Mines and Minerals, March 1899
Written for Mines and Minerals, by Prof. Arthur Lakes.
There is no doubt but a very rich strike of ore, perhaps the richest in the history of Cripple Creek, has lately been made in the Isabella mine at Cripple Creek. The exact nature and value of this at present are unascertainable, owing to the reticence of the company.
The nature of the ore appears to be very rich bodies of telluride and free gold, ores which, in a minor degree of richness, have characterized the mine from the outset.
The Isabella property is one of the oldest in the camp. It is situated on the Eastern slope of Bull hill, commanding a fine view of Pike's peak. A lofty castle rock of columnar phonolite crowns the hill above the mines, which was celebrated in the miners' strike as the "Strikers' fortress," commanding as it did a view of all the country round.
The rocks of this section are both andesitic breccia and phonolite. Most of the important mines on Bull hill are on veins in fissures in the massive eruptive rocks, a few such as the Victor, are on fissures in the breccia. The Bull Hill fissures are often faulted, owing to the brittle character of the massive rocks.
The veins are very numerous, trending in various directions. They vary from an inch to several feet in width, and show at times, well defined bodies of quartz in them, containing iron pyrites, iron oxide, fluorite, and other minerals forming the cement of the fragments of rock, occurring in the fissures. The country rock may also be impregnated with these minerals along the path of fissures.
The most well defined and continuous of these fissures or fissure systems are the veins of the Victor, Lee, and Isabella mines which are on the same, or closely allied veins, along the same zone of fissuring and ore deposition for a direct distance of upwards of a mile, and along these veins the mines are more or less in connection.
Two or more closely parallel veins are found in these mines, among which is the one in the Lee workings which has lately yielded such unprecedented riches. The course of the main vein fissures is often very circuitous and deflected. The country rock is partly massive trachy-phonolite and partly andesitic breccia. In the Lee shaft the vein is in the massive rock, in the Smuggler and Victor in the breccia.
The ore is in quartz, not unfrequently an opaline quartz like that deposited from a hotspring or geyser, showing the mode of genesis of the ore. This quartz may change into a yellow jasper. The precious metals are free gold, especially near the surface, and telluride ; the latter more with depth. The rich ore is in well defined shoots, continuing for considerable distance along the strike of the vein.