Chapter 9: Ore Genesis Constraints on the Agdarreh and Zarshouran Carlin-Style Gold Deposits in the Takab Region of Northwestern Iran
F. Daliran, A. Hofstra, J. Walther, D. Topa, 2018. "Ore Genesis Constraints on the Agdarreh and Zarshouran Carlin-Style Gold Deposits in the Takab Region of Northwestern Iran", Diversity in Carlin-Style Gold Deposits, John L. Muntean
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The neighboring sedimentary rock-hosted Agdarreh (~24.5 tonnes [~0.8 Moz] of Au in ore with an average grade of 3.7 g/t Au) and Zarshouran (~110 tonnes of Au [~3.5 Moz] in ore with an average grade of 4.5 g/t Au) Carlin-style gold deposits in the Takab region of northwestern Iran occur within the active geothermal field of the Miocene Urumieh-Dokhtar magmatic arc, which contains an assortment of intrusion-related deposit types (porphyry Cu, polymetallic, and epithermal Au-Ag).
Although Agdarreh is hosted in early Miocene reefal limestones and is strongly oxidized and Zarshouran is hosted in exhumed metamorphosed Neoproterozoic black schist, limestone, and dolomite and is relatively unoxidized, they have many features in common. Phyllic alteration is as young as 14 Ma, and the age of local volcanic rocks is between 16 and 11 Ma. Disseminated gold ore in jasperoid occurs as replacement bodies at intersections of normal faults in decalcified and dolomitized carbonate units as well as in schists at Zarshouran. Where jasperoid is fractured or brecciated, it is cemented by drusy quartz. Precipitation of early pyrite (±pyrrhotite), abundant sphalerite, and minor chalcopyrite and galena was followed by a variety of more abundant complex sulfosalt minerals, arsenian pyrite, stibnite, late orpiment-realgar, and cinnabar, with traces of native bismuth, tellurides, and the newly discovered daliranite (PbHgAs2S6). Most of the gold occurs as solid-solution or nanometer gold in arsenian pyrite and sphalerite and as native gold associated with late-stage As sulfides and cinnabar. Agdarreh has an abundance of barite, whereas Zarshouran contains fluorite; both deposits are crosscut by minor late calcite, and each deposit is overprinted by a younger Pleistocene to Holocene geothermal system that produced extensive travertine deposits and oxidized sulfide gold ore at Agdarreh. Whole-rock analyses indicate Al was immobile, Ca was depleted, and significant amounts of Fe, Zn, Pb, As, Sb, Hg, Tl, Ag, Au, Ba, ±F were introduced along with lesser K, W, Ni, Cd, Te, and Se. The paragenetic sequence of the ore minerals reflects a decrease in temperature and an increase in fS2. Geochemical data suggest that ore fluids transported base metals, trace elements, and Au together or that a saline, H2S-poor fluid containing Fe, Zn, Pb, and Ag mixed with an H2S-rich fluid containing Au and trace elements.
Primary fluid (LV/LV±CO2) inclusions in drusy quartz, fluorite, and barite with salinities up to 23 wt % NaCl equiv and eutectic first melting temperatures of –19 to –10°C, are suggestive of ore-stage NaCl-KCl solutions. Secondary inclusions have low salinities (<4 wt % NaCl equiv) and higher first-melting temperatures (–13 to 0°C) suggestive of Na2CO3-Na2SO4 solutions, and those with salinities close to zero and very low Tfm (–56 to –30°C) suggestive of CaCl2-MgCl2-FeCl2 solutions represent the latest population of one-phase liquid (L) inclusions. Homogenization temperatures of the primary fluid inclusions (200° ± 20°C) and calculated pressures (~150 bar) are indicative of depths around 1.5 km (assuming hydrostatic conditions). The maximum densities of the CO2 phase present in aqueous-carbonic inclusions (0.18 g/cm3 at Agdarreh and 0.29 g/cm3 at Zarshouran) are indicative of pressures between 100 and 260 bar and depths around 1 km (assuming hydrostatic conditions). These data suggest mixing of immiscible brine and CO2-bearing vapor with dilute groundwater.
Fluid inclusions in Zarshouran orpiment have δDH2O, Na/Cl, and Cl/Br values consistent with magmatic fluids. The δ18O of water in equilibrium with jasperoid and drusy quartz is either magmatic, as at Agdarreh, or extends toward meteoric water, as at Zarshouran. The δ13C of CO2 in equilibrium with calcite is characteristic of the host rocks with a minor magmatic component. The δ34S of H2S in equilibrium with pyrite and sphalerite (1–6‰) is likely magmatic, whereas H2S in equilibrium with orpiment, stibnite, cinnabar, and getchellite (8–10‰) was derived in part from sedimentary sources. Late barite has high δ34S values typical of marine sulfate. These data indicate that the deposits formed in different parts of a hydrothermal system at the interface between ascending magmatic fluids and local meteoric groundwater in sedimentary or metasedimentary rocks. The characteristics of Agdarreh and Zarshouran suggest that they are shallow manifestations of intrusionrelated hydrothermal systems and, therefore, are best classified as distal disseminated deposits.