Geologia e petrologia do plúton Saubinha e os depósitos de estanho associados, Rondônia
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2020-05-06
Autores
Farias, Vanderlei de [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
O plúton Saubinha (1.316 ± 16 Ma.) compõe uma das intrusões graníticas do maciço São Lourenço que em conjunto com outros maciços (Caripunas, Abunã e São Simão) representam as áreas de exposição de rochas graníticas, com ou sem depósitos de estanho associados, pertencentes a Suíte Intrusiva São Lourenço-Caripunas (1.32 – 1.30 Ga.). O núcleo de ocorrência dos depósitos primários de estanho ocorre na forma de um stock granítico alongado NW – SE. Constitui-se por biotita-álcali feldspato granitos, cartografados em duas unidades mapeáveis, 1:10.000, e em contatos, aparentemente, transicionais entre si. A Unidade A (UA) que integra tipos heterogranulares a equigranulares de granulação média e, subordinamente porfiríticos de matriz fina; e a Unidade B (UB), restrita a borda leste do plúton, com tipos equigranulares a porfiríticos de matriz fina. Adiciona-se ainda uma lente delgada e não contínua de granito miarolítico a sudeste do plúton e diques de microgranito e aplito que seccionam preferencialmente as rochas da UB. Geoquimicamente são granitos ferrosos, pós-colisionais, tipo A (subtipo A2), de caráter metaluminoso que permeia entre os campos peralcalino e peraluminoso com alcalinidade álcali-cálcica a cálcio-alcalina. Apresentam características de magmas oriundos de anatexia de protólitos quartzo-feldspáticos, de mais alta temperatura (~827,0ºC), pouco hidratados e que se posicionaram em níveis crustais rasos. A variação dos elementos maiores e traços por SiO2 sugere um trend evolutivo dos tipos porfiríticos (UB) para os heterogranulares (UA), o que aponta também para uma cristalização centrípeta do plúton, ou seja, mais acelerada nas bordas a mais tardia e lenta no centro. Ademais, apresentam valores elevados totais de ETR (Σ=558) e de terras raras leves sobre os pesados (La/Yb: 7,9), que em conjunto com a intensidade das anomalias negativas de Ba, Sr, Ti e Nb, e razões de K/Na, Rb/Sr; apontam características de biotita granitos precursores à mineralização e, portanto, sem vínculo genético com os depósitos polimetálicos, na forma de lentes de greisen e veios de quartzo subverticais, de Sn ± Cu, Pb e Zn descritos no Distrito Mineiro de São Lourenço-Macisa.
Saubinha pluton (1.316 ± 16 Ma.) composes one of granitic intrusions of São Lourenço massif, which together with other massifs (Caripunas, Abunã and São Simão) represent the areas of exposure granitic rocks, with or without associated tin deposits, belonging to the evolution of São Lourenço-Caripunas Intrusive Suite (1.32 – 1.30 Ga.). The core of occurrence of primary tin deposits occurs as an NW – SE elongated granitic stock. It consists of biotite-alkali feldspar granites, mapped in two mappable units, in scale 1:10.000, and in apparently transitional contacts. Unit A (UA) that integrates heterogranular to equigranular medium grain types and subordinately porphyritic of fine matrix; and Unit B (UB), restricted to the eastern edge of the pluton, with equigranular to porphyritic fine grain types. In addition, a thin non-continuous lens of miarolitic granite to the southeast of the pluton and dykes of microgranite and aplite that preferentially crosscut the rocks of UB. Geochemically, they are ferrous granites, post-collisional, A-type (subtype A2), of a metaluminous character that permeates between the peralkaline to peraluminous fields with alkali-calcic to calcic-alkali alkalinity. The granites have characteristics of magmas originated from anatexy of quartz-feldspar protoliths, of higher temperature (~827,0ºC), poorly hydrated and positioned in shallow crustal levels. Variation of major and traces elements by SiO2 suggests an evolutionary trend from porphyritic types (UB) to the heterogranular types (UA), which also points to a centripetal crystallization for the pluton, that is, more accelerated in the edges and later and slower in the center of the Saubinha pluton. Also, the granites have high total values of REE (Σ=558) and of light earth elements over heavy earth elements (La/Yb: 7,9) which together with intensity of negative Ba, Sr, Ti e Nb anomalies, and K/Na, Rb/Sr ratios; indicate characteristics of biotite granites precursors to mineralization and, therefore, without genetic link with the polymetallic (Sn ± Cu, Pb and Zn) deposits, in the form of subvertical greisen lens and quartz veins, described in the São Lourenço-Macisa mining district.
Saubinha pluton (1.316 ± 16 Ma.) composes one of granitic intrusions of São Lourenço massif, which together with other massifs (Caripunas, Abunã and São Simão) represent the areas of exposure granitic rocks, with or without associated tin deposits, belonging to the evolution of São Lourenço-Caripunas Intrusive Suite (1.32 – 1.30 Ga.). The core of occurrence of primary tin deposits occurs as an NW – SE elongated granitic stock. It consists of biotite-alkali feldspar granites, mapped in two mappable units, in scale 1:10.000, and in apparently transitional contacts. Unit A (UA) that integrates heterogranular to equigranular medium grain types and subordinately porphyritic of fine matrix; and Unit B (UB), restricted to the eastern edge of the pluton, with equigranular to porphyritic fine grain types. In addition, a thin non-continuous lens of miarolitic granite to the southeast of the pluton and dykes of microgranite and aplite that preferentially crosscut the rocks of UB. Geochemically, they are ferrous granites, post-collisional, A-type (subtype A2), of a metaluminous character that permeates between the peralkaline to peraluminous fields with alkali-calcic to calcic-alkali alkalinity. The granites have characteristics of magmas originated from anatexy of quartz-feldspar protoliths, of higher temperature (~827,0ºC), poorly hydrated and positioned in shallow crustal levels. Variation of major and traces elements by SiO2 suggests an evolutionary trend from porphyritic types (UB) to the heterogranular types (UA), which also points to a centripetal crystallization for the pluton, that is, more accelerated in the edges and later and slower in the center of the Saubinha pluton. Also, the granites have high total values of REE (Σ=558) and of light earth elements over heavy earth elements (La/Yb: 7,9) which together with intensity of negative Ba, Sr, Ti e Nb anomalies, and K/Na, Rb/Sr ratios; indicate characteristics of biotite granites precursors to mineralization and, therefore, without genetic link with the polymetallic (Sn ± Cu, Pb and Zn) deposits, in the form of subvertical greisen lens and quartz veins, described in the São Lourenço-Macisa mining district.
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São Lourenço-Macisa, Biotita granito, Greisen, Cassiterita, Estanho, Biotite granite, Cassiterite, Tin