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Abstract:

Barite occurs throughout the geologic record as massive beds, laminations, rosettes, and nodules. The most important scientific and economic occurrences of barite are stratabound and stratiform massive beds from the Early Archean and the Early to Middle Paleozoic. Paleozoic bedded barites are by far the most volumetrically significant deposits in the geologic record. Additional occurrences have been documented in some Middle Prolerozoic, Late Proterozoic, and Mesozoic rocks and in several localities on the modern ocean floor. Bedded barite is believed to have formed as emanations from seafloov sediments, as diagenetic replacements of preexisting minerals, or as direct precipitants due to biological fixation of barium in the water column. Direct field evidence to differentiate between these theories is often lacking or contradictory. Geochemical studies. particularly those that have employed δ34S and 87Sr/86Sr analyses of the barite, have proven very useful in understanding bedded barite genesis.

The low solubility of barite relative to other natural salts has helped barite survive as a pseuodomorph of stratiform evaporite minerals in some Archean sedimentary sequences. Other examples of Archean barite appear to have a shallow water delrital or authigenic origin. Very low δ34S values of Archean barite are interpreted as indicating a low-sulfate ocean. Large deposits of Paleozoic bedded barite are typically found in fine-grained, organic-rich siliciclastic sequences and are associated with massive and disseminated sulfides, cherts, phosphorites, and less frequently limestones and volcanic rocks. δ34S analyses indicate that almost all bedded barite had a seawater sulfate source. The genetic link between Paleozoic bedded barites and sedimentary submarine exhalative Pb-Zn sulfide deposits has been established by field and geochemical study of deposits in western Canada and western Europe. 87Sr/86Sr analyses suggest that these bedded barites have a continental barium source. Economically important bedded barites in China, Arkansas, and Nevada have 110 significant sulfides or other hydrothermal manifestations. The clear association of dissolved barium and barite with biological cycles in the modern ocean, associations with phosphorites and cherts, and 87Sr/86Sr analyses that are comparable to contemporaneous seawater suggest that the Chinese, Nevada, and Arkansas barite deposits formed as biological precipitates 011 the seafloor. Bedded barite formed by this mechanism holds promise as an indicator of high paleoproductivity and open ocean sulfate reduction during selected periods of the Paleozoic. The lack of world class examples of bedded barite in Mesozoic and Cenozoic black shale sequences indicates a lack of open ocean sulfate reduction during these periods of geologic time.

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