Occurrence of gold (electrum) in the Lousal mine, Iberian Pyrite Belt, Portugal

Recent exploration drill holes in the Lousal Mine, located within the Portuguese sector of the Iberian Pyrite Belt (IPB) yielded previously unknown and significant concentrations in gold (electrum) in a section of core consisting of hydrothermally altered banded sediments with massive pyrite and cobaltite. Research indicates that the gold (electrum) is present in veinlets within the more dominant massive pyrite and cobaltite associated with native bismuth and bismuthinite and is clearly late in the paragenetic sequence occurring also with fine chalcopyrite (± covellite). The gold (electrum) grains are small, seldom reaching more than 6 µm in length and half of that in thickness. EPMA results indicate that silver concentrations in the gold grains can be as high as 26 wt%.
These results show similarities with conclusions drawn from the IPB on the Spanish side where gold is found as electrum associated with abundant to common Co and Bi minerals. The gold is associated with pyrite, cobaltiteand/or chalcopyrite and hosted by sedimentary facies. This gold is interpreted to have formed at high temperature (>300 °C) during the initial phases of massive sulfide formation. At Lousal the gold seems to occur related with hydrothermal alteration of tthe sulfides, and remobilized in late-stage shear zones.