Potential of ophiolite complexes to host PGE deposits
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All six PGE have been recorded as concentrated at parts per million (ppm) in a number of ophiolite complexes. Occurrences of PGE concentrations in ophiolites are common in podiform chromitite where Os, Ir and Ru may be concentrated to give negative slope chondrite normalised patterns. If sulphur saturation of the magma occurred during chromite crystallisation then Pt and Pd + Rh will also be concentrated in the chromitite giving positive chondrite normalised patterns. PGE-rich ophiolite complexes are those where there has been sufficient mantle melting, in a subduction zone, to extract the PGE at a critical melting interval. Too much melting will dilute this melt with PGEbarren melt and too little melting will not extract the PGE. The Scottish Shetland ophiolite is an example in which all 6 PGE have been concentrated in base metal sulphide-bearing podiform chromitite. It is proposed that if all the PGE are concentrated in podiform chromitite then they crystallised from one PGE-rich melt that was close to sulphur saturation. Exploration for PGE within ophiolites is likely to be enhanced by growing evidence that there is a link between chromite composition and PGE concentration.Keywords:
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The Al ‘Ays ophiolite complex in Saudi Arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements (PGE) in podiform chromitite with maximum values of 2,570 ppb Pt, 6,870 ppb Pd, 840 ppb Rh, 5,800 ppb Ru, 6,200 ppb Ir, and 3,300 ppb Os. Smooth chondrite-normalized PGE profiles indicate igneous PGE ratios. These suggest that in situ alteration of the PGM caused only minor mobility of PGE during secondary modification of the mineralogy. Thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration.
Three main types of PGE mineralization are observed that are defined by their relative abundances of individual PGE. Type 1 has Ru > both Pt and Pd, with negative-slope chondrite-normalized profiles. Type 2 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 1 to 5, and convex upward chondrite-normalized profiles. Type 3 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 5 to 60, positive-slope chondrite-normalized profiles and is associated with elevated Cu and Ni concentrations. The unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition; for example, Cr2O3 varies from 39 to 69 wt percent. PGE mineralization types 1, 2, and 3 are related to the composition of the chromite. Type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent Cr2O3, type 2 occurs in chromitite having a range of 53 to 61 wt percent Cr2O3 , and type 3 occurs in chromitite having a range of 39 to 51 wt percent Cr2O3.
The PGE form a great variety of platinum group minerals (PGM) and they differ among the three types of PGE mineralization. Type 1 is characterized by euhedral Os, Ir, and Ru (IPGE) alloys and laurite, both commonly enclosed in chromite, as well as members of the irarsite hollingworthite solid-solution series and Pt-IPGE-bearing PGM, both commonly interstitial to the chromite grains. Type 2 PGE enrichment is characterized by IPGE-, Pt- and Rh-bearing PGM. Type 3 PGE enrichment hosts predominantly Pd- and Pt-bearing PGM associated with Ni- and Cu-bearing minerals. Where exposed to the serpentinization process, the PGM are altered to alloys, arsenides, antimonides, and oxides that form irregular shapes or may form pseudomorphs of former PGM. They are commonly associated with Ni- and Cu-bearing minerals, including ruthanian pentlandite, millerite, arsenides, and PGE-bearing awaruite.
Mantle melting and subsequent crystallization were at an optimum to concentrate PGE in the Al ‘Ays ophiolite complex. Crystallization of IPGE, commonly prior to chromite crystallization and latterly with some Pt and Rh, occurred across the range of chromitite composition. Crystallization of Pd with some remaining Pt occurred during sulfur saturation in chromitite formed from a more evolved magma. We propose that this crystallization was from a magma that was enriched in PGE because the degree of mantle melting was just sufficient to extract the PGE, but not dilute them in a melt that includes further mantle melting. This feature is likely to be common to other PGE-rich ophiolite complexes such as in Shetland in the United Kingdom, Leka in Norway, Thetford in Canada, Pindos in Greece, Tropoja in Albania and in New Caledonia. If equilibrium partial melting had continued in Al ‘Ays, then the magma would have been diluted by subsequent PGE-poor melt. This would have prevented sulfur saturation until much higher in the sequence, producing Pt- and Pd-bearing base metal sulfides in the crustal wehrlite and gabbro, as has occurred in the Cyprus and Oman ophiolites.
