Re-Os molybdenite geochronology relies on the aliquant analysis taken from a mineral separate. Although suitable for coarse-grained molybdenite samples, traditional mineral separation techniques are not ideally suited for samples possessing fine-grained molybdenite (<50 μm) and thus hamper the application of Re-Os geochronology for such samples. Here, we demonstrate a room-temperature hydrofluoric acid (HF) chemical separation technique that is capable of isolating ultrafine molybdenite (i.e., <50 μm) for Re-Os geochronology. Six Re-Os molybdenite model ages from four molybdenite in-house control and NIST reference material (RM8599) samples exposed to concentrated HF are indistinguishable from the Re-Os molybdenite model ages for aliquants not exposed to HF. Thus, HF exposure at room temperature has no effect on Re-Os molybdenite systematics. Our HF chemical separation technique was then applied to six ultrafine molybdenite samples from the Lupa goldfield, southwest Tanzania. Three samples from the Kenge deposit provide a weighted average Re-Os molybdenite model age of 1953 ± 5 Ma (MSWD = 0.6; n = 3), whereas three samples and one repeat analysis from the Porcupine deposit provide a weighted average Re-Os molybdenite model age of 1886 ± 5 Ma (MSWD = 1.5; n = 4). Our proposed analytical protocol has allowed us to determine reproducible ages from ultrafine molybdenite samples within the Lupa goldfield that were previously unsuitable for molybdenite Re-Os geochronology using conventional mineral separation techniques.

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