Geology of uranium and associated ore deposits, central part of the Front Range mineral belt, Colorado
The Central City district and adjoining mining areas in the central part of the Front Range mineral belt have supplied small quantities of uranium ore…
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c;;oo) T&7r ./)tf). 001 TEI-6ol GEOLOGY OF URANIUM AND A~SOCIATED ORE DEPOSITS> CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO By P. K. Sims and others Trace Elements Investigations Report 6ol UNITED STATES DEPAR.rME:NT OF THE INTERIOR GEOLOGICAL SURVEY
UNITED STATES · DEPARrMENT OF THE 'INTERIOR GIDIDGICAL SURVEY GEOLOGY OF URANIUM AND ASSOCIATED ORE DEPOSITS 1 CENTRAL PARI' OF THE FR'>NT RANGE MINERAL BELT 1 COIDRAOO by P. K. Sims .and others August 1959 Trace Elements Investigations RePort 601 This preliminary report is distributed wi about editorial and technical review for conformity with official standards and nomenela. ture. I.t is not for public inspection or ·quotation TEI-601 · *This report concerns work done on behalf of·the Division of Raw Materials of the S. Atomic Energy Commission. ' '
USGS TEI-6ol GlOOLOGY AND MINERAIOOY Distribution No. ot cQpies '·1 Division of Raw:Mater1als1 Salt Lake City , Division of· Raw·Materials, Washington ' U. s. Geological SU~y: Foreign Geology Branch~ WaShington Geophy'sics Branch, Washington .. ,
GEOLOGY OF URANIUM AND ASSOCIATED ORE DEPOSITS, CENTRAL PART OF THE FroNT RANGE MINERAL BELT, COLORAOO (Includes Central City 1 Id.a.ho Springs 1 Lawson-DumontFa.:ll River 1 Freeland-Lamartine 1 and Chicago Creek mining districts) by P. K. Sims and others Abstract The Central City district and adjoining mining areas in the central part of the Front Range mineral belt have supplied small quantities of uranium ore intermittently since the discovery of pitchblende at Central City in 1871. During the early years of development the uranium production from the region was of national importance 1 and until 1951 the region was this country's
principal domestic source of pitchblende. In recent years 1 however 1 the production bas been insignificant although the search fer uranium bas been greater than at any previous time. The pitchblende occurs as a. local minor constituent of gold- a.nd silver-bearing base-metal sulfide veins, chiefly valuable for .their gold content, which have yielded ores valued a.t about $200 million.
The mining districts of the eentra.J. pa.rt of the Front Range mineral belt are in a terra.p.e of complexly folded Precambrian crystalline rocks 1 which constitute the c"-re of the Front Range, and are near intrusive centers of porphyritic igneous rocks of early Tertiary age. The Precambrian rocks are an inter layered and genera.ll.y conformable sequence of gneissic 1 granitic 1 and pegmatitic units. The Tertiary intrusive rooks include a wide variety of rocks ranging in composition from granodiorite porphyry to a.la.skite po'rJ}byry. In general, the 'older Tertiary intrusive rooks occur as small irregular-shaped plutons, whereas the younger rocks form long narrow dikes. Although most of the Precambrian rocks have a low natural radioactivity, a local pha.se of metasedimentary biotite gneiss, two varieties of pegmatite, and biotite.-mua.c:evite granite are abnorma.J.J.y ra.d.ioa.c,tive. Except for one variety of pegmatite tbat contains uraninite, the radioactivity results largely from thorium and its disintegration products in the minerals monazite and xenotime. The Tertiary igneous sequence is one of _the most ra.dioa.cti ve groups of intrusive rocks 1n the world. The radioactivity of quartz bostonite porphyry--the most ra.dioa.cti ve rocks of the sequence--is 15 to 25 times as great as the average granitic rock, and results from both thorium and uranium, which according to George Phair of the Geological SUrvey occur largely in primary zircon.
The structural features of the region a.re of Precambrian and Laramide age. The Precambrian rocks are complexly folded as a result of two periods of Precambrian deformation. The dominant folds trend northeast and were formed during an early period of plastic deformation, which was accompanied by the developmeot of migmatites and the intrusion of granodiorite. Minor folds that trend about N. 55° E. and which a.re particularly abundant in the southeastern pa.rt of the region were formed by a later deformation that occurred a.:f'ter the formation of the youngest Precambrian rocks. The younger deformation was partly plastic and partly ca.ta.clastic. Some faults may have formed late in Precambrian time 1 but this has not been proved. The dominant Laramide structures a.re fa.ults 1 and these can be divided into six sets. Faults that trend northwest and north-northeast and locally contain siliceous breccia. "reefs" and low-angle faults related to the northwest-trending set a.re early Laramide (or older) in age; faults that trend northeast, east-northeast, and east developed after the emplacement of all the intrusive rocks except the youngest, biotite-quartz latite porphyry, and a.re late Laramide in age. With few exceptions the faults have a. dominant horizontal relative motion of one block to the o~her, but
displacements a.re small except along some of the persistent early northwest-trending faults.
The ore deposits in the mining districts of the region are gold-1 silver-, copper-, lead-1 zinc-, and uranium-bearing veins that formed as b:ydrothe:n:niu fissure fillings in the faults. The structure 1 mineralogy 1 and texture of the veins are similar to those described by Lindgren as mesothennal. The veins range from single 1 well-defined filled fissures to complex, branching lodes. 'consisting of subparallel fractures 1 loops 1 and "horsetailing" fractures; in general they, are 1 to 2 feet in width1 the lower figure .being the more common. The principal ore minerals are sulfides and sulfosalts of iron1 copper 1 lead, and zinc; pitchblende is a local constituent. Quartz is the domillant gangue minerali but carbonates of the calcite group, fluorite, and barite are present in some veins. The veins in the region differ in quantitative mineralogy, and they can be classified into two ~in types 1 one characterized by dominant pyrite and the other by dominant galena and sphalerite. The distribution of veins of the two contrasting types indicates a regional concentric zonal arrangement of the ores around a major center of mineralization. A large irregular core area (central zone) of pyrite type veins I, a few of which contain substantial amounts of copper minerals, is surrounded by a broad outer area (pe;ipheral zone) of galena-sphalerite type veins; transitional veins are pyrite type veins that contain galena and sphalerite as well as copper minerals and occur within an intermediate zone.
