TIMS VS MC-ICP-MS PB ISOTOPE ANALYSIS WITH APPLICATION TO ORE DEPOSIT STUDIES

A zone of polymetallic gold mineralization was discovered at Conical Seamount, located on the flanks of Lihir Island (Papua New Guinea). The seamount is a volcanic cone composed of trachybasalts to tephrites underlain by about 5 km of sediments deposited on Cretaceous oceanic crust. Lead isotopic analyses were carried out on mineralized samples and volcanic and sedimentary rocks using standard TIMS techniques in order to reveal the ore metal sources. The data indicated that the ores have more radiogenic Pb isotopic compositions than the host lavas, but similar to some of the sediments underlying the seamount, suggesting the sediments as the source of the ore metals. Reanalysis of the same sample set using a MC-ICP-MS (Nu-Plasma) with Tl-normalization revealed that the ores and the host lavas share similar Pb isotopic compositions, suggesting that the Pb in the mineralized zones was derived from the alkaline magmas in the area. Comparison between the two techniques reveals that errors generated during the traditional TIMS analysis of samples are not adequately compensated by application of correction factors derived from the analysis of pure standards. The mass bias-related error generated during the analysis of rocks and ores is, in some cases, much larger than the error observed for analyses of NBS 981. This suggests that the common TIMS practice of utilizing standard-based data for mass bias corrections can lead to inaccurate assessments of Pb isotopic compositions. The discrepancy between the analysis of the standards and rock/mineral samples may be related to many factors, including variable blanks, varying amounts of sample loaded for individual samples, and presence of trace amounts of other metals. All of these factors can affect the mass bias to varying degrees, and make application of the correction factors derived from standards inadequate for the development of precise data sets. Better accuracy and precision of the Pb isotopic data acquired by the MC-ICP-MS was confirmed by multiple analysis of NBS 981 and USGS rock standards. MC-ICP-MS Pb isotopic data, in combination with petrographic observations and geochemical data from the Conical Seamount suggest exsolution of metal-bearing fluids from a relatively shallow magma chamber as the most probable mechanism for the origin of the mineralization.