Fluid constraints for Au deposition at the Monges iron deposit, Ossa Morena Zone (Montemor-o-Novo, Portugal)

Abstract

The Monges deposit was an important iron mine during late 19th and early 20th century. In recent decades the region of Montemor-o-Novo has attracted the interest of several exploration companies due to high-grade gold anomalies found through soil geochemistry work. Exploration drilling carried out in the WNW-ESE and conjugated N-S shear-zones (≈30 km) identified interesting gold grades, including in the vicinity of the Monges iron deposit. The fluid inclusion study presented here is focused on constraining the fluids related to iron and gold mineralization processes. Fluid constraints for the iron mineralization are difficult assess in the early stage (SEDEX environment) of the Ossa-Morena Zone geodynamic evolution. An overprinting of these early fluids is found, and most of the fluids present may be related to late metamorphism. Gold in Monges area shows affinity to a H2O-CO2-NaCl fluid system showing evidence of immiscibility and trace concentrations of CH4. The geological settings hosting gold mineralization as well as the fluid characteristics with low-salinity, with an average of 7.72 wt. % NaCl Equiv. and homogenization temperatures between 150ºC and 350ºC, points towards an orogenic gold deposit model, corroborating and reinforcing the current accepted model for gold mineralization of Montemor-o-Novo region.