Summary:
Deposit Type
The deposits of the Neves-Corvo Mine are classified as volcanogenic massive sulphides (VMS). They are also known as volcanic-associated, volcanic-hosted, and volcano-sedimentary-hosted massive sulphide deposits. The deposits typically occur as lenses of polymetallic massive sulphides that formed at or near the seafloor in submarine volcanic environments, and are classified according to base metal content, gold content or host-rock lithology.
Mineralised Zones
Seven massive sulphide deposits are present and include: Neves, Corvo, Graça, Zambujal, Lombador, Monte Branco and Semblana.
The deposits lie on both flanks of the Rosario-Neves-Corvo anticline. Neves, Corvo, Graça, Zambujal and Lombador sulphide lenses are conformable with the stratigraphy and are connected by stockwork ‘bridge zones’ mostly over the crest of the fold. This has resulted in an almost continuous complex volume of mineralised rock showing a large range in both styles of mineralisation and geological structure. Drilling has demonstrated that no significant continuous mineralised ‘bridge’ exists from Zambujal to Semblana or Monte Branco. The mineralised zones are located at depths of 230m to 1,400m below surface.
The deposits occur as concentrations of high-grade copper and/or zinc mineralisation in massive sulphide pyritic lenses, and copper mineralisation within stockwork zones that typically underlie the massive sulphides. 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 and continuous. However, the geometry of the high-grade zinc and copper zones within the deposits can be very complex. In many cases, boundaries between economic mineralisation and barren pyrite may be almost parallel to the stratigraphic contacts of the sulphide lens.
Three styles of mineralisation are present:
• 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.
Due to the structural complexity of the orebodies, different mineralisation types are often juxtaposed, even over short distances both vertically and laterally. Zonation in the massive sulphide lenses is typically either copper or zinc, although they do occur together in some areas. Zinc-rich zones are typically found near the stratigraphic top of the massive sulphides while copper-rich zones tend to be found at the base. This zoning is interpreted to be a result of primary metal re-zoning caused by temperature, pressure and chemical gradients soon after deposition. In a general sense, grade continuity is better within the massive sulphide lenses than it is within adjacent stockwork and “bridge” zones. The geometry of the copper mineralisation tends to be more complex than that of the zinc mineralisation.
Massive, cassiterite rich, tin mineralisation is associated with the rich copper mineralisation and in the copper rich rubané. The tin mineralisation (now largely depleted by mining) was mainly found in the Corvo orebody, associated with north-south faults along a north-south oriented corridor. The underlying stockwork also contained tin mineralisation.
The following mineralogical and morphological descriptions relate to the orebodies in their original state, prior to mining.
Corvo
The Corvo orebody lies between 230m-800m below surface, dips to the northeast at 10-40° and has a strike of approximately 600m. The orebody attains a maximum thickness of 95m and consists of a basal layer of copper ore up to 30m thick, overlain by barren pyrite containing intermittent lenses of copper mineralisation.
The main massive sulphide orebody is overlain by a complex mineralised sequence known as Rubané which comprises an assemblage of chloritic shales, siltstones and chert-carbonate breccias that are all mineralised with cross-cutting and bedding-parallel sulphide veinlets and occasional thin lenses of massive sulphides.
Graça
The Graça orebody is up to 80m thick, extends for 700m along strike, 500m down dip and ranges in depth below surface from 230-450m. The orebody is linked to Corvo by a bridge of thin continuous sulphide mineralisation. As with Corvo, much of the copper ore occurs as a basal layer overlain by barren pyrite in which there are also intercalations of copper ore. Most of the copper mineralisation within the Graça orebody has been mined out with the exception of a small extension to the southeast that lies on the southern flank of the anticline and dips to the south at 10-70°.
Neves
The Neves orebody consists of two lenses of mineralisation which dip north at 0-35° and are joined by a thin bridge. The maximum true thickness is 55m with a strike length of 1,200m extending 700m down dip. The southern lens, Neves South, contains mostly zinc mineralisation with significant lead, silver and copper grades and minor barren pyrite underlain by copper ore which is locally tin-bearing. Zinc mineralisation tends to be very fine grained (<25microns) and does contain deleterious elements such as As, Sb and Hg.
Zambujal
The Zambujal orebody is located to the south of the Corvo orebody and is connected to Lower Corvo by a bridge of mineralisation. The upper part of the orebody is found at 380m below surface, has an average thickness of 55m and includes massive zinc and copper zones and a significant underlying copper stockwork. The mineralisation is found on two limbs of the anticline and are considered as Zambujal SW and Zambujal NE (based on the fold limbs). Zambujal SW limb dips at 15° to the southwest and extends for 250m down dip. Zambujal NE limb dips at 20-30° to the northeast. The massive sulphides extend for up to 290m down dip while the underlying stockwork extends further (up to 360m down dip) into Zambujal East with isolated zones identified beyond this.
Lombador
The Lombador orebody is the largest of the five massive sulphide zones at Neves-Corvo and is situated on the north-eastern flank of the anticline, to the north of Neves and Corvo orbodies. It is located at a depth of 400m at its western end and extends down to depths of 1,200m below surface. It dips to the northeast at approximately 35° but steepens at depth and has a shallow plunge to the northwest. The sulphide lens has dimensions of up to 70m in thickness and extends for approximately 1,400m down dip and at least 1,600m along strike and is open to the north. Lombador is connected by a bridge of mineralisation to the massive sulphide lenses of Corvo and Neves.
Monte Branco
The Monte Branco orebody was discovered in 2011 from surface exploration drilling. The deposit is located approximately 1.2km south of Semblana and to the west of the Cerro do Lobo tailings storage facility. The deposit consists of six discontinuous lenses that have been strongly affected by tectonic shearing. Monte Branco covers approximately 250m by 200m in area and is found at depths of between 540m and 700m below surface. The deposit contains copper sulphide mineralisation and includes both massive and stockwork types.
Semblana
The Semblana orebody is almost flat and has a gentle dip (15-20°) to the north and is located at a depth of 790m below surface. Drilling from surface has intersected mainly copper stockwork (FC), although several small zones of massive copper (MC) in lenses have also been identified. The hangingwall stratigraphy is identical to that at Corvo and Zambujal, but rhyolites are seen exclusively in the footwall at Semblana, with widths varying from a few metres to tens of metres. Mineralisation in the rhyolites is occasionally observed but is not considered economic.