MOLYBDENUM 



433 



also contain major molybdenum deposits, though 

 perhaps not all of minable grade. For example, the 

 cluster of occurrences next southwest of the Climax 

 cluster has been drilled by exploration companies 

 for many years, and although no minable deposit 

 has been discovered, a phenomenal amount of widely 

 dispersed molybdenum has been revealed, probably 

 greater in amount than is contained in the Climax 

 deposit itself (Ogden Tweto, oral commun., 1972). 

 From a geological standpoint, but not an economic 

 standpoint at this time, this cluster contains a major 

 molybdenum deposit. 



Molybdenum deposits known in the San Juan 

 Mountains in southwestern Colorado occur largely 

 in the volcanic pile overlying the intrusive bodies 

 that form the roots of the present San Juan vol- 

 canic field. Deposits in central Colorado in the main 

 segment of the Colorado Mineral Belt are associ- 

 ated with intrusive bodies that represent the root- 

 zone of a volcanic field now largely stripped away 

 by erosion. If major molybdenum deposits occur in 

 the San Juan field, presumably they lie at depth in 

 the root-zone of the present volcanic pile, analogous 

 to the central Colorado deposits. Occurrences known 

 at the surface in the San Juan field are likely "leak- 

 ages" from deeper seated deposits. The geological 

 probability of major molybdenum deposits at depth 

 in the San Juan Mountains seems good, although 

 the economics of mining them, even if they could 

 be found now, are at present unfavorable. 



The three known major molybdenum deposits in 

 and near Colorado have produced or contain un- 

 mined resources estimated at 5x10^ lbs molybdenum 

 each. Returning to the earlier conclusion that each 

 of the 10 clusters of molybdenum occurrences shown 

 on the map likely contains a major molybdenum 

 deposit, we can estimate a total resource for the map 

 area of roughly 5x10" lb molybdenum, occurring 

 in the vicinity of surface occurrences of molyb- 

 denum. 



By making further assumptions that volcanic and 

 other young rocks in the San Juan Mountains and 

 in the vicinity of Questa may cover unknown clus- 

 ters of molybdenum occurrences and that the spac- 

 ing of clusters of molybdenum occurrences in the 

 root-zone of a middle Tertiary igneous province is 

 similar to that shown in the central Colorado part 

 of the Colorado Mineral Belt where a root-zone is 

 exposed, it is logical to infer the presence of addi- 

 tional unknown clusters in the San Juan Mountains 

 and in the vicinity of Questa. Such unknown clusters 

 may nearly equal in number those already known 

 at the surface, judged from the areas of possible 

 molybdenum occurrence implied by the outlines of 



gravity lows and of middle Tertiary intrusive ac- 

 tivity shown on the map. If this assumption is cor- 

 rect, perhaps about 20 major molybdenum deposits 

 exist in and near Colorado, representing a total 

 resource of roughly 10" lb molybdenum. 



PROSPECTING 



The search for molybdenum is facilitated by the 

 physical properties of molybdenite as well as by the 

 geochemical characteristics of the element. Molyb- 

 denite does not oxidize readily, and because of its 

 bright silvery gray color and a strong platy crystal 

 habit, it can be seen by eye in concentrations 

 amounting to less than 100 ppm in light-colored 

 rocks. Molybdenum is a mobile element and is both 

 sulfophile and oxyphile. These characteristics work 

 both for and against the prospector. In chemically 

 acid environments, as in the presence of pyrite, 

 molybdenite commonly is fixed in place, chiefly as 

 ferrimolybdite, a recognizable molybdenum min- 

 eral, but also very commonly is attached to iron- 

 bearing clay minerals, and to goethite and other 

 limonitic minerals, in which forms it is not ordi- 

 narily recognizable. Fortunately, molybdenum in 

 its six-valence oxidation state is amenable to inex- 

 pensive and rapid detection by qualitative and 

 semiquantitative chemical and spectroscopic meth- 

 ods that have threshold sensitivities in the parts 

 per million range, approaching and even reaching 

 crustal-abundance values of molybdenum. In chem- 

 ically less acid environments or in alkaline environ- 

 ments, molybdenite may convert to ilsemannite, a 

 water-soluble blue eflSorescence. 



Because the economically significant molybdenum 

 deposits of today, and probably of the foreseeable 

 future, are the large volume stockworks and 

 porphyry-type deposits related to fractured and 

 hydrothermally altered granitic intrusive rocks, 

 various geophysical techniques are useful prospect- 

 ing methods (Sumner, 1969). Anomalously low 

 magnetism may be detected as a result of the re- 

 moval of magnetite during hydrothermal alteration 

 of host intrusives, and low gravitational anomalies 

 due to large intrusive stocks of granitic rocks may 

 be measurable. 



In terranes that are characterized by large vol- 

 umes of silicic intrusive rocks as young as Tertiary 

 in age, such as the Western Cordillera of the United 

 States, low regional gravity values are closely cor- 

 related with distribution of the young silicic rocks ; 

 because molybdenum seems to be preferentially 

 concentrated around silicic intrusives of Tertiary 

 age in this region, major regional gravity lows re- 

 flect regions favorable for molybdenum deposits. In 



