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M Zucchetti - One of the best experts on this subject based on the ideXlab platform.
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lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits
Ore Geology Reviews, 2007Co-Authors: O F Baltazar, M ZucchettiAbstract:Abstract The Rio das Velhas greenstone belt is located in the Quadrilatero Ferrifero region, in the southern extremity of the Sao Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquine Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–Graywacke, Graywacke–sandstone, Graywacke–argillite; (5) resedimented association, including three sequences of Graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures. Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilatero Ferrifero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.
O F Baltazar - One of the best experts on this subject based on the ideXlab platform.
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lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits
Ore Geology Reviews, 2007Co-Authors: O F Baltazar, M ZucchettiAbstract:Abstract The Rio das Velhas greenstone belt is located in the Quadrilatero Ferrifero region, in the southern extremity of the Sao Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquine Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–Graywacke, Graywacke–sandstone, Graywacke–argillite; (5) resedimented association, including three sequences of Graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures. Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilatero Ferrifero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.
Cristina Talavera - One of the best experts on this subject based on the ideXlab platform.
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ediacaran to lower ordovician age for rocks ascribed to the schist Graywacke complex iberian massif spain evidence from detrital zircon shrimp u pb geochronology
Gondwana Research, 2012Co-Authors: Cristina Talavera, Martinez D Poyatos, P Montero, Ian S WilliamsAbstract:Abstract New SHRIMP U–Pb ages of detrital zircon obtained from eight samples of Neoproterozoic to Lower Paleozoic Graywackes, schists, microconglomerates and shales provide the maximum depositional age and a new zircon age pattern for the Schist–Graywacke Complex (SGC) from the Iberian Massif (SW Europe). The ages of the youngest zircon grains found in four samples provide a maximum depositional age of latest Ediacaran–Lower Cambrian for the complex. Lower-Middle Cambrian fossiliferous formations on top of the lithologies correctly attributed to the SGC constrain its minimum depositional age. Unexpectedly, two samples attributed to the SGC yielded Cambro-Ordovician zircon populations. These must belong to younger Lower Ordovician sedimentary successions that, up to now, have not been differentiated from those of the SGC. The new age patterns are mainly composed of Neoproterozoic (73%) and Paleoproterozoic (15%) ages, with minor Neoarchean (7%), Mesoarchean (2%), Mesoproterozoic (3%) and Cambrian (1%) ages for the latest Ediacaran–Lower Cambrian successions, and Neoproterozoic (46%) and Cambro-Ordovician (46%) ages, with minor Neoarchean (1%), Mesoarchean (0.5%), Paleoproterozoic (6%), Mesoproterozoic (0.5%) and Carboniferous (1%) ages for the Lower Ordovician successions. The presence of Mesoproterozoic zircon points to the Saharan Metacraton as a contributing source for these sediments. Cadomian granitoids could have been a local Neoproterozoic source. The Cambro-Ordovician zircons may also indicate that Cambro-Ordovician magmatism contributed as a source. Cambro-Ordovician volcanism, the most probable source of the Cambro-Ordovician zircons, would have been coeval with the deposition of the Lower Ordovician successions.
Ian S Williams - One of the best experts on this subject based on the ideXlab platform.
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ediacaran to lower ordovician age for rocks ascribed to the schist Graywacke complex iberian massif spain evidence from detrital zircon shrimp u pb geochronology
Gondwana Research, 2012Co-Authors: Cristina Talavera, Martinez D Poyatos, P Montero, Ian S WilliamsAbstract:Abstract New SHRIMP U–Pb ages of detrital zircon obtained from eight samples of Neoproterozoic to Lower Paleozoic Graywackes, schists, microconglomerates and shales provide the maximum depositional age and a new zircon age pattern for the Schist–Graywacke Complex (SGC) from the Iberian Massif (SW Europe). The ages of the youngest zircon grains found in four samples provide a maximum depositional age of latest Ediacaran–Lower Cambrian for the complex. Lower-Middle Cambrian fossiliferous formations on top of the lithologies correctly attributed to the SGC constrain its minimum depositional age. Unexpectedly, two samples attributed to the SGC yielded Cambro-Ordovician zircon populations. These must belong to younger Lower Ordovician sedimentary successions that, up to now, have not been differentiated from those of the SGC. The new age patterns are mainly composed of Neoproterozoic (73%) and Paleoproterozoic (15%) ages, with minor Neoarchean (7%), Mesoarchean (2%), Mesoproterozoic (3%) and Cambrian (1%) ages for the latest Ediacaran–Lower Cambrian successions, and Neoproterozoic (46%) and Cambro-Ordovician (46%) ages, with minor Neoarchean (1%), Mesoarchean (0.5%), Paleoproterozoic (6%), Mesoproterozoic (0.5%) and Carboniferous (1%) ages for the Lower Ordovician successions. The presence of Mesoproterozoic zircon points to the Saharan Metacraton as a contributing source for these sediments. Cadomian granitoids could have been a local Neoproterozoic source. The Cambro-Ordovician zircons may also indicate that Cambro-Ordovician magmatism contributed as a source. Cambro-Ordovician volcanism, the most probable source of the Cambro-Ordovician zircons, would have been coeval with the deposition of the Lower Ordovician successions.
