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BEDDED BARITE IN THE GEOLOGIC RECORD

PAUL W. JEWELL*: Depcrr-ttirer~tof Grology nriil C;eo~~liy.sic.s,Urrii~ersifj~ of Utnl~,Srilt Lrrke Cir): UT 841 12

Abstract: Barite occurs throughout tlie gcologic ~.ecortlas massive beds, laminations, rosettes, and nodules. The niost importan( scientific and eco- no~iiicoccurrences of barite are stratabound and stratifor~nmassive betis from the Early Arcliean and tlie Early to Middle Paleozoic. Paleozoic bed- decl barites are by far thc most volu~netricellpsignificant deposits in tlie geologic record. Additional occurrences have been docu~net~tcdin 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~lgminerals, or as direct precipitants due to biological fixa- tion of barill111 in the water column. Dircct evidence to tlifferentiate between these theories is often Inciting or cootratlictory. Geochemical stud- ies. particularly tliose tliat liavc employed 6'" :end "7~/~'~ra~ialyses of tlie barite. 1i:lve proven very useful in understnnditig bedded barite genesis. The low solt~bilityof barite relntivc to other na(ura1 salts has Iicllied barite sur\li\,e as a pseuotlomorph of stratiform mi~leralsin some Arcl~eansedimentary sequences. Otlicr exanil,les of barite a11~earto liave a sliallow \trater detrital or authigenic origin. Very low s3's val- ues 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 , clierts, pliosphorites, a~itlless frequently 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~itarysubmerine exllalative Pb-Zn deposits lias been established by fielcl and geochemical study of deposits in western Canada ant1 western Errrope. "s~P""saanlyses suggest that these bedded barites have a continental source. Economically important bedtled barites in China, Arkansas. and Nevada have no significa~ltsulfides 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~nparableto con- temporaneous 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~niacas an indicator of high paleop~.oductivityand 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.

INTRODUCTION argue that transitions between anoxic, sulfate-reducing, and oxygenated waters are necessary for the formation of bedded Barite (BaS04) is found tl~roughoutthe geologic record and barite and associated miilerals in a variety of marine settings, is widely docurneilted in inodern oceanic environmei~ts.Several including a sulfate-poor Archean ocean (Perry et al., 197 I), deep factors make barite an econonlically and scientifically interest- sea hydrothermal emanations (Turner, 1992), and coastal ing . The high specific gravity (4.48 g/cn~3)and wide- upwelliilg zones (Jewell and Stallard, 1991; Jewell, 1994). spread occurrence of almost pure barite make it econonlically Barite has the useful characteristic of containing two ele- usefill, particularly ill the manufacture of drilling nluds for the ments (barium and sulf~tr)that call be interpreted withi11 the con- oil and gas industry. Barite is common gangue mineral in a a text of well-established isotopic techniques. Stable isotopes of variety of hydrothermal deposits including volcanic- and have a long history of being used to interpret both the sediment-hosted deep-sea hot springs associated with seafloor source and genesis of a wide variety of sulfide and sulfate min- volcanisnl. These associations are used as a nlodenl analog for erals. Within this context, barite lias played a role in under- bedded barite in sedimentary exllalative ("sedex") deposits, standing the global history of the (e.g., Schidlowski which are cornmoil in many fine-grained siliciclastic sedimen- et al., 1983). Also, the chemistry of barium is sufficie~ltlysimi- tary sequences in the geologic record. lar to that of strontiu~nthat 87~r/8%rtechniques have been Barium in modes11 seawater has long been recognized as fol- extensively used to understand the parentage (for example, con- lowing biological cycles in a inanner similar to elenlents such as tinental versus mantle versus seawater source) of barite. For pl~osphorousand silicon (Chow and Goldberg, 1960; Turekian instance, detailed geochemical analysis of the biologically pro- and Johnson, 1966). Barite and elevated bariu~nconcentrations duced barite fraction of deep-sea modern to Miocene sedi~ne~lts in modern deep water marine sediments have bee11 noted by shows that barite tracks the 87~r/"~rcomposition of contempo- oceanographers for decades (Goldberg and Arrhenius, 1958). raneous seawater (Paytan et al., 1993; Martin et al., 1995). On Careful research has shown that barite for~nsdiscrete particles in the other hand, barites that precipitate from fluids discharging the water column of the open ocean as a result of biological on the seafloor tend to reflect the 87~r/86~rvalues of lninerals processes (Dehairs et al., 1980; Bishop, 1988). This has led to that reacted with the formation fluids (e.g., Aquili~laet al., the development of barite as an important paleoproductivity 1997). These studies provide confidence that the large number tracer (e.g., Schmitz, 1987; Dymond et al., 1992). of 87~r/86~ranalyses of older bedded barites accurately reflect The of barite is dependent on the redox state of the parentage of the fluids from which they formed. water, which means that barite dissolves in reducing open water All of these features have co~nbinedto produce a wealth of and diagetletic environments. This feature has been used to studies on barite geology and geochemistry. 111 spite of this, many aspects of bedded barite genesis remain poorly under-

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FIG. 1.-Geogmphic distribution of major bedded barite deposits on the continents. stood. Although SUSllSnaSy articles of some aspects of barite for- PRECAMBRIAN BEDDED BARITE snation have been undertaken (e.g., Brodtkorb, 1989; Maynard Isolated bedded barite occurrences are described from a and Okita, 1991), no systematic attempt has been made to syn- small number of Precambrian shields throughout the world thesize theories of barite genesis throughout the geologic (Fig. 1, Table 1). Since inany Precarnbriatl barite deposits have record. This will focus on massive bedded or stratiform been extensively metamorphosed, it is often difficult to ascertain barite in sedimentary rocks in an effort to ullderstarld the gene- their precise genesis. Nevertheless, these barites have sparked sis of this irnportallt worldwide ecollornic and scientific co~lsiderablescientific interest because of their possible role in resource. Although the disc~issionis not meant to include all interpreting Archeall and the evolution of the redox bedded barite occurrences, specific examples are discussed from state of the Archean atmosphere and ocean. The best docu- all of the most important geologic eras and localities where bed- mented Archean barite occurrences are in Australia, South ded barite has been found. Particular attention is paid to Africa, and India, all of which have ages >3200 1n.y. The Paleozoic bedded barites, which constitute the largest deposits paucity of documeslted bedded barite in rocks from the Late in the world. The emphasis in this paper is on deposits that are Archean through the Late Proterozoic is puzzling and has been well described in mainstream scientific literature. Lesser noted by at least one author (Thorpe, 1979). emphasis is given to bedded barite occurrences that have o~lly been documented in abstracts or less accessible publicatio~ls.It should also be noted that nodular barite occurs with many of the massive, bedded barites that are discussed. In some cases, these two contrasting barite morphologies have been used to make Archea~~bedded barite from the Pilbara block of western specific inferences about environments of deposition (e.g., Australia (Lambert et al., 1978; Barely et al., 1979; Dunlop and Graber and Chafetz, 1990) and the isotope chemistry of the two Buick, 1981) and the Barberto11Mo~ultain Land of South Africa morphologies is often distinctly different (e.g., Rye et al., 1978). (Perry et al., 1971; Lowe and Kaauth, 1977) represent the old- A comprehensive discussion of ~lodularbarite would have sig- est bedded barites in the geologic record (Table 1). nificantly expanded the scope of this review and for the sake of Measurement of islterfacial angles of individual crystals of the brevity, the emphasis is placed on bedded barite. Australiall barite demonstrate that they are pseudomorphs of BEDDED BARITE Ih' THE GEOLOGIC RECORD

