism. In some cases, high-temperature metamorphism of graphitic schists can control the composition of the aqueous-carbonic fluids. A late-stage, lower temperature hydrothermal activity was responsible for both the propylitic and sericitic alterations and for the replacement of the high-temperature mineral assemblage. Stable isotope ( δ 18O and δD) data on the thermally metamorphosed rocks and granites indicate that these rocks underwent depletion in the heavy isotopes. Magma degassing and dehydration metamorphic reactions can explain the isotopic values of these rocks. Late-hydrothermal fluid of meteoric origin may also have contributed to the depletion of the heavy isotopes from the rocks. Under contact metamorphism conditions the rocks were plastic and impermeable. A transition from plastic to brittle conditions occurred as a consequence of the cooling of the system at depths of <4 km. The brittle-plastic transition at Larderello now occurs at a depth of 4.5–5 km, where present-day temperatures are in the range 400–450 °C. Deep-seated fluids probably occur in this zone, as suggested by the geophysical data.

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.

Hercynian Metamorphism in Nappe Core Complexes of the Alpine Betic–Rif Belt, Western Mediterranean—a SHRIMP Zircon Study

Ion microprobe U–Pb dating of a bimodal zircon population from INTRODUCTION a graphitic schist from the basal section of the Alpine Alpujarride The Betic–Rif mountain belt forms the southwestern nappe complex in the Betic Cordilleras, southern Spain, has yielded termination of the Alpine orogenic belt in Western Eurdifferent ages for zircons of contrasting morphology and composition. ope and North Africa. Although located in one of the Approximately half the grains are rounded and abraded, and have very regions where the foundations of modern geology ages ranging from >2·2 Ga to >350 Ma. These are detrital were laid (e.g. Argand, 1924), fundamental aspects of its grains from the schist’s sedimentary parent material, indicating its tectogenesis are still disputed. For example, there is no derivation from a composite basement and a Palaeozoic, possibly consensus on the direction of the regional collisional Carboniferous, age of deposition. The other half of the grains consist convergence and the location of the subduction zone. of euhedral cores of Hercynian age (305·3 ± 3·2 Ma) surrounded Most researchers (e.g. Egeler & Simon, 1969; Platt & by Late Alpine (19·7 ± 2·2 Ma) metamorphic overgrowths Vissers, 1989; Platt & Whitehouse, 1999) have envisaged ( = 0·05 errors). The Hercynian cores probably record the first, a north–south convergence within the framework of medium–high-grade amphibolite-facies metamorphism, relicts of an Iberia–Africa collision. Recently, an alternative was which are seen in the schist complex, whereas the rims were formed suggested (Zeck, 1996, 1997, 1999): an east–west-directed during the final tectono-metamorphic stage in the Alpine development convergence under the influence of the opening of the of the orogenic belt. The bimodality of the zircon population in age, North Atlantic. In this model, the Alpine collisional morphology and internal structure may mainly reflect the lithological nappe pile was derived from Tethys-realm Betic–Ligurian bimodality of the rock (thin quartzitic layers or schlieren alternating lithosphere in a SW–NE-trending subduction zone system obliquely underthrusting eastward-drifting Atlantic-realm with graphitic micaceous folia) and the control that that exerted on Iberia. fluid circulation in the metamorphic environment. In addition, the timing of the subduction is poorly constrained. Comparison of the scarce thermochronological data with tomographic information (Blanco & Spakman, 1993) and the sea-floor spreading record for the North Atlantic (Srivastava et al., 1990) suggest

The Palaeoproterozoic Kangerluluk gold-copper mineralization (southeast Greenland): Pb and Nd isotopic constraints on its timing and genesis

