Mining played a prominent role in the development of the New World. The Spanish march northward from Central Mexico was fueled by successive discoveries of silver at Zacatecas, Taxco, and Fresnillo and then at Durango and Santa Barbara. These were huge silver deposits. Silver from these mines powered the economy of the Spanish Empire.
By the early 1700's, Spanish prospectors were wandering the mountains and deserts of Sonora and southern Arizona. And then in 1736, a most unusual discovery occurred in the area between Guevavi and the Opata Indian village called Arissona. It was in that year that a Yaqui Indian found chunks of pure silver lying right on the surface of the ground. This "place where silver grew out of the ground" eventually became known as the "Planchas de Plata" or "Bolas de Plata" and produced some truly monumental silver nuggets, sheets, and boulders. One huge surface boulder of pure silver weighed nearly 3000 pounds. Many chunks weighing hundreds of pounds were discovered in the inevitable rush that followed. But the surface deposits gave out in only 5 years. By 1741, the area was abandoned.
The border country of Arizona and Sonora is silver country. For many years, persistent rumors of another surface deposit of native silver have circulated throughout the area. In the 1850's, chunks of native silver were again appearing at the trading post in Tubac. The local Opata Indians were selling them to the trader. Apparently the Opatas had discovered a new source of silver.
In the early 1880's, huge nuggets of native silver again began to turn up in the area. There was certainly no denying the chunks of silver lying on the bar of John Connor's saloon in Nogales. It was Opata silver alright. An old Opata Indian had sold the nuggets to a prospector who brought them in to Nogales. The Opata claimed he had found the nuggets just lying on the surface while hunting along Carrizo Creek, northwest of Nogales. It was another "Planchas de Plata"!
The prospector immediately set out for the desert country around Carrizo Creek, which drains the Atascosa Mountains west of the town of Rio Rico. Months later, he returned to Nogales with his burros straining under a load of silver. The old prospector had found the silver nuggets eroding out of a clay "hardpan" somewhere near Carrizo Creek. Unfortunately, the old man died before he could return to the silver field. John Connors made many prospecting trips into the desert in search of the silver but never found it.
Southern Arizona's "silver country" includes a number of famous settlements and mining camps and encompasses several mountain ranges extending from the Tumacacori and Atascosa mountain chains westward to the San Luis and Las Guijas Mountains. This historic section of southern Arizona embraces nearly 600 square miles of mountains and deserts and includes the old mining towns of Tubac, Arivaca, Oro Blanco, Las Guijas, and Ruby. The area was famous for its silver deposits, but gold is found here also.
The Oro Blanco District was named for the white gold or electrum found in the mines and placers there. Electrum is simply a natural alloy of gold and silver. In the Oro Blanco District, nuggets of electrum averaged 50% gold and 50% silver. Placer gold is said to occur in almost every ravine and gulch in the district. The area is highly mineralized and has been worked by Spanish, Mexican, and American miners for over two centuries. The best placer deposits were found in the gulches southwest of Ruby.
The Arivaca District was also a source of gold. Placer gold has been found in most of the gravels that mantle the flanks of the Las Guijas Mountains and in many gulches that head in the mountains. The term "Guijas" means rubble or conglomerate in which placer gold is found. Most of the placer gold mining was concentrated along the northeast flank of the range, south of Las Guijas Creek. Some placers also occur in the gravels that drape the southern slope of the Las Guijas Mountains, northeast of Arivaca Wash.