Gneissic Host Rocks Research Articles

Permeability Testing of Drill Core from Basement Rocks in the Fault-Hosted Gryphon U Deposit (Eastern Athabasca Basin, Canada): Insights into Fluid–Rock Interactions Related to Deposit Formation and Redistribution

Fluid percolation conditions in fault zones are often inferred in geologic studies, but the porosity and permeability of rocks are rarely tested during mineral exploration drilling. Routine permeability and porosity tests on drill core, with adequate detection limits and accuracy, can provide new insights into present- and paleo-fluid flow processes and deposit formation in sedimentary basins and permeable fault zones (e.g. uranium deposits, sub-sea hydrothermal ore deposits). In this study, geological and hydrogeological observations were combined with results of over a thousand permeability tests at spot-on unconfined drill core, extracted along transects through the Gryphon U deposit and fault zone, using a pressure-decay N2 gas probe with adequate seal to the rock surface. The new seal method allowed a wide range of permeability determinations under field conditions (over 5 × 10–20 to 10–12 m2). The penetrative macropore networks were observed directly from gas discharge patterns on drill core surfaces. For the tested crystalline basement rocks below the Athabasca Basin, we inferred that the matrix permeability and porosity distribution appear to be preserved since the late Paleoproterozoic/Mesoproterozoic, under the cover of a sedimentary basin. The permeability values at the present time, and the patterns of rock alteration and deformation, offer insights to paleo-hydrogeological conditions. The gneissic and pegmatitic host rocks have relatively low permeability at present time, except fractured (discrete flow channels) and/or intensely altered to a more porous rock. The alternating zones of silicification and desilicification appear to pre-date the intense argillic alteration, and it may have been a result of earlier hydrothermal activity. The Gryphon U deposit occurs in elongated narrow “lenses” along several fault strands of a fault zone (below the unconformity surface and within basement rocks). U-ore is a cementing mineral in fault-related fractures and fault rocks, but remnant flow channels in fractures in U-ore are still preserved. The most permeable and porous fault rocks are coarse gouges (~ 40% porosity, ~ 10–15 m2 permeability) with approximately three orders of magnitude higher rock matrix permeability than the gneissic host rocks. Such fault rocks locally contain carbonaceous matter, and similar fault rocks may have been preferentially mineralized by U to form the U ore lenses, but the role of brittle-ductile and ductile fault and shear zone rocks in U mineralization is not yet clear at Gryphon. Our review of U–Pb uraninite ages at Gryphon suggests a protracted faulting and U-mineralization history. In response to numerous episodic events of brittle deformation and hydrothermal alteration, the permeability and porosity of the host rocks and U-ores have evolved, as observed by multiple cross-cutting reaction fronts. We inferred from the test results and the alteration halo at the U deposit, affecting a wide part of the fault zone, that the permeability and porosity increased by the downward ingress of U-rich acidic basinal brine that percolated into the basement-hosted (rooted) shear/fault zone and nearby crystalline basement rocks. However, the younger prominent U roll/redox-fronts caused local-scale reduction in porosity and permeability by hematite cementation.

Mineral chemistry and deformation in a temperature gradient in the Sierras Pampeanas of Córdoba (Argentina): The Chicamtoltina Tonalite–Trondhjemite Orthogneiss

