| چکیده انگلیسی مقاله |
Introduction The Yeilaq Samanloo area is located 19 km southwest of Meshginshahr and 22 km west of Sablan in the West Alborz-Azarbaijan structural zone. Cenozoic igneous-pyroclastic rocks cover more than 95% of the area. The Eocene units are mainly composed of volcanic rocks, including andesite, trachy-andesite to trachy-basalt, tuff and shale layers. A granitoid intrusive body (granodiorite, monzonite, quartz monzonite) with Upper Oligocene age intruded the Eocene volcanic rocks and produced chlorite and epidote alteration in them, especially in the contact zone. A number of silica veins containing pyrite and chalcopyrite cross-cut both the granodiorite body and Eocene volcanic rocks, which host gold and copper mineralization. The youngest unit includes Sablan lavas of trachy-andesite, basaltic andesite and andesite with Quaternary age, which have flowed unconformably on the Eocene volcanic rocks. Materials and Methods In this research, 65 samples were taken from stream sediments for geochemical studies. In order to check the anomalies revealed from stream sediment studies, 30 rock samples were taken for lithogeochemical studies and 10 petrological samples from the igneous rocks and were analyzed by XRF and ICP-MS (petrological samples), ICP-OES (geochemical samples of stream sediments) and Fire Assay (for gold) at Zarazma lab. Results and Discussion Based on the petrological diagrams, the volcanic rocks of the region mainly have andesitic to andesi-basaltic composition, high potassium calc-alkaline and shoshonitic nature, and meta-aluminous to per-aluminous affinity. The tectonic setting of these rocks is an active continental margin, and their trace and RE elements pattern is similar to the subduction-related rocks. Remote sensing and field studies show that the distribution of various alteration zones is not extensive. The chlorite-epidote (propylitic) alteration zone is the most widespread zone, mainly observed in the northeast and southeast of the area. Argillic and sericitic alterations are present in the central and southwestern parts, and the distribution of alunite-pyrophyllite alteration is scattered and very limited. Stream-sediment geochemical studies and lithogeochemical investigations upstream the observed anomalies led to the introduction of several Cu-Ag and precious and base metal mineralization areas for the first time in this region. Coincidence of geochemical anomalies with alteration zones shows that Cu anomalies are mostly associated with argillic and sericitic. The association of Au with argillic and sericitic alteration zones in the south of the area is noteworthy. But Ag mineralization is associated only with propylitic and to some extent, argillic alteration. Microscopic studies of rock samples showed that the Cu-Ag mineralization in the Samanloo area is stratabound, being associated with andesitic units of the Upper Eocene and includes pyrite, chalcopyrite, bornite, malachite, azurite, chalcocite, native copper and to a lesser extent, covellite, which occur as disseminations, open space fillings and replacements, especially within the mega-porphyritic andesite unit. In the rock samples of this area, the highest anomalies of elements are: Cu (67800 ppm), Ag (18 ppm) and Au (1088 ppb). Based on the obtained statistical correlations, the anomalous elements were divided into three groups: (1) Cu-Ag, (2) As-Sb-S-Au and (3) Pb-Zn-Fe, which are attributed to three genetic-lithologic groups. The first group is related to the granitoid body, especially the halo around it. The second group is directly related to pyroclastic units and Eocene lavas, especially mega-porphyritic andesites and chlorite-argillic alteration zone within it. The third group can be attributed to the silicic veins/veinlets of the Neogene tectono-magmatic activities and the infiltration of hydrothermal fluids into fractures; Au displays more considerable anomaly among the elements of this group. Conclusion Based on the characteristics of mineralization, including host rocks, stratabound nature, mineralogy, metal content and alteration, it can be concluded that the mineralization at Yeilagh Samanloo area is of Manto-type copper deposits. According to the structure, texture and mineralogy of the mega-porphyritic andesite unit, two phases can be considered for the hypogene mineralization at the Samanloo area: primary diagenetic stage and burial stage. Early diagenetic processes led to the formation of pyrite within the porphyritic andesite unit, which is the host of mineralization, and as a result, reducing conditions have appeared in this unit. In the next stage, under the influence of the burial process, oxidant saline fluids have migrated and washed Cu from the underlying Cu-rich volcanic units (trachy-andesite, tuff with shale layers) and deposited it in the reducing mega-porphyritic andesites. |