Breccia pipe

A breccia pipe, also referred to as a chimney, is a mass of breccia, often in an irregular and cylindrical shape. When exposed at the surface, a breccia pipe may appear as an iron-stained knob, from several feet to several hundred feet in diameter. Breccia pipes may or may not be silicified. They usually consist of fragments of the host rock (the rock layer they are contained in) cemented together by silica. They can also be cemented by finely ground host rock called rock flour. These formations are often hosts for ore deposition especially in copper and uranium mining districts. They are sensitive to oxidation, and due to their porous nature, may be oxidized to depths far below the ground surface. Breccia pipes that have never reached the Earth's surface are referred to as "blind".