Metavolcano-sedimentary Sequence Research Articles

The mineralizing fluid in the Piaba gold deposit, São Luís cratonic fragment (NW-Maranhão, Brazil) based on fluid inclusion studies in quartz veins

Piaba is the first gold mine to operate in the Sao Luis cratonic fragment, NW-Maranhao, northern Brazil. The geological setting comprises chiefly metavolcano-sedimentary sequences (Au- rizona Group) and subduction-related granitoids (Tromai Intrusi- ve Suite), formed in island arc between 2240 and 2150 Ma. Gold mineralization is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesite of the Aurizona Group. The mineralized zone is confined within the limits of the Piaba fault (ENE-WSW-trending brittle-ductile shear zone) and con- sists of quartz veins and veinlets and accompanying hydrothermal haloes (chlorite + muscovite + carbonate + pyrite + chalcopyrite + gold) disposed in stockwork geometry. Petrographic, microthermo- metric and microRaman spectroscopic studies of quartz have defined two- and three-phase aqueous-carbonic fluid inclusions produced by heterogeneous trapping during phase separation, in addition to late aqueous fluids. The mineralizing solution is an aqueous-carbonic fluid composed of CO 2 (5 – 24 mol%), H 2 O (74 – 93 mol%), N 2 (≤ 1 mol%) and CH4 (≤ 1mol%). It presents low salinity (5.5 wt.% NaCl equivalent) and density of 0.96 – 0.99 g/cm 3 ). Ore deposition occurred at 267 – 302 oC and 1.25 – 2.08 kbars, corresponding to 4 – 7 km in depth, in agreement with the structural information. The P-T-X and reduced characteristics (log ƒO 2 -31.3 to -34.3) of the fluid, combined with host rock sulfidation, altogether indicate that gold has been transported as a sulfur complex and that ore deposition occurred in response to phase separation and lowering of the sulfur activity and ƒO 2 during fluid-rock interaction.

Extensional and colisional magmatic records in the Apiaí Terrane, south-southeastern Brazil: integration of geochronological U-PB Zircon ages

The aim of this work is the presentation of a synthesis of available geochronological data for the basement inliers and metavolcano-sedimentary sequences which occur in the southern part of the Apiai Terrane, south-southeastern Brazil. These data, especially those obtained during the last decade, have made substantial modifications to the tectonic scenario of south-southeastern Brazil with the recognition of the presence of extensional basins (continental rifts) with magmatism and sedimentation at the late Paleoproterozoic (1790 - 1750 Ma) and of the Mesoproterozoic (1600 - 1450 Ma). These processes started at the late Paleoproterozoic in the Betara, Perau and Apiai Mirim Nuclei, and the basins became widerduring the Mesoproterozoic with the deposition of the Betara, Perau, Votuverava and Agua Clara metavolcano-sedimentary sequences. A different pattern is found for the Itaicoca Sequence which occupies the northern part of the Apiai Terrain. Here two rock associations of different ages are found. The first is a metamorphosed carbonate platform association with subalkaline, tholeiitic metabasic rocks with minimum deposition ages at the end of the Mesoproterozoic or beginning of the Neoproterozoic (Itaiacoca Sequence; 1030 - 900 Ma), while the second mainly contains metapsammites and metavolcanic rocks including ultrapotassic trachytes deposited during the Cryogenian-Ediacaran transition between 645 and 628 Ma (Abapa Sequence). It is then proposed that the present scenario of the Apiai Terrane be representative of the distinct tectono-sedimentary histories, reflecting a polycyclic evolution. In the Mesoproterozoic, stable conditions prevailed, withthe deposition of the majority of the sequences (Lajeado, Agua Clara, Betara, Perau, Votuverava) in a passive margin context, succeded by a Tonian extensional phase (deposition of the Itaiacoca Sequence). In the Ediacaran the region was transformed into an unstable active margin, with the predominance of magmatic arcs (Tres Corregos, Cunhaporanga and Agudos Grandes) and restricted sedimentation (Abapa, Antinha and Iporanga Sequences).

