Search

A mineralogical mass balance has been calculated for the fraction of platinum group elements (PGE) incorporated as solid solution in sulfides and sulfarsenides and the fraction that occurs as major constituents in platinum group minerals (PGM) in the Kelly Lake Ni-Cu-PGE sulfide deposit in the Copper Cliff offset, Sudbury. Low levels of PGE in sulfides were determined quantitatively for the first time in Sudbury ores by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS), providing significantly more accurate data on the inventory and mineralogical distribution of the PGE than in previous studies. The majority of the Pt (98%), Rh (97%), and Ir (86–91%), and lesser but still large fractions of Pd (77–87%) and Ru (67–71%), occur as discrete PGM with the remainder occurring (in decreasing abundance) in solid solution in gersdorffite (1,700 ppm ∑PGE), pentlandite (1.35 ppm), chalcopyrite (0.5 ppm), and pyrrhotite (0.3 ppm). Although the PGE are present in low concentrations in sulfides, the abundances of these phases can be high enough to constitute a significant fraction of the total PGE. In particular, significant proportions of the Pd (14–23%) and Ru (~30%) in the Kelly Lake samples are hosted by sulfides and sulfarsenides. This has important implications for the genesis of the PGE, for the redistribution of the PGE during cooling and hydrothermal alteration, and for metal recovery during ore beneficiation.

We have performed six experiments in which we equilibrated monosulfide solid solution (mss) with sulfide melt in evacuated silica capsules containing solid buffers to fix oxygen and sulfur fugacity, at temperatures of 950°C, 1000°C and 1050°C at bulk concentrations of ∼50 ppm for each of the PGE and Au, 5% Ni, and 7% Cu. Concentrations of O, S, Fe, Ni and Cu were determined by electron microprobe, whereas precious metal concentrations were determined by laser-ablation inductively-coupled mass spectrometry. Partition coefficients of all elements studied show minimal dependences on oxygen fugacity from the IW to the QFM buffers when sulfur fugacity is fixed at the Pt-PtS buffer. Cu, Pt, Pd and Au are strongly incompatible and Ru remains moderately to strongly compatible under all conditions studied. At all oxygen fugacities, at the Pt-PtS sulfur buffer, Ir and Rh remain highly compatible in mss. In the single run at both low oxygen and low sulfur fugacity Ir and Rh were found to be strongly incompatible in mss. At QFM and Pt-PtS the partition coefficient for Ni shows weak temperature dependence, ranging from 0.66 at 1050°C to 0.94 at 950°C. At lower oxygen and sulfur fugacity Ni showed much more incompatible behavior. Comparison with the compositions of sulfide ores from the Lindsley deposit of Sudbury suggests that the sulfide magma evolved under conditions close to the QFM and Pt-PtS buffers. The compatible behavior observed for Ni, Ir and Rh at Lindsley and most other magmatic sulfide deposits hosted by mafic rocks requires equilibration of mss and sulfide liquid at moderately high sulfur fugacity and low temperatures near to the solidus of the sulfide magma. We argue that this constraint requires that the sulfide magma must have evolved by equilibrium crystallization, rather than fractional segregation of mss as is commonly supposed.

