The role of sulfidation of Fe-carbonate rocks in increasing gold contents at the Zarshuran deposit (northern Takab), Takab-Angouran metallogenic district

Document Type : Research Article

Authors

• Sharareh Heshmatnia ¹
• Ebrahim Tale Fazel ²
• Abbas Oroji ³

¹ M.Sc., Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

² Assistant Professor, Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

³ Ph.D. Student, Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

10.22067/econg.2022.75417.1042

Abstract

Iron and arsenic sulfides are considered as the most important gold hosts in the sediment-hosted disseminated invisible gold deposits. The Zarshuran gold deposit (155 tons Au with average grade of 2.63 g/t) is formed in the Lower Cambrian black shale and siltstone (Zarshuran unit) and Fe-rich carbonates (Chaldagh unit) host rocks. As-sulfide (e.g., realgar and orpiment) and arsenian pyrites are the most important host minerals of gold in this deposit. Based on EPMA data, pyrite with As content below the detection limit to 3.99 wt% occurs in six different types, respectively, Py0 (gold content of 0.01 ppm), Py1 (gold content of 0.02 ppm), Py2 (gold content of 0.03 ppm), Py3 (gold content of 0.02 ppm), Py4 (gold content of 0.04 ppm), and Py5 (gold content of 0.01 ppm). According to the evidences, gold can be present as participating in chemical bounded (Au⁺ and Au⁺³) or nanoparticle inclusions (Au⁰). The weak geochemical correlation (R² = −0.6) between As and S elements in pyrites indicates that there is pyrite with a complex composition [Fe(S,As)₂Au₂S⁰], which As⁻ has replaced S²⁻. Mineralogy and the abundance of Fe and S in the rock units suggest that gold mineralization in the Zarshuran deposit is well occurs in response to sulfidation process. Sulfidation occurs when H₂S-rich ore-forming fluids react with Fe-bearing carbonate host rock to form pyrite, marcasite, and pyrrhotite minerals.
 
Introduction
Sediment-hosted disseminated gold deposits with high gold grade (> 1000 g/t) are known to contain gold as participating in chemical bounded (Au⁺ and Au⁺³) or nanoparticle inclusions (Au⁰)in the pyrite composition (Deditius et al., 2014). The Zarshuran gold deposit (155 t @ 2.63 g/t Au; Madan-Zamin Company, 2020) is situated 35 km north of Takab in the Takab-Angouran metallogenic district, NW Iran. According to Mehrabi et al. (1999) and Asadi et al. (2000), gold mineralization in the Zarshuran deposit is in many respects like the Carlin-style and Carlin-like gold deposits, respectively. Gold substitution has been reported to be below detection limit to a maximum of 0.64 wt.% and 3 wt.% in arsenian pyrite and arsenopyrite lattice, respectively (Voute et al., 2019). Here, we examine the relationship between geochemistry and textures of pyrite, with a view to constraining the mechanism of gold precipitation. We use electron probe microanalysis (EPMA) to study the composition of different pyrite types. The results show that distribution of ferrous iron (Fe²⁺) is controlled by the initial diagenesis of Fe-C-S systems.
 
Materials and methods
During field studies, mineralization outcrops and their associated host rocks were identified. After preparing thin and thin-polished sections, mineralogical studies were carried out by transmitted and reflected polarizing ZIESS Axioplan2 microscope. To identify and classify different pyrite types and investigate the possibility of  presence of gold and trace minerals, eight thin-polished sections were prepared, and carbon coated. This study was carried out by scanning electron microscope (SEM) with EVO MA15 model in the Central Laboratory of Kharazmi University (Tehran, Iran). Then, to achieve the chemical composition of different types of pyrite, 104 points were examined by EPMA (model JEOL JXA-8530F) in the Laboratory of Geo Forschungs Zentrum (GFZ). Spot analysis was performed with a voltage of 20 kV, electron beam current of 10 nA, X-ray diameter of 2 μ and radiation time of 5 to 20 seconds. The standard examples used to calibrate the various elements in this experiment were as follows: FeS₂ (for Fe and S), CoAsS (for As and Co), and free gold (for Au). The detection limits of the elements are Fe (300 ppm), As (200 ppm), S (300 ppm) and Au (200 ppm). To identify rare minerals such as mackinawite (Fe₉S₈), HR microscopic confocal Raman technique was used in the Central Laboratory of Shiraz University (Shiraz, Iran). The experiment was performed with an X50 laser, a wavelength of 785 nm with a power of 100 mw and a radiation time of 20s.
 
