Document Type : Original Research Paper
Authors
1 Ph.D. Student, Department of Geology, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
2 Assistant Professor, Department of Geology, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
3 Associate Professor, Department of Applied Geosciences and Geophysics, University of Leoben, Leoben, Austria
Abstract
The Baghkhoshk porphyry copper system is located in the southern part of the Cenozoic Urumieh-Dokhtar magmatic belt of Iran. Mineralization in Baghkhoshk is associated with two shallow diorite-monzodiorite and granodiorite bodies that intruded the older andesitic volcanic rocks. Both intrusions are extensively altered by hydrothermal fluids into potassic, phyllic, and propylitic assemblages. Mineralization occurs as quartz-sulfide stockworks as well as sulfide disseminations in both intrusions and to a lesser extent in the host volcanic rocks. Two types of quartz veinlets can be distinguished in the Baghkhoshk porphyry system: 1) quartz+ chalcopyrite+ pyrite± molybdenite veinlets (mineralized veinlets); and 2) quartz± pyrite veinlets (barren veinlets). The intrusions display distinct enrichments in large-ion lithophile elements and rare earth elements relative to high-field strength elements and heavy rare earth elements, and distinct Nb, Ta and Ti troughs that are characteristic of subduction-related magmatic arcs. On various diagrams separating tectonic settings, the intrusions plot in the continental arc domain. Three main types of fluid inclusions were identified in quartz from mineralized veinlets: low salinity (4-8 mass% NaCl equivalent), vapor-rich inclusions; high salinity (mainly in the range of 32.5-35 mass% NaCl equivalent), poly-phase inclusions; and low to moderate salinity (mainly in the range of 4-10 mass% NaCl equivalent), liquid-rich inclusions. The vapor-rich inclusions yielded homogenization temperatures ranging between 325 and 434 °C, with a mode at 380-420 °C. Laser Raman microspectroscopic analysis confirmed the presence of CO2 gas and chalcopyrite daughter crystals in the vapor-rich inclusions. The high salinity, poly-phase inclusions were homogenized to liquid at temperatures of mainly 300-375 °C. The liquid-rich inclusions were homogenized at temperatures between 150 and 384 °C, with most of the measurements falling in the range of 200-250 °C and 300-375 °C. The salinities and homogenizaion temparatures for fluid inclusions in quartz from the barren veinlets were found to be 0.5-4 mass% NaCl equivalent and 200-240 °C, respectively. The fluid inclusion data suggest that Cu was originally accommodated into the system by a hot (380-420°C), low salinity (4-8 mass% NaCl equivalent), CO2-bearing, vapor-rich or supercritical fluid. A considerable drop in temperature at constant salinity from vapor-rich to liquid-rich inclusions in the mineralized veinlets suggests that cooling was the main control on ore formation as stockworks and sulfide disseminations. In the later stages of the hydrothermal activity, low salinity, Cu-poor fluids gave rise to abundant barren veinlets consisting essentially of quartz and pyrite.
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