Se Gilbert

Routine quantitative multi-element analysis of sulphide minerals by laser ablation ICP-MS: Standard development and consideration of matrix effects

ABSTRACT The paper describes a calibration standard for quantitative in-situ multi-element analysis of sulphide minerals by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Standard STDGL2b2 is a mixture of 25% Zn concentrate and 75% pyrrhotite doped with a number of additional trace elements and fused into an X-ray fluorescence (XRF) glass disk. The homogeneity of the disk has been tested for the 55 elements of interest. All elements except Se, Tl, Au and Pt are homogenous (< 5% variation). Accurate analysis for the above four elements requires averaging multiple analyses of the standard. Element concentrations in STDGL2b2 were quantified by XRF and standard solution ICP-MS using a Finnigan Element and an Agilent 4500 mass-spectrometers. For the analysis of pyrite, pyrrhotite, chalcopyrite, galena and sphalerite, analytical errors caused by matrix-dependent fractionation have been evaluated by analysing five pressed-powder pellets.. The compositions of the powders have been analysed by XRF and solution ICP-MS. When Fe or Pb can be used as the internal standard, errors for most elements are < 15%, but reach up to 50% for W, Zn and Cd, requiring correction factors to be introduced. However, when Zn is used as the internal standard, significant correction factors are required for most elements. Comparison of the results obtained with two different laser microprobes, a solid state 213 nm and an excimer 193 nm, indicates that either is well suited for LA-ICP-MS analysis of sulphide minerals using STDGL2b2 as the calibration standard. Use of STDGL2b2 significantly improves accuracy of sulphide analysis by LA-ICP-MS compared to silicate reference materials, such as the NIST 600 series.

Gold-telluride nanoparticles revealed in arsenic-free pyrite

Abstract Pyrite, the most abundant sulfide on Earth and a common component of gold deposits, can be a significant host for refractory gold. This is the first documentation of pore-attached, composite Autelluride nanoparticles in “arsenic-free” pyrite. Trace elements mapping in pyrite from an intrusionhosted Au deposit with orogenic overprint (Dongping, China) shows trails of tellurides overlapping Co-Ni-zonation. Intragranular microfracturing, anomalous anisotropy, and high porosity are all features consistent with devolatilization attributable to the orogenic event. The pyrite-hosted nanoparticles are likely the “frozen,” solid expression of Te-rich, Au-Ag-Pb-bearing vapors discharged at this stage. Nanoparticle formation, as presented here, provides the “smallest-scale” tool to fingerprint Au-trapping during crustal metamorphism

Multivariate Analysis of an LA-ICP-MS Trace Element Dataset for Pyrite

Application of multivariate statistics to trace element datasets is reviewed using 164 multi-element LA-ICP-MS spot analyses of pyrite from the Moonlight epithermal gold prospect, Queensland, Australia. Multivariate analysis of variance (MANOVA) is used to demonstrate that classification of pyrite on morphological and other non-numeric factors is geochemically valid. Parallel coordinate plots and correlation cluster analysis using Spearman’s coefficients are used to discover unexpected elemental relationships without making assumptions a priori. Finally, principal component analysis and factor analysis are used to demonstrate the presence of sub-classes of pyrite. Corroborated with geological data, statistical analysis provides evidence for successive generations of hydrothermal fluids, each introducing specific metals, and for partial or complete replacement of different minerals. The data permit reinterpretation of Moonlight as a telescoped system where epithermal-Au (± base metals) is superposed onto early porphyry-Mo mineralization.

Optimisation of laser parameters for the analysis of sulphur isotopes in sulphide minerals by laser ablation ICP-MS

The effects of laser type (Nd:YAG and excimer lasers) and their analytical parameters on 34S/32S isotopic fractionation during LA-ICP-MS analysis were investigated. Laser fluence has a larger fractionation effect when ablating pyrite with the New Wave Nd:YAG 193 nm laser, compared to the Resonetics 193 nm excimer laser which did not produce significant fractionation over the same range of fluence (1.3–3.7 J cm−2). Matrix effects occurred between pyrite and bornite on both laser systems, especially at low fluence. However, matrix effects can be reduced with increasing fluence lessening the need for matrix matched reference materials. The effects of interface tubing configuration were also investigated and the addition of a ‘squid’ mixing device, a coil of small diameter Tygon tubing and a small volume glass bulb, was found to improve signal precision and reproducibility and decrease the washout time of the S signal between analyses. The degassing of air from the inner surfaces of the interface tubing can produce significant isotopic drift (8‰ h−1), hence flushing the tubing prior to analyses is crucial for reproducible analyses. The isotopic composition and homogeneity of a range of sulphide minerals were characterised for use as potential reference materials. We present preliminary data for a large, isotopically homogeneous pyrite crystal (PPP-1) which could be considered as a new isotopic reference material (δ34SV-CDT = 5.3 ± 0.2‰).

