Summary:
The mineral deposits at Neves-Corvo are classified as volcano-sedimentary massive sulphide (VMS). They typically occur as lenses of polymetallic (Cu, Zn, Sn, Pb) massive sulphides that formed at or near the seafloor in submarine volcanic environments. They formed from accumulations of the focussed discharges of hot metal-enriched fluids associated with seafloor hydrothermal convection, typically in tectonic areas of active submarine volcanism, including rift spreading centres and island arc subduction zones. The massive sulphide lenses are commonly underlain by sulphide-silicate stockwork vein systems, although the stockwork systems may extend into the hanging-wall strata above the massive sulphide lenses. The immediate host rocks can be either volcanic or sedimentary. The deposits are overlain by a repetition of volcano-sedimentary and flysch units.
VMS deposits readily accommodate strain during regional deformation because of the ductile nature of massive sulphide bodies, and can therefore display much higher degrees of recrystallisation and remobilisation than the surrounding volcanic and sedimentary strata. The tectonic remobilisation may result in duplication of the stratigraphy further localising the sulphide mineralisation.
Six massive sulphide mineralised zones have been defined within the Neves-Corvo Mining Area and comprise Neves, Corvo, Graça, Zambujal, Lombador and Monte Branco. The Semblana massive sulphide zone is located within the Semblana Mining Area and is located 1.3km northeast of Zambujal.
The mineralised zones lie on both flanks of the Rosario-Neves-Corvo anticline. The mineralised zones of Neves, Corvo, Graça, Zambujal and Lombador are connected by thin massive sulphide “bridges” over the crest of the fold and are conformable with the stratigraphy. Within the area of these five main deposits this has resulted in an almost continuous complex volume of mineralised rock showing a large range in both style of mineralisation and geological structure. The mineralised zones are located at depths of 230m to 1,400m below surface.
The mineral deposits occur as concentrations of high-grade copper and/or zinc mineralisation within massive sulphide pyritic lenses, and copper mineralisation within stockwork zones that typically underlie the massive sulphide. Base metal grade distributions within the massive copper/zinc sulphide lenses typically show good internal continuity, but laterally can terminate abruptly in barren pyrite. The massive sulphide deposits are generally very large, regular, continuous and predictable. However the geometry of the high-grade zinc and copper zones within the deposits can be very complex. In many cases, boundaries between ore grade mineralisation and barren pyrite may be almost parallel to the stratigraphic contacts of the sulphide lens.
The base metal grades are segregated by a strong metal zoning into copper, tin and zinc zones, as well as barren massive pyrite. Three styles of mineralisation have been identified at Neves-Corvo:
- Rubané mineralisation - characterised by thin banded alternations of shales, breccias and massive sulphide or tin mineralisation (found mainly in Corvo but now predominantly mined out);
- Massive sulphide mineralisation; and
- Stockwork (fissural) sulphide mineralisation.