Gold in the Lousal Mine, Iberian Pyrite Belt, Portugal

Recent exploration boreholes in the Lousal Mine, located within the Portuguese sector of the Iberian Pyrite Belt (IPB) yielded marked concentrations in gold/electrum in a section of core consisting of banded metasediments with massive pyrite. Preliminary research indicates that the gold is associated with native bismuth and bismuthinite and is clearly late in the paragenetic sequence occurring in fine chalcopyrite (± covellite)-bismuthinite-gold filled veinlets within the dominant and more massive pyrite. The pale yellow gold grains are fine, seldom reaching more than 6 micron in length and half of that in thickness. EPMA results indicate that silver concentrations in gold grains can be as high as 27 wt.%. The results show similarities with conclusions drawn from the IPB on the Spanish side where gold of Co-Bi geochemical association is found as electrum with abundant to common Co and Bi minerals. These associated with pyrite and/or chalcopyrite are characterized by an abundance of sedimentary facies and show that the gold association formed at high temperature (>300 °C) during the initial phases of massive sulphide formation.