| چکیده انگلیسی مقاله |
Introduction
Molybdenum-copper deposit of Shele boran is located in East-Azarbaijan province, northeast of Ahar. The area is dominated by the oldest rocks including Paleocene-Eocene andesitic-dacitic units into which Oligocene sub-volcanic intrusions have penetrated. Quartz-sulfide, quartz-oxide, and sulfide vein-veinlets were developed within the intrusion bodies that exhibit typical stockwork texture. Three types of hydrothermal hypogene alterations, potassic, phyllic and propylitic are developed in these bodies. The Oligocene intrusive bodies range, in composition, from granite, tonalite to porphyry microdiorite. The major constituent minerals including plagioclase, alkali-feldspar, quartz, biotite and hornblende accompanied by minor amounts of clinopyroxene, apatite, sphene, zircon. and common textures granular porphyry to porphyroid, and porphyritic textures. The parent magmas are high-K calc-alkaline to shoshonite showing LILE positive anomalies with high LREE/HREE ratio. These bodies were emplaced in a post-collision volcanic arc and an active continental margin setting.
Regional Geology
The studied area, a part of Arasbaran exploration area, lies in the north-west of Iran and is one of the copper-molybdenum-gold metallurgical states, known as the Ahar-Arasbaran zone. The area, due to its special geological features and in terms of gold, copper mineralization, Molybdenum is of special importance. This zone is divided into two eastern and western parts by the Rasht-Takestan fault. The eastern part consists of basic to acidic tuffs belonging to shoshonite to alkaline magmatic series, and the western part contains andesitic to rhyodacite lavas and several granitoid masses with high-K calc-alkaline and shoshonite. Qaradagh, Shiverdagh batholiths and Haft-Cashmeh as well as Songun porphyry stocks are among the most important intrusions igneous masses related to mineralization in Arasbaran metallogenic zone, where skarn, porphyry, stockwork and epithermal mineralizations occurred. One of the prominent features of this zone is the extensive Tertiary magmatic activity, initiated in the Paleocene-Eocene and reached its peak along with the extensive folding of volcanic and pyroclastic units, and the emplacement of intrusive masses in the Oligocene (Pyrenean phase). The oldest rock units in this area are Paleocene-Eocene volcanic and pyroclastic deposits with andesite-dacite, andesite to basaltic andesite and associated tuffs, penetrated by sub volcanic Oligocene porphyroid masses.
Research methodology
Simultaneously with the preparation of the geological map of the region with the scale of, a number of samples were taken from the surface units and boreholes that were drilled for the exploration of molybdenum and copper. On the base of lithological diversity, 45 samples of intrusive rocks were collected. Following petrography study, 15 samples with the least amount of alteration were analyzed by XRF and ICP-MS methods and by combining the information obtained from field observations, microscopic studies as well as the main and rare elements analyses using the GCDkit software, petrogenesis and the formation of intrusive rocks of the region have been investigated.
Petrography
According to the petrographic studies, plagioclase, potassium feldspar, quartz, biotite and hornblende are the main rock-forming minerals. Clinopyroxene, apatite, zircon, and sphene as the minor and chlorite, sericite, calcite, and clay minerals as the secondary minerals. The presence of porphyric granular, porphyroid and porphyry textures microcrystalline in the background are notable.
Geochemistry
As the Co/Th, Ce/Yb versus Ta/Yb and Th/Yb versus Ta/Yb diagrams demonstrate, the parent magma has shoshonite and high-K calc-alkaline nature. On Nb vs. Y, Rb vs. (Ta+Yb), Ta vs. Yb, and Rb vs. (Y+Nb) the diagrams, the bodies under study were emplaced in a post-collision volcanic arc and an active continental margin setting.
On the spider diagram of intrusive masses normalized to chondrite, the studied samples show LREE enrichment, enrichment of LREE compared to HREE and negative anomaly of Nb and Zr indicate the dependence of these rocks on it shows calc-alkaline magmatic series. On the normalized diagram compared to the primary mantle, clear and distinct negative anomalies of Ti, Zr, P, Pr, Y, Nb and also positive anomalies of K, U, Pb, Cs, Nd are observed. The P, Ta, Ti and Nb negative anomalies on spider diagram with a specific enrichment of LILE (i.e. K, Sr, Sm, Th and Cs) are of the important characteristics of magmatic rocks associated with volcanic arcs, caused by the action of fluids derived from subduction. The negative Nb anomaly indicates the magmas related to the active continental margin environments and can be caused by the contamination of the crust and fluids released from the subducting lithosphere. The change process of these samples is compatible with the characteristics of the geo-structural environment related to subduction.
Discussion and Conclusion
The parent magma of the intrusion bodies has shoshonite and high-K calc-alkaline affinity. It should be noted that shoshonite and high-K calc-alkaline has also been reported in the volcanic units of Sonajil area of Harris. These masses display positive and distinct anomalies of K, Th, Sr, Sm, Cs, Rb, Ba as well as Ta, Ti, Zr, Nb, P, Pr, Y, Yb negative anomalies, the important features of the magmatic rocks related with volcanic arcs. The P, Ta, Ti and Nb negative anomalies on spider diagram along with a specific enrichment of LILE including K, Sr, Sm, Th and Cs are the important characteristics of magmatic rocks associated with volcanic arcs. Thus, the order of changes in these samples is consistent with the features of the geological environment related to subduction. These bodies were emplaced in a post-collisional volcanic arc and an active continental margin setting. The obtained data are consistent with the previous studies carried out on Oligocene granitoid intrusive masses of Western Alborz Zone-Azerbaijan (i.e., Moayyed, 2001, Aghazadeh et al., 2010).
Acknowledgments
We would like to thank all those who have cooperated in this research. Also, we would appreciate the editor-in-chief of the editorial board of Petrology magazine and the referees for their careful review and valuable suggestions. |