Carbonate-hosted Zn-Pb-Cu-Ba (-Ag) mineralization in the Mehdiabad deposit, Iran: new insights, new discoveries

The Mehdiabad deposit in the Yazd-Anarak metallogenic belt (YAMB), central Iran, is the largest car-bonate-hosted Zn-Pb-Ba-Cu (-Ag) deposit in the world, with a reserve of 630 Mt sulphide and non-sulphide ore. It was formed during the Early Cretaceous by the replacement of barite and hydrothermally dolomitized breccia bod-ies of the Taft and Abkuh formations. This deposit consists of different ore zones, including the feeder zone, massive ore (including sulphide-oxide parts), massive barite ore, and copper-rich sulphide-barite ore, formed in an exten-sional environment related to the Naein-Baft back-arc basin. The deposit is stratabound and comprises a wedge-shaped sulphide-barite orebody with complex replacement textures of sulphides and barite. The primary sulphide ore, including a copper-rich core (with a reserve of more than 50 Mt of copper ore), developed in a barite sheath and characterized by the replacement of barite and pyrite by an assemblage of chalcopyrite, bornite, sphalerite and galena. Several stages of barite and sulphide deposition in the Mehdiabad deposit are similar to those reported in other Irish-type and barite-replacement sediment-hosted Zn-Pb deposits worldwide (e.g., Red Dog deposit, Alaska, USA).Peer ReviewedPostprint (published version

An introduction to Irish-type Zn-Pb deposits in early Cretaceous carbonate rocks of Iran

Early Cretaceous carbonates are the most common host rocks for Irish-type deposits in Iran. They are largely concentrated in the Malayer-Esfahan metallogenic belt (MEMB) in southwestern Iran, and Yazd-Anarak metallogenic belt in Central Iran. They include some world-class ore deposits such as Mehdiabad, Irankuh, and Ahangaran. These stratabound deposits are hosted mostly in carbonates with minor siltstones and volcanic compo-nents, that formed in extensional and passive margin environments that are related to the Nain-Baft back-arc basin. The deposits are stratabound and comprise wedge-shaped to tabular sulphide-barite orebodies and occur in several different stratigraphic horizons. Dolomitization and silicification are the main wall-rock alteration styles. Replace-ment textures are common, and orebodies represent complex textures of sulphides and barite, such as brecciated, colloform, zebra, minor laminated, and banded replacement. Barite is an important gangue mineral in the MEMB and YAMB deposits, partly replaced by coarse-grained galena, sphalerite, and chalcopyrite. Sulphides from these Irish-type deposits have a wide spread of light d34S values (with the majority falling between -25 to +5‰) with mostly bacterial sulphate reduction (BSR) origin. Fluid inclusion studies show that homogenization temperatures of ore minerals are typically 120 to about 280°C (majority 225-275°C), and salinities range from 2 to 24 wt.% NaCl eq, with the majority falling between 8 and 22 wt% NaCl eq. Using the criteria outlined in this study, early Cretaceous extensional sedimentary basins (e.g., Nain-Baft) are highlighted as target areas for exploration of world-class Irish-type ore deposits and correspond well with the periods of expulsions of Cretaceous CaCl2-rich brines.Peer ReviewedPostprint (published version

The world-class Koushk Zn-Pb deposit, Central Iran: a genetic model for vent-proximal shale-hosted massive sulfide (SHMS) deposits - Based on paragenesis and stable isotope geochemistry

© 2020. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The Koushk Zn-Pb deposit is the largest known and least deformed and non-metamorphosed Early Cambrian shale-hosted massive sulfide (SHMS) deposit at Central Iran. The current remaining reserves are estimated to be greater than 14 Mt ore, averaging 7% Zn and 1.5% Pb; the primary resources ore of the deposit is estimated to be more than 60 Mt. At this deposit, different hydrothermal ore styles (bedded ore, vent complex, and feeder zone) are well preserved within the Lower Cambrian black siltstones and shales. According to fluid-rock interaction and different ore-forming processes in SHMS systems, these ore facies with extensive hydrothermal alteration provide unique conditions to understand critical textural and geochemical frameworks to present a genetic model. In this research, we focus on different paragenetic stages of sulfide mineralization and fluid-rock interactions in different ore styles from the Koushk SHMS deposit. The paragenetic relationship provides the context for the interpretation of stable isotopes (S, C, and O) in hydrothermal sulfides and carbonates. Detailed petrography and paragenetic studies represent three major generations of sulfide mineralizations at different ore zones: (1) stage I includes very fine-grained (<6 µm) framboids, spherulite pyrite (py1), associated with minor fine-grained disseminated sphalerite (sp1), and galena (gn1); (2) Stage II is composed of a diagenetic intergrowth of coarse-grained framboids and spherulite pyrite, packed polyspherulite aggregates and pyrite nodules (py2) replacing diagenetic barite and carbonate nodules, and are followed with coarse-grained sphalerite (sp2) and galena (gn2) that replace former sulfides and barite, deposited as disseminated, laminated and sulfide-rich banded textures; (3) stage III of sulfide mineralization is characterized by vent complex development (VCD) over the feeder zone, hydrothermal brecciation, dissolution of rock-forming minerals, and extensive replacement of earlier sulfides and barite, leading to deposition of stage III of ore sulfides. The oxygen and carbon isotopes values, for fluid in equilibrium with hydrothermal calcite and dolomite in this deposit range from d18O +8 to +16.7‰ and d13C from -8.3 to -4.3‰, are generally compatible with basinal brines and formation water as fluid sources. In addition, highly positive d34S values of hydrothermal sulfides (+6.5 to +36.7‰) in different ore stages of the Koushk deposit are consonant with other SHMS deposits. Textural relationships and S isotope data reveal that the contribution of bacterial sulfate reduction (BSR) in the Zn-Pb mineralization is not so significant, but the thermochemical sulfate reduction (TSR) nd direct barite replacement could provide sufficient sulfur for the main sulfide mineralization in the SHMS deposits. Also, the data presented in this paper are against a syngenetic, purely synsedimentary-exhalative model, and give prominence to that vent-proximal SHMS deposits formed predominantly during the diagenesis in the uppermost sediment pile and replacement of host rocks during vent complex development (VCD) processes.To memorialize one of the first author’s best teachers, Donald F. Sangster, for the love and support, and for the guidance and patience in dealing with the many problems arose during this and other projects about the sediment-hosted Zn-Pb deposits of Iran, from 2007 to 2018. The Serveis Científico-Tècnics de la Universitat de Barcelona and the research grant 2009SGR-00444 of the Departament d’Universitats, Recerca i Societat de la Informació (Generalitat de Catalunya) supported sulfur isotope analyses. The Instituto de Geofísica and Instituto de Geología of the Universidad Nacional Autónoma de México (UNAM) supported electron microprobe analyses, which were done with the assistance of Carlos Linares, and carbon and oxygen isotopes analyses. The authors thank Bafq Mining Company (BMC) for allowing access to the deposit and providing invaluable support on-site through access underground exposures. We sincerely thank Jan Peter for his helpful comments and advice, and discussions on the geology of Central Iran. The manuscript has benefited from helpful comments by David Lentz and an anonymous reviewer, and careful editorial handling by Maria Boni and Huayong Chen.Peer ReviewedPostprint (author's final draft