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


Por: Mahmoodi, Pouria, Rastad, Ebrahim, Rajabi, Abdorrahman, Alfonso, Pura, Canet, Carles, Peter, Jan M.

Publicada: 1 sep 2021
Resumen:
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 backarc paleotectonic setting as a result of the subduction of the Neo-Tethys oceanic plate beneath the SanandajSirjan 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 mu m 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 delta O-18 and delta C-13 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 degrees C and 6.6 to 29.5 degrees 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 delta S-34 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 (.galena-sphalerite) for two samples give temperatures in the range 209-224 degrees 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.

Filiaciones:
Mahmoodi, Pouria:
 Department of Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, 14115-175, Iran

 Tarbiat Modares Univ, Fac Basic Sci, Dept Geol, Tehran 14115175, Iran

Rastad, Ebrahim:
 Department of Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, 14115-175, Iran

 Tarbiat Modares Univ, Fac Basic Sci, Dept Geol, Tehran 14115175, Iran

Rajabi, Abdorrahman:
 School of Geology, College of Science, University of Tehran, Tehran, Iran

 Univ Tehran, Sch Geol, Coll Sci, Tehran, Iran

Alfonso, Pura:
 Dept. d'Enginyeria Minera, Industrial i TIC, Universitat Politècnica de Catalunya, Av. de les Bases de Manresa, o8242 Manresa, 61-73, Spain

 Univ Politecn Cataluna, Dept Engn Minera Ind & TIC, Ave Bases Manresa 61-73, Manresa 08242, Spain

Canet, Carles:
 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Del. Coyoacán, Ciudad de México, 04150, Mexico

 Univ Nacl Autonoma Mexico, Ctr Ciencias Atmosfera, Mexico City 04150, DF, Mexico

Peter, Jan M.:
 Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada

 Geol Survey Canada, 601 Booth St, Ottawa, ON K1A 0E8, Canada
ISSN: 01691368





ORE GEOLOGY REVIEWS
Editorial
Elsevier, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS, Países Bajos
Tipo de documento: Article
Volumen: 136 Número:
Páginas:
WOS Id: 000669448200001