TABLE 1.- SUMMARY OF AGE LITHOLOGIC AND DEPOSITIONAL CHARACTERTS'TICS OF IMPORTANT PRECAMBRIAN AND PALEOZOIC BEDDED BARITE DEPOSTTS

Location and geologic formation Age Lithologic associations Del~ositionalsetting

Western Australia Warrawoona ~roup' -3450 m.y. Chests, fine-grained Shallow water to subaerial metasiliciclastic rocks evapo~.iticshelf

South Africa Onverwacht Gro~p~,~ -3450 m.jf. Chests, quartzitic nlicaceo~~s Shallow water evaporitic amphibolites shelf Fig Tree Group ',4 -3200 1n.y. Cherts, dolo~niticconglomerates Submarine ridge

India Sagur Group 536 >3300 m.y Qua].tzites, talc schists. phyllites, Shallow water evaporitic ~uetavolcanics shelf Aravalli Supergroup -2000 m.y. Quartzites, metabasalts Alluvial, shallow marine shelf

Scotland Argyll Group ' 610-640 m.y. Quartzites, phyllites, scllists Epicratonic rift basin

Southern China Niutitang, Lujiaping, Early Cambrian Blaclc shales, cherts, Continental shelf (south), deep Longmaxi Formations phospl~orites water siliciclastic basin (north)

Western Canatla Mount Mye Unit Cambrian Nor~calcareousyhyllites, schists Epicratonic rift basin Vangorda Unit (' Cambro-Ordovician Calcereous phyllites, schists Epicratonic rift basin Road River Forrllation " Ordovician-Silurian Black shales, chests Epicratonic rift basin Lower Earn Group " Devonian Black shales, cherts Epicratonic rift basin

Nevada, U.S.A. Vinini Formation I' Ordovician Shales, chests, siltstones Deep water continental margin Slaven Chert ' ' Late Devonian Cherts, shales Deep water continental margin

Germany Meggan Beds l2 Middle Devonian Blaclc shales, siltstol~es Epicratonic starvecl basin Wissenbach Shales l3 Middle Devonian Blaclc shales, siltstones Shallow water shelf

Belgium Unnamed IJ Late Devonian Shales, argillaceous limestones Sl~allowwater evapositic shelf

Arkansas, U.S.A. Stanley Shale l5 Late Mississippian Shales, siltstones, limestones Deep water slope

Ireland Unnamed l6 Mississippian Limestones, dolomites Shallow water shelf

Jennings and Jilson (1986) I Barley et al. (1979); Dunlop and Buick (1981) lo Abbott er al. (1986) * Lo\ve and Knauth (1977) Stewart (1980) "eimel- (1 980) l2 fi-ebs (1981) '' Heinrichs and Reimer ( 1977) l3 Hanliak (1 98 1) Radhakrishna and Sreenivnsaiya (1974) I" Dejonglie anti Boulvnilr (1993) %eb et al. (1991) ''Morris (1974) Willa~land Coleman (1983); Hall et al. (1993) l6 Taylor and Andrew (1978) Wring and Li (1 991) PAUL l! JEllJELL

origin similar to that of the Australian barites (Lowe and Knauth, 1977; La~nbertet al., 1978). Reimer (1980) advocates a volcanic source of the barium on the basis of close stratigraphic proximity to large voluines of volcanic rocks and ininor amounts of associated sulfides. Detailed work on younger (-3200 lll.y.) bedded barite in the overlying Fig Tree Group sl~owsthat these deposits are largely detrital with some diage- netic recrystallization rather than being pseudomorphs of eval~- orites (Heinrichs and Reimer, 1977; Reimer, 1980) (Fig. 2B). An obvious source of the detrital barite would, of course, be the olcler Onverwacht barite. Archean bedded barite from central India is highly ineta- morphosed and occurs with a variety of Ba-bearing inetainor- phic . As with the South African and Australian barites, no significant sulfide mineralization is noted. Whereas the Indian bedded barites are clearly synsedimentary in origin, and their exact depositional environment is difficult to interpret (Radhakrishna and Sreenivasaiya, 1974; Deb et al., 1991). The Australian, South African, and Indian Archean bedded barite have all been studied isotopically. Assi~iningthat the pri- mary Rb-Sr signature of these barites has been retained, the Western Australian barite has the no st primitive Sr signature of all the bedded barites (Table 2). Co~nparisonof the 87~r/86~r coinpositioil of these barites with lunar and meteorite X7~r/86~r ailalyses have allowed collstraints to be placed on the origin of the Earth and the Moo11 (McCullocl~,1994). South African and Indian Archean bedded barites are on average slightly inore radiogenic than the Australia11 barite (Table 2), although both are consistent with ~nailtlebuffering of Archea11 seawater com- positioil (Perry et al.. 1971; Deb et al., 1991; Strauss, 1993). Sulfur isotopic analysis of Archean bedded barite has been a critical piece of evidence for uilderstallding the redox state and sulfate concentratio~lsof Archean seawater. 6% coinpositions of the Australian, South African, and Indian bedded barites arc all low relative to the minor amounts of coexistitlg sulfides (Table 2), wllicll suggests that the biologic fractionation of sul- fide sulfur observed throughout the post-Archean geologic record was not operative in the Archean ocean. Rathel; the low sulfide-sulfate fractionation is attributed to inorganic oxidatio~l of volcanic H2S or SO2 in an ocean with very small sulfate con- centrations (Perrv et al.. 1971 : Cameron. 1982: Schidlowski et % d FIG. 2.-Field exa~iiplesof Precambrian bedded barite. (A) Stratiform al., 1983; Strauss, 1993). barite-chert llorizon froill North Pole deposit of Western Australia. Universal stage measurement of barite crystals show that they have interfacial aiigles of gyl)s~n((fio~n Laiubert et a]., 1978). (B) Dctrital barite of the Arcliean Fig Tree Group, Swaziland, South Africa (from Heinrichs and Reimer, 1977). Exainples of bedded barite from the Proterozoic are rare gypsurn (Lainbert et al., 1978) (Fig. 2A) and that a portion of (Table 1). The lone example from the Early to Middle this sediinentary sequence was originally coinposed of evapor- Proterozoic is the -2000 m.y. bedded barite of the Aravalli ites. The high solubility of all common evaporite minerals rela- Supergroup of central India. These bedded barites are associated tive to barite has allowed these barites to survive subsequeilt with supracrustal quartzites, metavolcanics, and some carboilales tectonism, metamorpl~ism,and surface exposure. The Australian but no known tnassive sulfides (Deb et al., 1991). The Late Archean barites appear to be the oldest remnants of evaporite5 Proterozoic Dalradian Supelgroup in Scotlaild has an exceptional in tlle geologic record (Lambert et al., 1978). niunber of bedded barite occurrences with a clear associatioil to There are differences of opi~lioilregarding the origin of exllalative massive sulfide deposits as well as some barite barite in the -3450 m.y. Onverwacht Group of South Africa. deposits wit11 only ~ninor associated sulfides (Willan and Petrograpllic and ~nineralogicalanalysis suggests an evaporite Coleman, 1983; Hall et al., 1991). These deposits are fo'ound in BEDDED BAR17'E IIV THE GEOLOGIC RECORD 151

