The Iron King mine is about 2,000 feet west-northwest of the intersection of the 112 degrees 15' west meridian and the 34 degrees 30' north parallel in the Humboldt region in central Yavapai County, Arizona. The mine is approximately in the geographical center of the Humboldt region.Precambrian rocks form the bedrock. Late Cenozoic unconsolidated river wash and valley fill with some interbedded basalt locally mantle the Precambrian rocks, especially in the north-central part of the region.The Precambrian rocks consist of two metamorphosed volcanic formations and intrusive rocks that range in composition from quartz porphyry to gabbro. The volcanic formations originally were flows, volcanic breccias, and tuffaceous sedimentary rocks. Dynamothermal metamorphism of these rocks formed textures, structures, and mineral assemblages characteristic of low-grade metamorphic rocks, but sufficient relict textures and structures remain to permit delineation of formations on the bases of their original nature.All the Precambrian rocks are foliated, except those in the interior parts of the larger intrusive masses. This foliation has two major trends, (1) north to N 20 degrees W, and (2) N 20 degrees E. The northeast-trending foliation is younger and locally is superimposed on the north- to northwest-trending foliation. The Precambrian rocks strike north to northwest and dip steeply, chiefly westward. Duplication of stratigraphic units and determination of tops suggest two major northeast-trending folds, which probably plunge southward. The northeast-trending foliation appears to bear an axial-plane relationship to these folds. Some masses of igneous rock probably were intruded during development of foliation.The Iron King deposit supports the only active mine in the Humboldt region. It consists of 12 steeply plunging echelon veins arranged along the footwall of a sheared and altered zone in the metamorphosed andesitic tuffaceous sedimentary rocks. Narrow zones of more intense shear probably localized the veins.Solutions first introduced quartz, pyrite, ankerite, and sericite, forming a sporadically mineralized zone (in the hanging wall of the deposit) and probably veins in the Iron King fracture system. Intra-mineralization shear strongly brecciated these early minerals and formed the structures that localized and distributed the ore minerals in the veins. After this deformation, sphalerite, galena, chalcopyrite, tennantite, arsenopyrite, pyrite, quartz, and ankerite were deposited; the last three minerals may have formed through solution and the redeposition of earlier minerals. Formation of sericite either accompanied or followed the deposition of oreforming minerals. Silver is related closely in distribution to copper and probably is in the tennantite. Gold occurs chiefly in the pyrite.Banding, mimetic after foliation and planes developed by shearing, is pronounced in much of the vein material. Mineral zoning, generally similar in each vein, is a characteristic of the deposit.High-angle reverse faults of about 100 feet in maximum vertical separation offset the veins. These faults are nearly parallel to the veins in strike and dip.