Intermediate Metavolcanic Rocks Research Articles

Regional-scale mapping for determining geochemical background values in soils of the Itacaiúnas River Basin, Brazil: The use of compositional data analysis (CoDA)

This study aims at providing high quality soil geochemical data at a regional scale (1 sample per 25 km2) to understand the source and distribution of potentially toxic elements (PTEs: Al, As, Ba, Cd, Cu, Co, Cr, Fe, Hg, Mo, Mn, Ni, Pb, Se, U, V, and Zn) and to define their geochemical background threshold values (GBTVs) in the Itacaiúnas River Basin (IRB), Brazilian Amazon. A total of 2958 soil samples were collected at two depths (surface: 0–10 cm; bottom: 30–50 cm) and the fine fractions (<0.117 mm) were analyzed by ICP-MS and ICP-AES after aqua-regia digestion. The compositional nature of geochemical data was evaluated using the log-ratio approach and compared with traditional methods. Results show that compared to raw/log-transformation, centred log-ratio (clr) transformation improves spatial mapping and also better separated variables in Principal Component Analysis (PCA). Correlation analysis based on the isometric log-ratio (ilr) transformation in heat-maps provides reliable and better structure of the results. The spatial maps (clr) indicate that most of the elements in surface and bottom soils are mainly governed by underlying bedrock geology (parent lithologies and mineralization), while anthropogenic factors are insignificant. The resulting clr-based PCA and cluster analyses associated with geological information indicate that geochemical patterns of Fe-Ti-V-Cr-Sc can be linked to mafic to intermediate metavolcanic rocks and banded-iron formations, which are dominant in the Carajás basin, while Ni and Cr distribution pattern is attributed to mafic and ultramafic lithologies, and Cu anomalous values are mainly related to hydrothermal mineralized copper belts. Among the statistical methods applied for estimation of GBTVs, TIF produced the highest BTVs, followed by 98th and mMAD, with few exceptions, while mMAD was considered the more suitable for defining GBTVs. The number of potential anomalies of PTEs identified by these methods follow the order of MAD > 98th > TIF or MAD > TIF > 98th, and overall the anomalies are mostly related to the local lithology/mineralization with no indication of anthropogenic contamination. Compared with Brazilian guidelines (CONAMA Resolution No. 420/2009), the GBTVs determined for Cr, Co, Ni, and Cu by TIF, MAD, and 98th percentile mostly exceeded both IV and PV limits. However, these high values are due to the influence of local lithology. This reveals that the Brazilian CONAMA guidelines of these elements for the IRB are unrealistic because that suggests a large number of sites requiring investigation, which is not consistent with the geochemical evidence. Therefore, site-specific GBTVs must be considered for the accurate evaluation of anthropogenic contamination, as well as the ecological and human health risks of PTEs in this region.

Timing and origin of the host rocks to the Malmberget iron oxide-apatite deposit, Sweden

The northern Norrbotten region in Sweden hosts abundant iron-oxide apatite (IOA) deposits including Kiirunavaara, the type locality for Kiruna-type deposits, and Malmberget. Felsic and intermediate metavolcanic rocks hosting the Malmberget IOA deposit contain oscillatory zoned zircon which yield magmatic U-Pb SIMS ages of 1885 ± 6 Ma and 1881 ± 6 Ma, respectively. Metamorphic rims on zircon from these rocks yield 1797 ± 7 Ma and 1775 ± 6 Ma, respectively, and record the age of the latest Svecofennian regional metamorphic event in the Gällivare area, tentatively interpreted as regional contact metamorphism. Two granite dikes that cut the ore yield U-Pb zircon emplacement ages of 1790 ± 6 Ma and 1791 ± 7 Ma, respectively, overlapping with the metamorphic overgrowths, and set a lower age limit for ore formation in the Malmberget IOA deposit. Rocks hosting the Malmberget IOA deposit have an alkalic to alkali-calcic affinity with a geochemical signature that favors a continental-arc, transitional to extensional setting. These rocks are suggested to have been generated in a back-arc region, in response to subduction beneath the craton margin retreating to the SW or W. The obtained ages and geochemical signatures of these rocks coincide well with the regionally defined Kiirunavaara group rocks, hosting several other IOA deposits in northern Sweden.

