Evaluation of bitumen as a 187Re-187Os geochronometer for hydrocarbon maturation and migration: A test case from the Polaris MVT deposit, Canada

Re-Os analyses of bitumen from the Polaris Mississippi Valley-type Zn-Pb deposit demonstrate the potential utility of the Re-Os system for establishing the absolute timing of hydrocarbon migration and generation. The Polaris deposit is hosted by carbonate rocks of the Upper Ordovician Thumb Mountain Formation, and sulphide mineralization during Late Devonian time is constrained by a Rb-Sr sphalerite date of 366 ± 15 Ma and a paleomagnetic date. Bitumen occurs in open-space fillings and coatings, and is interpreted to have formed during the main and late stages of sulphide mineralization. Samples of bitumen were taken from a variety of locations from the main deposit at Polaris; Re and Os contents are ∼ 3-78 ppb and ∼ 55-1155 ppt, respectively. The 187Re/188Os ratios are very high, ranging from ∼ 300 to 1800, and the Os isotopic composition is moderately to highly radiogenic, with 187Os/188Os ratios ranging from ∼ 3.3 to 13.7. Calculated 187Os/188Os ratios at 366 Ma are variable, but an obvious grouping is defined by 11 of 16 analyses on a Re-Os isochron diagram, and these data yield a Re-Os date of 374 ± 9 Ma (MSWD = 12, Model 3), in agreement with Rb-Sr sphalerite and paleomagnetic dates for mineralization at Polaris. The combined dates suggest that the migration of fluids responsible for both Pb-Zn mineralization and bitumen formation at Polaris was broadly contemporaneous, and was coeval with the Late Devonian Ellesmerian Orogeny. Though considerable further research is required to understand the Re-Os isotopic systematics of migrated hydrocarbon systems, bitumen Re-Os geochronology may provide the possibility of establishing a more comprehensive understanding of MVT deposits, sedimentary basin and petroleum systems.