Geology and fracture-related hypogene hydrothermal alteration and mineralization of porphyry copper deposit at Sungun, Iran

摘要

The porphyry copper deposit at Sungun is located in East Azarbaidjan, NW of Iran. The magmatic suite in the Sungun area is apart of NW-SE trending Cenozoic magmatic belt of Iran. The Sungun porphyries (Oligo-Miocene) occur as stocks and dykes, and intrude a series of Oligocene dacitic breccias and tuffs and trachy-andesitic lavas, Eocene arenaceous-argillaceous rocks, and Upper Cretaceous carbonate rocks. The porphyry stocks are divided into two groups: (1) the porphyry stock I (quartz monzo-diorite) and (2) the porphyry stock II (quartz monzonite, granodiorite, and granite). The porphyry stock II hosts the copper ores. Four series of cross-cutting dykes (DK1, DK2, DK3 and DK4), ranging in composition from quartz monzo-diorite to granodiorite intruded the Sungun Porphyry stocks and surrounding enclosing rocks. The whole-rock chemical analyses of the samples for major and trace elements showed that the parental magma of the Sungun porphyries formed in continental margin tectonic setting, and was mainly calc-alkaline in character.Four types of hypogene alterations are present at Sungun: (1) potassic, (2) potassic-phyllic, (3) phyllic and (4) propylitic. The potassic, potassic-phyllic, and phyllic alteration zones were principally developed in the porphyry stock 11, whereas the propylitic alteration zone is chiefly restricted to the peripheral porphyry stocks I and dyke series DK2 and DK3. The phyllic and potassic-phyllic occurred in zones where the rocks underwent severe hydro-fracturing manifested by numerous and various cross-cutting veinlets and micro-veinlets of quartz, sulphides, silicate, carbonates, and sulphates. The hypogene mineralization is characterized by the introduction of hypogene sulphides such as pyrite, chalcopyrite, molybdenite, tetrahedrite, bornite, and chalcocite. Three stages of hypogene mineralization, though continuous, were recognized: (1) early, (2) middle and (3) late. The early stage was contemporaneous with the development of the early-type quartz veinlets and potassic alteration. The middle stage that is the main stage of copper mineralization, was contemporaneous with the phyllic alteration. Pyrite, chalcopyrite, and molybdenite along with trace amounts of tetrahedrite, bornite, and chalcocite are also associated with this stage. The sulphides are mainly concentrated in the phyllic and potassic-phyllic zones, and their abundance gradually decreases downward toward the potassic zone. The abundance of copper-bearing hypogene sulphides gradually decreases upward towards the upper parts of the phyllic zone where pyrite is relatively abundant. Only locally small amounts of sulphides (<2 wt%) are associated with rocks that underwent propylitic alteration. The XRF and electron-probe micro-analyses data combined with microscopic evidence suggest that the alteration of ferromagnesians liberated considerable Fe which in turn acted as a major factor for the fixation of the magmatically-derived sulphur in the ore-bearing solutions. Copper and molybdenum were introduced by the hydrothermal ore-bearing fluids.