Breccia Pipes

by Edgar B. Heylmun, PhD
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A breccia pipe, or chimney, is an irregular cylindrical mass of breccia that is often silicified and stands out as an iron-stained knob. They can be from a few feet to several hundred feet in diameter, and may or may not be mineralized. The breccia is usually, but not always, composed of altered angular to rounded fragments of the host rock, cemented by silica. Since breccia pipes often have good porosity and permeability when first formed, they may serve as conduits for mineralizing solutions, and can form important ore bodies. They are also more susceptible to oxidizing agents at the surface, and may be oxidized and enriched to considerable depths. “Blind” breccia pipes are those which have never reached the surface.

Origin

The origin of breccia pipes has been kicked around for years. The most commonly accepted idea is that they formed at the intersections of fractures, where hydrothermal solutions forced their way, sometimes explosively, toward the surface. Some breccia pipes are very clearly the result of solution collapse of limestone or other soluble rock types, and may not have any connection with hydrothermal systems.

Yellowstone National Park is an example of a hydrothermal system, or series of systems that has reached the surface. It is characterized by geysers, hot springs, steam vents, mud pots, fumaroles, and dormant volcanoes. Gold and other metals are slowly being deposited in some of the conduits. There are hydrothermal systems in various parts of the world that have never reached the surface. Volcanic eruptions testify to the explosive nature of systems that do reach the surface. We see many of the deposits long after the period of activity has ended.

Some breccia pipes are carrot-shaped and taper downward. Certain pipes may contain rich ore bodies, and knowledge of how they were formed may aid in the mining of the deposit.

Examples

Copper Creek, Arizona. There are up to 125 known breccia pipes in the Copper Creek Mining District of Pinal County, Arizona. Many are not mineralized, but some contain rich ore bodies. The richest are at the Childs-Aldwinkle Mine (see illustration) where two mineralized pipes join at depth and lose their identity at depths below 900 feet.

The pipes consist of altered fragments and blocks of dacite, with the richest masses of sulfide ore being at 820 feet, where the pipes join. The principal ore mineral is chalcopyrite which forms peripheral ore bodies at shallower depths, and which merges to form a central mineralized zone at depth. The phenomenon of peripheral ore bodies merging at depth has been observed at many other breccia pipes. Contacts with host rocks are often steep and sharp. Some breccia pipes in the Copper Creek district persist to depths over 3,000 feet. Most are cemented with quartz, sericite, and iron oxides. The pipes appear to be the result of explosive injections of hydrothermal solutions along intensely fractured zones of weakness. Many of the features seen in the Copper Creek Mining District may be seen in other mining districts, such as at Round Mountain, Nevada, and Delamar, Idaho.

Cripple Creek, Colorado. The Cripple Creek Mining District of Teller County, Colorado, is a nested diatreme of volcanic origin, but it still possesses characteristics of a breccia pipe complex. Some of the brecciated rocks involved have come from great depths. The most impressive feature is the famed “Cresson Blowout,” near the center of the district. The Cresson Blowout is up to 500 feet in diameter and extends to depths in excess of 2,300 feet. Angular to rounded fragments of altered basalt are cemented with sericite and iron oxides. Here again, at shallower depths, above 1,600 feet, the richest ore bodies were found along the periphery of the pipe whereas at depth, they merge to form a central mineralized core.

The ore minerals of interest are calaverite and other gold tellurides. The Cresson “Vug,” found along the periphery of the pipe at 1,200 feet, consisted of an almost solid mass of telluride ore that assayed at over 4,000 ounces of gold/ton. It had to be mined under armed guard. The stope eventually reached 400 feet in height. Open-pit and underground mining has continued, off-and-on, since its discovery in 1914. Over 22 million ounces of gold have been produced from the Cripple Creek Mining District since 1891.

Summary

Breccia pipes are commonly found in mining districts and many contain rich ore deposits. In the examples discussed, ore deposits tend to be found in peripheral contact zones at shallower depths, but are found in core zones at depth. There are undoubtedly many exceptions. There are probably examples where the entire breccia mass is mineralized, and other examples where the entire breccia mass is barren. Zones of enrichment are present in many pipes. There are examples of breccia pipes which grade laterally into stockwork, then into country rock.

The writer does not know of a comprehensive study of breccia pipes other than those done in regard to solution collapse in limestone. This article has been pieced together from a number of studies done by geologists in various mining districts. There are no doubt examples which do not fit this discussion. A definitive study is needed.