Electron microprobe and Raman spectroscopic investigation of monazite from chromitites of Cedrolina (Goias State, Brazil)

Monazite from chromitites of Cedrolina (Goias State, Brazil) was investigated by electron microprobe and Raman spectroscopy. Monazite has been rarely encountered in chromitites. In Brazil, it was previously reported from chromitites of the Campo Formoso layered intrusion. Comparison between the two occurrences indicates similar morphology and textural characteristics, but remarkable differences in chemical composition. In both cases, monazite occurs as irregular grains (up to 200 mu m) preferentially located in the chlorite-serpentine matrix of the chromitite, more rarely included in chromite. However, the monazite from Cedrolina is characterized by higher Ce/La ratio, and Pr, Nd, Th contents, compared with the monazite from Campo Formoso. The obtained Raman spectra are very similar in the two cases, suggesting that the compositional variation of monazites and the spectral resolution of the Raman do not allow a conclusive chemical analysis with Raman spectra. Textural evidence indicates that, in both occurrences, monazite precipitation did not take place at high temperature, concomitantly with the host chromitite. In the Campo Formoso chromitites, precipitation of monazite has been related with percolation of hydrothermal, aqueous and acid fluids emanating from a granite batholith. on the contrary, the Cedrolina monazite probably formed during one of the metamorphic events that affected the area in which the host chromitite occurs.

The 3.4-3.5 Ga Sao Jose do Campestre massif, NE Brazil: remnants of the oldest crust in South America

The oldest fragment of continental crust recognized in South America occurs as an isolated Archean enclave in northeastem Brazil's Borborema Province, ca. 600 Ma Brasiliano-Pan African orogenic belt. This Archean fragment, the Sao Jose do Campestre massif, is surrounded by large tracts of 2.2-2.0 Ga Paleoproterozoic gneisses and is located more than 600-1500 km from the much larger assemblages of Archean rocks found in the Sao Fransciso and Amazonian cratons, located to the south and west, respectively. Geochronological studies of the Sao Jose do Campestre massif show that its oldest rocks contain zircons with U-Pb ages up to 3.5 Ga and Sm-Nd T-DM model ages of more than 3.7 Ga, indicating that they represent reworked crust. This older nucleus is flanked by both reworked and juvenile 3.25 and 3.18 Ga rocks which arc intruded by both 3.00 and 2.69 Ga plutonic bodies. The protracted evolution the Sao Jose do Campestre massif is consistent with that of a larger continental mass as opposed to a small crustal fragment that grew in isolation. As such, the Sao Jose do Campestre massif is interpreted as representing a detached piece of an evolved craton that became entrained with younger rocks during a subsequent Paleoproterozoic accretionary-orogenic event. This hypothesis is bolstered by the presence of Paleoproterozoic gneisses that envelop the Sao Jose do Campestre massif, as well as the existence of ca. 2.0 Ga metamorphic zircon and monazite within its rocks. The occurrence of several different Archean cratonic basement inliers within the greater Paleoproterozoic crustal framework of the Borborema Province suggests that cratonic slices spalled off one or more larger Archean masses prior to the ca. 2.2-2.0 Ga Paleoproterozoic orogenic collage. A important challenge is to link these older fragments to their parent cratons. Although results are not unique, the pattern of ages and isotopic signatures observed in the Sao Jose do Campestre massif is similar to that seen in parts of the Sao Francisco Craton, and it is possible that the Sao Jose do Campestre massif is a fragment of an Archean continental fragment formed during an episode of continental breakup prior to 2200 Ma. (C) 2003 Elsevier B.V. All rights reserved.

Consolidation and break-up of the South American platform in southeastern Brazil: Tectonothermal and denudation histories

