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Journal of Petrology Volume 42 Number 10 Pages 1911-1926 2001
© Oxford University Press 2001
Phase Relations in the Fe–Ni–Cu–PGE–S System at Magmatic Temperature and Application to Massive Sulphide Ores of the Sudbury Igneous Complex*
1INSTITUT FÜR MINERALOGIE, UNIVERSITÄT MÜNSTER, CORRENSSTRASSE 24, 48149 MÜNSTER, GERMANY
2GEOLOGY DEPARTMENT, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
Received July 3, 2000; Revised typescript accepted March 1, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSPARTITION COEFFICIENTS (D)METAL SPECIATION IN SULPHIDE...DISCUSSIONCONCLUSIONSREFERENCES |
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Experiments in the Fe–Ni–Cu–S system were performed to identify the role of the metal/S atomic ratio on monosulphide–melt partition coefficients and closed-system fractionation paths. In accord with previous work, DCu is
0·2 at all temperatures and all metal/S ratios. DNi is highly sensitive to temperature and metal/S, and changes from 0·6 at high metal/S and high temperature to >2 at low temperature and low metal/S. The temperature at which the cross-over in DNi occurs is sensitive to S2 fugacity. The monosulphide solid solution (mss)–melt partition coefficients of the platinum group elements (DPGE) are determined with laser-ablation inductively coupled plasma mass spectrometry calibrated on synthetic sulphide standards. At trace element concentration levels, the DPGE are largely insensitive to metal/S, contrary to previous experiments with PGE concentrations in the percentage range. Pt and Pd are highly incompatible with mss (D < 0·1) whereas Ir, Ru, and Rh are compatible, ranging from >3 to 10. The chemical differentiation paths of Fe–Ni–Cu–S sulphide melts experiencing mss fractionation are determined by the metal/S parent melt ratio. Oxidized sulphide melts with metal/S < 1 solidify in the stability field of intermediate solid solution (iss) whereas reduced sulphide melts with metal/S > 1 may fractionate past iss stability. The latter will accumulate Ni together with Cu down to solidus temperature. Toward the end of their fractionation path, they are too depleted in S to crystallize iss. Instead, they will precipitate a copper sulphide with monovalent Cu and presumably solidify at an iss–bornite–millerite eutectic. The dataset is applied to massive sulphide ores of the Sudbury Igneous Complex fractionated with respect to Ni/Cu and (Ni + Cu)/
metal ratios. It is shown that the change-over in DNi may be used to retrieve parent melt compositions, fractionation temperatures, and magmatic fractionation paths of these deposits. KEY WORDS: Fe–Ni–Cu–S system; partition coefficients; laser-ablation ICP-MS; PGE; liquid immiscibility; Sudbury
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSPARTITION COEFFICIENTS (D)METAL SPECIATION IN SULPHIDE...DISCUSSIONCONCLUSIONSREFERENCES |
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The compositional range of sulphide melts parental to massive magmatic sulphide ore deposits is poorly constrained, for several reasons:
- at liquidus temperature, sulphide melts have viscosities several orders of magnitude lower than those of basaltic melts (Dobson et al., 2000
). This means that cumulus processes, i.e. the spatial separation of early crystalline phases from derivative sulphide melt, are more effective than in silicate melts. In fact, it has been shown repeatedly that magmatic sulphide ores are either cumulates of monosulphide solid solution (mss) or pooled derivative melts, or arbitrary combinations of these endmembers (Keays & Crocket, 1970; Naldrett, 1981, 1989; Naldrett et al., 1982, 1999; Li & Naldrett, 1994; Barnes et al., 1997).
- Sulphide melts never preserve high-temperature phase relations. Even the most rapidly chilled magmatic sulphide melt samples (droplets in basaltic glasses; Francis, 1990; Peach et al., 1990) invariably recrystallize to low-temperature assemblages such as pyrrhotite–pentlandite–chalcopyrite–pyrite, regardless of cooling rate. This may leave the primary metal ratios untouched, but there is no guarantee that more ephemeral parameters such as the metal/sulphur (metal/S) or metal/oxygen (metal/O) ratios are also preserved.
- Sulphide melts are extremely susceptible to oxidation. In nature, S2 fugacity (fS2) and O2 fugacity (fO2) are highly correlated and both variables control the metal/S and metal/O ratios of a sulphide melt. If oxygen is added to a sulphide melt, magnetite may appear as an early phase and the liquidus mss phase will become increasingly metal deficient. Given that metal/S and metal/O both control the vacancy concentration in mss, they also influence the capacity of liquidus mss to substitute chalcophile metals other than Fe, and thus alter the liquid line of descent (Ballhaus & Ulmer, 1995).
The only way to retrieve variables such as the metal/S melt ratio is from mss–sulphide melt phase equilibria. This requires precise knowledge as to how mss–melt partition coefficients of the major and minor chalcophile elements respond to variations in fS2, temperature, and metal spectrum of the sulphide melt. Of particular interest are elements whose partitioning behaviour is sensitive to the metal/S ratio.
