A Craton's Weakness
The Mengyin diamond field is located in the eastern part of the North China Craton, one of the planet's oldest and stable blocks of continental crust. Typically, diamond-bearing kimberlites erupt through the thick and stable centers of these ancient landmasses. The Mengyin deposits, however, are an exception. They are near the Tan-Lu Fault Zone, a massive and active fault that has significantly weakened and reworked this edge of the craton.
The kimberlite pipes and dikes, the volcanic conduits that carried diamonds from the mantle, punched through the Archean-aged basement rock during the Ordovician period between 450 and 480 million years ago. This magmatic event brought diamonds that had formed deep within the Earth, at depths between 147 and 176 kilometers, to the surface. The diamond field consists of three main belts: Changmazhuang, Xiyu, and Poli, which together contain dozens of individual kimberlite bodies. The richest of these is the Shengli-1 (Victory-1) pipe in the Changmazhuang belt, which produces high-grade diamonds, including valuable brown diamonds.
From Mantle to Plowshare
Hundreds of millions of years of erosion have taken their toll on the Mengyin kimberlite pipes. The softer volcanic rock has weathered away, leaving the incredibly hard diamonds to be washed into the surrounding landscape. This process created extensive secondary, or alluvial, deposits, scattering diamonds across the region's soil. As a result, some of China's most significant diamond discoveries were not made by geologists, but by local farmers.
The most famous of these finds is the Changlin Diamond. In 1977, a young farmer named Wei Zhenfang discovered a 158.78-carat, pale yellow diamond while working in her field in Changlin Village. It is the largest natural diamond ever found in China that is still extant and is housed in the Geological Museum of China. This was not an isolated incident. The region has produced all five of China's recorded natural diamonds over 100 carats. Another find was the "Jinji Diamond," an even larger stone weighing 281.25 carats, discovered by a farmer in 1937, though it disappeared during the Second Sino-Japanese War. Analysis of the Mengyin diamonds shows they are primarily Type Ia, containing nitrogen impurities, and their inclusions of minerals like olivine and pyrope confirm their origin in the Earth's upper mantle.