Abstract The Keshe area is a part of Cenozoic magmatic belt of Central Iran. Some volcanics in this area underwent hydrothermal alteration including kaolinitization and alunitization. The altered rocks are characterized by an assemblage of alunite, kaolinite and quartz, which is typical in advanced argillic alteration. The mineralogical study suggests that the hydrothermal alteration in this area occurred in a magmatic hydrothermal environment.
Abstract Alvand blue sapphire is related to the igneous rocks of Alvand pluton, located in the Sanandaj–Sirjan zone, Hamadan area. The main geological unit is the Alvand batholith including gabbro, granite, and granitoid. Sapphire occurs within the desilicated pegmatite dykes intruding magmatic-metamorphic lithologies, closely to metasomatic zones. Sapphire-bearing rocks are situated in the central zone of aplitic-pegmatitic composite dykes. In addition to sapphire, the rocks contain albite, microcline, and orthoclase as major minerals, the absence of quartz is noticeable. The whole-rock geochemical characteristics show a relatively silica-poor and alumina-rich composition of syenitic sapphire-bearing rocks. According to peraluminous and S-type signatures, aplite and pegmatites indicate crustal magma origin. The slight increase of Al 2 O 3 /TiO 2 ratio in the sapphire-bearing pegmatites may suggest mobilization of Al, which either by melt mobilization through alkaline hydroxide-complexation or by alkali-bearing high-temperature fluids such as Na-Al-Si-O polymers. Also based on strong enrichment in Th, sapphire may be formed by late-stage fluids. The high Ti, Fe, and Ga parallel to the low Cr and V contents indicate a magmatic-metasomatic origin of the Alvand sapphire.
Middle Cretaceous sedimentary carbonates of Alvand Mountain host Pb-Zn-Ba veins. This mineralization occurs mainly as veins, breccia fillings, and to a lesser extent disseminated and host rock replacements. Ore mineralogy is simple and consists of sphalerite, galena, and barite, with minor pyrite, rare chalcopyrite, and tetrahedrite. Host rock dolomitization and hydrothermal silicification are typically associated with ore. Two types of primary liquid–vapor fluid inclusions were distinguished in sphalerite, quartz, and barite. Type I (salinity, 17–23 wt% NaCl; Th, 130 ± 30 °C) is dominant in sphalerite, whereas type II (salinity, 4.5–10 wt% NaCl, Th, 190 ± 40 °C) is dominant in quartz and barite; these fluids have characteristics of basinal brines and show a negative mixing trend in an evolving process of sulfide and gangue saturation. The δ18O and δ13C values for host rock and altered minerals range from 22.0 to 12.9 ‰ and 3.8 to −3.2 ‰, respectively, which suggest the influence of increased temperature fluids, presence of organic carbon, and fluid mixing. The δ34S values of sulfides and barite varies from 2 to 17 ‰ and 23 to 24 ‰, respectively, suggesting that reduced sulfur could be derived by thermochemical reduction of Paleogene seawater sulfate in the presence of organic matter; however, the wide ranges of δ34S sulfide values (15 ‰) exhibits that reduction of sulfur probably originated by different sulfur reduction processes. The data present in this study suggest that during increased tectonic activity in the late Cretaceous-Paleogene, mixing of basinal brines and water–rock interaction resulted in Pb-Zn-Ba deposition in carbonate host rocks. Therefore, it may represent low-temperature mineralization, possibly analogous to MVT depositional systems.
The Pinavand fluorite deposit is hosted by lower Cretaceous carbonate rocks in the structural-geological transitional zone of Central Iran. The purple and white fluorite occur, respectively, as early replacement masses and late cross-cutting veins. Both fluorites have different and distinct physicochemical characteristics. The purple fluorite has higher homogenization temperatures of fluid inclusions (170–260 °C) and lower ∑REE (1.6 ppm) and Y (1.3 ppm) than the white variety (90–150 °C, 11.12 ppm, and 21.3 ppm, respectively). All fluorite samples show positive Y anomalies (Y/Y*) in the range of 1.15–3.5. The average values of La/Ho in the purple and white fluorites are 23.1 and 3.4, respectively. The purple fluorite samples have lower Y/Ho values (an average of 63.45) than the white fluorite samples (an average of 87.64). The Tb/Ca ratio in the Pinavand fluorites ranges between 0.0000000348 and 0.00000105, and the Tb/La ratio varies between 0.01 and 0.4; these values suggest that both fluorite types are “hydrothermal” in origin. The purple fluorites have a lower Sr and a negative Eu anomaly. These differences in concentrations and ratios of various REE suggest that the physico–chemical conditions of mineralization changed during fluorite deposition at the Pinavand. These changes correspond to an increase in oxygen fugacity and pH, which occurred during white fluorite mineralization at lower temperatures. The δ34S values of the Pinavand barite samples (an average of 23.25‰) are similar to those of seawater sulfate in the upper Proterozoic. The δ34S values of galena range from −0.2‰ to −3.7‰, compatible with bacterial sulfate reduction (BSR). These features are similar to those in the hydrothermal and magmatic deposits.
Abstract The Kaj-Rostam Abad, Dashtak and Doab diapirs are part of the Precambrian–Cambrian Hormuz series that are rich in igneous inclusions concentrated by dissolution of diapiric salt. They are situated in the Iran–Pakistan salt range and commonly associated with inclusions of basalt, trachyte, andesite, micro-gabbro, gypsum and anhydrite, with lesser amounts of carbonate rocks. The mineral assemblage in these inclusions developed in three stages: (I) magmatic stage (diopside, Ti-augite, kaersutite, plagioclase, apatite, biotite and opaque minerals), (II) late magmatic stage (biotite, quartz, chlorite, albite, calcite, titanite, epidote, actinolite and opaque minerals) and (III) vein mineralization (quartz, chlorite, albite, calcite, garnet, epidote, opaque minerals and actinolite). Clinopyroxene is diopside to Ti-augite. Actinolite, kaersutite, albite and pycnochlorite are constituents of the metasomatic rocks of the area. Chlorite geothermometry yielded a temperature of 330–500 °C for chlorite formation. Clinopyroxene thermobarometry ranges from 960 ≤ T ≤ 1440 °C and 1 ≤ P ≤ 10 kbar. The presence of halite-bearing fluid inclusions in hydrothermal quartz veins with homogenization temperatures between 320 and 350 °C points to strong evidence of hydrothermal events. The salinity of these fluids is 39.8–42.7 wt% NaCl. δ 18 O data on hydrothermal quartz veins range from 14.89 to 22.09 ‰ (SMOW), indicating that the studied samples were affected by fluids originated from sedimentary-evaporitic rocks. Meteoric water that penetrated the evaporitic rocks likely mixed with late magmatic fluids while subjected to magmatic heat, when buried to depths of several kilometres by the Phanerozoic cover sequence. Whole-rock geochemistry data for the studied rocks emphasize their alkaline to sub-alkaline affinities, in a transitional magmatic series.
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