In this study, we report new experimental partition coefficients for Ni, Cu, Ir, Ru, Rh, Pt, and Pd between mss and sulphide melt, determined over a wide range of temperature and metal/S ratio. We use the data to calculate the theoretical closed-system Rayleigh fractionation paths of magmatic sulphide melts in the Fe–Ni–Cu–PGE–S system. These fractionation paths are then compared with natural liquid lines of descent of magmatic sulphide ore deposits from the Sudbury Igneous Complex, Ontario, which are zoned with respect to major and trace element contents and thought to be differentiated by mss fractionation. We are able to derive, within limits, the metal/S ratio, the Ni and Cu contents, and the fractionation temperature of the parental sulphide melts.
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EXPERIMENTAL AND ANALYTICAL DETAILS |
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The sulphide starting mixes are listed in Table 1. The three compositions MS-10, MS-11 and MS-12 are designed to be in exchange equilibrium with a primitive tholeiitic basalt from a fertile mantle source (Rajamani & Naldrett, 1978
; Francis, 1990). All three compositions have the same (Ni + Cu)/metal atomic ratio of 0·2 but are variable in terms of the metal/S atomic ratio. Metal/S ranges from 1·2 to 0·9 to cover the range in fS2 reasonable for magmatic sulphide melts (Wallace & Carmichael, 1992). A fourth composition (MS-8) is an evolved sulphide melt: it represents a composition similar to MS-11 after 50% equilibrium mss fractionation, to monitor later-stage differentiation of sulphide melt after extensive mss fractionation.
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All compositions were synthesized from ultrapure base metal powders. The platinum group elements (PGEs) were added to the metal matrix as Ir2O3, RuS2, Rh2O3, PtS2 and PdS before addition of sulphur, to ensure identical PGE contents in all charges. All compositions were doped with traces of metallic Si and graphite, to capture any oxygen from trapped air and/or adsorbed moisture. The initially S-free metal mixture was homogenized under methanol to complete dryness.
After division in three aliquots, sulphur was added to match the desired metal/S ratios. All mixtures were reacted at 750°C for several days in evacuated silica glass tubes repeatedly flushed with neon. The products (polycrystalline sulphide aggregates) were then reground, dried, and stored in a desiccator to serve as starting mixes for the experiments.
Each experiment used 150 mg of sulphide charge welded under high vacuum in a 6 mm SiO2 glass tube (Kullerud, 1971). During evacuation, the tubes were flushed repeatedly with neon before final sealing to minimize oxygen incorporation in the melt. The glass tube was suspended on a retractable Pt wire in the hot zone of a vertical quench furnace. Temperature was controlled with a Pt–Rh (EL18) thermocouple at the outer wall of the ceramic tube to within ±2°C, and monitored with a second thermocouple placed next to the experimental charge. fS2 was buffered internally by the metal/S ratio of the charge. fO2 was left unbuffered but kept to a minimum by the presence of traces of metallic Si and graphite. Run times ranged from a few hours to 24 h depending on run temperature. Runs were quenched within 2–3 s to <100°C by dropping the charge in cold water.
The run products were polished under oil on lead–antimony discs to a relief-free finish. Coexisting mss–melt pairs were analysed for major elements with a JEOL electron microanalyser at 20 kV and 15 nA, calibrated on natural pyrite, chalcopyrite, and pentlandite standards. The PGEs were analysed with laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) calibrated on synthetic sulphide standards. As sulphide melts are impossible to quench homogeneously (see below), all melt analyses were carried out with electron and laser beams defocused to 50 µm, to integrate as much as possible dendritic quench growth.
Analysis of PGEs
The PGEs were added in roughly the same relative and absolute abundances as in the sulphide concentrate of the Merensky reef (Naldrett et al., 1987; Ballhaus & Sylvester, 2000). By adding the PGEs at part per million levels we ensured that no discrete PGE phases crystallized during the run, and we endeavoured to remain within Henry’s law range. The PGE standards for laser-ablation analysis were prepared by melting 500 mg of each starting mix in evacuated silica tubes for 24 h 50°C above their liquidus temperatures. One half of the quenched product—dendritically recrystallized melt—was polished to serve as laser standard. The other half was split in two aliquots, dissolved in hot aqua regia, and analysed in duplicate with solution ICP-MS. To correct for mass interferences we also analysed several PGE-free solutions with only Fe, Ni, and Cu in solution. Isotopes affected by interference are 103Rh and 105Pd (interference with 63Cu40Ar and 65Cu40Ar, respectively). Interference on Rh is unavoidable as Rh is mono-isotopic; however, the necessary correction is only 50 ppb apparent Rh per 5% Cu. Pd interference can be circumvented by quantifying the isotope 108Pd.
In situ laser analysis was carried out with a CETAC LSX-200 laser ablation module (frequency-quadrupled 266 nm Nd–YAG laser) connected to a Perkin Elmer Elan 6000 ICP-MS system. The pulse repetition rate was 10 Hz and the beam diameter was set to 50 µm. Ablation rate was 2000 µm3/s as approximated by measuring ablation craters with scanning electron microscopy (SEM). In addition to the five PGEs added to the charges, we also recorded 60Ni, 63Cu, and 34S for later normalization of PGE count rates to a major element.
