Formation Of Magmatic Sulfide Deposits Research Articles

Redox control of the partitioning of platinum and palladium into magmatic sulfide liquids

The partitioning behavior of platinum group elements in magmas is critical for their use as tracers of planetary accretion and in understanding magmatic sulfide deposits. Here we use laboratory experiments to determine sulfide liquid–silicate melt partition coefficients for platinum and palladium at 1.5 GPa, 1400 °C, and oxygen fugacity 1.5–2 log units above the fayalite–magnetite–quartz buffer. We find that the partitioning coefficients of these elements are 2.3 × 105 to 1.1 × 106 and are independent of the platinum and palladium concentration in the system. Combined with previous data obtained at oxygen fugacity below the fayalite–magnetite–quartz buffer, this indicates redox-controlled partitioning behavior whereby at oxidizing conditions platinum- and palladium-enrichments are achieved through their dissolution in sulfide liquids, while at reducing conditions the entrapment of platinum- and palladium-rich clusters in sulfide liquids is more critical. This redox-controlled partitioning behavior should be considered when studying crust–mantle differentiation and the formation of magmatic sulfide deposits.

Experimental Investigation on the Transport of Sulfide Driven by Melt‐Rock Reaction in Partially Molten Peridotite

AbstractExtraction of sulfides from the partially molten mantle is vital to elucidate the cycling of metal and sulfur elements between different geochemical circles but has not been investigated systematically. Using laboratory experiments and theoretical calculations, this study documents systematical variations in lithologies and compositions of silicate minerals and melts, which are approximately consistent with the results of the thermodynamically‐constrained model. During a melt‐peridotite reaction, the dissolution of olivine and precipitation of new orthopyroxene generate an orthopyroxene‐rich layer between the melt source and peridotite. With increasing reaction degree, more melt is infiltrated into and reacts with upper peridotite, which potentially enhances the concomitant upward transport of dense sulfide droplets. Theoretical analyses suggest an energetically focused melt flow with a high velocity (∼170.9 μm/hr) around sulfide droplets through the pore throat. In this energic melt flow, we, for the first time, observed the mechanical coalescence of sulfide droplets, and the associated drag force was likely driving upward entrainment of fine μm‐scale sulfide. For coarse sulfide droplets whose sizes are larger than the pore throat in the peridotite, their entrainment through narrow constrictions in crystal framework seems to be physically possible only when high‐degree melt‐peridotite reaction drives high porosity of peridotite and channelized melt flows with extremely high velocity. Hence, the melt‐rock reaction could drive and enhance upward entrainment of μm‐ to mm‐scale sulfide in the partially molten mantle, potentially contributing to the fertilization of the sub‐continental lithospheric mantle and the endowment of metal‐bearing sulfide for the formation of magmatic sulfide deposits.

Sulfur in ore formation

Two sections of general physics – thermodynamics and molecular physics – have common goals and objectives, but differ sharply in research methods. Thermodynamics studies the macroscopic properties of bodies and natural phenomena, abstracting from the essence of molecular processes. Molecular physics, on the contrary, strives to penetrate into this essence, but has difficulties with the analysis of complex macrosystems. The thermodynamics that absolutely dominates now, abstracting from the molecular essence and proceeding from the equality of macrophysical characteristics, identifies the supercritical state of a substance with a liquid. At the same time, the main property of supercritical gases is ignored – the absence of intermolecular bonds, and consequently, the high mobility of gases relative to the host condensed substances. In this paper we consider the physicochemical features of the behavior of sulfur. The critical temperature of sulfur is in the area of the possible existence of magmatic melts, and the isolines of its saturated vapors are propagated in the temperature range of postmagmatic and hydrothermal processes. The evolution of endogenous gas mixtures causes the possibility of sudden condensation of sulfur. As a result, a high-temperature and highly active reagent occurs in the path of endogenous gas flows. Sulfur condensate is capable of capturing metals from any compounds transported by the gas stream and accumulating them as part of sulfide ore concentrations. In the future, conditions may change, and sulfur may leave these ore concentrations, being replaced by metasomatic oxygen. This is how oxide ores are formed. The physicochemical properties of sulfur determine the formation of deposits of sulfide and post-sulfide oxide ores. This makes it possible to create a unified theory of the formation of endogenous ore deposits. We consider model schemes for the formation of magmatic sulfide deposits (Norilsk, Monchegorsk, Pechenga, Allarechensk), Bushveld oxide ores, iron deposits of the El Laco volcano and the Kambalda deposit, pyrite deposits and volcanic deposits of native sulfur. These schemes can be considered as the molecular-chemical basis of a possible theory of ore formation. However, in order to create a full-fledged theory of ore formation, it is necessary to clarify this basis using experimental and theoretical methods of thermodynamics. A small inaccuracy made in the physical sciences in the interpretation of the supercritical state of substance has grown into a big problem in the geological sciences, studying substance in supercritical (for volatiles) conditions. Now this problem affects the entire foundation of the geological sciences, hindering their further development. We are convinced that the elimination of the problem and the balanced combination of the molecular “eyesight” of physical science with its thermodynamic “power” will ensure significant progress of knowledge in geosciences.

