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UNITED STATES MINERAL RESOURCES 



or oval in plan and with steeply dipping sides. In 

 nature, however, the deposits are more likely to be 

 in complex forms, such as domal, funnel-shaped, 

 arcuate, and annular bodies (Brown, 1969; James, 

 1971). 



Almost invariably one or more major faults pass 

 through or close to the ore bodies. Evidence of both 

 preore and postore movement is abundant in these 

 faults, and localization of the ore bodies along major 

 faults is an accepted hypothesis. Intensive, wide- 

 spread and continued fracturing of host intrusives 

 and of enclosing country rocks is characteristic. 



The average mineral content of porphyry-type 

 deposits ranges from 0.1 to 0.5 percent molybdenite 

 in molybdenum porphyries, and from about 0.015 

 to 0.1 percent molybdenite in copper-molybdenum 

 porphyries. Currently minable porphyry deposits 

 range in size from a few hundred feet to several 

 thousant feet in horizontal dimension and may 

 extend to depths of several thousand feet. Fractures 

 and hydrothermally altered rocks extend many hun- 

 dreds of feet outward beyond the limits of minable 

 ore, decreasing in intensity outward from the cen- 

 tral zone. 



Three-fourths of the world's reserves of molyb- 

 denum are in the Western Cordillera of North and 

 South America, the mountainous region along the 

 western part of these continents. These reserves 

 consist of large low-grade disseminated deposits of 

 hydrothermal origin that occur in clusters through- 

 out the length of the Cordillera. They are typical 

 porphyry deposits, genetically associated with in- 

 trusive rocks of acidic to intermediate composition 

 in areas of intense volcanism and complex tectonic 

 history. The period of ore deposition extended from 

 Late Triassic to middle Tertiary. 



The remaining one-fourth of the world's reserves 

 of molybdenum are distributed among the U.S.S.R., 

 parts of Europe, southwestern Asia, and southeast- 

 ern Asia including China, the islands of the South 

 Pacific, and Australia; the U.S.S.R. reserves ac- 

 count for at least half of this fraction. 



Porphyry or stockwork deposits that yield molyb- 

 denum alone or as the chief metal are known to 

 occur principally in Alaska, British Columbia, and 

 in the Rocky Mountain region of the United States 

 (Clark, 1970) ; porphyry copper-molybdenum de- 

 posits from which molybdenum is obtained as a 

 byproduct occur over the entire length of the West- 

 ern Cordillera from Alaska to Argentina, and in 

 other parts of the world (Eimon, 1970). 



The Climax and Urad-Henderson deposits in 

 Colorado and the Questa deposit in New Mexico are 

 described here as three examples that illustrate the 



typical porphyry molybdenum ore deposits, their 

 geological relationships, and the resources connected 

 with this type of deposit. Comprehensive descrip- 

 tions of the porphyry copper-molybdenum deposits 

 in southwestern North America are given by Titley 

 and Hicks (1966), and of those in the Western 

 Hemisphere by Ridge (1972). 



CLIMAX, COLORADO 



For many years (1930-55), the Climax molyb- 

 denum deposit at Fremont Pass, about 100 miles 

 west of Denver, Colo., was unique in that it was 

 the only known porphyry molybdenum deposit in 

 the world. Large-scale bulk mining of the dissemi- 

 nated ore body began in the mid-1930's, about 20 

 years after its discovery and after the mining from 

 near the top of the deposit of small tonnages of 

 high-grade zones that contained from % to 1 per- 

 cent molybdenite (Butler and Vanderwilt, 1933). 

 The Climax mine, operated by the Climax Molyb- 

 denum Co. division of American Metal Climax, Inc., 

 exploits one of the three largest known molybdenum 

 stockwork deposits in the United States if not in 

 the world. The two other deposits of equivalent size 

 and character are the Questa molybdenum deposit 

 in northeastern New Mexico (Molybdenum Corp. of 

 America), and the Urad-Henderson molybdenum 

 deposit at Empire, Colo. (Climax Molybdenum Co.). 

 The Climax deposit has, according to the company's 

 annual report, reserves amounting to about 500 

 million tons of molybdenum ore which contain over 

 2 billion pounds of molybdenum, sufficient to sustain 

 the present rate of production through the remain- 

 der of this century. 



The deposit consists of a complex dome-shaped 

 mass of fractured, silicified, and mineralized Pre- 

 cambrian granite, gneiss, and schist that generally 

 overlies a composite porphyry stock of quartz mon- 

 zonite to granite composition, middle Tertiary in 

 age. Molybdenite associated with quartz, pyrite, 

 fluorite, topaz and lesser amounts of tungsten, scan- 

 dium, titanium, and tin minerals is dispersed in 

 fracture fillings, in veinlets, and as minute flakes 

 throughout the fractured host rocks forming a low- 

 grade (0.05 to 0.5 percent Mo) ore body of several 

 thousand feet in smallest dimension. Several com- 

 modities, such as tungsten, pyrite, and tin, are re- 

 covered as byproducts. Climax has become a lead- 

 ing United States producer of tungsten in recent 

 years. A small tonnage of monazite has been recov- 

 ered in the past, as has a small amount of topaz. 

 According to company reports, more than 150 mil- 

 lion tons of ore had been mined from the Climax 

 deposit by 1972. The ore deposit has not yet been 



