A sub-seafloor hydrothermal syn-sedimentary to early diagenetic origin for the Gushfil Zn-Pb-(Ag-Ba) deposit, south Esfahan, Iran
Konari, Mina Boveiri; Rastad, Ebrahim; Peter, Jan M.
published: Jan 1, 2017
The Gushfil stratabound Zn-Pb-(Ag-Ba) deposit is hosted by an Early Cretaceous siliciclastic-carbonate sequence deposited in an extensional back-arc basin setting in the Malayer-Esfahan Metallogenic Belt in the southern part of the Sanandaj-Sirjan Zone, Iran. Mineralization is mainly hosted in dolostone and black siltstone. Based on petrographic studies there are three sulphide ore facies: feeder zone, bedded and massive. Textures observed within these styles are semi-massive, massive, framboidal, col- loform, laminated and vein-veinlet. Sulphide minerals are predominantly sphalerite, galena, tetrahedrite, chalcopyrite and pyrite, with lesser bornite. Barite, dolomite and quartz are major non-sulphide gangue minerals. Based on microscopic studies, sulphide mineralization in the Gushfil deposit was emplaced in two stages: stage I (pre-ore) is comprised of barite and fine-grained, framboidal, colloform and laminated pyrite and minor sphalerite and galena, and stage II (main) comprises coarse-grained, semi-massive to massive, and vein/veinlet sphalerite, galena, tetrahedrite, chalcopyrite, pyrite and lesser bornite. Dolomitization, silicification and sericitization are the major hydrothermal alteration styles in the Gushfil deposit, and they occur in all host rocks, but only in the immediate vicinity of the Gushfil-Baghabrisham syn-sedimentary normal fault (GBF) that served as the main conduit for the mineralizing fluids. High contents of redox sensitive elements such as Mo, V, and, U in the black siltstone host rocks indicate that mineralization formed in an anoxic ambient environment. The enrichment of LREE compared to HREE (high ration of LaN /YbN) and the ΣREE content of dolomitized limestone host rocks of the Gushfil deposit indicate inter action of hydrothermal fluids with marine limestone. The host rocks lack Ce anomalies, in marked contrast to the mostly negative Ce anomalies of oxic seawater, also indicating deposition under reduced conditions for the siltstones and a reduced hydrothermal fluid overprint on the dolostone host rocks that occurred concomitantly with ore deposition. Sphalerite from the feeder zone have 160 to 239 °C trapping temperature with salinities ranging from 8.7 to 23.3 eq. wt.% NaCl whereas, those of sphalerite from the massive ore ranges from 125 to 220 °C, with salinities ranging from 16.8 to > 23.3 eq. wt.% NaCl. The data suggest that ore fluid temperatures decreased with increasing distance from the feeder zone (in the GBF) and toward massive ore. Abundant pyrite framboids, interlaminated organic matter (bitumen) with sulphide minerals, and negative sulphur iso- tope values of sulphides collectively indicate that sulphur was sourced from bacterially reduced seawater sulphate. Sulphide minerals precipitated due to mixing of this fluid with metal-bearing higher temperature hydrothermal fluid. Collectively, the host rocks, structural setting, ore styles (bedded, feeder zone, and massive), mineralogy, textures, and fluid inclusion micro-thermometric data indicate that Gushfil is a SEDEX-type deposit in which sulphide minerals were formed below the seafloor during sedimentation through early diagenesis.