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Chah Zard |
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Iran |
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Au Ag
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Super Porphyry Cu and Au
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The small Chah Zard breccia-hosted epithermal gold-silver deposit is located ~100 km SW of the city of Yazd and ~150 km NW of the Meiduk porphyry copper deposit in central Iran.
The deposit is hosted within a high-K, calc-alkaline andesitic to rhyolitic volcanic complex in the central section of the NW-SE aligned Urumieh-Dokhtar Magmatic Arc (UDMA).
For detail of the regional setting of the UDMA see the Iranian Porphyry Copper and Epithermal Gold Province record.
Magmatic and hydrothermal activity has been interpreted as being associated with local extensional tectonics in a strike-slip regime formed in transtensional structures of the Dehshir-Baft strike-slip fault system. This fault system is composed of two major strike-slip fault trends. The most prominent is a major NW-SE trending structural zone, with a second set of small, late, NE-SW trending faults, with only small inferred displacements, parallel to several shear zones, transtensional pull-apart basins, and other syntectonic structures SW of Yazd (Kouhestani 2011).
The Chah Zard deposit is hosted within a late Miocene volcanic complex that has been dated at a mean age of 6.2±0.2 Ma (LA-ICP-MS, U-Pb, zircon) and forms a range of tall steep-sided hills in an area that is >6 km long. It unconformably overlies Eocene rocks to the NW, west and SE, and on Miocene aged rocks to the north, although the contacts with these older rocks is mostly faulted. The Eocene rocks are mainly alternating beds of tuff, marl, sandstone and shale. Miocene volcanism commenced with the development of flow domes of dark grey andesite-trachyandesite, and continued with several co-axial dacitic–rhyolitic porphyries nestled in the core of the complex. The margins of the andesitic domes are typically weakly altered, although the extent and intensity of alteration (particularly sericitic and silicic) increases markedly into the eroded core of the rhyolitic porphyries. Hydrothermally altered breccias occur within the flow dome complex (Kouhestani et al., 2012).
At Chah Zard, the flow domes are the predominant volcanic facies, forming tall, steep-sided hills in the outer fringes of the system. They andesite to trachyandesite contains abundant phenocrysts of plagioclase and hornblende with lesser biotite in a fine-grained to glassy, frequently devitrified matrix. The felsic suite forms small hills in the central part of the complex and occur as variably altered and partially eroded dacitic to rhyolitic domes and lavas, intruding the earlier andesitic lavas. Their texture is porphyritic with cm-sized phenocrysts of plagioclase, and smaller phenocrysts of quartz, amphibole,
and biotite, all set in a fine-grained matrix that commonly contains alteration products. The breccia complex and related veining formed during and after the waning stages of explosive brecciation caused by the shallowly emplaced rhyolite porphyry, that may have been localised by strike-slip faulting. Three distinct breccia bodies have been recognised:
• volcaniclastic breccia with a dominantly clastic matrix, comprising poorly sorted, angular to subrounded polymict breccia that is clastic-matrix
supported;
• grey polymict breccia with a greater proportion of hydrothermally altered cement. This breccia varies from clast to matrix-supported with a fine-grained sandy matrix. The clasts are poorly sorted and angular to subrounded. They consist of 30 to 70% andesite and dacite; 15 to 30% Eocene rocks; 10 to 30% rhyolite porphyry; and 5 to 10% clasts of re-brecciated breccia. The matrix comprises rock flour ± mineral fragments and varies from 10 to 70%, although, it may locally occupy more than 90% of the breccia, giving it a more homogeneous appearance; and
• mixed monomict to polymict breccia with a clay matrix. This breccia is matrix-supported with a distinctive white clay groundmass. Lithic clasts are poorly sorted and composed of 40 to 70% rhyolite porphyry; 10 to 30% porphyritic dacite-rhyodacite; and 10 to 30% andesite. Clasts range from 1 to 20 cm in size and are subangular-angular to subrounded. The matrix comprises fine-grained material of the same lithological units (Kouhestani et al., 2012).
Mineralisation at Chah Zard is intimately associated with phreatomagmatic polymictic breccias (Kouhestani 2011) that are partially synchronous with, and partially subsequent to, the rhyolite porphyry emplacement. These breccias host the bulk of the mineralisation. In contrast, the volcaniclastic breccias that are relatively impermeable, are largely barren. The highest grade and most extensive mineralisation is found in two polymictic pipe-like breccia bodies with elliptical plan dimensions of 500 x 250 m and 300 x 200 m respectively that pass into adjacent mixed polymictic to monomictic breccias. Separate base metal sulphide (Zn, Pb) veins occur in the porphyritic dacite-rhyodacite lavas on the southeastern and southwestern sides of the deposit (Kouhestani et al., 2012).
Precious metals are disseminated, occurring with sulphide and sulphosalt minerals, as well as being found in veins and in breccia cements. There is a progression from pyrite-dominated (stage 1) → pyrite-base metal sulphide and sulphosalt-dominated (stages 2 and 3) → base metal sulphide-dominated (stage 4) breccias and veins (Kouhestani et al., 2012).
Hydrothermal alteration and deposition of gangue minerals progressed from illite-quartz → quartz-adularia → carbonate, and finally → gypsum-dominated assemblages. Free gold occurs in stages 2 and 4, where it is predominantly intergrown with pyrite, quartz, chalcopyrite, galena, sphalerite, and Ag-rich tennantite-tetrahedrite, and also as inclusions in pyrite (Kouhestani et al., 2012).
High Rb/Sr ratios in ore-grade zones are closely related to sericite and adularia alteration. Positive correlations of Au and Ag with Cu, As, Pb, Zn, Sb and Cd in epithermal veins and breccias suggest that all these elements are related to the same mineralisation event (Kouhestani et al., 2012).
The ore mineral assemblages and alteration at Chah Zard is typical of low- and intermediate-sulphidation epithermal systems (e.g., Hedenquist et al., 1996; Einaudi et al., 2003; Kodera et al., 2005; Gantumur et al., 2008; Kouzmanov et al., 2009).
The total measured resource at Chah Zard is ∼2.5 Mt @ 12.7 g/t Ag, 1.7 g/t Au containing 28.6 t and 3.8 t of recoverable silver and gold respectively (Kouhestani et al., 2012).
The most recent source geological information used to prepare this decription was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Kouhestani, H., Ghaderi, M., Chang Z. and Zaw, K., 2015 - Constraints on the ore fluids in the Chah Zard breccia-hosted epithermal Au-Ag deposit, Iran: Fluid inclusions and stable isotope studies: in Ore Geology Reviews v.65, pp. 512-521.
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Kouhestani, H., Ghaderi, M., Zaw, K., Meffre, S. and Emami, M.H., 2012 - Geological setting and timing of the Chah Zard breccia-hosted epithermal gold-silver deposit in the Tethyan belt of Iran: in Mineralium Deposita v.47, pp. 425-440.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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