AUTHORS: Benoit M. Saumur, Alexander R. Cruden, David Boutelier, Dawn Evans-Lamswood
ABSTRACT: The geometry of the Voisey’s Bay Intrusion (VBI) is strongly controlled by pre-emplacement ductile and pre-to-syn-emplacement brittle wall-rock structures. Only the final inputs of magma, associated with the feeder dykes of the VBI, contributed massive sulphide mineralisation. Emplacement occurred by a “caldron subsidence” process in which the roof of a lower chamber collapsed as the floor of an upper chamber subsided; dense sulphides, which were concentrated at the base of the lower chamber, were withdrawn last. Theory and analogue models of entrainment of sulphide liquid by mafic magma during forceful withdrawal suggest that draw-up of sulphide is highly sensitive to flow rates and mafic magma viscosity. Significant draw-up of sulphide can occur at high Reynolds numbers and flow rates (10-1000 m3/s). Finally, because of its high density, low viscosity and low solidus temperature, sulphide can percolate downwards late in the history of the system. This is controlled by a “critical accumulation height”, above which sulphide can percolate downwards into anisotropies of given widths. This process was important for the development of mineralisation in the Reid Brook Zone of the VBI, located below a now-eroded magma chamber, where injections of sulphide intrude dominantly in the down-dip directions of host-rock gneiss structures.
KEYWORDS: mafic magma, sulphide liquid, emplacement, withdrawal, entrainment, downward percolation
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