This article starts from the observation that, in order to ensure their legitimacy, the modes of governance in place in most Western democracies make more room for citizen participation in decision-making processes. The result is the implementation of various participatory mechanisms, many of which seek to stimulate a citizen's argumentative expression. Based on a case study (the public debate on a gold mine project in French Guiana), we observe the norms that govern such participation processes and their implementation in the argumentative exchanges.