TABLE 2.- SUMMARY OF SELECTED 87~r/8%and 634~DATA FOR BEDDED BARITE. VALUES REPORTED ARE - MEAN + ONE STANDARD DEVIATION OF ALL ANALYSES REPORTED IN THE LlTERATURE REFERENCE.

--Location and geologic formation R7~r~S6~r(barite) 87~d8"r (seawater) Western Australia, Warrawoona Group (Early A~chcan)

South Africa O~~verwachtGroup (Early Archean) Fig Tree Group (Early Archean)

India Sagur Group (Early Archean)

Al.avalli Supergroup (Middle Proterozoic) Scotland Argyll Group (Late Proterozoic)

Southern Chi~la Qinling Region (Early Cambrian) Jiangnan Region (Early Cambrian) Western Canada Fa1.o deposit (Late Cambrian) Grum deposit (Late Cambrian) DY deposit (Late Cambrian) Vulcan deposit (Late Silurian - Early Devonian) Jason deposit (Middle Devonian) Lower Earn Group (Late Devonian) Nevada. U. S. A. East Nol.thumberlancl Canyon (Late Devonian) Mountain Springs Deposit (Late Devonian) Argenta (Late Devonian) Miller (Late Devonian) Queen Lode (Late Devonian'?) Western Europe Megge11 deposit (Middle Devonian) Ra~riselbergdeposit (Middle Devonian) Chaudfontaine deposit (Late Devonian) Arkansas, U. S. A. Fancy Hill deposit (Late Mississippian) Millche~ndeposit (Late Mississippian) Chamberlain deposit (Late Mississippian) Italy (Mid-Late Triassic) Cuba (Jurassic) Mexico (Early Cretaceous) Paita (Holocene, Peru middle shelf) Chiclayo Ca11~1on(Holocene, P~~LIcontinental slope)

' Burke et al. (1982) " Wang and Chu (1 994); excludes one value 20 Rye et al. (1978) 'McCulloch (1 994); primary barite only (0,713661) fro111Jia~lg~ia~i area aiid another " Maynard et al. (1995) " Larnbert et al. (1978) (0.716710) from Qinling area 22 Barbieri and Masi (1983) 'Strausc; (1993) "Witiig and Li (1991): excludes analyses of con- 2"e~vell, unpublished Reimer (1980) cl.etionary barite 24 Mitcliell (1 980) "erry et al. (1971) l5 Slianlts et al. (1987); all samples described as 25 Buschendorf et al. (1963) Rei~ner(1980); detrital barite baritic rnassive sulfides '"nger et al. (1966) "eieier (1980); diagenetic barite l6 Mako and Shanks (1984); no reported standard *' Dejonglie et al. (1 989) 9~ebet al. (1991) deviations of data Hulen (1978) lo Heel-ing (1988) "T~lrner(1 986) '"~al-bieri et al. (1 984) 'I HsIII et SI~,(1991) IS Gardner and Hutcheon (1 985) "Kesler and Jones (198 1) l2 Willan and Coleman (1 983) "~ecileet al. (1983) " ilquilina et al. (1997) 152 I'AUL I\! JEIVELL siliciclastic sedi~nentarysequences deposited during a period of portion of the country (Wang and Li, 199 1). Barite deposits of extensional tectonics aloilg a continental margin. 111 this sense, the inore southera Jiangnan area are believed to have foriiied 011 the Proterozoic Scottish deposits are similar to the well-studied the interior of the Yangtze craton, while the Qinling deposits are Paleozoic barite of western Canada and western Europe. It is found in the suture zone that formed between the Yangtze and noteworthy that most large, well-known, Proterozoic sediment- Sino-Korean craton to the north during the early Paleozoic hosted nlassive sulfide deposits (for example, Sullivan in British (Maynarcl and Okita, 1991). Exact age deterillinatiorls and Columbia and the McArthur River area in ilortheastern Australia) stratigraphic conelations of barite-bearing strata are lacking, are devoid of bedded barite (Gustafson and Williams, 1981). although the massive barite beds appear to occur in stratigrapli- 87~r186~ranalyses of the ceiltral Indian and Scottish ically higher intervals in deep water facies to the northeast Proterozoic bedded barites show clear coiltinental affinities (Wang and Li, 199 1). (Table 2) and probably reflect the circulation of hydrothermal The Chinese barite deposits occur in massive beds, lamina- fluids through continental crust. All Proterozoic bedded barites tions, rosettes, and nodules and are apparently unique among have 634~coinpositions that are heavier than Archean barite, ~llajorbarite provinces of the ~vorldin being associated with which suggests that extensive biogenic fractioilation and liigh tliajor amounts of willlerite (BaC03) and sulfate oceans were common in the Proterozoic (Willan and (CaBa(C03)2). High concentrations of vanacliuill (up to 1 % Colernan, 1983; Hall et al., 1991: Deb et al., 1991) (Table 2). V205) have bee11 documented in associated sedi~neiltaryroclts. Additional occurrences of Proterozoic bedded barite froin Like many barite provinces, phosphatic and oganic southern Brazil (Cassedanne, 1989) and Ziinbabwe (Thorpe, -rich shales (up to 14% total organic carbon) are corn- 1979) are poorly docu~llentedwith respect to age, geologic set- inonly associated with the barite (Wang and Li, 1991). 87~r/86~r ting, and genesis. The latter is particularly intriguiilg because of values from ~llostof the Chinese deposits are very close to those its similarities to the South African and Australian Arcl~eanbed- of contemporaneous Cambrian seawater (Wang and Cl~u,1994) ded barites and its occurrence in rocks that could be anywhere (Table 2). However, single samples from the Sar~jiangdeposit of from Late Archean to Middle Proterozoic in age. the Jiailgnin area and Liulin deposit of the Qiilliilg area are ailo~nalo~~slyradiogeilic. 63" values of the barite grade from relatively heavy (+350/00) in the platform settings to somewhat PALEOZOIC BEDDED BARITE lighter values (+23%0)in the geosynclinal or coiltiilental margin Paleozoic bedded barites coilstitute the most widespread and deposits (Table 2). These observations are explained within the volu~netrically significant ainounts of barite in the geologic context of a euxinic iilterior basin that depleted seawater sulfate record. Major deposits are pre-Pennsylvanian in age aid found concentrations leadirig to relatively heavy sulfate that was then throughout the world (Fig. 1). The preponderance of Paleozoic incorporated into the barite (Wailg :und Li, 1991). bedded barites occurs in black shale sequences with associated cherts and phosphorites as well as lesser amounts of volcanic rocks, limestones, and (Table I). Significant amounts of barite occur wit11 Paleozoic -hosted massi\~e Western Canada (the Yukon, Northwest Territories (NWT), sulfide deposits, although inally large, ecoiiomically i~nportailt and northern British Columbia) are host to a large n~unberof bedded barites are not associated with sig~lificantalnouilts of sul- economically important sedimentary rock-hosted illassive sul- fide. Like Precambrian barite, the sulfur isotopic composition of fide (sedex) and barite deposits. These deposits are fouild in Paleozoic barite has been effectively used to track open and thick sequences of fine-grained siliciclastic rocks and chert closed ocean clleinistry for selected periods of time (e.g., deposited in an epicratonic rift basin that was active from the Goodfellow and Jonasson, 1984) and strontium isotopes have Cambrian through the Late Devonian (Table I) and bounded to proven effective it1 doculllenting the source of barium in a \7ari- the east and west by platforms (Abbott et al., 1986). ety of settings (e.g., Maynard et al., 1995). Paleozoic bedded barites have 634~signatures that are close to but not precisely the sailie as the Paleozoic evaporite curve (Fig. 3). P PP- M D Sorltlzer-11Chirlcr s m 0 Bedded basite deposits in southern China are ainong the largest barite reserves of the world. 111 spite of this, the Chinese e deposits are among the illost poorly documented of all barite pe provinces with respect to regional geologic setting and detailed studies of suecific deuosits. 10 15 20 25 30 40 50 Although Late Siniaii (Late Proterozoic) rocks host concre- sYs(9/m) tionary barite and , all of the major Chinese bedded FIG.3.-Mean values of 6"s (%r) of Late Protel-ozoic through Paleozoic barite deposits are found in Early Cambrian black shales and barite Table plotted h?lscul.ve of Cl;lypool et cherts of the Qiilliilg and Jiangnan regions in the soutl.1-central (1980). Circles ].eferto barite-only deposits; squares refer to Qb-211-Badeposits. REDLIED BARITE IiV THE GEOLOGIC RECORD 153

