THE OLON OVOOT OROGENIC GOLD DEPOSIT, SOUTHERN MONGOLIA: STRUCTURAL AND STABLE ISOTOPE CONSTRAINTS

Mesoscopic structural and stable isotope analyses of the mineralized vein systems at the Olon Ovoot gold mine reveal characteristics of an orogenic gold deposit. The Olon Ovoot mine has produced a total of 225K oz. of gold from predominantly fault-fill quartz-carbonate and extensional quartz-tourmaline veins. Highly deformed Silurian to Devonian sedimentary and volcanic rocks are the host for the gold-vein mineralization. The fault-fill veins are confined to steeply dipping brittle-ductile high-strain zones. Distinctive features of the fault-fill veins include wall rock slivers and septa enclosed within laminated veins, lenticular and irregular outline of individual laminae, and striated slip surfaces. The extensional veins, commonly occurring as horizontal and sub-horizontal vein arrays, display planar walls, internal compositional layering, and crack-seal and crystal fiber textures. Crosscutting relationships between the two sets of auriferous veins suggest that they contemporaneously formed. The axes of bulk strain were determined by the geometry of these contemporaneous vein sets and were compatible with their counterparts recorded by the sub-vertical transposition foliation and upright isoclinal intrafolial folds developed in the host rocks. This bedding transposition constitutes a main stage deformation of the study area. Measured δ¹⁸O values of the mineralized veins indicate an interesting history to the quartz vein systems. These isotopic results as well as the textural features of the veins, and the temporal relationship between the mineralization and regional deformation indicate that the Olon Ovoot gold deposit is a part of an orogenic gold system.