Origin of Sulfide and Phosphate Deposits in Upper Proterozoic Carbonate Strata, Irece Basin, Bahia, Brazil
KYLE, J. RICHARD, University of Texas at Austin, Austin, TX, and AROLDO MISI, Universidade 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. Shallow-water sedimentary structures include cryptalgal laminae, columnar stromatolites, mudcracks, teepee structures, collapse breccias, length-slow quartz nodules, and other pseudomorphs after evaporitic sulfates. Concentrations of fine-grained carbonate fluorapatite occur within stromatolitic carbonate units in the Irece-Lapao area. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphatic clasts and phosphatic units interbedded wit nonphosphatic 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. Barite is also present in nodular forms, as well as veinlets with sulfides. Replacement textures such as corroded grain boundaries between sulfide minerals are common. Sulfur isotope analyses indicate relatively uniform heavy S(34) values. Barite shows a S(34) range from +25.2 to +29.6%, CDT. Pyrite and sphalerite representative of a variety o textural types have a S(34) range of +20.2 to +22.6%. Late Proterozoic evaporite sulfates show a wide range of S(34) values from about +10 to +28%. Thus, the S(34) values for Irece barite could reflect original seawater sulfate values. However, the relatively heavy S(34) 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 S(34) evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)