Carbonized Wood From the Oligocene: Trapped in Cripple Creek’s Volcanic Complex

By Steven Wade Veatch and Tim Brown
CRIPPLE CREEK, LOCATED IN THE SOUTHERN PART OF TELLER COUNTY, Colorado is the premier gold mining district in the state. The gold camp has produced more than 21 million ounces of gold since 1891—almost half of Colorado’s gold production of 44 million ounces (Davis and Streufert, 1990).

Gold in the district is found in veins and surrounding rocks associated with a small (6 mi2) 32- million-year-old (Oligocene) volcanic complex (Kelley, 1996). The complex formed by explosive volcanism, development of a funnel-shaped breccia pipe or diatreme, episodic intrusion of alkaline igneous rocks (ranging in composition from phonolite to lamprophyre), and subsidence (Thompson et al., 1985; Thompson 1992, and Pontius, 1996). The volcanic complex has three principal vent areas containing breccias of different sizes and lithologies formed by volcanic and hydrothermal action. Local eruptive centers, small stocks, dikes, and sills formed in other parts of the complex. Magmatic and diatremal activity may have persisted 3 to 4 million years (Kelley, 1996).

Magmatism, diatreme emplacement, and mineralization were associated with the Rio Grande Rift system that may have started the melting of the upper mantle and lower crust, producing alkali-basalt magmas (Cappa, 1998).

A two-phased mineralizing event closely followed emplacement of the volcanic complex, starting 30 million years ago and lasting about 2 million years. Mineralization was linked to two major thermal events: (1) a high temperature epithermal event causing alteration and increased permeability of the surrounding rocks with little gold being deposited; (2) a low temperature epithermal event depositing gold in steeply dipping veins and disseminating gold into surrounding porous wall rocks (Kelley et al.,1998). Historic underground mining sought gold-silver telluride veins. Modern surface mining seeks broad zones of disseminated microscopic native gold and pyrite.

Today the Cripple Creek and Victor Gold Mining Company (CC&V) operates the Cresson surface mine clearly the most valuable deposit in the district, having produced over 3 million ounces of gold. This operation is a joint venture between AngloGold (Colorado) Corporation and Golden Cycle Corporation.




Overview of the Cresson Project.   The Cresson project and the lamprophyre pipe are in the foreground.  In the background is the valley leach facility where a dilute sodium cyanide solution removes gold from the surface of the crushed ore. Photo by S.W. Veatch 

Active exploration conducted by CC&V identifies where gold may be found in the district. Remote sensing techniques such as aerial photography and geophysical survey measurements of gravity and magnetic fields are used as exploration tools.




View of Cripple Creek & Victor Gold Mining Company Operations. Photo by S. W. Veatch.


Drilling with an exploration drill rig is another tool used in the exploration process and provides a view of what is below the surface of the district. During a recent deep directional drill program beneath the main Cresson surface mine, a core sample of Cripple Creek Breccia was obtained 3,527 feet down the hole (3,079 feet beneath the surface) that hosted a fossil wood fragment  The difference between the distance "down the hole" and "beneath the surface" is due to the fact that the drill hole was a directional hole. CC&V started the hole vertically, then used a directional motor to point the hole at a specific bearing and plunge, and finally drilled the core through the target zone. This resulted in a deviation from vertical that caused the differences in the distances. The Cripple Creek Breccia hosting the black carbonized wood is composed of tuff and angular to subangular fragments of rocks and is at least 3,300 feet thick (Thompson et al, 1985).


A carbonized wood fragment, from the Cresson surface mine, is circled in this core sample. This impressive sample was sliced from a piece of NQ-sized drill core (just under 2 inches in diameter).   Image courtesy of the Cripple Creek and Victor Gold Mining Company

 The carbonized wood fragment is solid and does not easily break apart. The rim around the wood appears to be an alteration halo of quartz and calcite, although identification is uncertain. The smaller black specs seen in the core sample are difficult to identify without the help of a thin section. However, most appear to be mafic minerals that are contained within other clasts and fragments of rocks incorporated into the breccia. Some of the small minerals also appear to be hosted within the matrix of the breccia.

Although the Cresson core sample is remarkable, fossil wood is not something new found in Cripple Creek mines. T. A. Rickard (1900) wrote that a number of tree parts, ranging from small pieces up to the size of a trunk, were found mixed in with the Cripple Creek Breccia. Rickard (1900, p. 384) reports: “In the Jack Pot mine, at 400 feet from the surface, in the Logan at 600 feet, and in the Doctor at 700 feet, there have been found fragments of coal, exhibiting traces of wood-structure. In the Independence, at 500 feet, a stump of a tree was discovered in the very midst of rich ore. In every case the enclosing rock was breccia. The specimen from the Independence is stone, the others are coal. In the former case, the tree-portion must have become buried under conditions free from access of air, and must have been subjected subsequently to the action of siliceous waters, which gradually replaced the fiber of the wood with a mineral precipitate. In the other cases, the tree must have become enclosed within the breccia and subjected to a slow oxidizing action which carbonized the wood, without permitting it to burn freely. Otherwise, it would have been destroyed, leaving only ashes. As it was, it became coal, carrying 60 per cent carbon, and having the other characteristics of a typical lignite.”