Structure of the Paleoproterozoic Kédougou-Kéniéba Inlier (Senegal-Mali) deduced from gravity and aeromagnetic data

The Kédougou-Kéniéba Inlier (KKI) corresponds to a window through Paleoproterozoic terranes of the West African Craton (WAC). This study presents, first of all, an interpretation of a regional Bouguer gravity map (14°W-9°W, 11°N-16°N) of the KKI and then introduces a new litho-structural map of the Malian part of the KKI using aeromagnetic data (12°0′W-10°80′W, 12°0′N-14°50′N). The KKI is limited to the west by a negative gravity anomaly forming a 30–100 km wide corridor, oriented N–S to NNE-SSW, and correlated to the Variscan Mauritanides orogenic belt. West of this belt, the Mesozoic to Cenozoic sedimentary deposits are marked by the highest positive anomaly of the region, attributed either to the presence of mafic rocks at intermediate to deep crustal levels or to a shallower Moho depth. Within the KKI, moderately positive to negative anomalies are correlated with exposed plutonic and mafic to intermediate metavolcanic rocks. Gravity data also reveal (i) two north-west trending lineaments attributed to crustal-scale shear zones, (ii) three north-east trending lineaments marked by gravity anomalies sub-parallel to the known Bissau–Kidira–Kayes Fault Zone, and named Kayes, Kédougou-Kéniéba, and South Shear Zones, respectively, (iii) and the extension to the south of the known Mauritanides Belt Thrust.The local aeromagnetic map of the Malian part of the KKI discriminates the Mako metavolcanic belt with granitic intrusions in the west characterized by heterogeneous anomalies, from the Kofi metasedimentary series delineated by magnetic lows in the east. ESE-WNW to ENE-WSW-trending alternation of magnetic anomalies in the metavolcanic and metasedimentary rocks are attributed to pervasive structures associated with ductile deformation. The Senegalo-Malian Shear Zone is marked at the regional scale by deflection of the geophysical lineaments but is not expressed by any offset of identified structures or lithological contacts.

Geochronological evolution of the Pitangui greenstone belt, southern São Francisco Craton, Brazil: Constraints from U-Pb zircon age, geochemistry and field relationships

The Pitangui greenstone belt (PGB) – located in the southern São Francisco Craton, (Brazil) – is a volcano-sedimentary assemblage that is tectonically imbricated over the orthogneiss and granitoid rocks collectively termed the Divinópolis Complex. One diorite gneiss sample from the northern Divinópolis Complex yields an igneous zircon age of 2878 ± 5.7 Ma, constraining the time of crystallization. The basal sequence of PGB predominantly comprise metabasalt with restricted bodies of metagabbro/metadiorite, metaultramafic volcanic rocks and BIF. The upper sequences of the PGB exhibit a maturation of the basin, grading to more felsic and sedimentary lithologies, such as clastic metasedimentary rocks, intermediate metavolcanic rocks, and metapyroclastic rocks. The first recognized deformation phase in the PGB (Dn) is associated with greenschist facies conditions and the development of the main tectonic foliation. The second deformation phase (Dn+1) is associated with amphibolite facies metamorphism, while a later (Dn+2) phase may be related to younger Proterozoic deformation (ca. 2.0 Ga). PGB metabasalts yield a crystallization age of 2740.2 ± 12 Ma and are characterized by a tholeiitic affinity and flat LREE patterns. Based on mixed MORB- and arc-affinities, a back-arc basin setting is suggested for the formation of PGB basalt. This mafic magmatism (2740 Ma) is slightly younger than the ages reported for the Rio das Velhas greenstone belt, with which the PGB is generally correlated. The metasedimentary rocks of the PGB have age intervals spanning from 2690 to 3250 Ma. One mica-quartz phyllite has a maximum depositional age of 2769.8 ± 8.9 Ma while a chlorite phyllite records and age of 2693.2 ± 7.9 Ma. The metasedimentary rocks are derived from a source area of mafic to felsic composition that is similar to TTGs, cratonic Archean shales and E-MORB. The predominant sources were likely the sialic Archean crust of the granite-gneiss complexes (e.g. the Divinópolis Complex and potassic granitoids of the Mamona Event) and the mafic sequences of the arc (e.g. PGB back-arc basalt). One late potassic pluton (Casquilho monzogranite) crosscutting the fabrics of the volcano-sedimentary assemblage and yields an intrusion age of 2694.7 ± 9 Ma, constraining the minimum sedimentation age assembly of the package. Collectively, a Neoarchean primitive island arc or back-arc basin is proposed to have generated the PGB, which is here interpreted as distinct from the Rio das Velhas greenstone belt. We suggest that PGB developed during an accretion-collision event in the southern São Francisco Craton before cratonic stabilization of the crust by the end of the potassic 2760-2680 Ma Mamona Event.

