Stairway to Gold: The stepped thermal evolution of Casas Novas deposit (Montemor-o-Novo, Portugal)

Abstract

The Iberian Peninsula is a geologically complex region divided into fault-bounded tectonostratigraphic zones, each characterized by a distinct internal stratigraphy and structural history. The Ossa-Morena Zone (OMZ) is one of these key divisions, located in the southwestern Iberian Peninsula and extending from the Alentejo region in Portugal into the Extremadura and Andalusia regions of Spain. The Casas Novas gold deposit is situated within the Escoural Gold District of the OMZ (Montemor-o-Novo, Portugal) and constitutes an orogenic gold system. The mineralization is present on structurally controlled mesothermal veins genetically related to the Montemor-o-Novo Shear Zone (MNSZ). Recent research by São Pedro [1], utilizing the laboratory facilities at the University of Évora, has defined the metallogenic sequence and characterized the mineralogical and geochemical signatures of the paragenetic assemblages. The mineralization reveals a complex evolution during regional metamorphism, categorized into three distinct stages(i) An early stage (potential peak metamorphic conditions), characterized by high-temperature, Co–Ni-enriched loellingite (FeAs₂) hosting Au-(Ag) alloys; (ii) an intermediate stage (retrograde metamorphism), defined by the replacement of loellingite (FeAs₂) by arsenopyrite (FeAsS) and the precipitation of native gold (Au), maldonite (Au₂Bi), and Bi–Te phases (e.g., hedleyite, Bi₇Te₃); and (iii) a late stage, represented by the crystallization of pyrite (FeS₂) and chalcopyrite (CuFeS₂). These new data, combined with the extensive expertise of the researchers at the Institute of Earth Sciences (ICT) regarding the geodynamic evolution of the OMZ, provide critical constraints for ongoing studies of similar auriferous systems in both Portugal and Spain. This contribution refines the metallogenic model for the region and enhances the potential for cross-border scientific networking and exploration strategies within the Iberian Massif.