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(Continued from Feb. 11, 1909.)
ORES IN VOLCANIC ROCKS.
Passing down along the Colorado range for 100 miles from the Clear Creek region south, few volcanic rocks of importance are met with and still fewer ore occurrences, until we reach the area characterized by the granitic upheaval of Pike's Peak.
In the great mass of coarse, red granite there are no ore deposits of importance and few igneous dikes other than varieties of granite.
Looking down from Pike's Peak on the low hills and rolling area on its western base, we note the mines and dumps of Cripple Creek, there congregated in a small oval area, not topographically suggestive of the ore producing center it has become. It is the seat and ground-plan, how-ever, of a once active volcano, and its ore bodies are principally confined to the site of the crater and its immediate surroundings.
The geology of Cripple Creek, as well as the first discovery of its ore deposits, and the later exploration, is well known; also, how the unpromising appearance of the area caused it to be overlooked by both geologists and prospectors. The area was originally one of small hills and ridges of granite, little differing from the surrounding country.
We may assume that at a late period in geologic history, earthquakes connected with some upward movements of Pike's Peak took place, growing in intensity until the forces below opened a series of fissures radiating from a central point which became the focus of eruption and the imprisoned gases found vent by blowing out a great crater hole and shooting the fragmentary material high into the heavens, mingled with a column of steam.
This so-called ash formed a dark, threatening cloud over the landscape. As the cloud of steam mixed with dust and fragments of rock condensed, it fell around the vent and over the country, filling valleys and covering hills with a liquid adobe or mud flow which, in its present hardened state, is known as tuff and breccia.
After the main eruption of steam had subsided, flows of andesite lava welled up in the crater and poured down the sides of the mountain. There was doubtless a cone over the orifice, as in other explosive volcanoes, but this being of incoherent material has been removed, exposing the roots and ground plan.
This consists of a rudely oval area in the granite with innumerable fissures radiating from it some of which are now filled with veins, some with igneous dikes. After the first main eruption of andesite, at a later period was one of the igneous rock called phonolite and some other varieties of lava.
This eruption either made new fissures for itself, or filled old ones already opened by previous eruptions with molten rock, which solidified and are known as dikes.
Still later there was an intrusion of little black dikes of the variety of lava known as nepheline basalt and these are found associated with the ores they, and their eruption, may have been one of the principal generators of the ore. We have noted before, the significance of the occurrence of little black dikes near to or in close association with ore bodies.
A later phase of these eruptions was what is known as the solfataric period. Enormous bodies of steam and fumarolic vapors issued from the shattered rocks and in some cases hot and mineral springs. The effect of these agents on the surrounding lavas was to decompose them, oxidizing any metal they might contain and leaching from them the diffused ore elements concentrating them in fissures as veins or in crushed and sheared zones of rock resulting from disturbances of the district.
The principal ore so deposited was in the form of gold tellurides, which near the surface were oxidized, setting free their gold. The early developments were consequently the richest, but the camp with depth still holds its own, though not on a par with its palmier days.
The mode of occurrence of the Cripple Creek ores is noteworthy. They are not generally in well-defined fissure veins as in the granitic areas described, but they occupy zones of weakness and shattering of all kinds, resulting from the great volcanic disturbances of the district.
A favorite locality for ore deposition is in a series of close fissures with sometimes a main fracture. Such fissures are known as shearage zones or fault planes of slight displacement accompanied by great compressive stress.
Some have a definite relation to the dikes because the vein fissure often followed the dikes as lines of least resistance. The dikes themselves are sheared and ore deposited in the cracks. Vein materials were carried into fissures and deposited as a replacement of the country rock after the alteration of the latter had considerably progressed.
When it occurs in shoots, it is due to a restricted circulation of solutions guided by the more permeable planes along fissures and by transverse fractures. In many cases it occurs as a zone in shattered rock, following irregularly the course of a dike which is itself mineralized.
The Gold Coin is a good example of ore occupying a zone of shearage planes 30 ft. wide in granite. Ore also occurs in true fissure veins in the massive eruptive rocks independent of dikes. In some cases veins cross each other and ore occurs at the crossing.
In the Independence the ore is along a fissure zone, both in breccia and granite, near or in contact with a phonolite dike, or in separate fissures in the country alone. The coarse granite near the ore has been acted upon by solutions which have removed many of its constituent minerals, leaving it as a honeycombed skeleton of quartz, kaolin and red feldspar. The cavities have been filled by later secondary quartz with iron pyrites and fluorite, among which are crystals of tellurides which are sometimes replaced by a pseudomorph of rusty free gold.
In the Anna Lee mine the ore body occurs in an oval cylindrical chimney crossed by a dike. The ore which forms the mineralized cement around little pebbles or lapilli was probably deposited by a spring which rose through the orifice.
Summary and Conclusions.
Cripple Creek is near the center of a great line of volcanic eruption which extends north into South Park and south to Rosita and Silver Cliff. These lines have along them points of special activity. Rosita and Silver Cliff is one, Cripple Creek another, Florence and perhaps Black Mountain a third. At these points there were once active volvanoes. They were not all known centers of mineralization.