Results and discussion
Based on SEM and optical-microscopic studies, six types of pyrite have been identified in the Zarshuran deposit. They are (1) framboidal pyrite (Py0) (avg. = 30 μm in diameter), (2) fine-grained disseminated pyrite (Py1) (< 20 μm in diameter), (3) coarse-grained euhedral pyrite (Py2) (avg. = 100 μm in diameter), (4) porous/sponge pyrite (Py3) (avg. = 300 μm in diameter), (5) colloform pyrite (Py4) (avg. = 550 μm in diameter), (6) vein pyrites (Py5) (avg. = 55 μm in thickness). Decalcification and sulfidation of host rocks are two important mechanisms in genesis of sediment-hosted gold deposits, and the importance of sulfidation depends on the reactivity of Fe²⁺ and H₂S rocks (Voute et al., 2019). The interaction of H₂S-rich hydrothermal fluids with reactive iron originating from the host rocks (Cail and Cline, 2001) or through hydrothermal fluids added to the environment (Reich et al., 2005) which results in formation of Au(-As)-rich pyrite. Arsenic in pyrite can occur in various oxidation states that correspond to different crystallographic sites in the lattice and different substitution mechanisms. As⁻ substitution for S²⁻ is found in reducing conditions and often in sediment-hosted gold deposits; while As cations (As²⁺, As³⁺, As⁵⁺) replace Fe²⁺ under oxidation conditions (Reich et al., 2005; Deditius et al., 2014). The diagram of arsenic versus sulfur in the Zarshuran deposit shows that the concentration of As in the formed pyrites is strongly related to the decrease in concentration of S. Based on this, it can be concluded that As replaces S in the pyrite structure and in the form of As^−..; Moreover, As species are common in pyrites of gold deposits with sedimentary host rocks. In the Zarshuran deposit the occurrence of gold in As-Hg-Sb sulfides in late drusy quartz veins (Daliran et al., 2018) is more important than the presence of gold in various pyrites. Finally, all evidence suggest that gold mineralization in the Zarshuran deposit occurs well in response to sulfidation process of Fe-bearing carbonate rocks. The correlation between gold content and degree of sulfidation in the Zarshuran deposit indicates that sulfidation process is a much more important mechanism to gold precipitation relative to addition of pyrite (pyritization).

Keywords

• Pyrite
• Disseminated gold
• Sulfidation process
• Fe-carbonate
• Zarshuran

---

References

Alavi, M., 2004. Regional stratigraphy of the Zagros Fold-Thrust belt of Iran and its proforeland evolution. American Journal of Science, 304(1): 1–20. https://doi.org/10.2475/ajs.304.1.1

Alipour, R., Tale Fazel, E. and Farhani Moghadam, M., 2020. The role of right-lateral shear zone and fractures related to folding in development of Zarshuran gold deposit, Takht-e-Soleyman complex, northern Takab. Journal of Economic Geology, 12(2): 131–155. (in Persian with English abstract) https://doi.org/10.22067/econg.v12i2.75702

Asadi, H.H., Voncken, J.H.L., Kühnel. R.A. and Hale, M., 2000. Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran. Mineralium Deposita, 35(7): 656–671. https://doi.org/10.1007/s001260050269

Bagby, W.E. and Berger, B.R., 1985. Geologic characteristics of sediment-hosted, disseminated precious-metal deposits in the western United States. In: B.R. Berger and P.M. Bethke (Editors), Geology and Geochemistry of Epithermal systems. Society of Economic Geology, Reviews in Economic Geology, pp. 169–202. https://doi.org/10.5382/Rev.02.08

Berner, R.A., 1981. A new geochemical classification of sedimentary environments. Journal of Sedimentary Research, 51(2): 359–365. https://doi.org/10.1306/212F7C7F-2B24-11D7-8648000102C1865D

Cail, T.L. and Cline, J., 2001. Alteration associated with gold deposition at the Getchell Carlin-type gold deposit, north-central Nevada. Economic Geology, 96(6): 1343–1361. https://doi.org/10.2113/gsecongeo.96.6.1343

Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M. and Hickey, K.A., 2005. Carlin-type gold deposits in Nevada: Critical geologic characteristics and viable models. In: J.W. Hedenquist, J.F.H. Thompson, R.J. Goldfarb and J.P. Richards (Editors), Economic Geology 100th anniversary volume. Society of Economic Geologist, Littleton, CO, pp. 451–484. https://doi.org/10.5382/av100.15