Fractionation of sulphur relative to iron during laser ablation-ICP-MS analyses of sulphide minerals: implications for quantification

In this study we investigate the effect that the mineral composition has on the quantification of sulphur by Laser Ablation ICP-MS (LA-ICP-MS) between a range of sulphide minerals: pyrite, pyrrhotite, bornite, chalcopyrite, sphalerite, pentlandite and tetrahedrite. The amount of S fractionation, relative to Fe, was compared between three different nano-second pulse width laser ablation systems: a 213 nm Nd:YAG, a 193 nm Nd:YAG and a 193 nm excimer. Significant matrix effects were seen for some minerals. With the 213 nm Nd:YAG laser, the yield (sensitivity per µg g−1) of S relative to Fe is up to 50% higher for tetrahedrite and approximately 30% higher for bornite and chalcopyrite when compared to the yields of pyrite, whereas no fractionation was seen between Cu and Fe. For analyses on a fixed position on the sample, significant down-hole fractionation (DHF) occurred where S/Fe ratios increased during an analysis. The rate of DHF is also mineral specific, enhancing the need for matrix matched standards for accurate S analysis. The ablation properties of the minerals were also investigated by characterising the shape of the ablation craters and the composition and morphology of the deposited aerosol material around the ablation site using a field emission scanning electron microscope (FE-SEM). At fluences below 3.5 J cm−2, pyrite is ablated efficiently by all laser systems with minimal melting around the ablation site, producing steep sided ablation craters. However, some melting occurs in and around the craters for most other sulphide minerals. The amount of melting is mineral specific and primarily dependent on its physical properties (e.g., bond strength and melting point). The greater the extent of melting, the more S fractionation occurs, consistent with the higher volatility of S relative to Fe.

Cu–Ni–PGE fertility of the Yoko-Dovyren layered massif (northern Transbaikalia, Russia): thermodynamic modeling of sulfide compositions in low mineralized dunite based on quantitative sulfide mineralogy

The geology and major types of sulfide mineralization in the Yoko-Dovyren layered massif (northern Transbaikalia, Russia) are presented. This study focuses on the structure, mineralogy, and geochemistry of poorly mineralized plagiodunite and dunite in the lower part of the intrusion. Assuming these rocks contain key information on the timing of sulfide immiscibility in the original cumulate pile, we apply a novel approach which combines estimates of the average sulfide compositions in each particular rock with thermodynamic modeling of the geochemistry of the original sulfide liquid. To approach the goal, an updated sulfide version of the COMAGMAT-5 model was used. Results of simulations of sulfide immiscibility in initially S-undersaturated olivine cumulates demonstrate a strong effect of the decreasing fraction of the silicate melt, due to crystallization of silicate and oxide minerals, on the composition of the intercumulus sulfide liquid. Comparison of the observed and modeled sulfide compositions indicates that the proposed modeling reproduces well the average concentrations of Cu, Cd, Ag, and Pd in natural sulfides. This suggests the sulfide control on the distribution of these elements in the rocks. Conversely, data for Pt and Au suggest that a significant portion of these elements could present in a native form, thus depleting the intercumulus sulfide melt at an early stage of crystallization.

Matrix dependency for oxide production rates by LA-ICP-MS

The production rates of polyatomic oxygen interferents (MO+/M+) during LA-ICP-MS analysis were investigated in a range of silicate materials and metals. The total amount of oxygen in the ICP is significantly lower for laser ablation analysis compared to solution nebulisation analysis resulting in lower oxide production rates. However, these interferents can still be significant for some elements. The contribution of oxygen from the material being ablated was found to influence the oxide production rate (OPR). When using a well degassed system to minimise the entrainment of atmospheric oxygen, the OPR for Al, Si and W was up to 4 times lower when ablating the elements as a metal compared to when ablating oxygen-bearing minerals. There is a relationship between the MO+/M+ production rate and the cation–oxygen dissociation energy for elements measured by solution and by laser ablation ICP-MS. However, for Hf and Th the OPR varied significantly depending on the mineral being ablated under the same analytical conditions (0.007–0.02% for Hf and 0.09–0.2% for Th), whereas UO+/U+ was more consistent (0.058–0.063%). The effects of carrier gas flow rate and resulting differences in aerosol breakdown and ionisation in the ICP were investigated for U and Th oxides in NIST610, NIST612, zircon (ZrSiO4), monazite ([REE,Th]PO4) and uraninite (UO2). Increasing the Ar flow rate had a larger effect on the Th OPR (0.05 to 0.5%) compared to U (0.04 to 0.09%) when ablating the NIST610 glass. The relative differences in the OPR between minerals compared to NIST610 were small for U, with all minerals having the same OPR except for uraninite at high carrier gas flow rates (35% higher). In contrast the OPR for Th was highly variable between all minerals and showed differing responses to changes in the Ar flow. This study highlights the complexities in oxide production for LA-ICP-MS compared to solution analyses, and that the OPR for some elements is strongly dependent on the material being ablated. Also, that there can be a significant contribution to the MO+ production from ionisation of an incompletely atomised sample aerosol in the plasma.