Summary:
The Cuajone deposit is considered to be an example of a porphyry copper–molybdenum deposit.
The Cuajone deposit is part of the Eocene porphyry copper belt of the main arc of the Peruvian Andes. The regional geology consists of the Upper Cretaceous/Lower Tertiary Toquepala Group, a sequence of basal volcanic flows and volcano–sedimentary rocks overlain by Miocene to Recent volcanic and volcano sedimentary rocks.
The deposit is approximately 2,300 m long, 900 m wide, and averages 1,000 m in thickness. Mineralization has been drill tested to a depth of 2,255 m. The deposit remains open at depth.
Mineralization and alteration at the Cuajone deposit is directly related to a multi-stage latite porphyry that intrudes basaltic andesites and the overlying 370 m of rhyolite porphyries of the Toquepala Group.
Supergene Mineralization
The 900 m wide hypogene ore zone was overlain by a secondary enrichment blanket that was about 20 m thick and averaged more than 0.75% Cu (Herrera, 2021).
The main chalcocite layer was overlain by 15–40 m of partially-oxidized upper zone averaging 0.60% Cu, where remnant chalcocite was apparent, but malachite and chrysocolla dominated. These were in turn overlain by a partially-preserved (maximum of 120 m thick) hematite-bearing leached cap that graded 0.01–0.12% Cu. Argillic alteration associated with the supergene ores included kaolinite, montmorillonite, illite and dickite.
Hypogene Mineralization
Hypogene mineralization is distributed as follows:
x Basaltic andesite ......................................... 51%
x Latite porphyry ............................................ 47%
x Toquepala Group rhyolite .......................... 1%
x Mineralized breccias ..................................... 1%
The mineralogy is typically simple and consists of pyrite, chalcopyrite, and bornite, with sparse sphalerite, galena, and enargite. Hypogene mineralization represents >98% of the remaining mineralization within the Cuajone open pit.
The major sedimentary and intrusive rock types in the general Project area, as well as the breccia types in the deposit area, are described below. A stratigraphic column is also available.
Intrusive Lithology and Mineralization Description
Latite Porphyry 3 – Age: 53 Ma
Monzogranite to granodiorite stock and dykes with bipyramidal quartz phenocrystals without Cu-Mo mineralization. It is weakly altered with a predominance of sericite and to a lesser extent, clays; weakly disseminated pyrite and sporadic veinlets.
Latite Porphyry 2 – Age: 56 Ma
Coarse-grained with hornblende phenocrysts and/or plagioclase to 1 cm and very low density of granular quartz veins. It is considered to be an intra-mineral intrusion. Weak to moderate argillic alteration and sericitization superimposed on earlier potassic alteration characterized by granular silica veins with K-feldspar halos.
Latite Porphyry 1 – Age: 55–51 Ma
The stock is elongate northwest to southeast. Cu and Mo mineralization are related spatially and temporally to this stock. It is characterized by a medium to coarse grain porphyritic texture, phenocrysts of plagioclase, hornblende, biotite, and quartz, with moderate to high density of granular quartz veins. Sulfides are mainly disseminated and in quartz veins. The ratio pyrite to chalcopyrite varies depending on the location within the system.
Granodiorite – Age: 65–58 Ma
This pluton extends to the west and northwest of the porphyritic stocks, cutting lava sequences of andesite and rhyolite (Toquepala Group) and is partially covered by pyroclastic deposits of the Huayillas Formation. Hydrothermal granodiorite breccias developed in the Cuellar sector are weakly mineralized in intra-clastic cavities, showing weak to no rotation of clasts.
Diorite – Age: 66 Ma
Crops out in an elongate north–south trend east of the current pit. Cuts most of the units of the Toquepala Group.
Breccia Type
Rupture breccia
Synonymous with “stockwork”, “shatter breccia”, “fracture breccia” and “crackle breccia” and is characterized by a multitude of randomly crisscrossing cracks, the same ones that when crossing and joining each other, divide the original rock into angular fragments caused by hydraulic fracturing. The most distinctive characteristic of the rupture breccia is that its individual fragments do not detach, displace, slide or rotate among themselves.
Hydrothermal breccia
Predominantly angular clasts, arranged chaotically in a matrix of strongly altered porphyritic latite and mineral sulfides.
Magmatic–hydrothermal breccias
Ore breccia, siliceous ore breccia, siliceous breccia, blind breccia in LP2 and cubes breccia. Form elongated sub-vertical chimneys with diameters that vary from 58 to 244 m. They are characterized by the rotation and/or transport of their angular to sub-rounded clasts in a matrix of granular quartz and sulfides. The breccia is typically inter-mineral and Cu-Mo mineralization is most common within the breccia itself. A molybdenite-bearing breccia is characterized by the fact that the upper part of the chimney has tabular fragments of latite porphyry 2, aligned parallel to the cupola, defining a “shingle breccia” with quartz-molybdenite cement. Its formation is attributed to exfoliation of the wall rock and its fall towards the interior of the magmatic chamber. Intra-clastic cavities contain calcite and ankerite due to the circulation of fluids with high calcium content and contain high-grade copper mineralization due to its intrinsic permeability.