Comparisons of PGE measurements by laser-ablation microprobe (LAM–ICP–MS) with micro-PIXE analyses on the same sulfide grains were made in order to assess data quality. Pyrrhotite, pentlandite and chalcopyrite from a few samples of disseminated ores of the Noril’sk I intrusion and massive ore from the Talnakh deposit, Russia, were analyzed for Pd and Rh by microPIXE and for all PGE by LAM–ICP–MS. Micro-PIXE analyses employed focused proton beams of 8 � 15 � m for grains in heavy-mineral separates and 15 � 20 � m for grains in hand samples. These represent analyzed volumes of about 2,350 � m 3 in pyrrhotite and 5,900 � m 3 for Pd in pentlandite. LAM analyses were performed with a focused, 266-nm laser beam fired at 0.3 mJ/ pulse and 10 Hz, resulting in 35–40 � m spots and analyzed volumes of about 85,000 � m 3 . Fused pyrrhotite standards separately doped with Pd, Ru, and Rh were synthesized for LAM–ICP–MS, and calibrated against sintered pyrrhotite PGE standards of Ballhaus & Sylvester (2000). The level of Pd, Ru and Rh varies by ~4 to 9% (1� ) in the fused pyrrhotite standards at a 50-� m sampling scale. The LAM and micro-PIXE results for the natural sulfides are quite comparable, with the large majority of determinations falling within 33% of each other for both Pd and Rh on the same spot. This agreement is considered encouraging for use of the LAM–ICP–MS for trace PGE analysis, considering the large differences in volumes analyzed, and in probable trace-element heterogeneity in the grains and standards. The reasonable concordance of most Pd and Rh results by LAM–ICP– MS and micro-PIXE indicates the potential of the LAM–ICP–MS technique. LAM analyses of the natural sulfides for Pt, Ir and Os were made using the standards of Ballhaus & Sylvester (2000). Platinum is more enriched in pentlandite (n = 10) than in pyrrhotite (3.2 ± 1.2 versus 0.73 ± 0.60 ppm) from Noril’sk (MP6–97), but Ir (0.94 ± 0.32 versus 1.38 ± 0.13 ppm) is only slightly higher in pyrrhotite (n = 10), whereas Os (~0.5 ppm) is comparably enriched in both minerals. Noril’sk chalcopyrite contains trivial amounts of all PGE except Pd (6.6–15 ppm).

L'indice de Lodestar, dans la partie orientale de Terre-Neuve, contient des concentrations importantes d'or dans des breches magmatiques-hydrothermales polymictes mineralisees en sulfures. La teneur maximale, 58.5 g/t d'or, caracterise un echantillon quelconque d'arsenopyrite massive. Les echantillons d'autres mineraux du minerai sulfure, la pyrite, par exemple, ne montrent pas de teneurs comparables. Nous en deduisons que l'or serait directement lie a l'arsenopyrite. Les echantillons contenant des sulfures ont ete etudies avec une batterie de techniques analytiques. Les developpements recents d'analyse in situ par ablation au laser avec plasma a couplage inductif et spectrometrie de masse (LAM-ICP-MS) ont mene a de nouvelles observations a propos de la distribution des elements traces dans les mineraux. Les analyses LAM-ICP-MS ont ete faites pour determiner si l'or est distribue de facon homogene dans les sulfures, en particulier l'arsenopyrite, ou s'il se presente sous forme de micropepites. Les resultats LAM-ICP-MS demontrent que l'indice Lodestar contient des concentrations d'or atteignant 201 g/t. L'or est reparti de facon homogene dans la structure de l'arsenopyrite, sans effet de micropepites. Les autres sulfures, la pyrite et la chalcopyrite, par exemple, contiennent des teneurs en or tres faibles. Les teneurs en or de l'arsenopyrite dans des echantillons individuels varient, probablement en fonction des teneurs en arsenic. Parce que nous n'avons pas pu deceler l'or par les autres techniques micro-analytiques, cet element doit etre incorpore chimiquement dans la structure; on pourrait donc le qualifier d'or "invisible".

X-ray diffraction (XRD), Mossbauer spectrometry (MS), secondary ions mass spectroscopy (SIMS) and laser-ablation microprobe–inductively coupled plasma–mass spectrometry (LAM–ICP–MS) were used to study mineral samples of Colombian auriferous ores collected from the “El Diamante” mine, located in the municipality of Guachavez-Narino, in Colombia. The samples were prepared as polished thin sections and polished sections. From XRD data, quartz, sphalerite and pyrite were detected and their respective cell parameters were estimated. From MS analyses, pyrite, arsenopyrite and chalcopyrite were identified; their respective hyperfine parameters and respective texture were deduced. Multiple regions of approximately 200 × 200 μm in each sample were analyzed with SIMS; the occurrence of “invisible gold” associated mainly with pyrite and secondarily with arsenopyrite could thus be assigned. It was also found that pyrite is of the arsenious type. Spots from 30 to 40 μm in diameter were analyzed with LAM–ICP–MS for pyrite, arsenopyrite and sphalerite; Au is “homogeneously” distributed inside the structure of the arsenious pyrite and the arsenopyrite (not as inclusions); the chemical composition indicates similarities of this “invisible gold”, forming a solid solution with arsenious pyrite and arsenopyrite. One hundred nineteen and 62 ppm of ‘invisible gold’ was quantified in 21 spots analyzed on pyrite and in 14 spots on arsenopyrite, respectively.