PETROLOGY
Year:2010 | Volume:1 | Issue:3|Papers Count:7

The Angouran Zn-Pb deposit is located about 120 Km Zanjan Province, within Sanandaj-Sirjan Zone.The deposit occurs in metamorphosed schists and marbles of Proterozoic age. The deposit contains about 4.7 Mt of sulfide ore, grading 27.7% Zn, 2.4% Pb and 110 g/t Ag and 14.6 Mt of oxidized carbonate ores, grading 22% Zn and 4.6% Pb. The Angouran Sedex deposit formed as a result of continental rifting and exhalation of seawater hydrothermal solutions into the seafloor and syngenetic deposition of sphalerite and galen within shales and carbonates. The deposit was then metamorphosed along with schists and marbles. Supergene oxidation of sulfide minerals and redeposition and replacement of zinc and lead formed the non-sulfide carbonate ores in Quaternary period. Based on structural, textural, mineralogical and geochemical evidence such as sulfides-schists foliation in marbles and schists, elongation of pyrite and sphalerite, high grade of Zn (28%), low grade of Cu (0.014%), and the absence of space filling and vein controlled mineralization, the Angouran Zn-Pb deposit resembles the exhalative sedimentary (Sedex) deposit, rather than Mississippi valley type (MVT) or volcanogenic massive sulfide (VMS) deposit.

The Abtorsh- Youzbashichai alteration zone, located in Tarom zone, is an obvious example of hydrothermal type alteration that found in trachyandesitic tuffs and lavas.Based on the field observations, mineralogical and geochemical investigations and satellite images there are argillic, advanced argillic, alunitic and silicic zones in this altered area. These zones exhibit a regular zonation from down to up. Some evidences such as following of alteration from rock fractures system and the patches of unaltered host rock in alteration body also confirm the hydrothermal type alteration in this area.Considering widespread existence of some minerals such as kaolinite, alunite, jarosite, and silica in alteration paragenesis and the trend of enrichment and depletion of elements (major & trace) observed in alteration stages, it seems that this area is influenced by acid- sulfate (high sulfidation) type alteration. The diagrams and alteration reactions confirm that enrichment or depletion of elements in alteration stages depends on their geochemical nature, their abundances in source rock, the type of altered minerals and their structures as well as physicochemical properties of alteration solutions.

Volcanic rocks with composition of basalt and possibly Lower Paleozoic (Devonian) age are present in the Kuh-e-Abdulhosein (Pol-e-Khavand, Anarak area). These rocks with 120 m thickness consist of clinopyroxene (augite), plagioclase (albite), alkalifeldspar (sanidine, anorthoclase), chlorite (corundophilite by alteration of olivine, and clinochlore scattered in the groundmass), amphibole (schermakite hornblende), garnet (spessartine), calcite, sericite and opaque minerals (magnetite and ilmenite). As the most of Paleozoic basalts of Iran, these rocks are not different in texture and mineralogy that are evidences of limited differentiation of their parental magma. On the basis of the geochemical studies and tectonic setting patterns, these rocks resemble as WPTB (within plate tholeiitic basalts) and transitional basalts. In chondrite-normalized spidergrams, they are more enriched in LREE than the HREE.

The Makran accretionary prism in southeastern Iran contains extensive Mesozoic melange zones and large intact ophiolites, representing remnants of the Neotethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism, the Jazmurian depression lies which is a subduction-related back-arc basin. The Ganj Complex is one of the ophiolitic complexes, located on the west side of the Makran accretionary prism and Jazmurian depression, and is bounded by the Jiroft fault system in the west. The Ganj Complex with an Upper Cretaceous age is composed mainly of lava flows, pillow lavas, acidic plutonic rocks and sedimentary rocks, which are intruded by northwest-southeast trending dykes and does not ressemble a classical ophiolitic sequence. It lacks the intrusive crustal and mantle sections. The Ganj Complex pillow lavas, mainly olivine basaltic, occur as normal and as mega-sized bodies and are mostly flattened - tubular in shape with bread crust crack surfaces. They show three textural zones from the top glassy (zone 1) through the intermediate (zone 2) to the holocrystaline interior (zone 3), with each characterized by varying assemblages of plagioclase and olivine that form different textures. The Ganj pillow basalts are characterized by variolitic, porphyritic, microlitic-porphyritic, intersertal, intergranular and amigdaloidal textures. Mineralogically, they consist of plagioclase ± olivine ±pyroxene+opaque. The outer glassy surfaces of pillows frequently consist of one, or rarely multiple rinds. The rinds consist of three layers, which from surface inwards are: (1) sideromelane, (2) dark tachylyte; and (3) tachylyte with elongated vesicles.