The different tectonic stages that occurred at the end of the Proterozoic and during the Phanerozoic have an important bearing on the tectonothermal history of the South American Platform and its consolidation. Geochronological data (U/Pb monazite, Ar-40/Ar-39 whole rock) and apatite fission-track analysis, from Precambrian rocks of the southeastern Brazilian coastline, permit the modeling of a long-term thermal history of the crust and constrain variable denudation rates.Using these data, a temperature-time diagram reflects a period of accelerated exhumation during the end of the Brasiliano Orogeny, followed by long stability and reactivation of the platform during the Rifting Phase of the South Atlantic Ocean.U/Pb zircon and monazite (blocking temperature of ca. 650degreesC) data from a series of igneous bodies suggest that a tangential and transpressional tectonic regime occurred between 625 and 610 Ma. During the following escape tectonics, between 610 and 590 Ma the exhumation process indicates cooling rates of ca. 12degreesC/Ma. Ar-40/Ar-39 biotite ages between 540 and 510 Ma (ca. 300degreesC) and a corrected fission-track age on apatites (100degreesC) of 480 Ma indicate an exhumation event related to block tectonics with huge vertical displacement along shear zones.A long stabilization phase, with low exhumation, and cooling rate around 0.25degreesC/Ma was recorded from the Cambro/Ordovician to the Mesozoic. At 65 Ma an acceleration of the exhumation through denudation and reworking of the South American surface with cooling rate of 1.5degreesC/Ma is observed.The uplift of the Mantiqueira and Serra do Mar mountain ranges along the southeast Brazilian coastline works as a climatic barrier provoking lateral erosional processes causing long-term scarp retreat, combined with intense, but progressive denudation towards the continent. A denudation of 2.5 to 4 km was calculated for such processes. This lateral retreat of escarpments and flexural response can provide important insights regarding marginal isostatic uplift and the evolution of offshore sedimentary basins of southeast Brazil.

Geological evolution of the basement rocks in the east-central part of the Rondonia Tin Province, SW Amazonian craton, Brazil: U-Pb and Sm-Nd isotopic constraints

On the basis of geologic, petrologic, and U-Pb geochronologic data the basement rocks in the east-central part of the Rondonia Tin Province (RTP, southwestern Amazonian craton) are grouped into five lithologic associations: (1) tonalitic gneiss (1.75 Ga); (2) enderbitic granulite (1.73 Ga); (3) paragneiss; (4) granitic and charnockitic augen gneisses (1.57-1.53 Ga); and (5) fine-grained granitic gneiss and charnockitic granulite (1.43-1.42 Ga). The first three are related to development of the Paleoproterozoic Rio Negro-Juruena Province and represent the oldest crust in the region. The tonalitic gneisses and enderbitic granulites show calc-alkaline affinities and Nd isotopic compositions (initial epsilon(Nd) = +0-1 to -1.5; T-DM of 2.2-2.1 Ga) that suggest a continental arc margin setting for the original magmas. The paragneisses yield T-DM values of 2.2-2.1 Ga suggesting that source material was primarily derived from the Ventuari-Tapajos and Rio Negro-Juruena crusts, but detrital zircon ages and an intrusive granitoid bracket deposition between 1.67 and 1.57 Ga. The granitic and charnockitic augen gneisses show predominantly A-type and within-plate granite affinities, but also some volcanic arc granite characteristics. The initial epsilon(Nd) values (+0.6 to +2.0) indicate mixing of magmas derived from depleted mantle and older crustal sources. These rocks are correlated to the 1.60-1.53 Ga Serra da Providencia intrusive suite that reflects inboard magmatism coeval with the Cachoeirinha orogen located to the southeast. The fine-grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1.43-1.42 Ga in northern Rondonia. Their geochemical signatures and the slightly positive initial epsilon(Nd) values (+0.7 to +1.2) are very similar to those of the most evolved granites of the calc-alkaline Santa Helena batholith farther southeast. U-Pb monazite and Sm-Nd whole-rock-garnet ages demonstrate that a high-grade tectonometa-morphic episode occurred in this region at 1.33-1.30 Ga. This episode attained upper-amphibolite conditions and is interpreted as the peak of the Rondonian-San Ignacio orogeny. The U-Pb and Sm-Nd data presented here and data published on rapakivi granites elsewhere indicate that the east-central part of the RTP is a poly-orogenic region characterized by successive episodes of magmatism, metamorphism, and deformation between 1.75 and 0.97 Ga. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Plutonism in three orogenic pulses, Eastern Blue Ridge Province, southern Appalachians

