Magnetite chemistry in the Dalli porphyry Cu-Au deposit, central Urumieh-Dokhtar Magmatic Arc (UDMA)

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.