Geochemistry of molybdenum

Abstract Chondrites have rather uniform molybdenum contents; the average of 14 specimens is 1.6 p.p.m. Mo, with the range 1·2–2·0 p.p.m. Composites of these contain an average of 0·6 p.p.m. Mo in the silicate phase, 8·0 p.p.m. in the metal phase and 5·7 p.p.m. in the sulphide phase. The Canyon Diablo and Henbury irons showed 17 and 10 p.p.m. Mo respectively. The siderophilic and chalcophilic characters of molybdenum are similar and much stronger than the lithophilic. The concentration of molybdenum in the silicate phase of chondrites is substantially the same as in ultramafic rocks. Silicic and mafic rocks differ but little in molybdenum content as shown by the averages: granitic rocks, 1·1 p.p.m. Mo (based on 135 samples); basalts and diabases, 1·1 p.p.m. (38 samples); gabbros, 0·6 p.p.m. (21 samples); ultramafic rocks, including 9 serpentines, 0·4 p.p.m. (23 samples). Ultramafic rocks are a little lower than other types in molybdenum. An average value of 1 ± 0.5 p.p.m. Mo may be assigned to the crustal rocks. The rather uniform distribution of molybdenum among the common igneous rocks is largely due to its ability to replace a number of elements in the lattices of rock-forming minerals. It is found in the feldspars, femic minerals (biotite, amphibole, pyroxene) and, especially, in magnetite and ilmenite, but is not present in significant amounts in quartz. Substitution of molybdenum for ferric iron, titanium, aluminium and possibly silicon is indicated. These substitutions are in general accord with the ionic radii of the elements involved. In mafic rocks molybdenum tends to vary with titanium. This relation is well shown in the asaltic flows of upper Michigan. It does not hold for differentiates of certain intrusives (Mt. Bohemia gabbro of Michigan, Endion Sill at Duluth and probably the main mass of the Duluth gabbro) in which concentration of the alkalies can take place. Molybdenum is markedly enriched in the silicic or alkalic differentiates of mafic magmas, as shown by its concentration in the granophyric red rock phase of the Duluth gabbro and of the Endion Sill. It occurs in greater than average amounts in rocks of alkalic affinities, as shown by syenites, trachytes and some basalts of West Texas and is doubtless to be classed among the elements concentrated in alkalic differentiation. In granitic rocks molybdenum appears to be a little less abundant (on the weight basis) than tungsten. Comparative data are not available for mafic and ultramafic rocks. Shales poor in organic matter contain about as much molybdenum as igneous rocks (average about 1 p.p.m.). Carbonaceous and pyritic shales are likely to be much richer in molybdenum; contents of 100 p.p.m. or more may be encountered. Limestones and dolomites usually contain less than 0·5 p.p.m. Mo except when organic matter is present. Because of the variable amounts of molybdenum in shales it is difficult to arrive at an average value for molybdenum in sedimentary rocks. The content may be appreciably higher than in igneous rocks because of the escape of molybdenum in hydrothermal solutions and volcanic emanations. Deep sea sediments contain an average of a few parts per million of molybdenum. The few metamorphic rocks studied averaged 0·5 p.p.m. No correlation between Mo content and metamorphic grade was observed.