Irankuh is located 20 km south of Isfahan. Late Jurassic black shale, silt and sandstones are the oldest rocks in the area that exposed in the northern flank of the Irankuh Mountain. Lower Cretaceous dolomitic limestone with some shale and marl starting with a basal conglomerate, overlay the Jurassic sediments in the area. Folds are trending NW-SE. In the northern flank of Irankuh a Kilometer scale Syncline with vertical axial plane and axis plunges of 35° to the west and an anticline with 32° plunges to the west and its exposure extended toward the southern flank of mountain. According to competency contrast of the rock units, abundant minor folds consistent with the geometry of the major structures are produced in couple of synclines and anticlines. Goushfil-Tapeh Sorkh reverse fault (75° dipping to the NE) thrust the Jurassic sediments over the Cretaceous rocks in Goushfil and Tappeh Sorkh1 Mines. Field observations indicate that it was originally a normal fault dipping to the SW and has been changed during the course of folding process to an inverted high angle reverse fault dipping to the NE close to the surface. NW-SE trending reverse faults are controlling the mineralization in the Irankuh area i.e. Goushfil-Tapeh Sorkh reverse fault.
The Eastern Haft-Savaran Zn-Pb-(Ba) deposit, located in the southeastern part of the Arak Mining District of the Malayer-Esfahan Metallogenic Belt, Iran, is hosted in the uppermost part of an early Cretaceous massive limestone unit that is capped by shale. The mineralization has a sheet-like geometry and is associated with intense dolomitization and silicification. The mineralization is mineralogically zoned: chalcopyrite-galena-pyrite-sphalerite-tetrahedrite occurs in the southern part of the deposit; sphalerite-galena-pyrite occurs in the central part of the deposit; and barite-galena-sphalerite-pyrite occurs in the northern part of the deposit. The thickest mineralization and most intense alteration are in the southern part of the deposit, associated with an aphyric rhyodacite flow containing minute euhedral barite crystals and Cu-bearing sulphides in vein/veinlets and disseminations, indicating this was the mineralizing fluid upflow site. Three hydrothermal mineralization stages are recognized. The first is mud lime sedimentation, framboidal pyrite, minute euhedral barite, sphalerite, galena and early dolomite. Minute euhedral barites are cut by microsparite formed by micrite recrystallization during diagenesis. First stage sphalerite and galena occur as inclusions in framboidal pyrite, and these were remobilized into inclusions in euhedral pyrite during recrystallization. The second (main) stage mineralization includes sphalerite, galena, chalcopyrite, tetrahedrite, pyrite and barite that with dolomite and quartz as gangue minerals replaced first stage mineralization and is associated with silicification and dolomitization. The third stage of mineralization comprises sphalerite and galena that is associated with dolomitization and calcitization. The occurrence of mineralization in an extensional back arc setting, massive limestone host rock, intense and pervasive host rock alteration, Fe-dolomite associated with main stage mineralization, the presence of mineralization load casts and minute euhedral barite, the remobilization of first stage of mineralization during diagenetic evolution of framboidal pyrite to euhedral pyrite, and timing of mineralization during early diagenesis of mud lime collectively indicate the Eastern Haft-Savaran deposit can be classified as an Irish-type deposit.
Abstract Most of the known large gold deposits in Iran are located along the Sanandaj–Sirjan Zone, western Iran, which hosts a wide range of gold deposit types. Gold deposits in the belt, hosted in upper Paleozoic to upper Mesozoic volcano‐sedimentary sequences of lower greenschist to lower amphibolite metamorphic grade, appear to represent mainly orogenic and intrusion‐related gold deposit types. The largest resource occurs at Muteh, with smaller deposits/occurrences at Zartorosht, Qolqoleh, Kervian, Qabaqloujeh, Kharapeh, and Astaneh. Although a major part of the gold deposits in the Sanandaj–Sirjan Zone are related to metamorphic devolatilization, some deposits including Muteh and Astaneh are related to short‐lived disruptions in an extensional tectonic regime and are associated with magma generation and emplacement. The age of gold ore formation in the orogenic gold deposits is Late Cretaceous to Tertiary, reflecting peak‐metamorphism during regional Cretaceous–Paleocene convergence and compression. The Oligocene to Pliocene age of most intrusion‐related gold systems is consistent with the young structural setting of the gold ore bodies; these deposits are sequestered along normal faults, correlated with Middle to Late Tertiary extensional tectonic events. This relationship is comparable to the magmatic‐metallogenetic evolution of the Urumieh‐Dokhtar magmatic arc, where the number of different types of gold‐copper deposits and the magnitude of the larger ones followed development of a magmatic arc. The appropriate explanation may be related to two different stages of gold mineralization consisting of a first compressional phase during the Late Cretaceous to Early‐Middle Tertiary, which is related to orogenic gold mineralization in the Qolqoleh, Kervian, Qabaqloujeh, Kharapeh, and Zartorosht deposits, and the extensional phase during the Eocene to Pliocene that is recognized by young intrusion‐related gold mineralization in the Muteh and Astaneh deposits.
