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 h ........