The geology, petrology, and geochronology of the Archean Côté Gold large-tonnage, low-grade intrusion-related Au(–Cu) deposit, Swayze greenstone belt, Ontario, Canada

The Archean Côté Gold Au(–Cu) deposit is the first large gold deposit discovered in the Swayze greenstone belt of the Abitibi Subprovince. The deposit is a low-grade, large-tonnage type with a combined indicated and inferred resource of 8.65 M oz Au (245.2 t Au). The deposit is hosted by the Chester intrusive complex (CIC), a multiphase, subvolcanic intrusion composed of low-Al tonalite, diorite, and quartz diorite, plus magmatic and hydrothermal breccia bodies. The age of the tonalite and dioritic phases is constrained at 2741–2739 ± 1 Ma using high-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon geochronology. Although these phases are co-temporal and co-spatial, they appear to be petrogenetically unrelated. The CIC was emplaced into mafic metavolcanic rocks of the Arbutus Formation whose geochemistry reflects a back-arc environment. The tonalite of the CIC is coeval and co-genetic with the felsic to intermediate metavolcanic rocks of the Yeo Formation. Emplacement of the CIC into a shallow crustal level is inferred based on the incorporation of screens and inclusions of the Yeo Formation and is supported by the presence of textures in tonalite and dioritic rocks (e.g., granophyres, miarolitic cavities, and pegmatites), as well as Al-in-hornblende geobarometry results of ≤1.3 ± 0.6 kbars (1 kbar = 100 MPa). The CIC is petrologically similar to other subvolcanic, low-Al tonalite–trondhjemite–diorite intrusions that underlie volcanogenic massive sulphide (VMS)-type deposits and which themselves may contain syn-intrusion mineralization. Several geochemically unrelated dykes and deformation events crosscut and postdate the CIC.

Visible and infrared properties of unaltered to weathered rocks from Precambrian granite-greenstone terrains of the West African Craton

In situ and laboratory 0.35 μm–2.5 μm spectra of rocks from a Paleoproterozoic granite-greenstone terrain along with its Neoproterozoic sedimentary cover and derived regolith materials were examined in western Burkina Faso. The reflectance spectra show the influence of typical arid to semi-arid weathering with the formation of desert varnish, iron films, and dust coatings. Fe and Mg–OH absorption features related to chlorite, amphibole, pyroxene, epidote, and biotite are observable in the mafic and intermediate meta-volcanic rocks as well as in the granodiorites and tonalites. Al–OH absorption caused by kaolinite, smectite, illite/muscovite are typical for meta-volcano-sedimentary schists, Tarkwaian-type detrital meta-sediments, sandstones of the Taoudeni basin, all of the weathered surfaces and regolith materials. Ferric and ferrous iron absorptions related to both primary rock-forming minerals and secondary weathering minerals (goethite, hematite) were observed in most of the sampled materials. The results show that although weathering alters the spectral signature of the fresh rock, indicative absorption features located in the short wave infrared region remain detectable. In addition, spectra of soils partially reflect the mineral composition of the weathered rock surfaces. The analysis of the hyperspectral data shows the potential of differentiating between the sampled surfaces. The library presents a primary database for the geological and regolith analysis of remote sensing data in West Africa.

