U-Th-He Geochronology of Pyrite from Alteration of the Au-Fe-Skarn Novogodnee-Monto Deposit (Polar Urals, Russia)—The Next Step in the Development of a New Approach for Direct Dating of Ore-Forming Processes

Olga Yakubovich,Mary Podolskaya,Ekaterina Ivanova,Eugenia Tyukova,Ivan Sobolev,Ilya Vikentyev,Alexander Kotov

doi:10.3390/geosciences11100408

Olga Yakubovich, Mary Podolskaya + Show 5 more

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https://doi.org/10.3390/geosciences11100408

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Abstract

We report on the application of the U-Th-He method for the direct dating of pyrite from the alteration halo of the Novogodnee-Monto Au-Fe-skarn deposit, Polar Urals. The deposit is genetically related to the formation of volcanogenic complexes of the Ural Paleozoic belt. A modification of the original methodology for measuring U, Th and He isotopes in a single grain allowed us to determine a U-Th-He age of 382 ± 8 Ma (2σ) based on six pyrite samples from the altered rocks of the deposit (U mass fraction ~0.2 mg/kg; Th/U ~3.5; 4He specific volume ~10−5 cm3·STP·g−1). This age is consistent with estimates of the age of ore formation and coeval with the end of the period of island arc magmatic activity. Our results indicate that U-Th-He dating for pyrite samples of ~1 mg in weight from the hydrothermal-metasomatic halo of ore bodies is possible, providing a crucial next step in the development of U-Th-He pyrite geochronology.

Highlights

In the past decade, a large body of work has been focused on understanding the behavior of radiogenic helium in various minerals that has not been traditionally studied for geochronology, such as hematite, magnetite, calcite, native metals, arsenides and sulfides ([1,2,3,4,5,6,7] and references within)
Scanning electron microscope (SEM) analysis of the pyrite grains revealed the presence of micron-sized inclusions of gold, galena, hessite and arsenopyrite (Figure 6)
It is possible that sample NM-17 contains pyrite of two generations, the first of which corresponds to the skarn stage of formation of the Novogodnee-Monto deposit, and the second pyrite generation corresponds to a lower-temperature— beresite stage (382–360 Ma)

Summary

Introduction

A large body of work has been focused on understanding the behavior of radiogenic helium in various minerals that has not been traditionally studied for geochronology, such as hematite, magnetite, calcite, native metals, arsenides and sulfides ([1,2,3,4,5,6,7] and references within). Some of these minerals have shown a high retentivity of helium, which may facilitate their use as He-geochronometers [1,3,8], including pyrite. The imprecise ages could be related to isotopic heterogeneity of the trapped Os or by disruption of the Re-Os system by subsequent processes such as metamorphism

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