Barite Nodules in Bedded Barite Within Phosphatic Devonian Slaven Chert, Toquima Range, Central Nevada

Karen K. Graber, Henry Chafetz

Bedded barite deposits in the Devonian Slaven Chert in East Northumberland Canyon, Nevada, are part of the Roberts Mountains allochthon. The barite was deposited in an oxygen minimum zone as indicated by abundant phosphate, organic matter, and radiolarian microfossils. The barite displays three main morphologies: massive, nodular, and composite. Petrographic analyses indicate that massive fine-grained (0.01-0.15 mm in diameter) barite formed as a rapid primary sedimentary precipitate at or above the sediment-water interface, whereas barite nodules formed as early diagenetic precipitates within the top few meters of barite and/or chert deposits.

The barite nodules display three internal morphologies: fine grained, fiber bundle, and spherulitic. Fine-grained nodules (0.04-6 mm in diameter) are composed of fine-grained barite similar to the massive barite beds. Fiber bundle nodules (0.75-1.3 mm in diameter) are composed of randomly oriented dipyramidal crystal bundles and lack a differentiated nucleus. Spherulitic nodules (0.1-2.5 cm) are composed of concentric layers of radially arranged dipyramidal crystal bundles around a nucleus of fine-grained barite and organic matter. The spherulites form through episodes of crystallization as indicated by remnant organic matter rims and patches of fine-grained barite between the concentric layers. Spherulites often contain phosphate, silt grains, mica, and radiolarians. SEM analyses ind cate that the dipyramidal bundles consist of bladed barite crystals oriented in the same direction.

Lab experiments indicate that fine-grained barite precipitated rapidly (in seconds) out of solution, whereas the dipyramidal crystals precipitated more slowly (2 to 3 days) after the fine-grained barite.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.