Abstract

Bedded barite deposits in the Slaven Chert in East Northumberland Canyon, Toquima Range, central Nevada, are part of the Roberts Mountains Allochthon which was thrust eastward onto the carbonate shelf by the Late Devonian to Early Mississippian Antler Orogeny. High concentrations of organic matter (up to 6.2%), well-preserved microfossil skeletal hash, and the presence of phosphate in interbedded barite and chert deposits indicate that the rocks formed in an anoxic environment. Rare earth element values from Early Paleozoic Cordilleran margin phosphates resemble patterns of phosphorites formed in Holocene zones of coastal upwelling at low latitudes versus phosphate formed in non-upwelling areas. These facts suggest that the barite, chert, and phosphate formed within an oxygen minimum zone associated with coastal upwelling currents. The barite displays three main morphologies: massive, nodular, and composite. Massive barite is fine-grained, lacks matrix chert, and contains phosphate nodules. The massive beds formed as a rapid primary precipitate at or above the sediment/water interface. Nodular barite is composed of barite nodules associated with matrix chert. Chert laminations are bent by the barite nodules, indicating that the nodules grew prior to lithification of the chert and are therefore not a later diagenetic replacement of a precursor. Composite barite consists of barite nodules within a fine-grained barite matrix and lacks brown matrix chert, indicating that the nodules formed as a synsedimentary precipitate within a previously deposited fine-grained barite bed. Phosphate nodules are common to abundant in the chert beds and rare to common in the barite beds. Phosphate nodules within massive barite beds commonly contain abundant fine-grained barite rimmed by phosphate masses, occasionally the opposite relationship of phosphate rimmed by fine-grained barite was present. Similar relationships were observed between nodular barite and phosphate. Therefore, the phosphate and barite were contemporaneous to penecontemporaneous precipitates. Rare microbial mats exhibit a range in growth orientation within barite samples. The various orientations indicate that the mats were not light dependent, and geochemical analyses support a bacterial origin. Lab experiments on barite precipitates from sulfuric acid solutions demonstrate that the fine-grained barite precipitates rapidly out of solution, whereas the dipyramidal and bladed crystals precipitate more slowly, after the fine-grained barite. Although the medium in which the barite precipitated is different from any natural fluid, this sequence is very similar to the patterns observed in the rocks.

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