جستجوی مقالات مرتبط با کلیدواژه « Magnetite chemistry » در نشریات گروه « زمین شناسی »

The Mahura placer magnetite deposit is located in the eastern part of the Miqan Arak sedimentary basin on the border between Urmia Dokhtar and Sanandaj Sirjan structural zones in Quaternary alluvial sediments. Magnetite grains have micron to milimeter in form free grains and also inside the minerals of volcanic rocks found in the alluviums. Based on petrography and mineralogy studies, these mafic volcanic rocks are named , andesite, andesite basalt to basalt, and tiny grains of magnetite are observed inside amphibole and pyroxene mineral grains and at the border between them. According to the microprobe Analysis data of the magnetite ,, this mineral has high TiO2 and is located as a titanomagnetite or a solid solution mineral between magnetite and ilmenite, and according to the diagrams, it is placed in the group of Fe-Ti-V magnetites. The evidence obtained from the chemistry of existing clinoproxen minerals indicates that the source rock of these placers has a calc-alkaline composition and is formed in a continental arc environment , campatible with tectonic setting of Urmia Dokhtar , pressure about 2-5 kilobars and a temperature of 1100 degrees Celsius and depth between 7 to 10 km are condition to form this rocks. High content of iron-titanium, high amount of water about 10%, high fugacity of oxygen according to FMQ+1.5 system, are among the most important factors that caused immicibility and partial crystallization of magma oxide liquids and the formation of magnetite grains in the source rock of this deposit.

The Sarkuh porphyry Cu deposit located in the Urumieh – Dokhtar Magmatic Belt (UDMB), is associated with the emplacement of Miocene intrusions mainly containing granite – granodiorite within Eocene units including tuff, andesite, basaltic andesite, and pyroclastic breccia. Potassic alteration of the deposit (dominated by magnetite, secondary and re-equilibrated biotites as well as anhydrite) is well extended. In this study EDX spectra of accessory minerals within potassic alteration was assessed to trace the occurrence of rare minerals. Additionally, EMPA results of magnetite paragenesis were linked to physicochemical attributes of rare minerals occurrences. Results indicate that unusual occurrence of Ce-La rich epidotes with REE-enriched monazites and Ce-bearing titanites are accompanied with apatite, anhydrite, hydrothermal biotite, quartz, and magnetite assemblages in the potassic alteration. Magnetite composition implies that early-stages high temperature magmatic – hydrothermal fluids (>500 °C) with low rates of fluid – rock interaction were contributed in the magnetite crystallization. Commonly, these magnetites are accompanied by hematite and anhydrite providing insights into the very high oxygen fugacity conditions (near magnetite - hematite buffers ~ ΔFMQ+4). Considering the chemistry of magnetite paragenesis, it seems that the occurrence of rare minerals are mainly associated pre-ore stages in the potassic alteration

The studied mining area is located in the northeast of the Lut block, within the Ahangaran mountain range. The rock units in the region generally include limestone and metamorphosed carbonate rocks that are penetrated by intrusive masses with the composition of diorite, monzodiorite, granodiorite and granite. Based on the structural control of the mineralization zone and the formation of metasomatism succession with the presence of low-temperature hydrous minerals such as chlorite and epidote along with magnetite, the iron mineralization of the area can be considered as a low-temperature skarn type. The mineral chemistry of magnetite and the amounts of Ca, Ti, Al, V, Cr, Ni and Mn are also similar to skarn deposits. Field studies indicate that another younger intrusive mass at depth is the origin of the iron mineralization of the ore zone. Based on analysis of components, calculated on the basis of REE, the ore solution with magmatic origin has moved upward along the faults and after mixing with atmospheric fluids, as a result, it has created a reaction with the carbonate rocks of the iron ore.