On the eastern margin of the Sierra Chica, in a gneissic host rocks metamorphosed under medium to high amphibolite facies (‘Sil-in’ isograd) during the Neoproterozoic–Cambrian M2-D2 event, a body of tonalitic-trondhjemitic composition was discordantly emplaced, cutting the S2 metamorphic foliation and the ‘Sil-in’ isograd. A strong deformation, with along the temperature gradient of the host rocks, transformed the igneous rocks into an orthogneiss during a M3-D3 event. Based on textural, microstructural and mineral chemistry changes along the mentioned metamorphic transition and age determination, the orthogneissification process is related to the deformational and thermal history in the Sierras Pampeanas of Córdoba. The M3-D3 event would have reworked the S2 foliation, generating an S3 foliation associated with folding and boudinage in the gneissic host rocks, forming a major intrafoliar fold in the tonalitic-trondhjemitic body, with its flanks on both sides of the ‘Sil-in’ isograd. The M3-D3 event is recorded in the textural and mineral chemistry variations, following a medium-high temperature gradient during deformation, from the southeast to the northwest of the body. In that direction, the ‘core and mantle’ texture in plagioclase increased the proportion of the recrystallized mantle, which was also enriched in anorthite content. Quartz textures evolved from polycrystalline ribbons, with internal deformation bands, to dismembered ones, with internal chessboard pattern. Recrystallization of the entire plagioclase-quartz aggregate ended in an interlobate to amoeboid texture, implying high-temperature conditions (~650-750 °C) along the mentioned temperature gradient during the M3-D3 event. Regarding micas, the primary biotite progressively increased in Mg and decreased in Fe towards the northwest, without appreciable textural changes. Concomitantly, the primary Ti-rich ferrimuscovite increases its content in Mg and becomes poor in Ti, at the same time that it developed symplectitic quartz-plagioclase intergrowths. During a final D4 deformational event, also under high ductility conditions, asymmetric intrafolial folds (drag folds, with geometrical S4 axial planes) were developed, associated with imbrication and folding of boudins within the S3 (// S2) foliation. The D2, D3, and D4 deformation events are part of a continuum (progressive deformation) in a combined simple shear and flattening regimen (transcompression = convergent oblique shear). The contrasting rheology between the stiffer Chicamtoltina igneous body and its gneissic host rocks explain the different behavior during high ductility deformation. Internally, this small igneous body, with plagioclase as stress-supporting mineral framework (plagioclase-quartz ratio 2:1), mirrored the rheological behavior of the middle to lower crust deformation. A concordia age of 535.81 ± 0.79 Ma (U–Pb LA-ICP-MS on zircon) obtained for the Chicamtoltina orthogneiss is interpreted as the crystallization age of the tonalitic-trondhjemitic protolith (igneous event Ig1). By correlation with the tonalitic orthogneisses from the Sierra Chica and Sierra Norte of Córdoba, this protholith is linked to the Pampean magmatic arc, with an age range of 541–515 Ma and a peak of magmatic activity at 530–535 Ma.

U-Pb zircon geochronologycal investigation on the Morro dos Seis Lagos Carbonatite Complex and associated Nb deposit (Amazonas, Brazil)

We present results of U-Pb dating (by MC-ICP-MS) of zircons from samples that cover all of the known lithotypes in the Seis Lagos Carbonatite Complex and associated lateritic mineralization (the Morro dos Seis Lagos Nb deposit). The host rock (gneiss) yielded an age of 1828 ± 09 Ma interpreted as the crystallization time of this unit. The altered feldspar vein in the same gneiss yielded an age of 1839 ± 29 Ma. Carbonatite samples provided 3 groups of ages. The first group comprises inherited zircons with ages compatible with the gneissic host rock: 1819 ± 10 Ma (superior intercept), 1826 ± 5 Ma (concordant age), and 1812 ± 27 Ma (superior intercept), all from the Orosirian. The second and the third group of ages are from the same carbonatite sample: the superior intercept age of 1525 ± 21 Ma (MSWD = 0.77) and the superior intercept age of 1328 ± 58 Ma (MSWD = 1.4). The mineralogical study indicates that the ∼1.3 Ga zircons have affinity with carbonatite. It is, however, a tendence rather than a well-defined result. The data allow state that the age of 1328 ± 58 Ma represents the maximum age of the carbonatite. Without the same certainty, we consider that the data suggest that this age may be the carbonatite age, whose emplacement would have been related to the evolution of the K'Mudku belt. The best age obtained in laterite samples (a superior intercept age of 1828 ± 12 Ma) is considered the age of the main source for the inherited zircons related to the gneissic host rock.

ALKALINE COMPLEXES AS POTENTIAL AREAS FOR INSTALLATION OF REPOSITORIES OF NUCLEAR WASTE: CRETACEOUS INTRUSIONS IN BRAZIL

Brazil initiated its nuclear program in the decade of 70 with the installation of the Thermonuclear Plant Angra I. The implantation of this type of enterprise involves activities which generate environmental risks manly due to disposal of nuclear waste. The storage of nuclear waste must follow strict international standards and faces the resistance of the civil society. Thus, complex technical and politically procedures are required to find a suitable place for this end. The use of geologic bodies that fit the required features for the installation of nuclear repositories is described in literature. This kind of geological units must be deep, structuralized and with low infiltration. These aspects should to be identified through geologic, hydrologic and geophysical studies and structural models as well.This study aims to report the results of the regional geophysical survey performed to delimit the gneissic host rocks of Ribeira Belt and the characterization of their magnetic characteristics main textural variations. This work also includes the analysis of the structural features of rocks (faults and fractures), in addition to how these structures change within the massif coupled with mineralogy and chemistry signatures.The magnetic anomaly map displays a series of semi-circular magnetic highs, following the general configuration of the alkaline intrusions. In Brazil, the intrusive bodies with potential to function as repository are the alkaline complexes, once they are isotropic, massive, have well defined geometry, compatible depth and with low or no human occupation. However special attention to the possible occurrence of water percolations in fractures and faults must be given. In this way, detailed studies of alkaline bodies that fit the expected features comprise the following intrusions: Mendanha, Tinguá, Tanguá, Soarinho, Rio Bonito and Morro de São João. This study intends to be a contribution for the knowledge of the properties of alkaline bodies with potential to be used for storage of nuclear waste, in the near future, in Brazil.

