Molybdenum deposits are classified by using magma series chemistry expressed as K (sub 57.5) (i.e., the K 2 O content at 57.5% SiO 2 in a magma series); the F, Nb, Rb, and Sr contents of the source pluton; and the abundance of F and Sn in the hydrothermal system. By plotting K (sub 57.5) values against the Nb, Rb, and F contents of the granitic source pluton, a natural dividing line emerges separating igneous rocks with K (sub 57.5) values below 2.5 from rocks with K (sub 57.5) values above 2.5. This metallogenically significant break forms the basis for our proposed classification.No molybdenum deposits have been found in calcic magma series. Calc-alkaline stockwork molybdenum deposits are associated with calc-alkalic and high K calc-alkalic magma series (K (sub 57.5) < or = 2.5). The peraluminous source pluton, as a rule, contains less than 20 ppm Nb, between 100 and 800 ppm Sr, and from 100 to 350 ppm Rb. Molybdenite grades in the hydrothermal system rarely exceed 0.25 percent and fluorine is only weakly enriched in the phyllic zone. Tungsten in the form of scheelite may be common, but tin is absent. Stock and plutonic deposit types are distinguished by taking into account depth of formation and hydrothermal and fluid inclusion characteristics. Examples of stock type calc-alkaline molybdenum deposits include Kitsault and Hall, whereas Endako and Adanac represent plutonic calc-alkaline molybdenum deposits.Alkali-calcic and alkalic molybdenum stockwork deposits are related to metaluminous to peraluminous granitic differentiates of high K calc-alkalic (K (sub 57.5) > 2.5), alkali-calcic, and alkalic magma series. Source plutons contain from 25 to in excess of 250 ppm Nb, 200 to 800 ppm Rb, less than 125 ppm Sr, and less than 0.2 percent TiO 2 and are enriched in F, Sn, and Mo. The hydrothermal system commonly contains molybdenite grades above 0.30 percent and is characterized by intense K metasomatism and an abundance of fluorite and/or topaz. Tungsten is common and occurs as huebnerite or wolframite. The deposits are enriched in tin. The alkali-calcic to alkalic deposit category has been subdivided into: (1) transitional deposits cogenetic with granitic differentiates of high K calc-alkalic (K (sub 57.5) > 2.5) and alkali-calcic magma series characterized by moderate enrichment in Nb, Rb, and F (Questa, Mount Hope); (2) Climax-type deposits associated with highly differentiated granites of alkali-calcic magma series (e.g., Climax, Urad-Henderson, Mount Emmons); and (3) alkalic deposits found with alkali-calcic and alkalic magma series. The alkalic molybdenum deposits are subdivided into (a) deposits cogenetic with metaluminous quartz-deficient syenitic and monzonitic stocks (e.g., Nogal Peak) and (b) deposits related to granites (e.g., Malmbjerg).Calc-alkaline molybdenum deposits occur in continents and older island arcs in volcanoplutonic arcs associated with plate convergence. Climax-type deposits are found in areas of crustal relaxation and extension above the deepest part of a subduction zone. Alkalic deposits are present in areas of back-arc spreading associated with converging plate margins, in intracratonic rifts, and in rifts associated with the opening of oceanic basins. Magmatism responsible for the formation of calc-alkaline molybdenum stockwork deposits may be a product of partial fusion of subducted oceanic lithosphere. Partial melting of enriched upper mantle in response to pressure release in areas of back-arc spreading and intracratonic rifting could form magmas related to Climax-type and alkalic deposits. We propose that the chemical differences between source plutons and hydrothermal systems in the two categories indirectly reflect the phlogopite stability field in the subducted slab and the overlying mantle wedge. We also propose that, based on the data presented, molybdenum contained in all stockwork deposit types is of subcrustal origin.The validity of the proposed classification has been tested by plotting radiometrically dated molybdenum deposits of the western United States on maps showing the distribution and magma chemistry of arc magmatism for the appropriate time span. The location of the deposit accurately predicts the type of deposit as determined independently by igneous trace element data and hydrothermal system characteristics. Our findings suggest that at any time during the development of the magma arc each molybdenum deposit category has its unique position within the arc. Geochemical data presented also show that a continuum exists between calc-alkaline molybdenum stockwork deposits and porphyry coppers. Calc-alkaline molybdenum deposits grade into Climax-type deposits through the intermediate transitional deposit type. No deposits transitional between alkalic porphyry copper deposits and Climax-type molybdenum deposits are known to us.The striking differences between calc-alkaline and Climax-type molybdenum deposits are a consequence of original magma chemistry, molybdenum concentration levels in the parent magma, and the behavior of molybdenum in the melt and hydrothermal system. The high K, F, Rb, and low Ti and Fe contents of granitic rocks associated with Climax-type deposits create conditions ideal for molybdenum concentration, initially through crystal fractionation and liquid state thermogravitational diffusion and later by means of volatile transfer or the formation of a hydrous alumina-deficient K-rich silicate melt. Following vapor saturation of this residual melt, molybdenum will be introduced into the hydrothermal system. Decomposition of the K silicate component in response to cooling and a pH decrease results in addition of potassium to the hydrothermal fluid or the wall rocks and the precipitation of quartz and molybdenite. The striking correlation between added K 2 O and MoS 2 grades in the deposits lends credence to this concentration mechanism. No quantitative correlation between fluorine and molybdenum introduced in the hydrothermal system has been documented. In our estimation, the importance of fluorine principally lies in its capability of altering magma characteristics that help to enhance molybdenum concentration processes. Differentiates of calc-alkaline magma series have lower initial molybdenum concentrations and this factor combined with the higher Ti and Fe levels, and the less extreme K and Rb and low F contents, prohibits extreme molybdenum concentration during the late magmatic stage. Consequently, most calc-alkaline molybdenum deposits are less intensely mineralized.The relationship between magma chemistry and metallogeny as exemplified by the metallogeny of molybdenum deposits may be extended to include other metals and could prove to be a valuable exploration tool.