The Dalli porphyry Cu-Au deposit is located in the central parts of Urumieh –Dokhtar Magmatic Arc (UDMA). This deposit is formed via the emplacing of Miocene intrusions mainly containing diorite and quartz diorite within the Eocene andesite and porphyritic basaltic andesite units. Main alterations in this region include potassic, propylitic and to a lesser extent phyllic. In this study, magnetite chemistry in the potassic alteration zone is investigated. The results of EMPA show that magnetites of this Au-rich porphyry system are characterized by enrichment in Ti, Al, V, Mg, and Mn values. Also, the magnetites formed via the hydrothermal processes. Evidences such as magnetite martitization and exsolution of ilmenite lamellae imply for magnetite crystallization in high oxygen fugacity conditions. Moreover, based on the Al + Mn vs. Ti + V diagram studied magnetites follow the trend of temperature decreasing which could be considered as an important factor in increasing the potentials of sulfide mineralization thorough the hydrothermal system evolution. Compared with Daralou and Regan porphyry systems, the Dalli magnetites contain higher concentrations of Ni, Mn, Cr, and Co presenting an exploration key for discovering the Au-rich porphyry deposits.

Porphyry copper deposits (PCDs) are related to the shallowly emplaced (5-10 km) oxidized magmatic systems in the subduction, syn-collisional, as well as post-collisional tectonic settings (Richards, 2011). They supply most of the world's Cu and Mo resources; ~80 % Cu and ~95 % Mo (Sun et al., 2015). In Iran, widespread occurrence of porphyry copper deposits has been discovered and mined in the Urumieh-Dokhtar Magmatic Arc (UDMA; Richards, 2015). These deposits are related to the evolution of the Neo-Tethys ocean starting with subduction in late Cretaceous to middle Miocene and subsequent prevailing the  syn- to post-collisional tectonic regimes during the Neogene (Richards, 2015; Zarasvandi et al., 2018). Most of the porphyry bearing intrusions of UDMA exhibiting a spectrum of mineralization from weakly to highly mineralized systems were emplaced during the Miocene (e.g., Sarcheshmeh, Meiduk, and Dalli). Recent studies concerning on the source of mineralized porphyry granitoids in the UDMA (e.g., Asadi, 2018) specified a model comprising the partial melting of subduction-modified thickened mafic juvenile lower crustal rocks responsible to generation of adakite-like-hydrous, relatively oxidized magmas (Sun et al., 2015) with the high potential to form porphyry Cu ± Mo ± Au systems.
During the last decade, magnetite geochemistry has been the focus of several studies trying to constrain the physicochemical attributes of igneous and hydrothermal ore systems (e.g., Dare et al. 2014). Magnetite can form under various conditions having the capability of fixing various minor and trace elements (e.g., Co, Cr, V, Ti, Mn, Mg, and Al) in its spinel structure (Nadoll et al., 2015). This makes magnetite able to record many environmental variables which are very important in mineralization potential of porphyry Cu-systems (e.g., oxygen fugacity, temperature, and ratios of fluid-rock interaction). Although the Dalli porphyry Cu-Au deposit has been the subject of many studies manly focusing on the magmatic evolution, fluid inclusion, silicate (plagioclase, biotite, and amphibole) and sulfide (pyrite and chalcopyrite) chemistry (Ayati et al., 2013; Zarasvandi et al., 2015a; Zarasvandi et al., 2018; Zarasvandi et al., 2019c); none focused on the oxide minerals (magnetite composition). The present work reports petrographic and chemical data of magnetite and tries to constrain the factors controlling the formation of Dalli deposit.

Sampling was carried out on drill cores and special care was undertaken to select the samples showing no obvious overprint of low temperature alteration. Polished thin sections were prepared from 1-2 cm sized blocks for microscopy and electron probe microanalyzer (EPMA) studies. Wavelength-dispersive (WDS) EPMA analyses of oxides were conducted at the Chair of Resource Mineralogy, Montanuniversität Leoben, Austria using the Jeol JXA 8200 instrument and the following analytical conditions: 15 kV accelerating voltage, 10 nA beam current and beam size to spot mode (of about 1μm). K lines were used for Mn, Fe, Ti, Mg, Al, Cr, and V. The counting times for element peaks and background (upper and lower) were 100 s and 20 s, respectively. The lower limit of detection for these elements (single standard deviation) as calculated by the integrated Jeol software.

