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, <2, 2–4, 4–10, and 10–40 μm, respectively. The results of potassium measurements by flame photometry in the K–Ar sample aliquots show a range in potassium content which reflects the amount of quartz present in each sample as an impurity. With one exception, a correlation was observed between potassium concentration in the samples, including the size fraction separates, and the calculated K–Ar age. The large range of calculated ages, from 348 to 206 Ma, is interpreted to reflect excess argon contained in fluid inclusions in the hydrothermal quartz present in the analyzed samples. We interpret the youngest age of the quartz‐free samples, 206 ± 4 Ma, as closest to the age of mineralization in the Vangtat gold belt. The age could be as young as ~170 Ma if the age versus potassium content relation is extrapolated to the average potassium content of white mica in the samples analyzed.