The Eastern Blue Ridge Province of the southern Appalachians contains, in part, remnants of an Ordovician accretionary wedge complex formed during subduction of an oceanic tract before mid-Ordovician accretion with Laurentia. The Eastern Blue Ridge Province consists of metapelite and amphibolite intruded by low-K plutons, high-temperature (T > 750 degrees C) Ordovician eclogite, and other high-pressure metamafic and meta-ultramatic rocks. Felsic plutons in the Eastern Blue Ridge Province are important time markers for regional-scale tectonics, deformation, and metamorphism. Plutons were thought to be related to either Taconian (Ordovician) or Acadian (Devonian-Silurian) tectonothermal events.We dated five plutonic or metaplutonic rocks to constrain pluton crystallization ages better and thus the timing of tectonism. The Persimmon Creek gneiss yielded a protolith crystallization age of 455.7 +/- 2.1 Ma, Chalk Mountain 377.7 +/- 2.5 Ma, Mt. Airy 334 +/- 3 Ma, Stone Mountain 335.6 +/- 1.0 Ma, and Rabun 335.1 +/- 2.8 Ma. The latter four plutons were thought to be part of the Acadian Spruce Pine Suite, but instead our new ages indicate that Alleghanian (Carboniferous-Permian) plutonism is widespread and voluminous in the Eastern Blue Ridge Province. The Chattahoochee fault, which was considered an Acadian structure, cuts the Rabun pluton and thus must have been active during the Alleghanian orogeny. The new ages indicate that Persimmon Creek crystallized less than 3 m.y. after zircon crystallization in Eastern Blue Ridge eclogite and is nearly synchronous with nearby high-grade metamorphism and migmatization. The three phases of plutonism in the Eastern Blue Ridge Province correspond with established metamorphic ages for each of the three major orogenic pulses along the western flank of the southern Appalachians.

Neoproterozoic tectonic and magmatic episodes in the NW sector of Borborema Province, NE Brazil, during assembly of Western Gondwana

The Medio Coreau domain of NE Brazil is located along the northwest margin of Borborema Province, the western branch of a Brasiliano/Pan-African collisional belt that formed during the assembly of Western Gondwana. The early Paleoproterozoic basement of the Medio Coreau domain is composed of migmatitic gneisses and juvenile granulites, overlain by late Paleoproterozoic and Neoproterozoic rocks intruded by syn- to post-tectonic Brasiliano granitoids. According to integrated structural and geochronological data (U-Pb zircon and monazite ages), the Neoproterozoic tectonic evolution of the Medio Coreau is characterized by low-angle thrusting and transcurrent deformation. U-Pb geochronological data from plutons intruded during this compressional regime indicate the collisional evolution began at approximately 622 Ma and continued until about 591 Ma. The continuation of convergence until approximately 560 Ma resulted in the formation of NE-SW and E-W shear zones within the Borborema Province and adjoining West African provinces. The final stage of the ductile tectonism was characterized by uplift and high-angle fault generation between approximately 560 and 545 Ma. The last tectonic event was an extensional phase, resulting in the formation of the Jaibaras graben and intrusion of post-orogenic granites at around 532 Ma. (c) 2007 Elsevier Ltd. All rights reserved.

U-Pb geochronology of the southern Brasília belt (SE-Brazil): Sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U-Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID-TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean- Paleoproterozoic ages (3.4-3.3, 3.1-2.7, and 2.5-2.4Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3-1.9Ga, with a peak at ca. 2.15Ga) and to the ca. 1.75Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2Ga, with a peak at 1.3Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6-1.2Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt. Whilst continent-continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634-599Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595-560Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545-500Ma in the Paraguay belt and ca. 500Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50-100 million years. © 2003 Elsevier B.V. All rights reserved.

Final stages of the Brasiliano Orogenesis in SE Brazil: U-Pb and 40Ar/39Ar evidence for overprinting of the Brasília Belt by the Ribeira Belt Tectonics

Around the southern margins of the São Francisco Craton, there is a zone of tectonic interference between the Brasília belt to the west and the younger Ribeira belt to the east. U-Pb monazite and 40Ar/39Ar cooling age determinations carried out in the area reveal the cooling histories of these belts and the timing of tectonic overprint, unraveling the final stages of Brasiliano Orogeny in SE Brazil. The U-Pb monazite data from migmatized paragneisses and late-stage pegmatites in the Socorro-Guaxupé Nappe System of the southern Brasília belt show that migmatization peaked between ca. 613±1 and 607±3 Ma. 40Ar/39Ar biotite and muscovite ages of paragneisses and schists in this area indicate that the northern high-grade core of the Nappe System (Guaxupé Domain) was uplifted and cooled through the 350°C isotherm between 599±1 and 587±1 Ma. In contrast, samples from the southern high-grade core of the Nappe System, the Socorro Domain, south of the Jacutinga shear zone, yields a broader and younger spectrum of 40Ar/39Ar biotite ages between 571±1 and 562±1 Ma, attributed to a later uplift and cooling of the crust. The cooling ages can be assigned to local resetting of the 40Ar/39Ar system during transpressive tectonic overprint due to reactivation as a result of collision of the Ribeira belt. A younger group of 40Ar/39Ar mica ages (537±1 to 521±1Ma) in schists of the Socorro Domain, are associated with transpressional structures of the Ribeira belt. Rock samples from the Jacutinga and Três Corações shear zones, yield 40Ar/39Ar biotite-muscovite ages around 520 Ma. These are typical cooling ages of the Ribeira belt, and are interpreted to mark the western limit of the Ribeira belt transpressional regime within the Brasília belt. The youngest biotite-muscovite cooling ages in schists of the Socorro Domain, between 510±2 and 491±1 Ma, mark the final cooling and exhumation of that part of the Brasília belt.

