Transport and deposition of immiscible sulfide liquid during lateral magma flow

Abstract

Magmatic sulfide deposits are generated by extraordinary accumulations of immiscible sulfide liquids in ultramafic-mafic magmas. While their associated geochemical and petrogenetic systems are well understood, the fluid dynamic processes that control the formations of ore bodies and textures are much less well known, particularly during lateral magma emplacement. Using a computational fluid dynamic modelling approach, we depict the transport and deposition of sulfides in two distinct laminar and turbulent horizontal flow settings that correspond to mafic intrusion-hosted and komatiite-hosted deposits, respectively. In horizontally flowing mafic magma, there is no vertical velocity component to offset the gravitational settling of sulfide liquid droplets, while the relatively high viscosity of mafic melt retards their deposition. Single droplets follow curved trajectories analogous to horizontal projectile motion and can be transported considerable distances before reaching the base of the flow, depending on their sizes and relative elevations in the flow, and the melt viscosity and flow velocity. Lateral motion of sulfide-rich mafic magma will eventually evolve into either stratified or plug flow with some pre-accumulation of sulfides in the lower parts of channels, which may be the precursors of economic ore bodies in mafic intrusions. While, volcanic or shallow subvolcanic komatiite systems are characterized by turbulent flows, in which the vertical velocity fluctuations can usually hold dense sulfide droplets in suspension, making long-distance transport of sulfides more feasible. The maximum diameter (d*sed) of an isolated sulfide droplet that can remain suspended in a turbulent flow with velocity U and melt viscosity μm is dsed∗ = (α ∙ μm ∙ U)0.5 for 1 ≤U≤ 15 m·s−1 and 0.1 ≤μm≤ 5 Pa·s, where α is a constant, 13.085 kg−1·s2. Stratification of suspended sulfides is rapidly achieved in the lower parts of sulfide-rich ultramafic flows, and the downward increasing gradients in sulfide proportions may partly account for the well-developed disseminated/net-texture/massive ore profiles commonly observed in komatiite-hosted deposits. When sulfide-laden ultramafic magma passes over a backward-facing step, representing the simplified geometry of a topographic low or embayment in a lava channel, sulfide droplet particle paths converge into the corner of this step, consistent with the field-based deductions that embayments are optimal dynamic traps for ore localization in komatiite flow. Trapping of sulfide droplets in embayments will be enhanced in cases with lower melt viscosity, slower flow velocity, larger droplet size and higher sulfide fraction. These model scenarios are comparable to the dynamics of dense crystals and/or xenoliths in flowing magmas, and their similarities and differences may shed light on some of the complex ore textures observed in magmatic sulfide deposits. The results herein further our knowledge of sulfide transport and deposition in lateral magma propagation and establish a link between dynamic processes and ore localization during ultramafic-mafic magmatism.