Extreme Late Oligocene to middle Miocene tectonic extension in central western Arizona was associated with exhumation of deep crustal rocks to form metamorphic core complexes, felsic to mafic magmatic activity, and formation and subsidence of sedimentary basins. Tectonic extension was accompanied by genesis of iron-oxide copper gold (IOCG) deposits along and near extensional detachment faults. Fluid-inclusion analyses from these deposits indicate derivation from saline fluids at moderate temperature, as identified in previous studies. Many other Miocene epithermal mineral deposits in the region were derived from high-salinity fluids, indicating that brines were characteristic of mineralizing fluids regionally and were not limited to areas affected by detachment faulting. Some outlying deposits have a range of fluid-inclusion salinities within a single sample, suggesting changing ratios of contrasting-salinity fluids that mixed during deposit genesis. Manganese mineralization and K-metasomatism were widespread over the same area and occurred at about the same time, but it has not been possible to demonstrate a relationship between these disparate geochemical processes even though such a relationship seems likely.

County geologic map of Yuma County and La Paz County, which was part of Yuma County in 1960. Map scale 1:375,000. Contour interval is 500 feet.

Geologic map of the a portion of the southern Kofa Mountains.

One of the things that the Arizona Bureau of Mines has always wished to do is to issue reports that cover the geology and mineral resources of parts of Arizona that are known to be mineralized, but concerning which little or no information is in print. There are certainly vast areas of this kind in the State, but the Bureau has been unable to do much along this line because the funds needed have not been available. Several bulletins of the type under consideration have been issued, however, and they have been very well received. It is to be hoped that, in the future, the Bureau will be supported more adequately and that it can much more rapidly secure and disseminate information concerning parts of the State that have never previously been described. There is a very great demand for such publications, and, if they are available, interest in the areas involved will undoubtedly be aroused and capital may be attracted to Arizona.

The Owl Head 7.5' quadrangle and the northern third of its southern neighbor, the Kofa Butte 7.5' quadrangle, represent an 80 mile' area of the northern Kofa Mountains, a remote, rugged range of southwest Arizona (Figure 1). The only part of the map area covered by significant amounts of Quaternary alluvium is an 8 mile' area in the northwest corner which is part of a narrow, Quaternary or younger Tertiary alluvial-filled valley that separates the Kofa Mountains from the New Water and Plomosa mountains to the north. Most of the bedrock exposures are Miocence calc-alkaline volcanic rocks of the Kofa volcanic field. They occur in three sequences: a series of outflow sheets of ash-flow tuffs that depositionally overly the crystalline basement rocks, a middle sequence of mafic to intermediate lava flows, and a younger sequence of basalt lavas. The crystalline basement rocks are exposed in the footwall of two major south-side-down normal faults which transect the north-central part of the map area. The basement consists primarily of Mesozoic(?) granitoids which intrude amphibolite-facies, Proterozoic(?), sedimentary rocks to the east. To the west, the granitoid has an indeterminate east-northeast striking contact with low-metamorphic grade argillites of probable Late Mesozoic age which underlie the northwestern peidmont of the Kofa Mountains, and which are probably correlative to extensive areas of similar rocks which make up large areas of the Plomosa Range to the north (Richard and others, 1993).