A TECHNIQUE FOR DETERMINING SELENIUM ISOTOPE RATIOS IN GEOLOGIC MATERIALS USING A CHG-ICP-DRC-MS, WITH APPLICATIONS TO SE-RICH VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSITS OF THE FINLAYSON LAKE DISTRICT, YUKON

Recent studies on selenium isotopes have demonstrated the potential application of ‘heavy stable isotopes’ to understanding sources, transportation and deposition in geological environments. However, many underlying problems have hindered the development of a facile technique to measure selenium stable isotopes in such materials. These include: 1) low selenium abundances of geologic materials, 2) scarcity and cost of MC-ICP-MS, N-TIMS instruments capable of performing such analyses, and 3) instrument discrimination and sensitivity. For these reasons, there is a paucity of selenium isotope studies in mineral deposit research. We have devised a technique for measuring selenium isotope ratios in geological samples by standard-bracketed Continuous-Hydride-Generation Inductively-Coupled Dynamic-Reaction-Cell Mass-Spectrometry (CHG-ICP-DRC-MS) using a Perkin Elmer-SCIEX Elan 6100® DRC with a unique continuous-hydride-generator. Instruments similar to the Elan 6100® DRC are readily accessible (>300 in North America) and have lowered polyatomic Ar-interferences with Se by several orders of magnitude, providing sufficiently precise isotopic measurements (delta 0.5‰). The analytical procedure was based upon published procedures to minimize: 1) problems of accessibility and cost, 2) isobaric interferences, 3) matrix affects, 4) memory affects, and 5) analytical and preparation time. An inverted fluorinated ethylene propylene  membrane-less gas-liquid separator hydride generator (HG) was designed to significantly reduce memory effects and excessive washout time characteristic of commercial HG systems. Matrix affects were reduced by pre-concentration of selenium and removal of interfering elements by thioglycollic fiber column separation. Natural variations of selenium isotopes were obtained for a diverse geological suite of samples, with a particular focus on volcanogenic massive sulfide (VMS) deposits from the Finlayson Lake District (FLD) of the Yukon, Canada. FLD samples range from -2 to -15 ‰ delta 82/76(MERCK), a larger range than previously found for hydrothermal ore-forming systems. These data help to assess sources of Se in the ore-forming fluids and interpreting the redox conditions of seawater at the time of ore-formation.