Investigation of Pb mineralization in Jurassic deposits of Horeh (north of Shahrekord) using geochemistry, fluids inclusion and geophysics data

 
 
1-Introduction
Pb mineralization in Horeh area is located in 25 km northeast of Shahrekord, the middle part of Sanandaj-Sirjan zone in the mineralization belt of Malayer-Isfahan, in the geology map of 1/100000 of Chadgan (Ghasemi et al., 2005). There are nine mineralization belts, and 120 index mineralization of Pb-Zn has been identified based on paragenesis of mineralogy, time and type of mineralization in the Sanandaj-Sirjan zone. The Malayer-Isfahan mineralization belt is in the middle part of the Sanandaj-Sirjan zone, which formed in Mesozoic in carbonate sequences along with deep faults (Shahabpour, 1385). Often, this type mineralization is similar to Mississippi Valley-type (MVT) Pb-Zn deposits that many of these deposits have been created simultaneously with orogeny so that topographic slope is an essential factor in the ore fluids displacement (Leach et al., 2005, 2003, 2001; Appold and Gruven, 1999). The lithostratigraphy units in Horeh area include dolomite and limestone Permian, conglomerate, sandstone and shale Jurassic, limestone cretaceous, low grade metamorphic and young alluvium. The primary trend of the structure of the Hore Pb mineralization is NW-SE as same as the trend of the Sanandaj-Sirjan zone and the Zagros fault. This paper aims to identify the geological, geochemical and petrogenesis of the Pb mineralization on the base of the mineralogy, geochemistry, geophysics and fluid inclusion data.
 
2-Methodology
There are taken 35 samples and the number of 10thin section and 5 polished sections were prepared and studied in order to petrography and mineralogy. Major oxides (XRF method) elements were analyzed for 5 samples. 3 samples (calcite) were selected for Fluid inclusion study by linkam THMS-600 in Isfahan University. Data Geophysics was taken by  IPRSw-888 set and was measured Rs, Ip, Sp.
 
3-Result and discussion
Horeh Pb Mineralization occurred as lens and veins with a thickness of several centimeters to several meters in sedimentary rocks, with slopes and stretches of NW-SE and angle of 45°. This deposit is sulfide-type consists of galena, pyrite, and chalcopyrite as the primary ore and malachite, calcite and iron oxides as gangue. There are observed galen as fine-coarse grain, euhedral to xenomorph with triangular cleavage cavities, pyrite, and chalcopyrite as finely-coarse-grain, calcite as open space filling and comb texture, and veins in other rocks. Malachite often is formed by oxidation of the pyrite and chalcopyrite. Also, there are goethite, hematite, magnetite, illite, dolomite, and quartz. The mineralogical paragenesis sequence in Horeh area is two stages: the initial phase of the reduction that caused to deposit the sulfide minerals such as galena, and the second phase of the oxidation, which led to the formation of oxides and hydroxides minerals by initial carbonate and silicate minerals. Based on geochemical data, SiO2 =38.31% indicates to low maturity of sedimentary rocks compared to the upper crust (Taylor and McLennan, 1985; SiO2 = 64.8%). The high mean value of CaO = 25.22% (upper crustal crust = 4.19%) indicates to high amounts of carbonate cement, which cause to decrease of the relative amounts of SiO2 and Al2O3 in the samples. Al2O3 amounts are due to the clay and mica and Al-rich mineralogy, especially illite (Elsass et al., 1997). Fluid inclusion data of mineral calcite indicate to the two-phase of the fluid include (L + V) with irregular shapes in the size of 4 to 10 μm, and 136.6 ° C average homogeneity, -14.5° to -20° ice melting and 20.15% average salinity ( weight equivalent to NaCl)( Bodnar,1993). The result of fluid inclusion indicates to the basinal brines that is similar to the Pb deposits of the Mississippi Valley type. Geophysical investigations identified 4 Pb anomalies in the region, which begins at depths of 10 m and extends along NS and steep slope toward the west to the depth of 50 m by measure chargeability (PI), electrical resistivity (RS) and metal coefficient map (FM).
 
4-Conclusion
The Pb mineralization in the Horeh area is as Galena with chalcopyrite and pyrite. Based on field study and petrography data, Galena is the main mineral and carbonate, and silicate minerals are gangue. Pb Mineralization has occurred as the replacement, bedding and tangential in the Jurassic formations by basin brine fluid.  The combinations of field study and mineralogy, geochemical and geophysical data indicated to the similarity Pb deposit of the Horeh to the Mississippi type that was formed during the two-stage reduction and oxidation. Geophysical data were indicated to the, 4 Pb anomalies from 10 m the topography level with 50m thick with the steep slope to the west.