Geology, mineralization, geochemistry, and fluid inclusion characteristics of the Bagh Khoshk copper deposit, Sirjan, southeast Iran

Bagh Khoshk deposit is located 35 km northeast of Sirjan in the southern Urmia- Dokhtar magmatic belt (Kerman metallogenic area). The magmatic activities and copper mineralization in this belt are attributed to the Eocene-Oligocene, middle-late Oligocene, and middle-late Miocene. Meanwhile, fertile porphyry copper deposits are genetically associated with middle-late Miocene granitoids (adakitic intrusive rocks). In Kerman metallogenic area, intrusive rocks are divided into productive with Miocene age (Kuh-Panj type) and semi-productive to barren groups with Eocene-Oligocene age (Jebal Barez type). Bagh Khoshk copper deposit has not been studied in terms of mineralization and genesis. In addition, it is not clear whether the Bagh Khoshk granitoid intrusion is a productive or semi-productive to barren magmatic system in the Kerman region. In this research, Bagh Khoshk deposit has been studied from the perspective of lithology, alteration, geochemistry, mineralization and fluid inclusion and by determining the geochemical nature of Bagh Khoshk granitoids, the origin of copper mineralization has been investigated.

In this research, the number of 21 samples from the outcrops and 24 samples of drilling cores have been selected for petrographic and mineralogical studies. 13 unaltered to less altered rock samples were taken from the outcrops and drilling cores for petrological studies and analyzed using XRF and ICP-OES/MS methods for major and trace elements. Ore geochemistry study has been done on 491 rock samples from drilling cores. To study the fluid inclusion, 4 mineralized samples were selected from the potassic and phyllic alteration zones and after preparation of double polished sections, micro-thermometry studies were done on quartz crystals.

The Eocene andesite to basaltic andesite lava flows and tuffs are the most widespread rock units in the Bagh Khoshk area. Late Miocene hypabyssal porphyry granodiorite and diorite stocks are intruded into the volcanic rocks. The alterations include potassic, prophylitic, phyllic, and argillic zones from the inside out. This deposit includes sulfide minerals (pyrite, chalcopyrite, bornite, molybdenite, chalcocite, and covellite), iron oxides (magnetite, olygiste, hematite, and goethite) and malachite which are mostly observed as disseminated and vein-veinlet forms in the potassic and phyllic zones. Copper is the major element, which has a positive correlation with molybdenum. Fluid inclusions in quartz crystals include LV, VL, and LVH types. The homogenization temperature of the LV, VL, and LVH fluid inclusions ranging from 180 to 289, 331 to 565, and 207 to 276 ⁰C. Their salinity varies from 0.35 to 10.24, 0.88 to 11.22, 33.55 to 42.66 wt.% NaCl eq., respectively. The Bagh Khoshk magmatic system in the Urmia-Dokhtar belt, was formed by partial melting of mantle source and thickening of lower crust, where the share of lower crust has been dominant. Finally, the Bagh Khoshk mineralization is a porphyry copper deposit, which is associated with adakitic and productive late Miocene magmas.

The late Miocene granodiorite intrusions host copper mineralization in the Bagh Khoshk area. These intrusions have the geochemical properties of adakitic magmas and are located in a normal continental arc environment. Enrichment in LREE, high Sr/Y and La/Yb ratios, enrichment in LILE and Sr, and depletion in HFSE are prominent geochemical features of Bagh Khoshk granitoids. Chalcopyrite is the most important copper-bearing mineral that is found as disseminated and vein- veinlets forms in the potassic and phyllic alteration zones. Based on fluid inclusion studies, the normal cooling of magmatic fluids and their mixing with meteoric waters has been one of the most important factors of metal deposition and the average depth of fluid inclusions entrapment and placement of the Bagh Khoshk porphyry stock is estimated at about 1200 m. The Bagh Khoshk magmatic system consists of partial melting of a mantle source with garnet amphibolite composition and a thickened lower crust, in which the share of lower crust has been dominant. The rapid rise of productive adakitic magma has led to the formation of economic copper deposit in this area.