فصلنامه زمین شناسی اقتصادی
سال چهاردهم شماره 4 (پیاپی 35، زمستان 1401)

The Arabshah Fe mineralization is the only known magnetite-apatite mineralization at the Takab–Takht-e-Soleyman–Angouran subzone in southeast of Takab. The oldest rock units in the mineralization area include sedimentary succession of the Qom Formation that was intruded by the Pliocene Ayoub Ansar volcanic dome. Magnetite- apatite mineralization at the Arabshah occurs as vein-veinlets with E-W stright within the Ayoub Ansar dacitic dome. Brecciated zones containing narrow magnetite vein- veinlets occur at footwall and hanging wall of the main vein. Hydrothermal alterations include sodic-calcic, silicification and argillic. Magnetite is the only ore mineral in this mineralization which is accompanied with apatite, clinopyroxene, albite and quartz as gangue minerals. Mineralization textures in the Arabshah deposit include vein-veinlet, brecciated, disseminated, and replacement. REEs concentration within apatite crystals are more than 1%, and demonstrate LREE enrichment with high LREE/HREE ratio and distinctive negative Eu anomalies which is indicative for Kiruna- type iron ores. The result of fluid inclusion studies indicates the presence of two-phase and poly-phase inclusions include LV, VL, LVH, LVS and LVHS fluid inclusions with homogenization between 230-550 °C. The salinity of halite bearing poly-phase fluid vary between 35-60 wt.% NaCl equiv. Fluid inclusion data indicates that Arabshah magnetite-apatite mineralization originated from magmatic fluids. Evidences like mineral assemblages, hydrothermal alteration, ore structure and textures, geochemical characteristics and fluid inclusion data, indicate that the Arabshah magnetite-apatite mineralization can be classified as Kiruna-type iron ores.

Petrographic and mineralogical data indicate the widespread presence of five generations of apatite, two generations of monazite with minor xenotime in the Esfordi deposit. The O-H isotopic studies on the 1st- and 2nd-generations of apatites and massive fine-grained and vein-type apatites as well as their Sr and Mn contents, showed that the source of phosphorous was the sedimentary phosphorites. The ratio of 143Nd/144Nd vs 147Sm/144Nd and εNd vs P2O, and the difference of Nd isotopic ratios in the massive fine-grained and vein-type apatites indicate that they are not reproductively related to the host rhyolite and diorite. The similarity of 143Nd/144Nd vs 147Sm/144Nd and εNd vs P2O5 in the 1st- and 2nd-generations of apatite and the host rocks indicated that recrystallization of the apatites occurred during the magmatic and hydrothermal fluids circulation which were derived from the felsic to intermediate subvolcanic rocks. Difference in the age of the 2nd-generation apatites and the paragenetic- monazites ( 238U/206Pb and 207Pb/206Pb dating), the crystalline apatites and magnetite, the ilmenite exclusions in the magnetites, the dissolution evidences of different apatites and monazites generations, the content of Ti vs V, Al+Mn vs Ti+V and Mg+Al+Si vs Ti, and the O-H isotopes of the magnetite-apatite ores, all indicate the mixing of high-temperature magmatic and hydrothermal fluids rich in REE, P with Ca ±Fe evaporatic brines in different time periods, which caused a polygenic origin for the Esfordi deposit.

Iron and arsenic sulfides are considered as the most important gold hosts in the sediment-hosted disseminated invisible gold deposits. The Zarshuran gold deposit (155 tons Au with average grade of 2.63 g/t) is formed in the Lower Cambrian black shale and siltstone (Zarshuran unit) and Fe-rich carbonates (Chaldagh unit) host rocks. As-sulfide (e.g., realgar and orpiment) and arsenian pyrites are the most important host minerals of gold in this deposit. Based on EPMA data, pyrite with As content below the detection limit to 3.99 wt% occurs in six different types, respectively, Py0 (gold content of 0.01 ppm), Py1 (gold content of 0.02 ppm), Py2 (gold content of 0.03 ppm), Py3 (gold content of 0.02 ppm), Py4 (gold content of 0.04 ppm), and Py5 (gold content of 0.01 ppm). According to the evidences, gold can be present as participating in chemical bounded (Au+ and Au+3) or nanoparticle inclusions (Au0). The weak geochemical correlation (R2 = −0.6) between As and S elements in pyrites indicates that there is pyrite with a complex composition [Fe(S,As)2Au2S0], which As− has replaced S2−. Mineralogy and the abundance of Fe and S in the rock units suggest that gold mineralization in the Zarshuran deposit is well occurs in response to sulfidation process. Sulfidation occurs when H2S-rich ore-forming fluids react with Fe-bearing carbonate host rock to form pyrite, marcasite, and pyrrhotite minerals.

