A 70-Million-Year-Old Treasure Vault
Deep beneath the surface of modern Cheonan, a geological process during the Cretaceous period set the stage for future kingdoms. Around 70 million years ago, intrusions of granitic magma pushed their way into the overlying Precambrian Gyeonggi metamorphic belt. This immense heat and pressure created a complex network of fractures in the older rock. Superheated, mineral-rich water—known as hydrothermal fluid was released from the cooling magma. These fluids, with a magmatic origin confirmed by sulfur isotope analysis, forced their way into the newly formed cracks and fissures.
As these hydrothermal solutions moved upward and cooled, they underwent a dramatic chemical change. The dissolved minerals began to precipitate out, forming veins of milky white quartz. Entrapped within this quartz, and associated sulfide minerals, were microscopic grains of gold. The gold itself is primarily electrum, a naturally occurring alloy of gold and silver. Fluid inclusion studies, which analyze tiny pockets of the original fluid trapped inside the quartz crystals, reveal the intense conditions of formation. The initial fluids reached temperatures near 400°C and were deposited at a depth of around one kilometer, at pressures ranging from 100 to 300 bars.
The Veins and Their Minerals
The gold-bearing quartz veins in the Cheonan area fill pre-existing fault zones, creating a series of deposits that include the formerly active Cheonbo, Ilbo, and Seonggeo mines. The geology shows at least three distinct stages of mineralization, with the most significant gold deposition occurring during the first stage. This initial phase laid down quartz and electrum but also a suite of sulfide minerals. Common associates include pyrite (fool's gold), arsenopyrite, sphalerite, and galena.
The highest concentrations of gold are often found where galena is present. The gold grains themselves are small, typically less than a millimeter, and are intimately mixed with the other sulfides. Analysis of the electrum shows it contains between 49.5 and 63.2 mole percent gold. Later stages of mineral formation involved cooler, more dilute fluids, indicating an increasing influx of meteoric water (rainwater) into the hydrothermal system over time. This entire geological sequence, from the initial magma intrusion to the final cooling of the veins, created the mineral deposits that would later be exploited for centuries. Historical records show gold and silver from the Korean peninsula were sent as tribute to Chinese empires for hundreds of years, and a domestic gold rush occurred in the early 20th century.