THE URANIUM ORE MINERALS
From "Minerals For Atomic Energy"
By Robert D. Nininger
Lindgren defines an ore mineral as "a mineral which may be used for the extraction of one or more metals." A uranium ore mineral is therefore a mineral possessing such physical and chemical properties and occurring in a deposit in such concentrations that it may be used for the profitable extraction of uranium, either alone or together with one or more other metals. There are only a few of the many uranium minerals that meet these qualifications and still fewer in which uranium is the major constituent. Pitchblende and uraninite contain theoretically up to 85 per cent uranium but actually between 50 and 80 per cent; carnotite, torbernite, tyuyamunite, autunite, uranophane, and brannerite, 45 to 60 per cent. In other minerals, uranium is an important but relatively minor constituent the minerals, davidite, samarskite, and euxenite, for example, contain only 1 to 18 per cent. The majority of uranium-bearing minerals, however, contain uranium in small or trace amounts as an accessory to other major constituents.
The uranium content of a mineral does not of itself, however, determine whether it is a uranium ore mineral. If the uranium is present in a mineral in such complex combinations with other elements that it is too costly to extract, or if the mineral does not occur in sufficient quantities to make extraction worthwhile, that mineral is not a uranium ore mineral. Thus, the definition for an ore mineral, like that for an ore deposit, is dependent upon economics and time upon the value of uranium and the results of future exploration and metallurgical progress. A uranium mineral that is not an ore mineral today may be one tomorrow.
Most of the uranium minerals in pegmatites and placers are refractory; that is, the uranium is present in combinations which are extremely difficult to break down chemically in order to recover the uranium. These minerals also usually occur scattered sparsely throughout the deposit so that recovery difficult and expensive. Therefore, even though some of the individual minerals may contain up to 50 per cent uranium, they are not ore minerals.
The fact that only a few of the numerous uranium minerals qualify as uranium ore minerals and form uranium ore deposits, whereas uranium in small amounts is widely spread throughout the rocks of the earth's crust, adds greatly to the problem of uranium exploration. The uranium prospector gets many "nibbles" but few "bites," and to avoid disillusionment and frustration, as well as waste of time, effort, and money, he must know his business well. This is one of the most important factors in searching for uranium, as it is for other metals-the ability to judge the importance of what is found and whether to discard it or follow it up. In this respect, it is of first importance to become familiar with the uranium ore minerals.
PRIMARY URANIUM ORE MINERALS
Primary uranium minerals have been found most commonly in veins or pegmatites, although in recent years extensive, flat-lying deposits of pitchblende in sedimentary rocks have also been discovered. The refractory primary uranium minerals are also found in placers.
The primary uranium minerals are generally black or dark brown, noticeably heavy, and often have a shiny or pitch-like luster. When they are exposed to weathering at or near the surface, they are sometimes altered to form the bright-colored secondary uranium minerals. At the present time, there are only three known primary uranium ore minerals, and the most important of these, uraninite and pitchblende, are really varieties of the same mineral.
Uraninite (combined UO2and UO3; 50-85 percent U308)1. Uraninite is a naturally occurring uranium oxide with cubic or octahedral crystal form. It has a specific gravity of 8-10.5 (iron = 7.85), a grayish-black color sometimes with a greenish cast and a hardness2 of 5-6, about the same as steel. Its streak3 is black. Its most widespread occurrence is in pegmatites 4, in which it is found in small amounts, throughout the world. However, it is also an important constituent of nearly all important primary deposits, occurring closely associated with its massive variety, pitchblende.
