THE THREE GENERATIONS OF GOLD IN THE PALAI-ISLICA EPITHERMAL DEPOSIT, SOUTHEASTERN SPAIN

Abstract

In the Palai‐Islica deposit, in southeastern Spain, gold is found associated with sulfide mineralization (particularly Fe sulfides). Grains of Au‐Ag alloy (gold of types A and B) occur with pyrite in mineralized veins, and grains of native gold (type C) are associated with areas of massive silicification at the uppermost levels of the deposit. The content of “invisible” gold in the various Fe sulfides is practically nil. Of all the textural varieties of pyrite studied, unzoned medium- to coarse-grained pyrite is the only one bearing gold. A clear distinction can be established between three types of gold (A, B and C), each with a different genesis. Grains of type-A alloy deposited as a result of variations in the thermodynamic parameters of the system, mainly a decrease in sulfur activity, whereas the appearance of the type-B alloy was mainly controlled by electrochemical factors. Type-C gold may have been produced from colloidal solutions in significantly different geochemical conditions. The chemical evolution of the alloys is characterized by Ag enrichment as precipitation continued. Type-A grains [mean Au/(Au + Ag) = 0.861] were the first to form, encased in the pyrite, with a relatively low Ag content and barely any zonation, followed by gold of type B, overgrowing pyrite, with a higher average Ag content [Au/(Au + Ag) = 0.756] and commonly zoned, with later zones richer in Ag. Finally, type-C native gold has practically no Ag [on average, Au/(Au + Ag) = 0.988].