MOLYBDENUM 



427 



undertaken at the Urad-Henderson porphyry mo- 

 lybdenite deposits in Colorado. Production at the 

 Urad mine began in 1967 at an annual rate of about 

 3 million pounds of molybdenum. In 1966, American 

 Metal Climax, Inc., announced reserves of more than 

 300 million tons of ore in its Henderson deposit, 

 some 2,000 feet below the Urad ore body, at a grade 

 of 0.49 percent molybdenite. The company expects 

 to put this mine into production by the late 1970's 



Only at Climax, Colo., has the recovery of associ- 

 ated minerals in a porphyry-type molybdenum de- 

 posit so far been successful; tungsten (huebnerite), 

 pyrite, topaz, uranium (brannerite), monazite, tin 

 (cassiterite), and molybdenum oxide have been re- 

 covered as byproducts. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY '■ 



Molybdenum is a metallic element of Group VIb 

 of the periodic table and has atomic number 42, 

 atomic weight 95.94, and occurs as six isotopes. In 

 nature it has valences of +4 and +6 in molyhy- 

 drates and possibly rarely has valences in the +3 

 and +5 states. Molybdenum is a rare element, its 

 abundance in continental crustal rocks being gen- 

 erally given as 1 to 1.5 ppm. It is rather uniformly 

 distributed among the igneous rocks but averages 

 slightly higher in basaltic rocks than in granitic 

 rocks. 



Chemically, molybdenum can function both as a 

 metal and as a nonmetal. In the geochemical cycle 

 it becomes concentrated in shales, clays, and phos- 

 phorites; in these rocks, there is a positive correla- 

 tion between the content of molybdenum and the 

 content of organic carbon. Molybdenum is also con- 

 centrated in coal and petroleum. Of the molybdenum 

 that is supplied to ocean waters, virtually all is 

 precipitated either mainly with the clay minerals 

 or partly in hydrous manganese oxides. 



Man's activities contribute molybdenum to the 

 environment in mining, in metallurgical processing, 

 in the addition of phosphate fertilizers to the soil, 

 and in the conversion of coal and oil to provide for 

 his various energy needs. 



MINERALOGY 



Molybdenum does not occur in nature in its free 

 or native state but is found only in combination 

 with other elements, such as sulfur, oxygen, tung- 

 sten, lead, uranium, iron, magnesium, cobalt, vana- 

 dium, bismuth, and calcium. The most common 

 molybdenum minerals are molybdenite (molybdenum 



' From material provided by Michael Fleischer, written commun., 1972. 



disulfide, M0S2), powellite (calcium molybdate, 

 CaMoOi), wulfenite (lead molybdate, PbMoOi), 

 ferrimolybdite (FeMoOsHjO), ilsemannite (mo- 

 lybdenum oxysulfate), and jordisite (amorphous 

 molybdenum disulfide). Of these, only molybdenite, 

 ferrimolybdite, and jordisite are of current eco- 

 nomic importance. Some molybdenum has been re- 

 covered in the past, however, from wulfenite- and 

 powellite-bearing ores. As the demand for molyb- 

 denum continues to rise, powellite, ferrimolybdite, 

 and jordisite may be looked upon as possible source 

 materials. 



TYPES OF DEPOSITS 



Molybdenum deposits are of five genetic types: 

 (1) porphyry or disseminated deposits including 

 stockworks and breccia pipes in which metallic sul- 

 fides are dispersed through relatively large volumes 

 of altered and fractured rock, (2) contact-meta- 

 morphic zones and tactite bodies of silicated lime- 

 stone adjacent to intrusive granitic rocks, (3) 

 quartz veins, (4) pegmatites and aplite dikes, and 

 (5) bedded deposits in sedimentary rocks. 



PORPHYRY DEPOSITS 



Probably more than 95 percent of the world's 

 supply of molybdenum has been obtained from por- 

 phyry molybdenum or porphyry copper-molybdenum 

 deposits. In these deposits primary copper sulfides 

 and (or) molybdenite occur as disseminated grains 

 and in stockworks of quartz veins and veinlets in 

 fractured or brecciated, hydrothermally altered 

 granitic intrusive rocks and in the intruded igneous 

 or sedimentary country rocks. 



Favorable host intrusive rocks with which the 

 ore minerals are generally assumed to be genetically 

 related range from intermediate to acidic and in- 

 clude diorite, quartz monzonite, and granite, and 

 their porphyritic equivalents. Metalization com- 

 monly took place both in the host intrusive and in 

 the surrounding or overlying country rock. 



In the porphyry molybdenum deposits, molybde- 

 nite is characteristically the sole ore mineral. Mo- 

 lybdenite commonly is accompanied by pyrite, 

 fluorite, and small amounts of tungsten, tin, lead, 

 and zinc minerals. In contrast, the porphyry-copper 

 or copper-molybdenum deposits contain only small 

 amounts of molybdenum, and in these, the molyb- 

 denum is of economic significance only because of 

 the recoverable copper content. The primary copper 

 mineral is chalcopyrite, which is intimately associ- 

 ated with pyrite. 



The porphyry or disseminated deposits are com- 

 monly considered to be pipe-shaped bodies, circular 