Uraninite is the principal uranium-bearing mineral in two newly developed types of deposits that produced for the first time in 1952: the very low-grade (in uranium) Witwatersrand and Orange Free State gold-bearing conglomerates of the Union of South Africa, and the medium-grade uranium and copper-bearing carbonaceous slates at Rum Jungle, Northern Territory, Australia. In both of these deposits uraninite occurs as finely disseminated crystals, usually invisible to the naked eye. Pitchblende (combined UO2 and U03; 50-80 percent U308) Pitchblende is the massive variety of uraninite, without apparent crystal form, that occurs most abundantly in the rich primary vein deposits of uranium. It is the chief constituent of nearly all high-grade uranium ores and has provided the largest part of all uranium produced throughout the world, forming the principal product of the Shinkolobwe mine, Belgian Congo; the Eldorado mine, Great Bear Lake, Northwest Territories, Canada; and the mines at Joachimsthal, Czechoslovakia.
Pitchblende is somewhat lighter than uraninite, having a specific gravity of between 6 and 9, but its other properties, with the exception of crystal form, are the same. It occurs as irregular masses often with a rounded, layered, botryoidal structure.
The principal occurrences of pitchblende are in primary (hydrothermal) vein deposits, usually of the mesothermal (medium temperature and pressure) type, in igneous and metamorphic rocks and in flat-lying bedded deposits in sedimentary rocks. Pitchblende is commonly associated with one or more of the primary ore minerals of iron, copper, cobalt, lead, silver, and bismuth; and the presence of these minerals in a mineral deposit is one indication of favorable conditions for pitchblende. It is usually accompanied also by bright colored secondary uranium minerals where subjected to weathering or other alteration. The commonly associated gangue1 minerals are quartz and other silica minerals, carbonates, fluorite, barite, and hydrocarbons. Quartz, calcite, and dolomite are usually the most abundant. Pitchblende, in vein deposits, is most likely to be deposited in existing open spaces in rock formations, rather than by replacement of the rock itself, and the richest deposits occur where large open fractures were available for filling by the mineralizing solutions. There are no important pitchblende replacement deposits like those of copper, lead, zinc, and silver, where rock formations have been substantially replaced by ore through solution of the original constituents and deposition of the ore minerals.
Deposition of pitchblende is usually accompanied by strong alteration of the wall rock along the veins. The presence of hematite (a red iron oxide mineral) extending from the pitchblende a few inches to a few feet into the wall rock is the most characteristic feature. The formation of hematite has occurred in all of the major pitchblende vein deposits and in many of the deposits of minor importance. Other alteration features often associated with pitchblende deposition are the formation of kaolin, chlorite, sericite, and silica minerals in the wall rock.1
In the recently discovered flat-lying deposits of pitchblende in sedimentary rocks, such as sandstones and conglomerates, the pitchblende is deposited between and around the grains of the rock and in available rock openings. The exact mechanics and chemistry of deposition, however, are not as well understood as they are in the case of the vein deposits. The two most important examples are the "copper-uranium" deposits in southern Utah and northern Arizona, in which pitchblende occurs with a variety of secondary uranium and copper minerals and copper and lead sulfides, and the deposits in Big Indian Wash near La Sal, Utah, in the central Colorado Plateau, where the pitchblende is associated with the vanadium mineral, vanoxite, and some secondary minerals, principally carnotite, tyuyamunite, and becquerelite.2
Pitchblende has also been found in smaller amounts disseminated in volcanic rocks in the southwestern United States, in some of the carnotite deposits of the Colorado Plateau, and in the deposits in limestone in the Grants district, New Mexico.
Davidite (rare earth-iron-titanium oxide; 7-10 percent U3O8). Davidite was not considered a significant uranium ore mineral until 1951, when additional exploration at the old Radium Hill mine near Olary, South Australia, an early producer of small quantities of radium, indicated a substantial uranium deposit. After World War II a few tons of davidite were produced from less important deposits near Tete in Mozambique (Portuguese East Africa). Davidite is a dark brown to black mineral with a glassy to submetallic luster. It has about the same hardness as pitchblende (5-6) and is somewhat lighter in weight (specific gravity, 4.5). It occurs most commonly in angular, irregular masses, sometimes with crystal outlines, but never in round, botryoidal shapes like pitchblende. When it is exposed to weathering, a thin yellow-green coating of carnotite or tyuyamunite may form on its surface. This is particularly true at Radium Hill, Australia, and it provides an easy means of tentative identification in the field.