Uranium occurs loca.J.ly in veins of different mineralogic types throughout muqh of the region, but it is a.bund.ailt in only a. few veins. Of a. total of 141 localities known to cont~in concentrations of ! uranium, 53 mines includinS 16 tba.t have produced uranium ore contain selected material tba.t assays .10 percent uranium or better. The remainder of the localities are not known to contain material of ore grade. Although :uranium-bearing veins are widely scattered, a large proportion of the occurrences and most of the minable deposits are in an area of less than a square mile in the Central City district. ' Pitchblende, a black oxide of uranium, is the primary uranium mineral in the veins. Oxidation of the pitchblende bas yielded a limited variety of hexavalent uranium minerals 1 which are hydrous uranium phosphates, silicates, sulfates, and carbonates. The most abundant secondary minerals are torbernite, autunite, and kasolite. The pitchblende varies from hard black dense material with a pitchy luster to soft porous sooty material with a dull luster. And all gradations between these extreme types exist. The hard pitchblende appears massive 1 but characteristically bas a colloform texture and occurs in spheroidal forms 1 vein forms 1 pelle~s 1 and rarely "cellular" or dendritic forms. The sooty pitchblende is loosely aggregated, and little is known of its texture. The change from a hard dense mineral to a sooty material is the result of oxidation, and is accompanied by a ,. decrease in the unit cell dimension of pitchblende and a loss of definition of x-ray diffraction powder patterns.
The pitchblende contains a distinctive suite of minor elements which can reasonabJ.¥ be inferred to occur in the mineral structure. Spectrographic analyses indicate that pitchblende from the Central City ' district and the Lawson area is relatively rich in zirconium, and probabJ.¥ in moJ.¥bdenum and tungsten; all pitchblende concentrates contain yttrium, and many also contain other rare earth elements. Pitchblende concentrates from Fall River are near:cy- devoid of zirconium but contain substantial ·nickel and some cobalt. The nickel and cobalt occur in inclusions of foreign minerals in the pitchblende. The pitchblende was deposited early in the sequence of vein filling, essentially contemporaneous with quartz and sparse earl.¥ pyrite, but be-fore the deposition of base-metal sulfides and sulfosalts. In the Central City district pitchblende was deposited during the first of''<three distinct stages of mineralization, and preceded a pyrite stage and basemetal stage mineralization; but at Fall River it appears to have been an early local variant of pyrite stage mineralization. The pitchblende occurs local:cy- in all fracture sets as small lenses, pods, or stringers, or rarely as larger ore shoots. The margins of the ore bodies are sharp, and the intervening vein material commonJ.¥ is -near:cy- devoid of uranium. Most ore bodies contain only a few tens to a few·hundred pounds of ore, but some of the larger shoots contain as much as 50 tons of ore. The bodies commonly are high in grade but much narrower than mining widths, and accordingly the grade of the shipping ore depends upon the care taken in mining and sorting.
As the pitchblende was deposited a.s fillings of open spaces, it was localized largely by structural features that produced openings along the fissures; but other factors of importance were proximity of the openings to the source of the ore-forming fluids and, perhaps loca.lly', the presence of mafic wall rocks along the vein-fissures. In contrast to the gold- and silver-bearing sulfide ores, the pitchblende deposits do not appear to extend over long vertical distances, and they rarely can be mined profi ta.bly over a. vertical range of more than 500 feet. At Central City all the lmown va.lua.b'le deposits are w1 thin 500 feet of the surface. The upper parts of pitchblende-bearing veins have been altered by surficial waters and atmospheric gases. Uranium was leached from pyrite type veins; but an assemblage of hexavalent secondary uranium minerals was fomed by the oxidation and solution of uranium in the primary pitchblende from ga.lena.-spha.lerite type veins. The secoo.da.ry uranium 1 minerals occur in the original vein with limonite and a.s disseminations and fracture coatings in wall rock adjacent to the originating vein. Sooty pitchblende occurs a.t the interface between the unoxidized and the oxidized parts of veins. At most localities secondary uranium minerals do not extend below a. depth of 150 feet.
Because of the close spa.tia.l a.nd temporal association of the ore deposits a.nd Tertiary igenous rocks and substantiating mineralogical and chemical da.ta.1 we infer that the uranium a.s well a.s the associated sulfide ore deposits were derived from the magmas that yielded the Tertiary igneous rocks. Uranium genera.J.l¥ was a. sparse constituent in the ore-forming fluids 1 but in certain environments, as at Central City1 substantial quantities probably were derived from local1 sha.llow --crystallizing quartz bostonite porpeyry dikes. These fluids mingled With the ore-forming fluids from the main deep-seated source to yield deposits of economic value. These deposits generally were formed near the separate dike sources a.nd many are Within 500 feet of a. presumed source. The uranium-bearing ore bodies tpat have been found in the region are small but generally high in sraae. There is little reason to expect the discovery of bodies that are larger than those previously mined, and accordingly 1 future production ot uranium from this region can be expected to come from scattered small pods and ore shoots distributed through the uranium-bearing veins. These bodies can be recovered most economically during the mining of precious- a.nd base-metal ores.