Calibration was achieved using averaged count rates of five analyses of each standard. PGE count rates were normalized to a major element whose concentration was known from previous electron probe analysis. The smallest standard deviation is obtained by normalizing to 60Ni, as Ni concentrations are least affected by quench redistribution. Relative cumulative errors in a calibration are 8% for Pt and Pd, and 14% for Ir, Ru and Rh.
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EXPERIMENTAL RESULTS |
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Backscatter images of polished run products are illustrated in Fig. 1. All starting compositions returned mss single crystals as the only liquidus phase. The melt quenched to metastable dendritic aggregates of mss and intermediate solid solution (iss). The most reduced (metal-rich) composition (MS-10) and the derivative composition (MS-8) also gave minor quench bornite, millerite, and sometimes micron-sized blebs of metallic Cu.
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Liquidus temperatures, reported in Table 1, are accurate to within 10–20°C and fall with rising metal/S. In near-liquidus runs it often proved difficult to distinguish between equilibrium and quench mss (eg. Fig. 1c). Solidus temperatures (Table 1) are even more imprecise. When the degree of melting falls below 5%, the melt tends to be consumed during quenching by back-reaction with mss crystals, leaving behind Cu-enriched grain boundaries in mss aggregates as the only evidence for former melt.
The major and trace element phase compositions of experimental mss–melt pairs are tabulated in Tables 2 and 3. We chose pseudobinary T–X projections with the metal/S and (Ni + Cu)/metal atomic ratios as compositional variables (Fig. 2). The major compositional trends are as follows:
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- at any given temperature and bulk metal/S ratio, mss is more S rich than the coexisting melt. This is contrary to the simple Fe–S and Fe–Ni–S systems (Craig & Kullerud, 1969; Craig & Scott, 1982) and probably is due to the fact that Cu dissolves in the melt mostly as the Cu2S complex (Ebel & Naldrett, 1996).
- With falling temperature, mss becomes increasingly tolerant toward Ni and Cu whereas the metal/S ratio of mss stays approximately constant within analytical error.
- The more oxidized (S-rich) the starting mix, the higher the defect concentration in mss and the more tolerant mss is toward impurities such as Ni and Cu. This is in agreement with the phase relations of the Fe–Ni–S system (Craig & Kullerud, 1969; Craig & Scott, 1982). A similar trend was noted by Ballhaus & Ulmer (1995) with respect to PGE substitution in disordered pyrrhotite.
- The higher the bulk metal/S ratio, the lower are the liquidus and solidus temperatures and the lower is the temperature to which a sulphide melt may fractionate and become enriched in Cu.
- Oxidized (S-rich) sulphide melts will solidify in the iss stability field whereas reduced (S-poor) sulphide melts may fractionate past iss stability and solidify at a bornite–millerite eutectic, as detailed below.
Quenching behaviour
It is well known that sulphide melts are difficult to quench (Fleet & Pan, 1994; Kress, 1997). The element most sensitive to quench redistribution is Cu. Unlike Ni, which is largely incorporated in quench-mss, Cu forms its own quench phases, which are iss and bornite. Quench modification is most severe in the metal-rich compositions MS-10 and MS-8. In agreement with Ebel & Naldrett’s (1996) observation, the metal-rich compositions (MS-10 and MS-8) appear to have depressed viscosities and better wetting characteristics toward the glass tube than the relatively more S-rich melts (MS-12 and MS-11).
Crystalline mss is also affected by quench modification, again most severely in the metal-rich compositions MS-10 and MS-8. Despite the high quench rates, many mss crystals exsolve sub-micron wide lamellae that we expect to be enriched in Ni and Cu (Fig. 1b and c). Mss grains are also modified by back-reaction with melt. Wherever in contact with melt they are distinctly zoned. As a rule, the smaller the mss grain, the larger the immersing melt pool, and the higher the metal/S bulk ratio, the more profound the compositional effect of back-reaction.
Figure 3 shows a microprobe traverse across a heterogeneous mss grain 200 µm in diameter from a 950°C run of composition MS-11. The grain was surrounded by dendritic quench phases and visibly zoned. The extent of element heterogeneity is surprising, notably the heterogeneity with respect to S (and metal/S). To demonstrate that the zonation forms during quenching and is not due to failure of reaching equilibrium, we made use of another ubiquitous quench phenomenon—the development of thermal contraction cracks. Figure 4 shows an element map of Cu over several mss grains. The peripheries of the grains, in contact with melt, are veined by a network of extremely narrow contraction cracks. These cracks apparently served as infiltration pathways for fractionated melt enriched in Cu and depleted in S. As such cracks would have been unstable at run temperature, the plot indicates that mss heterogeneity is related to quenching.
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We identify three stages of quench modification: (1) the Ni-rich overgrowth rims form when a liquidus mss grain equilibrates down-temperature along its respective solidus; (2) the broader zonation with respect to S [as well as metal/S and (Ni + Cu)/metal] is tentatively attributed to pervasive melt infiltration along minute thermal contraction cracks; the alternative, i.e. solid-state diffusive exchange during quenching, is considered unlikely at the high quench rates achieved; (3) the exsolution lamellae (Fig. 1b) form in the subsolidus temperature range.
