The giant Muruntau gold deposit geologic, geochronologic, and fluid inclusion constraints on ore genesis

We reinterpret the regional geologic setting of the giant Muruntau gold dep osit and report new Ar-40/Ar-39 isotope age determinations and a laser Rama n microprobe analysis of fluid inclusions. New Ar-40/Ar-39 isotope age dete rminations of hydrothermal sericite seh ages to gold-stage quartz veins are in excellent agreement with older Rb-Sr dates oil auriferous quartz veins. They suggest Triassic sericite formation at 245 and 220 Ma, some 30 m.y la ter than subjacent felsic intrusions as defined by Rb-Sr data. These dates call into question the role of magmatic fluid, metal, and heat input. Indee d, the Rb-Sr pluton ages may be a reflection of hydrothermal activity. rath er than cooling from magmatic conditions. The Muruntau deposit is situated adjacent to a major rift, which has been p eriodically active from the Devonian to the Tertiary and has accumulated a substantial thickness of chemical, volcaniclastic, and elastic sediments, i ncluding red beds and evaporites. We infer that the Muruntau deposit is so large because of the presence of a major source of reduced sulfur in these nearby evaporitic sediments, via thermochemical sulfate reduction. Some sup port for the role of reduced sulfur is provided by the presence of detectab le H2S in fluid inclusions from late-stage auriferous quartz veins. Reduced sulfur-rich fluids could have been focused into the depositional site alon g major northeast-trending structures, which have influenced rift-basin arc hitecture as recently as the Jurassic. Oxidation could, therefore, be an im portant ore-precipitating mechanism. A change from early CH4- to later syng old CO2-dominant fluid inclusions may be an indication of this process.