Bedded Barite in the Geologic Record

Barite occurs throughout tlie gcologic ~.ecortl as massive beds, laminations, rosettes, and nodules. The niost importan( scientific and econo~iiic occurrences of barite are stratabound and stratifor~n massive betis from the Early Arcliean and tlie Early to Middle Paleozoic. Paleozoic beddecl barites are by far thc most volu~netricellp significant deposits in tlie geologic record. Additional occurrences have been docu~net~tcd in some Middle Proterozoic, Late Proterozoic, and Mesozoic rocks and in several localities on the motlert~ ocean floor. Bedded barite is believed to have fornled as emanations from seafloor sediments, as diagclletic replacements of preexisti~lg minerals, or as direct precipitants due to biological fixation of barill111 in the water column. Dircct field evidence to tlifferentiate between these theories is often Inciting or cootratlictory. Geochemical studies. particularly tliose tliat liavc employed 6' " :end " 7 ~ / ~ ' ~ r a~ialyses of tlie barite. 1i:lve proven very useful in understnnditig bedded barite genesis. The low solt~bility of barite relntivc to other na(ura1 salts has Iicllied barite sur\li\,e as a pseuotlomorph of stratiform evaporite mi~lerals in some Arcl~ean sedimentary sequences. Otlicr exanil,les of Archean barite a11~ear to liave a sliallow \trater detrital or authigenic origin. Very low s3's values of Archean barite are interpreted as indicating a low-sulfate ocean. Large tleposits of Paleozoic becltled barite are typically found in fine-grained, organic-rich siliciclastic sequeticcs and are associated with massive and disseniiilated sulfides, clierts, pliosphorites, a~itl ess frequently limestones and volcanic roclts. 6"s aiialyses indicate that almost a11 bedded barite liad a seawater sulfate source. Tlie genetic link between Paleozoic bedded barites and sedirne~itary submerine exllalative Pb-Zn sulfide deposits lias been established by fielcl and geochemical study of deposits in western Canada ant1 western Errrope. "s~P""s aanlyses suggest that these bedded barites have a continental barium source. Economically important bedtled barites in China, Arkansas. and Nevada have no significa~lt sulfides or other Iiydrothermal manifest:~tions. Tlie clear association of dissolved bariiun and barite with biological cycles in (lie modern ocean, associations with phosphorites and clierts, and 8 ' ~ r / " ~ aa:~Ipses that are co~nparable to contemporaneous seawater suggest that thc Chinese, Nevad;~, and Arkansas barite dcposits formed as biological precipitates on tlie seatloor. Bedded harite formed by this mechanism holds pro~niac as an indicator of high paleop~.oductivity and open ocean sulfate reduction during sclected periods of the Paleozoic. Tlie lack of worltl class exatiiples of bedded barite in Mesozoic and Cenozoic black shale seclnences indicates a lack of open ocean sulfate reduction during these periods of geologic time.