Lindgren and Ransome (1906, p. 31) mentioned carbonized tree trunks and coal layers and provide this interesting account about the Elkton mine: “In July, 1905, a carbonized tree trunk was found on the 800-foot level of the Elkton mine. A letter from Mr. E.M. De la Vergne, the manager of the mine, dated November 25, 1905, states that the log is 18 inches in diameter and was at that time exposed for a length of 5 feet. It lies in hard unfissured breccia, about 40 feet west of the Elkton basic dike, and the matrix shows the impressions of knots and bark. A specimen from this tree trunk kindly supplied by Mr. De la Vergne, retains the rings of growth and other general woody structures, although the material is now altered to coal like that found in the Doctor-Jackpot mine. According to Prof. F.H. Knowlton the tree was undoubtedly a conifer and probably belonged to a species of Pinus.” Frank Hall Knowlton was a well-known paleobotanist and had been a professor of botany at Columbian University (now George Washington University), Washington, D.C. (White, 1927).

William Francis Hillebrand performed a chemical examination of the carbonaceous material that retained its original woody structure from the Doctor-Jackpot mine and determined it was bituminous coal (Lindgren and Ransome, 1906). Hillebrand published extensively on the composition of rocks and minerals and was the first chemist to be hired by the U.S. Geological Survey (Allen, 1932).

Loughlin and Koschmann (1935) note carbonized wood was found in several locations in the volcanic complex, including these mines: Cameron, Morning Glory, Doctor Jackpot, Logan, and Elkton. They also document a log found in the Cresson mine. The deepest wood was found at the 800 level of the Elkton mine; although Loughlin and Koschmann state the sample from the Cresson mine was from an “equal or greater depth.” The core sample recently obtained with the ancient wood fragment is significant as it was found at a depth lower than the deepest workings in the Cresson underground mine.

The mechanism that brings these surface materials to great depths within the volcanic complex is the subsidence that follows violent volcanic explosions. Lindgren and Ransome (1906), in their early investigation of the district, describe the Cripple Creek volcanic complex as explosive. During Oligocene time, when local intrusions encountered water, phreatic (steam) explosions resulted, shaking the landscape. These explosions resulted from the contact between magma and groundwater. Violent explosions, jets of volcanic ash, and billowing clouds of steam (driven by the expansion of super-heated water after contact with magma) brecciated the rocks and thoroughly mixed the shattered material. Magma rapidly ascended along zones of weakness while small eruptive centers or diatremes were enlarged.

Subsidence faulting, along steeply dipping faults, followed these violent explosions. Surface materials, through this active process of subsidence, were brought deep within the volcanic complex and mixed with shattered rocks of all sizes. Loughlin and Koschmann (1935) recognized the role of subsidence in the volcanic complex when sedimentary rocks were found in the deepest mine workings—3,200 feet below the surface. Miners created quite a stir in the gold camp when they discovered bird tracks in sedimentary rocks in one of the mines (Ed Hunter, personal communication, 2003).

Finding petrified wood deep underdround in mining drifts was a remarkable event. Photo by S. W. Veatch

The recently drilled core sample that contains a carbonized wood fragment came from a depth greater than 3,000 feet below the surface. The rock unit that this core was drilled from reveals the shattering of rocks and subsidence resulting from the volcanic eruptions and phreatic explosions that occurred here 32 million years ago. From Cripple Creek’s early days on into modern times, the district continues to yield earth’s fantastic treasures—from precious gold to incredible Oligocene fossil wood—helping the district maintain its title as the “World’s Greatest Gold Camp”.

Acknowledgments

We appreciate the help of Ed Hunter who greatly improved this article. Carol Edwards (U.S.G.S. Field Records Library) provided valuable assistance.

References Cited:

Allen, E.T., 1932. Pen Portrait of William Francis Hillebrand, 1853-1925. Journal of Chemical Education, vol. 9, no. 1: 73-83.

Cappa, J.A., 1998. Alkalic igneous rocks of Colorado and their associated ore Deposits. Colorado Geological Survey Resource Series 35, 138 p.

Davis, M.W. and Streufert, R.K., 1990. Gold occurrences of Colorado. Colorado Geological Survey Resource Series, 28, 101 p.

Kelley, K., 1996. Origin and timing of magmatism and associated gold-telluride mineralization of Cripple Creek Colorado (PhD dissertation). Colorado School of Mines, Golden, Colorado. 259 p.

Kelley, K.D., Romberger, S.D., Beatty, D.W., Pontious, J.A., Snee, L.W., Stein, H.J., Thompson, T.D., 1998. Geochemical and geochronological constraints on the genesis of Au-Te deposits at Cripple Creek, Colorado. Economic Geology, vol. 93: 981-1012.

Lindgren, W., and Ransome, F.L. 1906. Geology and gold deposits of the Cripple Creek district, Colo. U.S. Geological Survey Professional Paper 54, 516 p.

Loughlin, G.F. and A.H. Koschmann, 1935. Geology and ore of the Cripple Creek district, Colorado. Colorado Scientific Society Proceedings, vol. 13, no. 6: 217-435.

Pontius, J.A., 1996. Gold deposits of the Cripple Creek mining district, Colorado USA. Society of Economic Geologists, Guidebook Series vol. 26: 29-37.

Rickard, T.A., 1900. The Cripple Creek volcano. Transactions Am. Inst. Min. Eng., vol. 30: 367-403.

Thompson, T.B., Trippel, A.D. and Dwelley, P.C., 1985. Mineralized veins and breccias of the Cripple Creek district, Colorado. Economic Geology, vol. 80: 1669-1688.

Thompson, T.B., 1992. Mineral deposits of the Cripple Creek district, Colorado. Mining Engineering vol. 44: 135-138.

White, D., 1927. Memorial of Frank Hall Knowlton. Geological Society of America Bulletin, vol. 38, no. 1: 52-70.