A product of colliding continents
The Bajo de la Alumbrera copper-gold deposit results from immense geological forces. It sits within the Farallón Negro Volcanic Complex, the eroded remains of what may have been a stratovolcano active around 9 to 8 million years ago during the Miocene epoch. This volcanic activity was fueled by the Nazca oceanic plate subducting, or sliding, beneath the South American continental plate. The specific shallow angle of subduction at that time created conditions that allowed magma to form and ascend into the continental crust.
Multiple pulses of dacitic magma, containing in water and metals, intruded to within a few kilometers of the surface. As this magma cooled and crystallized, it could no longer hold its dissolved water and volatiles. A separate, superheated hydrothermal fluid phase exsolved from the melt. This metal-rich fluid, under intense pressure, fractured the surrounding rock, forming a dense network of tiny veins called a stockwork. Within these fractures, minerals precipitated out of the fluid, depositing chalcopyrite (a copper-iron sulfide), native gold, and pyrite. The process formed a vast, low-grade but economically significant orebody, with total resources estimated at over 600 million metric tons of ore averaging 0.54% copper and 0.64 grams per ton of gold.
Anatomy of a porphyry deposit
The name "porphyry deposit" comes from the texture of the igneous rocks involved. They are porphyritic, meaning they contain large, well-formed crystals (phenocrysts) set within a fine-grained groundmass, a texture indicating a two-stage cooling history. The process of mineralization also forms distinct alteration zones, like concentric rings around the intrusion. Geologists can read these zones to understand the deposit's history and locate the highest concentrations of ore. The core of Alumbrera has a potassic alteration zone, characterized by minerals like biotite and potassium feldspar, which corresponds to the highest copper and gold grades. Moving outwards, this gives way to phyllic (quartz and sericite) and propylitic (chlorite and epidote) alteration zones.
The entire mineralized system at Alumbrera originally formed a sub-circular body roughly 800 meters in diameter with a vertical extent of up to 1,000 meters. Subsequent faulting has displaced sections of the orebody. The deposit itself lies in a natural topographic bowl, which formed because the hydrothermally altered and fractured rock is softer and erodes more easily than the surrounding unaltered volcanic rocks. The open-pit mine, which began commercial operation in 1998, sits at an altitude of 2,600 meters.