USA > Colorado > History of the State of Colorado, Volume II > Part 7
Note: The text from this book was generated using artificial intelligence so there may be some errors. The full pages can be found on Archive.org (link on the Part 1 page).
Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14 | Part 15 | Part 16 | Part 17 | Part 18 | Part 19 | Part 20 | Part 21 | Part 22 | Part 23 | Part 24 | Part 25 | Part 26 | Part 27 | Part 28 | Part 29 | Part 30 | Part 31 | Part 32 | Part 33 | Part 34 | Part 35 | Part 36 | Part 37 | Part 38 | Part 39 | Part 40 | Part 41 | Part 42 | Part 43 | Part 44 | Part 45 | Part 46 | Part 47 | Part 48 | Part 49
Notwithstanding the study that has been given to the subject of ore-deposition, both in Europe and in the United States, and the many ingenious theories that have been advanced, we are still forced to acknowledge the fact that in all that relates to the conditions governing the formation of ore-bodies, we are yet in the speculative stage.
At the present time the following points only can be considered of general application:
That the most valuable ore-bodies occur inclosed by, or in direct contact with, either eruptive or highly metamorphic rocks; or if in sedi- mentary rocks, then in localities where these have been intersected or broken by eruptive or metamorphic rocks.
That they may occur as the material filling pre-existing fissures, or be deposited along contact,-or fault,-planes, by the partial or entire replacement of the constituents of the inclosing rock.
That they have been deposited from aqueous solutions, which have derived their metallic contents from the contiguous, subjacent, or not very remote rocks of the region in which they occur.
While these points cover the majority of known metalliferous occur- rences, they are very general in character, and, within the limits given, the ore-bodies themselves show great variation in mode of occurrence and mineralogical composition and association.
The eruptive rocks most frequently found associated with ore- deposits are porphyries, diorites, andesites, less frequently trachytes, and rarely rhyolites, all of which belong to the class of acid eruptives.
80
HISTORY OF COLORADO.
The basic eruptives, like basalt, dolerite, etc., are seldom, if ever, asso- ciated with important ore-bodies,-hence the character of the eruptive rocks of a region may furnish a valuable clue to those who explore for metalliferous veins.
The age of the rocks inclosing the veins of a district is of little or no importance, for we find productive veins in rocks of all ages from the Archæan to the Tertiary inclusive. Nor is it likely that the oldest veins are always contained in the oldest rocks; on the contrary, veins of quite recent origin may occur in the most ancient varieties of granite. The ore-bodies of Boulder, Gilpin and Clear Creek Counties, while contained in granite rocks, are more likely to have originated during the disturb- ances of the mountain-making period than at any earlier time. The ore-bodies of Leadville and Aspen are contained in rocks of Carbonif- erous Age, yet the association of these with eruptive rocks of Creta- ceous or Tertiary Age, warrants the supposition that the ore-bodies were formed at a much later period than the inclosing rocks. Con- cerning the other great vein-systems of the State, there can be little doubt of their Tertiary age.
The mode of occurrence of ore-bodies is likewise a feature of less importance than is usually supposed. All forms of deposits, whether fissures, gashes, bedded veins, segregations, or mineralized zones, have been found equally productive, and, in turn, marvelously rich. Nor is there any foundation, as the record of all our older mining districts will show, for the commonly entertained notion that veins increase in . richness with depth.
Lithological similarity of the inclosing rocks does not indicate, as a rule, that the veins of separate districts will be of similar mineralogical composition. Thus we find the auriferous veins of Boulder County to contain combinations of gold and silver with tellurium; those of Gilpin County to contain the gold in the free state, or mechanically mixed with pyrites ; while the Clear Creek County ores are largely argentiferous compounds of base metals,-yet all of these are contained in the same continuous granite formation of the Colorado Range.
81
HISTORY OF COLORADO.
It is not the intention here to enter fully upon the description of all the different mineral districts of the State; but for the purpose of illus- trating the main features of ore-deposition, as exemplified by the vein systems of Colorado, and already outlined above, brief reference will be made to the best known and most important.
The remarks on the veins of Northern Colorado, just given, require but little amplification to enable one to gain a general idea of their true character. They all belong to the class known as fissure veins,-that is, they extend, more or less vertically, for a considerable distance into the earth. The granite inclosing rock is often found to be traversed by eruptive dykes, and frequently impregnated with ore adjacent to a pro- ductive vein. The want of similarity in mineralogical composition may be partly due to the dyke rocks associated with each system of veins ; yet it would appear more probable that it was due to the latter having originated in separate zones of granite; each zone differing from the other in the composition of its contents, and the secretions it afforded.
