Summary:
Constancia mine includes the Constancia and Pampacancha deposits.
The Constancia deposit is a porphyry copper-molybdenum system which includes copper-bearing skarn mineralization. Multiple phases of monzonites and monzonite porphyry have intruded a sequence of sandstones, mudstones and micritic limestone of Cretaceous age.
The majority of the mineralization is associated with potassic alteration and quartz veining, occurring as chalcopyrite-(bornite)-molybdenite-pyrite mineralization in “A” and “B” type veinlets, and replacing ferromagnesian minerals or filling fractures. Copper grades are highest where fracture-filling style copper mineralization is superimposed on earlier disseminated copper mineralization. The higher-grade hypogene copper mineralization is hosted by a dense A-veinlet stockwork developed in an early porphyry phase. The pyrite/chalcopyrite ratio is typically below 2:1. Molybdenite commonly increases with depth in association to “B” veinlets. Bornite occurs sporadically especially at deeper levels, sometimes associated with some gold values.
Propylitic alteration is transitional to the potassic alteration and extends more than one kilometre from the porphyry intrusive contacts. The propylitic alteration mineral assemblage includes epidote-chlorite-calcitepyrite-rhodochrosite. Subordinate chalcopyrite is also present, filling fractures or replacing mafic minerals. Sphalerite-galena veinlets and veins are distributed as a halo to the copper-molybdenum mineralization within the propylitic alteration zone up to 3 km away from the porphyry copper system.
Phyllic alteration forms a pervasive carapace surrounding and sometimes overprinting potassic alteration. The phyllic alteration accompanies almost complete destruction of primary rock textures; the mineral assemblage includes sericite-quartz-pyrite, limited amounts of chalcopyrite and associated occasional “D” veins and veinlets.
At the contact between intrusions and limestones, a magnetite garnet skarn develops, while a pyroxene– diopside (garnet–epidote) association is more common in calcareous sandstones and arkoses of the Chilloroya formation. Skarn mineralization is volumetrically much smaller, but grades are normally higher.
Structural deformation has played a significant role in concentrating the hydrothermal alteration and the copper-molybdenum-silver-gold mineralization, including skarn formation. Major inter and post mineral fracture systems in the deposit area strike northeast and include the Barite fault system. This is represented by a number of nearly parallel vein-faults carrying base metal sulphides and barite which have been exploited by artisanal workings throughout the property. A second important system strikes northsouth. It appears to be more recent than the Barite system and controls part of mineralization and most of the silicified breccias (sometimes mineralized) in the system.
The Pampacancha deposit is a porphyry Cu-Mo-Au related Skarn system. Oligocene unmineralized basement diorite is intruded by the diorite porphyry cited as the source for skarn mineralization. This in turn, is cut by intra-mineral monzonite intrusions which provide minor local increases in Cu-Au and also locally replaces skarn Cu-Au mineralization which is most developed at the upper and lower margins of the limestone body. Magnetite-chalcopyrite-pyrite skarn ranges to marginal less well mineralized garnet and pyroxene skarn, locally overprinted by epidote-bearing retrograde skarn.
Epithermal mineralization as low sulphidation quartz-sulphide Au + Cu style accounts for common supergene enriched Au anomalies along with other features such as hydrothermal alteration and veins typical of near porphyry locations.
The Constancia porphyry copper-molybdenum system, including skarn, exhibits five distinct deposit types of mineralization:
1. Hypogene fracture-controlled and disseminated chalcopyrite mineralization in the monzonite (volumetrically small);
2. Hypogene chalcopyrite (rare bornite) mineralization in the skarns (significant);
3. Supergene digenite-covellite-chalcocite (rare native copper) in the monzonite (significant);
4. Mixed secondary sulphides/chalcopyrite in the monzonite (significant); and
5. Oxide copper mineralization (volumetrically small).
Molybdenite, gold and silver occur within all these mineralization types.
Two areas of porphyry-style mineralization are known within the project area, Constancia and San José. At Constancia, mineralization is deeper than that observed at San José which occurs at surface. The mineralized zone extends about 1,200 metres in the north-south direction and 800 metres in the east- west direction.
The Pampacancha deposit is located approximately three kilometers southeast of the Constancia porphyry. The stratigraphy unit in the area is the massive, gray micritic limestone of Upper Cretaceous Ferrobamba Formation; this unit in contact with the dioritic porphyry generates a magnetite skarn, hosts economic mineralization of Cu-Au-Mo.
The intrusive rocks are Oligocene age unmineralized basement diorite. Diorite porphyry is recognized as the source for skarn mineralization, which in turn is cut by mineralized monzonite intrusions which provide minor local increases in Cu-Au mineralization. Skarn Cu-Au mineralization is best developed at the upper and lower margins of the limestone body.
Epithermal mineralization of the low sulphidation quartz-sulphides Au + Cu style, accounts for common supergene enriched Au anomalies, and along with other features such as hydrothermal alteration and veins typical of near porphyry settings.