Kinetic controls on the sulfide mineralization of komatiite-associated Ni-Cu-(PGE) deposits

Abstract

Genesis of magmatic Ni-Cu-(PGE) deposits involves many interconnected and multiple-scale processes in the fields of thermodynamics, kinetics and fluid dynamics. The main focus of quantitative work on magmatic sulfide ore genesis has been on equilibrium thermodynamics since the initial studies over 50 years ago, while the significance of kinetics and dynamics to sulfide mineralization in dynamic magmatic systems has received little attention. Taking the komatiite-hosted Ni-Cu-(PGE) deposits of the Raglan area as an example, we have developed a quantitative model for assimilation of sulfidic substrates during lava emplacement through the consideration of these three fields. We show that the dominant control on the composition of externally derived sulfide xenomelts throughout their residence within the flowing magma is exerted by relative rates of diffusion of sulfur and metals. In rapid channelized flow of komatiites, the kinetically-controlled extraction of chalcophile elements from silicate melt into externally-derived sulfide droplets released from underlying sediments is the quickest way to produce orebodies with economic value, without the necessity of reaching either sulfide saturation throughout the mass of magma or indeed compositional equilibrium in the magmatic system as our long-held understanding would suggest. The kinetic upgrading of initially metal-poor sulfide xenomelts is enhanced for sulfide droplets of smaller size and increasing downstream distance from the source, approaching to equilibrium mass distribution for the fast-diffusing elements such as Cu during prolonged or especially vigorous processes (e.g., chaotic magma mixing). In this kinetically-driven process, differences in diffusivities between Pt and Pd may account for the anomalous excess Pd/Pt ratios (~2) of disseminated ores from many komatiite-related deposits compared with the compositions of the parental magmas. Sedimentation of sulfide on the bottom of the flowing magma terminates this kinetically-controlled process, while the number density of droplets imposes a restriction on sulfide compositions due to mass conservation. Therefore, sulfide mineralization in komatiite-associated deposits is mostly controlled by diffusion kinetics with additional interactions and restrictions from the dynamics of transport and settling of sulfide droplets. By analogy, multiple cycles of dynamic processes predominate in the conduit-type magmatic deposits. A quantitative grasp of the relative importance of these three fundamental mechanisms, i.e., thermodynamic driving forces, kinetics of diffusive equilibration, and fluid dynamics of transport and deposition, is of great value in understanding ore genesis in complex mineral systems and even the origin of igneous rocks in general.

Introduction

Magmatic Ni-Cu-PGE sulfide deposits are mostly related to mafic and ultramafic magmas, formed as the end product of formation, transportation and concentration of immiscible sulfide liquids that have scavenged chalcophile elements from silicate magmas (Rajamani and Naldrett, 1978, Campbell and Naldrett, 1979, Barnes and Lightfoot, 2005, Arndt et al., 2005, Naldrett, 2010, Mungall, 2014, Robertson et al., 2015a, Barnes et al., 2016, Barnes and Ripley, 2016). Mantle-derived mafic–ultramafic magmas are generally undersaturated in sulfide until immediately prior to or during emplacement (Keays, 1982, Lesher and Groves, 1986, Lesher et al., 2001, Fiorentini et al., 2010) due to the negative effect of decreasing pressure on the sulfur content at sulfide saturation (SCSS) (Wendlandt, 1982, Mavrogenes and O’Neill, 1999, Holzheid and Grove, 2002). In order to achieve sulfide saturation in mantle-derived magmas in low-pressure environments, either a reduction of SCSS or enrichment of sulfur content in melts, or both, should occur, which can be attributed to numerous processes, e.g., extensive crystallization, magma mixing, crustal contamination and introduction of external sulfur (Naldrett, 2011, Ripley and Li, 2013). Because Ni is strongly compatible in olivine, the formation of Ni-rich magmatic sulfide is favored by early sulfide saturation due to assimilation of S-rich siliceous metasediments (Mungall, 2005; Ripley and Li, 2013). Evidence for the addition of externally derived sulfur into ore-forming mafic–ultramafic magmas is supported by sulfur isotope evidence from many komatiite-associated deposits (Green and Naldrett, 1981, Ripley, 1981, Lesher et al., 1984, Lesher and Groves, 1986, Ripley et al., 1999, Ripley et al., 2003, Ripley et al., 2007, Ripley et al., 2010, Lesher, 1999, Lesher, 2007, Lesher, 2017). Melting of sulfidic wall-rocks and xenoliths derived from thermomechanical erosion by komatiite lava flows or stoping by mafic magmas is the quickest process for liberating crustal sulfur (Robertson et al., 2015b), generating economic sulfide liquids. However, it is commonly presumed that the stable coexistence of sulfide with host magma requires bulk dissolution of a sufficient amount of the external sulfide xenomelts to bring the entire magma volume to a fully equilibrated state of sulfide saturation, which is a time-consuming kinetic process due to the extremely low diffusivity of sulfur in basalts (Freda et al., 2005). This sulfide liquid is then assumed to extract the chalcophile elements from surrounding melts.

The deep-rooted assumption of thermodynamic equilibrium is often approximately valid, but not always. Transportation, deposition, re-entrainment and accumulation of sulfides to form the ore bodies are mostly controlled by various dynamic processes, e.g., conduit propagation, magma emplacement, channeled flow, and gravity-dominated backflow in the funnel-like openings of magma conduits (De Bremond d’Ars et al., 2001, Robertson et al., 2015a, Saumur et al., 2015, Mungall et al., 2015, Barnes et al., 2016, Barnes and Robertson, 2019, Yao et al., 2019). Nucleation and growth of sulfide droplets occurs via diffusion transport of sulfur and chalcophile elements from the host melt, under conditions that may remain far from equilibrium throughout the formation of magmatic Ni-Cu-PGE deposits (Mungall, 2002a). Relative to equilibrium thermodynamics, research efforts on the role of non-equilibrium thermodynamics in ore-forming processes are rare and sporadic despite their importance. At any given length scale, the process operating at the shortest time scale dominates complex magmatic processes (Barnes and Robertson, 2019), with the result that magma flow exerts critical controls on ore genesis.

Komatiite-associated magmatic Ni-Cu-PGE sulfide deposits contain ~8–10% of the global Ni resources (Hronsky and Schodde, 2006), and most of them formed in dynamic lava channels or magma conduits by incorporation of crustal sulfur via the erosion of underlying sediments (Lesher et al., 1984, Lesher et al., 2001, Williams et al., 1998, Williams et al., 2001, Williams et al., 2011, Lesher, 1999, Lesher, 2007, Lesher and Barnes, 2009). Their ore bodies are commonly localized in footwall embayments (Lesher et al., 2001, Lesher, 2007, Lesher and Barnes, 2009), which served as fluid dynamic traps. Komatiite-associated deposits provide penetrating insight into the complex ore-forming processes operating in vigorous magma flows, insights that may be transferrable to magmatic mineral systems in general. Here, we take the komatiite-associated deposits at Raglan area for instance, to develop a comprehensive model that addresses the approach to thermodynamic equilibrium by considering kinetics and dynamics in an erosive lava flow, making a solid step towards a more fully quantitative description of magmatic sulfide ore genesis.