TESTING THE ROBUSTNESS OF RE-OS LOW-LEVEL SULFIDE CHRONOMETERS: AN EXAMPLE FROM METAMORPHOSED VMS ORES, TRANS HUDSON OROGEN, CANADA

Re-Os geochronology of ‘low-level' sulfide minerals is emerging as a viable technique for dating hydrothermal ore-forming events. However, the ability of common sulfide minerals to retain primary age information through subsequent thermal events is currently unknown. To this end, we have analyzed Re-Os isotopes in pyrite, pyrrhotite, and chalcopyrite from variably metamorphosed volcanogenic massive sulfide deposits in the Paleoproterozoic Trans-Hudson Orogen, Canada. Independent age information indicates that peak metamorphism throughout the study region, which includes the low-grade Flin Flon belt and the high-grade Snow Lake belt and Kisseynew domain, occurred at ca. 1815 Ma, some 85 million years after deposition of the ca. 1900 Ma VMS ores.

The Re-Os sulfide results reveal a progression towards younger Re-Os ages and higher initial ¹⁸⁷Os/¹⁸⁸Os (Os_i) ratios with increasing metamorphic grade, consistent with resetting of primary Re-Os isotopic compositions. A mixture of pyrite and pyrrhotite from the greenschist grade Trout Lake deposit yields scattered Re-Os results with an ‘age' of < 1000 Ma; however, analysis of pure pyrite following the magnetic extraction of pyrrhotite yields an accurate mineralization age of 1893 Ma. The Re-Os chalcopyrite age for the Konuto deposit overlaps the peak metamorphic age and, along with a very radiogenic Os_i ratio (> 1.3), implies complete resetting of the Re-Os system in chalcopyrite during greenschist facies metamorphism. Isotopic disturbance is also apparent in pyrite from the upper amphibolite – lower granulite facies Harmin deposit in the Snow Lake belt, for which a ca. 1730 Ma age and Os_i > 1 was determined. However, a ca. 1860 Ma Re-Os pyrite age, interpreted here to date a later ore depositional event, was determined for the Sherritt Gordon deposit, Kisseynew Domain. This result demonstrates the robustness of the pyrite chronometer, since the deposit was metamorphosed at ~ 660°C during the 1815 Ma regional event.

These results indicate that the Re-Os chronometers in pyrrhotite and chalcopyrite are readily disturbed at relatively low temperatures, and should be avoided for reliable primary age information in most cases. In contrast, the Re-Os pyrite geochronometer is quite robust, probably remaining systematically closed through temperatures as high as 660°C.