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Critérios mineralógicos e microestruturais aplicados à determinação de pathfinders para prospecção aurífera (Mina de Escádia Grande, Góis)
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A antiga mina de ouro de Escádia Grande (Alvares, Góis, Coimbra) foi revisitada com o propósito de contribuir para o esclarecimento de algumas questões em aberto, nomeadamente no que diz respeito: (1) à sequência das várias fases de deposição de quartzo e respectiva mineralogia acessória, de modo a constranger o posicionamento cronológico relativo do evento mineralizante; (2) às razões pelas quais os teores de Au não se distribuem de forma consistente ao longo das estruturas interpretadas até agora como potencialmente mineralizadas (filonetes subparalelos de arsenopirite); e (3) ao controlo de natureza estrutural, envolvido no input aurífero no sistema, que condiciona a distribuição espacial dos teores, relacionando-o com a evolução do corredor de cisalhamento principal. Foram reconhecidas 8 gerações de quartzo que marcam diferentes eventos de deformação síncronos do dobramento regional (Qz I, II, III, IV e V), da propagação/reactivação e corredor de cisalhamento NW-SE/WNW-ESE em regime dúctil/semi-frágil (Qz VI e Qz VII), e do período tardio de reactivação frágil do maciço rochoso (Qz VIII). Os eventos mineralizantes relacionam-se com a propagação local de um corredor de cisalhamento NW-SE/WNW-ESE esquerdo, cujos eventos cíclicos de reactivação sob condições de elevada pressão de fluidos, concorreram para a génese de vários conjuntos de estruturas. Destaca-se a génese (diacrónica) de dois tipos de corpos filonianos relacionados com: (i) o desenvolvimento de fendas dilatantes (Qz VI), mais precoces; e (ii) o preenchimento de zonas de falha WNW-ESE, com pendor para SSW (Qz VII), desenvolvidos posteriormente. As associações minerais observadas em contexto filoniano contrastam fortemente com as preservadas nas sequências metassedimentares adjacentes. O contexto filoniano é o que melhor regista os efeitos decorrentes dos sucessivos impulsos mineralizantes e aquele onde preferencialmente precipitam fases portadoras de Au e Ag. Em contexto metassedimentar, estes impulsos concorrem para a neoformação de associações mineralógicas com agregados de clorite, cujas composições são adequadas à aplicação de alguns modelos geotermométricos. O evento mineralizante aurífero principal (~350-375ºC) é marcado pela deposição de Qz VII + Apy II + Py Ia + Mrc I + Ccp Ia + Sp I ± Po ± Cb I, imediatamente seguida de Py Ib + Gn I + Elect + Ttr/Frg (Prr) + Ulm I + Ccp Ib; subsequentemente, ocorre precipitação de Dol + Gn II + AgxSy (~350-340ºC a ~330-220ºC) marcando um evento argentífero subsequente. O evento aurífero principal sucede ao desenvolvimento dos filonetes subparalelos de arsenopirite (Apy I), pelo que estas estruturas não devem ser encaradas como potencialmente mineralizadas, embora possam, localmente, funcionar como armadilhas geoquímicas para o crescimento de partículas de Au. O input aurífero é essencialmente governado pelas falhas com pendor para SSW e os teores aumentam na intersecção com os corpos filonianos mais precoces (Qz VI). Os domínios de coalescimento destas estruturas com geometria releasing bend devem ser igualmente vistos como potenciais alvos, pois representam zonas de dilatação propícias à acomodação de precipitados hidrotermais (Qz VII) potencialmente mineralizados. Regista-se a ocorrência de um pico de rejuvenescimento térmico tardio do sistema (340-350ºC); contudo os precipitados hidrotermais (Qz VIII) resultantes são estéreis no que a Au(Ag) diz respeito. Em suma, o desenvolvimento das estruturas de suporte ao depósito Escádia Grande processou-se em regime constritivo forte, de carácter muito local e sob elevada pressão de fluidos; tal terá permitido a abertura de descontinuidades (pré-existentes e neoformadas) e escoamento (recorrente, embora irregular) de fluido hidrotermal. Os constrangimentos termomecânicos inferidos e o carácter restrito dos episódios de incremento da permeabilidade, limitam fortemente a continuidade espacial (lateral e em profundidade) do metalotecto estrutural portador de mineralizações com teores apreciáveis. Por esta razão, a menos que seja possível demonstrar a existência de outros corredores estruturais de maior continuidade espacial, se considera que futuros investimentos circunscritos ao depósito de Escádia Grande comportam risco elevado.
