The Early Cambrian Chahmir shale-hosted Zn-Pb deposit, Central Iran: An example of vent-proximal SEDEX mineralization
Abdorrahman Rajabi, Ebrahim Rastad*, Carles Canet, Pura Alfonso
Abdorrahman Rajabi, Ebrahim Rastad*
1-Department of Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, 14115-175, Iran
2-Department of Geology, Faculty of Basic Sciences, University of Birjand, Birjand, Iran
Ebrahim Rastad*
Department of Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, 14115-175, Iran
Carles Canet
Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., Mexico
Pura Alfonso
Departament d’Enginyeria Minera i Recursos Minerals, UniversitatPolitècnica de Catalunya, Avinguda Bases de Manresa 61-73, 08242 Manresa,Catalunya, Spain
Geochemical analytical methods
The geochemical characteristics and distribution of Zn, Pb, Cu, Cd, Ag, Ba and Fe in different parts of the depositwere determined by analyzing 899whole rock samples from drill cores by atomic absorption spectroscopy (AAS). For these analyses half of whole cores separated and samples were collected continuously based on their mineralogical and petrological characteristics. Approximately 1 to 3 kg of host and mineralized rocks were selected from the cores for each sample. Also, 15 samples from different ore facies were analyzed for whole-rock trace elements compositions by inductively coupled mass spectrometry (ICP-MS). Approximately 500 g samples of host and mineralized rocks from the Chahmir deposit were selected for ICP-MS analyses. All samples were crushed, and then about 50 g of sample pulps were ground into powders to 200-mesh size using Cr-steel pestle and mortar. The AAS analyses were performed at the Esfordi and AMG (Anguram Mining Group) laboratories of Iran, and the ICP-MS analyses of selected rock samples, representative for different parts of ore facies, were carried out at theALS-Chemex laboratories in Canada.
Trace elements were determined by ICP-MS following a Lithium metaborate fusion and nitric acid digestions on a 0.2 g sample. A lithium metaborate fusion of the sample prior to acid dissolution and ICP-MS analysis provides the most quantitative analysis for a broad suite of elements. This technique solubilizes most mineral species, including those that are highly refractory. The analyzed elements and its detection limits are: Ba (0.5 ppm), Ce (0.5 ppm), Co (0.5 ppm), Cr (10 ppm), Cs (0.01 ppm), Cu (1 ppm), Dy (0.05 ppm), Er (0.03 ppm), Eu (0.03 ppm), Ga (0.1 ppm), Gd (0.05 ppm), Hf (0.2 ppm), Ho (0.01 ppm), La (0.5 ppm), Lu (0.01 ppm), Mo (2 ppm), Nb (0.2 ppm), Nd (0.1 ppm), Ni (5 ppm), Pb (1 ppm), Pr (0.03 ppm), Rb (0.2 ppm), Sm (0.03 ppm), Sn (1 ppm), Sr (0.1 ppm), Ta (0.1 ppm), Tb (0.01 ppm), Th (0.05 ppm), Ti (0.5 ppm), Tm (0.01 ppm), U (0.05 ppm), V (5 ppm), W (1 ppm), Y (0.5 ppm), Yb (0.03 ppm), Zn (1 ppm), Zr (2 ppm).
Fluid inclusion microthermometry
Development of sphalerite and galena mineralization (stage II) within the massive ore facies and Stockwork zone of the Chahmir deposit is associated with silicification and precipitation of some fine to coarse-grained quartz grains. Figure 5 shows parallel dissections of a sample from boundary part of the massive ore and bedded facies of the Chahmir deposit. Mineralogical studies of these slices indicate intergrowth and co-precipitation of sulfide minerals (sp2 and gn2) and quartz within the ore vein and veinlets. A process referred as "zone refining", whereby the earlier laminated pyrites and associated materials (organic matter) are replaced by galena-sphalerite and quartz and form massive high-grade ore at the center of ore system. In all sample slices, quartz is paragenetically associated with stage II of sulfide minerals, hence the primary inclusions within quartz grains can represent the ore forming fluids during sulfide precipitation.
In this study, seven doubly polished wafer specimens of quartz grains were used for petrography studies of fluid inclusion, then five wafers were selected forfluid inclusion analyses: two samples (CHM-4a and b) from the CH1 drill core and three samples (CHM-3, CHM5 and CHM-6) from the CH6 drill core(numbers 1 and 6 drill cores in Fig. 5). Samplescontaining quartz grains and veinlets from the major massive sulfide ore zone in the deposit were selected for investigation. Only primary inclusions(according to the criteria of Roedder 1984) were examined. Microthermometry analyses werecarried out on the Department of Geology, Tarbiat Modares University, Iran. Samples were mounted on a Linkam TMS 94 heating-freezing stage, where each sample was cooled to -50°C at a rate of ~80°/min to freeze the aqueous inclusions contents. The sample was then heated at a steady rate of 5°/min to obtain the temperature of first melting (Tml) and the liquid–vapor homogenization temperature (Th). A total of 40 fluid inclusions were analyzed and in order to ensure reliable data, the measurements of phase transitions were duplicated. Calibration of the heating-freezing stage was carried out using the in-house melting point of the standard references (Chloroform CHCl3 for -63.5°, water H2O for 0° and Potassium Dichromate K2Cr2O7 for +398°C) of calibration, based on the calibration manual of Linkam TMS 94 heating-freezing stage.
