بررسی الگوی کانه‏ زایی کانسار مس دره تخت؛ نمونه‏ ای از یک کانسار چینه‏ کران نوع مانتو در جنوب شهرستان ازنا، لرستان براساس مطالعات کانی‏ شناسی و ژئوشیمی

Introduction

Manto-type (sediment-hosted) copper (Cu–Ag) deposits are of high economic and metallogenic importance due to their relatively high metal grades and significant contribution to copper production, particularly in the Central Andes (Wilson et al., 2003; Oliveros et al., 2008). These deposits are widely distributed along the circum-Pacific and Himalayan–Tibetan belts (Shen et al., 2020). In Chile, manto-type deposits are considered the second most important Cu deposits after porphyries (Wilson and Zentilli, 1999). In Iran, sediment-hosted copper deposits hosted in volcanic-sedimentary rocks, classified as manto-type deposits, are distributed across several structural provinces, including the Sanandaj–Sirjan, Urmieh–Dokhtar, Alborz, Sabzevar, Central Iran, Kopet Dagh, Tabas Block, and Lut Block (Salehi and Rasa, 2016; Maghfouri et al., 2020; Movahednia et al., 2022). The Darreh Takht Cu deposit is located in Lorestan Province within the Sanandaj–Sirjan structural-metasedimentary belt. This study aims to investigate the mineralogical, alteration, and geochemical characteristics of Cu mineralization and to elucidate the formation pattern of this deposit within the framework of manto-type Cu systems.

Methodology

To investigate the Cu mineralization at the Darreh Takht deposit, a large-scale geological map was prepared based on satellite imagery, field surveys, and integration with the 1:100,000 geological maps of Aligudarz (Soheili et al., 1992) and Dorud (Goodarzi, 2002). Systematic sampling of host rocks and ore was conducted according to lithological variations, alteration intensity, and mineralogy. For mineralogical and textural studies, thin sections, polished sections, and thin–polished sections were prepared and examined microscopically. Mineral identification was carried out using X-ray diffraction (XRD), while the chemical composition of ore minerals was determined using electron probe microanalysis (EPMA). Geochemical analyses of least-altered host volcanic rocks were performed for major and trace elements using XRF and for rare earth elements using ICP-MS. These datasets formed the basis for interpreting mineralization characteristics and reconstructing the formation model of the deposit.

Results and Discussion

The Sanandaj–Sirjan zone, with a length of approximately 1,500 km and a width of 150–250 km, extends from southeastern Iran (cities of Sirjan and Esfandqeh) to the northwest (Urmia and Sanandaj) and is considered one of the most important metallogenic regions of the country (Mohajjel and Fergusson, 2014). This zone comprises metamorphosed sedimentary, volcanic–sedimentary, and intrusive igneous rocks, ranging in age from the Paleozoic to the Mesozoic, and forms part of the Sanandaj–Sirjan metamorphic shear zone within the Zagros thrust-fold belt (Sarkarinejad and Azizi, 2008). Its tectonic evolution from the Late Paleozoic to the Middle Triassic involved faulting, carbonate deposition, basaltic lava flows, and coeval sedimentation, reflecting an extensional regime associated with the separation of the Central Iran block from Gondwana and the opening of the Neo-Tethys oceanic basin. The Early Cretaceous volcanic–sedimentary sequences in the southern part of the zone host Manto-type stratabound deposits, including Keshte-Mahki, Kal-Rizeh, Northeast and East Hassanabad, Khvorjan, and Simkan (Movahednia et al., 2020).

The Darreh Takht Cu deposit is situated in the northern part of the Sanandaj–Sirjan metamorphic belt, adjacent to the Zangros thrust sub-belt, and is hosted in a Middle to Late Triassic volcanic–sedimentary sequence. The main lithologies include andesitic to andesite–basaltic flows, pyroclastic units such as crystal tuff and lithic tuff, and Permian carbonate sedimentary rocks, all generally aligned with the regional structural trend and partially metamorphosed to green schist facies. Three main rock units were distinguished: volcanic units (andesite to andesite–basalt), pyroclastic units (crystal tuff, lithic tuff, and agglomerate), and sedimentary units (limestone, dolomitic limestone, and green schist). Volcanic and pyroclastic units predominantly host Cu mineralization, and all units are aligned NW–SE along the Sanandaj–Sirjan belt. This lithological sequence reflects a volcanic–sedimentary environment with low- to medium-grade metamorphism.