Structural interpretation of the Steenkampskraal monazite deposit, Western Cape, South Africa

The Steenkampskraal Monazite Mine was first established in 1952, to extract monazite ore for the production of thorium and rare earth element (REE) concentrate. Refurbishment of the mine in recent years has required the re-inspection and re-evaluation of the mineralized monazite zone (MMZ). This contribution presents a structural review of the MMZ and its emplacement, based on recent data and its setting at the southern extent of the Bushmanland Sub-province of the Namaqua-Natal Metamorphic Belt. New surface and underground mapping confirm that the MMZ is a moderately-dipping body within gneissic host rocks on the southern limb of a broad F3 antiform. Thickness variations, both down-dip and along-strike, are the result of D2 and D3 deformation. The MMZ has been locally transected and steepened by subsequent late-D3, “steep-structures”, which are typical of the Okiep copper district, ∼150 km north of Steenkampskraal. Geochronological data suggest that the MMZ was intruded, emplaced or formed at 1046 ± 7.5 Ma, at the start of the D3 Klondikean Episode (1040–1020 Ma). Unlike the analogous copper-bearing Koperberg Suite in the Okiep Copper district, the MMZ was not intruded into Klondikean-aged steep structures, but was rather transected and steepened by these. Local steepening of the otherwise moderately-dipping to flat-lying MMZ makes it locally amenable to detection by soil sampling and radiometric surveys.

Structural characterization of the Misajé granitic pluton (NW Cameroon): constraints from magnetic and field observations

The Misaje granitic pluton, emplaced between 569 and 560 Ma in an amphibolitic and gneissic host rock, comprises four petrographic units namely biotite-hornblende granite (BHG), granodiorite (Gd), biotite granite (BG), and leucocratic granite (LG). Four major tectonic events have been described in the studied area: a D1-early tectonic event, responsible of the E–W flat foliation which has been progressively transposed by a D2 tectonic event. A D2 event has developed heterogeneous simple shear in a dextral transpressive context with moderate to strong dipping NE–SW striking foliation; a D3 tectonic event has lead to a sinistral N–S ductile shear characterized by N- to ENE-striking foliation and E–W strike-slip shear corridors and a D4 tectonic event that developed N–S dextral ductile strike-slip deformation. The magnetic study of the pluton, based on the AMS parameters, reveals the coexistence of both paramagnetic (dominated by iron-bearing silicates; 54 % of sites) and ferromagnetic (due to the occurrence of PSD and MD grains of magnetite or other ferromagnetic minerals; 46 % of sites) behaviors. Magnetic foliation shows best poles at 55/82 for the whole pluton, 95/32 in BHG, and 273/83 in BG, and the magnetic lineation trends are mostly NNE–SSW with best lines at 210/8, 198/19, and 36/3, respectively. The trend of the magnetic lineation in BG indicates an S-shape trajectory, suggesting a sinistral sense of shear motion along discrete E–W corridors situated at the northern and southern ends. Kinematic indicators in BG point to a sinistral sense of shear, suggesting its emplacement during the D3 event. The close relationship between K1 and K3 points to a syn-kinematic emplacement and crystallization of the Misaje granitic pluton during the Pan-African event, and the tectonic evolution of the study area is considered to be coeval with the tectonic evolution of the trans-Saharan Pan-African belt of eastern Nigeria.