We suggest that the best strategy to retrieve equilibrium phase compositions is to opt for longest possible run times to promote large mss grain sizes, then to make every effort to maximize quench rates by using thin-walled containers and small quantities of charge, and finally only probe mss grains attached to the container walls partly isolated from melt.
Liquid immiscibility in the Fe–Ni–Cu–S system?
It was suspected for some time that magmatic sulphide liquids may not be fully miscible in all proportions (Distler et al., 1986; Naldrett, 1989). Distler et al. (1986) noted for the Noril’sk–Talnakh deposits that a large number of sulphide droplets trapped in silicate matrix were composite in nature. An FeS-rich lower part was found separated from a Cu-rich upper sulphide by a sharp horizontal meniscus. Distler et al. argued on textural grounds that the meniscus was an early magmatic phenomenon and developed before the melts crystallized. The first mention of incomplete miscibility in an experimental sulphide system was by Peregoedova (1998) but detailed compositional data were not reported.
We present here experimental evidence that liquid immiscibility may be widespread in magmatic sulphide melts. Superliquidus runs with MS-11 and MS-8 returned two discrete sulphide melts immersed as droplets in each other (Fig. 1d–f). Despite dendritic quench modification, the two melt phases are separated by an optically and compositionally sharp interface. Very often, the quench phases used this interface—in immiscible systems a discontinuity in the melt structure with positive interfacial energy—as a preferential nucleation site (Fig. 1d). In addition, many quenched melt droplets contain radially oriented vapour bubbles (Fig. 1f) that can only have formed in that arrangement when the phase carrying the bubbles was molten before quenching. On the basis of these textural characteristics, we propose that the interface separating the two melts was thermodynamically stable at run conditions and that there is stable liquid immiscibility in the Fe–Ni–Cu–S system, thus supporting Distler et al.’s (1986) proposition.
Conjugate melt compositions are listed in Table 2, and the two-liquid fields are illustrated in pseudobinary T–X plots in Fig. 5 using the metal/S and (Ni + Cu)/metal atomic ratios as compositional variables. In both starting compositions the exsolved droplets strongly fractionated the Cu2S melt component. The actual shape of the solvus and the extent of immiscibility still are uncertain. In the fractionated bulk composition MS-8 the two-liquid field appears to intersect the liquidus for 25°C. In the primitive MS-11 composition it closes before mss stability is reached. The upper consolute point is above 1000°C for the MS-8 composition but beyond experimental reach in MS-11.
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Unfortunately, both bulk compositions showing evidence for two liquids happened to be close to the composition of the matrix melts of the conjugate systems, so the proportion of exsolved droplets did not exceed 10 vol. %. Droplet sizes rarely exceeded 300 µm in diameter, and this resulted in large variations in individual electron probe analyses reflected in the 1
errors in Fig. 5.
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PARTITION COEFFICIENTS (D) |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSPARTITION COEFFICIENTS (D)METAL SPECIATION IN SULPHIDE...DISCUSSIONCONCLUSIONSREFERENCES |
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Average D values (mss–melt) are summarized in Table 4. DCu is
0·2 and essentially independent of metal/S ratio and temperature (Fig. 6), in agreement with previous work (Fleet & Pan, 1994; Ebel & Naldrett, 1996, 1997; Li et al., 1996). In contrast, DNi is highly dependent on the metal/S bulk ratio as well as temperature (Fig. 6). In the reduced, metal-rich composition MS-10, Ni is moderately incompatible (DNi 0·6) at all temperatures at which mss is the only liquidus phase. In the more oxidized compositions, DNi switches from less than unity above 1050°C (MS-12) to above unity around 950°C (MS-11), to reach a maximum of >2 near the solidus of MS-12. The temperature of the cross-over in DNi is sensitive to bulk metal/S. It should be noted that the maximum D in Fig. 6 (2·6) should be viewed with caution as it was measured on rather small melt pools in only one charge close to solidus temperature. None the less, the general conclusion that DNi is sensitive to both fS2 and temperature (Kullerud et al., 1969; Li et al., 1996; Barnes et al., 1997) seems robust.
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The DPGE are illustrated in Fig. 7 as a function of metal/S ratio of the mss liquidus phase. Also shown for comparison are the DPGE determined by Li et al. (1996). Our DPGE fall into two distinct groups. Ir, Ru, and Rh are highly compatible with mss, whereas Pt and Pd are highly incompatible with mss. DPt, DPd, and possibly DRu appear to be slightly influenced by variations in fS2 (metal/S ratio) but the general chemical affinity of the elements toward mss remains unaffected by fS2. The DPGE reported here are valid only for near-liquidus temperature. Reliable laser analysis of mss–melt pairs required melt pool sizes that exceeded the diameter of the laser beam (50 µm) by a factor of two, and this is guaranteed only in near-liquidus runs.
