Emerald from the Fazenda Bonfim Deposit, northeastern Brazil: chemical, fluid inclusions and oxygen isotope data

• Published in: Brazilian journal of geology 2018-07, Vol.48 (3), p.457-472
• Main Authors: Santiago, Judiron Santos, Souza, Valmir da Silva, Filgueiras, Bernardo de Carvalho, Jiménez, Federico Alberto Cuadros
• Format: Article
• Language: English
• Subjects: Borborema Tectonic Province; Chemical Mineral; Fazenda Bonfim Emerald Deposit; Fluid Inclusions; GEOLOGY; GEOSCIENCES, MULTIDISCIPLINARY; Oxygen Isotope; Seridó Mobile Belt
• ISSN: 2317-4889; 2317-4692; 2317-4692
• DOI: 10.1590/2317-4889201820170130

The Fazenda Bonfim emerald deposit, State of Rio Grande do Norte, is within the regional geological domain known as Seridó Mobile Belt, Borborema Tectonic Province. It was formed from metasomatic fluids interaction at along lithological contacts between Be-rich albite-granite intrusions and Cr (±V)-rich mafic-ultramafic host-rocks, enclosed in the lens-shaped “hornfels” phlogopite schist. Emerald crystals display relatively high contents of Mg and Na, as well as trace amounts of Ca, K, Cs, Li, P, Sc, Ti, Mn, Co, Ni, Zn, Ga and Rb. Cr is the main chromophore element, followed by Fe and some V. Display also concentric growth zones and randomly-oriented mineral micro-inclusions, indicative for static growth. This zoning is linked to cationic substitution of alkalis accompanied by Cr loss, which favors irregular coloration of crystals. Metasomatic fluids contemporaneous with emerald growth are aqueous (H 2 O+NaCl), with low to moderate salinity and low density, although trace amounts of CO 2 ± CH 4 were also observed. These fluids showed a field-trapped between 375-430ºC and 200-600 bars, based on combination of fluid inclusions isochores. In addition, oxygen isotope data (δ 18 O = 6.9-7.4‰) suggest an igneous-metasomatic source for fluids and emerald components.