The placers of Gilpin and Boulder Counties, which have yielded so largely in times past, no doubt owe their enrichment to the liberation of gold, through the constant and long-continued degradation of the aurif- erous veins of these districts.
Somewhat similar to the Northern Colorado deposits, but much less productive, are the auriferous veins of Independence on the head of Roaring Fork, and those of Granite on the Upper Arkansas.
In the Leadville district, which has been very thoroughly studied and described by S. F. Emmons, the ore-bodies lie in nearly horizontal position, between the floor of Carboniferous limestone and the roof of white porphyry,-or in what is termed by miners the "contact." Some- times the ore, in irregular form, replaces the limestone for a consid- erable distance below this contact. By a series of faults the ore-sheet, which was probably once continuous, has been cut up into several sepa- rate areas or benches.
The most characteristic ore consists mainly of calcareous earthy matter containing oxidation products of lead and iron ; these metals 6 II.
82
HISTORY OF COLORADO.
existing mostly as carbonates, frequently as oxides, and in exceptional cases large quantities of iron oxide are present. The silver in the ore is usually combined with chlorine, bromine and iodine, some of the mines producing specimens rich in horn-silver. In some mines the ores still exist as sulphides. Other ore-bodies of this district of an entirely different character,-like the Printer Boy, mainly auriferous,-have in times past served to enrich the placers of California Gulch, and probably those of the Upper Arkansas. According to Emmons the Leadville porphyry is of late Mesozoic Age (Cretaceous), hence the ore-bodies themselves must be referred to this age, or to a period still later,-that is to the early Tertiary.
Outside of the Leadville district, but yet in the same region, are many others of lesser note. At Red Cliff there are a number of product- ive ore-bodies, yielding oxidized silver-lead ores of similar character to those of Leadville, and occurring at nearly the same geological horizon, may be somewhat older. The deposits are found mostly in metamorphic strata, though near by are intrusive eruptives, and coarse granite is exposed in the cañon of Eagle River.
The veins of Ten-Mile district belong mainly to the class typified by those of Leadville, and illustrate very forcibly the influence of neigh- boring or adjacent eruptive masses on the formation of ore-bodies.
Along the Mosquito Range the same connection is apparent. The veins occur in Palæozoic strata, frequently cut by dykes of quartz-por- phyry, diorite or porphyrite. Some of the deposits yield auriferous ores, others argentiferous galena and oxidation products ; still others, as at Mount Lincoln, approach the Leadville ores in composition.
The district around Breckenridge,-one of the oldest in the State, -includes a number of valuable ore-deposits, which on the whole cannot be referred to any particular system, owing to variation in mode of occurrence and mineralogical composition. Nearly all the argentiferous veins contain simply argentiferous ores, usually of lead and copper. There are exceptions, for instance, on Shock Hill, where base metals are absent, and the silver exists in the chloride form. Some of the auriferous
83
HISTORY OF COLORADO.
ore-bodies afford exceedingly rich ore, notably the Ontario, which, in the joints and crevices of the rock, contains beautiful specimens of matted wire-gold. The rocks in the auriferous portion of the district are often traversed by eruptive dykes, which may partly explain why the ore is, in some instances, distributed through zones of altered and enriched country rock, without well defined boundaries. On the west side of Blue River, the rocks are granitic or metamorphic.
The placers near Breckenridge were noted for their richness in times past, and are still productive. The gold of French and Leaven- worth Gulches is often more or less crystalline, like that of the lodes from which it was derived.
The same region likewise includes the once rich and still productive placers of Alma, Fairplay and Tarryall.
The principal ore-bodies of Aspen lie in, or near, a highly inclined plane of contact in Lower Carboniferous limestone, or between what are locally termed the "blue" and "short-lime." The deposits of both Aspen and Smuggler Mountains, which are situated on opposite sides of Roaring Fork Valley, evidently belong to the same geological horizon, if not to the same contact-plane, and will probably be found more or less continuous in the intervening drift-covered valley whenever this shall be explored. In the vicinity of the Aspen Mountain ore-bodies, the strata appear to have been synclinally folded between the main Archæan area on the east, and an intrusive mass of granite at the western extremity of the mountain ; thus producing a second series of oppositely inclined beds, also containing a few ore-bodies. Intrusions of partly altered diorite, or porphyrite, occupy a prominent position in the intervening trough, and may have seriously faulted, or dislocated, the strata in the depths. The ore is not always confined to the "contact" between the "blue" and " short-lime," but may branch out irregularly for some dis- tance into these rocks, although such spurs or impregnations, are evidently related to the " contact" ore-bodies.