Along the Yukon-NWT boundary, this rift is known as the (Came and Cathro, 1982). The Anvil Camp on the western side Selwyn Basin and to the southeast in British Columbia it is of the basin hosts a n~llnber of important massive sulfide known as the Kechika Trough. The wide range of bedded barite deposits at the contact of Cambrian noncalcareous metapelites and sulfide deposit ages in western Canada has allowed deter- of the Mount Nye Unit and calcareous ~netapelites of the mination of secular changes of both sulfide and sulfate 6'''s val- Vangorda Unit (Jennings and Jilson, 1986). Barite mineraliza- ues during the Middle Paleozoic. This work suggests that the tion appears to be minor, although a nunlber of barite a3's Selwyn basin was anoxic, stratified, and hydrographically iso- analyses have been reported (Shanks et al., 1987) (Table 2). The lated from the open ocean during much of its existence (Cecile Howards Pass Camp deposits along the Yukon-NWT boundary et a]., 1983; Goodfellow and Jonasson, 1984). are hosted by Upper Ordovician-Lower Silurian carbonaceous Within the Selwyn Basin. three major districts of bedded shales and siltstones of the Road River Group and also have rel- barite and massive sulfide mineralization have been recognized atively small amounts of bedded barite (Carne and Cathro, 1982; Goodfellow et a]., 1983). Deposits of massive sulfide and bedded barite are reported in the Upper Silurian to Lower Devonian portion of the Road River Fortnation south of the Hotvards Pass Camp (Mako and Shanks, 1984) (Fig. 4A). The MacMillan Pass Camp located immediately north of the Howards Pass Camp hosts several econoinically important sedex Pb-Zn-Ba deposits (Gardner and Hutcheon, 1985; Turner, 1986), as well as several bedded barite deposits with no associ- ated sulfides (Lydon et al., 198%). The number of "barite-only" deposits from the MacMillan Pass is greater than those of either the Anvil or Howards Pass areas. The nlassive sulfide and bed- ded barite deposits are found in Upper Devonian carbonaceous shales of the Lower Earn Group. Bedded barite and shale hosted Pb-Zn-Ba deposits in the Kechika Trough of northeastern British Columbia are more spa- tially and temporally dispersed than those in the Selwyn basin. Deposits are found in host rocks that range in age from Ordovician through Devonian. Bedded barite deposits with no associated massive sulfides include the Ordovician Sika and the Devonian Kwadacha deposits (MacIntyre, 1982). Research in the Selwyn Basin and Kechika Trough has emphasized an exhalatiire origin for the bedded barites (e.g., Large, 1983; Lydon et al., 1985a). 111 addition to association with massive sulfide mineralization, evidence of an exhalative origin of the bedded barite includes hydrother~nalbreccias, con- teinporaneous volcanism, and 87~r/S"r values, which indicate that the barite was derived from continental crust (Table 2). It is noteworthy that some of the bedded barites (particularly from the Devonian section) with no associated massive sulfide miner- alization resemble other barite-only provinces of the world (China, Nevada, Arkansas) in terms of morphology and associ- ation with cherts and organic-rich shales (e.g., Mako and Shanks, 1984; Lydon et al., 198%). It should be further noted that 87~r/8"r data have been reported for only one deposit (the Devonian Jason Pb-Zn deposit) (Table 2).

