Hydrothermal Evolution of the Sonajil Porphyry Copper System, East Azarbaijan Province, Iran: The History of an Uneconomic Deposit

The Sonajil porphyry copper system formed in a small, relatively deep-seated (~5 km) granodioritic intrusion at Ahar, East Azarbaijan, Iran. Based on field geology, petrography and fluid inclusion studies, early hydrothermal alteration caused a potassic-type alteration in the deep, central parts of the intrusion, and generated a peripheral propylitic halo. Dominant phyllic alteration overprinted apical parts of the intrusion. Hypogene copper mineralization was associated with potassic alteration. The earliest fluid circulated in the complex had high salinity (35-60 wt% NaCl equiv.), and is interpreted to have been formed directly from the crystallizing magma. Major entrapment of this fluid, which is responsible for potassic alteration and deposition of chalcopyrite and bornite in thin, discontinuous Group I and II veins, occurred after cooling to about 400°C. The average Cu grade in the potassic alteration zone is between 0.01 and 0.07%. Circulation of a lower temperature fluid having a homogenization temperature of 220-350°C with meteoric H2O that originated in the peripheral parts of the system was responsible for propylitic alteration. During an intermediate phase of hydrothermal activity, meteoric water penetrated central parts of the system along a network of late fractures (Group III veins) and is partly responsible for the Group II veins. Mixing of the later fluid with the magma-derived high-salinity fluid created a fluid characterized by salinities ranging from 1 to 25 wt% NaCl equiv. The inflow of meteoric water, and a continuous temperature decrease within the system, produced more oxidized fluids, which caused phyllic alteration and intense copper leaching. Late boiling in the apical parts of the system favored deposition of sporadic chalcopyrite and bornite, but did not add significant copper. Supergene enrichment led to copper grades higher than that of phyllic zone, but not enough to be of economic significance. Based on the similarities in wall-rock alteration, mineralization style, petrography, and fluid evolution in Sonajil and those of typical productive porphyry copper deposits, the lack of economic mineralization in Sonajil is surprising. It is proposed that this is mainly due to the deep level of emplacement of the intrusion and the dacitic composition of the parental magma. These factors are considered to be the main causes preventing melt saturation with alkali chloride-enriched fluids until the late stage of crystallization, which restricted the amount of exsolved fluid and Cu extracted from the melt. Moreover, overpressuring of the upper parts of the system and a lack of wide apical stockwork veins failed to provide sites for the mineralizing fluids to concentrate copper to economic levels. Finally, the uneconomic nature of the Sonajil porphyry could reflect the lack of multiple intrusions and the intense Cu leaching of the phyllic alteration zone.