The Eagle Mountains are in the southeastern part of Hudspeth County, Tex., about 17 miles southwest of Van Horn and 100 miles southeast of El Paso, Tex. The fluorspar deposits are in the northern and northeastern parts of the mountains, except for the Rocky Ridge deposits, which are near the center of the mountainous mass. A good all-weather road leads south from Allamoore on U. S. Highway 80 to the mine and mill at Spar Valley. With the exception of the Rocky Ridge deposits, all deposits can be reached by ranch roads from the main Spar Valley road.

Fluorspar was first found in the Eagle Mountains in 1919, but no development was undertaken until 1942. Since then, mining has been done at Eagle Spring and at the various deposits in Spar Valley. Many other deposits have been found in the area. About 12,000 tons of fluorspar had been shipped previous to January 1949, most of which came from the North ore body in Spar Valley. A mill was built near the deposits In 1945. With the exception of the Eagle Spring and Tank Canyon deposits, both of minor importance, the fluorspar deposits as of 1950 are all controlled by Texas Fluorspar Mines, Inc., of Van Horn, Tex.

Cretaceous sedimentary rocks, which crop out on the flanks of the mountains, are overlain by a thick series of Tertiary volcanics that make up much of the central part of the mountains. Low on the northeast side, the Cretaceous rocks are underlain by Permian (?) limestones and the pre-Cambrian Carrizo Mountain schist. The Cretaceous sedimentary rocks range from the Yucca formation of Early Cretaceous (lower Trinity) age through the Eagle Ford formation of Late Cretaceous age. The rocks on the northeast side of the mountains dip southwest, and those on the southwest and west sides dip east-northeast and northeast. The axis of the large syncline thus formed roughly parallels the axis of the range. The igneous rocks occur within the trough of the syncline.

Both intrusive and extrusive rocks are present. The extrusive rocks have been separated into three divisions : the lower rhyolitic series, trachyte porphyry, and the upper rhyolitic series. Both rhyolitic series consist of flows, flow breccias, volcanic breccias, and tuffaceous sediments, all predominantly of rhyolitic composition, although tending toward andesite locally. These volcanics have been intruded by a small stock of syenite, named in this report the Eagle Peak syenite, which crops out in the central, higher parts of the mountains. Rhyolite sills have invaded the sedimentary rocks near the margin of the volcanics, and diabase and late rhyolite dikes have intruded both sedimentary and volcanic rocks.

Faults are common in the area, and six series of faults have been recognized. Thrusting from the southwest occurred both before and after the igenous activity and the subsequent downwarping of the central part of the mountains. The early thrust faults were followed by high-angle normal and reverse faults that trend northeastward and cut the volcanics. Later normal and reverse faults trending northwestward, and faults with large horizontal displacements trending roughly eastward, also are present, in addition to very late faults trending in a general northwesterly direction.

Fluorspar occurs in the Eagle Mountains both as replacement deposits in limestone and as fissure veins, chiefly in rhyolite. Chief among the fissure veins are those occurring along the Rhyolite fault, the Wind Canyon fault, the vein at Shaft 4, and the veins on Fox claims 9 and 10. The most important replacement deposits are in the Rocky Ridge area and in Spar Valley. • At the North ore body in Spar Valley, the fluorine-bearing solutions replaced a series of sandy limestones in the upper beds of the lower part of the Finlay formation. Structural conditions limited the extent of the replaceable beds and consequently of the fluorspar mineralization.

The fluorine-bearing solutions represent a very late stage of the igneous activity of the area. The large east-trending faults with their wide zones of gouge and breccia, typified by the Rhyolite and Wind Canyon faults, acted as the major channels for the solutions in their upward course. From these faults, the solutions spread outward into other faults and fractures, chiefly those with a northeasterly trend, and into the adjoining limestones. The physical and chemical nature of the surrounding rock, as well as structural conditions affecting the presence of openings in the rock, were the major controlling factors governing the size, extent, and position of the fluorspar deposits.