Alaskcr Alaska has a number of bedded barite occurrences (Clark and Poole, 1989; Schmidt, 1997). Interestingly, shale-hosted stratiform deposits in Alaska appear to be either massive sulfide deposits with no barite or deposits colnposed only of barite near the Vulcar~deposit. Northwest Territories, Canada (from Mako and Shanks, (Schlllidt, 1997). An exceptioll is the Red Dog deposit ill the 1984). Note pencil for scale. (B) Sequence of deformed barite, chert, and western Brooks Range, a world class (77 inillion metric ton) Pb- orgaiiic-rich sliales of the Slaven Chert (Late Devonian) in the Argcnta mine of Zn-Ba deposit \vith significant amounts of bedded barite hosted north central Nevada (from Jewel1 and Stallal.d, 1991). Jacob staff is 1.5-m long. by Lower Mississippian to Pernlian black shales and chert. nificant associated volcanisill and massive sulfides, analogs of Genetic relationships of the barite to sulride ~llineralizationat barite deposition in the ruoclern ocean, and major and minor geo- Red Dog are similar to sedex deposits or western Canada and chemical systematics. Jewell (1994) details a illode1 for the for- Germany (Moore et al., 1986). mation of a sulfate-reducing coastal upwelling system as a Deposits composed only of bedded barite are hosted by depositional mechanism for the Nevada bedded barite. Devonian through Triassic black shales, chests, and minor amounts of limestones and fine-grained siliciclastic rocks. Most of these deposits are found in the western Brooks Range or in Arkansas southwestern Alaska. Exact genetic relationships between the The Arkansas bedded barite province is hosted by the Late barite and various rocks types are poorly known due to lack of Mississippian Stanley Shale and has been an important producer exposure or detailed geologic investigations. Reported a3" val- of barite for much of this century. The deposits are found in an ues of the barite range frorn 21%0 to 28%0 (Schmidt, 1997). east-west-trending syncline in the central part of the state; the principle zones of bedded barite occur at Chainberlin Creek in the east and Fancy Hill to the west. The Stanley Shale consists of organic-rich black shales interbedded with turbiditic siltstones Central Nevada contains some of the largest bedded barite and sandstones that were deposited on the southern edge of a deposits in the world. Barite is found in more than 100 individ- continental margin that received sediment fi-om the Appalachian ual mines and prospects that have reserves of approximately 90 Mountains to the north (Morris, 1974; Maynard and Okita, million metric tons. An overview of the Nevada barite province 1991). De\~oniannovaculite (metamorphosed chert), shales, and can be found in Mitchell (1980) and Papke (1984). volcanic tuffs underlie the barite deposits. The barite occurs in a Most of the central Nevada bedded barite is found in variety of morphologies, including rhythmic bands of shale, Ordovician through Late Devonian chests and shales of the barite, and carbonate (Zimmerman and Amstutz, 1989). Roberts Mountain allochthon. These deep-water facies rocks The Arkansas bedded barites differ from the Nevada barites were thrust over shallow-water facies rocks during the Late in having dominantly shale and siltstone host rocks, larger con- Devonian through Early Mississippian Antler Orogeny. Tectonic centrations of carbonate, and trace amounts of anhyclrite. A overprinting during the Mesozoic and Cenozoic has greatly detailed trace element study of the Fancy Hill bedded barite complicated the host rocks of these barite deposits. The most deposit has also shown that the minor sulfide present was dia- common bedded barite bearing strata are the Ordovician Vinini genetic with little input frorn hydrothermal sources (Howard and Formation and the Late Devonian Slaven Chert (Papke, 1984). Hanor, 1987). 87~r/s6~ranalyses of the Arkansas barite are very The general lithologic similarities and extreme structural con- close to those of contemporaneous seawater (Maynard et al., plications of these two units make them difficult to distinguish 1995) (Table 2). in the field. Both are dominantly chert and shale, although the Vinini has higher amounts of coarse-grained siliciclastics (Stewart, 1980). Barite deposits in both formations are associ- Other Occur~zrzcesirl Nor.tl7 arlcl So~ithArrlericarz ated with chests and organic-rich shales with lesser amounts of Lesser known Paleozoic bedded barite deposits have been li~nestoneand igneous rocks (DubC, 1988; Graber and ChaTetz, noted in Sonora, Mexico (Poole, 1988; Clark and Poole, 1989), 1990; Jewell and Stallard, 1991) (Fig. 4B). Interestingly, all and northeastern Washington state (Mills et al., 1971). Neither deposits that have been studied in detail and produced reliable area has massive sulfide deposits that can be directly related to paleontological data are Middle or Late Devonian in age. Sulfur the bedded barite. A small number of Paleozoic nodular and isotopic data (Mitchell, 1980; Rye et al., 1978) show that mas- lensoid deposits have been described in west-central Argentina sive bedded barite has a seawater sulfate source, while concre- in a gray and black shale sequence of Ordovician age tionary and rosette barite have relatively heavy sulfur ratios that (Brodtkorb et al., 1982). are characteristic of a closed sulfi~rsystem. Analyses (Rye et al., 1978; Maynard et al., 1995) show that 87~r/86~rratios of mas- sive barites likewise are close to or slightly heavier than those of contemporaneous seawater (Table 2). The relatively well-studied nature of the Nevada bedded Well-documented bedded barite is associated with sedimen- barite has not yielded consensus about its origin. Structural com- tary rock-hosted Pb-Zn deposits in Germany (Meggen and plexity, poor stratigraphic control, and paucity of dateable fossils Rarnmelsberg), Belgium (Chaudfontaine), and Ireland contribute to this situation. Ketner (1963) advocated diagenetic (Silvermines and a number of other deposits). Lesser known replacement of on the basis of rare brachiopods found barite deposits occur throughout Europe. The German deposits within some of the barite beds. Most subsequent workers have have many of the same characteristics as the sedex deposits in argued for a synsedimentary origin, with deposition by exhala- western Canada, that is, a central core of massive stratabound tive processes being a popular mechanism (e.g., Poole, 1988; Pb-Zn illineralization surrounded by an apron of bedded barite DubC, 1988). Shawe et al. (1969) and Jewell and Stallard (1991) (Krebs, 198 1; Hannak, 198 1). Reported tonnages of barite at argue for deposition by biological processes, citing a lack of sig- Meggen are much higher than at Rarnilielsberg (10 million tons versus 0.2 million tons) (Fuchs, 1989). Deposition of the BEDDED BARITE IiV THE GEOLOGIC RECORD 155