Origin of pegmatites and fluids at Ponta Negra (RJ, Brazil) during late- to post-collisional stages of the Gondwana Assembly

The Ponta Negra Pegmatites (PNP), part of a pegmatitic province in Rio de Janeiro State, Brazil, crop out along an intensely deformed, medium- to high-grade metamorphic area that is proximal to a crustal-scale thrust zone developed during the Brasiliano/Pan–African Orogeny. Fieldwork shows that the pegmatites formed in two distinct stages: (i) syn-collisional leucosome veins (Group I) conformable with the tectonic foliation of the gneissic host rocks and (ii) late- to post-collisional dykes (Group II) that cross-cut the same tectonic foliation at a high angle.In this paper, we use geochemistry of whole-rock and mineral separates (alkali–feldspar and biotite), fluid inclusion microthermometry and stable isotopic (δ18O, δD, δ13C) determinations on minerals (quartz, alkali–feldspar, biotite and magnetite) and fluid inclusions to provide insights into the composition of the pegmatite-forming melts, associated fluids, and their geotectonic significance. U–Pb SHRIMP dating of the Cajú syenogranite was performed to evaluate and compare the timing of magmatic events along the Cabo Frio Tectonic Domain as this is the closest post-collisional pluton to the studied pegmatites.The calculated temperature for the Group I syn-collisional veins (740°C) is similar to previous estimates for the peak metamorphic conditions in the study area. Variations in the temperature of the Group II pegmatite dykes obtained from stable isotopes (380 to 720°C), and microthermometric data from primary fluid inclusions with traces of N2 (Th=280 to 360°C), may reflect the thermodynamics of the pegmatite crystallization, exsolution textures and isotopic exchange. The composition of fluids in equilibrium within the pegmatite dykes consists of magmatic and metamorphic components. The minimum pressures calculated for the emplacement of the pegmatites are equivalent to a shallow crustal depth between 1.7 and 3.5km, which corresponds to the exhumation of the orogen since the emplacement of the pegmatites. A linear trend of decreasing CO2 content and δ13CCO2 is consistent with mixtures between (i) carbon derived from organic matter or volatilization of skarns and (ii) inorganic carbon (carbonate).Based on the data obtained, we propose that the pegmatites of Ponta Negra are close to an LCT-type geochemical signature (highly peraluminous magmas with normative corundum), and originated by partial melting of the metasedimentary Palmital succession at depth, during the waning stages of the Búzios Orogeny. The primary melts of the PNP cross-cut both the Neoproterozoic supracrustals and the Paleoproterozoic orthogneissic basement during its ascent and emplacement at higher crustal levels. Variable melt sources explain the slight differences in geochemical compositions among the studied rocks within the metasedimentary succession, which probably include Mn-bearing exhalites, as well as differentiation processes.The 454±5Ma U–Pb (zircon) age of the Cajú syenogranite overlaps previous geochronological data of 440±11Ma obtained on a pegmatite dyke at Ponta Negra, bracketing and extending the time interval for the Gondwana assembly collapse magmatism in the region. The heat that triggered this magmatic event could still be a consequence of the collisional orogeny, increasing contents of heat-producing elements, or, a large intraplate extension that followed the Gondwana amalgamation and initiated the formation of Paleozoic basins.

Evolution of a hydrothermal fluid-rock interaction system as recorded by Sr isotopes: A case study from the Schwarzwald, SW Germany

Metasomatic and Sr-isotopic changes, associated with formation of zoned alteration halos along hydrothermal veins, are documented for a gneiss from the Artenberg quarry near Steinach (Kinzigtal, Schwarzwald, SW Germany). Veins are postorogenic, SW-NE-oriented, and cut straight through metaquartzdioritic Variscan gneiss, where flow of low-temperature fluids (∼100–200°C) caused adularia-quartz-sericite-type alteration. Fluid-rock interaction occurred nearly 50 Ma after Variscan metamorphism, as constrained by a Rb–Sr multimineral isochron for unaltered gneiss of 327.1 ± 3.1 Ma, and by two independent ages of 279.2 ± 3.1 Ma and 274 ± 13 Ma, based on Rb–Sr systematics of late-stage quartz from the veins. In a profile from unaltered gneiss towards a vein, alteration-induced mineralogical changes correlate with metasomatic net addition of K, Rb, and Cl to the alteration zone, combined with net loss of Na, Ca, and Sr. Strontium isotopes give a more detailed insight into the fluid-rock interaction process. 87Sr/86Sr ratios in a profile across the alteration zone are incompatible with simple Sr leaching but reflect partial replacement of the rocks’ Sr by fluid-derived Sr, the isotopic composition of which varied with time. Early fluids, with high 87Sr/86Sr ratios compared to unaltered gneiss, evolved into fluids with somewhat lower ratios, and finally reached a second maximum in 87Sr/86Sr ratios. This Sr-isotopic fluid evolution is equally revealed by the mineral sequence of the vein mineralization. It appears that the compositional evolution of the fluids correlates with the sequence of mineral breakdown reactions in the gneissic host rock, and that the Sr-isotopic evolution of the fluids can be fully explained as the result of internal, progressive reaction of fluid with the local rocks. Results also show that the spatial distributions of Sr isotopes in metasomatic alteration zones may reflect the complex evolution of fluid-rock interaction systems, and ultimately constrain the factors controlling both fluid compositions and alteration patterns.

