زمین‌شناسی و ژئوشیمی ایزوتوپی کانسار Cu-Mo پورفیری هفت‌چشمه با تکیه بر نتایج ایزوتوپ‌های Sr–Nd–Pb-S-O-H

Introduction The Haftcheshmeh porphyry Cu–Mo deposit is located in the NW part of the Arasbaran Metallogenic-magmatic zone (AMZ), NW Iran. The (AMZ), located in the southernmost of Lesser Caucasus subduction zone, extends from NW Iran to Armenia and Azerbaijan (Hassanpour et al., 2015). The Haftcheshmeh Cu–Mo porphyry deposit was developed synchronously with the emplacement of the Oligo-Miocene Haftcheshmeh porphyries, ranging in composition from gabbro-diorite to granodiorite. Based on the detailed field and petrography studies, four alteration zones from center to outward have been recognized in the Haftcheshmeh deposit, including early potassic and peripheral propylitic alterations, successively followed by sericitic and locally argillic alteration zones. According to the mineralogical, textural, and crosscutting relation of the quartz veins, three hypogene hydrothermal alteration-mineralization have been recognized. Stages I and II are associated with potassic alteration zone; and stage III is associated with sericite alteration zone. The purpose of this paper is to determine the characteristics and origin of the ore-bearing fluids, with particular focus on the results of S-O-H stable isotopes of the hydrothermal sulfide ores, phyllosilicate minerals (biotite and sericite) given from potassic and sericite alteration zones. The whole rock Sr-Nd-Pb radiogenic isotopes were undertaken to elucidate the possible origin of the parental magma of the ore-bearing Haftcheshmeh porphyries. Materials and methodsMore than 100 polished and thin sections from mineralized gabbro-diorite and granodiorite porphyries bore hole samples were studied by petrographic and mineralogical methods at the Shahid Beheshti University, Tehran. Two biotites from stage II; and five sericites from stage III and ten sulfide minerals (eight pyrite and two chalcopyrite) were separated from quartz–sulfide veinlets of II and III mineralization stages. They were used for δ18O, δD and δ34S stable isotope analysis; which was performed at the geochemistry and isotopic research Laboratory of British Colombia, Canada, using a Finnigan MAT 252 mass spectrometer. Whole-rock Sr-Nd-Pb isotopic compositions of the two least altered gabbro-diorite and granodiorite porphyries were performed at the geochemistry and isotopic research Laboratory of British Colombia, Canada, using Nu Multi-Collector Thermal Ionization Mass Spectrometer; (TIMS).Results and discussionThe calculated aqueous fluids δ18OH2O and δDH2O values of water in equilibrium with biotite samples range from +8.3‰ to +6 and from –76 to –74‰ respectively. The calculated δ18OH2O and δDH2O values of water in equilibrium with sericite samples range from 5.6 to 8.3 ‰ in δ18OH2O and from –100 to –84‰ in δDH2O, respectively. The δ34S values of pyrite and chalcopyrite from stage II range from -5.4 to -3.7 (n=4), and -3.2‰ (n=1) respectively, and δ34S values of pyrite and chalcopyrite from stage III range from +0.9 to +3.1 (n=3) and +0.7 (n=1), respectively. Gabbro-diorite and granodiorite samples at Haftcheshmeh have an initial 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb ratios, and εNd (t) values between 0.7044-0.7046; 0.5128-0.51277; 18.8-18.93; 15.60-15.61; 38.8-39 and +2.7 to +2.6, respectively.The δ18OH2O and δDH2O1 values of the biotite samples from stage II with potassic alteration halo and sericite samples from stage III, indicate that the initial ore-forming fluids were from a magmatic dominated origin and then mixed with a low component of the meteoric water. The δ34S values of pyrite and chalcopyrite minerals reflected a homogeneous magmatic and mantle-dominated sulfur source. The Pb isotopic compositions of the Haftcheshmeh porphyries show a relatively uniform magmatic origin during the compressional regime. Whole-rock initial 87Sr/86Sr, 143Nd/144Nd isotopic ratios and positive ɛNd(t) values indicated that the adakite-like Haftcheshmeh porphyries were generated from a dominantly depleted mantle-derived, thickened lower crust source, which was consequently contaminated by upper crustal materials during the ascent and crystallization of magma.ConclusionsThree alteration and Cu-Mo mineralization stages associated with potassic and sericite alteration zones of the gabbro-diorite to granodiorite phases have been recognized in the Haftcheshmeh porphyry deposit. The measured and calculated δ18O and δD values of the potassic to sericite minerals from stage II to III reflected that the magmatic hydrothermal fluids were progressively mixed with a meteoric water influx. The δ34S and the calculated δ34SH2S values of pyrite and chalcopyrite sulfides from stage II and III reflected that the magmatic sulfur and physico-chemicals contributed to sulfide mineral formation. The homogenous whole rocks 143Nd/144Nd, 87Sr/86Sr and initial Pb isotopes ratios of the gabbro-diorite to granodioritic porphyries indicated that the primary magmas were generated from a dominantly depleted mantle-derived, thickened, lower crust source. It was consequently contaminated by upper crustal materials either at the magma source or during the ascent and crystallization of magma during the compressional regime.