Summary:
The porphyry-style mineralization at Quebrada Blanca is considered to be typical of an Andean porphyry copper–molybdenum deposit. Common features of this subset of porphyry-style deposits include:
- Large zones (>10 km2 ) of hydrothermally altered rocks that commonly grade from a central potassic core to peripheral phyllic-, argillic-, and propylitic-altered zones;
- Mineralization is generally low grade and consists of disseminated, fracture, veinlet, and quartz stock-work controlled sulphide mineralization. Deposit boundaries are determined by economic factors that outline ore zones within larger areas of low-grade, concentrically-zoned mineralization;
- Mineralization is commonly zoned with a chalcopyrite–bornite–molybdenite core and peripheral chalcopyrite–pyrite and pyrite zones;
- The effects of surface oxidation commonly modify porphyry deposits in weathered environments. Low pH meteoric waters generated by the oxidation of iron sulphides will leach copper from hypogene copper sulphides and form oxide copper minerals such as malachite, chrysocolla, and brochantite, and redeposit copper as secondary chalcocite and covellite immediately below the water table in flat tabular zones of supergene enrichment.
A north–south trending corridor, bounded by the West and El Loa Faults, is preserved in the Quebrada Blanca–Collahuasi area. The corridor includes the Paleozoic volcano–sedimentary sequence Collahuasi Formation, generally consisting of Permian sub-aerial and sub-aqueous dacite and rhyolite volcanic flows of with minor volcaniclastic rocks, and voluminous Paleozoic quartz monzonite to granodiorite stocks (303–294 Ma).
To the northwest of the Quebrada Blanca deposit, sedimentary rocks of the Quehuita Formation unconformably overlie the Collahuasi Formation. The unit consists of deep to shallow marine limestone, calcareous sandstone, arenites, and conglomerates reflecting the extensional evolution of a Jurassic backarc basin in northern Chile. Within the same Mesozoic belt, the Cretaceous Cerro Empexa Formation crops out to the west and southeast of the Quebrada Blanca area, and consists of dacite lavas, volcaniclastic breccias, and inter-bedded arenite and conglomerates. Granite stocks have intruded the Cerro Empexa Formation and reflect the marginal shift of a Cretaceous arc into the Quebrada Blanca district.
During the late Eocene to Early Oligocene, pre- to syn-mineral feldspar porphyries (37.5–36.4 Ma) and synmineral hydrothermal breccias (37.4–34.5 Ma) were emplaced.
The deposit is hosted in nine individual or groups of rock types. These include a Paleozoic quartz monzonite to granodiorite, and Late Eocene to Early Oligocene pre- to syn-mineral feldspar porphyries and syn-mineral hydrothermal breccias that intrude Collahuasi Formation lithologies.
Supergene leaching of the upper portions of the deposit and subsequent remobilization of copper produced supergene mineralization consisting of chalcocite and, to a lesser degree, copper oxides (chrysocolla, copper wad, copper clays, and minor atacamite, cuprite, and locally brochantite).
The hydrothermal breccias are interpreted to be formed by a single event, with textural and hydrothermal facies representing different energy conditions and hydrothermal zonation. The eastern area, dominated by tourmaline, and biotite–magnetite and biotite–potassic feldspar cements, is interpreted to be a better conserved column in the hydrothermal system. The western breccias do not contain tourmaline at the top. Lower-temperature quartz-rich and molybdenum-rich breccias have been recognized in the shallower parts of the breccia body.
The leach cap varies from about 7–200 m in thickness, whereas the thickness of the secondary copper zone ranges from 10–200 m. Continuous supergene copper mineralization has been traced over a 2.5 x 1.5 km area.
Hypogene mineralization occurs over a 2 x 5 km area, extending to at least 1.5 km vertical depth. Hypogene mineralization remains open to the west, northeast, east, southeast, and at depth.
Alteration zoning patterns at Quebrada Blanca are typical of porphyry copper deposits, and a detailed paragenetic sequence has been established. The three major alteration stages include:
- Early stage potassic alteration event: defined by secondary K-feldspar and biotite with associated biotite veinlets (EB), dark mica veins (EDM with biotite and green mica), and A veins (mainly quartz with K-feldspar halos). Brecciation occurred during the potassic event, permitting these hydrothermal minerals to develop locally as breccia cement. Chalcopyrite ± bornite occur as disseminations, in veins and/or in the cement of the breccia;
- Transitional stage alteration event: consists of grey-green sericite (SGV) and quartz as cement in breccias and/or as veins in coherent rocks. Quartz veins with sulfides (chalcopyrite and molybdenite and B veins) occur in this event as well as a biotite-, or biotite–magnetite- and chalcopyrite-cemented hydrothermal breccia;
- Late stage alteration event: consists of sericite–quartz–pyrite (phyllic; QS alteration) mainly as planar veins showing a prominent alteration halo (typical of D veins) and intermediate argillic alteration with kaolinite–smectite clays dominant.
Mineralization consists of supergene (chalcocite and, to a lesser degree, copper oxides such as atacamite, cuprite, and locally brochantite) and hypogene (chalcopyrite, bornite, molybdenite) mineralization. Mineralization displays two major trends.
Within the east–northeast trend, bornite mineralization forms two distinct zones that are interpreted to represent two different mineralizing centres as they do not spatially coincide with the higher copper grade areas hosted in chalcopyrite. Bornite is mainly concentrated in the southwestern part of the pit and in the northeastern portion of the east–northeast corridor that controls the porphyry and breccia intrusions. Chalcopyrite is controlled by the east–northeast trend in early potassic alteration as well as in transitional grey–green sericite and magnetite–chalcopyrite. Molybdenite mineralization is controlled by the same east–northeast-trending structures controlling higher copper grades (>0.5% Cu), but is also associated with northwest-trending structures in the eastern portion of the deposit.
Supergene Mineralization: secondary mineralization appears to be preferentially concentrated close to structures and more permeable rocks. The lower portions of the secondary enrichment zone transition into primary copper mineralization, resulting in a mixed low-grade ore type that was processed through run- of-mine (ROM) dump leaching.
Hypogene Mineralization: in the hypogene environment, mineralization occurs mainly as disseminated, veinlet-like and breccia cement mineralization forming an east–northeast-trending area within the Paleozoic quartz monzonite to granodiorite, feldspar porphyry intrusions, and breccias.
Within the northwest trend, pyrite distribution is generally related to quartz–sericite alteration and highly concentrated in late veins, typical of the northwest-trending principal faults where they also control supergene copper mineralization. The supergene mineralization is characterized by chalcocite and minor covellite. Gold and silver distributions correlate with copper mineralization grading >0.5% Cu. The locations of higher metal grades also appear to be structurally controlled, with grades increasing towards the hanging wall of the main fault. These minerals can locally occur outside of the main trends.