A Record of Tectonic Plumbing
Deep within Myanmar's Sagaing Region, the earth's crust holds a detailed record of ancient hydrothermal activity. The Yinmabin deposits consist of countless veins of antimony ore that fill a network of fractures in the surrounding rock. These deposits were formed when superheated, mineral-rich water, driven by tectonic forces, forced its way up from deep within the earth. As these fluids traveled through cracks in older sedimentary rocks, they cooled and precipitated their dissolved mineral load. The result is a complex, three-dimensional network of ore that maps the exact pathways the fluids took millions of years ago.
The primary host rocks belong to the Pane Chaung Formation, a thick sequence of turbidites—sediments deposited by ancient underwater landslides. This formation dates back to the Late Triassic period, between 233 and 206 million years ago. The tectonic activity that created the fractures and provided the heat for mineralization is linked to the ongoing collision of the Indian and Eurasian plates. This immense geological pressure cracked the crust, allowing magmatic heat to drive the circulation of hydrothermal fluids at temperatures between 140 and 260 °C. These fluids dissolved metals from a deep source and carried them upwards. As the fluids ascended into cooler rock, the change in temperature and pressure caused the dissolved elements to crystallize, filling the fissures with a metallic bounty.
The Stibnite Veins
The principal ore mineral at Yinmabin is stibnite, a sulfide of antimony (Sb2S3). It appears as lead-gray, needle-like crystals, which can form radiating sprays or massive, granular aggregates. With a Mohs hardness of just 2, stibnite is remarkably soft and can be scratched with a fingernail. The veins themselves are a mineralogical mixture. They consist mainly of stibnite and quartz, but can also contain pyrite, arsenopyrite, and occasionally traces of gold and silver.
Geologists study the complex network of these quartz-stibnite veins to reconstruct the region's tectonic history. The orientation and thickness of the veins—some just millimeters wide, others up to a meter—reveal the stresses that were acting on the rock. Weathering of the primary stibnite ore near the surface creates a host of secondary oxide minerals, such as stibiconite and valentinite. These often colorful, earthy minerals show the deposit's more recent interaction with the atmosphere and groundwater. The entire system is an example of the dynamic, fluid-driven processes that concentrate metals within the Earth's crust.