Polyphase deformation of the massive sulphide ore of the Black Angel Mine, central West Greenland

Abstract

The massive Fe-Zn-Pb sulphide sheets constituting the Angel Zone ore body of the Black Angel Mine, show evidence of three phases of deformation at greenschist facies metamorphic grade. During an early phase an originally layered sulphide ore type was isoclinally folded. Subsequent thrusting parallel to the ore body transformed the layered ore into massive and porphyroclastic ore tectonites. Late, open folds refolded the earlier structures and caused localized differential mobilization of the sulphides. The microstructures of the layered ore tectonite indicate a period with static grain growth, interpreted as the result of prograde metamorphism, followed by a dynamic recrystallization under low stress and at low strain rates, which is correlated with the early isoclinal folding. The microstructures of the massive and the porphyroclastic ore tectonites indicate syntectonic recrystallization under high stress and at high strain rates, corresponding to the thrusting of the ore bodies. The microstructures of the mobilized sulphides show evidence of repeated plastic/cataclastic deformation and recrystallization, corresponding to highly variable strain and strain rate conditions during the mobilization. Post-deformational annealing took place at elevated temperature and was largely controlled by inhibition-dependent grain growth and to a minor extent by orientation-dependent grain growth.