Journal of Economic Geology (Dec 2022)
Alteration and mass balance of igneous rocks in the Taktelar area, northwest of Zahedan, SE Iran
Abstract
The Taktelar area is located in 140 Km northwest of Zahedan city in the Sistan Suture Zone. In this area Eocene flysch-like rocks such as shale, siltstone and sandstone are intruded and extruded by Oligocene-Miocene plutonic, extrusive and sub-volcanic rocks. The igneous rocks are dacite, andesite, and diorite porphyry and are mainly altered. The mineralization in the Taktelar area occurs as dissemination and vein types in sub-volcanic and sedimentary rocks. The disseminated mineralization is characterized with wide formation of pyrite with or without Cu minerals and vein mineralization includes malachite, galena and Au-bearing silicic veins. The hydrothermal alterations are potassic, phyllic, tourmalinitic, argillic, silicic and propylitic. The result of mass change calculation shows enrichments of SiO2 and depletion of other elements in phyllic alteration zone. In argillic alteration zone, contents of As, Mo, Zn, and Pb were enriched and other elements were depleted or were not changed. The alteration zones with quartz stockwork veins and enrichment of SiO2 and K2O have the most enrichment of Cu and Mo. The behavior of the elements in the propylitic alteration zone is different and generally increasing of the main elements is more significant. The ore-making elements have more enrichment in samples with silica veins so that the highest enrichment of copper and molybdenum occurs in rocks with higher SiO2 and K2O and quartz veins. Generally, in most altered zones, contents of copper, molybdenum, lead, zinc and arsenic show enrichment and rare earth elements show depletion. Light rare earth elements show more depletion than heavy rare earth elements, indicating their more mobility in hydrothermal systems. Introduction Alteration is characterized by mineralogical, and textural changes in the wall-rocks through which hydrothermal fluids circulate. There are several types of hydrothermal alterations, depending on the nature of the circulated fluid and wall-rocks and physico-chemical conditions of the formation environment. Although, the alteration types have been mainly investigated in terms of their minerals, recently they have been also described for geochemical characteristics and mass changes (Maanijou et al., 2012, Siahcheshm et al., 2014; Derakhshani and Abdolzadeh, 2009). The main aim of this paper is to study mass change and enrichment and depletion of the different elements in altered rocks in the Taktelar area. In addition, mineralogy and geochemistry of the alteration types will be investigated. Geology The Taktelar area is located 40 Km northwest of Zahedan city in the Sistan Suture Zone. This zone is characterized by Cretaceous ophiolites of the Neo-Tethys Ocean and Cretaceous to Eocene flysch-like rocks that have been intruded by a range of calc-alkaline to shoshonitic igneous rocks of subduction, collision and post-collision settings (Camp and Griffis, 1982). In the Taktelar and adjacent area, Eocene flysch-like rocks such as shale, siltstone and sandstone are intruded and extruded by Oligocene-Miocene plutonic, sub-volcanic, volcanic and pyroclastic rocks. The Oligocene plutonic rocks are mainly related to shoshonitic syenitic and mozonitc rocks (Piri, 2018). Sub-volcanic rocks occur as stock and dikes and are mainly post-collisional arc related, calc-alkaline dacite and diorite porphyry. The volcanic rocks range from basalt to dacite. The pyroclastic rocks are lithic tuff and ignimbrites. The volcanic, sub-volcanic and sedimentary rocks have been altered in the study area. The study area has been affected by several faults including NW-SE and N-S strike-slip and NE and E-S faults. Methods Several samples were collected from outcrops and cores of all rocks and alteration types. Thin, thin-polish and polish sections of the samples were studied by polarizing microscopes at the University of Sistan and Baluchestan in Zahedan, Iran. Some of them were analyzed by XRF (9 samples) and ICP-MS and ICP-OES (12 samples) to measure main and trace elements, respectively. The mass change of these elements is calculated by the Grant (2005), MacLean and Kranidiotis (1987) and MacLean (1990) methods. Result and discussion The igneous rocks in the Taktelar are mainly andesite, dacite and diorite porphyry that mainly contain plagioclase, quartz, amphibole and biotite. Other igneous rocks in this area are monzonite and syenite that contain mainly plagioclase, orthoclase, clinopyroxene, amphibole and biotite. The flysch-type rocks are sandstone, siltstone and metamorphosed shales (hornfels). The sedimentary flysch-like rocks and stocks are extensively and intensively altered and weakly mineralized. The mineralization occurs as dissemination and vein types. The alteration types are potassic, sericitic (phyllic), tourmalinitic, argillic, silicic and propylitic. The potassic alteration occurs in dioritic to granodioritic stocks in the north part of the Taktelar area and is characterized with orthoclase, biotite and quartz-pyrite stockworks and little malachite. The phyllic alteration including tourmalinitic and sericitic alteration as the most extensive alteration mainly occurs in dacite and is characterized with sericite, quartz, pyrite, tourmaline and clay minerals. The propylitic alteration occurs in dikes and sedimentary and igneous rocks mainly in marginal parts of the study area and is characterized with epidote, calcite, chlorite, quartz and pyrite. The argillic and silicic alterations are the most intensive and extensive alteration around silicic and mineralized veins in the Taktelar area. The mineralized veins are important for exploration of Au, Ag, Cu and Pb. The Isocon and MacLean and Kranidiotis (1987) methods monitor composition-volume relations by the following equations: CAi=MO/MA(COi+ΔCi) and DCi=(CAi/COi) of immobile element*CAi-COi, respectively, where Ci is concentration of the species i, O and A refer to original and altered rocks, respectively, MO and where MA represents equivalent masses before and after alteration and Δ means change. The results show enrichments of Cu, Mo, Pb, Zn and As and depletion of REE in most alteration zones. The alteration zones with quartz stockwork veins and enrichment of SiO2 and K2O have the most enrichment of Cu and Mo. Conclusion Isocon diagrams and gain and lost histograms show that the main, minor and rare earth elements were mainly depleted while Cu, Mo, Pb, Zn, As were usually enriched during alteration by hydrothermal fluids. The most enrichment of Cu occurs in amphibole-biotite diorite porphyry that was cut by quartz stockworks. The SiO2 and K2O were increased in this type of rock that may be a character of potassic alteration. The most depleted alterations in the Taktelar area are phyllic and argillic alterations.
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