530 Ma syntectonic syenites and granites in NW Namibia — Their relation with collision along the junction of the Damara and Kaoko belts

The Lower Ugab and Goantagab structural domains are located at the junction between the N–S trending Kaoko and the E–W trending Damara belts (NW Namibia), where Neoproterozoic metavolcano-sedimentary sequences were intruded by several syenitic/granitic plutons. We present here new U–Pb ages on zircon grains from the Voetspoor and Doros plutons. Together with petrological, geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of Gondwana amalgamation.The plutons are composed of three main rock types: hornblende quartz-syenite, syenodiorite and biotite granite. The two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures. The biotite granite occurs in the SW parts of the intrusions and clearly cuts the syenitic rocks. Although the plutons are mainly isotropic, the structures around them demonstrate that their intrusion occurred during a second deformation phase (D2) with a component of sinistral solid state rotation with respect to the wall rocks in response to D2 transpression. Four samples were dated using U–Pb SHRIMP methodology in single zircon grains. A hornblende monzodiorite from the Voetspoor pluton yielded an age of 534±4.5Ma. A hornblende monzonite from the Doros pluton produced an age of 528±5Ma. The biotite granite facies was sampled in the Doros intrusion and yielded an age of 530±4.5Ma. In addition, a granitic vein folded by D2 close to the northeastern contact of the Doros pluton with the encasing phyllites (Amis River Formation) was also dated, yielding an age of 533±6Ma. The data show that all granite–syenite from Doros and Voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522Ma within the errors. D1–D2 deformational phases took place under greenschist facies (biotite zone) conditions and during D3 the metamorphic grade was slightly lower. We interpret that the plutons are coeval to peak metamorphism of the region (530–520Ma) and that D2 and D3 sinistral transpressional phases are due to collision in the Damara Belt. The E–W compressional event and second metamorphic episode in the Kaoko Belt occurred between 580 and 560Ma and are apparently unrelated to the thermo-tectonic evolution described here, although D1 might be partially related to this event. The sinistral transpressional D2 phase resulted probably from the position of the area considered at the junction between the belts, and not in the frontal Damara collision further to the east. This new interpretation is consistent with the Ar–Ar ages for the region (about 500Ma), interpreted to reflect cooling of the orogen. The enrichment in LREE, K, Rb, Ba and Sr, and depletion in Nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination. We interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab-breakoff during collision between Kalahari and Congo cratons.

New Constraints from Pb-Evaporation Zircon Ages of the Méiganga Amphibole-Biotite Gneiss, Central Cameroon, on Proterozoic Crustal Evolution

The amphibole biotite gneiss (ABGn) in the Meiganga area forms part of a meta volcano-sedimentary sequence of the Adamawa Yade domain (AYD), Central African Fold Belt (CAFB). This sequence shows affinity with immature sediments (greywackes, arkoses) with intercalation of mafic lavas or tuffs. New 207Pb/206Pb zircon evaporation ages for two ABGn samples range from 1887 - 2339 Ma and from 675 - 889 Ma, respectively. These ages and evidence from internal zircon structures indicate that igneous rocks of Archean to Paleoproterozoic and of early Neoproterozoic age contributed to the detritus of the sedimentary sequence. The deposition of detritus took place prior to 614 - 619 Ma which represent the syntectonic emplacement of the Meiganga metadiorite. Leucogranites north to the Meiganga area were generated by melting of crust identical to that which provided the source of the ABGn. The metasedimentary sequence investigated in this study is similar to that of the southern part of the AYD and in the Borborema Province, NE Brazil. The tectonic and geochronologic characteristics of the AYD in the Meiganga area support the idea that during the Proterozoic, Central Africa and NE Brazil were part of the same continental landmass.

Provenance of the Novo Oriente Group, southwestern Ceará Central Domain, Borborema Province (NE-Brazil): A dismembered segment of a magma-poor passive margin or a restricted rift-related basin?

