Determination Of Ppt-level Platnium Group Elements In Geological Samples And Its Application

The Platinum Group Element(PGE) content in the mafic-ultramafic intrusion is an efficient geochemistry tool for study on the mantle-derived magmas and magmatic Ni-Cu-(PGE) sulfide deposits. East Tianshan area, NW Xiangjiang, is an important Ni-Cu deposits base. In the east part of this area, there exist many Ni-Cu deposits in the mafic-ultramafic intrusions, such as Huangshan, Xiangshan and Hulu deposit. Recently, many intrusions were reported with limited mineralization in the western part. But the ages and mineralization potentials of these intrusions are still unknown. A study on the PGE contents of these intrusions would be helpful for understanding the magma process and the mineralization potentials. From the study of Tarim Large Igneous Province(Tarim LIP), however, both the extremely low abundance and the heterogneous distribution(e.g. nugget effect) of PGE in the samples have been found. Even the commonly used nickel sulfide fire assay(Ni S-FA) technique is difficult to quantify the content of PGE in these sample.In this study, PGE was determined by isotope dilution-inductively coupled plasma-mass spectrometry(ID-ICP-MS) with combination of the sulfide fire assay method. Three key improvements were carried out with this method:(1) using ID method, the loss of PGE during sample preparation can be overcome;(2) a tandem of two columns packed with cation resin and Ln resin was employed to separate PGE from the interfering elements;(3) the resulting solution could be directly introduced by the membrane desolvation system and the detection signals could be increased for analytes. As a result, more accurate determination of PGE at very low content levels could be achieved.For sample preparation, appropriate amounts of 99Ru-193 Ir, 110Pd-198 Pt and 190 Os spike solutions were mixed with sample powders and flux in a clay crucible. The crucible was fused at 1000 °C for 1.5 h and a sulfide button was formed. The buton was dissolved with HCl and PGE residue was obtained. The residue was distilled in HNO3 medium for Os, which was simultaneusely absorbed by H2 O. This H2 O solution was analyzed by ICP-MS for Os. The remaining solution was converted to 0.24 M HCl medium and loaded onto the tandem columns packed with cation and Ln resins. The eluents were collected and directly analyzed by membrane desolvation-ICP-MS for Ru, Rh, Pd, Os, Ir and Pt. PGE contents are calculated with isotope dilution method whereas Rh by the conventional external standard calibration combined with 193 Ir as internal standard. Based on the separation by the tandem columns in the 0.24 M HCl solution, PGE over 99% could be recovered whereas the interfering elements were effectively revoved. This method was validated using a series of PGE reference materials, and the analytical data consistent with their recommended and literature values presented the method precision is better than 10%. The whole procedural blanks are within the range of 0.045 ng for Ir to 0.960 ng for Pd and limits of detection(3s) are 0.006 ng g-1 for Ir to 0.045 ng g-1 for Pd, indicating this method can be applicable to analysis of trace PGE in geological samplesBy using the improved method for the PGE analysis, four intrusions in Dacaotan area, western part of East Tianshan were investigated on the PGE geochemistry characteristics. Together with zircon U-Pb dating, mineral compositions, major and trace data and Sr-Nd isotopic compositions, the ages, the natures of mantle source, compositions of parental magma were discussed for the forming processes of these intrusions. At last, the potentials of Ni-Cu mineralization were also evaluated for these four intrusions.The No.1 and No.2 mafic-ultramafic intrusions in Dacaotan area located in the middle of Jueluotag suture zone and north of Kanggnertage fault. No. 1 intrusion consists of olivine gabbro whereas No. 2 is mainly composed of gabbro and dunite occurred in the center of its body. No.3 and No.4 intrusions in Dacaotan area located in the NW of Tuwu and south of Kanggnertage fault. No. 3 intrusion consists of gabbro while No. 2 is composed of diorite. The four intrusions were found with low PGE contents(∑PGE=0.15-8.418 ng g-1) and depleted PGE patterns. The Cu/Pd ratios were in the range of 1×104-5×106, indicating the sulfide-saturated parental magmas. The Pd/Ir and Ni/Cu ratios ranged in 10-90 and 1.6-63, respectively. The primary magmas had a similar composition with high Mg basalts.For No.1 and No.2 intrusions, MELTS simulations indicated that the primitive magmas were derived from depleted mantle source by low degrees of partial melting(5-8%) and sulfides remained in the mantle source, leading to the PGE depletion of the magma. Compared with the simulated results, the Ni contents in olivine of No.1 intrusion are significantly low and this could be explained by a small-scale sulfide segregation(<0.5%). Crust contamination was supposed to cause the sulfide segregation. Trace elements and Nd isotopic data show that the parental magmas of the two intrusions have been contaminated by small amounts(1-5%) of the coeval A-type granite and crust during ascent. The olivine fractional crystallization may also play a role in the sulfide segregation. Due to the low degree of partial melting of the mantle source and limited crust contamination, No.1 and No.2 intrusions in Dacaotan area seem to have no potential for an economic Ni-Cu sulfide deposit. For No.3 and No.4 intrusions, the parental magmas might experience the limited contaminations from crust(5-10%) and coeval A-type granite. Due to relatively low degrees of partial melting of the mantle source, sulfides might remain in the mantle source. The parental magmas are PGE-depleted and have little chance to form economic Ni-Cu sulfide deposits in the two intrusions.Compared with mineralized intrusions in East Tianshan, four items related to PGE contents were suggested to evaluate mineralization potentials for mafic-ultramafic intrusions in this area:(1) positive corrections were found between IPGE and PPGE in the rocks from mineralization intrusions;(2) besides the high Cu/Pd ratios, high Pd and Cu concentrations were related to mineralization;(3) lower Pd/Ir ratios indicated the promising mineralization;(4) higher PGE contents with constant(Th/Nb)N values indicated selective assimilation of crustal sulfides.