Summary:
The Marcona district of littoral south-central Perú represents the largest concentration of iron oxide-copper-gold deposits in the Central Andes. Hydrothermal activity occurred episodically from 177 to 95 Ma and was controlled by NE-striking faults.
At Marcona, emplacement of massive magnetite orebodies with subordinate, overprinted magnetite-sulphide assemblages coincided with a 156-162 Ma episode of eruption of andesitic magma in the Jurassic arc, but mineralization is hosted largely by underlying, Lower Paleozoic metaclastic rocks.
The magnetite orebodies exhibit smoothly curving, abrupt contacts, dike-like to tubular apophyses and intricate, amoeboid interfingering with dacite porphyry intrusions, interpreted as evidence for the commingling of hydrous Fe oxidic and silicic melts.
The Marcona mine comprises eight open pits in a ~ 25 km2 area elongated from WNW to ESE. Traditionally, a crudely en echelon array of 12 major magnetite orebodies (“minas”) and 55 smaller “cuerpos” are recognized. However, the three zones exploited by the largest, 3 km-long pit, viz. Mina 2, Mina 3 and Mina 4, represent interconnected segments of a single orebody. Approximately 60 per cent of the reserve, comprising the so-called “E-grid” orebodies, is hosted by the Marcona Formation, and the remainder, the N-13 type orebodies, by the lower members of the Río Grande Formation. The hypogene grades of the larger orebodies hosted by the Paleozoic metasediments average 57-58 per cent Fe, significantly exceeding those of 41-48 per cent for the orebodies in Jurassic strata. Whereas the total sulphur content of the mineralization is consistent at ~ 3 wt. percent, the copper content is more variable, averaging 0.06-0.18 percent, but attaining 0.4 percent in Mina 1 and 0.9 percent in the upper part of the easternmost, Mina 11, orebody.
Mineralization
Seven stages of alteration and mineralization are identified at Marcona. The earliest hydrothermal alteration (Stage M-I) comprises characterized by aggregates of 175-177 Ma old cummingtonite and 171 Ma phlogopite (-magnetite), which locally occur in the upper section of the Marcona Formation. Stage M-II Na metasomatism occurred throughout the mine area and is represented by albite and minor Na-rich scapolite. Stage M-III magnetite and Stage M-IV magnetite-sulphide mineralization, which together form the massive iron orebodies accounting for over 95% of the magnetite mineralization at Marcona, comprise a variety of opaque and gangue minerals including magnetite, calcic amphiboles, biotite, phlogopite, K-feldspar, calcite, apatite, diopside and sulphides. Major mineral assemblages assigned herein to Stage M-III are magnetite-actinolite (or tremolite±phlogopite±apatite) and magnetite-biotite (±actinolite). The main magnetite orebodies exhibit smoothly curving, abrupt contacts, dyke-like to tubular apophyses and intricate, amoeboid interfingering with dacite porphyry intrusions, and local vesicular texture, interpreted as evidence for the commingling of hydrous Fe oxidic and silicic melts. The main magnetite orebodies are haloed by biotite and K-feldspar alteration in metasedimentary rocks and dacites. Sulphide deposition was initiated at Stage M-IV in association with magnetite-actinolite (or tremolite±calcite) and magnetite-phlogopite (±actinolite or tremolite±calcite) assemblages. The major sulphides of Stage M-IV are pyrrhotite and pyrite, occurring commonly as subhedral to anhedral crystals intergrown with magnetite, silicates and calcite.
In Stage M-V, sulphide veins, commonly with calcic amphibole, formed in the upper levels of the orebodies and cut massive Stage M-III magnetite-calcic-amphibole assemblages. More commonly, Stage M-V sulphides and coexisting minerals occur as aggregates in cavities in Stage M-III and Stage M-IV magnetite-amphibole assemblages. Common Stage M-V assemblages include chalcopyrite-pyrite± pyrrhotite-amphibole and chalcopyrite-pyrite-calcite.
Late-stage, M-VII, hydrothermal veins developed in multiple episodes with ambiguous age relationships, and include tourmaline-quartz-sulphide, magnetite and sulphide, quartz (±calcite), calcite and hematite facies. Both Stage M-V and M-VII represent post-magnetite mineralization hydrothermal activites and only contain minor non-economic Cu mineralization.