The Ketilidian Mobile Belt (KMB) in South Greenland is dominated by the Palaeoproterozoic Julianehab Batholith (1,850–1,770 Ma) and is bordered by supracrustal sequences. The batholith is composed of granite, granodiorite, tonalite, diorite and subordinate gabbro and appinite and formed in a juvenile magmatic arc. The supracrustal rocks consist of psammites, pelites and subordinate, interstratified mafic volcanic rocks. A Rapakivi granite suite (1,755–1,735 Ma) intrudes them. Gold is an important metal in the KMB and occurs in various settings and element associations. Here, emphasis is placed on the Kangerluluk Cu–Au mineralization in the eastern part of the mobile belt. It is hosted within a remnant supracrustal succession dominated by redundant pillowed basalts. Structurally, a syn-volcanic association with gold can be distinguished from one that is clearly related to crosscutting quartz veins. In between, calc-silicate formation in the volcanic rocks occurred contemporaneously with a pervasive sericitization (bleaching) of leucocratic granitoids that are intrusive into the volcanic sequence. Bulk rock and mineral (sulfide) Pb isotope data are comparable in the Pb isotope space. The data define two groups of mineralization where one group is characterized by relatively higher 207Pb/204Pb at given 206Pb/204Pb ratios than the other. Pb stepwise leaching (PbSL) dating experiments performed on garnet and epidote associated with both groups of mineralization confirm the offset in the Pb isotope data. With the geochronological information from PbSL, the results indicate: (1) the initial gold mineralization was genetically related to metalliferous fluids associated with the emplacement of late intrusive stages of the Julianehab Batholith (1,800–1,770 Ma); and (2) local remobilization of the initial mineralization (1,700–1,750) involved a Pb isotopically evolved fluid, which tentatively can be correlated with the intrusion of the Rapakivi granites in the area. Radiogenic Pb was not only found in altered host rocks but also in various sulfides, suggesting that fluids may have percolated through and/or originated from uranium-bearing graphitic schists which are abundant in the sedimentary package of the KMB. Lead isotopic results suggest that the Rapakivi granites may have supplied heat for renewed circulation of fluids within the supracrustal rock pile.

THE MARA ROSA GOLD DISTRICT, CENTRAL BRAZIL

In the Mara Rosa District, three different magmatic series have been identified and all of them host gold mineralization. The Mara Rosa Metavolcano-Sedimentary Sequence constitutes a poorly differentiated calc-alkaline magmatic series typical of arc settings. It is arranged into three belts, forming synclinal structures and was affected by high-grade amphibolite facies metamorphism. Early intrusive rocks of the Posse Unit show an alkaline affinity and host the epigenetic Posse Gold Deposit, which is associated with a late hydrothermal event, preferentially where earlier shear zones are present. Early hydrothermal alteration has produced an aluminous, sulphide rich greisen rock, which is barren of gold. Slices of an allochthonous and more magnesian sequence are associated with metachert and graphitic schists that host the syngenetic Au-Ag Zacarias mineralization and extensions. Gold-bearing quartz veins were emplaced before the second main isoclinal folding event. Supergene ore has played an important role in the mining activity of the Mara Rosa Region, since it represents the majority of the ore exploited. It is possible that yet undiscovered deposits lay hidden underneath the extensive lateritic cover.

The Mara Rosa gold district, Central Brazil

In the Mara Rosa District, three different magmatic series have been identified and all of them host gold mineralization. The Mara Rosa Metavolcano-Sedimentary Sequence constitutes a poorly differentiated calc-alkaline magmatic series typical of arc settings. It is arranged into three belts, forming synclinal structures and was affected by high-grade amphibolite facies metamorphism. Early intrusive rocks of the Posse Unit show an alkaline affinity and host the epigenetic Posse Gold Deposit, which is associated with a late hydrothermal event, preferentially where earlier shear zones are present. Early hydrothermal alteration has produced an aluminous, sulphide rich greisen rock, which is barren of gold. Slices of an allochthonous and more magnesian sequence are associated with metachert and graphitic schists that host the syngenetic Au-Ag Zacarias mineralization and extensions. Gold-bearing quartz veins were emplaced before the second main isoclinal folding event. Supergene ore has played an important role in the mining activity of the Mara Rosa Region, since it represents the majority of the ore exploited. It is possible that yet undiscovered deposits lay hidden underneath the extensive lateritic cover.

Stable Isotopic Information on Calcareous Pelitic Rocks in the Tizapa Volcanogenic Massive Sulfide Deposit Area, the United Mexican States

Abstract: Tizapa volcanogenic massive sulfide (VMS) deposit is hosted in greenschist facies metamorphic rocks; footwall is green schist of felsic to mafic metavolcanic rocks and hanging wall is graphite schist of metasedmentary pelitic rock. Pb‐Pb dating of ore samples indicates 103. 4Ma to 156. 3Ma for the age of mineralization (JICA/MMAJ, 1991).