جستجوی مقالات مرتبط با کلیدواژه « MVT-type deposits » در نشریات گروه « زمین شناسی »

Separation of geochemical anomalies from the background has always been a major concern of exploration geochemistry. The search for methods that can make this analysis quantitative and objective aims not only at the reduction of but also at providing an automatic routine in exploration, assisting the interpretation and production of geochemical maps (Nazarpour et al., 2015). The Malayer-Isfahan metallogenic belt with the north-west-south-east trend is the largest and most important Pb-Zn belt of MVT type in Iran (Rajabi et al., 2012). In this study, separation of Pb and Zn geochemical anomalies was performed using the methods named further in the study area.

1. Classical statistics Various statistical methods have been used to process geochemical data in order to determine threshold values. Statistical quantities, such as the mean, standard deviation (SDEV) and percentiles, have been used to define thresholds for separating anomalies form the background. For example, geochemical anomalies have been defined as values greater than a threshold value defined as the 75th or 85th percentile, and Mean+SDEV or Mean+nSDEV (Nazarpour et al., 2015). The boxplot and median+2MAD techniques of the EDA approach have been widely used to separate geochemical anomalies from the background. In exploratory data analysis (EDA) of geochemical exploration data, the median+2MAD value was originally used to identify extreme values and serve as a threshold for further inspection of large data sets (Carranza, 2009). The MAD (is the median of absolute deviations of individual dataset values (Xi) from the median of all dataset values (Tukey, 1977). 2. Multifractal models Fractal and multifractal models have also been applied to separate anomalies from background values. These methods are gradually being adopted as effective and efficient means to analyze spatial structures in metallic geochemical systems (Cheng et al., 1994). The concentration-number (C-N), concentration-area (C-A) multifractal methods have been used for delineation and description of relations among mineralogical, geochemical and geological features based on surface and subsurface data (Nazarpour et al., 2015). Fractal/multi-fractal models consist of frequency distribution and spatial self-similar or self-affine characteristics of geochemical variables. These fractal/multifractal models have been demonstrated to be effective tools for decomposing geological complexes and mixed geochemical populations and to recognize weak geochemical anomalies hidden within strong geochemical background (Cheng et al., 1994).3. Singularity Index (SI) The Singularity technique is another important process for fractal/multifractal modeling of geochemical data (Zuo et al., 2015). This technique is defined as the characterization of the anomalous behavior of singular physical processes that often result in anomalous amounts of energy release or material accumulation within a narrow spatial–temporal interval. The Singularity can be estimated from observed element concentration within small neighborhoods based on the following equation (Cheng, 2007): (1) The Singularity Index is a powerful tool to identify weak anomalies, but it is influenced by the selection of the window size. (Zuo et al., 2015).

In this study, a total of 19946 stream sediment geochemical samples were analyzed using the ICP-MS and XRF methods. In the maps derived from the Singularity Index (SI) the higher accuracy of this method compared to other applied methods was employed. Therefore, the hidden and weak anomalies are better represented, and a better overlap with limestone as the major host rock of Pb and Zn deposits (MVT type) in the study area were observed. A comparison among all of the applied methods indicates that the concentration of Pb and Zn increased toward the and south east and northwest parts, respectively. In these regions there is a high potential for the occurrence of promising mining areas. Moreover, the obtained Pb and Zn anomalies have a good correlation with the exposure of limestone in the study area.

