Duas exposicoes complexas do granada-biotita granito anatetico Nazare Paulista e de suas rochas e... more Duas exposicoes complexas do granada-biotita granito anatetico Nazare Paulista e de suas rochas encaixantes foram estudadas para identificar os diferentes corpos de granito e suas relacoes. No afloramento NP74, tres corpos graniticos principais e corpos menores de aplito-pegmatito foram distinguidos, todos intrusivos mas subconcordantes com (granada)-biotita gnaisses migmatiticos encaixantes. O primeiro corpo (de NW para SE; 74g1), com espessura de ~ 43 m, e composto de granada-biotita granito cinza venulado; veios de granada leucogranito formam uma rede cortanto o granito cinza, e contornam os abundantes xenolitos de gnaisse e schlieren; a zona de teto e formada por ~8 m de granada leucogranito com abundantes xenolitos, que pode ter se formado por concentracao do material presente nos veios. O granito 74g2, com espessura de ~ 12 m, ocorre apos um estreito septo gnaissico; e constituido por um granada leucogranito com bolsoes pegmatiticos locais. O granito 74g3, com espessura de ~ 20 m, forma extremo sul do afloramento, e ocorre apos uma grande exposicao de rocha encaixante invadida por camadas de pegmatito; e formado por (±biotita)-granada leucogranito com fibrolita local. No afloramento NP40, os migmatitos encaixantes ocorrem apenas como rochas alteradas circundando a exposicao de granito. Aqui, um corpo com largura de ~ 15 m de granito cinza venulado (40g2) e invadido em sua porcao central por um leucogranito branco (40g3) possivelmente formado pela coleta de material diferenciado das venulas, e invadido em ambos os lados por um granada leucogranito aplitico-pegmatitico heterogeneo, rico em schlieren (40g1 e 40g4). Esses dois afloramentos mostram que uma ampla variedade de “granitos Nazare Paulista” existe mesmo na escala de detalhe, formando pequenos corpos cujas relacoes mutuas podem ser identificadas. O granito cinza venulado, em geral considerado como o granito Nazare Paulista “tipico”, tem distribuicao ampla na regiao, mas e localmente subordinado em relacao as variedades mais leucocraticas. Estas podem ser relacionadas diretamente a coleta de material proveniente das venulas em torno de xenolitos ou no teto dos corpos de granito cinza, mas tambem formam corpos independentes que podem ter genese totalmente distinta. Evidencias estruturais de extensao sin-magmatica no afloramento NP74 sugerem que alguma migracao de magma ocorreu em um cinturao colisional extensional; no entanto, o intervalo de tempo em que ocorreu anatexia na regiao pode ter sido grande (> 10 milhoes de anos?), e ainda requer determinacoes de idade precisas adicionais.
The potassium feldspar variety adularia was found in the Davi and Joel primary gold prospects loc... more The potassium feldspar variety adularia was found in the Davi and Joel primary gold prospects located in the Tapajos Mineral Province, southwestern Para State, Brazil. In both areas adularia occupies fractures and vugs intimately associated with mineralized quartz veins hosted by hydrothermally altered Proterozoic subvolcanic mafic rocks. Adularia constitutes small (1-4 mm) red to pink crystals of pseudo-orthorhombic form. The Felsobanya habit was identified in specimens from the Joel prospect. Microprobe analyses show that adularia has a composition near to pure KalSi 3 O 8 . X-ray diffraction data indicate that the feldspars correspond to predominantly triclinic types with structural state near that of maximum microcline. This high degree of ordering is relatively rare in adularia and may be attributed to the extensive action of late fluids over originally metastable, disordered crystals whose primitive monoclinic form is still preserved. Fluid inclusion studies in quartz crystals adjacent to adularia show that the feldspar formed under temperatures of 240° -320°C at Davi and 220° - 340°C at Joel. The trapped fluids are poor in or devoid of CO 2 . Fluid salinity was low, averaging 1.15 at Davi and 0.43 at Joel, attaining exceptionally values up to 11 wt.