A geochemical survey was conducted in the McCarthy quadrangle Alaska, to identify areas containing anomalous concentrations of various metallic and nonmetallic elements. This study incorporates the results of analyses for molybdenum from 825 rock samples collected in the quadrangle, and analyzed by the U.S. Geological Survey between 1961 and 1976 using semi-quantitative emission spectrophotometry. The samples included both unaltered and hydrothermally altered rocks. The hydrothermally altered rock consists of ore grade materials, gossans, fault gouge, vein materials, silica-rich boxworks, veins adjacent to faults, and fracture surfaces showing evidence of mineralization. Therefore, the analytical data set may be considered representative of most rock types known to occur in the study area.

The accompanying map shows the distribution and relative abundance of molybdenum in rocks collected. Geochemical analyses have been grouped and are represented by symbols on a base map, which includes topography and generalized geology. The range of analytical values and the symbol that represents it are shown on the histogram. Graphical representation of analytical values on the map permits easy observation of any large variation resulting from separate or duplicate samples collected at the same or nearby localites. All samples were crushed and ground to pass through a 180 micron opening sieve before being analyzed.