The bulk of the Aspen ores consists largely of oxidation products of argentiferous minerals, with true silver minerals, associated with calca-
84
HISTORY OF COLORADO.
reous matter and considerable heavy-spar; it is therefore what is called " dry ore," and requires to be mixed with silicious lead ores, or with matting ores, before it can be treated. A few veins, away from the main contact, yield ores containing a high percentage of lead ; but they are not as rich in silver as the dry ores, and as yet do not promise to become an important source of lead for smelting purposes in this district.
The ore-deposits of Southwestern Colorado, or what is known as the San Juan country, possess great interest for students of vein phenomena ; and economically considered, may eventually prove the most lasting and valuable in the State. In no part of the Rocky Mountains are metalliferous veins so numerous over such a wide extent of country.
The majority of the San Juan deposits are referable to the great system of veins common to all the mining districts of Hinsdale, Ouray, San Miguel and San Juan Counties. The origin of this system may be briefly explained as follows: During the early part of the Tertiary Period, an eruption on a grand scale, covered the higher region of the San Juan Mountains to a depth of 1,500 feet, with an overflow of brec- ciated andesitic lava, which on cooling, developed fissures of contraction (shrinkage-cracks) traversing the mass in all directions. The filling of these fissures corresponded to the formation of the existing system of veins, which, as a rule, terminate at the base of the breccia. Following the first grand overflow were others of less magnitude, consisting of non- brecciated andesites and rhyolites. The dynamic movement attending these later eruptions, produced in places, fissures which extend below the horizon of the breccia, into the stratified rocks, but usually cease to be productive below the eruptive zone. Again, there are ore-bodies, such as the Calliope, Boomerang, Trout and Fisherman, and Mineral Farm, which evidently do not belong to the main system, being situated far below the eruptive horizon. These deposits occur in the vicinity of dykes of andesite or diorite, which probably mark the channels of past eruptions, and apparently have had some connection with the origin of the neighboring ore-bodies.
85
HISTORY OF COLORADO.
There are still other ore-occurrences in San Juan which form, as it were, a group by themselves; among these the deposits of Red Mountain district are at present the most important. In typical cases the ore-bodies occupy a series of more or less connected irregular chambers, trending downward, which were probably at one time the channels of thermal, or mineral, springs. The action of the mineralizing water upon the surrounding eruptive rock, brought about complete sil- icification for some distance away from the chambers, so that the ore- bodies they contain are in each case virtually distributed through a huge irregular column of quartz extending to an undetermined depth. The ore-deposits of this district afford one of the few instances where the ascension theory can find logical application. The famous Bassick mine at Rosita has by some been cited as another; but this theory can hardly be applied to the great system which has its downward limit at the breccia.
At a number of localities in the San Juan Mountains there exist immense decomposed masses of eruptive and sedimentary material, of yellowish or variegated colors, which appear to have been acted upon by mineral waters, not confined, as at Red Mountain, to particular channels, but circulating everywhere, through joints and fractures, or along bedding-planes, producing, according to the nature of the rock acted upon, either kaolinization or local metamorphism. No doubt ore- bodies often exist in these altered masses; indeed, such have already been discovered near Ouray and elsewhere, which are considered quite important.
The bulk of the productive ore from the San Juan district consists of argentiferous gray-copper, copper pyrites, and galena, associated with some zinc-blende, and iron-pyrites in a quartz matrix. In particular districts, like Poughkeepsie Gulch, the ore often contains a high per- centage of bismuth. In Marshall Basin the most productive mines yield largely of the true silver minerals, pyrargyrite and polybasite, while the ore from the same mines carries considerable quantities of gold. Other mines on the San Miguel drainage, and a few in San Juan County,
86
HISTORY OF COLORADO.
afford auriferous ores only. Tellurium has been found, in combination with the precious metals, at the Hotchkiss Mine near Lake City.