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Our DPGE agree well with those of Fleet et al. (1993) considering that the latter were calculated only from secondary ionization mass spectrometry relative count rates. They do, however, deviate from the DPGE reported by Barnes et al. (1994, 1997) and Li et al. (1996). The latter workers proposed that DRh and DIr switch from <1 (incompatible with mss) at high metal/S, to 10 (compatible with mss) at low metal/S. This does not seem to be valid at trace concentration levels (see Fleet, 1996). The experiments of Li et al. (1996) were with percentage concentrations of PGEs, and were thus outside the validity of Henry’s law. Many of Li et al.’s charges were PGE oversaturated after quenching. Their most S-rich runs returned mss liquidus phases with up to 4·8 wt % combined Ir and Rh in solid solution, and this must have resulted in incalculable effect on the partitioning behaviour of the other PGEs.
Conjugate melt partition coefficients
Major element partition coefficients between matrix melt and exsolved droplets are DNi = 1·09 ± 0·04 and DCu = 0·6 ± 0·1, nearly identical for both starting compositions and apparently insensitive to temperature or sulphide melt composition. For the immiscible melts, the DPGE could only be approximated from count rate ratios because most of the exsolved droplets were too small and possibly not deep enough for reliable, contamination-free laser analysis. Figure 8 shows a laser step profile across the two droplets in Fig. 1e. The DPGE (matrix/droplet) calculated from intensities are DIr 12, DRu 12, DRh 3·5, DPt 0·4 and DPd 0·35. In terms of magnitude, these D values are similar to the respective mss–liquid DPGE, except that Pt and Pd do not seem to partition as strongly as between mss and melt.
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METAL SPECIATION IN SULPHIDE MELTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSPARTITION COEFFICIENTS (D)METAL SPECIATION IN SULPHIDE...DISCUSSIONCONCLUSIONSREFERENCES |
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It is not well known what complexes and oxidation states chalcophile metals attain in Fe–Ni–Cu sulphide melts. To formulate a solution model, one needs to know how metal/S varies with (Ni + Cu)/
metal and fS2. For the Fe–Ni–Cu–S system, activity–composition relations are established only for metal-rich bulk compositions with metal/S ratios >1·2 (Kongoli et al., 1998), which is outside the range of interest for most natural magmatic sulphide systems.
Some preliminary information on metal speciation can be obtained from the partitioning data between the immiscible melts. As the conjugate melts must have been in fS2 equilibrium, all activities and fugacities of all species in both melt phases are identical. Ni varies little between the conjugate melts (DNi 1·09) suggesting that >90% dissolves as near-stoichiometric NiS complex, with no effect on metal/S. Cu fractionates into the metal-rich liquid fraction (DCu 0·6), consistent with Ebel & Naldrett’s (1996) suggestion that Cu is predominantly monovalent and dissolves as Cu2S complex. It should be noted that a Cu–S melt saturated with liquid Cu has around 66 atomic % Cu, closely matching the stoichiometry of the Cu2S complex (Kongoli et al., 1998). We suggest that metal speciation in sulphide melts may be modelled by the three species Fe1-xS, NiS, and Cu2S (<20% CuS). In this preliminary model, the stoichiometry of the Fe1-xS complex is expected to compensate for variations in fS2 and fO2.
If this is correct then the valencies of metal cations in sulphide melts are the same as in silicate melts: in silicate melts Ni is divalent and dissolves as NiO (Holzheid et al., 1994), Cu is monovalent and dissolves as Cu2O (Ripley & Brophy, 1995), and Fe is divalent to trivalent (FeO1+x) depending on fO2 (e.g. Kilinc et al., 1983). Variations in fO2 are compensated by the stoichiometry of the FeO1+x complex, whereas other transition metal cations with higher oxidation potentials than Fe2+ (i.e. Ni2+ and Cu+) remain unaffected, at least within the range of natural O2 partial pressures.
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DISCUSSION |
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We have derived mss–melt partition coefficients of Ni, Cu, and five PGEs over a range in fS2 within which primitive natural sulphide melts fall. Out of all chalcophile elements included in the charges, only DNi is sensitive to fS2; its tendency to favour the liquid is reversed if fractionation occurs at high fS2. The notion that the DPGE are fS2 sensitive (Li et al., 1996
; Barnes et al., 1997) is not confirmed to hold at trace concentration levels (see Fleet, 1996) although there may be a minor trend with respect to DPt and DPd. In this section, we calculate model fractionation paths of the three primitive melt compositions in Table 1 as a function of temperature and metal/S ratio. We then apply the model paths to selected massive sulphide ores of the Sudbury Igneous Complex, to quantify parent sulphide melt compositions and fractionation temperatures of the Sudbury ores. Finally, we speculate on the role of liquid immiscibility in natural sulphide melts.
Fractional crystallization paths of experimental sulphide melts
With the set of D values in Table 4 and the equilibrium crystallization paths in Fig. 2, one may calculate how melts with differing metal/S parent ratios would evolve with closed-system mss fractional crystallization. A batch fractionation model is assumed that subtracts mss in 5 wt % increments from the melt fraction remaining after each fractionation step. Care was taken that all compositional characteristics of the model mss–melt pairs matched precisely those of the experimentally determined mss–melt pairs. The resultant melt fractionation paths are plotted in Fig. 9 against percent melt remaining after each batch fractionation step.
