Pressure-temperature condition and hydrothermal-magmatic fluid evolution of the Cu-Mo Senj deposit, Central Alborz: fluid inclusion evidence

The Senj deposit has significant potential for different types of mineralization, particularly porphyry-like Cu deposits, associated with subduction-related Eocene–Oligocene calc-alkaline porphyritic volcano-plutonic rocks. The study of fluid inclusions in hydrothermal ore deposits aims to identify and characterize the pressure, temperature, volume and fluid composition, (PTX) conditions of fluids under which they were trapped (Heinrich et al., 1999; Ulrich and Heinrich, 2001; Redmond et al., 2004). Different characteristics of the deposit such as porphyrtic nature, alteration assemblage and the quartz-sulfide veins of the stockwork were poorly known. In this approach on the basis of alterations, vein cutting relationship and field distribution of fluid inclusions, the physical and chemical evolution of the hydrothermal system forming the porphyry Cu-Mo (±Au-Ag) deposit in Senj is reconstructed.
Over 1000 m of drill core was logged at a scale of 1:1000 by Pichab Kavosh Co. and samples containing various vein and alteration types from different depths were collected for laboratory analyses. A total of 14 samples collected from the altered and least altered igneous rocks in the Senj deposit were analyzed for their major oxide concentrations by X-ray fluorescence in the SGS Mineral Services (Toronto, Canada). The detection limit for major oxide analysis is 0.01%. Trace and rare earth elements (REE) were analyzed using inductively coupled plasma-mass spectrometery (ICP-MS), in the commercial laboratory of SGS Mineral Services. The analytical error for most elements is less than 2%. The detection limit for trace elements and REEs analysis is 0.01 to 0.1 ppm. Fluid inclusion microthermometry was conducted using a Linkam THMS600 heating–freezing stage (-190 °C to  °C) mounted on a ZEISS Axioplan2 microscope in the fluid inclusion laboratory of the Iranian Mineral Processing Research Center (Karaj, Iran).
The Cu-Mo Senj deposit covering an area about 5 km2 is located in the central part of the Alborz Magmatic Arc (AMA). The Nb/Y versus Zr/TiO2 diagram (after Winchester and Floyd, 1977) illustrates a typical trend for the magmas in the Senj magmatic area–starting from basaltic and evolving to dacite/rhyodacitic compositions, with few data plotting in the alkali basalt field. Most of the igneous rocks plot within the medium- and high-K fields in the K2O versus SiO2 diagram. The igneous rocks from the Senj area define a typical high-K calc-alkaline on SiO2 versus K2O diagram (Le Maitre et al., 1989). All studied rocks show similar incompatible trace element patterns with an enrichment of large ion lithophile elements (LILE: K, Rb, Ba, Th) and depletion of high field strength elements (HFSE: Nb and Ti), which are typical features of magmas from convergent margin tectonic settings (Pearce and Can, 1973). At least three veining stages namely QBC, QM, and QP which are related to alteration and mineralization are distinguished at the Senj mineralized area. Three distinct alteration assemblages including K-feldspar-biotite-sericite-quartz, quartz-sericite-K-feldspar-pyrite, and K-feldspar-biotite-sericite-quartz, are distinguishable with these veins. About 80 % of the copper at Senj is associated with the early QBC-stage veins, with another 5 to 15 % in the QM-and QP-stage veins. About 70 % of the molybdenite occur in QM veins.
Fluid inclusion distribution, fluid chemistry, and homogenization behavior document that S2-type fluids are samples of magma-derived aqueous-saline fluids characterized by high salinity and temperature, and high Cu content. Such parental fluids scavenged Cu and Mo from the melt below and transported them to the hydrothermal system above. The increased abundance of S- and LV-types inclusion coinciding with the highest grade Cu mineralization (early QBC-stage veins) at the Senj deposit suggests that brine-vapor unmixing and phase separation plays an important role in Cu-ore precipitation and alteration zonation. In addition to unmixing, cooling and water-rock interaction also played important roles in chalcopyrite precipitation at the Senj deposit.
Compositions, deposit-scale distribution, and trapping conditions of fluid inclusions can be explained by the continued influx of a parental high salinity magmatic hydrothermal fluid, with no significant change in the bulk composition of the input fluid over the integrated lifetime of ore metal precipitation and vein formation. Fluid inclusion evidence and vein-cutting relationships indicate that molybdenum mineralization (QM vein) occurred at moderate temperatures coinciding with phyllic alteration, rather than from later, lower temperature fluids. Furthermore, early fluids decompressed rapidly relative to cooling, forming quartz-stockwork veins with K-silicate alteration at depth and QP veins at shallower levels in the Senj deposit. Later, as the hydrothermal system waned, the rate of decompression relative to fluid cooling slowed, causing the fluid to remain above its solvus, forming barren quartz-dominated veins with quartz-kaolinite±illite alteration which overprint much of the deposit.