Summary:
Deposit Types
During 2006 and 2010 geologists envisaged the emplacement of the Aguablanca mineral deposits through multiple injections of over pressured magma mainly controlled by the gradual opening of tensional fractures. According to this model, overpressure would have been the responsible of brecciation and transport upward of the dense sulfide- and fragment-charged silicate magmas from a deep level to a shallower site through low-density crustal rocks. The unmineralized igneous rocks of the Aguablanca stock (mostly, gabbrodiorites) would represent early injections of fractionated silicate melt from the uppermost parts of the deep magma chamber, whereas semi-massive mineralization would represent the latest injections containing a mixture of sulfide melt, partially consolidate mafic-ultramafic fragments and remaining silicate melt. The emplacement model envisaged is described as following. The first stage is the injection of sulfide-free differentiated silicate melts carrying minor mafic-ultramafic fragments. These melts would have flowed toward the south, taking on the inverted drop geometry of the Aguablanca stock and would have evolved by fractional crystallization giving rise to sulfide-free gabbronorite, norite and gabbrodiorite. Later, a new injection of silicate magma coming from the same magmatic source but containing droplets of unfractionated sulfide melt would have generated the disseminated mineral-bearing gabbronorites and norites, likely accreting inward from the sidewalls of the Aguablanca rocks. Geologists suggested that the barren igneous rocks and the disseminated mineral-bearing rocks may represent the product of turbulent and variable mixing between fragments and sulfide and silicate melts. Finally, the emplacement of the semi-massive mineral deposit would have taken place in the latest stages of emplacement of the intrusion. In this last injection, the sulfide-rich silicate melt charged with mafic-ultramafic fragments from deep, partially consolidated rocks, would have given rise to the semi-massive mineralization in the inner parts of the mineralized breccia.
Mineralization
The matrix to the breccias takes two main forms: (a) semi-massive mineralization, and (b) disseminated mineralization. In addition, chalcopyrite veinlets (chalcopyrite-veined ore) occur crosscutting both fragments and mineralized matrix.
Semi-Massive Mineralization
Semi-massive mineralization preferentially occupies the inner zones of the breccia, which are surrounded by disseminated mineralization. Texturally, semi-massive mineralization is characterized by euhedral to subhedral (up to 0.5 cm across) silicates enclosed by an assemblage of sulfides composed of pyrrhotite and pentlandite with minor amounts of chalcopyrite and pyrite. The modal proportion of sulfides typically ranges from 45 to 70% but locally reaches modal proportions as high as 85%. Silicates comprise variable proportions of orthopyroxene (<48%) , plagioclase (<30%), clinopyroxene (<27%) and minor amounts of hornblende (<9%), phlogopite (<3%) and olivine (<2%). This primary silicate assemblage is variably replaced to sericite, serpentine, bastite, talc, actinolite, chlorite, calcite and clay minerals.
Orthopyroxene occurs in form of euhedral crystals (0.3-4 mm, typically < 3 mm) variably altered to bastite along fractures and cleavage planes, talc, actinolite and calcite. In places, it shows thin clinopyroxene exsolution lamellae along (100) planes. Clinopyroxene forms subhedral grains (0.5-5 mm, typically < 3.5 mm) commonly replaced by actinolite and phlogopite. Some clinopyroxene grains show complex oscillatory zoning with respect to the Cr content, consisting of Cr-poor cores, concentric Crrich zones, and Cr-poor outermost rims. Some clinopyroxene crystals contain orthopyroxene inclusions, suggesting that orthopyroxene crystallized early, and small subrounded sulfide inclusions pointing the existence of an immiscible sulfide liquid before and/or during its crystallization. Tabular plagioclase crystals are irregularly altered to sericite and, at a lesser extent, chlorite.
Economic Minerals
The mineral assemblage is typical of magmatic Ni-Cu-Fe sulfide mineralization, consisting of pyrrhotite (Fe1-xS), pentlandite [(Fe, Ni)9S8], chalcopyrite (FeCuS2) and minor pyrite (FeS2). Magnetite (Fe3O4), minor amounts of violarite (FeNi2S4) and marcasite (FeS2) replacing pentlandite and pyrrhotite, respectively, and a complex assemblage of platinum-group minerals (PGM), mostly Pd-Pt-Ni bismutho-tellurides, complete the assemblage. Pyrrhotite is, by far, the most abundant sulfide in the semi-massive ore, occurring as large (1-2 mm) anhedral twinned crystals in proportions higher than 55 modal %. Pentlandite (10-35 modal %) occurs as polycrystalline, chain-like aggregates surrounding pyrrhotite, formed by relative high-temperature grain boundary exsolution, and as small exsolution flames along grain boundaries and fractures within pyrrhotite. Chalcopyrite (<10 modal %) forms anhedral grains of variable size randomly distributed between pyrrhotite and pentlandite. Magnetite (up to 2 modal %) occurs as individual euhedral crystals within sulfides (mostly, pyrrhotite) and in the contact between sulfides and silicates. The euhedral morphology of magnetite suggests early crystallization.
Disseminated Mineralization
In the disseminated mineralization, sulfides comprising less than 20 vol.% of the rock occur interstitially between primary silicate minerals with sharp and well-defined contacts. Gabbronorite is, by far, the most common lithology hosting disseminated ore, with norite, gabbro and amphibole-rich pyroxenite as other minor igneous rocks. These rocks show cumulate igneous textures formed by variable proportions of cumulus crystals of orthopyroxene (19-56 vol.%), clinopyroxene (<22 vol.%) and plagioclase (<52 vol.%), and intercumulus amphibole (<46 vol.%), phlogopite (<14 vol.%) and minor quartz. Actinolite, sericite, epidote, chlorite, carbonates, talc, serpentine and clay minerals are common secondary phases. Despite locally intense alteration, magmatic textures are usually well preserved and primary silicates are recognized from their morphologies. Chemically, pyroxene and plagioclase of mineralized gabbronorite are quite similar to these silicates from the unmineralized Aguablanca intrusion as is mainly indicated by the similarity in the Mg and An numbers, respectively. However, the compositions of pyroxene and plagioclase are slightly more primitive in semi-massive mineralization relative to disseminated mineral-bearing gabbronorite.
Economic Minerals
Sulfides have typical interstitial disseminated textures in the form of variably sized (from few millimetres to 1-2 cm) polymetallic aggregates between silicates with well-defined boundaries. The sulfide assemblage is quite similar to that of the semimassive ore although there are differences in the modal proportions. The most important is that the disseminated ore contains greater proportions of chalcopyrite than the semi-massive ore.
Chalcopyrite Veins
Chalcopyrite veins represent a very minor mineral-type in Aguablanca, comprising less than 5% of the mineralization present in the deposit. Chalcopyrite veins occur crosscutting both semi-massive and disseminated matrix and mafic-ultramafic fragments. Widths range from a few millimetres to about 2 cm and they extend up to several centimetres in length. They are made up of massive chalcopyrite, minor amounts of pyrrhotite and pentlandite, and traces of argentopentlandite located within chalcopyrite. Pentlandite has more Co (>2 wt%) and less Fe (<27.3 wt%) and pyrrhotite more Ni and Co (~1 and 0.1 wt%, respectively) than in semi-massive and disseminated ores.