Summary:
The Zaldívar porphyry copper deposit is situated on the western margin of the Atacama Plateau in northern Chile. The deposit is part of a large Tertiary porphyry copper system which includes the scondida porphyry copper deposit. This porphyry complex occurs within the large West Fissure structural system which controls most of the large porphyry copper deposits in Chile. The Zaldívar porphyry system is at the intersection of the West Fissure and a series of Northwest and Northeast striking faults. The deposit is generally centered on a Northeast striking granodiorite porphyry body that intrudes andesites and rhyolites, and cuts across the north-south striking Portezuelo fault. Although the geology and the Zaldívar mineral deposit are generally continuous from east to west, the orebody was arbitrarily divided into two zones: the Main zone (area east of 93,000E) and the Pinta Verde zone (area west of 93000E).
The alteration developed in the Zaldívar Porphyry correspond to an early potassic alteration event, represented by secondary K-feldspar, which affect the Llamo Porphyry and the Paleozoic rocks below the 3,050 m elevation, and by secondary biotite, which widely affects the andesitic rocks.
This alteration is overprinted by low temperature quartz-sericite hydrothermal alteration with two stages of development. An early wide spread stage of sericite-chlorite alteration followed by the principal stage with more penetrative quartz-sericite-pyrite alteration focused in the mid part of the deposit and with a clear association with the Llamo Porphyry intrusions. Strong sericite alteration is present as discontinuous bodies in the core of the quartz-sericitic alteration zones and typically destroys primary rock textures. Propylitic alteration with chlorite as the main mineral effects the andesitic rocks and the late Llamo Porphyry.
The final stage of the system is represented by very restricted advanced argillic alteration. Finally, as result of the leaching and oxidation of the rock column of the deposit, the supergene argillic alteration is developed, affecting all the lithological types with variable intensity, but being typically stronger in the andesitic rocks.
The mineralization has an elongated shape trending northeast-southwest. This is the result of the overprinted hypogene and supergene processes, where the latter with its leaching and enrichment process, gave the deposit a characteristic vertical profile with a superior leached zone, an oxide copper zone, and a secondary sulfide zone located between the oxide and the basal primary sulfide zone.
The leached zone is present as a continuous horizon in the upper part of the deposit, with local depths of up to 300 m. The typical mineralogy of this zone includes hydroxides and iron sulfates (hematite-goethite-jarosite) with copper phosphate (turquoise).
The oxide zone extends more or less continuously from the Portezuelo Fault and surrounding areas towards the southwest, covering an area of approximately 2 km by 1.5 km and with an average thickness of approximately 90 m. The oxide zone is incised locally by leached areas related to faults of the northwest-southeast structural system. The mineralogy varies according to lithology and dominant alteration. For example, brochantite-antlerite is found in the rhyolitic rocks and chrysocolla, “black copper”, and copper phosphate are found in the andesitic rocks with chlorite-biotite alteration.
The secondary sulfides cover an area of approximately 2.5 km by 1.5 km, with a variable thickness from a few metres in the southwest extremity up to over 300 m in the northeast extremity. Mineralogy in this area is represented by pyrite, chalcocite, covellite, chalcopyrite and minor sphalerite and molybdenite. Chalcocite is the dominant secondary sulfide mineral (over 80%). Covellite is the main sulfide mineral present in the transition zone to the underlying primary sulfides. Alunite dating indicates an age for the secondary enrichment process of approximately 18 My to 14.7 My (Monroy 2010).
The primary sulphide zone follows the Llamo Porphyry trends and over 70% of the mineralization occurs in veinlets. The primary sulfide body has a bornite-rich core with minor chalcopyrite that is located in the Llamo Porphyry and in the late magmatic breccias. The bornite-rich core is enveloped by a chalcopyrite-pyrite zone which in turn is surrounded by a pyritic halo. The primary mineralization age is 37.2 My (Morales 2010).