% NaCl equivalent. Considering that the presence of adularia is indicative of boiling, it is possible to estimate a maximum depth of 1.5 km for the formation of adularia and the associated quartz veins. The presence of adularia in the Davi and Joel prospects is important in the definition of the typology of primary gold of the Tapajos Province as this feldspar is a typical mineral of epithermal deposits of the adularia-sericite category. Other features also point to the epithermal nature of the mentioned occurrences. These include the non-metamorphic character of the host rocks and their hydrothermal alteration pattern; the open-space filling textures of the veins and their mineral assemblage; and the brittle nature of the deformation imprinted in both veins and wallrocks. The composition of fluids in inclusions also lies within the limits admitted for adularia-sericite epithermal deposits. The temperatures of formation of the adularia and quartz veins, in turn, belong to a high temperature rank in epithermal deposits, suggesting that the Davi and Joel prospects correpond to exposures of the deeper, higher temperature sectors of their original hydrothermal systems. Two other aspects should be emphasized concerning the definition of an epithermal typology for gold occurrences in the Tapajos region. The Proterozoic age is an unusually old one for this type of deposit, as the majority of their representatives in the world are Cenozoic; and the definition is interesting for gold exploration in connection with the potential for bulk-tonnage deposits.
... Geosciences Division, Centre for Earth Science Studies, Thiruvananthapuram-695 031, Kerala, I... more ... Geosciences Division, Centre for Earth Science Studies, Thiruvananthapuram-695 031, Kerala, India [e-mail: ajith.nair@mailcity.com] * Department of Geology, Sree Narayana College, Thiruvananthapuram-695 587, Kerala, India ** Departamento de Mineralogia e ...
Back-scattered imaging, X-ray element mapping and electron microprobe analyzer (EMPA) chemical da... more Back-scattered imaging, X-ray element mapping and electron microprobe analyzer (EMPA) chemical dating reveal complex compositional and age zoning in monazite crystals from different layers and textural positions in a garnet-bearing migmatite in SE Brazil. Y-rich ( ...
Duas exposicoes complexas do granada-biotita granito anatetico Nazare Paulista e de suas rochas e... more Duas exposicoes complexas do granada-biotita granito anatetico Nazare Paulista e de suas rochas encaixantes foram estudadas para identificar os diferentes corpos de granito e suas relacoes. No afloramento NP74, tres corpos graniticos principais e corpos menores de aplito-pegmatito foram distinguidos, todos intrusivos mas subconcordantes com (granada)-biotita gnaisses migmatiticos encaixantes. O primeiro corpo (de NW para SE; 74g1), com espessura de ~ 43 m, e composto de granada-biotita granito cinza venulado; veios de granada leucogranito formam uma rede cortanto o granito cinza, e contornam os abundantes xenolitos de gnaisse e schlieren; a zona de teto e formada por ~8 m de granada leucogranito com abundantes xenolitos, que pode ter se formado por concentracao do material presente nos veios. O granito 74g2, com espessura de ~ 12 m, ocorre apos um estreito septo gnaissico; e constituido por um granada leucogranito com bolsoes pegmatiticos locais. O granito 74g3, com espessura de ~ 20 m, forma extremo sul do afloramento, e ocorre apos uma grande exposicao de rocha encaixante invadida por camadas de pegmatito; e formado por (±biotita)-granada leucogranito com fibrolita local. No afloramento NP40, os migmatitos encaixantes ocorrem apenas como rochas alteradas circundando a exposicao de granito. Aqui, um corpo com largura de ~ 15 m de granito cinza venulado (40g2) e invadido em sua porcao central por um leucogranito branco (40g3) possivelmente formado pela coleta de material diferenciado das venulas, e invadido em ambos os lados por um granada leucogranito aplitico-pegmatitico heterogeneo, rico em schlieren (40g1 e 40g4). Esses dois afloramentos mostram que uma ampla variedade de “granitos Nazare Paulista” existe mesmo na escala de detalhe, formando pequenos corpos cujas relacoes mutuas podem ser identificadas. O granito cinza venulado, em geral considerado como o granito Nazare Paulista “tipico”, tem distribuicao ampla na regiao, mas e localmente subordinado em relacao as variedades mais leucocraticas. Estas podem ser relacionadas diretamente a coleta de material proveniente das venulas em torno de xenolitos ou no teto dos corpos de granito cinza, mas tambem formam corpos independentes que podem ter genese totalmente distinta. Evidencias estruturais de extensao sin-magmatica no afloramento NP74 sugerem que alguma migracao de magma ocorreu em um cinturao colisional extensional; no entanto, o intervalo de tempo em que ocorreu anatexia na regiao pode ter sido grande (> 10 milhoes de anos?), e ainda requer determinacoes de idade precisas adicionais.