Daliran, F., 2008. The carbonate rock-hosted epithermal gold deposit of Agdarreh, Takab geothermal field, NW Iran hydrothermal alteration and mineralization. Mineralium Deposita, 43(4): 383–404. https://doi.org/10.1007/s00126-007-0167-x

Daliran, F., Hofstra, A., Walther, J. and Topa, D., 2018. Ore Genesis Constraints on the Agdarreh and Zarshuran Carlin-Style Gold Deposits in the Takab Region of Northwestern Iran. In: J.L. Muntean (Editor), Diversity of Carlin-Style Gold Deposits. Society of Economic Geology, Reviews in Economic Geology, pp. 299–333. https://doi.org/10.5382/rev.20.09

Deditius, A.P., Rich, M., Kesler, S.E., Utsunomiya, S., Chryssoulis, S. L., Walshe, J. and Ewing, R.C., 2014. The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits. Geochimica et Cosmochimica Acta, 140: 644–670. https://doi.org/10.1016/j.gca.2014.05.045

Dickson, J. and Coleman, M.L., 1980. Changes in carbon and oxygen isotope composition during limestone diagenesis. Sedimentology, 27(1): 107–118.  https://doi.org/10.1111/j.1365-3091.1980.tb01161.x  

Giggenbach, W.F., 1990. Water and gas chemistry of Lake Nyos and its bearing on the eruptive process. Journal of Volcanology and Geothermal Resource, 42(4): 337–362. https://doi.org/10.1016/0377-0273(90)90031-A

Hajialioghli, R., Moazzen, M., Jahangiri, A., Oberhansli, R., Mocek, B. and Altenberger, U., 2011. Petrogenesis and tectonic evolution of metaluminous sub-alkaline granitoids from the Takab Complex, NW Iran. Geological Magazine 148(2): 250–268. https://doi.org/10.1017/S0016756810000683

Heshmatnia, Sh., 2021. Classification, chemical composition and sulfur stable isotope constraints of pyrites from the Zarshuran deposit (north Takab) with special reference to gold formation mechanism. M.Sc. Thesis, Bu-Ali Sina University, Hamedan, Iran, 178 pp. (in Persian with English abstract)

Hofstra, A.H. and Cline, J.S., 2000. Characteristics and models for Carlin-type gold deposits. In: S.G. Hagemann and P.E. Brown (Editors), Gold in 2000. Reviews in Economic Geology, pp. 163–220. https://doi.org/10.5382/Rev.13.05

Hofstra, A.H., Leventhal, J.S., Northrop, H.R., Landis, G.P., Rye, R.O., Birak, D.J. and Dalh, A.R., 1991. Genesis of sediment-hosted disseminated gold deposits by fluid mixing and sulfidation: Chemical reaction path modeling of ore-depositional processes documented in the Jerritt Canyon district, Nevada. Geology, 19(1): 36–40. https://doi.org/10.1130/0091-7613(1991)019<0036:GOSHDG>2.3.CO;2

Karimi, M., 1993. Petrographic-mineralogical studies and the genesis of the Au-As ore at Zarshuran, Takab. M.Sc. Thesis, Tarbiat Moallem University, Tehran, Iran, 264 pp. (in Persian with English abstract)

Kavoshgaran, Consulting Engineers., 2013. Prospecting and exploration preliminary study of the peripheral Zarshuran gold mine. Iranian Mines and Mineral Industries Development and Renovation Organization, Tehran, Report no. 98901, 499 pp. (in Persian) 

Kesler, S.E., Fortuna, J., Ye, Z., Alt, J.C., Core, D.P., Zohar, P., Borhauer, J. and Chryssoulis, S.L., 2003. Evaluation of the role of sulfidation in deposition of gold, Screamer section of the Betze-Post Carlin-type deposit, Nevada. Economic Geology, 98(6): 1137–1157. https://doi.org/10.2113/gsecongeo.98.6.1137

Kettler, R.M., Waldo, G.S., Penner-Hahn, J.E., Meyers, P.A. and Kesler, S.E., 1990. Sulfidation of organic matter associated with gold mineralization, Moore orebody, Pueblo Viejo, Dominican Republic. Applied Geochemistry, 5(1–2): 237–248. https://doi.org/10.1016/0883-2927(90)90051-6

