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The Canadian Mineralogist Vol. 49, pp. 1397-1412 (2011) DOI : 10.3749/canmin.49.6.1397
ORIGIN OF PLATINUM-GROUP MINERALS IN MAFIC AND ULTRAMAFIC ROCKS: FROM DISPERSED ELEMENTS TO NUGGETS Alexander V. OKRUGIN§ Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, 39 Lenin Street, Yakutsk 677980, Russia
Abstract Based on the studies of paragenetic assemblages of platinum-group minerals (PGM) from placers and chromitite ores associated with zonal-ring dunite intrusions in the Aldan Shield, on the Siberian platform, a novel model for the PGM formation is offered, which implies separation of a Cr-rich oxide liquid with significant concentration of the platinum-group elements (PGE) from a picritic magma. The presence of PGM alloys in sulfide-poor chromitite ores from layered intrusions and zonal gabbro–dunite massifs indicates that PGE mineralization is genetically related not just to sulfide formation but to chromite formation as well. A hypothetical (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) triangular diagram shows that fractional crystallization of olivine results in the enrichment of the residual melt in Cr, PGE, a range of metals, and easily fusible and volatile elements, and in magma separation into a light silicate-rich liquid and a dense PGE–Cr-rich oxide liquid. In fluidsaturated magma, a Cr-enriched oxide melt completely separates from a silicate melt, its crystallization temperature is lower, and the solubility of PGE is higher. It is likely that the presence of PGE in the Cr-rich oxide liquid takes the form of sybotactic domains, clusters surrounded by ligands of easily fusible and volatile elements, which prevents the PGE from removal via an untimely crystallization from the melt as micronuggets. Crystallization of a PGE-, Cr-rich oxide melt can be traced in the schematic chromite – Os – Pt ternary diagram proposed, which shows a wide field of immiscibility existing between chromite and the PGE. Owing to the low solubility of PGE in chromite, crystallization of the Cr-rich melt results in accumulation of PGE in the interstitial liquid. With IPGE prevailing over PPGE, the crystallization trend does not reach the immiscibility field, ending in synchronous crystallization of chromite and small grains of Pt–Ru–Ir–Os alloys. In case of Pt dominance, the crystallization trend reaches the immiscibility field, Pt gradually accumulates in the residual melt, with final crystallization occurring at a triple eutectic point, which creates favorable conditions for the formation of large Pt nuggets from tens of grams to a few kilograms in weight, which are characteristic only of PGE–chromite deposits of the Uralian–Alaskan and Aldanian types. Keywords: platinum-group minerals, chromite, oxide melt, liquid immiscibility, mafic-ultramafic rocks, Inagli massif, Siberian platform, Russia.
Sommaire Compte tenu des résultats d’études paragénétiques de minéraux du groupe du platine (MGP) des placers et des minerais de chromitite associés aux intrusions annulaires de dunite du bouclier d’Aldan, sur la plateforme sibérienne, on présente un modèle nouveau pour expliquer la formation des MGP qui implique la séparation d’un liquide à base d’oxydes contenant une quantité importante d’éléments du groupe du platine (EGP) à partir d’un magma picritique. La présence d’alliages de EGP dans les minerais de chromitite à faible teneur en soufre dans les complexes stratiformes et les massifs zonés de gabbro–dunite indique que la minéralisation en EGP serait génétiquement liée à la formation non seulement de sulfures, mais aussi de chromite. Un diagramme triangulaire hypothétique (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) montre que la cristallisation fractionnée d’une olivine mène à un enrichissement du liquide résiduel en Cr, EGP, une gamme de métaux, et les éléments volatils et facilement fusibles, et à une séparation du magma en un liquide léger siliceux et un liquide à oxydes denses enrichi en EGP et en Cr. Dans un magma saturé en composants fluides, un magma à base d’oxydes peut se séparer du liquide siliceux, sa température est inférieure, et la solubilité des EGP y est plus élevée. Dans le liquide à d’oxydes enrichi en Cr, la présence des EGP est sous la forme de domaines sybotactiques, ce qui aurait empêché les EGP de précipiter prématurément par nucléation de micropépites. On peut tracer la cristallization d’un magma à base d’oxydes enrichi en EGP et en Cr avec un diagramme schématique chromite – Os – Pt, qui montre un champ étendu d’immiscibilité entre chromite et les EGP. A cause de la très faible solubilité des EGP dans la chromite, la cristallisation du liquide enrichi en Cr mène à l’accumulation des EGP dans le liquide interstitiel. Dans le cas où les IPGE prédominent par rapport aux PPGE, le tracé de la cristallisation n’atteint pas le domaine de l’immiscibilité, ce qui termine la cristallisation synchrone de la chromite et des petits cristaux d’alliages Pt–Ru–Ir–Os. Dans le cas d’une dominance du Pt, le tracé de la cristallisation atteint le champ de l’immiscibilité, le Pt s’accumule graduellement dans le liquide résiduel,
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avec cristallisation finale à un triple point eutectique, ce qui crée des conditions favorables à la formation de grosses pépites de Pt, allant de dizaines de grammes à quelques kilogrammes, tout comme celles qui sont caractéritiques seulement des gisements de chromite enrichis en Pt de type Ourale–Alaska et Aldan.
