A Preliminary Model for the Migration of Sulfide Droplets in a Magmatic Conduit and the Significance of Volatiles

Abstract

<jats:title>Abstract</jats:title> <jats:p>A close relationship between Ni–Cu–(PGE) sulfide deposits and magmatic conduit systems has been widely accepted, but our present understanding still rests on empirical inductions that sulfide liquids are entrained during magma ascent and aggregated at hydrodynamic traps such as the opening of a conduit into a larger magma body. In this contribution, a preliminary quantitative model for the dynamics of mm-scale sulfide droplets in a vertical magmatic conduit is developed, examining such limiting parameters as the size, transport velocity and the magmas’ maximum carrying capacity for sulfide droplets. Addition of numerous dense sulfide droplets significantly reduces magma buoyancy and rapidly increases the bulk viscosity, and the resulting pressure gradient in the propagating conduit dyke restricts the maximum volume fraction of droplets that can be carried by ascending magma. For sulfide droplets alone, the maximum carrying capacity is low, but it will be improved dramatically by the addition of volatiles which reduces the density and viscosity of silicate melt. Potential volatile degassing during decompression further facilitates sulfide entrainment by reducing bulk magma density, and the formation of buoyant compound vapour-sulfide liquid bubble drops also greatly enhances the carrying capacity. The breakdown of compound drops by detachment of parts of the vapour bubble or sulfide droplet may occur at low pressure, which liberates sulfide liquids from rising compound drops, potentially to collect in traps in the conduit system. When sulfide-laden magma flows through a widening conduit, many droplets can be captured by the re-circulation flow just downstream of the expanding section, followed by sulfide liquid accumulation and enhanced chemical interaction via diffusive exchange with the recirculating magma, potentially resulting in an economic, high-tonnage ore body. We apply our models to the emplacement of sulfide-rich magmatic suspensions at Noril’sk and show that the disseminated mineralization in intrusions could have formed when magmas carrying re-suspended sulfide liquid entrained from pre-existing sulfide accumulations in the conduit system reached their limiting sulfide carrying capacity as dictated by buoyancy and were deflected into blind sills flanking the principal conduit for flood basalt volcanism.</jats:p>

The Rustenburg Layered Suite formed as a stack of mush with transient magma chambers

“…The resulting contaminated magma will therefore comprise approximately one-third ultramafic solids and two thirds low-MgO basaltic liquid. This solid fraction is well within the range of mobile crystal suspensions that can travel through the crust with essentially Newtonian rheology and density lower than most crustal rocks 70 . The assimilation process occurs so easily that uncontaminated komatiites are rare, and it works to prevent the existence of superheated magmas, which cannot fail to react with and dissolve their containers of host rock.…”

“…The operation of this density filter leads broadly to the overall trends of vertical changes in mineral compositions, e.g., Mg # in mafic minerals and An% in plagioclase that superficially resemble the results of fractionation within individual magmatic lineages even if the denser ultramafic layers are in some cases younger than the mafic rocks above them 12 , 13 . However, many complicated and apparently stochastic reversals in mineral compositions are common in detailed profiles 24 , 43 , 46 , which can be attributed to the disordered emplacements of mushy macrolayers from similar but temporally discrete magmatic lineages, especially given that their emplacements are not determined by neutral buoyancy alone 70 .…”

Linking the Siberian Flood Basalts and Giant Ni‐Cu‐PGE Sulfide Deposits at Norilsk

“…The general idea of an open magmatic system with direct feeders between mineralized intrusions and overlying volcanic rocks has gained mainstream acceptance by most Western geologists, but many Russian geologists have long maintained that Norilsk‐type intrusions are blind and did not have exits for magma outflux to surface. If the intrusions truly are blind, then the very high proportion of sulfide within them demands that the sulfide first segregated at depth and then was carried into its present location in a dense sulfide‐silicate emulsion (Yao et al., 2019), whereas if the intrusions are feeders then sulfide might have been carried in very low modal abundance and dropped within the mineralized sills by large volumes of through‐going magma (e.g., Li, Ripley, & Naldrett, 2009; Naldrett et al., 1995). In either case, the dynamic process of sulfide entrainment and transport is still poorly understood (e.g., Barnes et al., 2020; Yao & Mungall, 2020; Yao et al., 2019).…”

“…Transport of sulfide‐rich contaminated magmas from the staging reservoir into shallow‐crustal mineralized sills is envisioned as follows, based upon modeling using the approaches described by Yao et al. (2019). Upon the entry of new magma with volume Δ V into a steady reservoir of constant volume V 0 without outflow confined within an elastic solid, the increase of magma overpressure Δ P follows (Anderson & Segall, 2011): where the compressibility of magma ( β m ) and wallrock ( β w ) are estimated as 5.67 × 10 −11 and 3 × 10 −11 Pa −1 , respectively.…”

“…Here we propose a comprehensive forward model that is able simultaneously to accommodate the sum of published major, trace, chalcophile element, and isotopic data for both the lavas and the mineralized intrusions. A key contribution of our model is an application of our quantitative assessment of the fluid‐dynamic constraints on the entrainment and upward transport of sulfide liquid by basaltic magmas (Yao et al., 2019).…”