Oatman Mining District, Black Mountains, Mohave County, Arizona, USAi

Regional Level Types
Oatman Mining District	Mining District
Black Mountains	Mountain Range
Mohave County	County
Arizona	State
USA	Country

This is a Au mining area located on the western slopes, southern portion of the Black Mountains, 9 miles East of Hardyville, which is on the Colorado River. The district was discovered in 1863. It lies between 2,000 to 3,200 feet of altitude. The district had several obscure names as listed.

The southern portion of the Black Mountains consists of a very ruggedly dissected, gently eastward-dipping block of Tertiary volcanic rocks which rest upon a basement of Pre-Cambrian gneiss and granite. The most important ore-bearing formation is the Oatman Andesite, which Schrader termed the "green chloritic andesite." These formations are cut by numerous faults of prevailingly northwestward strike and steep northeastward dip.

Veins of the Oatman District occur within fissures along which faulting has taken place, as a rule before, during, and after the period of vein formation. In general, no distinction between faults and veins can be made, although some fissures, such as the Military fault, are younger than the veins. The veins are rather widely distributed, and the most productive ones are in the northeastern half of the district.

Vein-filled fissures are particularly abundant in the southern and central parts of the district, especially in the Oatman Andesite and Esperanza Trachyte. The general strike is NW, but the strike of individual fissures may range from nearly north to nearly west. The dip is high as a rule, over 60º, and to the NE. Many of the fissures branch, but there appears to be no recognizable rule in respect to this; some fissures diverge to the NW, and others to the SE. The veins vary in width, but few of them are wider than 50 feet at any point. As a rule the large ore bodies of the district have been mined from veins or parts of veins that are not prominent at the surface.

Some of the veins have tabular forms but the larger ones are essentially stringer lodes of complex structure. Compound veins, consisting of two or more veins separated by country rock, with stringer veinlets, are common. Many of the veins are lenticular in all dimensions. A strong vein may pinch out within a few tens of feet, and an insignificant stringer may thicken to considerable width within a distance of 30 feet (10 meters). The veins of the Oatman District are of the epithermal bonanza type characteristic of Tertiary volcanic activity. The ores were deposited by ascending hydrothermal solutions at depths of not more than about 3,000 feet (1,000 meters) before the then existing surface.

The Oatman veins are mineralogically of simple character, consisting mainly of quartz, calcite, and adularia, associated in the ore shoots with free gold. As a rule, only quartz and calcite are recognizable with the naked eye. Generally several generations of quartz and calcite may be discernible. The gold is characteristically fine grained and generally can be seen only in rich ore. The adularia occurs generally in microscopic crystals.

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