The successful drainage of a great gold mining district must be considered an event of importance in mining circles. Especially so in the case of the Cripple Creek district, where for so many years the chief item in mining expenses was the cost of pumping. To understand why this is so it will be necessary to describe briefly the general geological conditions.
The Cripple Creek district is an area of eruptive or volcanic rock of an irregular elliptical shape three to five miles in diameter and unknown but probably great depth, and entirely surrounded by impervious granite walls. This volcanic mass has been thoroughly fissured by many successive eruptions and contains innumerable cavities of all sizes and shapes. During the ages that have elapsed since the eruptions ceased surface waters have worked their way downward into this fissured, honeycombed mass until it was completely saturated up to the level of the lowest notches in the surrounding granite rim.
This great crater has therefore become, as it were, an immense reservoir three to five miles in diameter at the surface and extending to unknown depths. The total amount of water in this vast reservoir is almost incredible. It has been determined to be not less than 50,000,000 gallons per vertical foot and probably amounts to 5 per cent of the enclosing rock. No wonder the mines are wet.
After many years' constant struggle with the water and the expenditure of vast sums for pumping the mine owners of the district decided to resort to the tunnel method for drainage.
The El Paso tunnel driven in the year 1903 quickly demonstrated beyond all doubt the great economy of tunnel drainage. It saved its cost many times over within two years after completion. It was such a decided success that it seems strange a deeper tunnel was not at once begun. But, at any rate, the chief mine owners of the district finally got together and the Roosevelt drainage tunnel was begun on June 1, 1907.
So much has been written and published about the Roosevelt drainage tunnel in the various newspapers and technical journals of the state that I will not attempt here to give any of the details of its construction. Work upon it was vigorously pushed (part of the time from three headings) up to Jan. 1, 1911.
At that time the total length of the tunnel was 15,746 ft. At a point 14,500 ft. from the portal connection was made with the bottom of the El Paso main shaft by means of a drill hole 6 in. in diameter and 335 ft. deep and at a depth of 1,350 ft. below the surface. Through the drill hole a flow of about 4,000 gallons per minute was passing and the water in the El Paso shaft had been lowered some 350 ft. at the beginning of the present year.
The main heading had passed from the granite into the phonolite core of Beacon hill, where we had confidently expected to open a strong water course. We were somewhat disappointed in this, as a flow of only 1,500 gallons was obtained. It was then decided to drive a crosscut northerly from the main tunnel at a point some 500 ft. east of the El Paso main shaft for the purpose of opening the well-known C. K. & N. water course.
This crosscut (commonly known as the Fuller crosscut) was driven during the early part of this year (1911) and intersected the C. K. & N. water course and vein about 500 ft. from the main tunnel. At this place the vein proved to be a very small fissure in the granite and only about 500 gallons of additional flow was secured. The drainage tunnel company's funds were now exhausted and this work was suspended entirely about the end of March.
At this time the total flow from the portal had slightly declined and amounted to but little over 6,000 gallons per minute. Meanwhile, the general water level of the district was declining about 5 ft. per month. Finally, about Oct. 1, additional funds were raised and work was resumed in the main heading by the tunnel company, and shortly afterwards the El Paso Mining Co. began work upon the C. K. & N. vein in the Fuller crosscut.
On Nov. 17, while driving northerly on this vein, a water course was suddenly encountered which yielded a flow of probably 5,000 to 6,000 gallons per minute for a few hours, gradually subsiding to a regular flow of about 4,000 gallons per minute. This prevented any further work for the time in that particular heading.
Meantime the main tunnel heading was being pushed forward with one shift, cutting a little additional water at every round but finding no large water courses. The El Paso Co. then began driving southwesterly on the C. K. & N. vein, and about 50 ft. from the Fuller crosscut opened a very fine ore shoot.
After drifting on this ore shoot some 70 ft. another strong water course was encountered yielding a flow of about 5,000 gallons per minute, and immediately and completely draining all the El Paso workings directly above and taking away the flow through the churn drill hole. The total flow at the present time (Dec. 15, 1911) from the portal of the Roosevelt tunnel is fully 8,500 gallons per minute, of which about 6,000 gallons are from the Fuller crosscut.
The main heading is being pushed forward at the rate of 8 ft. per day towards the Elkton main shaft. It has passed entirely through the phonolite plug of Beacon hill and is now in the granite neck between that phonolite and the main eruptive area.
The tunnel company has sufficient funds to carry it about 1,000 ft. farther. It is expected that within 800 ft. of the present breast (16,225 ft. from the portal) that the well-known Gold Dollar water courses will be cut and a very heavy flow developed there. Such are the conditions at the present time.
What of the results so far accomplished? The general water level of the entire district has been lowered an average of 75 ft. since water was first encountered. Several of our largest mines have been able to sink their principal shafts and to open new levels, or recover drowned workings. Others will soon recover levels now under water. The general level is subsiding about 10 ft. per month.
The El Paso, Gold Dollar, Mary McKinney, Elkton, Portland, Granite, Strong, Independence, Cresson, Index, etc., are among those that are sinking, or preparing to sink. Most of them have done no sinking since 1904, seven years ago, and it can be readily understood that ore reserves of some of the developed mines above the water level are approaching exhaustion.
The Roosevelt tunnel has even now shown that it is an entire success. It is lowering the general water level of the district as rapidly as could be expected, and it has already saved fully one-third of its cost. It has completely drained the workings of the El Paso mine to a depth of 400 ft. below the former water level, and has enabled that company to reopen some of the bonanza ore shoots that were submerged six years ago.
They have also been able to demonstrate that these ore shoots extend downward at least to the tunnel level. They have been able to open up and mine ore at an elevation of 8,050 ft. above sea level, or at a horizon at least 500 ft. lower than any ore has heretofore been mined in the Cripple Creek district. If good ore is found in the El Paso ground at this horizon, why not also in other properties. It will be.
The principal ore zones of the district are now fairly well defined. The main ore shoots of the leading properties have been followed to their deepest levels and are there still productive. How can anyone assert they will go no deeper? How can anyone place a limit to their depths?
The Roosevelt tunnel will open up 750 (vertically) of virgin ground; 750 ft. additional depth of the richest area of its size found in this country. An area that has produced nearly $300,000,000 of the precious metal and that is still producing at the rate of $15,000,000 per annum from its old workings. What will it do when the new ground is fairly opened up?
Many estimates have been made of the gross output between the present water level and the Roosevelt tunnel level? Such estimates, of course, are the merest guesses. But, at any rate, conditions are now such that we may feel very optimistic regarding the coming years.
There has been some discussion of the advisability of driving a still deeper tunnel than the Roosevelt, and I have made surveys and prepared plans and estimates for three such tunnels. It is quite unlikely that any such work will be attempted in the near future, or until the conditions above the Roosevelt tunnel level are fully understood.