Mineralogy and geochemistry of sulfide zone of Tarz Pb-Zn mine, North of Kerman province: A case study of carbonate-hosted sedimentary ore deposits

The Tarz Pb-Zn deposit is one of the underground active mines in the Ravar-Kuhbanan area, located in the North of Kerman province. In this mine, mineralization was observed in two distinct sulfide and carbonate divisions within the dolomitic-limestone host rock units of the Middle Triassic succession (Shotori Formation). Field observations show that mineralization occurred mainly along the faults as veins or veinlets and in lesser extent as massive textures. Galena, sphalerite, pyrite and chalcopyrite are the most important primary sulfide minerals in the Tarz Pb-Zn deposit. Secondary minerals associated with the Pb-Zn deposits such as smithsonite, hemimorphite, cerussite and anglesite were also identified in some of the investigated samples. The average concentrations of Pb and Zn were 19 and 24% in the studied samples, respectively. Other elements such as Cd, Ag, S, Sb, Te, Se, As and Cu also showed significant enrichment in the investigated samples. Evaluation of the obtained results using multivariate statistical methods can reveal the possible relationships between mineralogical phases and geochemical analysis. For example, the correlation between Pb, Ag, Tl and Sb can be related with the galena mineralization or the geochemical relationship between As, Bi, Cu, P, Fe, Co, and S is related with the pyrite and chalcopyrite mineralization. Statistical relationships also showed that Zn has only a weak geochemical association with Se, U and Mo. The strong correlation of Ca and Mg is also due to the host-rock mineralogy, which is mainly composed of course and fine crystal dolomites at the margin and far distances of ore veins, respectively. Keywords: Mineralogy, Geochemistry, Tarz Pb-Zn deposit, Kuhbanan, North of Kerman  

Tarz Pb-Zn deposit is located at 30 km east of Kuhbanan and 5 km southwest of the Tarz village in the North of Kerman Province. From geological point of view, this mine is located on the southeastern margin of the Bahabad Pb-Zn belt, which is a part of Tabas-Poshte-e-Badam metallic belt of the Central Iran (Alavi 1991; Rajabi et al. 2013). In this area, there are a large number of Pb-Zn deposits that mostly occurs in the Triassic dolomite-limestone beds of the Shotori Formation. Major faults such as Kuhbanan and Behabad with NW-SE direction have been very effective role in the development of geology and mineralogy history of this region. However, The dominant trend of the faults in Tarz Pb-Zn deposit is about N40E, but field observations showed various faults and fractures in different directions, which demonstrate the active tectonic of this area. For example, one of the major mineralization parts of the mine was in a shearing zone produced by four major faults with NW-SE direction. Some authors such as Amiri et al. (2009) believe that based on the position of mineralized sections in the carbonate host rocks the Tarz Pb-Zn mine is classified as the Mississippi Valley Type (MVT) deposits. In the Tarz mine, mineralization is observed in two distinct sulfide and carbonate sections, which are located in the north and south of the mine, respectively. This study emphases on the texture, mineralogy and geochemistry of sulfide section of the Tarz Pb-Zn deposits in order to determine the elemental dispersion, and reconstruction of depositional history of Pb and Zn minerals.  

Fifty rock samples were collected from mineral veins and hosted-rock for mineralogical studies of sulfide section of the Tarz Pb-Zn mine. Ore mineralogy studies were done on the 30 polished-thin section and five polished-blocks by reflecting and polarizing microscope (OLYMPUS BH-2 model) at the Geology Department of Shahid Bahonar University of Kerman. Also, eight samples were selected for further mineralogical studies by X-ray diffraction (XRD) method in the Zarazma Laboratory, Tehran, Iran. After mineralogical studies, 10 samples were selected for major and trace elements analysis by the ICP-MS method in the Zarazma Laboratory. Geochemical data were used to (1) calculate enrichment factor; (2) identification of enriched elements; and (3) geochemical correlations of target elements. The spider diagrams of major and trace elements were plotted in order to determine their changes from ore veins to the surrounding hosted rocks. For drawing of spider diagrams, the data were normalized against the standard limestone.  

