Ore mineralogy, mineral chemistry and genesis of carbonate-hosted zinc-lead non-sulfide deposit, Mansourabad, central Iran

Introduction
Iran possesses vast regions with high potential for carbonate-hosted Zn–Pb deposits, largely due to its favorable geodynamic framework (Rajabi et al, 2012). The majority of the country’s sedimentary-hosted lead and zinc deposits are concentrated within four major metallogenic belts, spanning from the Early Cambrian to the Tertiary period and occurring in a diverse range of siliciclastic and carbonate rock formations. These belts include Central Alborz, Tabas-Posht-e-Badam, Malayer-Esfahan, and Yazd-Anarak (Fazli et al, 2019; Rajabi et al, 2012, 2023). Among these, the Yazd-Anarak Metallogenic Belt (YAMB), located within the Yazd Block in western Central Iran, hosts some of the country’s largest lead and zinc deposits, including Mehdiabad, Darreh Zanjir, and Mansourabad. This region is recognized as one of Iran’s most significant metallogenic provinces. The Mansourabad deposit, situated approximately 75 km southwest of Yazd city, lies within the Yazd-Anarak metallogenic belt and represents a key Pb-Zn mineralization site. Despite the previous geological, mineralogical, geochemical studies the Mansourabad sulfide ore, there are no detailed investigations on the Mansourabad non-sulfide ore. The main objective of this study is to comprehensively investigate the mineralogical, textural, and petrographic characteristics of the Mansourabad non-sulfide ores and to develop a genetic model.
 
Materials and Methods
Sampling and analytical methods
For geological investigations and the study of non-sulfide ore characteristics (identification of zinc-bearing minerals using Zinc Zap fluid), field visits were conducted, and 30 samples were collected from all sections. To carry out mineralogical studies and examine the structure and texture of the non-sulfide ore, 15 XRD analyses, 12 thin sections, and 15 polished thin sections were prepared at ZarAzma Laboratory and studied at Shiraz University. All thin sections and polished thin sections were examined using an Olympus microscope under both XPL and PPL light at magnifications of 4X and 10X. Additionally, to identify mineral phases and analyze the distribution and concentration of elements, 12 polished thin sections were studied using a scanning electron microscope (SEM) model TESCAN-Vega3, which has a resolution higher than 50 nanometers, at the Central Laboratory of Shiraz University. Before conducting the study of polished thin sections in this device, all sections had to be coated with gold using a sputter coater.
Geologic setting and Ore Mineralogy
Mansourabad deposit is hosted by the Lower Cretaceous sedimentary sequence in the south of the Yazd basin. This sequence is based on lithological differences characteristics from bottom to top classified into three parts: Sangestan Formation, Taft Formation, and Abkoh Formation.




Ore mineralogy, mineral chemistry and genesis of carbonate-hosted zinc-lead non-sulfide deposit                        Khadivar and Taghipour / 2




Sulfide and non-Sulfide Zn-Pb mineralization is formed in the upper part of the dolomites of the Taft formation. The mineral observed in the sulfide part is generally galena, Sphalerite is rarely occurred due to it leached into non-sulfide zinc minerals. The field observations, petrographic and XRD results, determined that the main minerals of the non-sulfide ore included smithsonite, hydrozincite, hemimorphite and cerussite along with iron oxides.
 
Results and Discussion
Dolomitization is the major hydrothermal alteration styles in the Mansourabad deposit. According to the textural and petrographical studies, three types of dolomites were identified in Mansourabad Zn-Pb deposit Among these, type III dolomite (hydrothermal dolomite) hosts sulfide and non-sulfide mineralization in this deposit. Seawater is the source of fluids in this deposit, which, by circulating in the detrital sedimentary rocks of the underlying unit (Sangestan Formation), leach metals such as lead, zinc, silver, and copper (metal-bearing brine), moves upward through the syn-sedimentary normal fault. Mixing of acidic metal-bearing brine with cold, reduce, sulfur-containing seawater causes the formation of hydrothermal dolomite and the deposition of sulfide mineralization sub-seafloor in the limestone unit (hydrothermal dolomite) of the Taft Formation. Like all non-sulfide deposits in the world, the Mansourabad non-sulfide ore was formed under conditions of uplift, dry climate, and fault development. In the Late Cretaceous, the closing of the Neotethys ocean and the dominance of pressure conditions in central Iran led to the creation of orogenic activities (the formation of the Laramide orogeny) and a structure that, together with the climatic conditions (hot and dry) of Central Iran, caused the weathering and oxidation of the sulfide part and its transformation and change into the non-sulfide part. Faults in the Mansourabad deposit have led to the penetration of oxidizing meteoric waters into the host rock (carbonate rocks) and their dissolution. This interaction between oxide fluids and sulfide ore has caused the formation of the non-sulfide part in the Mansourabad deposit in the form of host rock replacement (white ore), sulfide mineral replacement (red ore). In dry climates, dissolved oxygen in meteoric waters reaches its highest level compared to other climates.
 
Conclusion
The Mansourabad non-sulfide ore deposit is hosted within the dolomitized limestones of the Taft Formation, dating back to the Early Cretaceous. This deposit contains both sulfide and non-sulfide mineralization. In the non-sulfide zone, minerals such as smithsonite, hydrozincite, hemimorphite, and cerussite, along with iron oxides, are observed. The most significant type of alteration in this area is dolomitization, followed by calcitization. The formation of the non-sulfide ore in this region has been influenced by various factors, including the composition of primary minerals (which determines the type of secondary minerals), the nature of the host rocks, faults and fractures (which facilitate water infiltration and accelerate oxidation), and climatic conditions. In the Late Cretaceous, with the closure of the Neotethys Ocean and the Laramide orogeny, tectonic pressures, along with the warm and arid conditions of Central Iran, intensified the weathering and oxidation of sulfides. The rapid uplift of the crust brought sulfide minerals closer to the surface, accelerating their oxidation. Additionally, faults provided pathways for oxidizing meteoric waters, facilitating the dissolution of the host rock and the replacement of sulfide minerals. These conditions led to the extensive oxidation of sulfides and the formation of the non-sulfide ore deposit in the region.