Phase Relations in the Fe–S–C System at P = 0.5 GPa, T = 1100–1250°C: Fe–S–C Liquation and Its Role in the Formation of Magmatic Sulfide Deposits

Phase Relations in the Fe–S–C System at P = 0.5 GPa, T = 1100–1250°C: Fe–S–C Liquation and Its Role in the Formation of Magmatic Sulfide Deposits

Settling of Immiscible Droplets: A Theoretical Model for the Missing Link Between Microscopic and Outcrop Observations

AbstractLiquid immiscibility is a critical mechanism to diversify magma compositions. The physical separation of exsolved melt droplets is an essential process in generating new magmas. However, little attention has been paid to this physical process. In this study, we present a new model for segregation of immiscible melt droplets in which exsolution, settling, and coalescence are all considered. The separation of immiscible droplets is similar to that of crystals in magma when the discrete melt exsolves as large (millimeter‐size) droplets and/or when the magma cools slowly. However, when immiscible melt droplets are small (micrometer‐size) and/or magma cools rapidly, coalescence can significantly enhance their separation. The low interfacial tension between coexisting silicate melts leads to the exsolution of extremely small melt droplets. Furthermore, the high viscosity of silica‐rich melt suppresses the coalescence of droplets. Consequently, the separation of highly viscous silica‐rich melt droplets is slow but could have occurred in a slowly cooling magma such as the Skaergaard intrusion. By contrast, the coalescence rate of melt droplets with low viscosity is high. Iron‐rich melt droplets, which have low viscosities, could have been separated from hydrous andesitic melts to form magnetite‐apatite ore deposits at El Laco and Marcona. Furthermore, the viscosities of sulfide and carbonatitic melts are low. Therefore, immiscibility between sulfide and silicate melts may lead to the formation of magmatic sulfide deposits, and immiscibility between carbonatitic and silicate melts can support periodical eruptions of carbonatitic lava.

Concentrations of Te, As, Bi, Sb and Se in the Marginal Zone of the Bushveld Complex: Evidence for crustal contamination and the nature of the magma that formed the Merensky Reef

The association of platinum-group elements (PGE) and the chalcophile elements Te, As, Bi, Sb and Sn (TABS) has been extensively documented in several magmatic sulfide deposits over the past years. However, understanding the roles of TABS during the formation of magmatic sulfide deposits partially depends on constraining the concentration of TABS on the liquids from which they crystallized. This study presents the distribution of TABS (apart from Sn) in rocks of the Marginal Zone of the Bushveld Complex. These rocks record the composition of the parental magmas from which the Bushveld Complex have crystallized. The major and trace elements of the marginal rocks have been modelled as mixtures of komatiite, continental crust and a plagioclase-rich residuum. Similar mixtures are required to model the TABS in the marginal rocks, with the continental crust component contributing a large part of the As, Sb and Bi budgets in these melts. The concentrations of the TABS in the Merensky Reef can be modelled as a mixture of two of the magma types present in the Marginal Zone (the B-1 and B-2). The modelling also reveals that the distributions of Se, Te and Bi in the reef are essentially controlled by the presence of sulfide minerals, whereas As and Sb distributions are controlled by both sulfide minerals and melt component. This is because Se, Te and Bi are moderately to strongly chalcophile elements, but As and Sb are only slightly chalcophile elements. Thus, whole-rock As and Sb concentrations are not upgraded by the formation of the sulfide minerals, and may still be used to trace crustal contamination.

