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
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