Sulfide Liquid Entrainment by Silicate Magma: Implications for the Dynamics and Petrogenesis of Magmatic Sulfide Deposits

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Abstract

Many intrusion-hosted Ni–Cu magmatic sulfide deposits, such as Voisey’s Bay (Labrador, Canada) and Jinchuan (China), show evidence for late-stage mobilization of high-density, low-viscosity sulfide-rich melts, which range from sulfide-rich silicate magma pulses to coherent pulses of pure sulfide liquid. Here we investigate the possibility that such sulfide-rich pulses can be generated by the upward entrainment of pooled sulfide liquid by immiscible silicate magma within a two-phase, density-stratified staging chamber. The potential degree of entrainment is measured by the critical draw-up height d , defined as the height of the overlying silicate magma at which the lower sulfide liquid will be mobilized and entrained during withdrawal of silicate magma from the chamber. Previous analytical solutions for axisymmetric withdrawal from a density-stratified reservoir have focused on the volcanology of bimodal, felsic–mafic silicate magma systems. We present new laboratory experiments of axisymmetric withdrawal that are tuned to the physical properties of mafic–sulfide systems, and find that the draw-up scales previously applied to felsic–mafic systems can be applied to both viscous and inertial entrainment of sulfide liquid into silicate magma. Within viscous (Reynolds number, Re < 10) to transitional regimes (Re ∼ 10–1000) d is dependent on the viscosity of the mafic layer, but independent of the viscosity of the sulfide layer; at a given volumetric magma flow rate ( Q ), higher viscosity mafic magmas will be more efficient sulfide entrainers than lower viscosity magmas. Entrainment is independent of viscosity within inertial regimes (Re > 1000). In all dynamic regimes d is proportional to Q and inversely proportional to the density difference between the fluids. Significant mobilization and entrainment of sulfide liquid, as indicated by predicted draw-up cones with d between 1 and 10 m, can occur under inertial conditions at high ( Q ∼ 1–1000 m 3 s –1 ) magma volumetric flow rates that are plausible for mid- to upper-crustal magma chambers. Although entrainment can potentially generate sulfide-rich magma pulses, it probably represents only an initial step in the mobilization of sulfide liquid and the genesis of magmatic sulfide deposits. Entrained sulfide liquids must be transported upwards and/or laterally from their site of mobilization towards their site of deposition, which may occur as coherent masses or as droplets depending on the dynamics of the system. Although entrainment may be viable at the scale of sills, dykes, chonoliths and lava flows, it may not be significant in scenarios where significant (>100 m) thicknesses of molten mafic magma must be expelled prior to reaching 1–10 m scale critical draw-up heights; in such scenarios gravitational backflow of sulfide may be a more viable mechanism for producing economic concentrations of Ni–Cu sulfide mineralization.

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