Tectonic controls on Ni and Cu contents of primary mantle-derived magmas for the formation of magmatic sulfide deposits

Tectonic controls on Ni and Cu contents of primary mantle-derived magmas for the formation of magmatic sulfide deposits

Cu diffusion in a basaltic melt

Recent studies suggest a potential role of diffusive transport of metals (e.g., Cu, Au, PGE) in the formation of magmatic sulfide deposits and porphyry-type deposits. However, diffusivities of these metals are poorly determined in natural silicate melts. In this study, diffusivities of copper in an anhydrous basaltic melt ( 2 O) were measured at temperatures from 1298 to 1581 °C, and pressures of 0.5, 1, and 1.5 GPa. Copper diffusivities in anhydrous basaltic melt at 1 GPa can be described as: D Cu basalt = exp [ − ( 14 . 12 ± 0 . 50 ) − 11813 ± 838 T ] where D Cu basalt is the diffusivity in m 2 /s, T is the temperature in K, and errors are given at 1σ level. A fitting of all experimental data considering the pressure effect is: D Cu basalt = exp [ − ( 13 . 59 ± 0 . 81 ) − ( 12153 ± 1229 ) + ( 620 ± 241 ) P T ] where P is the pressure in GPa, which corresponds to a pre-exponential factor D 0 = (1.25 ×÷ 2.2)×10 −6 m 2 /s, an activation energy E a = 101 ± 10 kJ/mol at P = 0, and an activation volume V a = (5.2 ± 2.0)×10 −6 m 3 /mol. The diffusivity of copper in basaltic melt is high compared to most other cations, similar to that of Na. The high copper diffusivity is consistent with the occurrence of copper mostly as Cu + in silicate melts at or below NNO. Compared to the volatile species, copper diffusivity is generally smaller than water diffusivity, but about one order of magnitude higher than sulfur and chlorine diffusivities. Hence, Cu partitioning between a growing sulfide liquid drop and the surrounding silicate melt is roughly in equilibrium, whereas that between a growing fluid bubble and the surrounding melt can be out of equilibrium if the fluid is nearly pure H 2 O fluid. Our results are the first copper diffusion data in natural silicate melts, and can be applied to discuss natural processes such as copper transport and kinetic partitioning behavior in ore formation, as well as copper isotope fractionation caused by evaporation during tektite formation.

Toward a quantitative model for the formation of gravitational magmatic sulfide deposits

A preliminary quantitative model for the formation of magmatic sulfide deposits through gravitational sulfide droplet settling is developed. The model incorporates thermodynamic consideration of the oversaturation of sulfide liquid from a silicate melt, and the coupled growth kinetics and settling dynamics of sulfide droplets. The conditions for the sulfide droplets to have enough time to grow and settle to form a sulfide liquid layer at the bottom of a magma chamber are referred to as the necessary criteria for sulfide ore formation. Simulations are carried out for dry Etna basaltic melt because their melt viscosity and sulfur diffusivity have been measured so that the simulations are more realistic rather than parametric. Furthermore, the effects of empirical nucleation rate and condition, and of the variation of solubility, viscosity and diffusivity have been parametrically evaluated. The simulations show that for a given magma containing about 0.1 to 0.3wt.% sulfur, sulfide melt layer can form for realistic magma body size (with cooling time of 1000yr or more) and realistic solubility, viscosity and diffusivity. The ability of the sulfide melt to collect ore elements depends critically on the diffusivity of individual ore elements. When the diffusivity of an ore element is similar to or greater than that of sulfur, near equilibrium partitioning is reached. When the diffusivity of an ore element is much smaller than that of sulfur, which is often the case for trivalent and tetravalent ions, the concentration of the element in the sulfide melt would be far below the equilibrium concentration. It is hoped that the model will help both researchers and exploration geologists.

