The Organ Mountains are an elongated, block-faulted wedge of Tertiary-age plutonic rocks capped by volcanics of similar composition. The present mountain range is all that remains of the so-called Organ Mountains caldera, one of 12 or so calderas active in southern New Mexico during Oligocene times. The Organ Mountains caldera is the oldest known caldera in New Mexico. It is of interest to geologists because deep-seated plutonic rocks are exposed beneath the volcano itself. The very roots of the volcano can be seen.
Tertiary-age quartz monzonite forms the core and spine of the Organ Mountain fault block. The Organ Batholith extends from San Augustin Peak southward to Rattlesnake Ridge, a distance of about 12 miles. Rattlesnake Ridge itself is composed of ancient Precambrian basement rock. The primeval gneisses and granites that comprise Rattlesnake Ridge crop out at the southern edge of the Organ Mountains, along the eastern edge of the range, and in the northern part of the area, near Mineral Hill and Black Mountain.
The throat of the Organ Mountains caldera is filled to the brim with pyroclastic material, mostly rhyolitic ash-flow tuff. Nearly 10,000 feet of ash-flow tuff is exposed in the southwestern portion of the range.
Pennsylvanian-age sedimentary rocks are exposed along the southwestern edge of the mountains (on Bishop's Cap) and north of the range, in the San Andres Mountains. These rocks consist mostly of shales, limestones, and dolomites.
The mineral deposits of Dona Ana County are almost invariably associated with uplifted, igneous rocks formed prior to the mid-Tertiary Period. The most important metal deposits are those in and near the Organ Mountains. Of these there are 4 types.
The first type consists of silver-bearing pegmatites associated with the Organ Mountains quartz monzonite. These are found on the slopes of San Augustin Peak. The second type consists of silver-bearing fissure veins that are also found in the quartz monzonite. These contain cerargyrite and native silver and tend to be quite rich. The third type consists of metal-bearing fissure veins associated with ancient Precambrian basement rock. These include the Mineral Hill and Black Mountain deposits. The fourth type of deposit has produced the greatest amount of precious and base-metal ores in Dona Ana County. These deposits consist of cavity fillings and replacement bodies associated with and localized by fault zones. The most important of these deposits occur in a 1-mile long band along the famous Torpedo-Bennett Fault Zone. The rich Torpedo deposit and the Stevenson-Bennett Mine both lie within this fault zone. Most of the ore bodies in the Stevenson-Bennett Mine were found along the contact between Precambrian granite and younger Paleozoic sediments. Many of these deposits were tabular replacement bodies that formed within the Paleozoic quartzites and dolomites.
The eastern flank of the Organ Mountains is bounded by a major north-south trending fault zone which also served to localize ore-bearing solutions. Consequently, silver, lead, copper, and gold have all been mined in this area.
The Organ Mountains are wonderfully mineralized but have been heavily prospected for over 200 years. Although noted for its silver and lead deposits, the area has never really been a major gold producer. Prospectors must realize that most of the major ore deposits in this area have already been found. But of course, the Lost Padre Mine was well hidden by the Spanish colonists sometime during the 1790's. It is entirely possible that a well-concealed mine could lie hidden and undiscovered for over 200 years.
Several accounts of the story place the mine in a deep canyon somewhere west or southwest of Espiritu Santo Springs, now known as Spirit Springs. Father LaRue's colony was located at Spirit Springs. The mine itself was said to lie somewhere between the springs and La Cueva de las Vegas.
Prospectors may want to concentrate on the area described above and in particular focus on all major fault zones. The richest metal deposits in the Organ Mining District are all associated with faults. The fault contact between the Tertiary quartz monzonite intrusion and the older Paleozoic sediments on the western flank of the range should probably be traced in some detail. In addition, the fault contact between the uplifted Precambrian basement rock and adjacent Paleozoic sediments should also be investigated. Some writers have suggested that the Lost Padre Mine lies hidden somewhere on Mineral Hill. Prospectors may also want to include this area in their search.