New insights on the Escoural Orogenic gold district (Ossa-Morena Zone, SW Iberia): Geochemistry, fluid inclusions and stable isotope constraints from the Monfurado gold prospect

Maia, M., Roseiro, J., Nogueira, P., Noronha, F., Fuertes.Fuente, M., Cepedal, A., Mirão, J., 2022. New insights on the Escoural Orogenic gold district (Ossa-Morena Zone, SW Iberia): Geochemistry, fluid inclusions and stable isotope constraints from the Monfurado gold prospect. Ore Geology Reviews, 104736.

The Escoural gold district belongs to the Montemor-Ficalho metallogenic belt which is part of the Portuguese section of Ossa-Morena Zone (OMZ), at the SW of Iberia. The Escoural gold district includes twelve gold prospects and/or deposits largely controlled by the NW-SE Montemor-o-Novo Shear Zone (MNSZ) and associated fault zones, extending for approximately 30 km. Ubiquitously, gold-arsenopyrite-loellingite assemblages hosted in quartz-sericite-chlorite veins are found in most deposits, although, in the Monfurado prospect, the gold-bearing assemblages are more complex. This prospect is located in the vicinity of a Cambrian SEDEX-VMS iron deposit, from which massive and disseminated iron-ores hosted in marbles and calcsilicate rocks, were exploited. The interplay of the gold mineralizing processes with the iron-rich host rocks has favored gold deposition at the Monfurado prospect. Selected samples from six drill cores allowed to define two mineralizing events: the pre-ore and ore stages. Two gold mineralization styles characterize the ore-stage: i) massive sulfide horizons in which gold (Au= 85.6 – 86.3 wt. %; Ag= 13.1 – 13.6 wt.%) is hosted in arsenopyrite and pyrite or, seldomly, gold particles (Au= 91.8 wt.%; Ag= 7.1 wt.%) found in an arsenopyrite-rich layer; and ii) quartz-chlorite-pyrite veins crosscutting acid metavolcanic rocks with rhyolite-rhyodacite affinities, in which gold (Au= 80.5 – 82.9 wt.%; Ag= 16.8 – 18.7 wt.%) is found as fracture filling in pyrite, sometimes accompanied by Bi-Te phases. Arsenopyrite geothermometer suggests that for type i the overall deposition temperature falls within the range of 188°C to 372°C. Type ii mineralization lacks arsenopyrite, and for this reason, thermodynamic constraints were gathered from fluid inclusions and chlorite geothermometer. CH4-rich fluid inclusions are ubiquitous in transgranular fluid inclusion planes, suggesting that reduced fluids percolated the rocks that host type ii mineralization. The reduced fluids support the transport of gold in sulfide complexes, such as AuHS and Au(HS)2. Furthermore, secondary H2O-NaCl fluid inclusions (Lw2) were found, with mean salinities of 6.0 eq. w(NaCl) and mean homogenization temperature of 226°C, with corresponding pressures of 3.0 MPa, thus suggesting late hydrostatic regimes. Chlorite geothermometer results are in the range of 229 °C and 309 °C, agreeing with the fluid inclusion homogenization temperatures for Lw2 fluids.

Sulfur isotope (δ34S) analysis of representative sulfide phases collected from both mineralization types, revealed signatures ranging from 8.5 ‰ and 10.6 ‰, indicating a single sulfur source. The gathered results suggest that although fluid transport is structurally controlled by MNSZ activity, the sulfidation reactions promoted by fluid-rock interactions are the main control on gold deposition from type i mineralization. It is further suggested that a coeval gold-event can lead to the deposition of two different types of mineralization, related to distinct gold deposition mechanisms.

The tectonic and geodynamic settings in which the Escoural gold district developed correlate it to worldwide Palaeozoic orogenic gold deposits, with the Monfurado prospect being an example of the complexity of such geological settings. Read the full article.