H Kouhestani; M.H Ghaderi; M.H Emami; S Meffre; V Kamenetsky; J McPhie; Kh Zaw
Abstract
The Chah Zard Ag-Au deposit, a typical breccia-hosted low- to intermediate-sulfidation epithermal system, is located within late Miocene andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar magmatic belt. The orebodies are emplaced in breccia bodies dominantly hosted by ...
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The Chah Zard Ag-Au deposit, a typical breccia-hosted low- to intermediate-sulfidation epithermal system, is located within late Miocene andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar magmatic belt. The orebodies are emplaced in breccia bodies dominantly hosted by rhyolite porphyries. Systematic whole-rock geochemical investigations on the volcanic rocks show that both intermediate and felsic rocks are characterized by significant Large Ion Lithophile Elements (LILE) and Light Rare Earth Elements (LREE) enrichment coupled with High Field Strength Elements (HFSE) depletion. These geochemical data indicate subduction-related magmatic arc affinity for the volcanic rocks, and suggest that hornblende fractionation appears to be an important controlling factor on the evolution of mineralized subvolcanic rocks. Although the rhyolite porphyry has relatively high 87Sr/86Sr ratios, the volcanic rocks have similar Sr and Nd isotopic compositions, displaying 87Sr/86Sr range of 0.704910-0.705967 and εNd(i) values of +2.33 to +2.70. These data suggest that the rhyolitic magmas probably represent the final diffetentiates of parental andesitic magmas with minor crustal contamination. The andesitic magmas generated from partial melting of a mixture of an incompatible element depleted anhydrous asthenospheric mantle source and a hydrous LILE and LREE enriched lithospheric mantle source in response to slab-break-off and upwelling of asthenospheric mantle. The rhyolite porphyry is inferred to have supplied heat that drove the convective hydrothermal system at Chah Zard deposit, but also provided some of the fluid sources responsible for the development of the Chah Zard epithermal system.
Majid Ghaderi; H. Kouhestani; M. H. Emami; K. Zaw
Abstract
The breccia-hosted epithermal Ag-Au deposit of Chah Zard is located within an andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar magmatic belt. At this location, magmatic and hydrothermal activity was associated with local extensional tectonics, formed in the Dehshir-Baft ...
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The breccia-hosted epithermal Ag-Au deposit of Chah Zard is located within an andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar magmatic belt. At this location, magmatic and hydrothermal activity was associated with local extensional tectonics, formed in the Dehshir-Baft strike-slip fault system. The host rocks of the volcanic complex consist of Eocene sedimentary and volcanic rocks covered by Miocene sedimentary rocks. LA-ICP-MS U–Pb zircon geochronology yields ages between 6.36±0.14 and 6.19±0.24 Ma, and a mean age of 6.23±0.16 Ma for magmatic activity at Chah Zard. Breccias and veins were formed during and after the waning stages of strong explosive eruption of magmatic-hydrothermal and phreatomagmatic brecciation events due to shallow emplacement of the rhyolite porphyry. Detailed systematic mapping leads to the recognition of three distinct breccia bodies: the volcaniclastic breccia with a dominantly clastic matrix, the gray polymict breccia with a greater proportion of hydrothermal cements, and the mono-polymict breccia with argillic groundmass matrix. The polymictic breccias generated bulk-mineable ore, whereas the volcaniclastic breccia is relatively impermeable and largely barren. Variable hydrothermal alteration occurs in outcrops, covering about 9 km2 at Chah Zard. Hydrothermal alteration assemblages are zoned around the breccias and veins, consisting of secondary quartz, illite, pyrite, adularia, chlorite, various carbonate minerals, and minor K-feldspar. Iron oxide-hydroxide, jarosite, gypsum, kaolinite, halloysite and rare alunite are the supergene alteration minerals replacing primary minerals, and filling the fractures and vugs. Precious metals occur with sulfide and sulfosalt minerals as disseminations in the veins and breccia cement. There is a progression from pyrite-dominated (stage 1) to pyrite-base metal sulfide and sulfosalt-dominated (stages 2 and 3) to base metal sulfide-dominated (stage 4) breccias and veins. Deposition of gangue minerals progressed from illite-quartz to quartz-adularia, carbonate and finally gypsum-dominated assemblages. Free gold occurs in stages 2 and 4, principally as intergrown with pyrite, quartz, chalcopyrite, galena, sphalerite, and Ag-rich tennantite-tetrahedrite; and also as inclusions in pyrite. The U-Pb zircon age of 6.19±0.24 Ma for emplacement of the rhyolitic magmas represents the maximum age of mineralization at Chah Zard. It may indicate that there was a previously unrecognized mineralization event in Urumieh-Dokhtar at this time.