At Rico on the Rio Dolores there is an interesting group of veins entirely independent of other San Juan deposits. The ore-bodies are con- tained in carboniferous limestone, or in the contact between the limestone and a mass of porphyrite, or andesite; the intrusion of the latter having tilted up the sedimentary beds anticlinally, which has been deeply eroded by the river, down to and below the level of the intrusion and its associ- ated ore-bodies. Pyrargyrite, and a few other silver minerals, are present in some of the veins, but the bulk of the ore consists of argentiferous oxidation products of lead, copper, and manganese, with considerable galena at lower levels. Large quantities of carbonic acid are exhaled along the line of the eruptive intrusion, and by its superior density dis- places the air in sheltered hollows, and along the floors of tunnels, often to such an extent as to prove fatal to mice and other small animals.
The La Plata Mountains include a district which, while properly belonging in the San Juan region, has an entirely independent system of veins. It may be mentioned as one of the few localities in the world containing compounds of gold and tellurium. The mass of the La Plata Mountains is eruptive, and in places, the tilted sedimentary beds on its flanks have been partly or wholly metamorphosed. There appear to be two distinct groups of deposits in this district, viz .: Auriferous veins containing free gold, tellurides, pyrites, and sometimes, as at the old Comstock mine, argentiferous minerals like cosalite; argentiferous veins, containing galena and zinc-blende, with some silver. The matrix is usually quartz.
While small quantities of very rich ore have been produced from La Plata mines, the average is generally of low grade and often refractory. Still the district is not entirely without promise; valuable ore has been found there, and exploration may at any time develop more important ore-bodies.
Of much less value than the metal veins, yet nevertheless worthy of mention, are the San Juan placers. These are mainly on the San
87
HISTORY OF COLORADO.
Miguel, although some washing has been done on the Uncompahgre, near the mouth of Dallas Creek, and on the La Plata, below Parrott City. The gravel of the Animas also contains gold, but hardly in paying quantities. The San Miguel placers extend from Marshall basin nearly the entire length of the river, and include large quantities of auriferous drift that could be profitably worked with improved appli- ances and cheaper labor.
The district which includes Rosita and Silver Cliff contains some unique and interesting forms of ore-deposits, which appear to be either mineralized zones of country rock, or else ore-bodies without defined boundaries, like the Bassick. The latter extends to an unde- termined depth, apparently following an old channel of deposition. In the former the silver usually exists as chloride; while in the ore- bodies of the Bassick type, argentiferous compounds of lead, zinc, copper, and occasionally tellurium, are found coating, in concentric layers, detached boulders and pebbles. The ore-bodies of this character have been thought by some to have been deposited in the channels of ancient thermal springs; a view which may be open to question, since it is not improbable that the channels are simply old eruptive vents, choked up, so to speak,with worn fragmental ejectamenta, the result, possibly, of an outflow of mud and boulders. In this case the channels might merely perform the part of receptacles for lateral secretions, and the assumption of a deep-seated source would be unnecessary. Rhyolite, trachyte, and andesite are the common eruptive rocks, and rest immediately on the granite mass of the Wet Mountain Range .*
The numerous deposits of the Elk Mountain region, including those of Irwin, Slate River, Gothic, Schofield, Crystal, Ashcroft, White Pine, Pitkin, Tin Cup, with many others, may be cited as additional instances of the association of productive ore-bodies and eruptive or metamorphic masses; an association everywhere illustrated in the mining districts of Colorado.
*This district will be fully described in a forthcoming report by S. F. Emmons of the U. S. Geo- logical Survey.
88
HISTORY OF COLORADO.
Regarding the iron deposits of the State, little can be said until their extent has been better determined. So far as known, the workable ore- bodies are confined to the occurrences on the west side of the Sangre de Cristo near Villa Grove, at Calumet near Salida, at Ashcroft, and at several localities in Gunnison County, including those at White Pine and the deposits near Snow-Mass Mountain. The ores consist of the oxygen compounds of iron, magnetite, hematite and limonite, of average purity and richness.
Throughout the coal measures there is considerable low-grade iron- stone, and in the mountains many deposits of bog-iron ; but neither of these are sufficiently rich or pure to be utilized, except for fluxing pur- poses, even under the most favorable circumstances.
Tin-ore has not yet been found in Colorado in important quan- tities, but its existence has been proved in the Pike's Peak region, where a few specimens of tin-oxide have been obtained by mineralogists ; hence there is a possibility that deposits may be found somewhere in the great Archæan areas.
