Mineralogy, fluid inclusions and genesis of the Middle Triassic carbonate-hosted Chah-‌Mileh Pb-Zn deposit (NE Anarak), Central Iran

The Himalayan-Tibetan and Zagros Mountain ranges which are the youngest and most extensive continental-collision orogens in Tethyan domain host many important sediment-hosted Pb-Zn deposits, including the world-class Jinding, Huoshaoyun, Mehdiabad, and Angouran deposits (Reynolds and Large, 2010; Rajabi et al., 2012; Rajabi et al., 2015; Hou and Zhang, 2015; Song et al., 2017). More than 300 sediment-hosted Pb-Zn deposits and occurrences have been identified in Iran (Rajabi et al., 2013). Cretaceous and Triassic carbonate successions are the most common host rocks for these deposits, which are largely distributed in both the Malayer-Esfahan metallogenic belt (MEMB) and the Yazd-Anarak metallogenic belt (YAMB) (Rajabi et al., 2012). The YAMB is located at the Yazd Block, northern margin of the Central Iranian Plate. Several Pb-Zn deposits and occurrences such as Mehdiabad, Nakhlak, Hovz-e-Sefid, Darreh-Zanjir, Mansurabad, Chah-Kharboze and Chah- Mileh have been identified distinguished at YAMB. The Chah- Mileh deposit is in 30 km northeast Anarak, 220 km northeast of Isfahan, YAMB. The Chah- Mileh Pb-Zn district is located in the Anarak Metamorphic Complex (AMC). There are three Pb-Zn deposits that have been recognized at the Chah- Mileh district, including Kuh-e Mileh, Mazra-e Deraz, and Seilacho. In this paper, we investigate geology, texture, mineralogy, alterations, fluid inclusions and genesis of the Chah- Mileh Pb-Zn deposit. The present research study provides more insight into understanding of geology and mineralization conditions in the study area with an implication for future exploration. 

A total of 120 samples were collected from the host rocks and ore deposit. They were studied by a transmitted/reflected polarizing microscope, X-ray Diffraction (XRD) and Scanning Electron Microscope-Energy Dispersive X-ray analyzer (SEM-EDS). Thin sections were stained  to differentiate calcite and dolomite according to the method of Dickson (1966). Fluid inclusion microthermometry was performed using a Linkam THMS600 heating-freezing stage (-190 °C to +600 °C) mounted on a ZEISS Axioplan2 microscope at the Kharazmi University (Tehran). Fluid salinity (wt.% NaCl eq.) and density (g/cm3) were calculated using the FLINCOR v.1.4 (Brown, 1989) and FLUIDS (Bakker, 2012). 

The Chah- Mileh Pb-Zn is a stratabound and epigenetic deposit hosted in dolomitic marble of the Chah-Gorbeh Complex with Middle Triassic age. Mineralization is composed of sulfide minerals (e.g., galena, sphalerite, chalcopyrite and minor pyrite) and non-sulfide minerals (e.g., cerussite, mimetite, wulfenite, litharge, hemimorphite, smithsonite, malachite, hematite, goethite). The gangue minerals are mainly composed of quartz, dolomite, calcite, and barite. Silicification and dolomitization are the two main types of hydrothermal alterations. Three mineralization stages were recognized in the Chah-Mile deposit: 1) pre mineralization stage characterized by fine-grained disseminated pyrite, 2) main hydrothermal stage characterized by galena, sphalerite and chalcopyrite and 3) post-ore mineralization consisting of secondary sulfides and non-sulfide. Four types of fluid inclusions including two-phase liquid-rich (LV), two-phase vapor-rich (VL), monophase liquid (L), and monophase vapor (V) were observed in the dolomite, quartz and calcite. Microthermometric measurements show that ore minerals were precipitated from low-temperature (81 to 167 °C) and moderate salinity fluids (7.02-22.2 wt.% NaCl eq.). Basinal hydrothermal fluids were responsible for ore mineralization at the Chah- Mileh deposit. Ore mineralization at the Chah- Mileh deposit has been formed as a result of fluid mixing. The formation of large siliceous zones in an area is a sign of hydrothermal fluid rising to the surface and mixing and diluting with low-temperature meteoric waters. Considering all the geological evidence, mineralization style, orebody texture and structure, alterations and fluid inclusion microthermometry, it may be inferred that the Chah- Mileh deposit is similar to the Mississippi Valley-type deposits.