Estratigrafia do Batólito Florianópolis, Cinturão Dom Feliciano, na Região de Garopaba-Paulo Lopes, SC

In southern Brazil, the Florianopolis Batholith results from prolonged, mainly granitic magmatism, as part of the Neoproterozoic Dom Feliciano Belt. Plutonic associations in this belt are related to transpressive tectonism (650-580 Ma) in post-collisional setting, where the translithospheric discontinuities of the Southern Brazilian Shear Belt have triggered magmatism, acting as channels for melts originated in deep crustal or mantle sources. In the region of Garopaba-Paulo Lopes, a fraction of this magmatism was studied, resulting in a formal proposition for its stratigraphic organization in igneous suites. A small volume of gneissic host rocks is found as roof pendants. The Paulo Lopes Suite comprises the foliated Paulo Lopes Granite, Garopaba Granitoids and Silveira Gabbro. It is characterized as porphyritic granitoids of high-K tholeiitic affinity, coeval with mafic, tholeiitic magmatism. It is followed by metaluminous, alkaline series granitoids of the Pedras Grandes Suite, namely the Vila da Penha Granite, comprising heterogranular and porphyritic facies, and the Serra do Tabuleiro Granite, comprising heterogranular, equigranular and porphyritic facies. The Cambirela Suite, last magmatic episode in the study area, encompasses alkaline plutonic, volcanic and subvolcanic rock types, as the Ilha Granite, the Cambirela Rhyolite and the Itacorumbi Granite. Hypabissal rocks are also part of this suite, and result from recurrent acid and basic magma pulses emplaced either as idividual dykes or composite ones. The magmatic associations described in this region attest to magma emplacement under a moderate to low stress field, possibly extinguished by the time the Cambirela Suite crystallized. Their age values indicate that they are mostly contemporaneous to the post-collisional, syntectonic magmatism of the Southern Brazilian Shear Belt, and the preservation of magmatic structures such as modal layering, relatively uncomon in granitoids, may also be attributed to their emplacement far from the main deformation sites. Features indicative of coeval mafic and felsic magmas are described in all three magmatic associations, and are especially significant in the Paulo Lopes and Cambirela suites. They attest to continuous mantle participation in the batholith construction.

Mineralogical controls on the distribution of trace elements in metasomatized peridotite enclaves from Planany, Czech Republic

AbstractSmall (m-scale) peridotite enclaves at Planany (central Czech Republic) are separated from their gneissic host rocks by a narrow (cm-scale) reaction rim comprising an inner, tremolite + phlogopite zone and an outer, essentially monomineralic phlogopite zone. Both retain an Mg# very similar to that of the peridotite (Mg# = 81), but relative to this reference frame, show large increases in LILE (K, Rb, Ba) and radionuclides (U, Th). On a smaller scale, however, there has been a mineralogically-controlled decoupling of various components, particularly among the HFSE and REE, the former favouring the phlogopite-rich outer layer of the reaction rim, the latter the amphibole-rich inner zone. Taken together, however, the reaction zones preserve key compositional features of their inferred protolith.

Granite emplacement and its relation with regional deformation in the Aravalli Mountain Belt (India)—inferences from magnetic fabric

This contribution presents field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the Godhra Granite, a ca. 1 Ga pluton at the southern margin of the Aravalli Mountain Belt (western India) that lies near the Central Indian Tectonic Zone (CITZ). The northern and southern margins of the granite are high strain zones with north dipping, E–W striking foliations, and NW plunging stretching lineations. The presence of preferentially oriented feldspars suggests that the granite was subjected to magmatic deformation during its emplacement. High temperature solid-state deformation fabrics are recorded from microstructural studies, which imply that the granite is deformed. AMS data show magnetic foliations and magnetic lineations in the granite with orientations similar to the field data. The magnetic foliation in the granite is parallel to the D 3 foliation in the gneissic host-rock. Both the stretching lineations and the magnetic lineations at the granite margin high strain zones are parallel with the axes of D 3 reclined folds in the gneiss. Therefore, the emplacement of the Godhra Granite is inferred to have taken place syntectonically, while the gneiss was undergoing the D 3 phase of deformation. The orientation and character of these lineations and the vergence of the D 3 folds imply oblique simple shear, which is inferred to have controlled the emplacement of the granite. The magnetic fabric data are compared with the regional structural trends of the metasedimentary rocks of the southern Aravalli region and the available geochronological data from rocks of the southern Aravalli region are summarised. Based on this information, it is suggested that the emplacement of Godhra Granite, deformation of the southern parts of Aravalli Mountain Belt and, tectonothermal events along the CITZ and Rodinia Supercontinent assembly were related events.