Mineralogy Mineralogical studies using ore microscopy and XRD showed that both primary and secondary minerals are present in the sulfide section of the Tarz deposit. The most important primary minerals were galena (about 60%), sphalerite (less than 40%), pyrite and chalcopyrite. These minerals were often found in the mineralized sections in the northern part of the mine, where mineralization occurs mainly as veins and veinlets, breccia and in lesser extent as pore-filling structures. The secondary minerals including smithsonite, hemimorphite, cerussite, anglesite and iron oxides also were identified in different parts of the mine. In addition to the mentioned minerals, dolomite and calcite were also observed as the main minerals of the hosted-rocks. The identified dolomites minerals can be classified into two groups including fine-crystalline and sudhedral, coarse crystalline or saddle dolomite. It seems that fine-crystalline dolomites have formed as primary minerals during the early time of diagenesis, while the coarse-crystalline dolomites have formed by hydrothermal fluids. Calcite in the hosted rocks was mainly in the form of microcrystalline or micrite type and it seems that this mineral is not affected by the Pb-Zn mineralization process.   Structure and texture The structures and textures of mineralization in the Tarz mine, were as veins and veinlets, massive, breccia, pore-filling and replacement types. These structures are mainly related to the active fault system of the region. Galena, sphalerite, pyrite and dolomite were the most important primary minerals in most of mineralized textures, however, secondary minerals such as smithsonite, hemimorphite and cerussite were also observe in pore-filling and replacement textures.  

According to the results obtained from normalized enrichment factor Pb, Cd, Zn, Ag, S, Sb, Te, Se, As and Cu have moderate to high enrichments in the investigated samples, respectively. Also, U, Tl, Bi, Mo, Co and Cr showed low enrichments in some samples. The geochemical relationships between enriched elements was well compatible with the mineralogical results. For example, identified sulfide minerals (galena, sphalerite, pyrite and chalcopyrite) have a great potential for replacing of several of enriched elements in their crystalline structures as impurities. The geochemical associations of most of the target elements in sulfide minerals can be considered as possible mechanisms for enrichment of these elements in the investigated samples. These geochemical association can be revealed using multivariate statistical methods. For examples, the following groups of geochemical associations were observed in the investigated samples:

1-     The correlation between Pb, Ag, Tl and Sb, which can be related with the galena mineralization; 2-     The correlation between P, Cu, Bi, As elements with Fe, Co and S can be related with the pyrite and chalcopyrite mineralization; 3-     The strong correlation between Ca and Mg is due to the mineralogy of the host-rock minerals; 4-     Zinc only showed a weak correlation with Mo, U and Se, which may be due to the alteration and weathering of its primary sulfide minerals mainly into the secondary carbonate minerals; The spider diagrams of major elements showed that except iron, other elements have the same values with standard limestone. Ore forming processes, especially the presence of sulfide minerals such as pyrite, chalcopyrite and even sphalerite are responsible for iron enrichment in the investigated samples. Contrasting to the most of major elements, trace elements, especially Pb, Cd, Zn, Ag, S, Sb, Te and Se, As and Cu, showed strong deviation from their values in the standard limestone. Concentration of these elements were decreased by increasing distance from ore veins to the host rock.   Possible mineralization model The Pb-Zn mineralization in the Tarz mine as well as other ore deposits in the Ravar-Bahbad region is often as veins and veinlets. A variety of lithological and structural factors control Pb-Zn mineralization in this region. According to Leach et al. (2005) diagenesis and tectonic processes have a fundamental role in the formation of MVT type Pb-Zn deposits. Field observation demonstrate that active tectonic and formation of a complex fault system have more important role than other factors in the formation of Tarz Pb-Zn deposit. Comparison of the obtained mineralogical and geochemical data in this study with the previous researches revealed that the Pb-Zn mineralization in Tarz area is more similar to the MVT type. Nevertheless, to determine the source of ore-bearing fluids and to draw up a comprehensive mineralization model, isotopic and structural data are required.