Geochemistry of hydrothermal alteration at the Qolqoleh gold deposit, northern Sanandaj–Sirjan metamorphic belt, northwestern Iran: Vectors to high-grade ore bodies

The Qolqoleh orogenic gold deposit in the northern part of the Sanandaj–Sirjan metamorphic belt in northwestern Iran is hosted by a steeply dipping sequence of greenschist facies Cretaceous volcano–sedimentary rocks, including mafic to intermediate metavolcanic rocks, sericite and chlorite schist, and marble. Geochemical and petrochemical data including the ∑REE, (La/Yb)N and Eu/Eu* ratios were obtained from country rocks, ore-enveloping alteration zones, and mineralized zones to assess the nature of the trace element and rare earth element (REE) interaction between the wall rock and the mineralizing fluid.Quartz–sulfide veins at the deposit are characterized by a pyrite–pyrrhotite–chalcopyrite–sphalerite–arsenopyrite–native gold assemblage. Alteration halos border the mineralized zones and broadly comprise: (1) an outer carbonate–chlorite alteration zone in all rock types, particularly in chlorite schist; (2) a middle sericite–carbonate alteration zone in the sericite schist; and (3) an inner quartz–sulfide alteration zone in sericite schist and mafic to intermediate metavolcanic rocks.The geochemical data indicate that the concentrations of Al2O3, P2O5, TiO2, Y, and Zr are relatively constant, suggesting that these elements were the least mobile during hydrothermal activity. Using Al2O3 as the immobile component, there is evidence for mobility of trace elements, particularly light REE, TiO2, and Zr in the altered wall rocks. The altered rocks show a relatively light REE depletion ((La/Yb)N≅9.41), which clearly correlates with the grades of gold mineralization and intensity of the alteration (3ppm Au). The depletion of light REE is best indicated by a decrease in (La/Yb)N as shown by ratios of 10.5 to 11.8. Wall rock decarbonation reactions during infiltration of the mineralizing fluid resulted in differential mobilization of REE, from a fluid with initially low REE content.The overall trace element geochemistry of the altered wall rock is controlled by the initial composition of the wall rocks and the ore-fluid composition. Hydrothermal ore-forming fluids are recognized as CO2-rich near-neutral reduced fluids with high values of H2S, K, and S content. Observed variability in alteration halos at the Qolqoleh deposit points to major differences in REE and trace element content in original host rocks that have interacted with a relatively similar ore fluid. Therefore, depending on the composition of each host rock lithology, the geochemistry of hydrothermal alteration (e.g., ∑REE content and (La/Yb)N ratios) and alteration mineralogy including the carbonate–sericite–quartz–sulfide assemblages may be used as a primary tool for lithogeochemical exploration for gold deposits in northwestern Iran.

Zircon evaporation ages and geochemistry of metamorphosed volcanic rocks from the Vinjamuru domain, Krishna Province: evidence for 1.78 Ga convergent tectonics along the southeastern margin of the Eastern Dharwar Craton

Palaeoproterozoic intermediate to potassic felsic volcanism in volcano‐sedimentary sequences could either have occurred in continental rift or at convergent magmatic arc tectonic settings. The Vinjamuru domain of the Krishna Province in Andhra Pradesh, SE India, contains such felsic and intermediate metavolcanic rocks, whose geochemistry constrains their probable tectonic setting and which were dated by the zircon Pb evaporation method in order to constrain their time of formation. These rocks consist of interlayered quartz–garnet–biotite schist, quartz–hematite–baryte–sericite schist as well as cherty quartzite, and represent a calc‐alkaline volcanic sequence of andesitic to rhyolitic rocks that underwent amphibolite‐facies metamorphism at ~1.61 Ga.Zircons from four felsic metavolcanic rock samples yielded youngest mean 207Pb/206Pb ages between 1771 and 1791 Ma, whereas the youngest zircon age for a meta‐andesite is 1868 Ma. A ~2.43 Ga zircon xenocryst reflects incorporation of Neoarchaean basement gneisses. Their calc‐alkaline trends, higher LILE, enriched chondrite‐normalized LREE pattern and negative Nb and Ti anomalies on primitive mantle‐normalized diagrams, suggest formation in a continental magmatic arc tectonic setting. Whereas the intermediate rocks may have been derived from mantle‐source parental arc magmas by fractionation and crustal contamination, the rhyolitic rocks had crustal parental magmas. The Vinjamuru Palaeoproterozoic volcanic eruption implies an event of convergent tectonism at the southeastern margin of the Eastern Dharwar Craton at ~1.78 Ga forming one of the major crustal domains of the Krishna Province. Copyright © 2012 John Wiley &amp; Sons, Ltd.

