Computational Fluid Dynamic Modeling of Magma Evolution in Cooling Dykes and Sills with the Object of Understanding the Deposition of Economic Mineral Deposits Within Them
Finite element computational fluid dynamic modeling of heat and mass transport during the cooling of intrusive sills and their feeder dykes was undertaken to assess their evolution with time. Particular concern was given over to manifestations of convective scavenging as a mechanism leading to the formation of minerals of economic interest, e.g., the Platinum Group Elements (PGEs). The program sought to predict regions of high shear within the magma where one would expect enhanced collisions between the immiscible sulphide liquid particles and PGEs. These collisions scavenge PGEs from the primary melt, aggregating and concentrating them. Bagnold or dispersive pressure then transport the PGE enrichment into zero shear zones. Increasing the size of the sills, dykes in the models enhances scavenging and the time evolution calculations show that increasing the size of the dyke, sill or magma chamber results in stronger initial convection in contrast to smaller magma bodies. However, convection takes approximately the same time to cease for both models. This because the convective heat transfer is significantly more rapid for the larger geometries. The conclusion is that the time evolution of convective heat transfer is influenced more by viscosity than size. However, on cessation of convection (brought about by cohesive freezing), conductive heat transfer to e-folding temperatures were almost six times as long for the largest models then the smaller ones. Video animations that simulate the cooling process for these models are demonstrated. The simulations of smaller sized dykes replicate some deposits in Finland, the much larger ones that of the Great Dyke of Zimbabwe. The multi- physics Ansys Finite Element Code was employed in this effort.
Size and Spatial Distribution of VMS Deposits Produced by Hydrothermal Systems Driven by the Convective Cooling of Sill Intrusions
VMS districts are typically ∼40 km in diameter and contain about a dozen regularly spaced Cu-Zn orebodies, one or two of which contain more than half of the district's resources. We numerically investigate this deposit size and spatial distribution through two-dimensional finite element modeling of the convection in systems driven by a sill of a simple geometry and the system above the Bell River sill in the Matagami district, Quebec. In the heuristic models, convection is strongest at the edge, and the edge convection induces a subsequent progression of convection cells towards the center. The Matagami simulations are based on a sill that tapes from 6.5 to 0 km thickness over a distance of 30 km. The zinc transport across the seafloor is dominated by those hydrothermal plumes driven by the strong horizontal gradient in temperature alongside the vertical portion of the retreating 350°C isotherm of the edges of the cooling intrusion. Convection occurs both above the sill and along its underside, and metal is extracted from both sides of the cooling sill.
Evaluation of the Chemical Reactivity of the Fluid Phase Through Hard-Soft Acid-Base Concepts in Magmatic Intrusions with Wpplications to Ore Generation
Ore genesis, when associated with felsic magmatism, develops from metals scavenging from the melt into the fluid, gaseous, phase. Its composition includes water, CO2, sulphur under several possible species, and halogens, mainly F and Cl. The respective influence of those elements is examined by computing the theoretical electronegativity and chemical hardness of the fluid phase. Those parameters are commonly larger for the fluid phase than for the silicate melt. Indeed a common hardness value for the melt is around 3.5 eV, whereas the computed values for the fluid phase are around 7.5 eV for pure water, with departure ranging from 6.9 eV in case of S-rich fluid, to 8.8 eV in case of a F-rich fluid. Since metals show tendency to present electronegativity above 15 eV and high hardness, the fluid phase is very attractive for metals. The influence of S, under its various non-detailed species, is to decrease both electronegativity and hardness. It therefore favours segregation of soft metals, as Cu, Ag and Au. Since F- is the hardest base, it increases both electronegativity and hardness, making the fluid phase attractive to Sn and W. Cl- present contrasted effects, since it decreases the hardness, but increases the electronegativity. It could be of influence in the segregation of Fe in iron-oxide- copper-gold (IOCG) porphyry deposits, though mixing between magmatic and evaporitic fluids make the situation quite complex. The chemical character of the fluid phase also explains the discrepancy existing for metal solubility, as well as for the redox conditions, between the melt and the fluid phase. The change in oxidation state induced by a hard fluid, i.e. F-rich, promotes oxidation, for instance from Sn(II) to Sn(IV) or reduction in case of a soft, i.e. S-rich, fluid phase, from Mo(VI) to Mo(IV). The bulk electronegativity and hardness of the fluid phase modify the redox state of the metals during transportation, before condensation. The semi-quantitative model provides a new insight on the chemical conditions of metals segregation and transportation through the magma before ore formation.