Nickel-ore occurs in limited quantities at the Gem Mine, near Silver Cliff ; and a small amount of uranium has been taken out near Central City.
To describe here all the mineral deposits of the State, which have a present, or prospective value, would be out of the question ; certain occurrences, however, possess too much interest to be entirely overlooked.
In the remarks on the Green River Eocene, allusion was made to the richness of the shales of this group in condensible, i. e., liquid, hydrocarbons. The great thickness and extent of this formation in the Book Cliffs Plateau, invites the consideration of it, as a future source of mineral oil. It must not be supposed that the present petroleum supply of the United States can be maintained indefinitely, and a serious decline in the production might so advance the price of the commodity as to render profitable the distillation of the richer shales of the Green River beds. From the tests that have been made it is known that in the Book
89
HISTORY OF COLORADO.
Cliffs exposures, along Grand River, there are as many as fifty bands of marly limestone, ranging from two to ten feet in thickness, which will yield twenty per cent. of crude oil ; while of the remaining 1,200 feet there is much that will yield ten per cent. The richer carbonaceous material can, if necessary, be used as fuel in the distillation process, since it burns readily when ignited. The distillation of the Scotch shales, yielding from ten per cent. to fifteen per cent., is one of the most profit- able industries in the British Isles, and it is only a question of time when a similar industry will be developed in Colorado.
Among other noteworthy occurrences may be mentioned the great bed of white marble on Yule Creek, in Gunnison County. On both sides of the creek this bed is exposed, dipping westward about 30°, and finally disappearing under partly metamorphosed limestone of the same age (Upper Silurian ?). The marble stratum, denuded of its limestone covering by erosion, is shown resting on the slope of White House Mountain, and reaching half way to its summit.
On the weathered surface the rock has been acted upon by frost, and crumbles readily ; but shallow excavations develop the solid marble intact. Like the product of similar deposits in Vermont and elsewhere, the Yule Creek marble varies in quality, from grades suitable only for architectural purposes, to the highly prized "statuary."
A diamond drill hole, normal to the planes of bedding, showed the thickness of workable marble to be about eighty feet ; and the core clearly demonstrated its firmness and excellent quality.
The other rocks of economic value have already been mentioned in connection with the formations containing them,-it only remains to note the most interesting and valuable of our non-metallic minerals.
The Pike's Peak region has long been celebrated for its beautiful specimens of bluish-green microcline (amazon-stone) which have been exported in large quantities to different parts of the world. The crystals of smoky-quartz, associated with the amazon-stone, are cut into gems, in which form the mineral is much used for cheaper kinds of jewelry, finding a ready sale under the trade name of "smoky-topaz."
90
HISTORY OF COLORADO.
Among gem-stones of a higher grade, occurring in the same region, may be mentioned true topaz and phenacite. The latter, notwithstanding its rarity, is not often cut into gems, owing to want of hardness. The topazes furnish very fair stones when cut, which are generally limpid or of a bluish or wine-colored tint, and range in weight up to 150 carats, or more. The beryls (aquamarines) from Mount Antero, furnish small gems, up to three carats weight. Corundum occurs in small crystals in a band of schist (corundum-schist) near Calumet. The crystals are usually of a bluish tint, and in places, they possess sufficient clearness and depth of color to entitle them to be called sapphires.
Zircon crystals, which are abundant in some Pike's Peak localities, have been exported to the Eastern States for the extraction of the earth zirconia ; but the bulk of the supply of this substance comes from the Southern States.
Colorado contains many other beautiful and rare minerals, and not a few that are new to science. Many of them are highly valued as min- eralogical specimens, but find little or no application in the arts.
In concluding the foregoing brief sketch of our geological history, there remains but to emphasize the most important part of the record. In the time intervening between the beginning of the Laramie and the close of the Tertiary,-a period very short indeed, when compared with the whole geological time,-the great coal measures were deposited, and the entire region elevated above the ocean by the continental revolution. Following this came the great eruptions, the deposition of nearly all the valuable ore-bodies and the final upheaval and crumpling of the Rocky Mountains. The same period witnessed, also, the sudden appearance, and gradual development in Colorado and Wyoming, of some of the most remarkable types of mammals the world has ever seen.
Need help finding more records? Try our genealogical records directory which has more than 1 million sources to help you more easily locate the available records.