Plutonismn in three orogenic pulses, Eastern Blue Ridge Province, southern Appalachians

The Eastern Blue Ridge Province of the southern Appalachians contains, in part, remnants of an Ordovician accretionary wedge complex formed during subduction of an oceanic tract before mid-Ordovician accretion with Laurentia. The Eastern Blue Ridge Province consists of metapelite and amphibolite intruded by low-K plutons, high-temperature (T >750 °C) Ordovician eclogite, and other high-pressure metamafic and meta-ultramafic rocks. Felsic plutons in the Eastern Blue Ridge Province are important time markers for regional-scale tectonics, deformation, and metamorphism. Plutons were thought to be related to either Taconian (Ordovician) or Acadian (Devonian-Silurian) tectonothermal events. We dated five plutonic or metaplutonic rocks to constrain pluton crystallization ages better and thus the timing of tectonism. The Persimmon Creek gneiss yielded a protolith crystallization age of 455.7 ± 2.1 Ma, Chalk Mountain 377.7 ± 2.5 Ma, Mt. Airy 334 ± 3Ma, Stone Mountain 335.6 ± 1.0 Ma, and Rabun 335.1 ± 2.8 Ma. The latter four plutons were thought to be part of the Acadian Spruce Pine Suite, but instead our new ages indicate that Alleghanian (Carboniferous-Permian) plutonism is widespread and voluminous in the Eastern Blue Ridge Province. The Chattahoochee fault, which was considered an Acadian structure, cuts the Rabun pluton and thus must have been active during the Alleghanian orogeny. The new ages indicate that Persimmon Creek crystallized less than 3 m.y. after zircon crystallization in Eastern Blue Ridge eclogite and is nearly synchronous with nearby high-grade metamorphism and migmatization. The three phases of plutonism in the Eastern Blue Ridge Province correspond with established metamorphic ages for each of the three major orogenic pulses along the western flank of the southern Appalachians. © 2006 Geological Society of America.

Tipo de cimentação em materiais duros do perfil de alteração da mina de fosfato da Anglo American, Catalão-GO

One of the main challenges in the mining sector at present, related to the optimization of mineral assets, consists in to find the best proportion of each geological material to be mined and homogenized to ensure high productivity and stability in ore processing plants. The stability of a mineral processing plant is factor for achieving operational excellence, as a stable performance of the plants ensures economy and adherence to financial planning businesses. Is already well Know that the feed rate of the processing plants is dependent on the hardness and quality of the ore. Therefore, the ore behavior predictability is the solution to ensure the stability of mineral processing plants. This work was developed in Anglo American Phosphate Brazil Ltda., at the Fazenda Chapadão Phosphate Mine, located in the southeastern portion of the state of Goiás, in the municipalities of Catalão and Ouvidor. The phosphate ore is the result of supergene apatite concentration in the weathering profile of the alkaline ultramafic phlogopitized rocks, intruded by foscoritic and carbonatitic rocks series. The weathering profile has about 100 meters thick and is divided into three basic types, from top to bottom, coinciding with decreasing intensity of weathering: oxidized ore, micaceous ore and weathered rock. In each portion of this weathering profile, mainly near the contacts between these three divisions, are hard materials, cemented by minerals such as apatite, quartz, goethite, monazite and barite. The different features presented by these materials makes to be vital the correct identification, quantification and location of different hard materials in the mine area, to maximize its use. The main objective of this study was to define the type of cementation of hard materials which predominates in each portion of the weathering profile in the mine, as well as their relationship with the protolith. The work involved geological mapping, channel sampling...

Dissolução redutiva de minério de ferro por Acidithiobacillus ferrooxidans para a recuperação de metais de interesse econômico

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