New data on the age (SHRIMP II) of protolith and paleoproterozoic transformations of the Archean Keivy terrain (Kola Peninsula)

The Keivy terrain is a unique structure principally different from adjacent territories. It comprises four complexes of magmatic and supracrustal rocks: the supposed basement composed of the tonalite- trondhjemite association (TTG), the Lebyazhinskaya Series of acid and intermediate metavolcanic rocks, the Keivy Series of highaluminum schists (1), and the complex of alkaline granites, sometimes associating with ultrabasic and alkaline rocks (2). The tectonic boundaries of the terrain control the location of basic and utrabasic intrusions including the layered plutons of the Panskaya and Fedorovskaya tundra. Of special importance for the solution of the problems of peri� odization of endogenous problems are the studies in which the U-Pb study of zircons provided the Neoarchean age of metavolcanic rocks of the Leb� yazhinskaya Series (2750 Ma) (3), alkaline granites and granosyenites of the western part of the Keivy ter� rain (2630-2674 Ma) (4, 5), and plutons of the Pan� skaya and Fedorovskaya tundra (2500 Ma) (6). This demonstrates that the considered structure is not only entirely composed of Archean formations, but its sep� aration took place at the end of the Neoarchean. This study was aimed at local U-Pb (SHRIMP) dating of the processes of Paleoproterozoic tectonic and meta� morphic transformation of Archean structures within the considered terrain. The structural geological investigations of the Keivy terrain demonstrating that later it underwent radical transformation allowed us to make a conclu� sion on the presence of nappe structures in the studied area of the Keivy terrain and on the formation of this terrain with the Bolshie Keivy nappe as one of its com� ponents (7-9) in the course of the thrust formation (10) during the Neoarchean-Paleoproterozoic colli� sional processes in the southwestern boundary of the Kola fragment of the Archean craton. The Bolshie Keivy nappe is the upper part of the tectonostratigraphic section of the Keivy terrain and composed of plates and lenses of blastomylonitic, mainly highaluminum metamorphic and metaso�

Geology and Fluid Characteristics of the Mina Velha and Mandiocal Orebodies and Implications for the Genesis of the Orogenic Chega Tudo Gold Deposit, Gurupi Belt, Brazil

