Journal of Economic Geology (Aug 2024)
Physicochemical Evolution of Hydrothermal Fluids in the Kuh-e-Esfand porphyry Copper System, South of Jiroft, Kerman Province
Abstract
The Kuh-e-Esfand copper deposit is located in the southernmost part of the Urmia-Dokhtar magmatic belt. The Oligocene-Miocene intrusive bodies, ranging from diorite to quartz diorite and granodiorite, are emplaced within the Eocene volcanic complex. Based on the classification of veins- veinlets, the main mineralization stage consists of quartz pyrite chalcopyrite associated with potassic alteration. Based on petrographic studies, fluid inclusions in quartz minerals are categorized into three main groups and seven subgroups: 1- Vapor-rich fluid inclusions comprising single-phase vapor inclusions (V), vapor-rich two-phase inclusions (VL), and vapor-rich inclusions with a opaque phase (VLS), 2- Liquid-rich fluid inclusions including liquid-rich two-phase inclusions (LV) and liquid-rich inclusions with a opaque phase (LVS) and 3- Saline fluid inclusions consisting of simple brine three-phase inclusions (LVH), and multi-phase brine inclusions (LVHS) containing solid phases of halite± hematite± anhydrite± sylvite± chalcopyrite. The multi-phase saline inclusions with high temperature and salinity (358-598°C and 42-70 wt.% NaCl equivalent) of magmatic origin are the primary fluid inclusions forming the deposit. The two-phase liquid-rich inclusions with lower temperature and salinity (290-490°C and 11-20 wt.% NaCl equivalent) of magmatic-meteoric origin are related to the final stages of hydrothermal fluid circulation and mixing with lower salinity fluids. The temperature decrease due to secondary boiling and mixing of magmatic and meteoric fluids led to the instability of the chloride complex carrying copper and subsequent mineralization under favorable conditions. Introduction Porphyry deposits are the major global source of Cu, Mo, and Re, along with being noteworthy reservoirs of Au and Ag (Sillitoe, 2010; Arndt and Ganino, 2012; Crespo et al., 2020). Exploration techniques aimed at optimizing the discovering new deposits are evolving towards a deeper understanding of ore genesis. Fluid inclusion studies serve as an enhanced technique to delineate the nature of ore-forming fluids and the processes governing deposit formation (Wilkinson, 2001), alongside other key geological aspects such as tectonic setting, mineral alteration, vein structure, ore-forming zones, and metal transportation and concentration dynamics (Singer et al., 2002; Sillitoe, 2010; Zajacz et al., 2017). Extensive studies have examined the physicochemical conditions, origins, and evolution of hydrothermal fluids in porphyry deposits globally, including in Iran, through fluid inclusion studies. The Kuh-e-Esfand porphyry copper deposit is located in Kerman province, Iran, approximately 90 kilometers southeast of Jiroft. Currently, the deposit is under exploration, and drilling activities are underway to obtain precise information on the type, composition, quantity, and economic potential of mineral reserves for evaluation and extraction purposes. Since fluid inclusion studies contribute to understanding hydrothermal processes as mineralizing agents, in this study focuses on detailed fluid inclusion studies, including petrography and microthermometry, to understand the nature and evolution of ore-forming fluids, as well as the physicochemical processes influencing mineral precipitation in the Kooh-Esfand deposit. Materials and methods In this study, 15 surface samples and 48 drill core samples were utilized for detailed investigations, with BH2, BH3, and BH4 boreholes being drilled at depths of 506 m, 475 m, and 496 m respectively. BH2 and BH3 were drilled into the intrusive mass, while BH4 was drilled into the volcanic unit, encountering a quartz diorite intrusive mass at 340 m depth. Among the selected samples, 42 thin section samples and 11 polished thin sections were prepared and examined. Petrographic studies of fluid inclusions were conducted using optical microscopy, and samples were separated from the veins in mineralogy and fluid laboratories. Temperature and salinity parameters of fluid inclusions in quartz minerals were measured at Pamukkale University in Denizli, Turkey, and part of it was conducted at Tarbiat Modares University in Tehran. In Pamukkale University's laboratory, fine grain size measurements were carried out using a Linkam THMSG 600 freeze-thaw stage equipped with an Olympus microscope. This stage was calibrated using H2O-CO2 fluid inclusions at temperatures of 1.1°C, 0.0°C, and -56.6°C. The upper and lower temperature thresholds for fine grain size measurements were 600°C and -120°C respectively. The heating rate was set at 1°C per minute for determining the homogenization temperature or ice melting temperature. At Tarbiat Modares University, temperature measurements on sections were conducted using a THMCG600 heating-cooling stage equipped with a Leitz microscope, with a temperature range of -196°C to +600°C. Calibration of the stage was performed using C4H3CH3 at 95°C and KNO3 at 335°C. Result The study area encompasses three distinct geological units: volcanic, volcaniclastic, and intrusive units. The intrusive unit