Meggen and Raminelsberg ore bodies also appears to have esis (Diaz, 1990). The geology and isotopic geocheiiiistry of occurred in sediment-starved rift basins, althougl~these basins si~nilarmanto-type barite and celestite (SrS04) deposits are were not nearly as large as the Selwyn and Kechika Basins in described in Middle Cretaceous lirnestolles of ~lorlheastern Canada. 634~analysis of these barite deposits indicate a seawa- Mexico (Kesler and Jones, 1081). Interestingly, this barite has ter source for the sulfi~rand 87~r/"~raallalyses suggest a conti- 87~dY6~rtliat is distinctly Inore radiogenic than the celestite. nental source for the barium (Table 2). a3's of the celestite is also lighter than tliat of the barite. Bedded The Chaudfontaine deposit is a relatively recent discovery in barite from a Cretaceous chalk-chert sequence in the Judean the Late Devonian shales and of western Belgium Desert of Israel has an associatioli wit11 biologically produced (Dejonghe, 1979). The deposit has a high percentage of bedded sediments (cl~erts,phosphorites) tliat is sirnilar to that of barite relative to Pb-Zn tnassive sulfides and the barite has a Paleozoic deposits (Bogoch and Shirav, 1978). wide variety of crystal morphologies (Dejonghe, 1990) that appear to have grow~zin a semi-restricted evaporite basin (Dejonghe and Boulvain, 1993). The source of the sulfur was Becllled Barite or1 the Seqfloo~.arlc1,fr-otii Deep-Sea Drilling seawater, while other metals including barium have a contiaen- Sites tal source (Dejonghe et al., 1989) (Table 2). Bedded barite associated with seafloor spreading on the Pb-Zn-Bn deposits are also associated with Carbo~liferous modcrn ocean floor has been doculnented in a numbel. of local- carbonate rocks in a variety of localities in Ireland (Morrisey et ities (Church, 1979; Lonsdale, 1979; Koski et, al., 1985, 1988; al., 1971; Coo~nerand Robinson, 1976; Taylor and Andrew, Peter and Scott, 1988). Other localities such as the California 1978). Stratiform bedded barite deposits have been described in borderla~lds(Cortecci and Longinelli, 1972) and the Peru con- Sardinia from Middle Cambrian dolo~nites (Barbieri et al., vergent ~liargi~l(Aquilitla et al., 1997) are not associated with 1984b; Padalino et al., 1989) and it1 associatio~lwith stratiforrn seafloor spreading. The latter setting appears to be an example Pb-Zn mineralization in Devonian black shales, siltstones, and of fluid circulation through an accretioilary prism. limesto~lesin the Pyrenees of France (Lhegu et al., 1988). These Deep-sea drilling over the past several decades has led to the deposits have relatively low tonnage and have not attracted discovery of a number of bedded barite occurrences (Dean and extensive scientific scrutiny. Schrieber, 1977). U~ifortunately,it is ge~lerallyi~npossible to dcter~niilethe morphology and areal extent of barite beds inter- MESOZOIC TO MODERN BEDDED BARITE sected by these cores. Nevertheless, analysis of the barite and associated pore fluids in deep-sea drilling sites holds the Note\vorthy bedded barite occurrences are relatively rare in proinise of providi~lgconsiderable insight into how bedded Mesozoic and younger rocks. None have tlie large size and asso- barite may have formed in siinilar en\lironments tl~roughoutthe ciation with black shale sequences typical of Paleozoic bedded geologic record. A particularly interesting analysis of this sort is barites (Table 1). As discussed below, this inay be attributed to the given by Torres et al. (1996) who inodel dissolution and repre- possibility that sulfate reducing ocean conditions were present cipitation of barite along diagenetic fronts where sulfate con- during the Paleozoic tliat were not present during the Mesozoic. ccntration of the pore fluids shows large gra d'leilts. A number of massive barite occurrences have been docu- mented in deep sea drilling sites and on the seafloor of the mod- ern ocean (Dean and Schrieber, 1977). Some of these deposits THEORIES OF BEDDED BARITE GENESIS have beell used as ailalogs for a variety of older bedded barite The wide variety of bedded barite and associated lithologies deposits (Torres et al., 1996). has led to several theories of bedded barite genesis. For the pur- poses of this review, these are placed into three categories: dia- Mesozoic Beclcletl Barite genetic replacement, biologic precipitation, and exhalative. All three are supported to greater or lesser degrees by field relation- Barite and a small ainount of Pb-Zn inineralization is found ships, geochemical data, and coinparisol~sto ~iiodernanalogs. in Middle to Late Triassic rocks of southern Italy (Barbieri et al., Of tlie three categories, exhalative is perhaps the most diverse. 1984a). 87~1.186~ranalyses of this deposit suggest that the barite 111 the present context, sedimentary exhalative (sedex) is meant precipitated from seawater (Table 2). Early Jurassic dolo~nites to include all fluids expelled from the underlying sedi~nentcol- host small bedded barite deposits in France whose paragenesis umn. No distinction is made between fluids heated by magma, appeass related to evaporite basins (Fuclis, 1989). A lumber of by circulation along a deep seated fault, or sitllply by geother- similar Early Jurassic barite deposits are found in evaporite mal gradients in an overpressured basin. sequences in central Argentina (Brodtkorb et al., 1989). Pb-Zn- Ba deposits Srorn Jurassic sedimentary rocks in Cuba have been described by M:lynard et al. (1995). 87~s/86~rdata Sro~nthese deposits suggest a continental source (Table 2). An early Replacement of evaporite sequences by barite in some Cretaceous bedded barite dcposit from the Atacania region of Archea11 bedded barite occurrences is well-documented on the Chile occurs as lnantos in a sequence of limesto~~esand cal- basis of crystal morphologies of the barite and surrounding sed- carenites that were exte~lsivelyaltcrecl during subaerial diagen- imentary associations (Lambert et al., 1978; Rei~ner, 1980). Diagenetic replacement of limestone has also been suggested vatcd 87~r/X6~rvalues, which indicate derivation from a conti- for Paleozoic bedded barite deposits of Nevada (Ketner, 1963), nental source (Aquilina et al., 1997). This form of the sedex although the evidence is far less convincing because of the deposition has been advocated for barite-only deposits in st~ataboundnature of the barite as well as the fact that liinestone Arkansas (Howard and Hanor, 1987), although 87~d86~rsigna- and barite are seldotn found in close spatial proximity (Papke, tures would seem to indicate that the barium was derived from 1984; Poole, 1988). Most subsequent researchers have adopted contemporaneous seawater rather than continental material a synsed~~nentarymechanism for thc Nevada as well as \ii~tually (Maynard et al., 1995; Table 2). all other Proterozoic through Paleozoic bedded barite deposits. In spite of the lack of evidence for the formation of bedded barite deposits by wholesale diagenetic- replacement, there is Biological Plucil~itcitiorl considerable evidence that diagenesis is important on local A biological origin for rnarine barite has been advocated scales (Papke, 1984). 111 fact, diagenetic reorganization of bed- since oceanographic sur\leys documented the association ded barite would be a logical consequence of any early sediment bet\veen marine biogoechemistry, elevated concentrations of diagenesis that included sulfate reduction. S~ilfiitereduction has b~iriumin modern ocean sediments, and the occurrence of dis- been documented in organic-rich marine sediments associated crete barite particles in biogenic matter of the water colu~nn with diagenetic barite deposits in the modern ocean (Torres et (Goldberg and Arhennius, 1958; Chow and Goldberg, 1960; al., 1996). Diagenesis is also considered the primary genetic Church, 1979; Dehairs et al., 1980). Barite fornlation has been mechanism for bedded barites in the carbonate-hosted Mesozoic documented in the ii~icroen\/~ne~ltsof organic particles mantos deposits of Mexico (Kesler et al., 1981). falling through the water column (Bishop, 1988), which thereby accounts for the correlation between surface productivity and barite accumulation in the underlying sediments (Dymond et al., 1992). Seawater is the source of the barium as well as the A sedimentary exhalati\le origin of bedded barite has been 87~r/8%rsignature of these particles (Martin et al., 1995). Barite applied to virtually all deposits that have temporal and spatial accu~nulatio~lsof several weight percent are observed below associations wit11 sediment-hosted ~nassi\resulfide deposits as zones of high biological productivity and low siliciclastic sedi- well as to many bedded barites without these associations. This mentation rates (e.g., Dymond, 1981). model evolved following the discovery from the 1960s through Biologically produced barite particles that fall into sulfate the 1980s of hydrothermal vents at seafloor spreading centers reducing waters of restricted basins of the modern ocean dissolve and continental margins. These disco\leries led to a consensus and enrich those waters in dissolved barium (Falkner et al., that many ancient volcanic-hosted massive sulfide deposits were 1993). If sulfate-reduction were present in the open waters of syngenetic rather than hydrothermal replaceine~lt deposits ancient oceans, then the barite would dissolve and be reprecipi- (Franklin et al., 1981). This exhalative model subsequently tated as bedded barite in the ocean's oxic-anoxic transition zone became a popular although by no means universally accepted (Fig. 6). Formation of massive bedded barite would require high inodel for sedimentary rock-hosted ~nassivesulfide and bedded surface producti\lity, significant water mass flux (that is, strong barite deposits throughout the geologic record (Gustafson and currents), and open ocean sulfate reductio~lto exist in the same Williams, 1981). The most widely recognized modern analog locality (Jewell, 1994). In modern coastal upwelling zones such for sedex deposits is the Guyamas Basin in the Gulf of as the coast of Peru, these first two criteria are met while the third California, where a large number of submarine hot springs and (sulfate reduction) has only been observed under rare circum- nlassive sulfide deposits l~a\~ebeen documented in sedimentary stances (Dugdale et al., 1977). As discussed in more detail below, basins near seafloor spreading zones (Koski et al., 1985; Peter and Scott, 1988). However, in many ancient sedex deposits a clear link with volcanis~nis lacking (Carne and Calhro, 1982; Large, 1983; Lydon et al., 1985a). For these deposits, the source of hydrothermal fluids is believed to be circulation along the normal faults of continental rift basins (Fig. 5). In this sense, no modern analog has been documented for sedex deposits, such as those of western Canada and Europe. A variation of the sedex nlodel that does have documented modern analogs in\~olvesexcess sediment pore pressure rather than l~ydrother~nalheating driving the exhalative processes. These seafloor discharges are characterized by relatively modest temperatures and tend to form in basins with relatively high sed- imentation rates (for example, the Gulf of Mexico) or in those FIG.S.-Dii~gl.alii of setlimentarp exhalative model of betlded barite ilepo- that are undergoing subduction (for example, the Peru margin). sition. hqassive sulfitle minernli;ration occurs near seafloor hydrothermal \,ents f?luidsthat precipitate barite along the peru margin are tliat are oftmi asmciated wit11 mi~ajorfault. Restl-ictetl circulation in the subma- contain high concentrations of dissol\/ecl barium, and have ele- line basin promotes anoxic \vater, allowing dissol\'ed barium to ~nigrateto tile site of betlded barite deposition at the 02-H2Stra~lsitiori Lolie. BEDDELI BARITE lili THE GEOLOGIC RECORD