Davidite is deposited in hydrothermal veins, presumably at a higher temperature and pressure than pitchblende. The veins have many of the characteristics of pegmatites. The associated vein minerals are ilmenite, hematite, biotite, mica, quartz, calcite, and pink feldspar. The rocks enclosing the veins at Radium Hill are largely gneisses or schists, with chloritic and sericitic alteration near the veins. At Tete, davidite veins are found in more basic1 rocks like gabbro and anorthosite. Davidite is almost never found as the "pure" mineral, but rather in complex inter-growths with ilmenite which has very similar physical properties and chemical composition. SECONDARY URANIUM ORE MINERALS
The secondary uranium minerals are by far the most spectacular in appearance of the uranium minerals. Instead of the dull black, gray, and brown colors of the primary minerals, they present an array of bright yellow, orange, green, and all of the combinations and in-between shades of those colors. Some of them also have the property of fluorescence under ultraviolet light, resulting in even more brilliant coloration. Rather than being heavy and massive, they occur as earthy or powdery materials or as fine, delicate, needle-like or platy, flake-like crystals. As a group, they are probably more beautiful than the minerals of any other element. This, of course, is an important factor in their recognition in the field, although the inexperienced prospector may often confuse them with other colorful minerals, such as malachite (copper carbonate), limonite (iron hydroxide), and sulfur, to name a few.
The secondary uranium ore minerals have represented only a small proportion of the total world uranium production to date. However, their deposits are more numerous and widespread than those of the primary ore minerals and, as a result of intensive prospecting activity, their importance is steadily increasing. The secondary minerals have two major modes of occurrence:
1. In the weathered or oxidized zones of primary deposits, where they are formed by decomposition of the primary minerals in place.
2. As irregular, flat-lying deposits in sedimentary rocks, primarily sandstones, but also conglomerates, shales, and limestones, formed by precipitation from solutions that may have carried the uranium some distance away from the original source.
The secondary uranium ore minerals also occur frequently along with a large variety of other secondary uranium minerals, mainly the uranium phosphates, carbonates, sulfates, hydrous-oxides and silicates, in what may be considered a third type of secondary mineral deposit. These have been referred to as oxidized secondary deposits or simply as oxidized deposits. Most of these deposits are probably oxidized vein deposits, the complete oxidation of the primary minerals in place making it difficult to prove the original primary character. On the other hand, they may be formed by ground-water solutions that have dissolved uranium from a broad area of slightly mineralized rocks and concentrated it by precipitation in veins and fracture zones. These deposits are numerous throughout arid and semi-arid regions, such as the western and southwestern United States, the west coast of South America, the Mediterranean area, and southern Russia, and, although a few of them have produced ore, they provide most of the troublesome traces or nibbles that often confound uranium prospectors. In some cases they have proved to be the oxidized upper portions of primary deposits from which primary ore has eventually been mined at depth.
The secondary minerals in the weathered zones of primary deposits have at some places contributed significant uranium production, particularly where weathering has been deep, as at Shinkolobwe in the Belgian Congo; at Urgeirica, Portugal; at Marysvale, Utah; and in some of the copper-uranium deposits of the southwestern United States. However, the major significance of such occurrences to the prospector is the indication of the presence of primary mineralization which, at important deposits, produces in the end the preponderance of the uranium. The flat-lying deposits in sedimentary rocks represent the most important occurrence of the secondary minerals, and the most important deposits of this type are the carnotite deposits of the Colorado Plateau area of Colorado, Utah, Arizona, and New Mexico, which have been radium, vanadium, and uranium producers since 1898.