Emplacement setting of the granite sheeted pluton of Esperança (Brasiliano orogen, Northeastern Brazil)

Anisotropy of magnetic susceptibility (AMS) and isotopic (U–Pb, Sm–Nd) data were combined to study the emplacement setting of the granite sheets that constitute the Esperança pluton in the Borborema Province (Northeastern Brazil). The sheets dip moderately to the SE along the contact zone between the Paleoproterozoic basement rocks and Early Neoproterozoic orthogneisses and metasediments. Granite fabrics were determined mainly using AMS in 136 sites distributed within the central and western part of the pluton. The sheets normally have susceptibility lower than 0.35 mSI but, locally, where a Ti-poor magnetite appears with titanite, the susceptibility increases up to 5 mSI. Comparison between the silicate fabric and AMS showed inconsistencies between the shape of mineral and magnetic ellipsoids despite of their orientations that fit fairly well to each other. AMS indicated the deformation was partitioned between the lower (tonalite, syenogranite) and upper (leucogranite and coarse porphyritic granite) sheets. In the lower sheets the curvilinear lineation trajectory is attributed to a dominant heterogeneous pure shear event that flattened laterally the still molten tonalite and syenogranite into the regional foliation. The associated microstructures are typically magmatic. Zircon U/Pb data of the syenogranite yielded a crystallization age of 592 ± 5 Ma. In the upper sheets the fabric recorded a component of simple shear deformation that displaced the coarse porphyritic granite and the top gneissic host rocks to the southwest. Microstructures are mostly of post-full crystallization type. T DM model ages and ɛ Nd ( t = 0) values indicate that the magma contaminated by partial melting of the regional host rocks. Sheet propagation at the emplacement level would have exploited the contact zone between crustal blocks of different rheologies when the melt pressures would be able to tensionally fail the anisotropy of the host rocks.

Structural appraisal of the Gadag schist belt from gravity investigations

Semi-detailed gravity investigations were carried out over an area of approximately 2750 sq km with maximum N-S and E-W extents of 55 and 50 km respectively in the Gadag region in the Dharwar craton with a view to obtain a clearer perception of the structural configuration of the region.From qualitative analysis of the gravity data, several tectonic features are inferred: the high density Gadag schist belt is characterized by a gravity high and occurs in two discontinuous segments — the main N-S trending segment, and its thinner NW-SE trending extension, the two separated by a NE-SW trending deep seated fault. While the N-S trend of the Gadag schist belt is bounded on its east by the NW-SE trending Chitradurga thrust fault and on its west by another major NNWSSE trending fault, the NW-SE extension is likewise bounded by two other NW-SE major faults.Quantitative evaluation from forward modeling/inversion of five profiles in the region, assuming a density contrast of 0.29gm/cc of the anomalous schistose body with the gneissic host rocks indicated a synclinal structure plunging to the southeast along its axis for the Gadag schist belt. The maximum width and depth from surface of the schist belt are 22 km and 5.6 km respectively.

Emplacement of the Nain anorthosite: diapiric versus conduit ascent

Estimation of settling velocities of large orthopyroxene megacrysts, found within anorthosite intrusions, are calculated and compared with ascent rates achieved by diapirism and conduit propagation. Calculations suggest that diapirism is far too slow to be an appropriate ascent mechanism for anorthositic crystal mush and favour conduit emplacement. The intrusions of the Nain Plutonic Suite (NPS) are located along the Abloviak shear zone, which marks the boundary between the Nain and Churchill provinces, and within the zone of juxtaposition of the Saglek and Hopedale blocks of the Nain Province. These crustal weaknesses have probably controlled the emplacement and distribution of the intrusions. Contact relations between intrusions of anorthosite and their gneissic host rock provide evidence for two emplacement styles within the NPS, the first typified by strongly deformed and recrystallized rocks, and the second by an outer border zone of mafic rocks. It is proposed that these differences in intrusive style are due to differences in ductility contrast between the magma and its surrounding host rocks, such that those intrusions emplaced into the thermally softened shear zone have deformed margins, whereas those intruded into the cooler Archaean crust have undeformed margins.