open ocean sulfate reduction was probably more common ill DISCUSSION Paleozoic oceans (Berry and Wilder, 1978) and, in fact, could Despite being a ininor ele~nentin nlost crustal rocks, beds of have been a natural consequence of the relatively large size of essentially pure barite with thicknesses of tens of ineters and Middle to Early Paleozoic oceans (Jewell, 1995). Under these strike lengths of several kilometers are found in selected sedi- circumstances, bedded barite sedimentation rates could have mentary sequences throughout the world. Pure bedded barite bee11 comparable to those of bedded chert (Jewell, 1994). represents ail l~pgradeof -1000 tiines over co~lceatrationsof Associatioils with biologically produced sediments s~ichas eleinental barium in average continental and inarine sediments. chests and phosphorites and parallels with biogenic barite in the What peculiar circumstances inobilized and transported large modern ocean were notecl for the Northurnberland Canyon bed- quantities of dissolved bariurn over long distances before it ded barite deposits of central Nevada (Shawe et al., 1969). A could react with sulfate to deposit beds of pure barite up to tells biological origin was later advocated for all of the Nevada of meters thick at various times in the geologic record in a inan- deposits (Jewell and Stallard, 1991). The biological precipita- ner not obser\red ill the modern ocean'? tion i~~echanist~~is appealing for barite-only provi~lcessuch as Sulfate reduction of seawater and concurrent dissolution of Nevada, China, and Arkansas because it explains the lack of barium-bearing mineral phases is the nlost logical way of trans- massive sulfide deposits in the host rocks as well as why these porting elevated concentrations of bariunl lo a suitable site of three barite-only provinces have a dominantly seawater depositioi~.111 seafloor hydrothermal and diagenetic systems, sea- Y7~r/86~rsignatwe (Table 2). water is reduced and barium is leached during circulation of sea- water through crustal rocks. H2S-bearing fluids \vith elevated conce~ztrationsof dissolved barium discharge into oxygenated water to forin barite-rich deposits near seatloor vents in the mod- em ocean (e.g., Koski et al., 1985; Peter and Scott, 1988; Aquilina et al., 1997). If similar discharges occurred in sulfate-reducing waters of ancient oceans, then the dissolved barium could migrate long distances from the exhalative source and produce the barite "apron," \vhich is a common feature of sedex deposits (e.g., Large, 1983; Lydoil et al., 1985a) (Fig. 5). In a similar fashion, if barite-bearing particles produced by biogenic processes it1 the upper portions of the water colurnn entered sulfate-reducing water, they would dissolve and be reprecipitated at the edges of the sulfate-reducing zone (Fig. 6). Three critical questions thus emerge. (1) What conditions would to areally extensive sul- fate reductioil in ancient oceans given modern oceanographic conditions in which sulfate reductioil is rare? (2) Why are massive bedded barite deposits most common in Early and Middle Paleozoic rocks? (3) How can a distinction be made between the sedimentary exhalative and biogenic models of barite fonnation? In modern marine settings, sulfate reduction occurs when oxidation of organic matter exhausts the supply of and nitrate in the water column. This can occur as a result of physi- cal limitations on the exchange of oxygenated water with the open ocean, high fluxes of organic matter, or some combination of these two factors. The most common sulfate reducing envi- roainents in the inodes11 ocean involve settings where the exchange of water with the open ocean is restricted (for exam- ple, silled basins such as the Black Sea, Baltic Sea, and fjords). The sulfate-reducing waters of these basins have relatively high dissolved bariutn concenlrations (Falkiler et al., 1993). The Cariaco Trench is a fault-bounded graben off the coast of Venezuela that has restricted circulation and anoxic bottonl waters (Richards? 1975). This setting inay be a good modern FIG. 6.-Diagrammatic representation of modern and ancient upwelling zones ant1 their ~~elationsliipto bedded barite fi~~.rnation.(A) Modern upwelling atlalog for the ailcieilt Sel\vyn, Meggen, and in \vhicIi equntorwal.d winds induce upivelling of nutrient-rich water. (B) basins, where active exteasional tectonics inight have produced Hypothesized Late Devonia~iup\velling zone. Disareobic or ariosic deep ocean seafloor hydrothermal circulation along normal faults as well as xuses water to be sulfidic below the photic zone. Biogenically produced barite restricted the exchange of water with the open ocean. particles settle into the H2S-rich \vatel.. Barite is deposited at the 02-H2Sbo~~nd- Exteilsiollal tectoilic basins and the subsequent restrictioil of Irp due to the interaction of barium-rich water \\,it11 sulfate-rich water (fi-om water colunln circulatioil on a localized scale is less appealing lcwell ant1 Stallartl, 1991). a been put forward, however. Widespread evidence of anoxia on continental shelves suggests that the global ocean was oxygen deficient during the early Paleozoic (Berry and Wilde, 1978). Jewell (1995) outlined a mechanism whereby sulfate reduction was a commo~~placefeature of the equatorial regions of the very large ocean Illat existed conc~~rrel~tlywith the large, closely spaced corltinellts of the Middle Paleozoic through the Middle Mesozoic. Elevated concentrations of surface ~lutrientsalong the equatorial axis of the modem Pacific Ocean have long been noted (Fig. 7). This is a result of equatorial upwelling, which causes high surface productivity, in co~~junctionwith the very FIG.7.-Surface liltrate concentrations (pmols/L) in the equatorial Pacific strong, eastward-flowing equatorial undercurrent. Sinking ocean (fi-om Toggwieler et nl., 1991) organic matter is remineralized in the undercurrent and then

