Summary:
Agua Rica Project is integrated with Alumbrera Mine (MARA Project).
Agua Rica
The Agua Rica deposit is interpreted as a substantial gold-rich porphyry copper-molybdenite deposit, formed by multistage porphyry intrusions of Miocene age.
Mineralization
Three major stages of alteration and polymetallic mineralization have been recognised at Agua Rica: early porphyry copper-molybdenum- gold (Cu-Mo-Au) associated with the emplacement of porphyritic intrusions, later overprinted by epithermal copper-gold-silver-arsenic-lead-zinc (Cu-Au-Ag-As-Pb-Zn) related to complex multi-stage hydrothermal brecciation; and supergene copper (Cu) enrichment. These are described in the subsections below.
Porphyry Stage
In a first pulse of Miocene magmatism, the emplacement of the Melcho intrusions in the southern portion of the Project area was accompanied by minor potassic and propylitic alteration and weak pyrite ± chalcopyrite ± molybdenite mineralization.
The subsequent intrusion of the Seca and Trampeadero feldspar porphyries resulted in the development of intense quartz stockwork veining, potassic alteration, and chalcopyrite-molybdenite mineralization with rare bornite and pyrrhotite. Remnants of this potassic alteration can be observed in patches in those two porphyries. Chalcopyrite and bornite are especially important at Seca Norte, however, are difficult to observe macroscopically. Rare magnetite-chalcopyrite veins in which magnetite has been partially altered to hematite are present in Seca Norte. Bornite is generally intergrown with and sometimes forms the cores of chalcopyrite grains and coincides with an area of higher primary copper grades.
Epithermal Stage
The emplacement of the hydrothermal breccias is associated with phyllic and advanced argillic alteration that carry precious metals and copper sulphosalts. The phyllic alteration assemblage, which is most intense in the Seca Norte porphyry and carries disseminated pyrite, destroys primary rock textures, including evidence of previous porphyry mineralization.
The central Quebrada Minas hydrothermal breccia body, which separates the Seca and Trampeadero porphyries, and the Trampeadero porphyry are most affected by the epithermal overprint. Mineralization is associated with an advanced argillic alteration assemblage that includes pyrophyllite and is characterized by zones of vuggy silica and massive silicification. The advanced argillic assemblage is intimately associated with pyrite, covellite, enargite, sphalerite, galena, and minor molybdenite. These sulphides occur in veins in the porphyries and metasedimentary rocks and as open-space fillings in the hydrothermal and igneous breccia.
Pyrite is the most abundant sulphide throughout the deposit, averaging 3% vol to 7% vol in the mineralized zones. Covellite is the dominant copper sulphide. Spectacular specimens of coarse-grained covellite occur as euhedral hexagonal plates up to 1 cm in veins and as cement or open-space filling in the hydrothermal and igneous breccias (Figure 7-8). Copper grades in areas with abundant, coarse-grained covellite exceed 2% Cu, whereas average disseminated copper grades throughout the Quebrada Minas breccia pipe are about 0.5% Cu (Landtwing et al., 2002).
Enargite and rare tennantite and tetrahedrite occur in association with galena, covellite and sphalerite in epithermal pyrite veins or in the matrix of hydrothermal breccias. Pyrite-covellite ± enargite mineralization is interpreted to represent the start of the epithermal stage of polymetallic mineralization, and two distinct stages are proposed as enargite is observed replacing covellite. High gold and silver grades show some spatial relationship to the hydrothermal breccia. Tagaki and Brimhall (1998) have described a very late stage of sphalerite-galena-anomalous enargite with pyrite, marcasite, and melnikovite, which contribute copper and gold at upper elevations of the Agua Rica polymetallic deposit.
Supergene Stage
There is an immature supergene copper enrichment of hypogene chalcopyrite and covellite by supergene chalcocite and covellite, forming a well-developed blanket at Seca Norte and Trampeadero. Chalcocite typically forms discrete particles and rims around chalcopyrite and pyrite grains and is the dominant supergene enrichment mineral at Trampeadero and Filo Amarillo and is subordinate to supergene covellite at Seca Norte. Covellite is the dominant copper mineral at Agua Rica and differentiating between supergene and hypogene textures is difficult. Enriched copper grades are typically twice and rarely three times as high as underlying primary grades. If significant copper enrichment was ever developed over the Quebrada Minas hydrothermal breccia pipe, it has been removed during later erosion processes.
The leached capping mimics topography, except for the structurally thickened area on the west side of Seca Norte, in a number of anomalously deeply leached areas in Trampeadero, and in local surface outcrops in Seca Norte and Trampeadero. The leached capping at Agua Rica is dominated by jarosite with lesser amounts of goethite and minor hematite. The lower boundary of supergene enrichment is subhorizontal and flatter than present valley topography. No evidence for multiple episodes of enrichment has been observed. Chalcocite is the dominant supergene enrichment mineral at Trampeadero, but fine-grained covellite is the most common at Seca Norte.
Alumbrera
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 hornblende. Postmineral porphyries, the youngest, truncate all such mineralization and alteration, and none of their hornblende is biotitized. Los Amarillos porphyry and igneous breccia, along the western periphery of the porphyry cluster, is between P2 and Early P3 in age but shows little relationship to mineralization.
A low-grade core zone consists in large part of barren K-feldspar-magnetite alteration and quartz veins in Early P3 porphyry, and in part consists of later barren porphyry, so is mostly younger than the Cu-Au deposited with P2 porphyry.