Conditions for Enrichment of Platinum Group Elements in Magma

      Platinum group elements (PGA) exhibit both chalcophilic and siderophilic geochemical properties. In silicate magmas, the partition coefficients of PGA in sulfide melts/silicate melts are very high (0.01 × 10⁶ to 1 × 10⁶), decreasing with increasing magmatic oxygen fugacity. The partition coefficients of different elements are ordered as Pd > Rh > Pt > Ru-Os-Ir, leading to different geochemical behaviors between Ir group (IPGE, including Os, Ir, and Ru) and Pd group (PPGE, including Rh, Pd, and Pt). Differentiation between Ir and Pd groups occurs during sulfide segregation.

      Under normal f(O2)(FMQ±1) conditions, Pt and Pd exhibit similar geochemical behaviors and are mainly found in sulfides. During the crystallization of monosulfide solid solutions (MSS) under sulfur saturation conditions, the partition coefficients of platinum group elements in monosulfide solid solutions and sulfide melts are ranked as follows: Ir (3.4~11)>Os (4.3)>Ru (4.2)>Rh (1.17~3.03)>>Pt (0.05~0.2) and Pd (0.09~0.2). Os, Ir, Ru, and Rh preferentially enter monosulfide solid solutions, while Pt and Pd remain in sulfide melts and exhibit differentiation.

      The proportion of platinum group elements extracted into the magma during partial melting in the magma source region, the conditions and controlling factors of unconventional PGE enrichment during magma evolution and emplacement, etc., are related to the occurrence form of PGE in the magma source region, the degree of partial melting, and the sulfur saturation of the magma.

Magma source region enrichment

      The composition and content of polygemonic gases (PGEs) in mantle-derived magma formed by partial melting of the magma source region mainly depend on the composition and content of PGEs in the mantle source region (i.e., the occurrence state of PGEs) and the release ratio (i.e., the degree of partial melting). The PGE content in mantle-derived magma primarily depends on the occurrence state of PGEs in the source region and the degree of partial melting.

1.Occurrence Forms of Platinum Group Elements

      Under high temperature and pressure conditions in the mantle, platinum group elements (PGEs) are found in PGE minerals, microparticle inclusions (element clusters), or alloys (such as Pt-Fe alloys), and their distribution is uneven. Mineral phases that control the distribution of PGEs include sulfides (including PGE sulfides and arsenides), nanoscale microparticle alloys, chromite (spinel), and some silicate minerals. The occurrence form of PGEs in mantle xenoliths and mafic rocks can reveal the occurrence state of PGEs in the mantle.

      Platinum group element (PGE) independent minerals mainly include sulfides (arsenides, antimony compounds, tellurides, bismuth tellurides, and sulfarsenides), native elements, intermetallic compounds, and alloys, as well as PGE-bearing minerals. PGE minerals include arsenopyrite (PtAs₂), native platinum, tellurite (PtTe₂), bismuth tellurite-palladium-nickel platinum[(NiPdPt)(TeBi)₂], bismuthate-palladium (PdBiTe), and hexagonal antimony-palladium (PtPd)Sb. Os, Ru, and Rh mainly substitute for Fe in isomorphous forms, occurring in pyrite and pyrrhotite; small amounts of Ir, Pt, and Rh exist primarily as tiny mineral particles in pyrite.

      PGE sulfides exist in two different forms: inclusions and intergranular deposits. PGE sulfides are present in the middle and lower parts of the Merenskey ore layer and the UG2 chromite ore layer in the Bushveld Complex in South Africa. In the Platreef contact zone, PGE mainly exists as half-metallic compounds such as tellurides and arsenides in silicate minerals.

