Abstract

The Chehelkureh base metal deposit in southeast Iran is in an abandoned historic copper district that is fault controlled and hosted in a sequence of slightly Eocene interbedded graywackes, siltstones, and shales. Several stocks and dikes of Oligo-Miocene granodiorite to quartz monzodiorite and granite composition, oriented parallel to the dominant northwest-southeast fault system, intruded into the sedimentary sequence. The sedimentary rocks were metamorphosed to the hornfels facies in some outcrops. The intrusions are spatially and temporally related to mineralized faults. The deposit consists of numerous irregular lenses and veins located along faults that cover an area 1,500 m long by 80 to 280 m wide. There are two periods of primary Cu-Zn-Pb mineralization that crosscut each other. The first stage includes quartz, calcite, dolomite, ankerite, siderite, ilmenite, rutile, molybdenite, pyrrhotite, arsenopyrite, pyrite, and chalcopyrite. The second stage consists of quartz, dolomite, ankerite, siderite, chalcopyrite, sphalerite, pyrite, galena, selenian galena, marcasite, nevskite, and paraguanajuatite. The gangue minerals are dominated by quartz and various carbonates, locally associated with chlorite. Hypogene alteration consists of silicification, carbonatization (ankerite, magnesite, siderite, and dolomite), chloritization, kaolinitization, sulfidation, and, less commonly, sericitization.

The intrusive rocks at Chehelkureh are calc-alkaline and have chemical features typical of I-type granitoids. Based on Nb-Y and Ta-Yb discrimination diagrams, the geotectonic environment of Chehelkureh granitoids is an intracontinental volcanic arc (Nb/Y ~ 0.4), which is the same setting as that in the multiphase granitoid batholiths of Zahedan, located southeast of Chehelkureh. Granitoids of the Chehelkureh area have moderate REE (rare-earth element) contents (∑REE = 110–174 ppm; average 153 ppm), moderate light REE/heavy REE ((La/Lu)cn = 7–8), and strong negative Eu anomalies (Eu/Eu* ~ 0.2), but no Eu anomaly is evident in monzodiorites. Spider diagram patterns for samples of igneous rocks show Nb and Ta, Sr, and Ti-V negative anomalies, with high amounts of Cu, U, and Th.

Fluid inclusion studies of quartz intergrown with sulfides in quartz veins show three main types of fluid inclusions: type 1 inclusions consist of liquid + vapor + daughter crystal (solid); type 2 inclusions are composed of liquid + minor vapor, with vapor/liquid ratios of 0.1 to 0.4; and type 3 inclusions are vapor rich with liquid/vapor ratios up to 0.05 (liquid phase is minor or absent). Homogenization temperatures (Th) for type 2 inclusions vary between 330° and 480°C, whereas their salinities range from 5 to 15 wt % NaCl equiv.

The δ18Owater values of quartz in sulfide-bearing veins are 8.8 to 11‰, with an average of 10‰. Most of the δ18Owater values of quartz veins are in the range of typical magmatic, metamorphic, and connate waters, so the source of the ore fluids cannot be distinguished based on oxygen isotopes alone. The δ18Owater and δDwater values calculated from chlorite range from 5.6 to 10.6‰ and −31 to −23‰, respectively. These data lie in the metamorphic water box, in the formation water box, or about midway between the magmatic water field and typical seawater. For kaolinite at ~200°C, the calculated δ18Owater varies from 4.2 to 10.7‰ and the δDwater values range from −88 to −49‰. The carbon isotope values (Pee Dee Belemnite) of carbonates vary between −5.7 and −0.9‰, whereas the δ18O values (Standard Mean Ocean Water) are between 12.4 and 14.9‰. The coupled oxygen and carbon isotope values shift from magmatic values and can be related to metasedimentary decarbonation reactions and metasomatic exchange of oxygen with magmatic fluid. The range of δ34S values of sulfide minerals is small (2.0–4.2‰, with a mean value of 3‰), consistent with a magmatic origin for sulfur. Isotope thermometry, based on quartz-carbonate pairs, yields an average of 450°C, which is consistent with fluid inclusion temperatures. Isothermal fO2-pH diagrams constructed at 450°C and 1 kbar suggest that the ore formed in the H2S-predominant field with fO2 and pH ranging from 10−29 to 10−24 and 5.0 to 6.2, respectively. The petrogenetic model proposed for the Chehelkureh deposit suggests that during the Oligocene a quartz-monzodiorite stock intruded the Eocene turbidites and provided a magmatic component to the ore-forming system, but also acted as the heat source for hydrothermal convection cells, although metamorphic and connate waters may also have contributed to the ore-forming fluids. In this model, magma is the main source of sulfur, base metals, and carbon, although Eocene turbidites also contributed some metals and carbon.

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