The use of reflectance spectroscopy and the prehnite spectral index to target gold mineralization at the Bonfim polymetallic skarn deposit, Serido Mobile Belt, Borborema Province, Brazil

The Serido Mobile Belt hosts numerous mineralized skarns in the Borborema Province, northeastern Brazil. The Bonfun W-Mo-Au-Bi-Te skarn occurs within a Neoproterozoic metasedimentary sequence (Serido Group), within marble and schist lenses, or at the contacts between both. An ASD-FieldSpc (TM) 3 High-Resolution spectrometer was used to characterize the mineralogy of a representative drill core interval (181 m) from the Bonfim deposit, in order to generate the spectral characterization to define the gold mineralization. Skarns are characterized by the presence of diopside, actinolite-tremolite, clinozoisite-zoisite, muscovite and prehnite. Prehnite and epidote constitute the main components of the talc-silicate assemblage associated with late-stage gold mineralization. The diagnostic absorption feature of prehnite at 1478 nm (OH) can be used to identify its presence, and the prehnite index proposed in this case study (a reflectance ratio of 1465 nm/1478 nm) allows highlighting the prehnite enriched sections along the core. Geochemical data show a positive correlation between gold and the presence of a prehnite absorption at 1478 nm. The results suggest that the prehnite spectral signature can be used as a reliable and efficient (rapid) indicator for gold skams, perhaps under geologically similar circumstances to those at Bonfim.