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Rio Blanco Project |
Rio Blanco Resources |
Geology |
History
 
Rio Blanco - Geology
The Rio Blanco Porphyry Copper Project is located close to the frontier with Ecuador; it represents the northern extremity of the copper-gold belt of the northern Peruvian Sierras. This Tertiary porphyry/epithermal metallogenic belt, which extends for over 1,000 km, hosts a number of world class deposits including: Yanacocha, Santa Rosa, Alto Chicama, Cerro Corona, Peirina, Sipan, La Granja, Michiquillay, and Carpa. As is common in the circum-Pacific, the epithermal gold deposits are hosted by Tertiary continental arc-volcanics, while the copper and copper-gold porphyry systems are characterised by small porphyritic intrusions.
Regional Geology
Two principal rock units occur in the region, the Portocello batholith, and siliceous Palaeozoic metasediments. The Portocello batholith is of granodioritic composition and is part of a sequence of Tertiary batholiths that characterise southern Ecuador and northern Peru. The batholith intrudes the tectonised Palaeozoic metasediments. These comprise principally phyllites, but also quartzites and gneisses.
Into these units, a porphyry complex, the Rio Blanco Porphyry System, was intruded. Its contacts with the Portocello Batholith are convoluted, ill-defined, and brecciated. The contact with the phyllites is a faulted contact.
Project Geology
The Rio Blanco porphyry system is a multi-phased intrusive complex. The porphyry complex has a quartz porphyry core (exposed in Quebrada Majaz), which intruded an earlier feldspar porphyry complex.
An extensive development of igneous breccia is present in the porphyry complex; it appears to have formed around the margins of the quartz porphyry, when it intruded. This breccia forms an important locus for the better grade copper mineralisation at Henry's Hill. To the north of Quebrada Majaz, there is an extensive area of phreato-magmatic breccia. This outcrops poorly, forming steep, almost inaccessible cliffs; these have formed where the breccia was silicified, and are almost featureless.
The dominant alteration at Rio Blanco is a huge phyllic zone, some 5 km2 in area, with evidence of an overprinted potassic zone. The phyllic alteration at Rio Blanco is not typical of porphyry systems, and is intense. The pyrite content is low for a phyllic zone.
There is evidence for the existence of a widespread advanced argillic overprint at Rio Blanco. But the extent of the clay development is not great, typically in the range 5-15% of the rock by volume. The diagnostic minerals are kaolinite, alunite (although not abundant), and pale epithermal rutile. Rare andalusite is present and energite has been observed.
The phyllic alteration at Rio Blanco extends only a limited distance into the granodiorite batholith (seldom more than 100m), giving way to weakly developed propylitic alteration. To the north of Quebrada Majaz, the phyllic zone extends into the phyllites.
Known limits of hydrothermal alteration and elevated copper values found in stream silt sediments and rock chip samples from bedrock indicate that the mineralised system may have a large areal extent of 25 km2.
Oxidation Profile
The oxidation profile is of paramount importance in most porphyry systems. At Rio Blanco, the oxidation profile is exceptional in several ways. Firstly, Rio Blanco displays one of the few well-developed supergene copper blankets in Northern Peru and Ecuador. Secondly, the dominant supergene copper minerals in porphyry systems worldwide are chalcocite, digenite and covellite, in that order of abundance. The dominant mineral at Rio Blanco is covellite, and supergene blankets that consist dominantly of covellite are uncommon. Thirdly, the leached cap at Rio Blanco is dominantly goethitic. But the most striking anomaly at Rio Blanco is the extremely steep topography, and the obvious correspondence of the better supergene copper intersections with the ridge crest of Henry's Hill (they occur 50 to 150 meters below the ridge crest).
From the surface, the oxidation profile at Rio Blanco is as follows:
- The Leached Cap is typically from 50 to 150 metres thick, although it varies from 12 to 240 metres in thickness. Oxidation is almost complete (dominantly goethite). The Leached Cap typically contains < 0.2 % total Sulphur.
- The Transition Zone is of limited extent. It is typically only a few metres to a few tens of metres thick, but because it is interdigitate with the Oxide Zone and the Supergene Zone its shape is irregular. By definition, it contains both oxides (dominantly goethite), and supergene copper minerals (covellite, chalcocite, digenite). But it contains little or no malachite, azurite, cuprite, or neotocite. Essentially, 'oxide' copper minerals are absent (as they are in most porphyry copper deposits in northern Peru and Ecuador).
- The Supergene Zone varies from a few metres to 240 metres in thickness. The most striking control of the supergene mineralisation is topographical. The better intersections are aligned with the ridge-crest of Henry's Hill; not only that they correspond well with the wider parts of it. The existence of perched supergene sulphides in RB-21 indicates that an originally much more extensive blanket has been destructively dissected.
The dominant minerals in the Supergene Zone are covellite (typically the covellite grains have relict cores of chalcopyrite), chalcocite and digenite. The reported solubility is high (averaging 88%).
- The Mixed Zone is usually a few metres thick, but in places it is tens of metres thick. By definition is contains chalcopyrite together with covellite, chalcocite and digenite (and rare bornite).
- The Hypogene Zone contains chalcopyrite, with minor quantities of other copper minerals (principally covellite and bornite).
Mineralogy
Mineralisation in the zone of secondary enrichment is characterised by chalcocite and covellite, which are found as disseminations, in veinlets and as fracture fillings, forming halos and sub-halos around chalcopyrite crystals and as patinas coating pyrite crystals. It has been estimated that the ratio of chalcocite to covellite is approximately 1.2. Pyrite is abundant and molybdenum grades in this zone are similar to those of the underlying primary ore.
In the transitional zone the blanket is partially destroyed. In this transition zone, limonite (including haematite) coexists with chalcocite, covellite and pyrite. Average copper grades are generally less than those of the corresponding primary ore. In the case of the mixed zone, a generally immature mixed mineral zone is formed, composed of pyrite, chalcopyrite and covellite/chalcocite.
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