Abstract

Lead isotope analyses of samples from ore deposits of the West African shield trace the metallogenic evolution of the Early Proterozoic or Birimian period. The selected deposits, which include the Loulo, Poura, and Banora gold deposits, the Faleme iron deposit, and the Perkoa, Gan, Hanhui, and Pilimpikou base metal deposits, are representative of each stage of the so-called Eburnean evolution. The study was completed by an analysis of gold deposits in the French Guianan Shield, which is a rifted remnant correlative of the West African craton.The isotopic compositions of most of the galenas cluster in a narrow field ( 206 Pb/ 204 Pb = 14.60-15.15, 207 Pb/ 204 Pb = 14.96-15.23, 208 Pb/ 204 Pb = 34.34-34.71) corresponding to the initial lead-isotopic composition of Birimian mineralization. Secondary isochrons, obtained on cogenetic lead-depleted sulfides, all converge toward the same narrow field; it is likely that they reflect an in situ addition of radiogenic lead to older lead from the Eburnean event. The calculated ages (2236-2741 Ma) have no clear geologic significance.The elongated field of initial lead compositions requires a mixing between juvenile, mantle-derived lead as expressed in the syngenetic deposits (Perkoa, mu = 9.11), and a more radiogenic component, such as old (>2.150 Ma) crustal lead, as expressed in the late orogenic deposits (Dorlin-THR stockwork, mu = 10.06). This increase in mu values reflects a gradual increase in the older recycled crustal lead component, favored by the tectonic change from D 1 thrusting to D 2 gold-bearing vertical strike-slip faulting. The age of the old recycled crust would almost certainly be earliest Proterozoic (2300-2145 Ma, Dabakalian); no perceptible Archcan lead influx is required to explain the genesis of these deposits. Geochronological data from other sources bracket most of the Eburnean metallogenesis between 2153 + or - 13 Ma (2Sigma ) age of the Dabakalian metamorphism, and 1949 + or - 55 Ma (2Sigma ) age of the last post-tectonic granites. Pb/Pb model ages, respectively 2113 Ma for the early so-called syngenetic ore deposits and 2014 Ma for the late-orogenic mesothermal gold deposits, are consistent with these independent geochronologieal data and probably reflect fairly well the approximate geologic ages of these deposits.

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