GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 65-4
Presentation Time: 9:00 AM-5:30 PM


PETERSON, Joshua R., Geological Science, University of Texas at El Paso, 500 W University Ave, EL PASO, TX 79968, ARNOLD, Gail Lee, Department of Geological Sciences, University of Texas at El Paso, 500 W. University Ave, El Paso, TX 79968, BRUNNER, Benjamin, Institute of Tectonic Studies, Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968 and GOODELL, Philip C., Geological Sciences, University of Texas at El Paso, 500 W University Ave, El Paso, TX 79968,

More than half the world's copper (Cu) and more than 95% of the world’s molybdenum (Mo) is mined from porphyry deposits. Despite the importance of these deposits, major questions about their formation, in particular with respect to the metal enrichment and mineralization, remain unanswered. Based on molybdenum isotope trends, which are interpreted as Rayleigh isotope fractionation, it is often presumed that molybdenum partitioning takes place in a partially closed-system from which molybdenum is removed.

Sulfur (S) species, in particular sulfide (H2S, HS-, S2-), are fundamental in the formation of porphyry deposits, and it can be speculated that sulfur partitioning coincides with molybdenum sequestration. Based on this assumption, we developed a series of different scenarios for coupled Mo and S isotope fractionation, which resulted in Mo and S isotope patterns that are diagnostic for the chosen respective scenario.

We collected a suite of molybdenite and pyrite samples from the Sierrita Mine. In a first phase, we confirmed that there is enough variability in the sulfur isotope composition of molybdenite (MoS2) to observe such diagnostic patterns. In the second phase, we measured the sulfur isotope composition of paragenetic MoS2 and pyrite (FeS2), which allows us to narrow down the number of S isotope fractionation scenarios, and to draw conclusions with regards to equilibrium or non-equilibrium isotope fractionation. We will generate the first coupled Mo-S isotope dataset for a porphyry deposit, allowing for scalable testing and analysis of Rayleigh fractionation and relationships that are diagnostic to the precipitation and formation of MoS2, and applicable to porphyry Cu-Mo deposits in general.

We present a pilot study of a traditional-non-traditional isotope system applied to an ore system and exploration. This study will stimulate further research into ore related elemental systems (e.g. Cu-S) and a new look into S isotope research in ore deposits.