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Melts as oxidized as MS-12 enrich only Cu. Ni may experience slight enrichment at fractionation above 1050°C then depletion, reflecting the cross-over in DNi with falling temperature (Fig. 6). The potential of such melts to fractionate past iss stability is negligible. After 80% mss fractionation, the derivative melt has reached 33 wt % Cu and Fe and 30 wt % S, closely matching the composition of iss. All Ni left at that stage (2 wt %) is easily accommodated in iss. With 5% mss batch fractionation increments, the amount of iss-saturated melt that can be generated from an oxidized parent with metal/S 0·9 is at most 20%.
The reduced composition MS-10 evolves differently. It accumulates Ni together with Cu over the entire interval within which mss is the only liquidus phase. MS-10 is unlikely to reach iss saturation. The S content at 33% Cu, the presumed threshold concentration for iss saturation (Ebel & Naldrett, 1996, 1997), is too low. This melt may instead become saturated with a sulphide containing monovalent Cu, possibly bornite Cu5FeS4 and/or digenite Cu2-xS, or even metallic copper. By that time, the NiS content is too high to be relieved by incorporating Ni as minor constituent in a copper sulphide, but may instead stabilize millerite. Melts as sulphur poor as the MS-10 composition are able to fractionate past the iss stability field. Their last fraction presumably solidifies at an iss–bornite ± digenite–millerite eutectic with little primary mss.
Application to selected Sudbury sulphide ore deposits
Having outlined the differences in fractionation paths exerted by metal/S, the paths are compared with differentiation paths of three sulphide ore bodies of the Sudbury Igneous Complex (Fig. 10). The Sudbury intrusion formed after a mega-impact at 1850 Ma into Early Proterozoic metavolcanic and metasedimentary rocks of the Southern Province and Archaean gneisses and granitoids of the Superior Province (Card et al., 1984). Following the impact the crater filled with an overheated silicate melt sheet (Marsh & Zieg, 1999) that exsolved an immiscible sulphide melt, perhaps as a result of massive silica contamination (Naldrett, 1981). The sulphide melt then segregated to the bottom of the intrusion, collected to large pools, and differentiated by mss fractionation to chemically zoned ore bodies ranging in composition from Ni-rich mss cumulate ore to evolved Cu-rich ore (e.g. Golightly & Lesher, 1999). Detailed geological descriptions of the Sudbury complex and its sulphide deposits have been given by Hoffman et al. (1979), Naldrett (1981), Coats & Snajdr (1984), Pye et al. (1984), Li et al. (1992, 1993), Naldrett et al. (1999) and therein.
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To compare the experimental fractionation paths with natural liquid lines of descent we select three deposits from Sudbury, well documented in terms of bulk sulphide composition (Fig. 11). Bulk analyses (Ni, Cu, S, the six PGEs, and Au) were kindly provided by A. J. Naldrett. All analyses with >5 wt % bulk S were recalculated to 100% sulphide by setting S = 36 wt %, a common upper limit for massive sulphide samples in the database. Ni and Cu concentrations were normalized to that value. Fe had to be computed by difference (100 – S – Ni – Cu) because it was not directly analysed. All compositions were then calculated to normative chalcopyrite CuFeS2, pentlandite (Fe,Ni)9S8, and pyrrhotite FeS. Any deficit in Fe after Ni and Cu allocation to pentlandite and chalcopyrite was compensated for by recasting the appropriate proportions of these normative minerals to bornite Cu5FeS4 and millerite NiS, respectively, according to the schematic reactions
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The extent of mss fractionation is monitored by a fractionation index (FI), which is defined as normative pyrrhotite minus bornite minus millerite (in wt %). FI will become negative only if there is a deficit in Fe after Cu and Ni allocation to cpy and pent.
On the basis of the norm calculations, the general classification of ore types observed is as follows (Fig. 11):
- ore with FI > 60 is classified as mss cumulate ore. The amount of Cu in cumulate ore may in part reflect the proportion of a trapped melt component, in part the amount of Cu in solid solution in former mss.
- Ore with FI around zero and Cu contents exceeding 30% may have crystallized from pooled iss-saturated derivative melts after extensive mss fractionation.
- Ni–Cu-rich ore with FI < 0 is Fe1-xS deficient, S poor, and enriched in bornite, cubanite, and millerite. Provided this type of ore is magmatic in origin (see Farrow & Watkinson, 1992), it may have crystallized from evolved melts that fractionated past the iss stability field.
Approximate parent melt characteritics can be deduced as follows. The oxidation state, i.e. the metal/S ratio, is estimated from the degree of Ni depletion if fractionation took place at temperatures below 1000–1050°C. Ni enrichment together with Cu at unusually primitive fractionation degrees (FI > 80) is taken to indicate that mss fractionation commenced at a temperature above which Ni was incompatible with mss. In this case it is impossible to derive the equilibrium metal/S melt ratio. The Ni and Cu contents of the parent melts are approximated by dividing averages of the most primitive cumulates of each suite by the appropriate values for DNi and DCu. For Cu, this will give maximum values, as the contribution of trapped Cu2S-rich liquid in mss cumulate to bulk Cu cannot be discriminated from Cu originally in solid solution in mss.