Mississippi Valley-Type (MVT) deposits are epigenetic zinc and lead deposits with minor copper hosted by dolostone, limestone, and locally sandstone in platform carbonate sequences inboard of major orogenic belts (Leach and Sangster, 1993; Leach et al., 2010). The Irankuh-Ahangaran Belt, which is the most important Pb-Zn mineralized zone of Iran, is situated within the Sanandaj-Sirjan tectonic zone. This belt is 400 km in length and 100 km in width. Three deposits including Irankuh mininig district, Ahangaran and Hosseinabad deposits were studied in this article (Fig. 1). The aim of this research is study of thermal gradient of subducted slab and age of formation of Pb-Zn deposits at Irankuh-Ahangaran belt, which is contrary information has been published so far on the type and their formation. Also, chemistry of ore-fluid in MVT deposits and impact of dolomitic and shale host rock on paragenesis, alteration, style, reserves and grade of deposits were discussed.These parameters will certainly be useful for exploration of the hidden MVT type deposits in the Irankou-Ahangan belt. Result and Discussion: The Irankuh mineralization is hosted by Cretaceous dolostone and minor Jurassic shale rocks as epigenetic. The constructive thrust fault, which has been cut the Jurassic and Cretaceous host rocks, has played a major role in the rising of fluid and formation of mineralization. Mineralization is occurred as replacement and open space filling (fault breccia, veinlets and cavity of rock) in dolostone and breccia, veinlet and open space filling in shale host rock. The mineral assembelages are Fe-rich sphalerite, Feand Mn-rich dolomite, ankrite, galena, minor pyrite, bituminous, calcite ± quartz ± barite within carbonate host rocks, whereas quartz, pyrite, Ferich sphalerite, galena, minor chalcopyrite, low Fe-dolomite, bituminous, ± barite ± calcite are important primary minerals at clastic hos rocks (Karimpour et al., 2018). The Ahangaran deposit is very similar to Irankuh in host rock, alteration, paragenesis, and form of mineralization. Thrust fault has a constructive role for occurrence of mineralization and later destructive strike slip and normal faults have caused the displacement and destruction of mineralization. The Hosseinabad deposit is hosted by Jurassic shale, siltstone, and sandstone rocks as veinveinlets, breccia and open space filling with structural control. Alteratin consists of silicification, chlorite, bituminous, and minor siderite, dolomite and ankerite similar to mineralization hosted by shale in Irankuh district. The mineral assemblages are galena, Fe-rich sphalerite, pyrite, chalcopyrite and minor phyrotite. Due to the lack of a proper dolostone unit in the Husseinabad deposit, mineralization is concentrated in particular areas with low-grade and low-reserves. Based on lithology, alteration, mineralization style, structural control by thrust faults, mineral paragenesis, and comparison with differnet types of Pb-Zn deposits, all deposits of Irankuh- Ahangaran belt are MVT-type. Deep-seated thrust faults formed during the early stages of subduction (~ 70 to 75 Ma), and played an important role in the upward migration of hydrothermal fluids from the basement to shallow depths. The geochronology of pyrite in Irankuh district based on Re-Os method indicate age of Irankuh Pb-Zn mineralization is 66.5 ± 1.6 Ma (Liu et al., 2019). Since the thrust faults have been cut the Jurassic to Upper Cretaceous rocks, and according to the absoulte age determined in Irankuh, the mineralization of this belt have been formed in the age range of 66 to 56 million years ago, mainly in the Paleocene (Fig. 15). Karimpour and Sadeghi (2018) suggested the hydrothermal fluid originated from the dehydration of a hot and young oceanic subducted slab, which liberated Pb, Zn, and other metals, and may have removed metals from rocks and organic material of the continental crust. More than 90% of all the water within the oceanic slab was released in the depth zone of the forearc region (depth of 30 to 50 km) (Karimpour and Sadeghi, 2018). In the depth zone, Mg-rich silicate minerals (such as antigorite, hornblende, chlorite, talc) have broken and the produced fluid is rich in Mg and Fe (Fig. 17). The ore-fluid of MVT deposits is Si-poor and Feand Mg- rich. Such fluid is mineralized on the hosts of the dolstone (Irankuh and Ahangaran) or Shale-Siltstone (Hossein Abad, and part of Irankuh and Ahangaran). There are significant differences in the type of paragenesis, alteration, shape, dimensions, reserves and grade in the deposits of this belt, which is controlled by the host rock type. Based on all lithological evidence, alteration, shape of mineralization, existence of thrust faults, mineral paragenesis and specific geological and geographic location, it can be used to exploration of the hidden MVT deposits in this belt.