Role of Sulfur in the Formation of Magmatic-Hydrothermal Copper-Gold Deposits
Sulfur plays essential roles in hydrothermal ore-forming processes , which calls for precise and accurate quantitative sulfur determination in fluid inclusions. Feasibility tests for sulfur quantification by comparing data from both LA-Quadrupole (Q) - ICP-MS and LA-High Resolution (HR) - ICP-MS show that reliable sulfur quantification in fluid inclusions is possible , provided that a very careful baseline correction is applied. We investigate the metal transporting capabilities of sulfur by measuring sulfur together with copper and other elements in cogenetic brine and vapor inclusions ('boiling assemblages') in single healed crack hosted by quartz veins. Samples are from high-temperature magmatic-hydrothermal ore deposits and miarolitic cavities of barren granitoid. Clear compositional correlations of sulfur with copper and gold were found. A molar S/Cu ratio commonly close to 2 but never above 2, indicates sulfur-complexed metal transportation in the high-temperature hydrothermal vapor, and probably also in the Na-Fe-K-Cl-enriched brines. Vapor/brine partitioning trends of the S and Cu are shown to be related with the chemistry of the fluids (possibly by various sulfur speciations in varying pH, fO2) and causative magma source. In the boiling hydrothermal environments, higher vapor partitioning of Cu and S is observed at reduced and peraluminous Sn-W granite, whereas oxidized and perakaline porphyry-style deposits have a lower partitioning to the vapor although the total concentration of S, Cu, Au in both fluid phase is higher than in the Sn-W granite . Vapor inclusion in the boiling assemblages from magmatic-hydrothermal ore deposits and granitic intrusions generally contain an excess of sulfur over ore metals such as Cu, Fe, and Mo. This allows efficient sulfide ore precipitation in high-temperature porphyry-type deposits, and complexation of gold by the remaining sulfide down to lower temperatures. The results confirm earlier interpretations  and recent laboratory experiments , indicating that sulfur is the key component determining the efficiency of ore formation in porphyry-style and epithermal systems.  Heinrich et al. (1999) Geology  Guillong et al. (2008) J.Anal. At. Spectrom.  Seo et al. (2009) Earth Planet. Sci. Lett. in review.  Pokrovski et al. (2008) Earth Planet. Sci. Lett.
Analog Experiments on Sulfide Foams in Magmatic Ore Deposits
Metal sulfides form as an immiscible phase from silicate magmas. Dynamic mingling and unmingling of the two phases is important for the development of economic deposits: mingling promotes enrichment of the sulfide in valuable metals, and subsequent unmingling generates massive sulfide. Analog experiments were carried out to investigate mingling processes in immiscible systems, using oil, water and small beads to represent magma, sulfide liquid and silicate crystals. Stirring or injection led to the formation of a foam of analog sulfide droplets within an analog silicate framework. We propose that the partial collapse of such a foam explains massive sulfide lenses at the Voisey's Bay magmatic sulfide deposit, and that crystallization of silicate crystals in the remaining foam walls generates 'net-textured' ores. In the experiments, solid particles had a profound effect on unmingling: analog sulfide droplets were stably contained within analog crystal-rich magma and did not coalesce. We therefore suggest that 'net' and 'leopard' textures in disseminated sulfides indicate mingling of sulfide with crystal-poor magma, whereas isolated disseminated patches of sulfide indicate mingling with a crystal-rich magma.