Demetrius Pohl - One of the best experts on this subject based on the ideXlab platform.
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regional setting structure and descriptive geology of the middle proterozoic syama gold deposit mali west africa
Economic Geology, 1992Co-Authors: Steven F Olson, Kassoum Diakite, Lawrence Ott, Ambogo Guindo, Christopher R B Ford, Nicholas Winer, Eric Hanssen, Nilda Lay, Robin Bradley, Demetrius PohlAbstract:The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold mineralization is hosted by a thin (0.5-2.0 km) but regionally extensive sequence of basalt and andesite, interbedded Graywacke and argillite, and andesitic-lamprophyric intrusions. This sequence is bounded on the east by andesitic conglomerate and Graywacke and on the west by interbedded Graywacke and argillite. In the Syama mine area, this ore-bearing sequence is structurally layered, overturned, and east vergent. Massive competent basalt and conglomerate bound the hanging wall and footwall, respectively, of the mineralized interval.Lateritic weathering developed an oxide orebody that overlies sulfide mineralization. The oxide orebody extends to 35 to 40 m below the surface; sulfide mineralization extends to a depth of at least 500 m. Mineable oxide reserves are 3.0 million tons with an average grade of 3.20 g/ton. Geologic sulfide reserves from the top of sulfides to 320 m below the surface are 21 million tons with an average grade of 4.02 g/ton.The oxide deposit was expressed on the surface as a series of 3- to 15-m-deep ancient mine workings, which extended 800 m along the strike of the deposit, and sparse outcrops of silicified rock. Oxide ore occurs as two deposits: a surficial deposit consisting of ancient tailings and dump material which are 0.1 to 5 m thick and extend up to 200 m away from the ancient workings; and an in situ "lode" or saprolite deposit which lies immediately below the ancient workings and extends downward into sulfide mineralization.The sulfide deposit consists of lenticular bodies of intense ankerite-quartz veinlet stockworks, zones of sheeted ankerite-quartz veinlets, and breccia bodies. Pyrite is the principal gold-bearing mineral and occurs disseminated in veinlet halos and breccia clasts. Green chloritic basalt is the preferred but nonexclusive host of the mineralization; it is commonly bleached by ankerite, albite, and sericite alteration in the pyritic gold-bearing zones. Silica, Na 2 O, Au, CO 2 , S, and C have been added to mineralized zones hosted by basalt; Fe 2 O 3 , Al 2 O 3 , and trace metals have been removed. Andesitic-lamprophyric intrusions and Graywacke-argillite layers also host disseminated pyrite mineralization. Faulted beds of graphitic Graywacke-argillite and zones of silicification bound most individual orebodies.The volcanic rocks at Syama were probably originally deposited in a narrow rift or graben in an intra-arc setting. Gold mineralization overlapped with compressional deformation and intrusive activity. The mineralized horizons at Syama lie within a sequence of rocks which has been either stacked or attenuated by layer-parallel reverse faults. These faults coincide with abrupt regional- and mine-scale contrasts in rock competency. Intrusive rocks form a volumetrically significant part of the mineralized sequence and may have aided hydrothermal alteration as a source of heat. Both the Graywacke-argillite layers and the altered basalt and intrusions may have been a source for some of the elements and fluids responsible for the mineralization and alteration.The mineralization at Syama is similar to other middle Proterozoic gold deposits in the Birimian Shield of west Africa and to Archean gold deposits hosted by mafic volcanic rocks in terms of tectonic setting, wall-rock alteration, form of mineralization, and associated structures.
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Regional setting, structure and descriptive geology of the Middle Proterozoic Syama gold deposit
1992Co-Authors: Steven F Olson, Kassoum Diakite, Ambogo Guindo, Christopher R B Ford, Nicholas Winer, Eric Hanssen, Nilda Lay, Robin Bradley, Lawrence Otr, Demetrius PohlAbstract:The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold mineralization is hosted by a thin (0.5-2.0 km) but regionally extensive sequence of basalt and andesite, interbedded Graywacke and argillite, and andesitic-lamprophyric intru-sions. This sequence is bounded on the east by andesitic onglomerate and Graywacke and on the west by interbedded Graywacke and argillite. In the Syama mine area, this ore-bearing sequence is structurally layered, overturned, and east vergent. Massive competent basalt and conglomerate bound the hanging wall and footwall, respectively, of the mineralized interval. Lateritic weathering developed an oxide orebody that overlies sulfide mineralization. The oxide orebody extends to 35 to 40 m below the surface; sulfide mineralization extends to a depth of at least 500 m. Mineable oxide reserves are 3.0 million tons with an average grade of 3.20 g/ton. Geologic sulfide reserves from the top of sulfides to 320 m below the surface are 21 million tons with an average grade of 4.02 g/ton. The oxide deposit was expressed on the surface as a series of 3- to 15-m-deep ancient mine workings, which extended 800 m along the strike of the deposit, and sparse outcrops o
