The tellurium isotope compositions of naturally-occurring tellurides, native tellurium, and tellurites were measured by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) and compared to theoretical values for equilibrium mass-dependent isotopic fractionation of representative Te-bearing species estimated with first-principles thermodynamic calculations. Calculated fractionation models suggest that 130/125 Te fractionations as large as 4 ‰ occur at 100° C between coexisting Te(IV) and Te(II) or Te(0) compounds, and smaller, typically < 1 ‰ fractionations occur between coexisting Te(-I) or Te(-II) (Au,Ag)Te2 minerals (i.e., calaverite, krennerite) and (Au,Ag)2Te minerals (i.e., petzite, hessite). In general, heavyTe/light Te is predicted to be higher for more oxidized species, and lower for reduced species.

Tellurides in the system Au-Ag-Te and native tellurium analyzed in this study have values of δ130/125Te = -1.54 to 0.44 ‰ and δ130/125 Te = -0.74 to 0.16 ‰, respectively, whereas those for tellurites (tellurite, paratellurite, emmonsite and poughite) range from δ130/125 Te = -1.58 to 0.59 ‰. Thus, the isotopic composition for both oxidized and reduced species are broadly coincident. Calculations of per mil isotopic variation per amu for each sample suggest that mass-dependent processes are responsible for fractionation. In one sample of coexisting primary native tellurium and secondary emmonsite, δ130/125 Te compositions were identical. The coincidence of δ130/125 Te between all oxidized and reduced species in this study and the apparent lack of isotopic fractionation between native tellurium and emmonsite in one sample suggest that oxidation processes cause little to no fractionation.

Because Te is predominantly transported as an oxidized aqueous phase or as a reduced vapor phase under hydrothermal conditions, either a reduction of oxidized Te in hydrothermal liquids or deposition of Te from a reduced vapor to a solid is necessary to form the common tellurides and native tellurium in ore-forming systems. Our data suggest that these sorts of reactions during mineralization may account for a ~3 ‰ range of δ130/125 Te values. Based on the data ranges for Te minerals from various ore deposits, the underpinning geologic processes responsible for mineralization seem to have primary control on the magnitude of fractionation, with tellurides in epithermal gold deposits showing a narrower range of isotope values than those in orogenic gold and volcanogenic massive sulfide deposits.