The Chega Tudo gold deposit is located in the Gurupi belt of northern Brazil. Gold mineralization in the Mina Velha (upper level) and Mandiocal (intermediate and lower levels) orebodies is hosted in highly strained Paleoproterozoic (2148‐2160 Ma) felsic to intermediate metavolcanic rocks and schists that record greenschist facies metamorphism. The orebodies are broadly concordant with the regional northwest-oriented foliation, and mineralization comprises thick quartz vein sets and small quartz-carbonate-sulfide veinlets and enclosing hydrothermally altered host rocks. The hydrothermal mineral assemblage is postpeak metamorphism, syn- to late tectonic, and includes quartz, calcite, chlorite, white mica, pyrite, subordinate chalcopyrite, and traces of sphalerite and galena. Free-milling gold occurs in sulfide-poor veins in the upper and lower zones of the deposit, whereas refractory gold was deposited in fractures of pyrite and quartz in the more gold-enriched intermediate (±lower) level. Fluid inclusion and stable isotope (O, H, C, and S) data indicate that the two styles of gold deposition were produced by a combination of fluid immiscibility, fluid-rock reactions (sulfidation, carbonatization), and probably mixing, occurring in two discrete stages within a single mineralizing event associated with fluid flow within and around an active shear zone under fluctuating pressure conditions. The refractory gold was deposited at 340° to 370°C from a CO2-CH4-H2O‐NaCl fluid having salinity of 5.8 ± 2.7 wt percent NaCl equiv, variable CO2 contents (typically 12‐22 mol %), and up to 6 mol percent CH4. The δ18O and δD values of this fluid are 7.9 to 9.4 per mil and ‐29 to ‐37 per mil, respectively, indicating a metamorphic origin. Carbon isotopes show contrasting values in fluid inclusion CO2 (δ13C = ‐24.1‰) and calcite (δ13C = of ‐3.8‰). The strongly negative value of fluid inclusion CO2 is interpreted to be an organic signature acquired at the site of deposition by reaction of the deeply sourced ore-forming fluid with carbonaceous schists present in the deepest part of the deposit. The higher value for calcite probably records a crustal source. Oxygen fugacities calculated for the whole range of T-P-XCO2 conditions yielded log fO 2 between ‐28.7 and ‐30.5, indicating reduced conditions for the fluid, which is in agreement with the mineral paragenesis, fluid inclusion compositions, and sulfur isotope values. The second fluid, responsible for the deposition of the free-milling gold in veins and probably part of the gold of the lower zone of the deposit, had a lower temperature (330°‐340°C). This CO2-H2O‐NaCl fluid had a salinity of 1.6 to 2.5 wt percent NaCl equiv and contained 11 to 13 mol percent CO2. The δ18O and δD values of this fluid are 5.3 to 7.2 per mil and ‐12 to ‐30 per mil, respectively, also indicating a metamorphic source. The δ13C value of fluid inclusion CO2 is ‐6.9 per mil, likely representing a mantle-derived carbon source. Log fO 2 values between ‐30.5 and ‐31.5 also indicate slightly more reduced conditions for this fluid. Both fluids had near-neutral pH, between 5 and 6.2, and δ34S values of H2S between ‐0.2 and ‐1.0 per mil, interpreted to reflect derivation of sulfur from magmatic sulfides.

Palaeoproterozoic mafic and intermediate metavolcanic rocks in the Turku area, SW Finland

Twenty two samples were analysed from mafic and intermediate volcanic rocks from the Turku area along ~ 60 km long north-south geotraverse. Geographically and lithologically the samples were divided into the Pargas, Turku and Vahto groups. The Pargas and Turku groups consist of mafic units and intercalations within migmatitic gneisses. Their geochemi cal compositions are very similar. In average, they show quite flat REE curves and the mul tielement diagrams show moderate LILE enrichments, minor negative Ta-Nb anomalies and straight HFSE curves. We infer these as transitional between MORB and VAB. The Vahto group consists of thicker volcanic sequences in connection with synorogenic plutonic rocks. The data show enriched LREEs, LILEs, Th and P combined with negative Ta-Nb anomalies, i.e. the Vahto group shows subduction component. We interpret that the Vahto volcanic rocks belong to the Hame belt. The geochemical data indicate that the Pargas and Turku groups were formed in extensional, possibly back-arc or intra-arc setting while the Vahto group was formed at continental volcanic arc.

Pukala intrusion, its age and connection to hydrothermal alteration in Orivesi, southwestern Finland