The Koushk zinc–lead deposit in the central part of the Zarigan–Chahmir basin, central Iran, is the largest of several sedimentary–exhalative (SEDEX) deposits in this basin, including the Chahmir, Zarigan, and Darreh-Dehu deposits. The host-rock sequence consists of carbonaceous, fine-grained black siltstone with interlayered rhyolitic tuffs. It corresponds to the upper part of the Lower Cambrian volcano-sedimentary sequence that was deposited on the Posht-e-Badam Block due to back-arc rifting of the continental margin of the Central Iranian Microcontinent. This block includes the late Neoproterozoic metamorphic basement of the Iran plate, overlain by rocks dating from the Early Cambrian to the Mesozoic. Based on ore body structure, mineralogy, and ore fabric, we recognize four different ore facies in the Koushk deposit: (1) a stockwork/feeder zone, consisting of a discordant mineralization of sulphides forming a stockwork of sulphide-bearing dolomite (quartz) veins cutting the footwall sedimentary rocks; (2) a massive ore/vent complex, consisting of massive replacement pyrite, galena, and sphalerite with minor arsenopyrite and chalcopyrite; (3) bedded ore, with laminated to disseminated pyrite, sphalerite, and galena; and (4) a distal facies, with minor disseminated and laminated pyrite, banded cherts, and disseminated barite. Carbonatization and sericitization are the main wall-rock alterations; alteration intensity increases towards the feeder zone. The δ34S composition of pyrite, sphalerite, and galena ranges from +6.5 to +36.7‰. The highest δ34S values correspond to bedded ore (+23.8 to +36.7‰) and the lowest to massive ore (+6.5 to + 17.8‰). The overall range of δ34S is remarkably higher than typical magmatic values, suggesting that sulphides formed from the reduction of seawater sulphate by bacteriogenic sulphate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulphate reduction likely played an important role in the feeder zone. Sulphur isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal SEDEX ore deposit.
The Gandy gold-base metal deposit is located in Tertiary Torud-Chahshirin volcano-plutonic range, north of Central Iran. Various styles of gold mineralization occur throughout the range. Mineralization at Gandy occurs in close spatial relation with rhyolitic domes in a caldera setting in a series of narrow brecciated veins. Two mineralogically and spatially different argillic alterations have affected the volcano-sedimentary host unit. The first is a pervasive advanced argillic alteration (kaolinite + quartz) and the other is a vein-controlled quartz with illite assemblage. Ore minerals comprise gold, silver, base metal sulfides and sulfosalt minerals are accompanied by carbonates, quartz and barite. Fluid inclusions in coarse-grained sphalerite have homogenization temperatures ranging from 139 to 345 °C and salinities from 7.9 to 16 wt% NaCl equiv. Data suggest that the main ore deposition mechanism is the mixing of a 300 °C magmatic fluid. Mineralization at Gandy is the product of ascending gas and saline fluid exsolved from crystallizing magma, possibly due to transition from plastic into brittle regime resulting from emplacement of the rhyolitic intrusion, consequent condensation in ground water and mixing. Characteristics of the Gandy deposit are similar to those of gold and base metal-rich epithermal deposits of intermediate-sulfidation state.
The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma.
Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite.
The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.
Early Cretaceous carbonates are the most common host rocks for Irish-type deposits in Iran. They are largely concentrated in the Malayer-Esfahan metallogenic belt (MEMB) in southwestern Iran, and Yazd-Anarak metallogenic belt in Central Iran. They include some world‐class ore deposits such as Mehdiabad, Irankuh, and Ahangaran. These stratabound deposits are hosted mostly in carbonates with minor siltstones and volcanic components, that formed in extensional and passive margin environments that are related to the Nain-Baft back-arc basin. The deposits are stratabound and comprise wedge-shaped to tabular sulphide-barite orebodies and occur in several different stratigraphic horizons. Dolomitization and silicification are the main wall-rock alteration styles. Replacement textures are common, and orebodies represent complex textures of sulphides and barite, such as brecciated, colloform, zebra, minor laminated, and banded replacement. Barite is an important gangue mineral in the MEMB and YAMB deposits, partly replaced by coarse-grained galena, sphalerite, and chalcopyrite. Sulphides from these Irish-type deposits have a wide spread of light δ34S values (with the majority falling between -25 to +5‰) with mostly bacterial sulphate reduction (BSR) origin. Fluid inclusion studies show that homogenization temperatures of ore minerals are typically 120 to about 280°C (majority 225-275°C), and salinities range from 2 to 24 wt.% NaCl eq, with the majority falling between 8 and 22 wt% NaCl eq. Using the criteria outlined in this study, early Cretaceous extensional sedimentary basins (e.g., Nain-Baft) are highlighted as target areas for exploration of world-class Irish-type ore deposits and correspond well with the periods of expulsions of Cretaceous CaCl2-rich brines.
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