Khentii Gold Belt
The Mongol-Okhotsk suture zone extends for about 3’000 km from the present-day Sea of Okhotsk to Transbaikalia and as far as central Mongolia. It separates the Siberian and Sino-Mongol (Amur-Mongol) continents.
The suture zone is bordered on both sides by Mesozoic magmatism and mineralization forming an about 3’000 km long and 1’000 km wide belt, which contains over 900 granitic intrusions. The Khentii mountain range and associated magmatic arcs formed as a result of subduction during the closure of the Mongol-Okhotsk ocean at the end of Jurassic and lower Cretaceous. A main metallogenic zone with gold formed so in the Khentii mountain range in late Jurassic.
The highest concentration of gold occurrences in Transbaikalia is along the Mongol-Okhotsk suture, and epithermal gold veins are exposed and widespread in the Khentii mountains after fluvial erosion has been active over a long period of time since late Jurassic. Many stream valleys in Mongolia are filled so with eroded gold and form very rich gold placers.
Gold Precipitation
During the Mesozoic granitic intrusions the magma crystallises into granitic rock in the subsurface and metal-rich fluids are expelled. The over-pressured metal-rich fluid pulses travel fast from the magma chamber upwards through hydro-fractured rock.
When a magmatic fluid pulse crosses the transition zone between the magmatic and cooling meteoric fluid regimes, pressure drops from above lithostatic to hydrostatic, and temperature decreases dramatically.
As a result the fluid releases most of its metal and gold cargo along fractures. Heat removal by steady convection of meteoric fluids stabilizes the plume at a depth of about 2’000 m.
Complete dewatering of the magma takes around 50’000 years.
(Pressure - Temperature Model after Weis et al. ETHZ, 2012)
Note: There are several other metallogenic zones with gold in Mongolia.
