The Cripple Creek gold deposits occur within a seven square mile (18 sq. km), 30 million year old, volcanic-intrusive complex that erupted and intruded through rocks that are over one billion years old. The rocks within the complex consist mostly of breccias, dikes, sills, and flows with minor amounts of fossil-bearing lake sediments, sandstones, conglomerates, and fresh-water limestones.
The gold mineralization followed the emplacement of the volcanic rocks and may have occurred over a time period that lasted as long as two million years. Most of the gold was localized along major structural zones within the volcanic complex. The ore bodies occur as (1) deposits of rich, narrow, gold-telluride veins with quartz, pyrite, and fluorite and (2) deposits of low-grade, disseminated, microcrystalline, native gold attached to pyrite. Most of the gold mined in the early days of the district came from the high-grade gold telluride veins.