      On the other hand, PGEs are encapsulated in crystalline minerals in the form of atomic bundles. When PGEs are supersaturated in silicate melts, heavy platinum group elements (PGA) tend to form microparticle inclusions. For example, olivine, chromite, and metal sulfides all contain PGE microparticle inclusions with a diameter of <100 nm, existing as clusters of PGE ions. The microparticle inclusions in olivine and chromite are composed of Pt-Pd±Au, while those in sulfides are composed of Os-Ir-Pt and Pt-Pd-Au.

      Alloys are also a common occurrence form of PGE. Under high f(O₂) conditions, PGE forms refractory Pt-Fe alloys. Individual euhedral alloys form on the surface of chromite, while Rh-Pt alloys exist between the grain surface and the melt. In the lower middle layers of the Merenskey and UG2 layers of the Bushveld Complex in South Africa, besides sulfides (such as sulfide-nickel-palladium-platinum ore, sulfide-platinum ore, and sulfide-ruthenium-osmium ore), the vast majority of PGE (65%–85%) occurs as Pt-Fe alloys within intergranular sulfides, with a small amount occurring as solid solutions in silicate minerals. Pt-Bi-Te alloys are preserved between silicate minerals, and Pt-As microparticle inclusions are preserved in pyrite, the main mineral containing PGE.

      Sulfides and atomic beams/alloys are likely the main occurrence forms of platinum group elements in the mantle, with sulfides having the strongest control over platinum group elements.

2. Partial melting enrichment of platinum group elements

      PGE in the mantle source region is preferentially hosted in sulfides. The PGE content in mantle-derived magma depends on the depth of magma origin and the proportion of sulfide melting, i.e., the depth of the source region and the degree of partial melting. Mantle plume magmas originating from the Earth's core or core-mantle boundary, as well as komatiites and picrites with high degrees of partial melting, have higher PGE contents. For example, mantle plume magmas such as Bushveld, Noril'sk, and Stillwater have formed super-large PGE magmatic deposits. Therefore, it is inferred that the Jinbaoshan type of PGE associated with the Emeishan mantle plume has great potential for mineralization.

      When the degree of partial melting in the mantle source region is low, Ni, PGE, and Cu remain in sulfides or olivine and are difficult to release into the magma. As the degree of partial melting increases, olivine begins to melt, releasing Ni into the mantle-derived magma. The melting of sulfides releases PGE and Cu into the magma, leading to a continuous increase in the Ni, Cu, and PGE content in the magma. Simulations show that when the degree of partial melting in the mantle source region is 10%, the Pt content in the original magma is 1.11 × 10⁻⁹, and the Pd content is 0.63 × 10⁻⁹. When the degree of partial melting is 18%, the sulfides in the mantle are nearly completely melted, and the Pt content in the magma is 18 × 10⁻⁹, and the Pd content is also 18 × 10⁻⁹.

      The main ore-forming magma types of platinum group metallogenic deposits are komatiitic magmas and tholeiitic basaltic magmas. Komatiitic magmas have a high degree of partial melting and are confined to the Archean (e.g., the Kambalda deposit) and Paleoproterozoic (e.g., the Thompson and Raglan deposits) periods of early Earth evolution. They exhibit a flat PGE distribution model, with (Pt+Pd)/(Os+Ir+Ru) = 3~3.5.PGE resources in tholeiitic basaltic magma deposits are higher than those in komatiitic magmas. These deposits are mainly developed in cratonic regions and exhibit a Pt-Pd enriched PGE distribution pattern, with (Pt+Pd)/(Os+Ir+Ru) = 5.6~55.6.During partial melting, IPGEs (Os, Ir, Ru) exhibit compatibility in olivine and remain in mantle peridotite, while Pt and Pd show incompatibility in olivine. Under certain temperature and oxygen fugacity conditions, the solubility of heavy platinum group metals (Os, Ir, Pt) in silicate melts is lower than that of light platinum group metals (Ru, Rh, Pd). In mantle-derived magma formed with a low degree of partial melting, Pt and Pd may differentiate, and the degree of Pt and Pd differentiation is closely related to the degree of partial melting.