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

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.