Integrated field, LA-MC-ICP-MS geochronology and geochemical data from the Novo Oriente Group have been investigated in order to determine their provenance and possible tectonic setting. This group in the southwestern portion of the Ceará Central Domain, is a well-preserved metavolcanosedimentary sequence, in part exhalative, composed of two distinct formations. The proximal coastal Bonsucesso Formation comprises mainly quartzite and minor basic metavolcanic rocks and is in gradational contact with the distal Caraúbas Formation which constitutes a metapelitic–volcanic-carbonate sequence, including metabasic rocks with pillow structure and sheared serpentinized ultrabasic rocks. The youngest detrital zircon found in the quartzite of the Bonsucesso Formation yielded an age of ca. 2.1 Ga and well-defined peaks around 2.2, 2.3–2.4 and 2.5 Ga. In addition, zircons from a related metabasalt yielded an upper intercept age of 2083 ± 28 Ma. We interpret the zircons found in this metabasalt as inherited from the host quartzite. This fact is supported by depleted-mantle Nd model ages ( T DM age) from correlated metabasic and metaultrabasic rocks from other places in the Novo Oriente sequence, which have yielded ages between 1.36 and 1.69 Ga, that is younger than the zircon age obtained in the metabasalt extruded over the Bonsucesso quartzite. Geochemical data of the metasedimentary rocks from the Caraúbas Formation indicates a provenance composed of a mixture of felsic and intermediate sources typical of old stable cratons and older continental roots of active tectonic settings. The rocks of the Granja (Médio Coreaú Domain) and Bacajá Complexes (southeastern Amazonian Craton) exposed to the west of the Novo Oriente Group, and largely covered by the Phanerozoic Parnaíba intracratonic basin, are the most likely source rocks. Since the precise age of the Novo Oriente Group is not yet well constrained, two distinct settings may be proposed: (1) it may represent a more evolved basin developed from extensional processes during the Mesoproterozoic (ca. 1.5–1.3 Ga); and (2) it could constitute part of the passive margin systems developed during the break-up of the Rodinia crown copyright supercontinent (ca. 0.95–0.8 Ga), associated with a pre-Brasiliano/Pan-African ocean (e.g. Pharusian Ocean).

Geochemistry and zircon geochronology of paleoproterozoic granitoids: Further evidence on the magmatic and crustal evolution of the São Luís cratonic fragment, Brazil

The Tromaí Intrusive Suite is the predominant exposed unit of the São Luís cratonic fragment in northern Brazil. The suite forms batholiths and stocks of granitoids that were emplaced between 2168 ± 4 Ma and 2149 ± 4 Ma and intruded a 2240 ± 5 Ma old metavolcano-sedimentary sequence. The batholiths are composed of a variety of petrographic types that have been grouped in three sub-units, based on the predominant petrographic type, and named Cavala Tonalite, Bom Jesus Granodiorite, and Areal Granite, from the more primitive to the more evolved phases, in addition to subordinate shallow felsic intrusions. The Tromaí Suite is an expanded magmatic association comprising minor mafic rocks to predominantly intermediate and felsic, low- to high-K, and metaluminous to weakly peraluminous granitoids that follow a Na-enriched calc-alkaline trend. Combined rock association, geochronology, Nd isotopes, and geochemical signature indicate that the Tromaí Suite formed from magmas derived from juvenile protoliths modified by fractional crystallization. The juvenile protoliths included ocean plate, mantle wedge, and minor sediments. The data also indicate an intra-oceanic arc setting that possibly transitioned to a continental margin and that the Tromaí Intrusive Suite records the main accretionary stage of the Rhyacian orogen (ca. 2.24–2.15 Ma) that culminated with a collision stage at about 2.1 Ga and gave rise to the present day São Luís cratonic fragment. This time interval is coincident with the main period of crustal growth in the South American Platform and in the Paleoproterozoic terranes of the West African Craton. The beginning of this period is also coincident with the end of a period in which only minor amounts of juvenile crust is found worldwide. The Negra Velha Granite is a distinct unit that forms a few stocks that intruded the granitoids of the Tromaí Suite between 2076 and 2056 Ma ago. Negra Velha is an association of monzogranite and subordinate quartz–monzonite and syenogranite with an alkaline signature that shows high Rb–Sr–Ba enrichments, resembling shoshonitic associations. This granite represents the post-orogenic phase of the Rhyacian orogenesis.

