Temperature condition, sulfidation state, and gold formation mechanism of the Atash- Anbar polymetallic deposit (south Qazvin) based on mineralization, alteration and chemistry of ore minerals

The NW-trending Urumieh-Dokhtar Magmatic Belt (UDMA) in the central-western Iran is a part of the Eastern Tethyan orogenic belt (Deng et al., 2018) and over past decades, it has become one of the most significant polymetallic (Au-Cu-Fe-Pb-Zn) provinces in Iran (Rabayrol et al., 2019; Ismayıl et al., 2021). While the Cenozoic magmatism and metallogeny of the UDMA are well known, its ore-forming potential during this period is still poorly understood (Alipour-Asll, 2019; Tale Fazel et al., 2019; Zamanian et al., 2020). Despite well-known Eocene to Miocene hydrothermal systems associated with extension-related and/or arc magmatism from the UDMA, the temporal and spatial association between continental-arc setting, Middle-Eocene magmatism and epithermal Au mineralization in the Buin-Zahra Range is not well understood. The Atash-Anbar polymetallic deposit (35°44′ N and 49°35′ E) is located ca. 70 km south of Qazvin city, central-northern Iran. A drilling program ~1000 m (including ten drill-holes) identified about 2 Mt of proven reserves grading at 2.13 g/t Au (locally up to 14 g/t) and 4.11% Pb + Zn + Cu (Pirooz, 2015). Herein, we focus on the textural, paragenetic relationships, and mineral chemistry of the Atash-Anbar polymetallic deposit. We focus on: (1) documenting the chemical composition of the different sulfides, (2) determining the chemical state of gold in iron sulfides, and (3) determining the sulfur activity

About 70 rock samples were collected from various parts of the deposit to determine the mineralogy, mineral textures, and mineral chemistry. The chemical composition of ore minerals was analyzed at the Analytical Center for multi-elemental and isotope research of SB RAS in Novosibirsk, Russia using a JEOL JXA-8100 electron microprobe (Japan) with five wavelength dispersive spectrometers and an energy dispersive spectrometer. Operating conditions were: 15-20 kV accelerating voltage, 30 nA beam current, and 20-30 s counting time. Sulfides and sulfosalts were analyzed for Fe, S, As, Cd, Sb, Te, Ag, Au, W, Zn, Pb, Bi, Hg, and Cu. The detection limits for major and minor elements are approximately 0.05 and 0.01 wt.%, respectively. Ten rock powdered samples were also analyzed using the X-ray diffraction (XRD) spectrometry (X′ pert Philips) in order to identify the mineralogy of clay minerals at the Iranian Mineral Processing Research Center (IMPRC), Karaj.

Au-polymetallic veins in the Atash-Anbar deposit with an epigenetic nature and an approximate length of 5 to 80 meters and a maximum depth of 45 meters have been formed in the Middle Eocene andesite, dacite, and rhyodacite host rock (Eord unit). Mineralization with vein and veinlet, crustiform, colloform, breccia, and disseminated textures were occurred in three stages: pre-mineralization (disseminated pyrites), main mineralization (chalcopyrite vein (II-A), quartz-sulfide breccia veins (II-B) and barite-sulfide vein (II-C)), and post mineralization (late carbonate vein (III-A) and supergene (III-B)). Argillic alterations (kaolinite-illite±dickite mineral assemblage) together with silicification alteration (quartz± jasper assemblage) are the main alterations in the area. At Atash-Anbar deposit, gold is present both as native gold (Au0) and invisible solid-solution (Au+) in sphalerite and pyrite compositions. According to Co/Ni ratio (1.2 to 45), pre-mineralized disseminated pyrites have volcanic affinity and zoned pyrites have a hydrothermal origin. As the result of changes in Cd concentration (4657 ppm on average) and Zn/Cd ratio (18.68 on average) in the sphalerites, mineralization considered as high-medium temperature hydrothermal system (200 to 250 °C). Also, based on FeS mol% content of sphalerites (which varies from 0.11 to 0.4), the LogfS2 changes are between -10 and -14, which are consistent with the high to intermediate sulfidation deposit. Due to the paragenetic association of gold with pyrite and sphalerite ores, the absence of oxide minerals, the medium temperature of ore-forming fluid, near-neutral to acidic nature of pH (presence of argillic±sericitic alteration), and the high activity of sulfur show that the gold solubility and its transfer were conducted by Au(HS)2– bisulfide complex. Evidence suggests that the increased logfO2 is caused the decreased S2– activity and it has resulted in instability of bisulfide complex and gold deposition in quartz-sulfide breccia vein.