Madan-Zamin Company, 2020. Modeling and updating report of rock estimation at the Zarshuran mine. Iranian Mines and Mineral Industries Development and Renovation Organization, Tehran, Report no. MO-9812-04, 35 pp. (in Persian)

Maynard, J.B., 1982. Extension of Berner's "New geochemical classification of sedimentary environments" to ancient sediments. Journal of Sedimentary Petrology, 52(4): 1325–1331. https://doi.org/10.1306/212F812F-2B24-11D7-8648000102C1865D

Mehrabi, B., Yardley, B.W.D. and Cann, J.R., 1999. Sediment-hosted disseminated gold mineralization at Zarshuran, NW Iran. Mineralium Deposita, 34(7): 673–696. https://doi.org/10.1007/s001260050227

Palenik, C.S., Utsunomiya, S., Reich, M., Kesler, S.E., Wang, L. and Ewing, R.C., 2004. “Invisible” gold revealed: Direct imaging of gold nanoparticles in a Carlin-type deposit. American Mineralogist, 89(10): 1359–1366. https://doi.org/10.2138/am-2004-1002

Pirajno, F., 2009. Hydrothermal processes and mineral systems. Springer, London, 1273 pp.

Qian, G., Brugger, J., Skinner, W.M., Chen, G. and Pring, A., 2010. An experimental study of the mechanism of the replacement of magnetite by pyrite up to 300°C. Geochimica et Cosmochimica Acta, 74(19): 5610–5630. https://doi.org/10.1016/j.gca.2010.06.035  

Reich, M., Kesler, S.E., Utsunomiya, S., Palenik, C.S., Chryssoulis, S.L. and Ewing, R.C., 2005. Solubility of gold in arsenian pyrite. Geochimica et Cosmochimica Acta, 69(11): 2781–2796. https://doi.org/10.1016/j.gca.2005.01.011

Samimi, M., 1992. Recognizance and preliminary exploration in the Zarshuran. Kavoshgaran Consulting Engineers, Tehran, 220 pp. (in Persian)

Shenberger, D.M. and Barnes, H.E., 1989. Gold solubility in aqueous sulfide solutions from 150 to 350 °C. Geochimica et Cosmochimica Acta, 53(2): 269–278. https://doi.org/10.1016/0016-7037(89)90379-7  

Simon, G., Kesler, S.E. and Chyssoulis, S., 1999. Geochemistry and textures of gold-bearing arsenian pyrite, Twin Creek, Nevada: implication for deposition of gold in Carlin-type deposits. Economic Geology, 94(3): 405–422.  https://doi.org/10.2113/gsecongeo.94.3.405

Tale Fazel, E., 2022. Major and trace elements geochemistry of pyrite: Implications for exploration of invisible gold deposits in the Takab-Angouran district, NW Iran. Kharazmi Journal of Earth Sciences, 7(2): 57–69. (in Persian with English abstract) Retrieved July 20, 2022 from https://gnf.khu.ac.ir/article-1-2800-fa.html

Voute, F., Steffen, H., Noreen, E. and Carlos, V., 2019. Sulfur isotopes, trace element, and textural analyses of pyrite, arsenopyrite and base metal sulfides associated with gold mineralization in the Pataz-Parcoy district, Peru: implication for paragenesis, fluid source, and gold deposition mechanisms. Mineralium Deposita, 21(7): 1–24. https://doi.org/10.1007/s00126-018-0857-6 

Whitney, D.L. and Evans, B.W., 2010. Abbreviations for Names of Rock-Forming Minerals. American Mineralogist, 95(1): 185–187. https://doi.org/10.2138/am.2010.3371

Wong, P.K. and Oldershaw, A., 1981. Burial cementation in the Devonian Kaybob Reef Complex, Alberta, Canada. Journal of Sedimentary Research, 51(2): 507–520. https://doi.org/10.1306/212F7CC5-2B24-11D7-8648000102C1865D

Ye, Z., Kesler, S.E., Essene, E.J., Zohar, P.B. and Borhauer, J.L., 2003. Relation of Carlin-type gold mineralization to lithology, structure and alteration: Screamer zone, Betze-Post deposit, Nevada.   Mineralium Deposita, 38(1): 22–38. https://doi.org/10.1007/s00126-002-0265-8

Zhou, Y.J. and Wang, K., 2003. Gold in the Jinya Carlin-type Deposit: Characterization and Implications. Journal of Minerals and Materials Characterization and Engineering, 2(2): 83–100. https://doi.org/10.4236/jmmce.2003.22008