Aspectos tectônicos das intrusões dos granitos do cerne, passa três e Rio Abaixo, sudeste do Pré-cambriano Paranaense. Estudo baseado em datações 39Ar-40Ar em Micas

The Cerne, Passa Três and Rio Abaixo granitic stocks are NE-SW-striking elliptic bodies intrusive in the metavolcano-sedimentary sequences of the southeastern portion of the Ribeira Belt (Paraná State, Brazil). These stocks are mainly biotite syenogranite, monzogranite and quartz-syenite with porphyritic textures containing microcline megacrystals. They are usually isotropic, but may also contain magmatic flow foliation and deformation restricted to the contact with the host rocks. These granites present petrological, geochemistries and geocchronological similarities, with many A-type characteristics, but they are enriched in Ba Sr and Zr. 40Ar-39Ar analyses of the Cerne Granite biotites indicate an integrated age of 557 ± 2 Ma, which is quite close to the lower Concordia intercept age interpreted as the crystallization age of about 563 Ma obtained by TIMS analysis of zircon from the Cerne and Rio Abaixo intrusions by Cury (2003). The 40Ar-39Ar determinations in the host muscovite schists from the vicinity of the Cerne granite are older (800-1200 Ma) than crystallization ages, indicating that the temperatures of the granite intrusion and of the regional metamorphic event that took place during the Brasiliano Cycle (c.a. 600 Ma) were not high enough to erase the isotopic record from these muscovites (T < 300ºC). These granitic stocks represent a late-orogenic magmatism (~560 Ma) in the context of the deformations phases registered in the Apiaí Domain, being different and subsequent to the event of Três Córregos - Cunhaporanga magmatic arc (c.a. 630-590 Ma). Their emplacement was closely associated with transcurrent shear zones and the development of the major antiforms and synforms during the Upper Proterozoic/Cambrian, associated with the final adjustments of the agglutination of the Gondwana Supercontinent.

Geology and petrography of paleoproterozoic granitoid rocks from Uatumã- Anauá Domain, central region of Guyana Shield, southeastern Roraima, Brazil

Regional mapping carried out in the Uatumã-Anauá Domain (UAD), Guyana Shield, southeastern Roraima (Brazil), has showed widespread calc-alkaline granitic to alkaline magmatism and local peraluminous granites. These granitoid rocks are distributed into several magmatic associations with different Paleoproterozoic (1.97-1.89 Ga) ages, structural and geochemical affinities. In this paper, detailed mapping and petrographic studies have distinguished two main subdomains in UAD. In the northern UAD, the Martins Pereira I-type, calc-alkaline granitoid rocks and Serra Dourada S-type granite (1.97-1.96 Ga) are deformed and affected by NE-SW and E-W shear zones. Basement inliers (2.03 Ga) crop out to the northeastern part of this area, and are formed by rocks from metavolcano-sedimentary sequence (Cauarane Group) and TTG-like calc-alkaline association (Anauá Complex). Younger and undeformed (with local discrete NE-SW shear zones) Igarapé Azul and Caroebe granites (I-type, calc-alkaline Água Branca suite) crop out in the southern UAD. A-type granites such as Moderna (1.81 Ga) and Mapuera (1.87 Ga) plutons, cross cut both areas of UAD. Gold mineralization is observed in the Martins Pereira granitoid rocks (northern UAD), alluvial columbite-tantalite is related to the Igarapé Azul granitoid rocks (southern UAD), and amethyst is associated to pegmatites of the Moderna A-type granites.

Geology and petrography of paleoproterozoic granitoid rocks from Uatumã- Anauá Domain, central region of Guyana Shield, southeastern Roraima, Brazil