In the Dalli porphyry Cu-Au deposit, hypogene mineralization mostly includes pyrite, chalcopyrite, and magnetite with minor chalcocite and bornite. Ore minerals occur as aggregates, in veinlets or disseminations within the potassic alteration, and to a lesser extent in the phyllic alteration zones. In the all analyzed samples, the values of Al2O3, V2O3 and MnO were upper then detection limit. Conformably, detectable values were mainly obtained for TiO2, Cr2O3 and MgO. On the contrary NiO, SiO2, CuO were mainly below the detection limit. The FeO content (wt. %) in the analyzed magnetites varies between 91.01 to 98.57 (average; 97.01), Al2O3 between 0.063 -5.07 wt. % (average 0.62 wt. %), and the lowest and highest values of V2O3 are 0.02 and 0.34 (wt. %), respectively. The average of MnO and MgO in the analyzed samples is 0.25 and 0.07 (wt. %), respectively. Additionally, the TiO2 content varies between 0.01 and 2.45 (wt. %); averaging 0.34 (wt. %).

On the Ti (ppm) vs. V (ppm) discrimination diagram, most of the analyzed magnetites extended to the field of hydrothermal field providing insight into the formation of magnetite owing to the exsolving of hydrothermal fluids through the potassic alteration. Comparison of the magnetite composition in the Dalli deposit with other PCDs in the UDMA implies that there are higher average contents (wt. %) of Mn, Fe, Mg, and Cr compared with Daralou (an example of pre-collisional porphyry intrusion) and Keder porphyry systems (an example of weakly mineralized collisional porphyry deposit). These features may highlight the importance of magnetite composition in establishing the discrimination diagrams of Au-rich porphyry Cu-deposits using magnetite composition. The documented oxy-exsolution of ilmenite as well as hematite intergrown with magnetite in the Dalli samples are the indicative of high  in the potassic alteration stage. Under such highly oxidized conditions sulfur is present as oxidized species (such as  ) rather than as reduced species (such as ) preventing the extensive sulfide deposition in magmatic and early stages of potassic alteration (Zarasvandi et al., 2022). This process could enhance the mineralization potential of the system by preserving the sulfur content, especially before the main mineralization stages. Besides optimum tectonomagmatic conditions, the physicochemical attributes of potassic alteration may also have a decisive role in predicting the mineralization potential of porphyry systems (Zarasvandi et al., 2018). Because bulk sulfide mineralization occurs at the end of potassic alteration (Richards, 2011). Prevailing of the high temperatures in potassic alteration could prevent the disproportion SO2 to H2S which is necessary for sulfide precipitation (Richards et al., 2017; Zarasvandi et al., 2018). Conformably, the inability of hydrothermal systems for cooling could be linked to the low mineralization degree of the porphyry deposits. Based on the Al + Mn vs. Ti + V diagram (Zhao et al., 2018), samples of Dalli deposit follow the trend of temperature decreasing which indicate the desirable conditions for enhancing the sulfide mineralization in the Dalli porphyry Cu-Au deposit.

Pade-Bid iron occurance is located at northeast of Kashmar-Kerman zone, which is one of the most important iron metallogenic zone in Iran. Lithology of the area includes alternation of metamorphosed carbonate rocks, slate and phyllite, which are intruded by dioritic and gabbroic intrusions. Based on structural controls of orebody, metasomatic replacement and formation of low temperature H2O-beraring minerals and occurrence of magnetite and pyrite associated with chlorite, epidote, calcite and quartz, the iron mineralization is low temperature skarn-type. Magnetite chemistry and Ti, Ca, V, Al, Mn, Ni, and Cr contents are similar to those of skarn deposit. REE patern indicate positive Eu and Ce and negative Gd, Yb and Sm anomalis. The source of Fe mineralization is probably a younger intrusive body at depth due to field observations. Based on principal analysis on REE, the source of ore fluid is mostly magmatic water, which was ascended within fault zone and reacted with carbonate units to form the orebody. The northeastern part of Kashmar-Kerman zone has a great potential for Fe skarn-type deposits, which should be given more attention.