The Rio Blanco porphyry copper deposit lies in the lower Andes of far northern Peru, close to the border with Ecuador, approximately 800 km north of Lima, 300 km NNW of the Antamina mine and 150 km east of the Pacific coast.
Rio Blanco lies at the northern extremity of the Cajamarca mineral belt in the Western Cordillera of the northern Peruvian Andes, a generally north-south trending belt of Oligocene to Miocene porphyry copper deposits that extends for 350 km from Cajamarca in the south to the Ecuadorian border and includes two geochemically distinct groups of deposits along this trend namely: i). porphyry Cu-Mo deposits which include Rio Blanco (1257 Mt @ 0.57% Cu, 0.0228% Mo), La Granja (3.5 Gt @ 0.55% Cu), Michiquillay (630 Mt @ 0.69% Cu, 0.02% Mo, 0.15 g/t Au), El Galeno (430 Mt @ 0.6% Cu, 0.015% Mo, 0.2 g/t Au) and Cañariaco; and ii). porphyry Cu-Au deposits which include Cerro Corona (91 Mt @ 0.53% Cu, 1.0 g/t Au), Minas Conga (190 Mt @ 0.3% Cu, 0.8 g/t Au) and La Carpa. These systems are mostly associated with dacite to monzonite to diorite intrusions, which intrude sediments and volcanic rocks of Mesozoic to Early Tertiary age.
The geology of the Rio Blanco district is dominated by the granodioritic Portocello batholith, one of a series of Tertiary batholiths that characterise southern Ecuador and northern Peru, intruding a sequence of siliceous Palaeozoic metasediments, composed principally of phyllites, with lesser quartzites and gneisses. The Rio Blanco Porphyry Complex has convoluted, ill-defined, and brecciated contacts with the Portocello Batholith, and is in faulted contact with the phyllites.
The multi-phase Rio Blanco Porphyry Complex has a quartz porphyry core (exposed in Quebrada Majaz), which intruded an earlier feldspar porphyry complex. Igneous breccia is extensively developed within the porphyry complex, and appears to have formed around the margins of the quartz porphyry during intrusion. This breccia constitutes an important locus for the higher grade copper mineralisation in the Henry's Hill segment of the deposit. To the north of the quartz porphyry core at Quebrada Majaz, an extensive area of phreato-magmatic breccia is poorly exposed on steep, almost inaccessible cliffs which have formed where the breccia was silicified, and are almost featureless.
The mineralisation at Rio Blanco is surrounded by a relatively low pyrite, but otherwise intense phyllic alteration zone over an area of some 5 sq. km which overprints a potassic zone. The phyllic alteration is centred on the Rio Blanco Porphyry Complex and only extends for around 100 m into the granodiorite of the batholith, while to the north it extends into the country rock phyllites.
The phyllic zone has been overprinted by a widespread argillic phase, although only around 5 to 15% of the rock by volume is represented by clays. The diagnostic argillic minerals are kaolinite, alunite (not abundant) and pale epithermal rutile, with rare andalusite and some enargite.
Hydrothermal alteration and geochemical sampling indicates that the mineralised system covers an area of up to 25 sq. km.
The Rio Blanco deposit has a well-developed supergene blanket in which the dominant mineral is covellite, overlain by a dominantly goethitic leached cap. The supergene blanket is developed in an area of strong relief, with the better accumulations below a steep ridge at the Henry's Hill sector where the best intersections are 50 to 150 m below the crest of the ridge.
The oxidation profile from the surface downwards may be summarised as follows:
i). Leached Cap - generally containing <0.2% S, which is typically 50 to 150 m thick, but varies from 10 to 240 m, and is represented by almost complete oxidation, dominantly goethitic.
ii). Transition Zone - which is typically a few to a few tens of metres in thickness, and is irregular in shape and interfingers with both the leached cap and supergene sulphide zone. It contains oxides, mainly goethite but also hematite, and supergene covellite, chalcocite and digenite, with little or no malachite, azurite, cuprite or neotocite. Average copper grades are generally less than those of the corresponding hypogene ore.
iii). Supergene Zone - which varies from a few to as much as 240 m in thickness, with the best developments following the trace of the crest in the Henry's Hill segment of the deposit. Perched supergene sulphides indicate an earlier extensive blanket which has been destructively dissected. The dominant supergene sulphides are covellite (typically with relict chalcopyrite cores), chalcocite and digenite.
iv). Mixed Zone - which is usually a few metres thick, but locally may be as much as a few tens of metres. It contains chalcopyrite with covellite, chalcocite, digenite and rare bornite.
v). Hypogene Zone - which is characterised by chalcopyrite with minor amounts of covellite and bornite.
Within the secondary enrichment zone the ore is characterised by chalcocite and covellite, occurring as disseminations, in veinlets and as fracture fillings, forming halos and sub-halos around chalcopyrite crystals and as patinas coating pyrite crystals. The ratio of chalcocite:covellite is around 1.2:1. Pyrite is abundant in the supergene blanket, while molybdenum grades are similar to those of the underlying hypogene ore.
Published resources and reserves at a 0.4% Cu cut-off are (Monterrico Metals plc, website 2006):
Measured Resource - 146 Mt @ 0.73% Cu, 0.0235% Mo,
Indicated Resource - 670 Mt @ 0.56% Cu, 0.0234% Mo,
Inferred Resource - 441 Mt @ 0.52% Cu, 0.0216% Mo,
Total Resource - 1257 Mt @ 0.57% Cu, 0.0228% Mo,
Total Supergene Resource - 358 Mt @ 0.69% Cu, 0.0134% Mo
Total Hypogene Resource - 899 Mt @ 0.52% Cu, 0.0265% Mo
The resource includes:
Proven Reserve - 133 Mt @ 0.74% Cu, 0.0232% Mo,
Probable Reserve - 365 Mt @ 0.59% Cu, 0.0210,
Total Reserve - 498 Mt @ 0.63% Cu, 0.0216% Mo
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2007)