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Chromites forming giant orebodies in the southern part of the Early Palaeozoic ophiolite sequence of the Kempirsai Massif (Kazakhstan, Urals) contain a large number of inclusions, i.e. silicates, sulphides, alloys, arsenides, and fluids. The chromite orebodies are surrounded by dunite envelopes of variable thickness, which show transitional boundaries to harzburgite host rocks. The composition of ore-forming chromites in depleted mantle rocks of the southern part of the massif (Main Ore Field) is rather uniform, showing high cr-number [100Cr/(Cr+Al), 78–84] and mg-number [100Mg/(Mg+Fe2+), 51–85] values. Smaller bodies of Al-rich spinel in the northern and western part of the massif (Batamshinsk) have variable cr-number (38–60) and mg-number (50–88) values. Three textural types of inclusions in chromite are distinguished: (1) In Main Ore Field chromites, primary silicate inclusions generally have high mg-number (>95), Cr and Ni, and are dominated by pargasitic amphibole, forsterite, diopside, enstatite and Na-phlogopite. Chromite formed over a temperature range from 1200° to <1000°C at oxygen fugacities 1–2 log units above the fayalite-magnetite-quartz (FMQ) buffer. A diversity of primary and secondary platinum-group mineral (PGM) is described from the chromitites, including alloys, sulphides, sulpharsenides, and arsenides of Ru, Os, Ir, Rh, Ni, Cu, Fe and Co. Alloys, sulphides and arsenides free of platinum-group elements are attributed to serpentinization of chromitite. (2) In addition to primary PGM and hydrous silicates, fluid inclusions of up to 50 μm size are frequently included in chromite within chromite-amphibole veins discordant to massive chromitite in the Main Ore Field. The fluids are low to moderately saline, sodium-dominated aqueous solutions with complex gas contents. Variable amounts of water, hydrogen, hydrocarbons, carbon dioxide and nitrogen have been determined in inclusion-rich samples. (3) In the northern and western part of the Kempirsai massif, complex silicate-oxide assemblages formed in small orebodies of orbicular Al-rich chrome spinel. Chlorite, amphibole, hydrogarnet, sphene, manganoan ilmenite and Ca-Ti oxide are documented in addition to Ni sulphides and rare PGM. The formation of chromitite in the Kempirsai Massif is explained in terms of a multi-stage process involving mantle fluids. Low-Cr, high-Al spinel present in small orebodies in the northern and western part of the massif formed from mid-ocean ridge basalt (MORB)-type melts extracted from fertile mantle in an extensional tectonic setting. The large orebodies and the amphibole-chromite veins in the southern part formed later from interaction of hydrous, second-stage high-Mg melts and fluids with depleted mantle in a convergent tectonic setting. Metasomatic alteration of the mantle wedge above subducted crust by fluids played an important role in generating second-stage melts and in releasing metals.
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The platinum-group element (PGE) contents of komatiites have been used to estimate the PGE contents of their mantle sources and investigate temporal variations in the PGE systematics of the mantle throughout Earth history. However, the use of mantle melts to investigate source characteristics is critically dependent on understanding the fractionation of the elements in question between mantle sources and their derived magmas. We present new PGE and Re abundance data, together with Re-Os isotope data for the pristine 1.9 Ga Winnipegosis komatiites, Canada. Whole rock Re-Os isotope data define a 1865 ± 40 Ma isochron, consistent with previous age estimates, with initial γOs of +0.9 ± 1.4. The PGE and Re variations in these komatiites were controlled by fractionation of olivine, chromite, Os-Ir alloys, a Ru-Os rich phase (possibly laurite) from the komatiite melt. The calculated parental melt composition at 23.6 ± 1.6 wt% MgO has 1.1 ± 0.1 ppb Os, 0.91 ± 0.08 ppb Ir, 4.0 ± 0.5 ppb Ru, 7.0 ± 0.8 ppb Pt, 7.2 ± 0.7 ppb Pd, and 0.53 ± 0.06 ppb Re. The calculated PGE contents are low relative to many komatiites; using existing methods for calculating the PGE contents of mantle sources suggests the Winnipegosis source had PGE contents from 25 ± 19 wt% to 57 ± 7 wt% of primitive mantle values. This surprising finding substantiates a growing ‘PGE paradox’, where almost all komatiites appear to have formed from PGE-depleted mantle sources. We suggest the simplest resolution to this paradox is that current methods systematically underestimate mantle source PGE contents. We test the assumptions implicit in these methods by comparing detailed models of PGE behaviour during mantle melting to a large database of parental melt compositions of high degree melts. Ruthenium behaviour in high degree melts can be adequately modelled using our current understanding of Ru partitioning. However, the Ru contents of high degree mantle melts are strongly dependent on the degree of melting, as Ru-rich melts formed after sulphide exhaustion are increasingly mixed into early-formed Ru-poor melts. Platinum and Pd behaviour during mantle melting cannot be successfully modelled under the widespread assumption that they are strongly incompatible following sulphide exhaustion, suggesting that these elements cannot be used to estimate PGE contents of the mantle sources of high degree melts. Parental melt Pt and Pd in many high degree lavas are correlated with Al2O3/TiO2, suggesting a pressure control on their partitioning. We attribute the relatively low PGE contents of the Winnipegosis komatiites to their moderate degrees and depth of melting compared to other komatiites, rather than invoking a PGE-depleted mantle source. More broadly, the complex controls (T, P, fO2, S content, prior depletion) on the PGE contents of komatiites mean that most komatiites can be reconciled with being derived from sources with primitive mantle-like PGE contents, and few komatiites can be confidently asserted to come from PGE-depleted mantle sources. Pending a better understanding of PGE behaviour during mantle melting, we recommend caution when using komatiite PGE systematics to infer the highly siderophile element evolution and accretionary history of Earth’s mantle.
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Abstract This paper reviews the compositional data (major elements, platinum group element [PGE] concentrations, Os‐ and O‐isotopes) for chromites from the mantle section of the Oman ophiolite. Chromites in chromitite from the Oman ophiolite lie on a compositional spectrum between high‐Cr♯, boninite‐like and low‐Cr♯, mid‐oceanic ridge basalt‐like end‐members. The high‐Cr♯ end‐member is low in Ti, has a fractionated PGE pattern and is enriched in iridium group‐platinum group elements (IPGE). The low‐Cr♯ end‐member has higher Ti and an unfractionated PGE pattern. The compositional variation in the chromitites reflects their crystallization from a range of different melt compositions. It is proposed that this wide variation in melt compositions was produced by the process of a melt–rock reaction, whereby a basaltic melt has reacted with harzburgitic mantle to yield successively more Cr‐rich melts. In contrast to previous models, this approach does not require a change in the tectonic environment to explain the different chromite types.
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Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new high-precision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores. Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion, and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion, commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiite-hosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within ore-bearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors. Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes.
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