Regional mapping carried out in the Uatuma-Anaua Domain (UAD), Guyana Shield, southeastern Roraima (Brazil), has showed widespread calc-alkaline granitic to alkaline magmatism and local peraluminous granites. These granitoid rocks are distributed into several magmatic associations with different Paleoproterozoic (1.97-1.89 Ga) ages, structural and geochemical affinities. In this paper, detailed mapping and petrographic studies have distinguished two main subdomains in UAD. In the northern UAD, the Martins Pereira I-type, calc-alkaline granitoid rocks and Serra Dourada S-type granite (1.97-1.96 Ga) are deformed and affected by NE-SW and E-W shear zones. Basement inliers (2.03 Ga) crop out to the northeastern part of this area, and are formed by rocks from metavolcano-sedimentary sequence (Cauarane Group) and TTG-like calc-alkaline association (Anaua Complex). Younger and undeformed (with local discrete NE-SW shear zones) Igarape Azul and Caroebe granites (I-type, calc-alkaline Agua Branca suite) crop out in the southern UAD. A-type granites such as Moderna (1.81 Ga) and Mapuera (1.87 Ga) plutons, cross cut both areas of UAD. Gold mineralization is observed in the Martins Pereira granitoid rocks (northern UAD), alluvial columbite-tantalite is related to the Igarape Azul granitoid rocks (southern UAD), and amethyst is associated to pegmatites of the Moderna A-type granites.

The New Consort Gold Mine, Barberton greenstone belt, South Africa: orogenic gold mineralization in a condensed metamorphic profile

The New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa is one of the oldest recognized orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units that occur at, or close to, the contact between the mafic and ultramafic volcanic rocks of the c. 3,280 Ma Onverwacht Group and the mainly metasedimentary rocks of the overlying c. 3,260–3,230 Ma Fig Tree Group. This contact, locally referred to as the Consort Bar, formed during ductile D1 imbrication of the metavolcanosedimentary sequence and predates the main stage of the gold mineralization. The imbricate stack is situated in the immediate hanging wall of the basal granitoid–greenstone contact along the northern margin of the greenstone belt. It is characterized by a condensed metamorphic profile in which the metamorphic grade increases from upper greenschist facies conditions (510–530°C, 4 kbar) in rocks of the Fig Tree Group to upper amphibolite facies grades (600–700°C, 6–8 kbar) in the basal Onverwacht Group. Detailed structural and petrological investigations indicate that the Consort Bar represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural sequence. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc–silicate alteration assemblage characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht Group. This early mineralization may correlate with the local D1 deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D2. The D2 deformation resulted in the reactivation of earlier structures, and the formation of a NNW trending, steeply dipping shear zone system, the Shires Shear Zone, which separates two regional SE plunging D1 synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes that have previously been dated at c. 3040 Ma. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of gold mineralization, which affected a crustal profile of ca. 1.5 km, was characterized by increasing temperatures (c. 520 to 600°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade, ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest to arsenopyrite + pyrrhotite + chalcopyrite + loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly syn-peak metamorphic with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally underlying Onverwacht Group. This indicates that the mineralization coincided with the juxtaposition of the two units. As the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence can be ruled out as the source of the mineralizing fluids.

ASPECTOS GEOLÓGICOS, PETROGRÁFICOS E GEOQUÍMICOS DOS MÁRMORES DOLOMÍTICOS COM NÓDULOS DE QUARTZO DA SEQUÊNCIA METAVULCANO-SEDIMENTAR DE ACARAPE-CE

The Acarape metavolcano-sedimentary sequence, Ceará State, Northeastern Brazil, consists of Neoproterozoic terrigenous metasedimentary rocks containing intercalations of marbles and calcsilicate rocks, as well as of metabasic and intermediate alcaline rocks. Deformation and metamorphism under amphibolite facies conditions affected the whole set. The marbles ocurr as lens-shaped, descontinuous layers that can reach more than 50 km and show locally (in some quarries) quartz concentration along the bedding planes. Quartz occurs as i) nodulate, subspheric structures; ii) centimetric-thick, metric-lenght beds; and iii) folded structures. Lozenge- and/or monoclinic-shaped molds and inclusions of sulfates, sometimes pseudomorphosed by dolomite in quartz crystals are common, suggesting a diagenetic origin for the silica as a result of precipitation in empty spaces generated by dissolution of sulfate in an evaporation-related, shallow-water marine environment. Deposition of the sulfate occurred probably on siliceous carbonatic mud, but later geochemical changes in the sedimentation setting, characterised by variations in the pH values, have caused its dissolution and quartz precipitation. The marbles present dolomitic- to calcitic-dolomitic composition, showing discreet variations inside the calcsilicate-rock field, which are reflected in the major-element diagrams CaO-MgO-SiO2. Some of the samples that plot in the calcsilicate-rock field correspond to siliceous marbles depleted in Al2O3 (&lt;1%), Na2O (&lt;0.1%), and K2O (&lt;0.04%), incompatible for calcsilicate rocks. The rare earth elements show an irregular pattern with a slight enrichment of LREE in relation to the HREE, strong positive Ce and moderate positive Eu anomalies. δ13C values are relatively homogeneous, ranging from +1.7 and -0.1, whilst the δ 18 O SMOW values vary between +18.3 and + 25‰, but one of the samples, collected next to a recent carstification zone and affected by intemperic changes, shows strong isotopic fluctuation (δ 18 O SMOW + 27.2‰ and δ 13 C –7.6‰). The δ 13 C values from the best preserved samples are compatible with the carbonates isotopic curve for the beginning of the Neoproterozoic.