The Pukala intrusion is situated in the Paleoproterozoic Svecofennian domain of the Fennoscandian Shield in the contact region between the Central Finland Granitoid Complex and the Tampere Belt. The acid subvolcanic intrusion, which is in contact or close to several altered domains, mainly consists of porphyritic granodiorite and trondhjemite. The Pukala intrusion was emplaced into volcanic sequence in an island-arc or fore-arc setting before or during the early stages of the main regional deformation phase of the Svecofennian orogeny. On the basis of the geochemical data, the Pukala intrusion is a peraluminous volcanic-arc granitoid. After crystallisation at 1896±3 Ma, multiphase deformation and metamorphism caused alteration, recrystallisation, and orientation of the minerals, and tilted the intrusion steeply towards south. The 1851±5 Ma U-Pb age for titanite is connected to the late stages of the Svecofennian tectonometamorphic evolution of the region. Several hydrothermally altered domains are located in the felsic and intermediate metavolcanic rocks of the Tampere Belt within less than one kilometre south of the Pukala intrusion. Alteration is divided into three basic types: partial silica alteration, chloritesericite±silica alteration, and sericite alteration in shear zones. The fi rst two types probably formed during the emplacement and crystallisation of the Pukala intrusion, and the third is linked to late shearing. Intense sericitisation and comb quartz bands in the contact of the intrusion and the altered domain at Kutemajarvi suggest that the hydrothermal system was driven by the Pukala intrusion.

Tourmalinites from the Eastern Sierras Pampeanas, Argentina

In the eastern Sierras Pampeanas, Central Argentina, tourmalinites and coticules are found in close association with stratabound scheelite deposits in metamorphic terranes. In Sierra Grande (Agua de Ramón and Ambul districts) and Sierra de Altautina, tourmalinites are associated with stratabound scheelite deposits related to orthoamphibolites. In the Pampa del Tamboreo area, tourmalinites are located in biotite schists stratigraphically related to acid to intermediate metavolcanic rocks and scheelite-bearing quartzites. The mineral chemistry and boron isotopic compositions of tourmalinite-hosted and vein-hosted tourmalines are investigated. Overall, the tourmalines belong to the dravite-schorl series and are generally aluminous; Fe/(Fe+Mg) ranges from 0.33 to 0.85, Al/(Al+Fe+Mg) from 0.66 to 0.76 and the amount of X-site vacancy (0.12–0.48) indicates significant foitite components. Their boron isotopic compositions ( δ 11B) are from −24.0‰ to−15.0‰. Similar mineral chemistries and boron isotopic values for tourmaline in tourmalinites related to stratabound scheelite mineralisation and in tungsten-bearing quartz veins suggest a common source for the boron and probably the tungsten. The field, chemical and isotopic relationships are consistent with tungsten and boron in quartz-vein deposits being remobilised from stratabound scheelite and tourmalinite, dominantly by liquid-state transfer associated with regional shear zones. Tungsten and boron in the original sedimentary sequence (now meta-exhalites) are ascribed to volcanogenic exhalations.

Fluid evolution of the late Archaean Rämepuro gold deposit in the Ilomantsi greenstone belt in eastern Finland

The late Archaean (ca. 2.7 Ga) Ilomantsi greenstone belt hosts a large number of small mesozonal gold occurrences. The Ramepuro deposit is spatially related to a feldspar porphyry dyke which has intruded the contact between metagreywackes and intermediate metavolcanic rocks. It consists of gold-bearing quartz–tourmaline–sulphide veins located mainly within an intensively altered and sheared zone about 20–30 m in width. Two types of fluid inclusions were distinguished in the quartz veins: (1) H2O–CH4 (≤9 equiv. wt% NaCl), and (2) H2O–CO2 (≤12 equiv. wt% NaCl). The two compositionally different fluid-inclusion types occur in separate veins. The compositions of decrepitate residues indicate that the type 1 fluid inclusions consist predominantly of Na, S, Ca, Cl, and in lesser proportions of K and Fe. The residues of the type 2 fluid inclusions are dominated by Na, Ca, Cl and minor K. For the type 1 and type 2 fluid inclusions, microthermometric experiments indicate average homogenisation temperatures of 310–350 and 220–250 °C respectively. Both types of fluid inclusions contain 1–3 anisotropic solids (Ca/Mg carbonate?, nahcolite?, tourmaline?). Sulphide daughter minerals occur only in the type 1 fluid inclusions. This fluid type is also often associated with short trails of sulphide inclusions which suggest mobilisation and redistribution of ore constituents. Some type 1 fluid inclusions display distinct morphological features ("implosion textures") related to post-trapping re-equilibration. The presence of the fluid-inclusion re-equilibration textures in some auriferous quartz veins and their absence in other gold-bearing quartz veins suggest an age difference between them. This conclusion is also supported by the compositionally different fluid regimes associated with the different quartz veins, indicating a change from reducing (CH4) to a more oxidising (CO2) fluid environment. The fluid-inclusion evidence suggests two separate gold mineralisation events, one related to late Archaean regional metamorphism, the other to Palaeoproterozoic thrusting and metamorphism.