The potassium feldspar variety adularia was found in the Davi and Joel primary gold prospects loc... more The potassium feldspar variety adularia was found in the Davi and Joel primary gold prospects located in the Tapajos Mineral Province, southwestern Para State, Brazil. In both areas adularia occupies fractures and vugs intimately associated with mineralized quartz veins hosted by hydrothermally altered Proterozoic subvolcanic mafic rocks. Adularia constitutes small (1-4 mm) red to pink crystals of pseudo-orthorhombic form. The Felsobanya habit was identified in specimens from the Joel prospect. Microprobe analyses show that adularia has a composition near to pure KalSi 3 O 8 . X-ray diffraction data indicate that the feldspars correspond to predominantly triclinic types with structural state near that of maximum microcline. This high degree of ordering is relatively rare in adularia and may be attributed to the extensive action of late fluids over originally metastable, disordered crystals whose primitive monoclinic form is still preserved. Fluid inclusion studies in quartz crystals adjacent to adularia show that the feldspar formed under temperatures of 240° -320°C at Davi and 220° - 340°C at Joel. The trapped fluids are poor in or devoid of CO 2 . Fluid salinity was low, averaging 1.15 at Davi and 0.43 at Joel, attaining exceptionally values up to 11 wt.% NaCl equivalent. Considering that the presence of adularia is indicative of boiling, it is possible to estimate a maximum depth of 1.5 km for the formation of adularia and the associated quartz veins. The presence of adularia in the Davi and Joel prospects is important in the definition of the typology of primary gold of the Tapajos Province as this feldspar is a typical mineral of epithermal deposits of the adularia-sericite category. Other features also point to the epithermal nature of the mentioned occurrences. These include the non-metamorphic character of the host rocks and their hydrothermal alteration pattern; the open-space filling textures of the veins and their mineral assemblage; and the brittle nature of the deformation imprinted in both veins and wallrocks. The composition of fluids in inclusions also lies within the limits admitted for adularia-sericite epithermal deposits. The temperatures of formation of the adularia and quartz veins, in turn, belong to a high temperature rank in epithermal deposits, suggesting that the Davi and Joel prospects correpond to exposures of the deeper, higher temperature sectors of their original hydrothermal systems. Two other aspects should be emphasized concerning the definition of an epithermal typology for gold occurrences in the Tapajos region. The Proterozoic age is an unusually old one for this type of deposit, as the majority of their representatives in the world are Cenozoic; and the definition is interesting for gold exploration in connection with the potential for bulk-tonnage deposits.
... Geosciences Division, Centre for Earth Science Studies, Thiruvananthapuram-695 031, Kerala, I... more ... Geosciences Division, Centre for Earth Science Studies, Thiruvananthapuram-695 031, Kerala, India [e-mail: ajith.nair@mailcity.com] * Department of Geology, Sree Narayana College, Thiruvananthapuram-695 587, Kerala, India ** Departamento de Mineralogia e ...
Back-scattered imaging, X-ray element mapping and electron microprobe analyzer (EMPA) chemical da... more Back-scattered imaging, X-ray element mapping and electron microprobe analyzer (EMPA) chemical dating reveal complex compositional and age zoning in monazite crystals from different layers and textural positions in a garnet-bearing migmatite in SE Brazil. Y-rich ( ...
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