Abundance and distribution of gold in the orogenic-type spinel peridotites from Ariège (Northeastern Pyrenees, France)

Abstract

Fifty-one spinel peridotite samples from the Eastern Pyrenean massifs (Ariège, France) have been analyzed for gold by instrumental neutron activation (INA) and radiochemical neutron activation (RNA). Two groups of peridotites are distinguished. The first one includes all the samples except the amphibole-rich peridotites from Caussou in the northeastern Pyrenees of France. In this group, the Au content varies widely between 0.1 and 4.0 ppb, irrespective of the refractory character of peridotites and Ce Yb ratios. LREE-rich peridotites which have been percolated by alkali basaltic melts (harzburgites and peridotite wall-rocks of amphibole-pyroxenite veins) are lacking in Au-enrichment. The second group of analyzed peridotites (amphibole-rich samples from Caussou) displays significant Au-enrichments in the concentration range of 5 to 18 ppb. We infer from these data that gold may have a significantly different behaviour during mantle metasomatism: gold behaves as a relatively inert element during mantle metasomatism dominated by silicate melts. In contrast, H 2O-CO 2 fluids leading to amphibole crystallization could have transported gold.