Summary:
The Cuajone deposit is considered to be an example of a porphyry copper–molybdenum deposit.
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. The Cuajone porphyry deposit exhibits a zoned alteration pattern that includes potassic, propylitic, sericitic and intermediate argillic hydrothermal alteration styles.
Hypogene mineralization represents >98% of the remaining mineralization within the Cuajone open pit. The mineralogy is typically simple and consists of pyrite, chalcopyrite, and bornite, with sparse sphalerite, galena, and enargite.
The first, pre-mineral intrusive in the mine area, situated approximately 1–2 km to the west of the deposit, was a north–south-elongated, 0.7 x 0.35 km, grey to grey-green holocrystalline to equigranular, medium grained, porphyritic diorite stock. This was followed by emplacement of three latite porphyry stages, producing a 2.5 x 0.7 km, northwest–southeast-elongated intrusive body. The latite multiphase intrusion hosts the mineralization.
The first magmatic pulse of the latite porphyry was concentrated in the southeastern part of the multiple intrusive mass and was responsible for the introduction of the bulk of the hypogene copper and molybdenite mineralization in the Cuajone orebody and the associated intense alteration of both the latite and surrounding Toquepala Group andesites and rhyolites. The intrusion is a porphyry with phenocrysts of quartz to 4 mm in diameter and laths of feldspar in a cryptocrystalline matrix.
The second intrusive phase formed two bodies, a larger, ovoid 850 x 550 m mass immediately to the northwest of the first pulse, while a smaller 300 x 200 m plug occurs within the outcrop of the first pulse. Both exposures have only weak associated alteration and very minor, low-level copper and molybdenum mineralization. Breccia bodies were developed along the intrusive contacts with the other latite pulses and country rocks.
The third magmatic pulse covers a surface area of around 800 m in diameter immediately to the northwest of the main primary latite porphyry outcrop and has only weak associated alteration and no copper or molybdenum mineralization.
At a late stage in the emplacement of the latite porphyry complex, and during an initial erosive period, the interaction of downward-percolating meteoric waters with the rising hypogene hydrothermal fluids produced an intense phyllic silica–sericite–pyrite zone that was superimposed on the upper parts of the mineralized system associated with the first latite porphyry pulse to develop a higher grade zone of copper–molybdenum ore with grades of >0.4% Cu as chalcopyrite and molybdenite. This alteration and mineralization style is principally developed within the Latite Porphyry and the Toquepala Group rhyolites, and only to a minor degree in the underlying andesites.
The Huaylillas Formation was deposited after the mineralizing event ended. It consists of conglomerate, tuff, vitrophyre, and trachyte, and is as thick as 230 m.
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.
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:
• Basaltic andesite: 51%
• Latite porphyry: 47%
• Toquepala Group rhyolite: 1%
• Mineralized breccias: 1%
Hypogene mineralization represents >98% of the remaining mineralization within the Cuajone open pit.