Hydrothermal evolution in the Calabona porphyry copper system (Sardinia, Italy); the path to an uneconomic deposit

McGill University, Department of Earth and Planetary Sciences, Montreal, PQ, Canada

The Calabona porphyry copper system is developed in a small, relatively deep seated ( nearly equal 5 km) dacitic intrusion located in northwestern Sardinia (Italy). Early hydrothermal alteration produced a potassic assemblage in the deep and central parts of the complex, and a peripheral propylitic halo. Sodic alteration was subsequently superimposed on the potassic zone and volumetrically dominant phyllic alteration overprinted the apical parts of the intrusion. Hypogene copper mineralization (chalcopyrite and minor bornite) was associated with potassic alteration. The earliest fluid that circulated in the Calabona porphyry complex had high salinity (40-60 wt % NaCl equiv), and is interpreted to have been exsolved directly from the crystallizing magma. However, major entrapment of this fluid only occurred after it had cooled to temperatures of about 400 degrees C. This fluid was responsible for potassic alteration and for precipitation of chalcopyrite and bornite in thin, discontinuous group 1 veins and irregular and widely spaced group 2 veins. The average Cu grade in the potassic zone is between 0.05 and 0.1 percent. The circulation of a lower temperature (270 degrees -330 degrees C), Ca-enriched fluid of meteoric origin in the peripheral parts of the system caused propylitic alteration. At an intermediate stage of hydrothermal evolution, waters of external origin entered the central parts of the system along a network of late fractures (group 3 veins) or reopened group 2 veins. Partial mixing of this meteoric-formational water with the high-salinity fluid already circulating in the system, created a fluid characterized by salinities ranging from 2 to 23 wt percent NaCl equiv. Circulation of this mixed fluid at relatively low fluid/rock ratios along a prograde thermal path caused sodic alteration. The continuous inflow of meteoric water, and the general temperature decrease in the system, produced progressively more oxidized and acidic fluids, which caused phyllic alteration and intense copper leaching. Late boiling in the apical parts of the phyllic alteration zone favored deposition of chalcopyrite and bornite but did not add significant copper (Cu grade <0.03%). Supergene enrichment led to copper grades higher than those in the phyllic zone (0.07%), but not enough to permit mining. The very low concentration of Cu in the Calabona deposits is surprising, in view of wall-rock alteration, style of mineralization, and a fluid evolution typical of those of many productive porphyry copper systems. We propose that this is mainly a consequence of the raltive deep level of emplacement of the intrusion and the dacitic composition of the magma. These factors are interpreted to have combined to retard melt saturation with alkali chlorife-enriched fluids until late stages of crystallization, which restricted the amount of exsolved fluid and Cu extracted form the melt. As a result, overpressuring in the apical parts of the system was limited, the related fracture density was low, and the system therefore failed to provide the focus for the mineralizing fluids needed to permit bulk concentration of copper to economic levels. Further reasons for the uneconomic nature of the Calabona porphyry were the lack of multiple intrusive events and the sulfide-destructive, Cu-leaching effects of phyllic alteration.

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