Nickeliferous gabbroic intrusions of the Pants Lake area, Labrador, Canada: Implications for the development of magmatic sulfides in mafic systems

The Pants Lake intrusions are located some 80 kilometers south of the world-class Voisey's Bay Ni-Cu-Co sulfide deposit in Labrador, Canada. They are dominated by olivine gabbro, with subordinate troctolite, melagabbro, peridotite and leucogabbro. They form several sheet-like bodies that were emplaced into metasedimentary gneisses that locally contain sulfides. The largest of these, termed the North and South intrusions, have been dated by other workers at 1322 ′ 2 Ma and 1337 ′ 2 Ma respectively, indicating that they formed in two discrete events. The North intrusion is more varied petrologically, and its three units show ambiguous contact relationships implying that they partly coexisted as liquids. Disseminated sulfide mineralization is widespread near the basal contacts of both intrusions, but massive sulfide mineralization is rare. In the older South intrusion, sulfides are hosted by melagabbro and peridotite of cumulate origin, and probably represent a gravitational accumulation. In the younger North intrusion, they are hosted by a complex mineralized sequence, interpreted to represent two or more influxes of magma charged with sulfides and reacted country-rock fragments. The older South intrusion is geochemically distinct from the younger North intrusion, and more closely resembles the Voisey's Bay Intrusion. Mineralized rocks within both Pants Lake intrusions are geochemically similar to the associated unmineralized rocks, and must be closer related to them. The mafic rocks almost all have low Ni contents and low Cu/Zr ratios that imply extraction of metals by sulfide liquids. This pervasive depletion signature, coupled with consistent Ni/Cu and Ni/Co, and clustering of sulfide metal contents, suggests that sulfide liquids were developed on an intrusion-wide scale, probably at depth, rather than by local processes. Sulfides at Pants Lake typically contain less than 2 percent Ni and 2 percent Cu. These values agree with predictions from the low Ni contents of silicate rocks, and imply a low mass ratio (R factor) of silicate magma to sulfide liquid (R 1000). The pervasive Ni and Cu depletion contrasts with the more localized metal depletion seen at Voisey's Bay. This may indicate that the Pants Lake sulfide liquids were not as effectively upgraded by later batches of undepleted magma. From the perspective of metallogenesis, data from the Pants Lake intrusions support several key concepts proposed m models for the formation of the Voisey's Bay deposit, suggesting that these may apply more widely as controls on the formation of magmatic sulfide deposits in broadly gabbroic magma systems.

Analog experimental insights into the formation of magmatic sulfide deposits

Abstract Magmatic sulfide deposits result from transport and settling of dense sulfide liquid droplets in basic magma. To model these processes, we carried out analog experiments in which dense silicone oil droplets were transported in a viscous aqueous polymer solution. The results of the experiments indicate that sulfide droplets can be carried in suspension by magma flowing at typical velocities. Due to the large surface tension of sulfide droplets, coalescence does not occur during transport. Transport ceases when upward magma velocity decreases because of conduit enlargement or change from vertical to horizontal flow. Coalescence of droplets is controlled by magma viscosity and the presence of crystals.

Critical factors for the formation of a nickel-copper deposit in an evolved magma system: lessons from a comparison of the Pants Lake and Voisey's Bay sulfide occurrences in Labrador, Canada

The Voisey's Bay nickel–copper deposit and the Pants Lake sulfide occurrences are the most important mineral systems discovered to date within the Nain Plutonic Suite in northern Labrador. There are many intriguing similarities at both locations. Both are hosted by relatively small troctolite/gabbro bodies that intrude the sulfide-bearing paragneiss of the Churchill Province, and these intrusions contain inclusions of the paragneiss. Similar chemical reactions of the gneissic inclusions with the host magmas are observed at both locations. The reactions resulted in the addition of SiO2, K2O, Na2O and sulfur to the magmas, and are responsible for sulfide-saturation and resultant segregation of immiscible sulfide liquids from the magmas. The initial sulfide liquids in both cases were relatively poor in metals, containing <2.5 wt% Ni and 2 wt% Cu. The sulfides at Pants Lake remained poor in metals because of a lack of subsequent interaction with new, chalcophile-undepleted magma. At Voisey's Bay, the initial sulfides segregated in a dynamic conduit, and were subsequently upgraded in metals to ∼6 wt% Ni and 3 wt% Cu by a new surge of undepleted magma using the same conduit. These sulfides were then concentrated to form large sulfide bodies in the wider parts of the conduit and its entry to an upper chamber in response to a sudden change of liquid velocity in these environments. This study confirms three of the most important factors for the formation of magmatic sulfide deposits in an evolved magma system: (1) contamination of magma with sulfide-bearing country rock to achieve sulfide saturation; (2) a dynamic magmatic system such as a magma conduit to transport large volume of sulfide liquid and to concentrate them in limited localities, and (3) upgrading of metals in the sulfide by new, chalcophile-undepleted magma.