Stable isotope (S, O, H and C) studies of the phyllic and potassic–phyllic alteration zones of the porphyry copper deposit at Sungun, East Azarbaidjan, Iran

Abstract

The porphyry copper deposit (PCD) at Sungun is located in East Azarbaidjan, NW of Iran. The magmatic suites in the Sungun area are a part of the NW–SE trending Cenozoic magmatic belt of Iran. The Sungun porphyries occur as stocks and dikes. The stocks are divided into two groups, I and II. Porphyry Stock II, ranging in composition from quartz monzonite through granodiorite to granite, hosts the Sungun PCD. Four distinct types of hypogene alterations were recognized at Sungun: (1) potassic; (2) potassic–phyllic; (3) phyllic; and (4) propylitic. Stable isotope (S, O, H, and C) studies were restricted to within the phyllic and potassic–phyllic alteration zones, where numerous cross-cutting quartz, sulfides, carbonates, and sulfate veinlets are present. The objective of these studies was to determine the origin of the ore-forming solutions, and their important components (e.g. sulfur and carbon). Twenty sulfide and four sulfate samples were taken from sulfide and gypsum veinlets within Porphyry Stock II and the associated skarn zone for sulfur isotopic analyses. The δ 34S values of sulfides (pyrite, chalcopyrite, molybdenite, galena, sphalerite) and sulfate (gypsum) range from −4.6 to −0.2‰ (mean of −1.5‰) and from 10.9 to 14.4‰ (mean of 12.9‰), respectively. These values are almost analogous to those from El Salvador (Chile) and Ajo (Arizona), and Twin Buttes (Arizona), and strongly suggest a magmatic source for the sulfur at Sungun. Twenty-eight fluid inclusion-rich quartz samples from quartz veinlets beneath the supergene zones of the Porphyry Stock II were chosen for O and H isotopic analyses. The δ 18O (of quartz) and δD (of fluid inclusions in quartz) values range from 8.3 to 10.2‰ (mean of 9.2‰) and −58 to −75‰ (mean of −66‰) relative to Standard Mean Ocean Water (SMOW), respectively. The calculated δ 18O values of the fluids range from 4.4‰ (T=375 ° C) to 7.6‰ (T=570 ° C) with a mean of 6.4‰. The δ 18O and δD values of the fluids lie within the typical range of magmatic waters. Based upon the lowest obtained δD values, the calculated maximum exchangeable meteoric water that might be involved in the phyllic and potassic–phyllic alteration processes was ∼20%. Fourteen calcite samples from carbonate veinlets in the Porphyry Stock II and two samples from associated marble aureole were collected for carbon and oxygen isotopic analyses. The δ 13C (calcite) from Porphyry Stock II and associated marble aureole varies from −0.4 to 2.9‰ and 0.06 to 2‰ relative to the PDB Standard of the University of Chicago, respectively. The corresponding δ 18O (calcite) values range from 6 to 13.2‰ relative to SMOW. The δ 13C values lie within the sedimentary range, suggesting that there was little or almost no magmatic carbon in the hydrothermal system. In contrast, The δ 18O (calcite) values are far lower than that of Cretaceous sedimentary carbonates which suggest that the values were influenced by those of magmatic fluids through isotopic re-equilibration. These isotopic investigations demonstrated that: (1) in contrast to some well-known PCDs, about 80% of the fluids involved in the phyllic and potassic–phyllic alterations at Sungun had a magmatic O and H isotopic signature; (2) the Upper Cretaceous limestone was the potential source of C; and (3) the sulfur had a magmatic origin.