Origin of sulfide and phosphate deposits in Upper Proterozoic carbonate strata, Irece basin, Bahia, Brazil

Conference · Fri Mar 01 04:00:00 EST 1991 · AAPG Bulletin (American Association of Petroleum Geologists); (United States)

OSTI ID:5676649

Kyle, J R ^[1]; Misi, A ^[2]

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1. Univ. of Texas, Austin (United States)
2. Univ. Federal da Bahia, Salvador (Brazil)

Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece basin of north-central Brazil. Stratabound sulfide- and phosphate-rich units occur within a 50-m thick tidal flat sequence of dolomitic limestone and cherty dolomite. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphate clasts and phosphatic units interbedded with non phosphatic dolomites suggest early diagenetic replacement of algal carbonate units. Local stratabound Zn-Pb-Ag sulfide concentrations at the Tres Irmas prospect occur within silty dolomite with shallow water sedimentary structures and local disturbed laminae, synsedimentary faults, and breccias. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite. Pyrite crystal aggregates commonly show bladed forms. Nodular aggregates of length-slow quartz are locally associated with sulfides. Sulfur isotope analyses indicate relatively uniform heavy {delta}{sup 34}S values. Barite shows a {delta}{sup 34}S range from +25.2 to +29.6{per thousand}, CDT. Pyrite and sphalerite representative of a variety of textural types have a {delta}{sup 34}S range of +20.2 to +22.6{per thousand}. Late Proterozoic evaporite sulfates show a wide range of {delta}{sup 34} S values from about +10 to +28{per thousand}. Thus, the {delta}{sup 34}S values for Irece barite could reflect original seawater sulfate values. However, the relatively heavy {delta}{sup 34}S values of the associated sulfides suggests that the original seawater sulfate was modified by bacterial sulfate reduction processes in shallow sea floor sediments. Textural and {delta}{sup 34}S evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.

• OSTI ID: 5676649
• Report Number(s): CONF-910403--
• Conference Information: Journal Name: AAPG Bulletin (American Association of Petroleum Geologists); (United States) Journal Volume: 75:3
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
BRAZIL
CARBONATE ROCKS
CHEMISTRY
DEVELOPING COUNTRIES
DIAGENESIS
GEOCHEMISTRY
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
GEOLOGIC STRUCTURES
ISOTOPE RATIO
LATIN AMERICA
LEAD ORES
MINERALIZATION
MINERALOGY
MINERALS
ORES
PHOSPHATE MINERALS
ROCKS
SEDIMENTARY BASINS
SEDIMENTARY ROCKS
SOUTH AMERICA
SULFIDE MINERALS
ZINC ORES