Three-quarters of the more than one hundred uranium minerals are secondary minerals, but of these only six may logically be considered ore minerals. Most of the others, many of them extremely rare, occur primarily as the weathering products in the oxidized zones of primary deposits, but some are found associated with the secondary ore minerals in deposits in sedimentary rocks. Unlike the primary uranium ore minerals, the secondary ore minerals seldom occur singly or only two to a deposit. They usually occur together in groups of several of both the ore and non-ore minerals, although, as in the case of the carnotite deposits, one mineral may be predominant. The dominant colors of the secondary uranium ore minerals are yellow and green, orange being confined primarily to the non-ore minerals.
Carnotite (K20*2UO3*V2O5*nH20; 50-55 percent U3O8). Carnotite, a potassium uranium vanadate, is the most important of the secondary uranium ore minerals, having provided possibly 90 percent of the uranium production from secondary deposits. It is a lemon-yellow mineral with an earthy luster, a yellow streak, and a specific gravity of about 4. It occurs most commonly in soft; powdery aggregates of finely crystalline material or in thin films or stains on rocks or other minerals. Its powdery nature gives the impression of even greater softness than its hardness scale rating of 2-3 would indicate. It can be easily scratched with the fingernail. Carnotite is not fluorescent.1
The most noted occurrences of carnotite are in the Colorado Plateau area of the United States, where it was first identified in 1898 and has since provided the major domestic uranium production, on the western edge of the Black Hills, South Dakota, and in the Ferghana basin, U.S.S.R. It occurs in sandstones in flat-lying, irregular, partially bedded ore bodies of from a few tons to a few hundred thousands of tons in size. In the higher-grade deposits (more than one-third of 1 per cent U3O8), the carnotite is present in sufficient quantity to color the rock a bright yellow; but in poorer deposits, particularly below 0.20 per cent U3O8, it is often difficult to distinguish it from the sandstone itself. Its color is also often masked by iron staining or by the dark-colored vanadium minerals usually associated with it. Most carnotite deposits range in grade from 0.10 per cent to 0.50 per cent U3O8.
Although carnotite is the principal mineral in the carnotite deposits, nearly twenty other secondary uranium minerals are found associated with it. The most common of these is the secondary ore mineral, tyuyamunite, described below. All of the other secondary ore minerals, torbernite, autunite, schroeckingerite, and uranophane, have also been found in carnotite deposits. The other associated secondary minerals are the rare oxides, carbonates, arsenates, vanadates, phosphates and silicates. The most common non-uranium minerals found associated with carnotite are the vanadium minerals, corvusite (hydrous-vanadium oxide), hewettite (calcium vanadium oxide), and roscoelite (vanadium mica-silicate). Minerals of the common metals, such as copper, lead, zinc, and manganese, have also been identified in carnotite deposits, as well as pitchblende and uraninite, but their occurrence in most cases is only of academic interest.
One other important association of carnotite should be mentioned, for it has an important bearing on prospecting for these deposits. An evident general affinity of uranium for certain organic materials, which has had some effect on its deposition in almost all types of deposits, is perhaps most clearly displayed in the carnotite deposits of the Colorado Plateau area. In a large number of these deposits, the carnotite is intimately associated with silicified or carbonized wood fossil wood ), and a variety of coal-like and asphaltic materials, all of which are good indicator substances for carnotite. In the Temple Mountain district, Utah, carnotite occurs in sandstones so impregnated with asphaltic material that the deposits are considered a special type and are called uraniferous asphaltite deposits. Elsewhere, fossil wood in the form of logs or accumulations of branches and twigs, locally called trash pockets, is the most common type of associated organic material.
Although occurrences of the type described represent the only ore deposits of carnotite, this mineral is one of the most widespread of the uranium minerals. It is present in varying amounts in nearly all of the other secondary uranium deposits and is the principal mineral in some of the noncommercial oxidized deposits, like those at Jean and Erie near Las Vegas, Nevada, and near San Carlos, Chihuahua, Mexico. Carnotite is found also in small amounts in the oxidized zone of any primary uranium deposit containing even trace amounts of vanadium, for example, the davidite deposits at Tete, Mozambique, and at Radium Hill, South Australia, in the fluorite deposits of the Thomas Range, Utah, and other parts of the southwestern United States, and in places as thin stains and coatings at the outcroppings of the very low-grade, uranium-bearing shale, phosphate, and lignite deposits.
