| چکیده انگلیسی مقاله |
Introduction
Today, alkaline rocks are a more important perspective for the largest gold deposits relative to normal calc-alkaline andesites (Müller and Groves, 1993; Sillitoe 1993, 1997, 2002). Specifically, four of the nine largest epithermal gold-silver deposits and four of the ten largest porphyry copper-gold deposits are associated with high-K calc-alkaline and shoshonitic rocks (Sillitoe, 1997, 2002). The high-K igneous rocks only comprise between 5 and 10 vol.% of volcanic arc rocks and are associated with 40% of the largest epithermal and porphyry deposits, indicating their importance for mineral exploration (Müller and Groves, 2019). The Siahouki Cu-Au deposit is located 50 km north of Bam, in the southern part of the Urumieh-Dokhtar magmatic belt of Iran. Considering the potential of shoshonitic volcanic rocks as suitable host rocks for epithermal Au-Cu deposits and also the vastness of the rocks in the area, the petrography, and geochemistry of volcanic rocks in the Siahouki Cu-Au deposit is chosen as the subject of the present research. It is hoped that the study of changes in the Cenozoic volcanism from the Siahouki area can reveal part of the geodynamic and subsequently metallogenic development in this part of Iran.
Research method
Field investigations including the preparation of a 1:5000 geological map (Figure 3) as well as collection of rock samples. were performed for this study. At this stage, over 50 specimens were taken, of which 25 samples were selected for preparation of thin sections. Using XRF and ICP-MS techniques, the abundance of major oxides, minor, and trace were determined on 22 representative samples. All the samples were then crushed in a jaw crusher in sizes smaller than 5 mm and were then sent to the laboratories. The analysis of major oxides was carried out by XRF at the Geology Department of Tarbiat Modares University and the analysis of major, minor, and trace elements was performed using ICP-MS at the laboratory of Zarazma Minerals Studies Company. Excel, Minpet, and GCDkit 6.2 software were used to process and analyze the data obtained from the geochemical analyses of the major oxides, minor, and trace elements (Table 1) and drawing diagrams as well.
Regional Geology
The Siahouki deposit located in the Urumieh-Dokhtar Magmatic Belt of Iran (Eftekharnejad et al., 1993), is mainly covered by Eocene volcano-sedimentary rocks and Quaternary alluvium, based on the 1:100,000 Bam geological map.
Ev2d unit: includes dacitic lavas, which are partly associated with rhyolitic and rarely andesitic lavas. The unit locally has undergone chlorite alteration. In the places where the argillic alteration is intense, the lavas are light gray to white, and appear cream to light brown in satellite images. The phenocrysts of the unit are mainly replaced by sericite or opaque minerals.
Unit (d) consists of banded-welded tuff associated with Eocene dacitic lava and well-layered sediments including conglomerate, sandstone, and siltstone.
Unit (t) is also mainly composed of dacite and partly associated with rhyolitic to andesitic lavas, tuff, and ash intercalations.
The Eocene volcano-sedimentary units are surrounded by Quaternary alluvial fans (Qf1 and Qf2). These units are cut by younger faults.
Alteration and Mineralization
Based on field observations along with drilling core data, Cu-Au mineralization in the Siahouki area occurred as quartz and carbonate veins with stockwork and breccia textures, accompanied by silicic, carbonate, and argillic alteration types. The mineralization is exposed along NW-SE structures in the dacitic crystal tuff (Ed) and andesitic lithic tuff (Elt) units, as ore zones up to 200m length and 0.1-2m (average 1 meter) thickness. Field and microscopic studies indicate that ore mineralization is controlled by silicic, carbonate, argillic, and propylitic alteration features. Silicic and carbonate are the most important types of alteration associated with ore (sulfide)-bearing veins. The hypogene ore minerals formed with the quartz and carbonate veins including chalcopyrite, tetrahedrite, bornite, pyrite, and native gold, respectively. During supergene processes, oxidation, and decomposition of sulfide minerals gave rise to the formation of malachite, azurite, chalcocite, and iron oxide-hydroxides as well.
Discussion
Based on the chemical analytical results of the host rock (Ed unit) in the Siahouki area, the composition of Ed unit plots on the dacite and rhyolite domains (i.e., 61.17 to 76.28 wt.% SiO2) (Table 1). The samples are characterized by the high Al2O3 contents (0.9 to 17.33 wt.%), very high K2O (˃ 4.26 wt.%), and a high ratio of K2O/Na2O, consistent with the chemical characteristics of typical shoshonitic rocks (Morrison, 1980). The studied samples show negative Ta anomalies, and low Nb/Ti ratios, as well as high contents of Al2O3 and P2O5 and the high LILE/HFSE ratios. Also, enrichment in Pb can be related to crustal contamination. The formation of potassic magmatism took place with the release of fluids from the subducted Neo-Tethyan crust, the metasomatism of the lithospheric mantle, and the subsequent melting of the metasomatized mantle, which is the result of an extension phase and fault systems in the region, the parent shoshonitic magma rose to the earth's surface.
Conclusion
The Eocene volcanic units at Siahouki are mainly composed of lavas and tuffs with an acidic to intermediate (rhyolite, dacite, and andesite) composition associated with intercalations of intermediate to basic (trachy-andesite, basaltic trachy-andesite and basaltic andesite) lavas. The chemical features of the host rocks (Ed unit) at the Siahouki deposit, which are dacite and rhyolite in composition, are comparable with typical shoshonitic rocks. Other geochemical characteristics of the volcanic-hosted Cu-Au mineralization in the Siahouki deposit (including low Nb/Ti ratios, negative Ta anomalies, and the high contents of Al2O3, P2O5, and LILE/HFSE ratios) suggest that the deposit generated in a continental volcanic arc environment and display close genetic relationship with shoshonitic composition. |