The Effect of Sulfur Fugacity on Pt, Pd and Au in Magmatic-Hydrothermal Systems
We have constrained experimentally the effect of sulfur fugacity (fS2) and sulfide saturation on the fractionation and partitioning behavior of Pt, Pd and Au in a felsic silicate melt + sulfide crystal/melt + oxide + supercritical aqueous fluid phase + Pt + Pd + Au system. Experiments were performed at 800°C, 150 MPa, with oxygen fugacity (fO2) fixed at approximately the nickel + nickel oxide buffer (NNO). Sulfur fugacity in the experiments was varied five orders of magnitude from approximately logfS2 = 0 to logfS2 = -5 by using two different sulfide phase assemblages. Sulfide assemblage one consisted initially of chalcopyrite plus pyrrhotite and assemblage two consisted of chalcopyrite plus bornite. At run conditions, in both assemblages, pyrrhotite transformed compositionally to monosulfide solid solution (mss), chalcopyrite to intermediate solid solution (Iss), and in assemblage two chalcopyrite and bornite formed a sulfide melt. Run- product silicate glass (i.e., quenched silicate melt) and crystalline materials were analyzed by using both electron probe microanalysis (EPMA) for major elements and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for major and trace elements. The measured concentrations of Pt, Pd, and Au in quenched silicate melt in runs with logfS2 values ranging from approximately 0 to -5, do not exhibit any apparent dependence on the dissolved sulfur content of the melt. The measured Pt, Pd and Au concentrations in mss vary as a function of fS2. The measured Pt, Pd and Au concentrations in Iss do not appear to be dependent on fS2. The system variables fS2 and fO2, working in concert with each other, control the stable magmatic sulfide phase assemblage. Additionally, the system fS2 strongly influences the solubility of Pt, Pd, and Au as lattice bound components in some common crystalline magmatic sulfide phases. Both the stable magmatic sulfide phase assemblage and the solubility of Pt, Pd, and Au as constituents in stable crystalline sulfide phases vary as a function of fS2. Our data suggest that the dynamic, temporal evolution of fS2, inherently linked with fO2, plays a determinate role in the Pt, Pd, and Au budget in sulfide-oxide saturated, ore-genitive magmatic systems.
The Effect of Initial Concentrations on R- and L-Factor Upgrading of Magmatic Sulfides
A popular model for explaining enrichment of valuable metals in magmatic sulfide deposits is the "R" factor model, which is a simple mass-balance formulation wherein metals in the sulfide phase are scavenged from a silicate magma of R times the mass of the sulfide [Campbell & Naldrett, Econ Geol, 1979]. If the mass of sulfide stays constant - that is, the magma is exactly sulfur-saturated so sulfide neither dissolves nor precipitates - the maximum sulfide metal concentration Xmax = D*X0, where D is the distribution coefficient and X0 is the metal concentration in the magma. This maximum is reached when R ∼ 10*D. Large R-factors, allowing greater enrichment, may be achieved if the sulfide equilibrates with several smaller batches of silicate, for example in a flowing conduit. An extension of the R-factor model, the "L" factor model [Kerr & Leitch, Econ Geol, 2005], showed that if the magma is sulfur under-saturated so that the sulfide dissolves, then greater enrichment of the remaining sulfide, Xmax > D*X0, is possible, and that "runaway" upgrading may occur if dissolution rates and D are both high. Here we present a new, improved formulation of the L-factor model in terms of the R-factor and magma undersaturation, dS. We show that runaway upgrading occurs if the product D*dS is over a critical value. We include the influence of a given initial metal concentration Xi in the sulfide, to allow for the situation where the magma that deposited the sulfide is different from the one that dissolves it. We show that 'washing out' of the initial metal concentration Xi in favour of the limiting concentration Xmax occurs over the same range of R-factors as the upgrading, even when Xi > Xmax.