Gold mineralization in recent and ancient volcanic-hosted massive sulfides: The PACMANUS field and the Neves Corvo deposit

Gold mineralization in the Neves Corvo copper stockworks occurs in two geochemical associations: (1) Au+Co+Bi(±Te) and (2) Au+Cu+Ag(±Hg). Type 1 gold occurs in the deeper parts of the feeder zones. Its associated mineralogical assemblage is consistent with low sulfur activity of the fluid and high temperature (360°–400°C) conditions. The second type of gold geochemical association reflects pH increase and/or temperature decrease as the fluid moves upwards in the stockwork and reaches the base of the massive sulfide Neves Corvo is accompanied by high sulfidation parageneses. These might have resulted either from the long-lived maturation of the ore-forming system, which would have led to extreme zone refining effects, and/or from the late input of an external fluid component (possibly magmatic?) in the Neves Corvo hydrothermal system. In the PACMANUS hydrothermal field, native gold also occurs in both high and low sulfidation mineral associations. Gold occurs at the seafloor level either associated with: (1) chalcopyrite+sphalerite +tennatite, or (2) chalcopyrite+bornite ± covellite. At depth, instead, gold occurs in sphalerite as small native gold inclusions (silver-poor). Following the process of copper enrichment by zone-refining, gold frequently concentrates in the borders of the sphalerite grains.