The Galiuro Mountains are one of many northwest-southeast trending mountain ranges that comprise the Basin and Range Province of southern Arizona. The Galiuros are predominantly composed of Tertiary rhyolites and andesites; these volcanics occur as lava flows, tuffs, and agglomerates. The western edge of the Galiuros, north of Schoenholzer Canyon, is a complex mélange of faulted Precambrian, Paleozoic, and Mesozoic sedimentary, igneous, and metamorphic rocks. This sequence of rocks forms a block or wedge 2 to 3 miles wide and about 24 miles long exposed along the western edge of the range. The southern end of the block consists of Cretaceous rhyolites and andesites intruded by small Laramide-age granitic stocks. This wedge of igneous rock is fault-bounded on its northern and western edge and covered by younger Tertiary volcanics on its eastern and southern edge. The exposure extends northward from Schoenholzer Canyon to Dry Camp Canyon, a distance of about 9 miles. A wedge of ancient Precambrian schist bounds the Cretaceous volcanics and Laramide granites on their northern edge. This block of Precambrian basement rock is fault-bounded on its northern, eastern, and southern margins. Along its eastern edge, a sequence of Cambrian, Ordovician, Devonian, and Mississippian quartzites and limestones butts up against the older schist. As mentioned above, this contact is fault-bounded. The Precambrian schist has a total outcrop area of about 5 square miles.
A large exposure of Precambrian diabase borders the block of older schist along its northern edge. This mass of ancient mafic igneous rock has an outcrop area of about 14 square miles. A faulted wedge of this diabase plus some early Paleozoic sediments forms the northern boundary of the ancient Precambrian schist described above. The sedimentary sequence consists of Precambrian, Cambrian, Ordovician, Devonian, and Mississippian quartzites and limestones. Much of the rugged country surrounding Aravaipa Creek is underlain by Precambrian diabase. A small Precambrian granite pluton is exposed south of the creek, immediately adjacent to the faulted wedge of diabase described above. This area encompasses the Table Mountain / Mining Mountain region, on the western flank of the Galiuros.
The mountain flanks north of Aravaipa Creek are composed of Precambrian diabase and Paleozoic sedimentary rocks ranging in age from Cambrian to Permian. (All periods of the Paleozoic are represented except Silurian.) Further north, as one approaches the Gila River, large exposures of Laramide andesite dominate the landscape.
Whereas the Galiuro Mountains east of the San Pedro River are mostly composed of younger Tertiary volcanic rocks, the Tortilla Mountains and adjoining hills on the west side of the river are almost entirely made up of ancient Precambrian granites. The Tortilla Mountains are a north-south trending range of uplifted basement rock which forms the western boundary of the San Pedro river valley for some 25 miles in this part of Arizona. Although the Tortilla Mountains are predominantly composed of Precambrian granitic rocks, there are minor exposures of Precambrian sedimentary rocks and dark-colored diabase in the southern portion of the range. The sedimentary rocks consist of ancient limestones, quartzites, and shales of the Apache group. They crop out in a narrow band along the eastern flank of the Tortillas.
The Black Hills also lie on the west side of the San Pedro River, between the mining town of Tiger and the area near old Camp Grant. Like the Tortilla range, the Black Hills are mostly composed of Precambrian granitic rocks. At least the northern portion is. The southern part of the Black Hills, near the Tiger mines, consists of early Tertiary sedimentary rocks. A sequence of truly ancient sedimentary rocks crops out in the northernmost part of the Black Hills. This small exposure ranges in age from Precambrian to early Paleozoic and is one of several scattered along the eastern flanks of the Tortilla Mountains and Black Hills.
The area of interest includes the western flanks of the Galiuro Mountains and the Tortilla Mountains on the other side of the San Pedro River. This is a vast amount of country to prospect, but some areas seem to offer greater potential than others. Most accounts of the story describe the deposit as a vein of gold-bearing rose quartz. The vein seems to be emplaced in sedimentary rock. According to most accounts, the gold lode lies at the bottom of a small, crater-like depression lined with shale. The vein itself is partially covered by loose shale debris. Any search for the Lost Aravaipa Mine should probably include a survey of all exposed sedimentary rocks.
Prospectors should focus on the western flanks and foothills of the Galiuro Mountains, from Schoenholzer Canyon north to Aravaipa Creek, a distance of about 16 miles. Mineralized blocks of sedimentary rock may lie hidden under a blanket of Tertiary-Quaternary alluvium in this area. Mineralization does occur on the western flank of the Galiuros, especially near the head of Copper Creek. This area is underlain by Cretaceous volcanics and Laramide-age granitic rocks.
It may be that the more recent Quaternary sands and gravels that drape the western flank of the Galiuros south of Schoenholzer Canyon may conceal a mineralized block of sedimentary rock. This apron of fairly recent Quaternary alluvium extends southward to Redfield Canyon, a distance of about 18 miles. Mineralized bedrock may have been exposed by downcutting in some canyon, steam bed, or arroyo.
In the Tortilla Mountains, the eastern flanks of the range offer similar opportunities. A thin belt of ancient sedimentary rocks crops out along the eastern edge. The belt is well-exposed in the mountains northwest of Camp Grant.