According to this scheme, the most primitive and highest-temperature cumulates occur at Strathcona. In this deposit Ni accumulated with Cu in the melt, restricting fractionation temperature to above the cross-over in DNi, possibly as high as 1100°C. No conclusion is possible about metal/S because in this temperature range DNi is insensitive to fS2. Calculated Ni and Cu contents of the melt parental to the most primitive cumulates are around 4·8 ± 0·8 and 0·5 ± 0·2 wt %, respectively. Cu is low for an early magmatic sulphide melt, and the same is true for bulk PGEs (see Naldrett et al., 1999). Naldrett et al. (1999) attributed the low PGE contents to anomalously low R factors with silicate melt. Perhaps the residence time of the Strathcona sulphide melts in silicate melt was shorter than elsewhere in Sudbury, resulting in higher segregation and in turn higher crystallization temperature.
The Lindsley deposit consists of mss cumulates with variable trapped Cu-rich melt component. For the most part, Ni is concentrated in the cumulate portion whereas Cu is accumulated in the melt. In contrast to the cumulates at Strathcona, Ni becomes depleted with protracted mss fractionation. This trend requires the parent melts to have been as oxidized (S rich) as, or more oxidized than, the composition MS-11. The ores also display well the cross-over in DNi that occurs at FI > 80, constraining the beginning of mss fractionation to above 1050°C. The Ni concentration in a parent melt in equilibrium with the most primitive cumulate endmembers is calculated to around 4·8 ± 0·3 wt %. Calculated Cu (5·8 ± 1·2 wt %) appears to be rather high for a magmatic sulphide melt, perhaps reflecting a high proportion of trapped melt component in the Lindsley cumulates.
McCreedy West is the most diverse deposit, as it features all types of ore identified so far: primitive and fractionated mss cumulates, iss-saturated ores, and highly differentiated bornite–cubanite–millerite–iss ores (FI < -20). One may relate this assemblage to mss fractionation of a melt more reduced than at Lindsley, to explain apparent fractionation past the iss field and enrichment in NiS and Cu2S–FeS. On the other hand, the cumulate trend at McCreedy West argues against an excessively reduced parent as it shows evidence for a change-over in DNi at an FI of 80. Naldrett et al. (1999) suggested on other grounds that the deposit may have received contributions from more than one parent melt. According to our scheme, one melt would have to be S rich to explain the cumulate trend, and another one metal rich to account for Ni enrichment in late-stage ores. Below we speculate that the range in ore types at McCreedy West may in principle be rationalized by large-scale liquid immiscibility of one common parent.
PGEs in Sudbury ores
If mss fractionation controls the major elements and normative ore compositions it should be reflected even more strongly in the PGE ratios. The DPGE deviate more markedly from unity than DNi and DCu, so their ratios are more sensitive to fractional crystallization than Ni/Cu ratios. Unfortunately, this is not evident at Sudbury. Neither do the compatible PGEs correlate well with the degree of fractionation (FI), nor do the incompatible PGEs follow Cu. The general picture is that the PGEs in Sudbury ores appear to be extremely heterogeneously distributed. Perhaps they were more affected by hydrothermal remobilization than the major elements (Farrow & Watkinson, 1996, 1997; Marshall et al., 1999), or a large PGE proportion occurs in discrete PGE phases (see Li & Naldrett, 1993), or the sample sizes were too small to be PGE representative.
Figure 12 compares the average PGE ratios between primitive cumulate ore and iss-saturated ore with the experimental PGE partitioning data. To extend the dataset, we include PGE averages from the Little Stobie mine quoted in the literature (Hoffman et al., 1979; Naldrett, 1981). The experimental PGE partitioning data are superimposed in Fig. 12 in three different ways; as straight DPGE (labelled ‘D’), as mss/melt model ratios after 70% mss fractionation in 20% increments (labelled ‘20’), and as mss/melt model ratios after 70% mss fractionation in 5% increments (‘5’).
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Clearly, agreement between ore ratios and experimental DPGE is disappointing. The only agreement is in terms of the general geochemical affinities of the PGEs, in that Os, Ir, Ru, and Rh are compatible and Pt and Pd incompatible with mss, both in experiment and in nature. We concur with Fleet et al. (1993) that is impossible to match in detail the experimental DPGE with PGE ore ratios. None the less, one should not go as far as Fleet et al. (1993) and discount on this basis that mss fractionation was the principal differentation mode, or deny the principally magmatic nature of the Sudbury ores (see also Li & Barnes, 1996). The differences among individual deposits and local PGE heterogeneity within single ores (1 error bars in Fig. 12) by far outweigh the poor agreement with the experimental data and do not warrant such far-reaching conclusions.
Liquid immiscibility in natural sulphide melts?
Liquid immiscibility may occur if there is a phase transition in the liquid state (see Chadwick, 1972; Coulet et al., 1999; Katayama et al., 2000) and if the sum of the free energies of the conjugate melts is less than the free energy of the mixture. In the present case, the major compositional contrast between the conjugate melts is with respect to Cu2S contents, so if the phase separation illustrated in Fig. 1 is indeed due to a change in melt structure, then it is probably induced by the metal/S ratio. We may infer from the compositional data that a sulphide melt may shift from a largely molecular structure at low metal/S, to a more metallic coordination as the metal-rich Cu2S complex becomes accumulated in the melt.
