Magmatic gold grains in the El Tale lamproite, Fortuna, SE Spain

Abstract

Abstract Gold of magmatic origin has been discovered in the lamproitic dyke of El Tale (Fortuna, Province of Murcia), a small outcrop of ultrapotassic rocks of tile Tertiary magmatic province of southeastern Spain. It is the first finding of gold grains in lamproite magmas from southeastern Spain, and indeed worldwide. Pure gold occurs only in the glassy groundmass as very rare spherules and globular grains (up to 5 µm). Despite the occurrence of gold grains, the bulk rock contains not more than ∼19 ppb Au. The magmatic origin of gold is inferred from the heterogeneous distribution of the grains in the rock, the shape of the grains (which is comparable to that of gold grains crystallized experimentally from mafic liquids) and from the As-Sb-S-Cl-poor composition of the bulk rock. The Pd/Ir and Au/Pd ratios, little affected by the early segregation of olivine (∼10 vol.%), support an Au- and PGE-rich composition of the mantle source. The low fO2 (≈IW) and the complexing with F and alkalies probably favoured partitioning of Au and PGE into the lamproite magma.