La franja aurífera de Xallas (Galicia occidental,
España): Las mineralizaciones de Albores dentro
de un contexto tectónico y metalogénico evolutivo.
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1992
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Instituto Geológico y Minero de España (Madrid)
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Abstract
Las mineralizaciones de oro de Albores (Galicia occidental) se encuadran dentro de una franja metalogénica N-S a NE de edad hercínica-tardihercínica (franja aurífera de Xallas). Dicha franja comprende una serie de indicios auríferos caracterizados por la presencia de una fase sulfurada rica en arsenopirita y otra metálica con oro/electrum. Estas mineralizaciones encajan generalmente en estructuras frágiles desarrolladas en zonas de cizalla de componente horizontal y vertical, que incluyen mílonitas, ultramilonitas, cataclasitas y brechas.Las mineralizaciones de Albores incluyen una paragénesis compleja consistente en arsenopirita, oro/electrum, pirrotina, marcasita, pirita, bismuto, bismutina, esfalerita, galena y calcopirita. Estudios de inclusionesfluidas en cuarzos acompañantes de la mineralización sugieren que ésta fue depositada por fluidos acuoso-carbónicos complejos de moderada baja salinidad (<56-9 por 100 eq. NaCI; salinidades máximas) operando en un rango de temperaturas epi a mesotermal (Th: 18Cl°-36Cl° CJ. Estas mineralizaciones se formaron durante un régimen extensional desarrollado durante el período comprendido entre la fase deformativa 3 hercínica (D3; Carbonífero Medio) y el episodio frágil tardihercínico (Carbonífero Superior/Pérmico Inferior}. La etapa extensional se caracterizó por el desarrollo de corredores miloníticos principalmente extensionales, de bajo ángulo (zona de cizalla de Xallas) con superposición de estructuras frágiles sobre la fábrica dúctil, estructuras antiformes, y fallas normales de gran ángulo de carácter frágil.
The gold mineralization of Albores (western Galicia, NW Spain) is located within a N-S to NE metallogenic belt of Hercynian - late Hercynian age (Xallas gold belt). This belt comprises a series of gold deposits characterized by the association gold-arsenopyrite. The mineralizations are associated to brittle structures developed within strike-slip and normal shear zones. The shear zones include mylonites, ultramylonites, cataclasites, and breccias. The Albores mineralization has a complex paragenesis comprising the following minerals: arsenopyrite, gold/electrum, pyrrothite, ·marcasite, pyrite, bismuth, bismuthinite, sphalerite, galena, and chalcopyrite. Fluid inclusion studies on quartz belonging to the gangue suggest that the mineralization was precipitated by complex aqueous-carbonic fluids of moderate low salinity (<56-9 eq. wt. % NaCI; maxima salinities), that circulated under epithermal-mesothermal conditions of temperature (Th: 180º-360º CJ. The mineralizations formed during an extensional episode that took place between the last episode of Hercynian (s.s.) deformation (D3; middle Carboniferous) and the so-called •late Hercynian brittle episode• (late Carboniferous/early Permian). The extensional episode was characterized by the development of low-angle extensional mylonitic corridors (Xallas shear zonel with brittle-on-ductile fabrics, antiform structures, and high-angle brittle normal faults.
The gold mineralization of Albores (western Galicia, NW Spain) is located within a N-S to NE metallogenic belt of Hercynian - late Hercynian age (Xallas gold belt). This belt comprises a series of gold deposits characterized by the association gold-arsenopyrite. The mineralizations are associated to brittle structures developed within strike-slip and normal shear zones. The shear zones include mylonites, ultramylonites, cataclasites, and breccias. The Albores mineralization has a complex paragenesis comprising the following minerals: arsenopyrite, gold/electrum, pyrrothite, ·marcasite, pyrite, bismuth, bismuthinite, sphalerite, galena, and chalcopyrite. Fluid inclusion studies on quartz belonging to the gangue suggest that the mineralization was precipitated by complex aqueous-carbonic fluids of moderate low salinity (<56-9 eq. wt. % NaCI; maxima salinities), that circulated under epithermal-mesothermal conditions of temperature (Th: 180º-360º CJ. The mineralizations formed during an extensional episode that took place between the last episode of Hercynian (s.s.) deformation (D3; middle Carboniferous) and the so-called •late Hercynian brittle episode• (late Carboniferous/early Permian). The extensional episode was characterized by the development of low-angle extensional mylonitic corridors (Xallas shear zonel with brittle-on-ductile fabrics, antiform structures, and high-angle brittle normal faults.