The term "Mother Lode" refers to a rich source of valuable minerals, particularly gold. It originated during the California Gold Rush in the mid-19th century, when miners discovered a significant vein of gold in the Sierra Nevada mountains. This discovery led to a massive influx of prospectors seeking fortune in the region. In a broader context, "Mother Lode" can also describe any primary source of wealth or resources. The term is often used metaphorically in various fields, such as business or literature, to indicate a major source of valuable information or opportunities, similar to how gold was sought after during the Gold Rush.