range in composition from diorite to quartz-diorite and granodiorite. Various alteration zones, such as potassic alteration, quartz-sericite-feldspar alkaline ± chlorite alteration, phyllic alteration, argillic alteration, and propylitic alteration, have significantly influenced the lithological units in the study area. On the basis of vein classification, the early stage of mineralization predominantly is characterized by quartz ± chalcopyrite ± magnetite ± pyrite veins. The main mineralization stage is characterized by quartz + pyrite + chalcopyrite veins associated with potassic alteration and quartz-sericite-alkali feldspar-chlorite zone. Post mineralization stage is characterized by quartz-pyrite veins. Due to pressure variations from lithostatic to hydrostatic conditions, substantial copper mineralization likely occurred during the main mineralization stage, with comparatively lesser molybdenum mineralization observed in quartz-pyrite-chalcopyrite and quartz-pyrite-chalcopyrite-molybdenite veins. These mineralization stages, often accompanied by abundant vapor-rich and multi-phase fluid inclusions, initiated ore formation through fluid boiling processes. Based on petrographic studies, fluid inclusions in quartz minerals are categorized into three main groups and seven subgroups: 1- Vapor-rich fluid inclusions comprising single-phase vapor inclusions (V), vapor-rich two-phase inclusions (VL), and vapor-rich inclusions with a opaque phase (VLS) (including chalcopyrite, possibly magnetite, and unidentified opaque phases), 2- Liquid-rich fluid inclusions including liquid-rich two-phase inclusions (LV) and liquid-rich inclusions with a opaque phase (VLS) containing opaque minerals (such as chalcopyrite and unidentified opaque phases), and 3- Saline fluid inclusions consisting of simple brine three-phase inclusions (LVH) containing liquid+ vapor+ halite, and multi-phase brine inclusions (LVHS) containing vapor+ liquid+ halite± hematite± anhydrite± sylvite± chalcopyrite. Discussion The relationship between different types of fluid inclusions in the Kuh-Esfand deposit is established through detailed petrographic and micrometerometric investigations. In microthermometric studies, the relationship between different types of fluid inclusions, including liquid-rich, vapor-rich, three-phase, and multiphase inclusions, is investigated to examine the origin and evolution process of the hydrothermal fluid. This investigation is based on variations in homogenization temperature and salinity content. By analyzing variations in homogenization temperature and salinity, these investigations provide valuable insights into the processes governing fluid evolution in the Kuh- e- Esfand copper deposit. On the basis of the microthermometric analyses, the observed changes in homogenization temperature and salinity indicate a systematic decrease from multiphase fluid inclusions to liquid-rich fluid inclusions. Interestingly, vapor-rich fluid inclusions exhibit homogenization temperatures comparable to the upper end of the temperature range observed in multiphase fluid inclusions. Primary fluid inclusions of magmatic origin encompass vapor-rich inclusions characterized by elevated homogenization temperatures (330-600 °C) and diminished salinities (12-22 wt.% NaCl eq.), multisolid fluid inclusions exhibiting extended temperature ranges (385-598 °C) and heightened salinities (42-70 wt.% NaCl eq.), and three-phase fluid inclusions demonstrating significant temperature variations (230-590 °C) alongside elevated salinities (35-65 wt.% NaCl eq.). The presence of multi-phase fluid inclusions is indicative of the initial hydrothermal fluids responsible for the formation of the Kuh-e-Esfand deposit. Conversely, fluid inclusions of magmatic-meteoric source encompass liquid-rich inclusions characterized by homogenization temperatures and reduced salinities (290-490 °C and 11-20 wt.% NaCl eq., respectively). This particular fluid inclusion type signifies the terminal phase of hydrothermal fluid circulation, characterized by interaction and dilution with lower salinity meteoric fluids. The depth of the Kuh-e-Esfand deposit ranges from 0.8 to 1.7 kilometers, with an average depth of 1.4 kilometers (equivalent to 1400 meters). This translates to pressures ranging from 215 to 603 bars on average, with an average hydrostatic pressure of 412 bars and a lithostatic pressure of 1112 bars. In the Kuh-e-Esfand deposit, fluid inclusions exhibit a sequence of influential processes, including secondary boiling phenomena, fluid immiscibility, the interaction of magmatic fluids with meteoric waters, and isothermal mixing, throughout the hydrothermal fluid evolution. Vapor-rich fluid inclusions, characterized by the presence of opaque minerals (e.g., chalcopyrite), are infrequently observed in the Kuh-e-Esfand deposit, suggesting that the brine phase predominantly facilitates the transport of copper metal. Finally, the decrease in temperature due to secondary boiling and mixing of magmatic fluids with meteoric fluids has led to the destabilization of the chloride complex, the primary carrier of copper in the studied deposit, and its deposition under favorable conditions.
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