Geochemistry of Host and Altered Rocks in the Nahran Area , Tarom Zone ( NW Iran ) : Implication for Determining of Mineralization Processes in the Alteration Environment

The Nahran area is located in the Northeast of Zanjan in the Northwest of Iran. This area with 20,000 km is part of the Tarom volcanic-plutonic zone which lies between the longitudes 49 ̊7'7.8"E and 36 ̊41'25.74"E near to the Nahran village. The Nahran altered zone is part of large-scale syncline, which is oriented from Sirdan to Altinkosh with an axial length of 9 km. A systematic study of petrographical and geochemical data of Nahran samples reveals they are andesite, trachyandesite to dacite composition and have metaluminous to peraluminous and calc-alkaline affinity. Four alteration zones of propylitic, medium and advanced argillic and silicific zones could be identified in the area. According to our finding, combination of both supergene and hypogene process has an effective role in the development of alteration in the Nahran alteration zone.


Introduction
The Tarom Mountains, which cover most of the Zanjan quadrangle, are western extention of central Alborz Mountain.According to structural divisions of Iran proposed by Nabavi (1976), this area is part of western Alborz structural zone (Alborz-Azerbaijan zone) [1].The Nahran alteration zone is located approximately 100 km northeast of the city Zanjan and covers 20,000 km 2 area within the Tarom volcano-plutonic zone (Figure 1).This zone lies between the longitudes and widely affected by hydrothermal alteration.Alteration haloes are important in mineral exploration, as they increase the size of the target mineralized zone.For example, some of the altered host rocks in the Tarom contain anomalous concentrations of gold, which are far-removed from the mineralized veins.Despite to their economic importance for alunite and kaolinite, the Nahran alteration zone is less studied so far and there are many unknown genetic processes involved in the development of alteration areole.This study normally focuses on the alteration zone of Nahran area with the aim of identifying the compositions of source rock (protolith) and the nature of physicochemical changes during alteration [2].

Methodology and Analytical Procedures
In this research, extensive field works include preparation of new geological map were combined with systematic and random sampling to aim multidisciplinary approach of this research.After petrographical observation, 25 samples were selected for geochemical analysis.Trace and Minor element for selected samples were analyzed using combined ICP-AES and inductively coupled plasma mass spectrometry (ICP-MS) methods, at the Geological Survey of Iran.However, major elements were determined by wavelength dispersive XRF, using an automated Philips PW 1480 spectrometer in the Geological Survey of Iran.

Geology of Nahran Kaolinite-Alunite District
The Nahran altered zone is located in the NWW border of a large-scale, syncline which oriented from Sirdan to Altin Kosh with an axial length of 9 km.The Dip of layer planes ranges between 40 to 60 towards the NE (Figure 2).The larg intrusion massif (Kohe Ghajar) is a which situated in the contact zone of Nahran alteration and extent southeastern to Sirdan, Hasan Abad and Kamar roud villages (Figure 3).In the Nahran area, there are several huge alunite deposite within the Eocene volcano-clastic rocks.It is considered that the emplacement of larg intrusion massif (Kohe Ghajar) into Eocene volcano-clastic rocks promotes the hydrothermal alteration and alunite formation.The alteration areole with 2 km long and 1 km width and with average thickness of 265 m have developed within the black to grey tuffs of E and EK1 unit in the proximity of quartz monzonite and quartz syenite of intrusion massif of the Kohe Ghajar [3] [4].In the Nahran area, there are regular hydtothermal alteration zonations from bottom to top including: 1) Propylitic unit which observed in the proximity of intrusion massif and bottom of the Nahran altered zone within the andesitic tuffs.