Emplacement of the Nain anorthosite: diapiric versus conduit ascent

Estimation of settling velocities of large orthopyroxene megacrysts, found within anorthosite intrusions, are calculated and compared with ascent rates achieved by diapirism and conduit propagation. Calculations suggest that diapirism is far too slow to be an appropriate ascent mechanism for anorthositic crystal mush and favour conduit emplacement. The intrusions of the Nain Plutonic Suite (NPS) are located along the Abloviak shear zone, which marks the boundary between the Nain and Churchill provinces, and within the zone of juxtaposition of the Saglek and Hopedale blocks of the Nain Province. These crustal weaknesses have probably controlled the emplacement and distribution of the intrusions. Contact relations between intrusions of anorthosite and their gneissic host rock provide evidence for two emplacement styles within the NPS, the first typified by strongly deformed and recrystallized rocks, and the second by an outer border zone of mafic rocks. It is proposed that these differences in intrusive style are due to differences in ductility contrast between the magma and its surrounding host rocks, such that those intrusions emplaced into the thermally softened shear zone have deformed margins, whereas those intruded into the cooler Archaean crust have undeformed margins.

Ultracataclasis, sintering, and frictional melting in pseudotachylytes from East Greenland

Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along ∼400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe 2O 3, MgO, and TiO 2 relative to the host rock, with minor increases in CaO, K 2O, and small decreases in Na 2O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe 2O 3, MgO, TiO 2, and Al 2O 3 relative to the host rock, with minor depletion in Na 2O and K 2O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO 2, Al 2O 3, K 2O, Fe 2O 3, MgO, CaO, and Na 2O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin.

Emplacement and deformation of granite pegmatite dykes in a mid-crustal regime of late-orogenic extension, southwest Grenville Province, Ontario, Canada

The horizontal opening of vertical fractures during emplacement of pegmatite-dyke swarms is an important mid-crustal mechanism of large-scale horizontal extension. This is documented in the south-western Grenville Province, a deeply eroded part of the collisional Grenville orogen. In the Georgian Bay region of central Ontario, Grenville gneisses host c. 990 Ma old, angular dykes which attest not only to horizontal extension but also to vertical thinning. The original dilation dykes probably varied in strike and were statistically vertical. However, many dykes had subhorizontal or inclined segments that were oblique or quasi-concordant to the gneissic foliation in the host rocks. The number of dykes exposed per 0.25 km2 varies on the scale of a few kilometres, and this is indicative of heterogeneous late orogenic extension of the Grenville gneisses. The apparent absence of regional gradients of peak palaeo-pressure, at the present erosion level, suggests that the extension was horizontal and initially unaccompanied by vertical contraction of the host gneisses. Subsequent buckling of the pegmatite dykes led to gentle, open or close folds with vertical enveloping surfaces. The geometric effects of gentle buckling of pegmatite dykes can be difficult to recognize in the field, especially where the late-stage vertical thinning is relatively weak. Among the geometric indicators of buckle-shortened dykes, the characteristic deflection (‘fanning’) of coplanar, inherited folia in gneissic host rocks is most sensitive. Systematic changes in the local degree of vertical shortening are indicative of heterogeneous vertical thinning, and may be associated with pull-apart structures at the horizontal scale of several kilometres.

Chemical composition of hard- and softrock groundwaters from central Norway with special consideration of fluoride and Norwegian drinking water limits

Groundwaters from crystalline and metamorphic rocks (hardrocks) and from Quaternary deposits, i.e., alluvial and glacial deposits (softrocks) from the counties of Nord-Trondelag and Sor-Trondelag were analyzed for major and minor elements and ions including fluoride. The median concentration of F− in water from the hardrock aquifers is 0.28 mg/l (14.7 μeq/l) in contrast to water from softrock aquifers in which it is found to be 0.05 mg/1 (2.6 μeq/l). More importantly, ca. 15% of the locations where water was abstracted from hardrock wells contain 1.5 mg/l (78.9 μeq/l) F− or more. Thus, 15% of all hardrock wells returned F− results that are at or above the maximum recommended value for drinking water. Of the softrock wells, none are above 1 mg/l. Geologists would normally expect higher F−contents in groundwaters derived from acid rocks, e.g., in granitic or gneissic areas. When comparing the host lithology with the observed F−contents, however, no clear relationship between F− content and lithology is visible. The highest observed F− values actually occur in gneissic host rocks. However, wells drilled in amphibolites/greenstones, mica schists, calcareous rocks, and sedimentary rocks all returned some analytical results above 1.5 mg/l F−. These results suggest that all hardrock wells drilled should be tested for F− and the users informed about the results and advised to take any necessary precautions. When applying the recently proposed Norwegian drinking water limits to our data, 51% of all softrock well waters and 56% of all hardrock well waters are unfit for consumption without prior treatment, although we analyzed only for about half of the proposed elements/parameters. This result seriously questions the concept of fixed action levels—many of them with totally unproven health implications—for so many parameters/elements for hardrock groundwaters.

