Abstract

The two main problems of ore genesis are the origin of metals and the process(es) of emplacement. Lead isotopes have proven to be a powerful tool for studying these questions. We have studied Pb-Zn mineralizations from Montagne Noire and Cevennes (southern Massif Central, France) by means of lead isotopes. This study shows the existence of two populations of ore deposits defined by 17.76 < 206 Pb/ 204 Pb < 17.94 and 18.25 < 206 Pb/ 204 Pb < 18.63. A general model of lead evolution in these deposits is proposed. After evolving in the earth's mantle until approximately 3 b.y. ago, the lead became incorporated into a segment of continental crust. Then, the less radiogenic population of ore deposits was emplaced contemporaneously with the formation of Cambrian carbonate host rocks. The second population is related to the emplacement of Hercynian granitoids or, in more general terms, to fluid circulations associated with the Hercynian orogeny. It can also be shown that some of the deposits are remobilized from preexisting ore deposits.

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