Structural controls on Neoproterozoic mineralization in the South Eastern Desert, Egypt: an integrated field, Landsat TM, and SIR-C/X SAR approach

The Arabian–Nubian Shield represents a complex amalgam of arcs and microcontinents assembled during Neoproterozoic closure of the Mozambique Ocean. The 750–720 Ma Allaqi suture is an arc/arc collision zone, formed when the Gerf terrane in the north overrode the circa 830–720 Ma Gabgaba terrane in the south, prior to closure of the Mozambique Ocean. Neoproterozoic rocks include ophiolitic ultramafic–mafic rocks, metasediments, intermediate metavolcanic rocks, intrusive gabbro-diorite rocks, granodiorites, biotite granites, and leucocratic granites. High-pressure/low-temperature metamorphism has been documented in rocks of the suture zone. Mineral deposits include nickel–copper–platinum and podiform chromite in ultramafic rocks, marble, gold-bearing quartz-veins in D 2 and D 3 shear zones, and radioactive mineralization associated with late leucocratic granitic rocks. Integrated field mapping and remote sensing techniques are used to distinguish and map the relationships between rock units, structures, and alteration zones associated with mineral deposits along the Allaqi suture of Egypt's SE Desert. Landsat TM images processed using a band ratioing technique show different rock types remarkably well, and are able to distinguish between alteration zones associated with ultramafic rocks (listwaenites) and those associated with leucocratic granitic rocks (greisenization, silicification and albitization). Black and white L-band SIR-C/X SAR images outline foliations, faults and folds that control mineralization at several deposits in the area, whereas color composite multiband Chh-Lhh-Lhv SIR-C/X SAR images reveal some elliptical granitic bodies that host radioactive mineralization. E-trending, tight to isoclinal, gently dipping folds, thrust faults and subvertical shear zones related to the Allaqi suture are overprinted by N-oriented structures related to the Wadi Ungate shear zone, formed during collision of east and west Gondwana during closure of the Mozambique Ocean. The location of the Wadi Ungate shear zone in the Wadi Shellman area was previously unknown due to burial of basement rocks beneath thin dry sands. A new structural map was prepared using Landsat TM ratio images and SIR-C/X SAR imagery. SIR-C/X SAR data conveys considerably more information about rocks and structures beneath the thin sand cover than discernible from aerial photographs or Landsat TM images.

Geochemistry and origin of the Ore Chimney Formation, a transported paleoregolith in the Grenville Province of southeastern Ontario, Canada

The Ore Chimney Formation is a mid-Proterozoic metapelitic rock unit that separates mafic to intermediate metavolcanic rocks of the Grenville Supergroup from an overlying clastic metasedimentary succession, the Flinton Group. It has been interpreted alternatively as a paleoregolith or a product of hydrothermal alteration of the volcanic rocks, in a major shear zone. The formation is thin (<10 m), massive to poorly layered, crops out over 40 km along strike and is typically composed of alternating layers of biotite schist, muscovite-biotite schist, hornblende-biotite schist and hornblende schist. It typically contains coarse porphyroblasts of garnet and less commonly of staurolite, kyanite, epidote and/or hornblende. Thirty-five samples of the Ore Chimney Formation, fifty samples of the underlying metavolcanics and fourteen samples of the overlying clastic metasediments were analysed for major and twenty-eight trace elements (including REE). Immobile trace-element ratios (Ti/Zr, Zr/Y, Nb/Y, La/Yb) indicate that the Ore Chimney Formation is locally derived from the underlying metavolcanics. Bulk chemistry, field relations and petrographic observations strongly suggest that the Ore Chimney Formation is a locally-reworked weathering product of the metavolcanic rocks.