Major- and minor-element mobilization, with implications for NiCuFe-sulphide remobilization, during retrograde metasomatism at the Vammala Mine, southwest Finland

Granitic melt, derived from migmatitic host rocks, intruded a D 3-related fracture system in a 1.89-Ga NiCuFe-sulphide-bearing peridotite sheet, near Vammala, southwest Finland. Zones of wall-rock alteration, mainly comprising amphibole-phlogopite/chlorite, developed in the peridotite adjacent to the leucocratic dykes. The minerals in the alteration zones and in the gneissic host rocks, together with the production and intrusion of granitic melt, are consistent with metamorphic conditions of at least 600°C and 4.5 kbar. Later hydration under condtions of lower temperature and pressure resulted in partial serpentinization of the peridotite and dykes. This later transition precluded direct investigation of the earlier transition that developed the medium- to high-grade alteration zones in the peridotite at the interface with the leucocratic dykes. Because serpentinization was largely accommodated by volume increase, rather than mass loss of MgO and SiO 2, the relative concentrations of most components in the serpentinized peridotite should mirror those in the peridotite. Based on this contention, two methods are used to document the composition-volume behaviour of the transition from now-serpentinized peridotite to alteration zones ( apparent transition), and hence infer the mass-balance trends of the igneous peridotite-to-alteration zone transition ( real transition). The methods collectively suggest that, in the alteration assemblages, K 2O, Rb 2O, P 2O 5, TiO 2 and Al 2O 3 form a group of relatively enriched components, and that Al 2O 3 was particularly mobile. An estimated volume increase of 10-10% accompanied serpentinization, and a calculated volume decrease of 10–15% accompanied the apparent transition; thus, a volume increase of < 5% was involved in the real transition. For “best-fit” volume factors, Gresens' equation demonstrates that the apparent transition involved mass gains in SiO 2, Al 2O 3, K 2O, P 2O 5 and TiO 2, and mass losses in FeO, MgO, CaO, Ni, S and volatiles (loss on ignition, LOI), these amounting to net losses of 32.5 kg of volatiles and 10.8 kg of other components per 1000 kg of serpentinized peridotite. The real transition involved similar relative gains and losses, except for SiO 2 gain (slightly less), MgO loss (slightly more) and LOI loss (negligible). It respectively records inward transfer of components from a feldspar-rich source to the peridotite, and outward transfer to that source. This is consistent with metasomatic exchange between the peridotite and the probably supra-solidus granitic intrusions. During the real exchange, NiCuFe-sulphides were retained and redistributed (internally remobilized) in the outer alteration zones, but transferred into the dyke from the zone abutting it (externally remobilized). The same genetic interpretations probably apply to similar dyke and alteration systems in other NiCuFe-sulphide deposits hosted in ultramafic rocks in southwest Finland.

Effects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland

Petrological studies of fault-generated pseudotachylytes, formed during coseismic slip, need to consider cataclastic processes as well as rapid, closed-system melting and freezing in order to account for the observed similarities and the variations between the chemical compositions of pseudotachylytes and their host-rocks. Whole-rock analyses of pseudotachylytes and their gneissic host-rocks from the Lewisian Complex of northwest Scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk host-rock composition but also to its mineralogy and fabric. In anisotropic (e.g., gneissic) rocks, selective cataclastic and melting effects may both contribute to determining the whole-rock composition of pseudotachylytes. Electron microprobe analyses of pseudotachylyte matrices indicate more rapid decomposition and fusion of hydrous ferromagnesian phases, such as biotite and amphibole, and to a lesser extent feldspar relative to quartz. The pseudotachylytes of this study appear to have formed neither by ultracataclasis alone nor by classical equilibrium partial melting or total fusion, but through the interaction of selective cataclastic and melting processes controlled by the lithological layering and mineralogical heterogeneity in their host-rocks. This interpretation may be widely applicable to pseudotachylytes generated by faulting in foliated host-rocks.