Classification and Nomenclature of Less Altered Host Rock
Several rock classification diagrams are used for classification and nomenclature of less altered extrusive rocks of the Nahran area.They are including: -Middlemost (1994) classification: According to Na 2 O + K 2 O vs. SiO 2 diagram of Middlemost [7] [8] [9], the samples fall in the andesite, trachyandesite and near to dacite boundary line field (Figure 4).
- Winchester and Floyd (1977) classification: The Zr/TiO 2 *0.0001 vs. Nb/Y diagrams of Winchester and Floyd (1977) [10] is more accurate then Na 2 O + K 2 O vs. SiO 2 diagrams since Zr and Ti are not mobile elements.According to this diagram all samples fall in the andesite, Trachy-andesite and dacite regions (Figure 5).

Determination of Magmatic Series
One of the main objects in the petrological studies is determination of magmatic series.According to Kuno (1968), a magmatic series is a group of rocks that share some chemical (and perhaps mineralogical) characteristics and shows a consistent pattern on a variation diagram [11], suggesting a genetic relationship.Hovewer, new funding shows other parameter such as magma assimilation, different rates of partial and magmatic contamination could be create false magma series and grouping of different magmatic rock in the same magmatic association which not The "AFM" (Alkalis-ΣFeO-MgO) diagram proposed by Irvin and Baragar (1971) can be discriminate tholeiitic, calc-alkaline or Shoshonite affinity of Subalkaline rocks [12].On the AFM diagram, the Nahran samples plot within the calc-alkaline field.One sample plot on the tholeiit field (Figure 7).-Aluminium saturity index: The volcanic rocks divided to peraluminous, metaluminous and peralkaline on the basis of the molecular proportions of AI, Ca, Na and K, expressed in the form A/CNK > 1, A/CNK-1, A/CNK < 1, respectively.The Nahran volcanic rock has metaluminus and peraluminus affinity (Figure 9 and Figure 10).

Tectonic Setting of Nahran Volcanic Rocks
In order to define the tectonic setting of Nahran volcanic rocks several tectonic

Spider Diagrams
The spider diagrams are used for determination of the petrological process and comparison of different tectonic setting.The rare earth element concentration in the Nahran rocks are normalize to a common reference standard such as chonderite, primitive mantle and MORBs, in order to recognization of magma evolution process such as fractional crystallization, partial melting and magma assimilation.
Thampson et al. (1980; 1984) [13] [14] are used common chonderites values to proposed a chonderite normalized spider diagrams.Thompson (1982) [15] proposed that normalization to chondrite values may be preferable to primitive mantle compasation since chondrite value are directly measured rather estimited.In this diagram, compatibility increase from left to right.Chonderite normalized spider pattern of Nahran volcanic rocks show relative enrichment in the incompatible element respect to chonderite (Thamson, 1982) [15].Relative enrichment in the incompatible element indicates parent magma was not primary magma and direct product of mantle melting.As mentioned above partial melting of parent rocks was medium.The concenterations of LILE depend to aqueous fluid phases in contrast concenterations of HFS element controlled by chemical composition of source rocks and magmatic evolution reactions and crystal-melt processes, respectively.The LILE enrichment of Nahran volcanic rock in the Chonderite normalized diagram indicate role of crustal contamination in the magmatic evolution.The LILE and HFS amount of Nahran volcanic rock show lowest difference which indicates medium partial melting of source region of parent magma (Figure 13).
Pearce (1983) determine normalizing values of MORB and provided most appropriate pattern for evolved basalt, andesite and mid ocean ridge basalts [16].
The element are ordered so that the most mobile elements (Sr, K, and Ba) are and more of 10 times enrichment in Ba, Rb, Th and K respect to MORB.The relative depletion in the Nb, has been explained by retention of a refractory phase in the source in which those elements are highly compatible, such as rutile.However, it can be in related to melting or fractional crystallization of Nb bearing mineral phases (Figure 14).
-Normalization Nahran volcanic rock to Primary mantle.Wood et al. (1979) are used estimated composition of mantle before continental crust formation as mean for comparing composition variations between basic lavas and intrusions [18] [19] [20].In this diagram, elements are arranged in order of increasing compatibility with respect to a small percentage melt of the mantle.Primary mantle normalized pattern for Nahran volcanic rocks show descending trend with relative enrichment in the compitable element.The week enrichment in the Cs and K indicate negligible crustal contamination and low crystal fractionation.Negative Nb anomaly is typical of continental crust rocks and demonstrate role of contamination process in the magma evolution.According to Wilson (1989) [21] enrichment of U, K and Th respect to primary mantle may indicate mantle metasomatism by released fluids from subducting slab (Figure 15).    1) Advanced argillic alteration with Na + K content between 0% to 2%.
The dequartzification filed is restricted to right segment of Na − K diagram where the Na + K content of samples are more of 7. In the Na + K diagram, the Nahran alteration samples located in argillation field toward the K-metasomatism and Na-metasomatism field this consistent with high leaching of Na and K by hydrothermal and supergene fluids (Figure 18).

