T63 Diverse Origins of Sedimentary Rock-Hosted Disseminated Gold Deposits: A Global Perspective
Monday, 28 October 2002
8:00 AM - 12:00 PM
AGDARREH & ZARSHURAN SRHDG DEPOSITS, TAKAB REGION, NW-IRAN
Paper number: T63-9
Program: Technical Sessions
Time: 10:10 AM – 10:30 AM
Authors
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A. Hofstraahofstra@usgs.gov, N/A, N/A
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J. WaltherInstitut für Mineralogie und Geochemie, N/A, N/A
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D. StübenInstitut für Mineralogie und Geochemie, N/A, N/A
Abstract
The Agdarreh & Zarshuran (50 & 80t Au at 3-4 g/t) prospects occupy a 20x20 km area of the NW-trending Lt. Cret.-Tert. Zagros orogenic belt produced by collision of the Afro-Arabian and Iranian plates along a NE-dipping subduction zone. They are in a Tert. volcanic belt that contains Miocene to Recent ore deposits and a geothermal field with high Au (up to kg/t). The SRHDG deposits are in the lower plate of a major NW-trending thrust fault, subsidiary to the Zagros thrust, and within a fault-bounded Tert. sedimentary basin. Au ore is controlled by NW and NE to E striking faults. Agdarreh is in Miocene reefal limestones and Zarshuran is in Precambrian carbonaceous metapelites and carbonates intruded by Miocene quartz porphyry. Altered metapelites yield K-Ar dates > or=14 Ma. Also present are Au-bearing As & Sb veins, Au-As anomalies in volcanics, and deposits of Pb-Zn and Fe. Ore fluids introduced S, Fe, Au, As, Sb, Hg, Te, Se, Tl, Ba, F, Zn, Ag, Cd, Bi, and Pb, but scant Cu. Au occurs in As-pyrite and sphalerite with minor sulfides and sulfosalts of As, Sb, Pb, Hg, Tl, ±Cu, and rare arsenopyrite, Pb & Hg tellurides, native Bi and Au. Quartz, calcite & barite are common, fluorite is only at Zarshuran, and Agdarreh is in an oxide blanket that contains % Mn-Fe. Alteration consists of carbonate dissolution, ± dolomitization, silicification, and argillic alteration. Fluid inclusions indicate ore formed at P<150 bars and T=200-140°C from CO2-bearing fluids with <1-25 wt. % NaCl. Stable isotopic data for water (dD=—50 to —66‰, d18O=—2.9-15.1‰), CO2 (d13C=—0.3-4.6‰), H2S (d34S=4.7-13.0‰), and sulfate (d34S=25.1-32.1 ‰) suggest ore fluids consisted of magmatic water that evolved by interactions with country rocks and meteoric ground water. Thus, the SRHDG deposits in the Takab region are intrusion-related.