Marcona and Pampa de Pongo: Giant Mesozoic Fe-(Cu, Au) Deposits in the Peruvian Coastal Belt

Located approximately 400 km south of Lima, Peru, the Marcona and Pampa de Pongo deposits are the largest iron accumulations with associated copper and gold along the western coast of South America. Approximate resources include more than 1400 Mt of iron ore at Marcona and 1000 Mt of magnetite mineralisation at Pampa de Pongo. Both deposits contain some copper and gold and exhibit numerous features that allow their inclusion in the "Iron Oxide-Copper-Gold" clan of deposits, alongside such examples asCandelaria and Mantos Blancos in Chile. The two deposits form part of a cluster of similar occurrences that together define the "Marcona Fe-Cu District".

The Marcona iron deposits were first identified in 1915 and mining commenced in 1953, while artesenal copper mining had been carried out in the district from the late 19th century. The larger iron bodies are hosted by the Lower Paleozoic Marcona and Middle to Upper Jurassic Rio Grande Formations. The Marcona Formation is dominated by arenites and both calcific and dolomitic carbonates, whereas the Rio Grande Formation comprises a thick sequence of basaltic andesites and andesites (sills and flows), volcaniclastics and minor limestones. Although including major carbonate replacement fades, the iron deposits widely exhibit previously undocumented, intra-mineralisation hydrothermal breccia textures and multistage iron oxide ± sulphide/copper mineralisation. Copper mineralisation is mainly associated with magnetite and lesser specularite. The iron oxide bodies strike northeast and north-northwest and show both fault and lithological controls on ore geometry. Intra- and post-iron mineralisation igneous activity in the Marcona Mine area included dacitic/granodioritic dykes and andesitic "ocoite" dykes.

The hitherto undocumented Pampa de Pongo Fe (-Cu-Au) deposit, covered by at least 20 m of sand, was discovered in 1994 by drilling a large magnetic anomaly 30 km southeast of Marcona. Host rocks to the mineralisation are dolostones and andesitic volcanics of the Oxfordian-Tithonian Jahuay Formation, which are higher in the sequence than the Marcona iron deposits. The iron mineralisation exhibits both replacement and breccia-fill facies within a steeply northwest-dipping fault corridor. Magnetite mineralisation in the andesites is associated with Fe-chlorite, talc and clinochrysotile, whereas replacement of the underlying dolostone includes magnetite-amphibole-serpentine associations.

The age of the Pampa de Pongo deposit is uncertain, although the preferred genetic model for the district involves a large metal flux coeval with deep-seated Jurassic, and probably Cretaceous igneous intrusive activity. This was triggered by the introduction of mantle-derived melt along the root zone of the extensional faults within an active continental arc. At a local scale, iron oxide-associated mineralisation at Marcona, Pampa de Pongo and the surrounding district probably fonned in an environment characterised by repeated crustal extension over a +20-60 my period. The anomalous concentration of thick andesitic volcanics or sills and dykes at Marcona, and evidence for district-scale thermal anomalies preceding and during the main introduction of iron oxide mineralisation, indicate that the area was also an important volcanic centre and the site of a long-lived thennal anomaly.