THE OCCURRENCE OF GOLD-RICH PYRITE IN THE ITAJUBATIBA SKARN DEPOSIT, BORBOREMA PROVINCE, NORTHEASTERN BRAZIL: A DISCOVERY BY PIXE ANALYSES

Resumen

The Itajubatiba deposit represents a gold skarn, in northeastern Brazil. Since its discovery in the 1940s, it was exploited during about 30 years and it has produced 5 tons of gold. Its average ore grade ranges from 0.5 to 2.0 ppm and has reached 6.3 ppm. Regional geologic units consist of a gneissic-migmatitic basement (Archean to Paleoproterozoic), a meta-sedimentary sequence (marble and schist) and Neoproterozoic intrusive igneous rocks. Later a metasomatism transformed marble, meta-tonalite (gneissic-migmatites), and meta-syenogranite into skarns. Gold mineralization is typically later, associated with sulfides, which appear either filling veins crosscutting the skarns or disseminated throughout the skarns. In order to verify this hypothesis regarding the formation of gold, a detailed search for the metal was carried out using several polished thin sections. Gold, however, was not observed. A subsequent detailed investigation of trace elements in sulfides of the Itajubatiba ore was conducted. Proton-Induced X-ray Emission (PIXE) analyses revealed that some elements occur in relatively high amounts in these sulfides, such as Mn (up to 0.9 wt.%), Au (up to 690 ppm), As (up to 1,360 ppm), Bi (0.5 wt.%), Pb (up to 1,870 ppm), Ni (up to 3,050 ppm), Co (0.6 wt.%), Se (up to 1,000 ppm), and W (up to 1,420 ppm). Pyrite shows the highest amounts of gold among the sulfides studied. Considering the quantity of gold detected in pyrite, the pyrite abundance in the study rocks, and the average densities of the main minerals occurring in skarns, their gold grade varies from 1.5 to 10.4 ppm (for gold hosted in pyrite only). The results reveal economically significant gold amounts linked to sulfides, and could provide a new perspective for the exploitation of the Itajubatiba deposit.