THE POTENTIAL OF PYRITE AS A (U-TH)/HE THERMOCHRONOMETER

Pyrite (FeS₂) is a common mineral found in igneous, sedimentary, and metamorphic rocks that offers extensive opportunities for study within Earth science. Depending on the closure temperature of helium in pyrite and the depth of crystallization, Pyrite could be useful as a thermochronometer for constraining the timing and rates of tectonic processes such as ophiolite obduction or regional orogenesis, or it could directly date crystallization associated with metasomatism or sulfidic mineralization of rare metal deposits.

One such outstanding query pertains to the enigmatic relationship between magmatism, magmatic fluids and economic mineralization of rare metals deposits. Sulfide mineralization and metasomatism are often difficult to directly date because many metasomatized rocks lack phases that are suitable for U/Pb geochronology or Ar/Ar thermochronology. Pyrite has been shown to be retentive of He from fluid inclusions on geologic timescales, yet radiogenic helium produced in-situ in pyrite has only been investigated in a few preliminary studies.

To test pyrite’s suitability for U-Th/He thermochronology, we dated pyrite from the Franklin zinc oxide mine in the Highlands terrane of New Jersey, which yielded a U-Th/He age of 501 ± 54 Ma (2σ) by the multi-aliquot method. We interpret this date to be a cooling age due to exhumation coincident with the Taconic orogeny, the closure of the Iapetus Ocean, and the deposition of the local Kittatinny Supergroup in the Appalachian foreland basin. Previous thermochronology of amphibole and biotite of the area yielded Ar/Ar ages of 906 ± 30 and 806 ± 24 Ma respectively, suggesting that pyrite has a closure temperature below 350˚C but significantly above ambient temperatures. Our future work will measure helium migration in Pyrite using conventional step-heating techniques to assess whether the system follows a thermally activated diffusivity model or a single-jump migration model.