Late recirculated to the surface in the equatorial upwelling system. Permian 4--4 The result is a "nutrie~lt-trap" in which nutrie~lts become enriched in an eastward direction (Fig. 7). Equatorial water at intermediate depths also shows pronoullced oxygen depletion in an eastward direction, for example, near the South Alnericall coast, where it is close to anoxic (Fig. 8). Sulfate-reducing water has been observed during rare occasiolls in this area (e.g., Dugdale et al., 1977). The much larger size of the ocean during the Middle and Late Paleozoic would have enhanced this nutri- ent-trapping effect, thereby leading to widespread sulfate-reduc- ing co~lditionsin the equatorial ocean during this period (Fig. 8). These equatorial waters, in turn, would feed coastal upwellillg at FIG.8.-Oxygen concentrations as a function of ocean basin width (in low latit~tdesalong the west-facing portions of continents, f~~r- changes in degrees of longitude in eastern direction for Pacific Ocean (data fro111 Levitus, 1982). Dashed lines represent linear extrapolations of modern data to ther enhancing the possibility of sulfate reduction and the for- ocean widths which are believed to liave existed in the Late Paleozoic. Negative mation of bedded barite deposits (Fig. 6). oxygen concer~trationsrefer to nitlate ant1 sulfate reduction (from Jewell, 1995). The biogenic inodel for bedded barite formation requires a seawater source of barium and strontium within the barite. This is supported by ~nuchof the data f1.0111 China, Nevada, and Arkansas, which tend to have 87~1./S6~rvalues close to that of co~~te~nporaneousseawater (Fig. 9, Table 2). It is also significant that paleogeographic recollstructions of these three localities plot very close to the paleoequator (Fig. lo), ~fllerethe tlutrient- trapping (and barium-enricl~~nei~t)effect would have been great- est (Fig. 7). In summary, ulldersta~ldillgthe genesis of bedded barite thro~~gho~~tthe geologic record col~tinuesto be a useful and inter- esting avenue of research. Clear association of bedded barites with sedimentary exhalutive deposits holds out the promise of 8 7 sr1865r barite -8hr/86Srseawater finding new base metal deposits in the geographic vicinity. Likewise, a biologic origin of bedded barite allows the possibil- FIG.9.-Histogram of measured 87~dS6~rof bedded barite relative to con- ity of reconstructing productivity and redox conditiolls in ancient teniporaneous seawater 87~r/S6~rvalues (modifier1 froni Maynard et al., 1995). Contemporaneous seawater values are from Burke et al. (1982). Barite-only oceans. Several aclditio~lalresearch possibilities are also possible data are from China, Nevada, ant1 Arkansas and Pb-Zn-Ba values arc from to realize these potentials, including more detailed comparison of Scotland. Gcrmany, Belgium, and westerli Canada (Table 2). areas that co~ltaillboth Pb-Zn-Ba and barite-only deposits (for example, Alaska and the Selwyn Basin) to understand specific mechanism for the genesis of barite-only deposits such as those differences between the two. 111 this regard, 87~r/8G~rst~ldies of Nevada and Arkansas. Although evidence of extensional tec- would be particularly useful in understanding sources of the bar- tonics has been found in some of these bedded barite deposits ium (Fig. 9). More detailed descriptions of the morpl~ologyand (e.g., DubC, 19881, rift basins capable of restricting seawater cir- form ol' barite occurrences on the modern ocean for which the culation have not been systeinatically documented in the same genesis is clearly understood would also be useful for makin2 fashion as the important sedex provinces of western Canada and meani~lgf~~lco~nparisolls with ancient deposits. Europe. Various arguments for widespread sulfate reduction in the open ocean during the Early and Middle Paleozoic have BEDDED BAR178 liV THE GEOLOGIC RECORD 159

lower Paleozoic deposits from Tcnncssee and Nevacli~(U.S.A): klinerologica Petrographica Acta, v. 27, p. 251-254. BARBIERI,M., DE Vivo, B.. PERRONE.V., AND TURCO,E., 19840. Strontium geo- chemistry of the Sa~iDonato barite mineralization (Calabria, Italy): Chemical Geology, v. 45, 11. 779-288. BARBIERI,M., MASI.U., AND TOLORIEO,L., 1984b. Strontium geochemical evi- dence for the origin of the barite deposits from Sardinia (Italy): , v. 79, 11. 1360-1365. BARLEY.M. E., DUN[-OP,J. S. R., GLO\J~H,J. E., AND GROVES,U. I., 1979. Sedimentnry evidence for an Archean shallow \\rater oolcanic sedimentary facies, castcrn Pilbara Block, \Vestern Ausrmlin: E;~rthand Planetary Science Letters, v. 43, p. 74-84. BEI~RY,W. B. U.,AND WILDE,P., 1978, Progressive ventilation of the oceans - an explanation for the distribution of the lo\\,er Palcozoic black shales: Amcrican Journal of Scicnce, v. 278, p. 257-275. BISHOP,J. K. B.. 1988, The barite-opal-organic carbon association in oceanic particulate matter: Nature, v. 332, p. 341-343. BOGOCH,R.. AND SHIRAV,M., 1978, Petrogenesis of a Senonian barite deposit, Judean Desert, ISI-acl:M~nemlium Depositn, v. 13, p. 383-389. BRODTKORB,k1, K., 1989. Nonmetalliferous Strataboulld Ore Fields: New, York, Van Nostl-~lndReinllold, 332 p. BRODTKORR,hf. K., RAMOS,V., BARBIERI,h4., AND ARIETRANO,S., 1982, The evnpolitic celestite-barite deposits of Nzuquen, Argentina: Miileralinln Deposita, v. 17. p. 423-436. BRODTKOI

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