Immiscibility in the Fe–Ni–Cu–S system means that natural sulphide melts may evolve by means other than mss fractionation, i.e. by large-scale separation of conjugate melts. The only question is how to identify immiscibility after a sulphide melt has crystallized. Unmixing on a small scale would cause a highly correlated heterogeneity in bulk Cu2S and metal/S ratio among individual sulphide droplets. This was described and illustrated by Distler et al. (1986) for droplet ore in the Noril’sk–Talnakh deposit (Czamanske et al., 1992), and another example may be represented by droplet ore in the Frood deposit at Sudbury (Fleet, 1977). In both cases, we may be looking at a superliquidus phase separation of a bulk sulphide composition inside a two-phase field, arrested in situ by a rapidly crystallized silicate matrix.
Phase separation on a large scale can be identified only on the basis of fractionation paths. Large-scale separation may be accomplished by the physical properties of the conjugate melts. Ebel & Naldrett (1996, 1997) noted that Cu2S-enriched sulphide melts wet silicate material more readily than FeS-rich melts. Indeed, in Sudbury, Cu-rich ores are in general farther away from the intrusive contact than Ni-rich cumulate ore, perhaps because the derivative Cu2S-rich melts were drawn away from the contact into country rock by capillary forces. Once the conjugate partners have separated, they are deprived of the opportunity to exchange melt components and one degree of freedom is gained. Each melt will then follow its own fractional crystallization path, largely determined by its metal/S bulk ratio. The FeS-rich melt fraction—equivalent to the matrix melt in Fig. 1d and e—should experience Ni depletion and follow the ‘oxidized fractionation path’ of Fig. 9, i.e. precipitate mss cumulate ore and solidify in the iss stability field. A pooled Cu2S-rich melt—equivalent to the exsolved droplets in Fig. 1d and e—may follow the ‘reduced fractionation path’ in Fig. 9 owing to its high metal/S, and accumulate the NiS component alongside Cu2S while fractionating mss. We doubt that it ever crystallizes major amounts of iss when it reaches the critical Cu content. Our model fractionation paths in Fig. 9 imply that the S content may be low enough to precipitate bornite–digenite assemblages in addition to, or instead of iss. Final solidification will be at an iss–bornite–millerite eutectic.
In slowly cooled magmatic sulphide deposits that suffer multiple episodes of hydrothermal overprint (Marshall et al., 1999) it will remain speculative if large-scale liquid immiscibility may contribute to sulphide ore heterogeneity and if it will ever be identified. None the less, large-scale superliquidus phase separation in natural pooled sulphide melts would probably produce an ore diversity as seen at the McCreedy West deposit.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSPARTITION COEFFICIENTS (D)METAL SPECIATION IN SULPHIDE...DISCUSSIONCONCLUSIONSREFERENCES |
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Mss–melt partition coefficients determined over a range of temperature and fS2 may be used to recover from massive sulphide ores parent melt characteristics as ephemeral as the metal/S ratio. Bulk analysis of sulphide ore for metal and sulphur will fail to provide this information because cumulus processes, postmagmatic oxidation, and hydrothermal overprint will not leave the primary metal/S ratio intact. Particularly useful in this regard is Ni. If a sulphide melt fractionates at high fS2, the tendency of Ni to favour the melt is reversed and Ni begins to partition into mss. The temperature of the cross-over in DNi is shown to be highly sensitive to fS2. Contrary to previous assertions the DPGE seem not to be particularly sensitive to the metal/S ratio at trace concentration levels.
The discovery of liquid immiscibility in the Fe–Ni–Cu–S system allows us to outline major element oxidation states and metal speciations in sulphide melt. Liquid immiscibility in a system as important as Fe–Ni–Cu–S, if it operates on a large scale alongside mss fractionation, would also add a new dimension to chemical fractionation of pooled natural sulphide melts. However, its positive identification in nature will always be hampered by the long and complex cooling and recrystallization histories of massive sulphide ores.
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ACKNOWLEDGEMENTS |
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C.B. thanks Anton LeRoex for his invitation and hospitality at UCT. Tony Naldrett generously shared his dataset of sulphide analyses of ore deposits from the Sudbury Igneous Complex. Sergey Matveev and Philipp Pöml helped with microprobe analysis and SEM imaging of experimental runs, on polished sections prepared by P. Löbke. The paper benefited greatly from comments by James Brenan, Peter Lightfoot, Astrid Holzheid, Jean-Pierre Lorand, and particularly Mike Lesher. This work was supported by DFG grant Ba 964/15 and funding from the South African National Research Foundation.
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FOOTNOTES |
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*This paper is dedicated to Eugen F. Stumpfl on the occasion of his 70th birthday.
Corresponding author. Telephone: +49-251-8333047. Fax: +49-251-8338397. E-mail: chrisb{at}nwz.uni-muenster.de
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REFERENCES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSPARTITION COEFFICIENTS (D)METAL SPECIATION IN SULPHIDE...DISCUSSIONCONCLUSIONSREFERENCES |
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