The Bajo de la Alumbrera porphyry Cu-Au deposit, Argentina, is in the eastern Andes, near the north edge of a region of reverse fault-bound basement uplifts that overlie a low-angle segment of the subduction zone.

The deposit is centered on a closely spaced cluster of small felsic porphyry stocks and dikes, emplaced into andesites during seven phases of intrusion. Dikes of several phases define a radial pattern. Most of the porphyries are very similar to one another, with phenocrysts of plagioclase, hornblende, biotite, and quartz, in a matrix of fine grained quartz, K-feldspar, and minor plagioclase, biotite, and magnetite. Individual porphyries are distinguished mainly on the basis of intrusive contact relationships.

Highest Cu-Au grades are associated with abundant quartz veins, secondary K-feldspar, ±magnetite, ±biotite, ±anhydrite, in the earliest porphyry (P2), and adjacent andesite. P2-related mineralization is truncated by porphyries of the second phase of ore-related intrusions (Early P3 and Quartz-eye porphyry), which contain similar but generally less intense mineralization and alteration. Porphyries of the next phase (Late P3) truncate mineralization associated with earlier phases and are weakly mineralized with Cu-Au, sparse quartz veins, and secondary biotite. The still later Northwest porphyries truncate most Cu-Au, quartz veins, and potassic alteration, and themselves contain only traces of such mineralization and partially biotitized hornblende. Postmineral porphyries, the youngest, truncate all such mineralization and alteration, and none of their hornblende is biotitized. Los Amarillos porphyry and igneous breccia, along the western periphery of the porphyry cluster, is between P2 and Early P3 in age but shows little relationship to mineralization.

A low-grade core zone consists in large part of barren K-feldspar-magnetite alteration and quartz veins in Early P3 porphyry, and in part consists of later barren porphyry, so is mostly younger than the Cu-Au deposited with P2 porphyry.

The 600-hectare lease encompasses all mineralized areas of the deposit. Immediate mine infrastructure and other mine facilities cover an additional permitted surface area of 5,200 hectares.

Standard truck and shovel mining techniques are employed in the open pit mine. Mining is carried out on 17-metre benches and two-metre sub-drills, and ore is hauled to processing facilities nearby.

The processing plant produces three products: a copper flotation concentrate containing the major gold credit, Dore bullion from gravity recovery of coarser free gold, and molybdenum concentrates.

Molybdenum concentrates are trucked to Chile for processing. Copper and gold concentrates are pumped through a pipeline over the Nevados del Aconquija mountains to the province of Tucuman. The concentrates are transported by rail to Alumbrera’s port facility in Puerto General San Martin, located near Rosario in the province of Santa Fe. The concentrate is then shipped overseas for refining.