The measurement of the Se/S ratios in sulphide minerals and their application to ore deposit studies
Fitzpatrick, Alexander John
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New analytical techniques have been developed for the determination of selenium concentrations in sulphide minerals to assess the utility of the Se/S concentration ratios in tracing fluids associated with ore deposit formation. This has been accomplished via a new hydride generation (HG) sample introduction method for the determination of selenium contents in sulphide minerals, development of solid calibration standards by a sol-gel process, and the establishment of protocols for the measurement of selenium/sulphur ratios in sulphides using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The low concentrations of selenium in sulphides require the use of hydride generation, which also requires the removal of metals to be effective. A process for the determination of selenium in sulphide minerals (~ 50 mg sample weight) wherein metals are removed by precipitation under alkaline conditions, followed by further removal by chelating resin, was developed. Determinations by HG-ICPMS on reference materials showed quantitative recoveries (100±5 %). Precision is 10 % relative standard deviation and the detection limit is 4 µg g-1 in a sulphide mineral. A sol-gel method for the fabrication of multi-element calibration standards, suitable for laser ablation, was developed. The addition of selenium and sulphur to a normal sol-gel method does not introduce detectable heterogeneity. Xerogel heterogeneity is less than that of the NIST glass standards. Calculated sulphur contents in the NIST glass SRMs are comparable to published data. Xerogels are potentially useful as standards in studies of glasses, minerals, and other materials. The development of a laser ablation technique allowed measurement of Se/S ratios in sulphide minerals. Sulphide minerals were sampled from volcanic hosted massive sulphide (VHMS) (Flin Flon area, Canada), high-sulphidation epithermal (Pierina, Peru), and iron oxide copper gold (IOCG) (Mantoverde, Chile) deposits. Thermodynamic data, mineral assemblages, and isotopic compositions are used to define depositional conditions and Se/S ratios of ore generating fluids. Although the elements can fractionate from each other, high Se/S ratios and low δ34S values generally reflect magmatic fluids, typical of VHMS and epithermal deposits, whereas the opposite is true for basinal or evaporitic sources, such as recorded by sulphides at the Mantoverde deposits. The Se/S ratios aid in the identification of ore generating fluid sources and can indicate mixing of fluids.