Hydrothermal Evolution of the Miduk Porphyry Copper System, Kerman, Iran: A Fluid Inclusion Investigation

Abstract

The Miduk porphyry copper deposit is located in SSE central Iran in the elongated NW-trending median mountain range of Kerman Province (85 km northwest of the Sar-Cheshmeh porphyry copper deposit). It is associated with Miocene quartz-diorite to quartz-monzonite which intruded Eocene volcanosedimentary rocks. Copper mineralization was accompanied by both potassic and phyllic alteration. Field observations and petrographic studies demonstrate that emplacement of the Miduk pluton took place in several intrusive pulses, each with associated hydrothermal activity. Four main vein groups have been identified: (I) quartz + K-feldspar ± anhydrite ± molybdenite (0.002%), with sporadic pyrite, chalcopyrite, and bornite; (II) quartz + chalcopyrite + pyrite ± sporadic bornite; (III) quartz + pyrite + calcite + chalcopyrite ± anhydrite; and (IV) quartz, and/or calcite, and/or gypsum ± pyrite. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, and propylitic alteration in its periphery. Later phyllic alteration overprinted these alteration zones. Early hydrothermal fluids are represented by high-temperatures (380 to 550°C), high-salinity (up to 60 wt% NaCl equiv.) liquid-rich fluid inclusions, and hightemperature (320 to 550°C), low-salinity, vapor-rich inclusions. These fluids are interpreted as orthomagmatic fluid that boiled episodically; the brines evidently caused potassic alteration, and deposition of Group I and II quartz veins containing chalcopyrite and very rare molybdenite. Propylitic alteration is attributed to a liquid-rich, lower temperature (200 to 350°C), Ca-rich, evolved meteoric fluid. Influx of meteoric water into the central part of the system, and mixing with magmatic fluid produced salinities ranging from 1 to 20 wt% NaCl equivalent in Group II and III veins. Phyllic alteration and copper leaching resulted from the inflow of oxidized, acidic meteoric waters during cooling of the system. Late boiling occurred in the upper part of the phyllic zone, and was associated with significant copper deposition.