The old gold mine of Escádia Grande (Alvares, Góis, Coimbra) was revisited aiming the elucidation of a few open questions, namely those regarding: (1) the sequence of quartz depositional phases and accessory mineralogy, in an attempt of constraining the relative chronologic positioning of the mineralizing event; (2) the reasons that could explain why Au grades are inconsistently distributed along the structures so far interpreted as potentially mineralized (subparallel fillets of arsenopyrite); and (3) the nature of the structural control involved in the Au input into the system, regulating the spatial distribution of grades, and relating it with the evolution of the main shear zone. Eight generations of quartz tracing different deformation events were recognized; synchronous to regional folding (Qz I, II, III, IV e V), propagation/reactivation of NW-SE/WNW-ESE shear zones under ductile/brittle-ductile regime (Qz VI e VII), and late brittle reactivation of the system (Qz VIII). The mineralizing events correlate to the local propagation of a NW-SE/WNW-ESE left-lateral shear zone, where cyclic reactivation events under high fluid-pressure conditions led to the formation of several sets of structures. Noteworthy are the two (diachronic) lode types related to: (i) the early development of dilation gashes (Qz VI); and (ii) filling of late SSW-dipping WNW-ESE fault zones (QZ VII). The observed mineral assemblages in lode-type systems strongly contrast with those preserved in neighbouring metasedimentary sequences. The former tend to better record the effects of multiple mineralizing pulses, thus representing the preferential deposition sites for Au and Ag phases. In metasedimentary context these pulses triggered the neoformation of mineral associations with chlorite aggregates, whose composition allows geothermometry modelling. The main auriferous mineralizing event (~350-375ºC) is outlined by the deposition of Qz VII + Apy II + Py Ia + Mrc I + Ccp Ia + Sp I ± Po ± Cb I, followed by Py Ib + Gn I + Elect + Ttr/Frg (Prr) + Ulm I + Ccp Ib; subsequently the precipitation of Dol + Gn II + AgxSy (~350-340ºC to ~330-220ºC) indicates a late argentiferous event. Because the main auriferous event follows the development of the subparallel arsenopyrite fillets (Apy I), these structures should not be interpreted as potentially mineralized, although they can work as geochemical traps for the growth of gold particles. The gold input is essentially controlled by the SSW-dipping faults and the grades increase at intersections with the early lodes (Qz VI). The releasing bend coalescence domains should be equally considered as potential targets, because they represent dilation zones prone to the accommodation of potentially mineralized hydrothermal precipitates (Qz VII). Several lines of evidence suggest the occurrence of late thermal rejuvenation of the system (reaching up to 340-350ºC), although resulting barren hydrothermal precipitates (Qz VIII) concerning Au(Ag). Concluding, the development of the structures that support the Escádia Grande deposit took place under high fluid-pressure and strong strain constriction conditions of local character; this allowed the opening of (pre-existent and neoformed) discontinuities and (recurrent, although irregular) hydrothermal fluid flow. The inferred thermomechanical constraints and the restrict character of permeability increasing episodes highly limit the spatial continuity of the structural array (laterally and in depth) potentially hosting high ores grades. Considering all the compiled information, and unless proven the existence of other shear zones of wider spatial continuity, future investments confined to the Escádia Grande deposit bear high risk.
The old gold mine of Escádia Grande (Alvares, Góis, Coimbra) was revisited aiming the elucidation of a few open questions, namely those regarding: (1) the sequence of quartz depositional phases and accessory mineralogy, in an attempt of constraining the relative chronologic positioning of the mineralizing event; (2) the reasons that could explain why Au grades are inconsistently distributed along the structures so far interpreted as potentially mineralized (subparallel fillets of arsenopyrite); and (3) the nature of the structural control involved in the Au input into the system, regulating the spatial distribution of grades, and relating it with the evolution of the main shear zone. Eight generations of quartz tracing different deformation events were recognized; synchronous to regional folding (Qz I, II, III, IV e V), propagation/reactivation of NW-SE/WNW-ESE shear zones under ductile/brittle-ductile regime (Qz VI e VII), and late brittle reactivation of the system (Qz VIII). The mineralizing events correlate to the local propagation of a NW-SE/WNW-ESE left-lateral shear zone, where cyclic reactivation events under high fluid-pressure conditions led to the formation of several sets of structures. Noteworthy are the two (diachronic) lode types related to: (i) the early development of dilation gashes (Qz VI); and (ii) filling of late SSW-dipping WNW-ESE fault zones (QZ VII). The observed mineral assemblages in lode-type systems strongly contrast with those preserved in neighbouring metasedimentary sequences. The former tend to better record the effects of multiple mineralizing pulses, thus representing the preferential deposition sites for Au and Ag phases. In metasedimentary context these pulses triggered the neoformation of mineral associations with chlorite aggregates, whose composition allows geothermometry modelling. The main auriferous mineralizing event (~350-375ºC) is outlined by the deposition of Qz VII + Apy II + Py Ia + Mrc I + Ccp Ia + Sp I ± Po ± Cb I, followed by Py Ib + Gn I + Elect + Ttr/Frg (Prr) + Ulm I + Ccp Ib; subsequently the precipitation of Dol + Gn II + AgxSy (~350-340ºC to ~330-220ºC) indicates a late argentiferous event. Because the main auriferous event follows the development of the subparallel arsenopyrite fillets (Apy I), these structures should not be interpreted as potentially mineralized, although they can work as geochemical traps for the growth of gold particles. The gold input is essentially controlled by the SSW-dipping faults and the grades increase at intersections with the early lodes (Qz VI). The releasing bend coalescence domains should be equally considered as potential targets, because they represent dilation zones prone to the accommodation of potentially mineralized hydrothermal precipitates (Qz VII). Several lines of evidence suggest the occurrence of late thermal rejuvenation of the system (reaching up to 340-350ºC), although resulting barren hydrothermal precipitates (Qz VIII) concerning Au(Ag). Concluding, the development of the structures that support the Escádia Grande deposit took place under high fluid-pressure and strong strain constriction conditions of local character; this allowed the opening of (pre-existent and neoformed) discontinuities and (recurrent, although irregular) hydrothermal fluid flow. The inferred thermomechanical constraints and the restrict character of permeability increasing episodes highly limit the spatial continuity of the structural array (laterally and in depth) potentially hosting high ores grades. Considering all the compiled information, and unless proven the existence of other shear zones of wider spatial continuity, future investments confined to the Escádia Grande deposit bear high risk.
Descrição
Relatório de estágio de mestrado, Geologia Económica (Prospecção Mineral)Universidade de Lisboa, Faculdade de Ciências, 2019
Palavras-chave
Controlo estrutural de mineralização Escádia Grande Metalogénese aurífera varisca Teses de mestrado - 2019