The Pb-Bi-(Sb)-S sulphidic melts from the VMS Besshi-type Tisová deposit, Czech Republic: mineralogy, chemistry and implications of their formation

AbstraktAbstract

Two types of crystallised melt were identified as tiny, sharply bounded, ellipsoidal inclusions (typically 20–80 µm) in metamorphosed massive pyrrhotite ore (12–14 kbar, ~530–550 °C) from the VMS Besshi-type Tisová deposit. The Pb-Bi-S melt is more abundant than Pb-Bi-(Sb)-S melt. Many inclusions exhibit complex internal eutectic-like intergrowths with enclosing sulphides. Electron-backscatter diffraction revealed crystallographic continuity of pyrrhotite/chalcopyrite across the boundaries of the inclusions, indicating that the Fe-(Cu)-S sulphides enclosed within the blebs were formed by the replacement and recrystallisation of the host sulphides, rather than by the direct crystallisation of the melt. Micro-computed X-ray tomography confirmed the dominance and random distribution of smaller inclusions. It also showed a strong, unidirectional preferred orientation of the longest axis of the inclusions within the metamorphic foliation plane, which is consistent with their ductile deformation shortly after melt crystallisation. Larger and more irregular inclusions are often oriented at an acute angle to the foliation, suggesting their link to the onset of brittle deformation. The estimated bulk compositions of the Pb-Bi-S inclusions are shifted towards galena relative to the experimentally constrained melt fields. We interpret this as selective bismuth loss during a phase of enhanced late metamorphic fluid activity. Our observations support the formation of melts at or near peak metamorphic conditions during the climax of the Variscan orogeny, followed by fluid-related modification(s) during the post-orogenic uplift phase. Our results show that melts may represent an important mechanism for the redistribution of Bi, Pb, Sb during metamorphism of sulphide ores.

Two types of crystallised melt were identified as tiny, sharply bounded, ellipsoidal inclusions (typically 20–80 µm) in metamorphosed massive pyrrhotite ore (12–14 kbar, ~530–550 °C) from the VMS Besshi-type Tisová deposit. The Pb-Bi-S melt is more abundant than Pb-Bi-(Sb)-S melt. Many inclusions exhibit complex internal eutectic-like intergrowths with enclosing sulphides. Electron-backscatter diffraction revealed crystallographic continuity of pyrrhotite/chalcopyrite across the boundaries of the inclusions, indicating that the Fe-(Cu)-S sulphides enclosed within the blebs were formed by the replacement and recrystallisation of the host sulphides, rather than by the direct crystallisation of the melt. Micro-computed X-ray tomography confirmed the dominance and random distribution of smaller inclusions. It also showed a strong, unidirectional preferred orientation of the longest axis of the inclusions within the metamorphic foliation plane, which is consistent with their ductile deformation shortly after melt crystallisation. Larger and more irregular inclusions are often oriented at an acute angle to the foliation, suggesting their link to the onset of brittle deformation. The estimated bulk compositions of the Pb-Bi-S inclusions are shifted towards galena relative to the experimentally constrained melt fields. We interpret this as selective bismuth loss during a phase of enhanced late metamorphic fluid activity. Our observations support the formation of melts at or near peak metamorphic conditions during the climax of the Variscan orogeny, followed by fluid-related modification(s) during the post-orogenic uplift phase. Our results show that melts may represent an important mechanism for the redistribution of Bi, Pb, Sb during metamorphism of sulphide ores.