Geological controls on the stable tellurium isotope variation in tellurides and native tellurium from epithermal and orogenic gold deposits: Application to the Emperor gold-telluride deposit, Fiji

摘要

Natural variability of stable Te isotope compositions in terrestrial settings, including hydrothermal ore deposits, and the geochemical conditions that control their compositions are poorly understood. Previously published Te isotope data on tellurides, sulfides, and native tellurium are limited in number and derived from samples that were geologically unconstrained with regards to their location within an ore deposit. Here, we have obtained a range of values of delta Te-130/125 of -0.47 +/- 0.13 to 0.66 +/- 0.13 parts per thousand (n = 13), relative to an in-house, gravimetrically prepared Te standard of 99.9999% Te metal, for located samples of sylvanite ((AuAg)(2)Te-4), krennerite ((Au,Ag)Te-2), and native tellurium from the alkaline igneous rock-related Emperor gold telluride deposit, Fiji. Based on similarities in the geologic setting of the alkaline-igneous rock related gold-silver telluride deposits in Colorado (Cripple Creek, Boulder County, and La Plata) with Emperor, and the narrow range of delta Te-130/125 values for tellurides and native tellurium from Emperor and previously published, unlocated samples from Colorado deposits (Cripple Creek, delta Te-130/125 = -0.53 to 0.44 parts per thousand, n = 13; Boulder County, delta Te-130/125 = -0.25 to 0.16 parts per thousand, n = 7; La Plata, delta Te-130/125 = -0.78 to 0.0.17 parts per thousand, n = 11), it is likely that the main factor controlling the range of isotope values in these epithermal systems is the Te isotope composition of the genetically-related alkaline igneous source rocks. In comparison, previously published unlocated samples of tellurides from the giant orogenic Golden Mile Au deposit, Kalgoorlie, Australia, show an isotopic range of delta Te-130/125 = -1.00 to 0.51 parts per thousand (n = 6) and appear to have been more affected by the metamorphic nature of the ore fluids and/or the higher degree of water-rock interaction during hydrothermal fluid transport. It is speculated that the zonation from isotopically heavy to light Te isotopic compositions with distance from the likely causative intrusion within the Emperor deposit is primarily controlled by Rayleigh fractionation of isotopes within the ore fluid.