HYDROTHERMAL EVOLUTION IN THE CALABONA PORPHYRY COPPER SYSTEM (SARDINIA, ITALY) - THE PATH TO AN UNECONOMIC DEPOSIT

The Calabona porphyry copper system is developed in a small, relativel y deep seated (approximate to 5 km) dacitic intrusion located in north western Sardinia (Italy). Early hydrothermal alteration produced a pot assic assemblage in the deep and central parts of the complex, and a p eripheral propylitic halo. Sodic alteration was subsequently superimpo sed on the potassic zone and volumetrically dominant phyllic alteratio n overprinted the apical parts of the intrusion. Hypogene copper miner alization (chalcopyrite and minor bornite) was associated with potassi c alteration. The earliest fluid that circulated in the Calabona porph yry complex had high salinity (40-60 wt % NaCl equiv), and is interpre ted to have been exsolved directly from the crystallizing magma. Howev er, major entrapment of this fluid only occurred after it had cooled t o 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 g roup 2 veins. The average Cu grade in the potassic zone is between 0.0 5 and 0.1 percent. The circulation of a lower temperature (270 degrees -330 degrees C), Ca-enriched fluid of meteoric origin in the periphera l parts of the system caused propylitic alteration. At an intermediate stage of hydrothermal evolution, waters of external origin entered th e central parts of the system along a network of late fractures (group 3 veins) or reopened group 2 veins. Partial mixing of this meteoric-f ormation 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 relative ly low fluid/rock ratios along a prograde thermal path caused sodic al teration. The continuous inflow of meteoric water, and the general tem perature 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 z one favored deposition of chalcopyrite and bornite but did not add sig nificant 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 dep osit is surprising, in view of call-rock alteration, style of minerali zation, and a fluid evolution typical of those of many productive porp hyry copper systems. We propose that this is mainly a consequence of t he relatively deep level of emplacement of the intrusion and the dacit ic composition of the magma. These factors are interpreted to have com bined to retard melt saturation with alkali chloride-enriched fluids u ntil late stages of crystallization, which restricted the amount of ex solved fluid and Cu extracted from the melt. As a result, overpressuri ng 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 co pper to economic levels. Further reasons for the uneconomic nature of the Calabona porphyry were the lack of multiple intrusive events and t he sulfide-destructive, Cu-leaching effects of phyllic alteration.