(Traduit par la Rédaction)
Mots-clés: minéraux du groupe du platine, chromite, liquide à base d’oxydes, immiscibilité liquide, roches mafiques-ultramafiques, massif d’Inagli, plateforme sibérienne, Russie.
Introduction Mineralization in the platinum-group elements in magmatic rocks is associated mainly with sulfide (Sudbury, Noril’sk, etc.), sulfide–chromite (Bushveld and analogues) and chromite deposits (mafic-ultramafic complexes of Uralian–Alaskan and Aldanian types). The formation of PGE-bearing sulfide deposits in S-saturated magmas can be explained by a high degree of PGE solubility in the immiscible sulfide liquid (e.g., Bezmen et al. 1994, Fleet et al. 1999) and by accumulation of PGE in sulfide globules already at an early magmatic stage. In S-deficient magmas, sulfide minerals make their appearance in interstitial spaces at a late stage of magma crystallization. However, significant concentrations of PGE occurring as alloys in sulfidepoor chromitite ores hosted in layered intrusions, zonal gabbro – clinopyroxenite – dunite massifs and alkaliultrabasic complexes indicate that PGE mineralization is genetically related not only to sulfide formation, but to chromite formation as well. Even high values of the distribution coefficient of Ru, Rh and Ir between spinel and melt, which were obtained in experiments (e.g., Righter et al. 2004), cannot adequately explain the high concentrations of PGE in chromitite ores, in which they are present as alloys and, in some cases, as large Pt nuggets. To understand this phenomenon, alternative models for the formation of PGE mineralization accompanying magmatic processes are needed. There are three main hypotheses for the genesis of PGM deposits: crystal fractionation, liquid immiscibility and hydrothermal–metasomatic remobilization. The main criterion for the validity of the hypotheses is their ability to explain the mechanism of PGE concentration from ppb levels in magmas to ppm levels in orebodies, up to the appearance of platinum-group minerals (PGM) mainly occurring as high-temperature alloys. In this paper, the author presents, briefly and schematically, his own understanding of important aspects of Pt mineralization in a magmatic process based on the results of studies of PGM (Okrugin 1998, 2001, 2004, Okrugin et al. 2006) from placer deposits in the Siberian platform and their bedrock sources, the Inagli and Kondyor alkaline-ultramafic massifs. The reader will find a new version of the model for PGM formation through the separation of a highly chromium-enriched
immiscible oxide liquid, which can concentrate significant quantities of the PGE.
Immiscibility of Silicate and Oxide Melts Where sulfide droplets are present in the magma, the immiscibility process is quite obvious, but the problem of the separation of a silicate magma into immiscible liquids of close composition is generally underestimated in spite of the evidence provided by the presence of glass-in-glass droplets (globules). Upon slow cooling, the immiscibility field of two liquids progressively narrows, and they become homogeneous, i.e., subsequent crystallization will not depend on the previous history of the immiscible melts. However, gravity-induced settling of the droplets of immiscible liquid from the melt may give rise to a magma of different composition (Roedder 1979). The presence of sybotactic domains (submicroscopic clusters) with a crystalline structure (Stewart 1931, Frenkel 1955) and protracted crystallization of large volumes of magma can lead to their layering. It has long been thought that in mafic-ultramafic rocks, chromitite nodules, lenses and veins form from an immiscible oxide liquid separated from a silicate magma (e.g., Pavlov et al. 1979). The main arguments in favor of this hypothesis are sharp boundaries between the chromitite deposits and the host rocks, a massive structure of the ores characterized by close packing of polyhedral grains of chromian spinel, practically without intercumulus phases, and the specific composition of mineral parageneses, which are discussed later. Marakushev & Bezmen (1992) established experimentally that at T = 1300°C, ore droplets rich in Cr (up to 9% Cr2O3) begin separate from a picritic melt containing 0.1–0.2% Cr2O3. Experiments with fluidbearing melts have shown the possibility of superliquidus differentiation of magma with the formation of a chromium-enriched oxide melt rich in PGE and Au. As a result of the gravity settling of clusters, cryptic layering of the magma may lead to the separation of layers with contrasting compositions, leading to the appearance of monomineralic rocks and massive ores (Bezmen 2001). At f(O2) values typical of mantle-derived magmas, the solubility of the PGE in silicate melts is insignificant, and normally at a ppb level, but with increasing
