Dynamics of Withdrawal and Backflow of Sulfide Liquids and the Formation of Magmatic Ni-Cu-PGE Sulfide Deposits: Theory and Analogue Modeling

Sulfide liquids precipitated from silicate magmas after sulfide saturation represent a volumetrically small portion of magmatic systems, but are responsible for the formation of orthomagmatic Ni-Cu-PGE sulfide deposits. Compared to silicate melts, immiscible magmatic sulfide liquids have lower viscosities (by 1 to 3 orders of magnitude), higher densities ( >1500 kg/m3 higher; also significantly higher than surrounding crust) and lower solidus temperatures (>150°C lower). These properties hinder the withdrawal of sulfide liquid from magma staging chambers and its ascent to higher structural levels, and allow for late-stage mobility and downward percolation along grain boundaries and fractures within wall rocks. Prior to emplacement towards higher crustal levels, sulfide liquid will have a tendency to accumulate and pond at the bottom of staging chambers. Later mobilization and upward or lateral withdrawal of sulfide can occur by viscous entrainment within relatively buoyant (mafic) magma. Analytical solutions (e.g., Blake and Ivey, 1986, JVGR, 27, 153-178) applied to the low viscosities, high densities and the likely relatively high volumetric flow rates (Q) in mafic melt-sulfide liquid systems predicts that significant draw-up of sulfide liquid within mafic magmas can occur at high, yet realistic flow rates (10-1000 m3/s), at reasonable Reynolds (>1000) and Weber numbers (0.07-700). At lower Q and Re draw up is hindered by interfacial tension, as indicated by the low capillary numbers of the system. We evaluate the dynamics of withdrawal and entrainment of sulfide liquid using analogue models scaled to mafic-sulfide system, focusing on flow rates and the viscosity and density ratios between sulfide and mafic magmas, in order to further explore the conditions under which sulfide liquids can be effectively entrained to promote the generation of deposits elsewhere in the magma system. The dynamics of sulfide percolation are analogous to the behavior of dense non-aqueous phase liquids (DNAPL). In fractured systems sulfide backflow is strongly controlled by the column height of the dense liquid. Because dense fluids flow downwards, this column generates a higher vertical body force if the network of down-flowing fluid remains interconnected. To enter a fracture, the fluid must overcome a capillary pressure which is inversely proportional to the width of a fracture; thinner fracture widths can be intruded as a body of dense fluid percolates downwards. Such a process, along with late stage mobility of sulfide liquid, could explain the presence of thin (cm to m sized) massive sulfide injections into wall rocks observed at Ni-Cu deposits such as Voisey's Bay (Labrador, CAN), Eagle (Michigan, USA), and Sudbury's late offset dykes.