Elsevier

Ore Geology Reviews

Volume 124, September 2020, 103654
Ore Geology Reviews

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

https://doi.org/10.1016/j.oregeorev.2020.103654Get rights and content

Highlights

  • •

    Vent-proximal SHMS deposits formed predominantly during the diagenesis and replacement of host rocks during VCD processes

  • •

    The O and C isotopes values, for fluid in equilibrium with hydrothermal carbonates are generally compatible with basinal brines and formation water as fluid sources.

  • •

    The S isotope data reveal that the contribution of TSR and direct barite replacement are so significant in Zn-Pb mineralization of SHMS deposits.

Abstract

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 δ18O +8 to +16.7‰ and δ13C from −8.3 to −4.3‰, are generally compatible with basinal brines and formation water as fluid sources. In addition, highly positive δ34S 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.

Introduction

Shale-hosted massive sulfide (SHMS) ore deposits, that previously described as sedimentary exhalative (SEDEX; Carne and Cathro, 1982, Goodfellow et al., 1993), are significant sources of zinc and lead, and usually form laterally extensive stratiform to stratabound accumulations of sulfides hosted in black shales and siltstones. SHMS mineralization is considered to form by deposition of sulfides at the seafloor from hydrothermal fluids that are thought to exhale from a vent complex. Preservation of feeder and vent complex is rare at most of these deposits, so-called vent-distal SHMS, and only very few deposits, known as vent-proximal SHMS deposits, show a clear relationship between the feeder pipe and the overlying sulfide mineralization (e.g., Scott et al., 1985, Shanks, 2001, Rajabi et al., 2015a, Rajabi et al., 2015b, Magnall et al., 2016a). The Zarigan–Chahmir basin (ZCB, Fig. 1), located in Posht-e-Badam block of the Central Iranian Microcontinent (CIM), hosts several SHMS deposits, including the Koushk, Chahmir, and Zarigan deposits, and Darreh-Dehu, Wedge, and Cheshmeh-Firouz occurrences (Fig. 1b), hosted in Early Cambrian volcano-sedimentary sequence (ECVSS). Koushk is the largest and economically most important SHMS Zn-Pb deposit in Iran. Although there are some ancient mining activities at the Koushk area, this deposit was first mined for zinc and lead in 1939. There is no accurate data on the primary resources of the deposit, but it is estimated to be more than 60 Mt, and its size (1500 m length, 65–150 m thickness) comparable to that of the Meggen (Germany), Anniv, and XY (Howards Pass, Canada) Zn-Pb deposits. Rajabi et al. (2012b) reported 20 Mt ore reserves for this deposit in 2012 and classified it as a vent-proximal SHMS mineralization. Its current remaining reserves are greater than 14 Mt ore, averaging 7% Zn and 1.5% Pb.

Most SHMS deposits worldwide have experienced extensive recrystallization and destruction of primary textures during post-ore deformation and metamorphism. Only a few deposits, such as McArthur River, Australia (Large et al., 1998), Macmillan Pass, Canada (Magnall et al., 2016a) and the Koushk and Chahmir deposits, Central Iran, preserve most of their primary, synsedimentary features. Herein, we are going to focus on mineral phases from the vent complex and bedded ore, which are particularly well preserved in the Koushk deposit, in order to develop a better understanding of fluid rock interaction in the central up-flow zone of SHMS systems. We also present stable isotope (S, O, and C) data of specific mineral phases at the Koushk deposit, new data that can improve our understanding from the Cambrian SHMS mineralizations and ore-forming processes in vent-proximal SHMS deposits.

Section snippets

Geological and tectonic setting

The Zarigan-Chahmir basin (ZCB) is a significant back-arc rifting succession in the central lithotectonic domain of Posht-e-Badam block, also known as the Kashmar-Kerman structural zone, in the CIM. The basin consists of non-metamorphosed, Early Cambrian marine sedimentary, and intercalated volcanic rocks (ECVSS; Rajabi et al., 2015a), covering the oldest Precambrian continental basement of Iran, and it is bordered by the Kuhbanan Fault to the east and the Posht-e-Badam Fault to the west (Fig. 1

Stratigraphy of the ECVSS and geology of the Koushk deposit

The geology of the basin and stratigraphy of the ECVSS have been described in detail by Rajabi et al. (2015a). The stratigraphic correlation scheme (Fig. 3) is applicable throughout the basin, based on sequence stratigraphic analysis of the ECVSS and geochemical data. The scheme is subdivided the system into two sub-sequences, defined from contemporaneous depositional and structural events recognizable across the basin (Fig. 3). Two main stages of a rift event for the ECVSS are proposed: (I) A

Mineralization

The Koushk deposit is located in the central part of the ZCB, within rocks deposited during the sag phase of the ECVSS; these include black siltstones and shales of the Koushk member that are overlain by 200 m of carbonates. The mineralization occurs as a tapering, wedge-shaped orebody within the black siltstones and shales, reaching a maximum thickness of about 111 m in the SE portion of the deposit and decreasing to <60 m at its northwestern limit (Fig. 5a). The Koushk orebody strikes SE–NW,

Sampling and methods

Representative samples of all mineralization styles and host rocks were collected from underground tunnels and drill cores from the Koushk deposit. The mineralogical, textural, and paragenetic analyses were carried out using reflected and transmitted light microscopy and scanning electron microscopy (SEM) at the University of Tehran, Iran, the Universidad Nacional Autónoma de México, Mexico, and the Universitat de Barcelona, Spain.

