Nature and genesis of the Zaglic Au deposit, NW Iran: Constraints from geochemical studies

Abstract

The Zaglic Au deposit is located in a Late Oligocene magmatic arc within the Arasbaran Magmatic Belt of NW Iran, which formed during the subduction of Mesotethys oceanic crust. Country rocks comprise intermediate to mafic volcanic and volcanoclastics of andesitic to trachy–andesitic composition, and exhibit late granite plutonism. Host assemblages display medium– to high–K calc–alkaline affinities with fractionated REE profiles, including Nb and Ti depletion and Rb, Ba, Th, U enrichment, consistent with subducted terranes. Wall–rock alteration comprises multi–stage silicic, sericitic, argillic and/or propylitic domains, associated with (i) pre–, (ii) main–, and (iii) post–ore emplacement. Pyrite is the dominant sulfide with spatially associated chalcopyrite and bornite in quartz vein and veinlets, with free gold occurring as microscopic grains in quartz, or as inclusions in pyrite. Fluid inclusion analysis of pre– (quartz I), main– (quartz II) and post–ore stages (quartz III) identifies liquid–rich, multi–stage, and vapor–rich varieties with decreasing temperatures and salinities, with the following Th and NaCl wt% equivalent ranges: pre–mineralization: 230–382 °C and 6.1–8.2; main–ore: 179–311 °C and 0.35–3.22%; and post–ore: 150–195 °C and 2.2–2.9%. Stable isotope analysis identifies variable δ18Ofluid and δDfluid signatures, including: quartz I: 2–3.4‰; −86 to −78‰; quartz II: −0.6–0.2‰; −89 to −85‰; sericite: −0.2 to 3.4‰; −107 to −91‰; and chlorite: −0.6 to 1.4‰; −115 to −104‰. Such ranges imply a mixture of magmatic and meteoric fluids, characteristic of a wide range of epithermal Au deposits. Sulfur isotope composition (δ34S) of this deposit's two principal sulfide minerals is consistent with compositionally reduced mineralizing fluids including: pyrite: −1.4 to 2.7‰ and chalcopyrite: −2.6 to 1.8‰. Geochronological analysis records 40Ar/39Ar dating of biotite from andesite porphyry with a plateau Eocene age of 48.20 ± 0.73 Ma, together with ~8 Ma or Late Miocene ages from fine–grained sericite.