STABLE ISOTOPIC EVIDENCE THAT BARITE DEPOSITS IN THE WESTERN BROOKS RANGE, ALASKA MARK THE SITES OF MISSISSIPPIAN METHANE SEEPS

In the western Brooks Range of Alaska, black shale, chert, and carbonate of the Mississippian-age Kuna Formation host numerous barite deposits. One of these, Anarraaq, may represent the largest barite accumulation ever discovered (~1 billion metric tonnes). These deposits were discovered mainly as a consequence of exploration programs targeting massive sulfide deposits that occur in the Kuna Formation. A sulfide deposit is currently being mined at Red Dog where barite directly overlies the sulfide ores. Elsewhere in the region barite and sulfide deposits are separated either geographically or stratigraphically.

Many of the Brooks Range barite deposits show S- and O-isotope systematics similar to barite samples and sediment pore waters that have been recovered from methane seeps on the modern seafloor. These systematics include: 1) linear correlations between d³⁴S and d¹⁸O with most samples exceeding the values for contemporaneous marine sulfate, 2) variable d³⁴S vs d¹⁸O slopes ranging from 1 (laminated or massive textures) to 4 or greater (nodular textures), 3) d³⁴S reaching extreme values of 50 per mil or greater, and 4) d¹⁸O for a few samples falling below the value for contemporaneous marine sulfate. The data suggest that the Mississippian barites contain marine sulfate that was isotopically modified by bacterially-mediated anaerobic methane oxidation, and also secondary sulfate produced by H₂S oxidation or S° disproportionation. The barite deposits formed, as at modern seeps, at a redox boundary between fluids carrying Ba and methane and bottom waters carrying sulfate. Barite precipitation occurred both on the seafloor and within shallowly-buried sediments. Red Dog shows evidence for vent fauna in abundant burrow- and pellet-like structures, possibly formed by annelid-like worms. Whether other barite localities show similar trace fossils is not yet certain. Authigenic carbonates, which at modern seeps provide isotopic confirmation of anaerobic methane oxidation, are conspicuously absent.