Genetic model for Jurassic shale-hosted Zn-Pb deposits of the Arak Mining District, Malayer-Esfahan metallogenic belt: Insight from sedimentological, textural, and stable isotope characteristics

Abstract

The genetic model, including conditions under which mineralization formed, and relative timing of mineralization are critical questions for SEDEX (or shale-hosted massive sulfide, SHMS) deposits. There is increasing awareness that sub-seafloor replacement is an important process in the formation of some SEDEX deposits. We have studied the Hossein-Abad and Western Haft-Savaran Zn-Pb SEDEX deposits located in the Arak basin of the Malayer-Esfahan Metallogenic Belt, Iran, to address these questions of genesis. This metallotect formed in a back-arc paleotectonic setting as a result of the subduction of the Neo-Tethys oceanic plate beneath the Sanandaj-Sirjan Zone. The rocks that host the mineralization are Jurassic organic matter-bearing, fine-grained sandstones, siltstones, and shales. Asymmetric lenticular bedding, unidirectional flow (based on oblique silt lamination direction relative to horizontal bedding), graded bedding, and clay-rich interbeds indicate sediments were deposited from turbidity currents in a low-energy basin environment.There are three ore facies in the Hossein-Abad and Western Haft-Savaran Zn-Pb deposits: 1) bedded ore; 2) massive ore; 3) feeder zone. Bedded ore contains pyrite framboids and polyframboidal clusters. The size range of the pyrite framboids (3 to 6 µm in diameter) indicates they formed in the water column and not in the subsurface. Characteristic structures in bedded ore are: 1) sulfide-bearing silt injections into clay-filled burrows, 2) injection of sulfide-bearing silt into flame structures of claystone laminae, and 3) organic matter in claystone oriented obliquely relative to bedding. These structures are the result of seismic deformation induced by synsedimentary earthquakes, whereby sulfides that formed in permeable unconsolidated sediment were injected into the organic matter-bearing claystone unit.The δ18O and δ13C values of siderite, calcite and dolomite in veins from the feeder zone and massive ore range from 12.2 to 23.8‰ and −16.7 to 1.7‰, respectively. These values indicate that formational water, seawater and organic matter oxidation-decomposition all played a role in hydrothermal carbonate formation. Melting and homogenization temperatures for CO2 for CO2-bearing fluid inclusions range from −57.5 to −60 °C and 6.6 to 29.5 °C, respectively, and indicate the presence of < 15 mol percent CH4. The CO2 homogenization temperature range suggests the CO2-rich phase is a CO2-CH4 mixture. The CH4 and CO2 in the H2S-bearing fluid were likely generated by biodegradation and oxidation of organic matter via BSR and methanogenesis.The sulfur isotope compositions of pyrite, galena, sphalerite and chalcopyrite from the feeder zone and the massive ores range from δ34S −4.3 to + 7.2‰ and display equilibrium fractionations, indicating that the sulfur originated from two processes: (a) bacterial reduction of seawater sulfate (BSR) (distal from mineralization site), and (b) thermochemical reduction of seawater sulfate (TSR) (in both massive ore and feeder zone). Sulfur isotope geothermometric calculations (Δgalena-sphalerite) for two samples give temperatures in the range 209–224 °C for the massive ore facies. Such high temperatures negate a contribution of sulfur from BSR. However, BSR likely occurred in sediments distal to the feeder zone, where seawater sulfate was bacterially reduced to H2S, and this H2S migrated to the mineralization site during diagenesis. Collectively, the stable isotope data indicate mineralization formed in response to mixing occurred between ascending hot metalliferous hydrothermal fluid that rose up the fault and fracture network, with H2S-bearing fluid and sulfate-bearing percolating seawater, triggering sulfide deposition.