Determination of Mineralization
In order to evaluate of chemical characteristics of fluids associated with alteration episodes several discriminate diagrams have been tested for Nahran samples.According to studies [22] [23], the Ba + Sr/Ce + Y + La, Zr/Ti and Cr + Nb/Ti + Fe ratios could be used for determine the role of Hypogene and Supergene processes in the genesis of Kaolins.
- which represent effect of hypogene and supergene processes in the kaolin mineralization (Figure 19).
-The Ti + Fe versus Cr + Nb binary diagram.
The Ti + Fe versus Cr + Nb binary diagram can be used to distinguish supergene from hypogene kaolinization [24].The supergene kaolin deposit has high content of Cr and Nb (Cr + Nb > 100) respect to hypogene deposits.The Cr during the supergene alteration could be substitute of Ti 4+ and Fe 3+ in the geotite  Nahran Kaolin samples fall in the both supergene and hypogene field (Figure 21).
-The Pb -Ba + Sr -Ce + Y + La ternary diagram.The Pb -Ba + Sr -Ce + Y + La ternary diagram are used for determination of physic-chemical properties of mineralization fluids.Pb is derived from decomposition of K-feldspar in the bedrock; however, it is very low in the weathering zone.Pb does not substitute for other cations in the APS (aluminium, Phosphate, sulphate) minerals but can replace Ba in barite.The Pb represent of plumbogummite [24].Plumbogummite was not detected in the Nahra deposit.All Nahran samples lay away from Pb and arrange parallel to the Ba + Sr -Ce + Y + La side in the Pb -Ba + Sr -Ce + Y + La ternary diagram.The proximity of some samples to Ba + Sr side indicates their hypogene source and supergene for Ce + Y + La side adjacency samples (Figure 22).

Conclusion
The Nahran area is located in the Northeast of Zanjan in the Northwest of Iran.This area with 20,000 km 2 is part of Tarom volcanic-plutonic zone which lies between the longitudes 49˚7'7.80"Eand 36˚41'25.74"Enear to Nahran village.

2 )
Medium argillic alteration unit with 210 m thickness.3) Advanced argillic alteration unit containing quartz bearing alunite with 25 m thickness.4) Silicic alteration unit which lie as Hard and brittle cap rock in the top of other alteration unit.The total thickness of this cap rock range between 25 to 30 m and consist of silica with minor alunite.The Nahran silicic unit has low purity

-K 2 O
versus SiO 2 diagram Peccerillo and Taylor (1976): The K 2 O versus SiO 2 plot showing a rock suite may be subdivided geochemically
Different alteration units can be distinguished and discriminated based on chemical features.Hydrothermal fluid changes of major and minor elements in the host rocks of the Nahran alteration zone.For classification of altered rocks used different diagram include: -AKF diagram.The AKF diagram discriminate Argillic, Advanced Argillic and sericitic zones.

Figure 16 .
Figure 16.Opportunity altered samples of Nahran in the diagram AKF.