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‰

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.

Tectonostratigraphy and base-metal mineralization controls, Aravalli province (western India): New interpretations from geophysical data analysis

Analysis and synthesis of multi-disciplinary geoscience information from geological literature/maps and from digitally-processed aeromagnetic and gravity data pertinent to the Aravalli province were carried out to address some hitherto unresolved questions about the tectonostratigraphy of this Archaean–Proterozoic metallogenic province. Based on the magnetic anomalies, several tectonic domains were identified. These domains, bounded by regional-scale geophysical lineaments, have distinct crustal, lithological, metamorphic, and metallogenic characteristics and correlate broadly with lithostratigraphic belts identified by several earlier workers. New interpretations on the tectonostratigraphy and the base-metal mineralization controls in the Aravalli province are as follows. The Hindoli sequences, in the eastern parts of the province, constitute an independent Palaeo–Proterozoic tectonic domain and do not form part of the Archaean basement complex. The base-metal-bearing metasedimentary enclaves in the central parts of the province also constitute an independent Palaeo–Proterozoic tectonic domain, which is quite distinct from the surrounding (basement complex?) rocks. The base-metal-bearing metavolcano-sedimentary sequences in the western parts of the province constitute an independent Neo–Proterozoic tectonic domain. The base-metal deposits in the province are spatially associated with the regional-scale lineaments and with the mafic metavolcanic rocks deduced from the aeromagnetic data. The regional-scale lineaments, which possibly represent Proterozoic crustal-scale faults, are plausible structural controls on the base-metal mineralization in the province. The mafic metavolcanic rocks are plausible heat-source controls on the SEDEX- and/or VMS-type base-metal mineralizations and are possible metal-source controls on the VMS-type base-metal mineralization in the province.

MINERALIZAÇÕES DE OURO DO TIPO OROGÊNICO EM ARCO MAGMÁTICO PALEOPROTEROZÓICO, BORDA OESTE DO CRATON SÃO FRANCISCO, REGIÕES DE SÃO DOMINGOS (GO) E CORRENTINA (BA)

The metavolcanosedimentary sequences and associated plutonic rocks from the Sao Domingos area, State of Goias, and Correntina area, State of Bahia, located at the border of the Sao Francisco Craton, were emplaced during the Paleoproterozoic in an oceanic, island-arc tectonic environment. The metassedimentary rocks from the sequences display different phases of deformation related to the Transamazonico Orogeny, reactivated during the Brasiliano Orogeny only in the Correntina region. The sequences studied and the oceanic island-arc type Almas-Dianopolis Terrane suggest a paleoproterozoic magmatic arc at the western border of the Sao Francisco Craton. The gold mineralizations are hosted by quartz veins which are fillings of Transamazonic fractures. In the Sao Domingos area, the quartz veins occur in phyllite and tonalite and are bordered by hydrothermal alteration zone, mainly kaolinization, while in the Correntina area, the veins crosscut tonalite and basic-ultrabasic rocks and are bordered by hydrothermal alteration zone enriched in biotite and carbonate. Gold generally ocurrs as millimetric grains and pyrite is the only sulphide present in the auriferous quartz veins. Fluid inclusion studies in vein quartz from Sao Domingos revealed the presence of aqueous and carbonic fluids, minimum fluid trapping temperature and pressure of about 300oC and 2 kb, respectively, and low salinity, wich suggests a metamorphic origin for the fluid. Trace element composition of the hydrothermal alteration zones indicates that granitoids and pyrite-rich carbonaceous phyllite in the Sao Domingos area, and monzogranite and basic-ultrabasic rocks in the Correntina area, were the main sources of gold to the hydrothermal metamorphic fluids. The gold mineralizations of the Sao Domingos and Correntina areas and also those of the paleoproterozoic Almas-Dianopolis Terrane, indicate a very good potencial of the paleoproterozoic magmatic arc in the western border of the Sao Francisco Craton for orogenic lode-gold deposits.