The Natanz plutonic complex is located 120 km north of Isfahan and in the Orumieh -Dokhtar magmatic belt. These Oligocene - Miocene plutons are intruded into the volcanics of Eocene. Based on petrographic studies, the Natanz complex is composed of six types of rocks, including gabbro, diorite, quartz-diorite, quartz-monzonite, granodiorite and granite. Field, mineralogy and geochemical studies indicate that the parental magma of this complex is I-type in nature, calc-alkaline and meta-aluminous.Magma produced gabbro resulted from melting of metasomatized upper mantle which have undergone differentiation prior to emplacement. The parental magma of granitoid rocks is diorite that produced from mixing of mantle-derived mafic magma and crustal melting magma. The geochemical characteristics of rare earth elements are comparable to other subduction-related magmatic rock suites. The flat REE pattern in these magmas may require their derivation by partial melting from a garnet-poor source. Petrological evidence show that fractional crystallization from dioritic magma to monzogranitic magma was the main role in the genesis of these rocks. Tectono-magmatic setting of this complex may be subduction related magmatism or post-collisional magmatism due to extensional phases after collision of Arabic and Central Iran micro continent.

The middle-Jurassic Astaneh granitoid body is composed of mainly of granodiorite.From the ancient time, Au mineralization and prospecting site have been known. The Astaneh granitoid have been altered in some parts and phyllic and silicaceous alterations can be recognized in some areas, especially in Shir Mazd Mountain which is vast area.Other alteration zones are: propylitic, chloritic, argillic, tourmalinzation, sodic metasomatism and spinel - phlogopite. The most widespread alteration zones are phyllic (quartz- sericite) and propylitic. There are two types of fluid inclusions in the quartz veins. On the bases of hydrothermometry study, the thermal phase of hydrothermal is from 337-341oC to 350-355oC. The density of fluid inclusions is low to intermediate (10-17 percent NaCl) while there is no daughter crystal. Temperature decrease and density of fluid inclusions are the result from mixing of magmatic with meteoric waters.The necking down of fluid inclusion is in consequence of temperature lower than 220oC. The depth of vein was controlled by dense fluid, temperature and pressure, also mineralization occurs in the depth 400-900 meter. The spider diagram show enrichment of LREEs in contrast to HREEs in unaltered samples with lack of HREEs differentiation. Tourmaline alteration displays a flat REE pattern and does not show enrichment but silicaceous alteration shows depletion of LREEs and enrichment of HREEs.

Very low-grade metamorphism (hydrothermal metamorphism) in Kamal Abad (west of Naein) affects Eocene basalts, and led to the formation of: prehnite, malachite, azurite, zeolite (mesolite, scolecite and laumontite), quartz and calcite in cavities and fractures.Chlorite (brunsvigite), amphibole (ferro-actinolite) and saussurite appear as secondary phases in primary rock. Two series prehnite occurs in the study area: The first has high relief and interference color and the second has low relief and interference color. This may be due to difference in Fe3+content in their crystallization structure. Using mineral chemistry as well as the abundance of hydrothermal minerals (prehnite, zeolite, quartz, calcite, malachite, azurite and laumontite), the nature of fluid should be enriched in Ca, Si, Al and OH and poor in Fe, Na, K, Mn and Cu. The very low-grade metamorphic minerals formed in neutral to slightly alkaline pH, low CO2, temperature of 150-350oC and less than 3 kbars pressure. The studies show prehnite-pumpellyite facies, zeolite facies and laumontite subfacies formed in the area. Moreover, the rate of alteration owing to the small volume of magmatic and hydrothermal solution is not high in the study area.