K–Ar geochronology of orogenic gold mineralization in the Vangtat gold belt, southeastern Laos: Effect of excess argon in hydrothermal quartz

The Thongkai‐Ok gold deposit is one of several actively mined deposits in a newly discovered orogenic gold belt in southeastern Laos. At a district scale, gold mineralization occurs within the Vangtat shear zone, one of the tectonic structures of the Poko suture zone that marks the collision between the Indochina Terrane and the Kontum Massif in southern Laos‐central Vietnam. High grade gold mineralization is associated with quartz–pyrite–white mica veins and silicified zones up to 10 m thick, hosted by strongly deformed and sheared pelitic schist. Lower‐grade gold mineralization occurs in a graphite–carbonate–quartz–sulfide hydrothermal alteration envelope that grades into sub‐economic/barren altered pelitic schist. The objective of this study is to establish the age of mineralization in the Vangtat orogenic gold belt by K–Ar dating of hydrothermal white mica in the gold‐bearing quartz‐sulfide veins in order to better understand its spatial and temporal relation to other gold deposits in Southeast Asia and to regional tectonic events. Three‐white mica bearing quartz–pyrite samples were collected from the open pit at Thongkai‐Ok. The dominant white mica phase in the samples is 2M1 muscovite and the K2O content measured by electron microprobe techniques in individual white mica crystals ranged between 8.61 and 9.79 wt%. Two of the three vein samples were separated by gravity settling into four size fractions,