MINERALIZAÇÕES DE OURO DO TIPO OROGÊNICO EM ARCO MAGMÁTICO PALEOPROTEROZÓICO, BORDA OESTE DO CRATON SÃO FRANCISCO, REGIÕES DE SÃO DOMINGOS (GO) E CORRENTINA (BA)

The metavolcanosedimentary sequences and associated plutonic rocks from the São Domingos area, State of Goiás, and Correntina area, State of Bahia, located at the border of the São Francisco Craton, were emplaced during the Paleoproterozoic in an oceanic, island-arc tectonic environment. The metassedimentary rocks from the sequences display different phases of deformation related to the Transamazônico Orogeny, reactivated during the Brasiliano Orogeny only in the Correntina region. The sequences studied and the oceanic island-arc type Almas-Dianópolis Terrane suggest a paleoproterozoic magmatic arc at the western border of the São Francisco Craton. The gold mineralizations are hosted by quartz veins which are fillings of Transamazonic fractures. In the São Domingos area, the quartz veins occur in phyllite and tonalite and are bordered by hydrothermal alteration zone, mainly kaolinization, while in the Correntina area, the veins crosscut tonalite and basic-ultrabasic rocks and are bordered by hydrothermal alteration zone enriched in biotite and carbonate. Gold generally ocurrs as millimetric grains and pyrite is the only sulphide present in the auriferous quartz veins. Fluid inclusion studies in vein quartz from São Domingos revealed the presence of aqueous and carbonic fluids, minimum fluid trapping temperature and pressure of about 300ºC and 2 kb, respectively, and low salinity, wich suggests a metamorphic origin for the fluid. Trace element composition of the hydrothermal alteration zones indicates that granitoids and pyrite-rich carbonaceous phyllite in the São Domingos area, and monzogranite and basic-ultrabasic rocks in the Correntina area, were the main sources of gold to the hydrothermal metamorphic fluids. The gold mineralizations of the São Domingos and Correntina areas and also those of the paleoproterozoic Almas-Dianópolis Terrane, indicate a very good potencial of the paleoproterozoic magmatic arc in the western border of the São Francisco Craton for orogenic lode-gold deposits.

Turmalinitos vulcanogênicos da formação Morro da Pedra Preta do Grupo Serra do Itaberaba (SP): petrografia, composição química da turmalina e implicações metalogenéticas

Tourmalinites associated with amphibolites, metachert, iron formation, metatuffs, metasedimentary, metavolcaniclastic and calc-silicate rocks were found in the Morro da Pedra Preta Formation, basal metavolcano-sedimentary sequence of the Serra do Itaberaba Group (São Paulo State, Brazil). The syn-sedimentary origin of the tourmalinites is attested by the deformation of the alternating tourmaline-rich and quartz-rich layers, the presence of rip-up clasts, and whole-rock chemistry similar to that of surrounding metassedimentary rocks. Microprobe analyses show that the tourmaline compositions are: 1) within the schorl-dravite series, for the examples associated with metatuffs, iron formation, metachert metavolcaniclastic and metasedimentary rocks, and 2) within the dravite-uvite series, for those associated with amphibolites, metatuffs and calc-silicate rocks. The former are characterized by low Ca and high Na and Al contents, with variable Fe and Mg contents (schorl-dravite series), whereas the latter, a F-bearing tourmaline, has low Na and Al and high Mg and Ca contents. The schorl component of the tourmaline of the proximal tourmalinites is indicative of the potential of Morro da Pedra Preta Formation for gold, which contrasts with the conditions required for the formation of volcanic massive sulphide deposits (hypersaline fluids and association with magnesian tourmalinites). The tourmaline of the various tourmalinites of the Morro da Pedra Preta Formation is not typically magnesian and its composition was more strongly influenced by the composition of the neighboring rocks than that of the hydrothermal fluids, especially when the tourmalinite was deposited in an intermediate to distal position in relation to the volcano-exhalative center.

