| The distribution and concentrations of platinum group elements (PGEs; Ru, Rh, Pd, Os, Ir and Pt) in sulfide minerals, such as chalcopyrite, are useful for understanding genesis of ore deposit, mantle evolution, magmatic crystallization differentiation and providing important information for mineral exploration. They are indentified as a set of geochemistry "indicators" by geologists. However, the concentrations of PGEs and Au are extremely low (usually from several ng g to hundreds of ng g-1) and dispersive. In the past decades, two traditional methods were used for preprocessing of sulfide minerals. First, sulfide minerals were often digested by acid or alkali for solution nebulization ICP-MS (SN-ICP-MS) determination of PGEs and Au. Secondly, NiS fire assay was used as a collection procedure of PGEs and Au before SN-ICP-MS or LA-ICP-MS. Yet, these two methods are just suitable for bulk analysis of PGEs and Au in sulfide minerals, which means that these measurements can not distinguish between PGEs actually present in solid solution in the sulfides and those present as discrete phases included within or adhered to the sulfides. Sometimes, this information is helpful for accounting for some geological research and interesting for geologists. Therefore, it’s significant to develop a microanalysis method for determination of low concentrations of PGEs and Au in sulfide minerals.Since1980s, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become a powerful technique for the in situ analysis of minerals and other geological samples at a fine spatial resolution (tens of micrometers) and at sub ng g-1detection limits. It has been successfully applied to the determination of the concentrations and spatial distribution of PGEs and Au in natural sulfide minerals. Nevertheless, the application of LA-ICP-MS for determination of PGEs and Au in natural sulfide minerals presents several challenges:(1) the ablation behaviors of sulfide minerals are different from other geological samples such as silicate samples;(2) the concentrations of PGEs and Au are much lower than those of NIST glasses, and therefore NIST glasses certified reference materials are not suitable for calibration when determination of PGEs and Au in sulfide minerals;(3) but so far no ideal, commercial sulfide calibration standard can be available for the calibration of PGEs and Au;(4) moreover, base metal argide interferences as well as isobar interferences are a well-known potential problem for quantification of PGEs. Hence, it’s necessary to exclude these interferences and establish a reliable quantitative method for determination of PGEs and Au in sulfide minerals.In this paper, we carried out our experiments as following to try to overcome the problem described above:1.Five natural sulfide minerals were analyzed using LA-ICP-MS with the193nm ArF excimer laser system. Elemental fractionation effect and influences of spot size, repetition rate and laser energy on laser behavior of different sulfides were investigated based on different laser ablation profiles and transient signal response. The results showed that laser ablation characteristics and elemental fractionation effect of sulfides varied significantly with the physical properties and chemical components of sulfides. The ablation halos of pyrite, molybdenite and sphalerite were approximately10times the diameter of the corresponding spot size, whereas chalcopyrite and pyrrhotite were about14times. The elemental fractionation indexes (EFI) indicated that pyrite and molybdenite behaved apparent elemental fractionation while chalcopyrite, pyrrhotite and sphalerite, EFI of which approached to1.0, did not. For sulfide minerals microanalysis by LA-ICP-MS, the accurate analysis results can be obtained under the laser conditions of large spot size, low repetition rate and low laser energy.2.This paper develops sulfide reference materials for quantitative PGEs and Au in natural sulfide minerals by LA-ICP-MS. A series of sulfide reference materials doped with different concentrations of PGEs and Au were prepared using a home-made high temperature furnace. Homogeneity of element distribution was expressed as the RSD of signal intensity of PGEs and Au, which was less than10%. The detected concentrations of PGEs in synthetic sulfide reference materials were obtained both by SN-ICP-MS and that of Au was tested by LA-ICP-MS. The effective concentrations of PGEs and Au were considered as reference value for calibration and compared with the added concentrations of PGEs and Au. The detected concentrations of PGEs and Au are all higher than the added ones, but there are good liner relations between two values. Accordingly, it’s reliable to accurately add PGEs and Au into the sulfide minerals after calibration.3.To develop a method for determination of PGEs and Au, base metal argide interferences and isobar interferences were subtracted from the total signal intensity by mathematical calculation in two ways. One is the method summarized by Sylvester for determination of PGEs and Au in our synthetic sulfide reference materials, which contain Fe, Ni, Cu and Zn. The other one is a simplified mathematical algorithm. Interference ratio (IR) was defined and used to evaluate the interference degree of base metal argides. IR values were>10%when the concentrations of PGEs in sulfide reference materials were low than1.0μg g-1. As the concentrations of PGEs were>10μg g-1, the IR values were less than10%, which can be ignored. Then, our synthetic sulfide reference materials added with10,25,75μg g-1PGEs and Au were treated as unknown samples and detected by LA-ICP-MS with synthetic sulfide reference material added with50μg g-1PGEs and Au as a calibrator (S-50). The obtained results were compared with those detected by SN-ICP-MS. The relative percentage differences these two results were≤11%, which demonstrated that determination of PGEs using our synthetic sulfide reference material as calibrators could be successfully achieved. The synthetic sulfide reference materials were then employed as calibrators for quantitative determination of PGEs in two natural chalcopyrite samples. The valid results demonstrated that our synthetic standards can be considered for quantitative microanalysis of PGEs in natural sulfide minerals.4. Nine pyrite and six chalcopyrite samples collected from Baizhangzi deposit were analyzed by LA-ICP-MS using our synthetic sulfide reference material S-50as a calibrator. Ten single spot analyses on each sample were conducted to assess the concentrations and distribution of PGEs and Au in these sulfide minerals. Gold appeared high concentrations in both pyrite and chalcopyrite samples. Two of the nine pyrite samples and five of the six chalcopyrite samples contained more than1.0μg g-1Au, and another3pyrite samples contained ca.0.50μg g-1Au. Rh and Pd were concentrated in chalcopyrite samples, in which the concentrations of Rh and Pd were≥1.0μg g-1. While the concentrations of Rh and Pd in pyrite samples were much lower than those of chalcopyrite samples. The concentrations of Ru and Pt in both pyrite and chalcopyrite samples were less than1.0μg g-1. |
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