THE GIANT MURUNTAU GOLD DEPOSIT GEOLOGIC, GEOCHRONOLOGIC, AND FLUID INCLUSION CONSTRAINTS ON ORE GENESIS

摘要

We reinterpret the regional geologic setting of the giant Muruntau gold deposit and report new ~(40)Ar/~(39)Ar isotope age determinations and a laser Raman microprobe analysis of fluid inclusions. New ~(40)Ar/~(39)Ar isotope age determinations of hydrothermal sericite selvages to gold-stage quartz veins are in excellent agreement with older Rb-Sr dates on auriferous quartz veins. They suggest Triassic sericite formation at 245 and 220 Ma, some 30 m.y. later than subjacent felsic intrusions as defined by Rb-Sr data. These dates call into question the role of magmatic fluid, metal, and heat input. Indeed, the Rb-Sr pluton ages may be a reflection of hydrothermal activity rather than cooling from magmatic conditions.The Muruntau deposit is situated adjacent to a major rift, which has been periodically active from the Devonian to the Tertiary and has accumulated a substantial thickness of chemical, volcaniclastic, and elastic sediments, including 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 support for the role of reduced sulfur is provided by the presence of detectable H_2S in fluid inclusions from late-stage auriferous quartz veins. Reduced sulfur-rich fluids could have been focused into the depositional site along major northeast-trending structures, which have influenced rift-basin architecture as recently as the Jurassic. Oxidation could, therefore, be an important ore-precipitating mechanism. A change from early CH_4- to later syngold CO_2-dominant fluid inclusions may be an indication of this process.