Abstract

The Stillwater Complex, Montana (USA), is one of the world’s largest layered intrusions and hosts the Johns-Manville (J-M) Reef, a world-class platinum group element (PGE) deposit. In this study, zircon and baddeleyite were successfully extracted from three mineralized samples of the J-M Reef. The samples, which were collected over 15 km of strike length of the deposit, provide high-precision crystallization ages for the reef and the Stillwater Complex. All samples are coarse grained to pegmatitic and range in composition from olivine-bearing gabbronorite to anorthosite and troctolite. Irregular (anhedral) morphology and prominent sector zoning characterize interstitial zircon. U-Pb crystallization zircon ages determined by chemical abrasion thermal ionization mass spectrometry (CA-TIMS) for the three samples are indistinguishable within uncertainty: Frog Pond adit (2709.11 ± 0.56 Ma), East Boulder mine (2709.28 ± 0.32 Ma), and West Fork area (2709.00 ± 0.45 Ma). A U-Pb baddeleyite date from the Frog Pond adit sample (2708.85 ± 0.46 Ma) is identical within uncertainty with the zircon dates. Collectively, the results illustrate that the J-M Reef is an intrusion-wide time marker that formed at ca. 2709 Ma during crystallization of the Lower Banded series of the Stillwater Complex. The recognition that zircon can be successfully extracted from mafic-ultramafic rocks associated with magmatic ore deposits provides new opportunities for testing the timing of mineralization and duration of mineralization processes in layered intrusions worldwide.

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