The geology and mineralisation of the world-class Alpala porphyry copper-gold deposit in northern Ecuador.
Dr Steve Garwin
The recently discovered Eocene Alpala porphyry copper-gold deposit is located in the under-explored northern section of the Andean Copper Belt. The maiden mineral resource estimate (January 2018) includes 1.08 billion tonnes at 0.68% Cu equivalent, using a cut-off of 0.3% Cu equivalent, for 5.2 million tonnes of copper and 12.3 million ounces of gold. This resource includes a high-grade core that contains > 120 million tonnes at 1.82% Cu equivalent (1.2 Mt Cu and 4.7 Moz Au; cut-off of 1.1% Cu equivalent). Recent drilling has added significantly to the tonnage and Cu-Au grades of the core to the deposit.
The recognition of geochemical zoning has assisted in drill-targeting within the deposit and tenement-wide exploration. This zoning is characterized by central Cu-Au; proximal Mo; proximal to distal Bi, Se and Te; and distal As, Mn and Zn. Within the Alpala deposit, variations of Au/Cu in drill-hole assist in the delineation of different intrusion stages.
The applications of the Anaconda method to geological mapping and drill-core logging have facilitated the identification of more than six major intrusion stages, a diagnostic vein sequence and sulphide-oxide mineral paragenesis that allows for the prediction of copper-gold grades. The most important indicators of high-grade include the presence of the early-stage causal intrusion(s), elevated porphyry-style quartz vein abundance and an increased ratio of chalcopyrite to pyrite.
Radiometric U-Pb SHRIMP dates on zircons return 39.4 + 0.6 Ma (2 σ) for an early-mineralisation quartz diorite intrusion and 38.7 + 0.6 Ma (2 σ) for a late-mineralisation quartz diorite dike. A Re-Os date of molybdenite in a late-stage, sulphide-bearing anhydrite-quartz vein associated with quartz-sericite-pyrite alteration indicates 38.6 + 0.2 Ma (2 σ). The ages of the quartz diorite intrusions and late-stage molybdenite are consistent with a time-span of 800 + 800 Ka (2 σ) for deposit development.