THE AGE OF GOLD MINERALIZATION AT THE MURUNTAU AU DEPOSIT, UZBEKISTAN, FROM RE-OS ARSENOPYRITE GEOCHRONOLOGY

The Muruntau gold deposit, located in the Kyzylkum desert, Uzbekistan, is one of the largest known hydrothermal gold deposits worldwide (~170 Moz.). The deposit, one of several economic gold deposits situated within the Southern Tien Shan fold and thrust belt (Hercynian Tien Shan orogen), is hosted by lower greenschist grade sedimentary rocks of the Early Paleozoic Besapan suite. Main stage gold at Muruntau is present within a network of steeply dipping quartz veins, and displays a cogenetic relationship with arsenopyrite. The Muruntau deposit is also spatially associated with two Late Carboniferous to Early Permian granitic intrusions, one ~15 km southeast of the deposit (Sardarin Pluton) and the other discovered by drillhole at ~4 km depth near the SE margin of the deposit (Murun Granite). The origin of the deposit is still debated, and the possibility of a genetic relationship between the intrusions and gold mineralization is a highly contentious issue. Re-Os geochronology was performed on arsenopyrite from Muruntau to determine the absolute age of sulfide mineralization in the ore system, and by proxy, gold mineralization. Nine Re-Os analyses were performed on arsenopyrite obtained from five different crystal locations on a single hand specimen derived from a stockwork veinlet from the Muruntau open pit. These analyses yielded a range of ¹⁸⁷Re/¹⁸⁸Os ratios up to maximum values of ~6000, with correspondingly highly radiogenic Os values (¹⁸⁷Os/¹⁸⁸Os ~ 30). The Re contents range up to a few tens of parts per billion (ppb) with common Os abundances ranging up to ~ 100 parts per trillion ¹⁹²Os. The nine analyses, when regressed together, yield an initial Re-Os age of 286 ± 5 Ma (Model 3; MSWD=5). This initial age result is similar to previously reported Sm-Nd scheelite and Rb-Sr ages for mineralization at Muruntau, but older than reported Ar-Ar data from sericite. The Re-Os age is interpreted to represent both the age of arsenopyrite formation and, by proxy, main stage gold mineralization at Muruntau. The overlap of the Re-Os arsenopyrite age with previously determined ages for granitic intrusions in the Muruntau region suggest that the plutonism may have played an important role in the generation of widespread gold mineralization in the southern Tien Shan belt.