Tyuyamunite (CaO*2UO3 *V2O5*nH20; 48-55 percent U3O8).Tyuyamunite is closely related to carnotite as indicated by the chemical formula, which is the same except that calcium substitutes for the potassium of carnotite. The physical properties of tyuyamunite are the same except for a slightly more greenish color than carnotite and, in some cases, a very weak yellow-green fluorescence not found in carnotite.
Tyuyamunite is found in small amounts in almost any deposit or with any occurrence of carnotite. It is, as one would suspect, more abundant where there is an appreciable amount of calcium, usually in the form of calcite or limestone. Tyuyamunite first obtained importance as an ore mineral because of its occurrence in a deposit in southeastern Turkistan, U.S.S.R., near the town of Tyuya Muyun, for which it was named. It occurs there, and at other localities in the region, associated with other secondary uranium minerals, particularly carnotite and torbernite, in fractures in limestones, dolomites, and shales. It is also an important constituent of the deposits in limestone at Grants, New Mexico, and has been identified in the deposits at Big Indian Wash, Utah.
Torbernite and Meta-torbernite (CuO*2UO3 *P2O5* nH20; 60 percent U3O8) . Torbernite and meta-torbernite are hydrous copper uranium phosphates, the only difference between the two being the number of water molecules present; their physical properties are identical. They have a bright emerald color, a pearly luster, hardness of 2-2 1/2 (about the same as the fingernail), and specific gravity of about 3.5 (a little heavier than quartz). They occur in flat, square, translucent crystals which usually fluoresce with a faint green color.
Torbernite and meta-torbernite are the most common of the secondary uranium minerals that are found associated with primary deposits where oxidation has occurred. They are common in nearly all such deposits except pegmatites, which usually do not contain the necessary copper to form them. They are most noted for their abundance in the oxidized zones at Shinkolobwe, Joachimsthal, and in the copper-uranium deposits of Utah and Arizona. They have provided a substantial uranium production from the Urgeirica mine and nearby deposits in Portugal and from Marysvale, Utah, and they occur in the oxidized zone at Rum Jungle, Northern Territory, Australia. In addition, they occur with the other secondary uranium minerals in the oxidized secondary deposits whenever copper has been present in the depositing solutions or surrounding rocks. They are associated with tyuyamunite in Turkistan and with autunite at Bukhova, Bulgaria, and at Mt. Painter, South Australia. The principal non uranium minerals associated with torbernite are the clay minerals, limonite, quartz, pyrite, and the copper sulfides and carbonates.
Elsewhere in this book these two minerals will be referred to simply as torbernite, although actually the most common of the two is probably Meta-torbernite.
Autunite and Meta-autunite (CaO*2UO3* P2O5* nH2O; 60 percent U308. Reference to the chemical formula will show that these two minerals have the same composition as torbernite, with calcium substituting for copper. Because of this similarity, they are commonly found together, the proportion of torbernite being dependent upon the amount of copper available to the uranium-bearing solutions. In some instances, where copper is completely lacking, only autunite or meta-autunite is formed. Like torbernite and meta-torbernite, autunite and meta-autunite are identical in their physical properties, the distinction being made on the basis of the number of water molecules present. Also, as in the case of torbernite, meta-autunite is probably the most common. For simplification, however, they will be referred to as autunite.
The physical properties of autunite are similar to those of torbernite, except for its color, which is predominantly lemon or sulfur-yellow, although occasionally apple-green, and its brilliant yellow to greenish-yellow fluorescence in ultraviolet light. Autunite has a hardness of 2-2 1/2, is slightly heavier than quartz (specific gravity, 3.1), has a colorless to pale yellow or green streak, and occurs in small square, rectangular, or octagonal flat, translucent crystals or as thin coatings or stains on rock or other mineral surfaces. It is seldom found in large masses but rather as small spots scattered throughout the enclosing rocks. A good autunite exposure is a brilliant sight at night under ultraviolet light, and the inexperienced prospector is apt to overestimate the grade of a deposit seen under those conditions.