Paleoproterozoic Crustal Evolution of the São Luís Craton, Brazil: Evidence from Zircon Geochronology and Sm-Nd Isotopes

The São Luís Craton, northern Brazil, is composed of a few granitoid suites and a metavolcano-sedimentary succession. New single zircon Pb evaporation ages and Nd isotope data, combined with other available information, show that the metavolcano-sedimentary succession developed from 2240 Ma to approximately 2200-2180 Ma from juvenile protoliths. The subduction-related calc-alkaline suites of granitoids, spatially associated with the metavolcano-sedimentary sequence, formed in an oceanic island arc setting between 2168-2147 Ma. Most of these granitoids are tonalitic and formed from juvenile, mantle- or oceanic plate-derived protoliths, whereas minor true granites are the product of the reworking of the juvenile island arc material. These arc-related successions represent an accretionary event around 2.20±0.05 Ga, which is coincident with one of the main periods of crustal growth in the South American Platform. This accretionary orogen has subsequently been involved in a collision episode, at ca. 2100-2080 Ma, which is mainly recorded in the nearby Gurupi Belt. The rock associations, inferred geological settings, and the crustal evolution detected in the São Luís Craton are similar to what is described in Paleoproterozoic domains of major geotectonic units of the South American Platform, such as part of the São Francisco Craton, southeastern Guyana Shield, and of the West African Craton.

U-Pb (Zircon) ages of metavolcanic rocks from the Itaiacoca Group: tectonic implications

The main aim of this work is to present and discuss the U-Pb ages obtained for zircon grains from metavolcanic rocks of the Itaiacoca Group. The Itaiacoca Group is a metavolcano-sedimentary sequence, which occurs as a narrow belt between the Cunhaporanga granitic batholith to the northwest and the Itapirapua shear zone to the south and southwest, which separates the sequence from the Tres Corregos granite batholith and metasedimentary rocks of the Acungui Group. Geological studies of the southern part of the Itaiacoca belt led to the recognition of three units, represented (from base to top) by metawackes with an important volcanic component, metacarbonate, and metapelitic and metapsammitic rocks. The U-Pb geochronological analyses of zircon grains from two outcrops of metavolcanic rocks yield ages of 628 ± 18 Ma (SHRIMP) and 636 ± 30 Ma (conventional multi-grain analyses). These ages are quite close to the metamorphic event recorded in the Itaiacoca Group (628 - 610 Ma), suggesting a short interval between the formation of these rocks and closure of the basin. Furthermore, this volcanism is very close to the age of formation of the Tres Corregos (630 Ma) and Cunhaporanga (590 Ma) granitic batholiths, admitted as associated with a probable magmatic arc. Such an isotopic pattern characterizes a Neoproterozoic tectonic scenario involving volcanism, metamorphism and granitic plutonism, interpreted here as the final stages in the evolution of the Itaiacoca Basin.

Aproveitamento Industrial do Mármore da Sequência Metavulcano-Sedimentar Marmeleiro, Rio Grande do Sul, Brasil. Dados Petrográficos e Químicos

This study presents chemical and petrographic properties of marble of the Marmeleiro metavolcano-sedimentary sequence, Southern Brazil. This rock is characterized by 80% of the pure calcite, 48 to 53.3 wt.% of CaO, 0.32 to 0.86 wt.% of MgO and neutralizing value between 75 and 91%. The elevated loss on ignition this rock makes possible the production of considerable amounts of CaO after calcinations. These characteristics are adequated for manufacture of Portland lime, as well as for agricultural lime.