Column No. 48

Oro Blanco Geology and Geography

Bob Ring, Al Ring, Tallia Pfrimmer Cahoon

When we started these columns on the history of mining along the Ruby Road, we pretty much jumped right into the American history of the region. So let’s go back a few years and talk about the geology of Oro Blanco and what happened before the Americans got there.

Arizona’s Oro Blanco region is just south of Arivaca, along the border with Mexico. Oro Blanco’s precious metals (gold and silver) and the base metals (lead, zinc, and copper), date from the current Cenozoic Era, during the early Tertiary geologic period - probably some 50 to 60 million years ago.

Explosive volcanism marked that period in southern Arizona. Deep underground, hot aqueous solutions, enriched with metals and other elements, remained as molten rocks cooled. Over thousands of years, these solutions, driven by heat from the molten rocks, circulated upward through fissures in the earth's crust and gradually cooled as they approached the surface.

As the solutions cooled and the pressures decreased, certain mineral constituents of these hydrothermal solutions became insoluble and deposited in the open spaces (e.g., fractures and faults) through which the solutions were moving. The base metals deposited a little deeper below the surface than the precious metals. In the Oro Blanco region, quartz usually deposited along with the precious and base metals. Constant weathering and erosion from water and wind gradually wore away the upper part of the mineral deposits, exposing the top of a zone of potentially economically “mineable” ore.

Early miners found some of the free gold liberated from “host” rocks by this erosion in placer deposits in the gravels at the bottoms of the arroyos draining the area. This “placer” gold from the Oro Blanco region contained an unusually high amount of silver, giving the gold a whitish color - hence "Oro Blanco,” or "white gold" in Spanish.

“Lode” deposits of precious and base metals remained in veins within solid rock. The lode ore from the Oro Blanco region was "complex,” i.e., intimately mixed or combined mineral constituents, and therefore difficult to cleanly separate from each other into their individual metal concentrates for shipment and sale. (The ore was also very hard, making it difficult to crush and grind.) This complexity caused difficulties in maintaining a high percentage of recovery of specific minerals from the raw ores. Because the Oro Blanco ore was relatively low grade (small percentage of precious or base metals) to start with, maintaining profitability in mining was a constant challenge in the Oro Blanco area.

But that’s getting ahead of our story. Well before mining in Oro Blanco, in the 1500s, Spanish conquistadors vigorously searched what became Arizona for Cìbola, a mythical province supposedly containing seven cities of silver and gold. Though the Spanish never located Cibola, missionary expansion, from what is now western Mexico, brought Jesuit Eusebio Kino to the Pima settlement of Tumacácori in 1691. Historian Herbert Eugene Bolton thinks that Father Kino first entered the present day United States from Tucubavia Mexico through Sycamore Canyon and Bear Valley, in today's Oro Blanco country.

In the next few years, Father Kino founded missions along the Santa Cruz River at Tumacácori; Guevavi, a few miles south; and San Xavier del Bac, to the north, near present day Tucson. In addition to their spiritual duties, settlers, priests, and Indian converts started mining nearby shallow deposits of silver.

In 1736 Spanish prospectors made a unique silver discovery, Planchas de Plata (Sheets of Silver), just south of the present international boundary and just west of present day Nogales, Arizona. They found large chunks of almost pure silver near the surface of the ground. Thousands of Spanish fortune seekers rushed to the area, only 15 miles from the Oro Blanco area.

Some of the unsuccessful would-be miners drifted northward and began to settle in present day south-central Arizona, along the Santa Cruz River, at Tubac, just north of the mission at Tumacácori. Prospecting in the region intensified. Spanish prospectors from the Santa Cruz Valley, initially working only close-by silver mines, probably first entered the Oro Blanco region to mine placer gold around 1740.

Small-scale Spanish mining continued, but constant Apache raids forced closure of most southeastern Arizona mines by 1787. The Spanish did not resume mining operations until 1804, this time with emphasis on new gold placers, instead of reopening the old silver mines. The Oro Blanco gold placers may have been of particular area interest.

Mexico achieved its independence from Spain in 1821, but it is not clear that this affected the intermittent mining in Oro Blanco. Apache raids continued to harass miners, ranchers, and settlers in the Santa Cruz Valley.

The land-hungry United States now began to show interest in Mexico. In 1846 the U.S. went to war with Mexico over boundary disputes and the U.S. desire to acquire the northern Mexican provinces. The war ended with the Treaty of Guadalupe Hidalgo in 1848. The U.S. paid Mexico 15 million dollars to give up claims to Texas and acquired lands that would eventually become the states of California, Utah, Colorado, Wyoming, New Mexico, and the part of Arizona, north of the Gila River. The U.S. and Mexico continued to argue about where to draw the southern international boundary. Finally, in the Gadsden Purchase of 1853, the U.S. paid Mexico 10 million dollars for the balance of the Arizona Territory, south of the Gila River.

The Gadsden Purchase brought the Oro Blanco region into the United States (at that time into the new Territory of New Mexico). But it was to be another 20 years – of international boundary haggling, American exploration, reopening of some of the old outlying silver mines, the U.S. Civil War, creation of the Arizona Territory, and continuing raids from the Apache and Mexican bandits - before the first Americans started gold mining in Oro Blanco.

(Sources: Discussions with retired geologist Bill Daffron; Stanton B. Keith, “Index of Mining Properties in Santa Cruz County;” Herbert Eugene Bolton, Kino’s Historical Memoir of Primeria Alta; John P. Wilson, Islands in the Desert)

Map of 1853 Territory

At the time of the Gadsden Purchase, southern Arizona became part of the new Territory of New Mexico. (Map by Bob Ring, 2005)

Next Time: Mining Technology Transfer from the Far West

Back to List of Columns