Autunite is found in varying amounts in almost all deposits of the other secondary uranium minerals. It is an oxidation product of pitchblende and uraninite and most of the other primary minerals, and may also be derived from some other secondary minerals, like gummite and uranophane. As such it is an important constituent of the oxidized zones at Shinkolobwe and other important primary ore deposits and is a common secondary uranium mineral in most pegmatites. It is present in small amounts in many of the carnotite deposits of the Colorado Plateau area and in larger amounts in the tyuyaunite deposits of Turkistan.
The greatest significance of autunite to the prospector lies in the fact that it is the most common uranium mineral in the oxidized secondary deposits in igneous rocks of arid regions, both those related to primary mineralization and those of unknown origin. It is an important constituent of the oxidized ores at Urgeirica, Portugal, and at Marysvale, Utah, and the most prominent mineral in the White Signal, New Mexico, district, at Mt. Painter, South Australia, and in the numerous low-grade secondary occurrences in the Mojave Desert and at other localities in southern California and Nevada. In addition, it frequently occurs as thin stains on fracture surfaces in granite and pegmatites in the Appalachian region of the eastern United States from Stone Mountain in Georgia to New England. The associated non-uranium minerals are the same as for torbernite, except that the copper minerals may be absent.
Uranophane (CaO*2UO3*2SiO2*6H2O; 65 percent U308 Uranophane is a hydrated calcium uranium silicate containing silica in place of the phosphate of autunite. It is slightly lighter in color and somewhat heavier than autunite (specific gravity 3.85) and has a different crystalline form; it may occur as stains or coatings without apparent crystal form or as finely flbrous or radiating crystal aggregates.
The origin and occurrence of uranophane are very similar to autunite and torbernite. At least two of these three minerals are almost always found together, in proportions varying with availability of copper and phosphorus, uranophane becoming predominant where these two elements are scarce or absent. Although it has as broad a geographic occurrence as the other two, uranophane, with a few exceptions, is usually present in smaller quantities. It is an important constituent of the secondary deposits in limestone near Grants, New Mexico, where it earned its reputation as an ore mineral, and in recently discovered deposits in sandstone in southern Carbon County, Wyoming. It is also the most common secondary uranium mineral found in the noncommercial deposits in granite and pegmatites in the eastern United States. Its most noted occurrences of this type are at Stone Mountain, Georgia (granite), and at the Ruggles mine at Grafton, New Hampshire (pegmatite).
Schroeckingerite [NaCa3 (UO2) (CO3)3(SO4)F*1OH20; 30 percent U308]. Schroeckingerite is a complex hydrated sulfate, carbonate, and fluoride of calcium, sodium, and uranium. It has a yellow to greenish-yellow color with a pearly luster, a bright yellow-green fluorescence, and a paler yellow or greenish yellow streak. It is very soft (less than 1 on the hardness scale)1, easily water soluble, and is the lightest of the uranium minerals (specific gravity, 2.5). It occurs as globular coatings on rock fracture surfaces or as small rounded masses composed of aggregates of flaky crystals distributed through soft rocks or soil.
Schroeckingerite is the least important of the uranium ore minerals and barely qualifies as such. It is a significant constituent of the secondary ores at Marysvale, Utah, and probably occurs in small amounts in the oxidized zones of most of the important primary deposits. The only known occurrence in which schroeckingerite is the principal mineral is at Lost Creek near Wamsutter, Wyoming. It occurs there as small pellets distributed through clay beds at or near the ground surface over a considerable area to form a low grade uranium deposit that is presently submarginal. In this type of deposit there are no significant associated minerals.