Tables
Table 1 Selected element geochemistry for samples from the Chahmir deposit (depth in meter; Ag, Cu and Cd in ppm; Fe, Zn and Pb in percent).
Table 2 Selected minor and trace elements whole rock contents (in ppm).
Table 3 δ34S values for the Koushk (Rajabi et al. 2012) and Chahmir deposit. The “I” and “II” suffixes denote Stage I and Stage II, respectively, as in Figure 11.
Table 4 Summary of the microthermometric characteristics for the quartz fluid inclusions within SSC.
Fig.1General stratigraphical correlation of the sedimentary dominated upper sub-sequence of the ECVSS in Chahmir-Darreh Dehu, Koushk and Lakeh-siyah areas and the location of the Chahmir and Koushk deposits with some Zn-Pb occurrences within the black siltstone unit (ECVSS: Early Cambrian volcano-sedimentary sequence, M.: Member).
Fig. 2BSE images. a and b Spherical framboidal pyrite (Py1) grains with galena (Gn1) core. Furthermore, one micron-sized gold (Au) grain was found. Ap, apatite; Qz, quartz. c botryoidal-layered Pyrite (Py) aggregates with scattered grains of galena (Gn) and sphalerite (Sp). d and e Rhythmic layered aggregates of pyrite (Py1) and Galena (Gn1) that are replaced by stage II galena (Gn2) in some parts of the sulfide-carbonate sub-facies of the massive ore. Cc: calcite. f Inclusions of arsenopyrite (Apy) in galena (Gn2). g Galena (Gn2) mineralization associated with silicifacation in the sulfide-silica-carbonate sub-facias of massive sulfide ore.
Fig. 3Distal facies of the Chahmir deposit. a Microscopic photograph of disseminated barite (Ba) and Fe- and Mn-bearing dolomite (Do) within silty crystal tuff. b Handspecimen photographs of cherty bands in distal facies.
Fig. 4 Handspecimen (a, b and e), microscopic (c) photographs and BSE image (d) SSC sub-facies illustrating close relationship and intergrowth of galena (Gn2) and sphalerite (Sp2) with quartz (Qz).
Fig.5 Development of sulfide mineralization (stage II) and silicification within different parallel dissections of a sample from of the massive ore-bedded facies boundary part of the Chahmir deposit. Figures b to g show the transactions of each slice in a and show intergrowth of sphalerite and galena within sulfide-quartz veins that cut the pyrite laminae. Sp: sphalerite, Gn: Galena, Om: organic matter, Qz: quartz.
Fig. 6Reflected light microscopic photographs (a and c) of bedded ore that indicate the growth of py3 on small spherical grains of framboidal pyrite (Py1) within pyrite and pyrite-sphalerite laminae. These textural relations reveal that py3 nucleated on preexisting pyrite (py1) and had grown by displacement within the preexisting framboidal pyrite laminae. b and d show schematic photographs of a and c respectively.
Fig. 7Lateral zonations of Pb (a), Zn (b) and Fe (c) from northeast to southwest across the deposit based on AAS geochemical dada (ESM Table 1).
Fig. 8Vertical zonation of metals and metal ratios in DDH1 that intersects the massive ore (see Fig. 5). The Zn/(Zn+Pb), Zn/(Zn+Fe) ratios and Zn and Pb contents increase, and Cu/(Zn+Pb), Cu/(Zn+Cu) and Ag/(Pb+Ag) ratios decrease, upward. (S: Siltstone and shale, M: Massive sulfide, T: Tuff, Sd: Siltstone and shale with dolomitic nodules and debris felow)
Fig. 9Core photos showing microbial mat-like textures preserved in pyrite and laminated base metal sulfide ore. Polished slabbed core showing crinkly laminations in bedded pyrite (Clp in a and b) and microbial nodular pyrites (Np in a). Om: organic matter, Bms: laminated base metal sulfides including sphalerite and pyrite, Flp: framboidal laminated pyrite.
References
Rajabi A, Rastad E, Alfonso P, Canet C (2012) Geology, ore facies and sulfur isotopes of the Koushk vent-proximal sedimentary-exhalative deposit, Posht-e-Badam block, Central Iran. Intl Geol Rev 54: 1635-1648,DOI:10.1080/00206814.2012.659106
Roedder E (1984) Fluid inclusions. Reviews in Mineralogy 12:644 pp