Thirty-one sulfide and eleven carbonate samples from different

Sulfide textures and paragenesis

Rajabi et al., 2012b, Rajabi et al., 2015b) interpreted the textural relations of sulfides at the Koushk and Chahmir deposits to indicate that the sulfides formed in two main stages, namely, (I) synsedimentary depositional sulfides, and (II) hydrothermal replacement mineralization. Further detailed SEM and reflected light ore microscopy reveal multiple generations of sulfide growth in the Koushk black shales and siltstones. New studies of the relationship between the sulfides and sedimentary

Sulfur isotopes

The new sulfur isotope data for this deposit (Table 1) span the range of δ34S values of previous studies (Rajabi et al., 2012b) and are most positive (Fig. 12). All the δ34S values for samples from the Koushk deposit are between 6.5 and 36.7‰ (avg. 22.0‰). Significant differences in δ34S values occur among different sulfide generations and ore facies.

In Koushk, the highest δ34S values correspond to the bedded ore, where they vary between +9.7 and +36.7‰ (avg. +28.7‰). The δ34S values from the

Oxygen and carbon isotopes

The O and C isotope compositions of hydrothermal carbonates (calcite and dolomite) from the Koushk deposit are listed in Table 2. The δ18OvSMOW values in calcite (n = 4) are between +17.1 and +19.8‰ (avg. +18.5‰) and in dolomite (n = 4) between +22.7 and +24.0‰ (avg. +23.5‰). The δ13CvPDB values in calcite are between −8.3 and −7.0‰ (avg. −7.6‰), and in dolomite from −4.9 to −4.3‰ (avg. −4.6‰). The δ18OvSMOW values in calcite from the host rock (Koushk member shales) in the basin (Rajabi et

Sources of sulfur

Sulfide minerals in almost all SHMS deposits show wide ranges of δ34S, with values from negative to highly positive (Fig. 13), especially in pyrite from carbonaceous shales hosting the sulfide mineralizations (Leach et al., 2005, Lyons et al., 2006). But, a striking feature of the Koushk deposit is the broad range (Fig. 12, Fig. 13) of positive δ34S values of sulfide minerals. This broad range of δ34S values in Koushk is similar to that for other SHMS deposits worldwide (e.g., Anger et al., 1966

Stratabound ore and sulfate reduction

The stratabound ore system in vent-proximal SHMS deposits includes the vent complex (massive ore) and an underlying feeder (stockwork) zone, with an alteration halo around these zones. According to the replacement ore textures and microthermometric data, the main stage of mineralization in vent-proximal SHMS deposits form from relatively high-temperature fluids. Fluid inclusion microthermometry and geochemical investigations provided evidence of a steep thermal gradient (300 to 100 °C) within

Carbon and oxygen isotopes

Both carbon and oxygen isotopic compositions of the calcite samples from the Lower Cambrian limy siltstone (δ13CVPDB from −1.5 to +0.4‰, δ18OVSMOW from + 18.1 to + 20.5‰) are within the range of values reported from Lower Cambrian limestones (Veizer et al., 1999), including those of Lower Cambrian carbonate rocks of the Alborz region in Iran (Kimura and Watanabe, 2001). The δ13CVPDB and δ18OVSMOW values of one sample from the Koushk member are −4.8 and +16.5‰, respectively, which is comparable

A genetic model for vent-proximal SHMS deposits

Rajabi et al., 2012b, Rajabi et al., 2015a, Rajabi et al., 2015b) relied on tectono-sedimentary and textural evidence to describe the origin of the Early Cambrian SHMS deposits in Central Iran, and suggested that these deposits formed as a result of buoyant plume rising hydrothermal fluids to the seafloor (Fig. 16). More recently, other studies (Gadd et al., 2016, Magnall et al., 2016b) have revisited these genetic models for SHMS deposits and suggested diagenetic models for these deposits. But

Conclusions

An exciting feature of vent-proximal shale-hosted massive sulfide (SHMS) deposits, as Koushk in Central Iran, is the development of vent complex over the feeder zone and the preservation of sulfide mineralization with high positive δ34S values. In this study, we have recognized multiple generations of sulfides in different mineralization stages in both stratiform bedded ore and stratabound vent complex at Koushk. Detailed textural and S isotopic data from Koushk support a poor

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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

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