The study area is located about 30 km south of Ramsar, in the central Alborz zone. In addition to the Nusha granitoids (with an age of about 56 million years), the outcrops in this area, mainly include Paleozoic and Mesozoic rock units. Petrographically, the Nusha granitoids have diorite, syenite, monzonite, monzodiorite, granodiorite and quartz monzonite compositions. Moreover, mineralogically, feldspar is the principal mineral, and the texture superiority in them belongs to the granular type. In terms of magmatic series these rocks are metaluminous and range from high K calcalkaline to shoshonitic. The geochemical characteristics of the major and rare elements, as well as the petrographic ones indicate that these granitoids are I type granites, and at the same time they belong to high temperature ones based on the behavior of Ba, Ce and Y elements. Enrichment in LILE and LREE and low concentrations of heavy rare earth elements HREE and high field strength elements HFSE, together with Nb and Ti negative anomaly in the spider diagrams are signs of magmas related to the subduction zone. The high temperature nature and characteristics such as Y/Nb, Rb/Sr and Rb/Ba ratios show that the Nusha granitoids have the geochemical properties of both crustal and mantle origin materials with different ratios. Based on tectonomagmatic discrimination diagrams and trace element compositions, these granitoids belong to an active continental margin environment. The parental magma has originated from melting of an enriched mantle source and contaminated with continental crust during ascent.

The Masjeddaghi porphyry-epithermal Cu-Au deposit has located in the western part of the Alborz-Azarbaijan zone; in the south margin of Lesser Caucasus. The Eocene porphyritic quartz diorite intrusion has intruded into the andesite volcanic rocks and formed the main host rock of Cu-Au mineralization. Hydrothermal alteration types consisted dominantly of potassic, phyllic, argillic, and propylitic, and local silicification around the veins. Electron microprobe studies indicated that the Masjeddaghi biotites has been located in the phlogopite field and fall into the field of re-equilibrated primary biotite. Moreover, these biotites indicate the tectonomagmatic setting and magma characteristics related to calk-alkaline granitoids which were originated from mantle sources. The temperature of biotites from Masjeddaghi indicated a range between 417 ºC -641ºC. The conditions of oxygen fugacity in the magmatic biotites are in the range of hematite-magnetite (HM) and nickel-nickel oxide (NNO), which indicate high oxygen fugacity of the magma in this ore deposit. In the Masjeddaghi biotites, there is no linear/parallel trend between halogen fugacity,d log (ƒH2O/ƒHF), log (ƒH2O/ƒHCl) and log (ƒHF/ƒHCl) lines. Therefore, it is possible that biotites have not formed under the same conditions and were in equilibrium in a wide range of temperatures and compositions with hydrothermal fluids.

In the Shourestan area, 14 kilometers west of Sarbisheh city, in South Khorasan province, volcanic rocks with pyroxene-andesite composition belonging to Eocene-Oligocene are exposed. The constituent minerals of these rocks include plagioclase and pyroxene. The composition of plagioclases have range from Ab32, An68 to Ab58, An42 and are andesine-labradorite type. Clinopyroxene and orthopyroxene have diopside-like augite and enstatite composition, respectively. The crystallization temperature for clinopyroxene and orthopyroxene were about 1175 and 1200 °C respectively and the pressure (for both types) was 2 to 5 kb. The geochemical data of whole rocks show that the andesitic lavas of Shourestan have high potassium calc-alkaline nature and the amount of Mg# in them varies from 40.97 to 60.97, which indicates the role of mantle components in their formation. These rocks show signs of differentiation including LREE/HREE ((La/Yb)N) between 9.95 to 12.42, LREE/MREE ((La/Sm)N) between 3.53 to 6.55, MREE/HREE ((Sm/Yb)N) between 1.89 to 2.99. High ratios of Zr/Nb (9.81-22.10), Th/Nb (0.68-1.79), Th/Ta (7.29-24), and Nb/Ta (9.69-15.66) along with the pattern of LIL elements, support the possibility of different degrees of crustal contamination-assimilation of magma during its ascent to the earth's surface. The studied rocks have low ratios of Ce/Y (2.44-3.48), (Tb/Yb)N (1.17-1.39), Sm/Yb (1.92-2.78), and relatively flat MREE-HREE pattern that confirms the melting of the subcontinental lithospheric mantle in the field of spinel stability and at a depth of fewer than 75 kilometers.