Mineralization at Darreh Takht occurs primarily in pyroclastic units (andesite to andesite–basalt) with sedimentary and metamorphic rocks located in the upper levels. Cu mineralization is mainly vein- and stringer-type and fills fractures, faults, and voids. The copper mineralization is predominantly vein-type, stockwork, and cavity-filling, and is structurally controlled by faults, fractures, and joints. The main factors influencing the mineralization process include: (1) suitable lithology of the host rocks, (2) effective structural features such as faults, fractures, joints, and related porosity and permeability characteristics, (3) the circulation of hydrothermal fluids through chemically favorable rocks leading to copper enrichment, and (4) the presence of granitoid intrusions in the area that provided a thermal source. Alteration types include chloritic, epidotic, sericitic, silicic, carbonatitic, and Fe-oxide/hydroxide zones. Chloritic and epidotic alterations are the most widespread and correspond to early hydrothermal activity, while sericitic and silicic alterations are restricted and associated with late hydrothermal phases.

Mineralogical studies (microscopy, XRD, and EPMA) reveal five main mineral groups: sulfide Cu–Fe minerals (chalcocite, covellite, bornite, pyrite, chalcopyrite, tetrahedrite), Cu-carbonates (malachite and azurite), Cu oxides and native Cu (cuprite and native copper), Fe oxides/hydroxides (goethite and limonite), and gangue minerals (quartz, calcite, and gypsum). The dominant textures are vein, stringer, disseminated, void-filling, and replacement, reflecting the synoptic progression of sulfide, carbonate, and oxide mineralization.

Geochemical studies of host rocks indicate andesitic to andesite–basaltic and trachyandesitic compositions, belonging to the calc-alkaline series. The Zr/Y versus Zr (Pearce, 1979) and Th–Co (Hastie et al., 2007) diagrams classify the rocks as continental arc-related, consistent with subduction of the Neo-Tethyan oceanic lithosphere beneath the Central Iranian continental block during the Late Triassic–Early Jurassic. Spider diagrams normalized to chondrite show enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) with positive Sr and Eu anomalies, indicating plagioclase fractionation, high oxidation states, and crustal contamination of arc magmas. Negative Nb anomalies coupled with Sr enrichment are characteristic of subduction zone magmas and crustal assimilation.

Among base metals, Cu and Ag show a very high correlation (r = 0.97), indicating a common source and geochemical linkage, likely due to their occurrence in sulfosalts (tetrahedrite, pyrargyrite, and prostite) and covellite.

Formation Conditions and Genetic Stages

Based on field, petrographic, and geochemical studies, the mineralization and deposit formation can be divided into three genetic stages:

1. Early diagenesis (primary volcanic–sedimentary stage): Extensive volcanic activity formed pyroclastic units and lava flows, producing andesitic and pyroxene-andesitic host rocks. Early diagenesis also led to thin hematite layers from the breakdown of Fe-bearing minerals (pyroxene and amphibole).

2. Secondary diagenesis (primary sulfide mineralization): Basin-derived Cu released from Fe–Mg minerals and altered feldspars migrated and precipitated as primary sulfide minerals (pyrite, chalcopyrite, and chalcocite) in voids and as disseminated grains. The main source of Cu is the volcanic–sedimentary host rocks.

3. Uplift and hydrothermal activity (secondary oxide–carbonate stage): Regional uplift and faulting focused sulfide and oxide–carbonate mineralization along fractures and voids in pyroclastic units. Hydrothermal fluids of meteoric–magmatic origin formed oxide and carbonate Cu minerals (malachite, azurite) and Fe oxides/hydroxides in veins and stringers.

Conclusion

Cu mineralization at Darreh Takht occurs along the northern margin of the Sanandaj–Sirjan belt within andesitic to andesite–basaltic volcanic–pyroclastic sequences. Host rocks are calc-alkaline and related to a continental arc setting. Mineralization is vein- and stringer-dominated, controlled by tectonic structures, and post-dates host rock formation (epigenetic and stratabound). The positive Cu–Ag correlation indicates co-enrichment through metal-bearing hydrothermal fluids. The formation model comprises a multi-stage evolution from primary volcanic activity, diagenetic sulfide precipitation, to secondary hydrothermal concentration, demonstrating that Darreh Takht is a typical manto-type Cu deposit.

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