origin of platinum-group minerals in mafic and ultramafic rocks
f(O2), the solubility of PGE grows to a ppm level (e.g., Farges et al. 1999, Amossé et al. 2000, Borisov & Palme 2000). A sharply increased Pt and Pd solubility (by a few orders of magnitude) in H2O-saturated silicate melts under reducing conditions, compared with the solubility of these elements in dry melts, may be explained by the formation of Pt hydride complexes or Pt–fluid–silicate clusters (Bezmen et al. 2008). These clusters may consist of a metallic nucleus that could be stabilized by an outside “envelope” of ligands (e.g., S, As, Sb, Te) (Tredoux et al. 1995). Thus, the author believes that the high PGE content of chromitite ores is due to the initial accumulation of PGE in the dense Cr-rich oxide liquid upon separation of the melt into two immiscible liquids. When discussing a model for the formation of chromitite ores from immiscible liquids, many researchers turn to the phase-equilibrium diagram for the system MgO–SiO2–Cr2O3 constructed by Keith (1954), which exhibits a wide field of liquid immiscibility. But as noted by Pavlov et al. (1979), it is impossible to reach, by way of crystallization-induced differentiation, the oxide–silicate immiscibility field since it is on the other side of cotectic line. However, experiments showed that the addition of components and volatiles to the system results in an expanded field of immiscibility and a significant decrease in the phase-equilibrium temperature (Lapin & Solovova 1979). As a result of a comprehensive analysis of phase diagrams for silicate systems (Toropov et al. 1972), the author constructed, on the basis of the MgO–SiO2– Cr2O3 (Keith 1954) and CaO–SiO2–Cr2O3 (Glasser & Osborn 1958) diagrams, a generalized hypothetical diagram taking into account the effect of the accessory oxides FeO, CaO, Fe2O3 and Al2O3 (Fig. 1). Though schematic and approximate, the diagram adequately reflects the real picture of crystallization-induced differentiation of a basic-ultrabasic magma. In this and the following diagrams, the boundaries between the fields of different phase-compositions are shown on an enlarged scale to make the path of crystallization of the system studied clearer. The cited temperatures correspond to dry systems, such that those temperatures for naturally occurring magmas will clearly be lower. In the liquid state, silica is practically immiscible with Cr2O3, Fe2O3 and P2O5. The solubility of these oxides in liquid SiO2 does not exceed 0.1 wt.%. In other SiO2 – MeO systems (where MeO = MgO, FeO or CaO), the immiscibility field, with a width between 20 and 40 wt.%, is immediately adjacent to silica. In the systems Al2O3–SiO2–Cr2O3 and CaO–SiO2– Cr2O3, wide fields of immiscibility extend from the SiO2–Cr2O3 side toward the central parts of the ternary diagrams, coming close to cotectic lines of synchronous crystallization of Cr2O3 and other phases with spinel, pyroxene and olivine structures. Swisher & McCabe (1964) studied the layering of liquids in the system CaO–SiO2–Cr2O3, where already at 1600°C a dense
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CaO–SiO2 liquid accumulates with more than 40 wt.% and a light one with less than 5 wt.% Cr2O3. In the ternary SiO 2 – Cr 2O 3 – MeO diagrams, two fields of immiscibility unite into a wide band. Of the major oxides, only Al2O3, Na2O and K2O can form homogeneous melts with silica. With the addition of these oxides to the SiO2 – MeO systems, the immiscibility field becomes much narrower, leading to its disappearance where the amount of Al 2O3 or alkalis exceeds 5 wt.% in the systems. Therefore, even insignificant amounts of Al2O3 and alkalis will result in the miscibility gap on the SiO2 – MeO side of the SiO2 – Cr2O3 – MeO ternary system, as seen from the diagram discussed. Besides, the addition of Al2O3 significantly expands the field of crystallization of olivine. The same effect is achieved in the presence of alkalis in the system, which increase the activity of olivine. Thus one has a triangular diagram (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) that shows a wide field of immiscibility on the SiO2 – Cr2O3 side, coming close to the cotectic line of simultaneous crystallization of chromian spinel and olivine or pyroxene (Fig. 1). As seen from the diagram, the crystallization of a picritic magma (A) begins with olivine, and the residual liquid becomes richer in silica and Cr oxide owing to the low (
