The Arizona Geological Survey (AZGS) has just published its study of the mineral potential of the Sonoran Desert Heritage Area (see full study here, 25Mb). Establishment of the heritage area, which consists of several detached conservation areas and wilderness areas, would prevent mineral exploration and development on almost one million acres west of Phoenix. A general location map, with mineral potential is shown below:
The AZGS assessment was limited to the following types of deposits: (1) sand and gravel (aggregates), which are used for concrete and asphalt, (2) porphyry copper deposits, which may yield large amounts of byproduct molybdenum, silver, and gold, (3) gold deposits in veins, some of which contain substantial silver, and (4) manganese deposits.
Some highlights from the report:
“Rapid population growth in the greater Phoenix metropolitan area over the past several decades required enormous amounts of aggregate for construction of new homes, buildings, roads, and other facilities and infrastructure. Once urban development occurs, however, underlying aggregate resources are no longer accessible, and nearby resources may become inaccessible because of concerns about noise, dust, and truck traffic associated with quarry operations. Transportation costs, a major factor in aggregate costs, increase greatly if aggregate must be imported from distant areas.”
AZGS has identified five areas within and adjacent to the Heritage Area that are especially favorable as a supply for much needed aggregates.
Precious metal deposits:
The Harquahala – Big Horn Mountains area, which is included in the Heritage plan, has geology favorable for gold vein deposits. The map below shows the northern part of the area. On the map, orange areas show existing wilderness areas, purple areas show Sonoran Desert Heritage proposed wilderness areas, and the light blue line shows the boundary of the proposed Sonoran Desert Heritage National Conservation Area.
Small mines and prospects within the proposed conservation area have produced 506,000 ounces of gold and 549,000 ounces of silver.
Arizona is the center of one of the world’s three great clusters of porphyry copper deposits
(the other two are in northern and central Chile). At the current copper price, Arizona total copper production from 1874-2010, would be worth $472 billion.
The Big Horn Mountains contain rocks of the right type and age for large copper deposits. Historic mining in the Big Horn Mountains yielded millions of pounds of copper and lead as might be expected for mineralization peripheral to a porphyry copper deposit.
The Gila Bend Mountains to the south are largely covered by younger volcanic rocks. The uncertainty in evaluating the possibility of porphyry copper deposits is high because of this cover, but there are enough indications of mineral deposits, including minor production from the Webb district (27,000 lbs. historic copper production) and scattered evidence of historic prospecting and small mines, that this area is considered to have moderate potential for porphyry copper deposits.
Manganese is used in the production of steel. Currently, the U.S. imports 100% of its manganese. Numerous manganese deposits are scattered over a large region in western Arizona and extend into southeastern California. These deposits are low grade (generally only a few percent Mn) and formed during the past 25 million years. Most historic production occurred between 1953 and 1955 when the US Government purchased manganese at above-market prices. Total historic production is ~100,000 metric tons of manganese (226 million lbs.) from 24 mining districts.
The Aguila manganese mineral district, with 42 million pounds of historic manganese production, is within the proposed SDH National Conservation Area in the northern Big Horn
Mountains. The Black Dome district, with 344 thousand pounds of historic manganese production, is located north of the Hieroglyphic proposed Special Management Area.
Manganese in these districts has not been in economic concentrations historically, but could become so if serious supply disruptions occur and/or prices increase sufficiently, see map below.
The study should be considered an overview assessment. Note that the Arizona Geological Survey is serving as a scientific advisor and is not advocating any position, but I will. The Sonoran Desert Heritage Area proposal, if passed at all, should be reduced to exclude areas with medium to high mineral potential as a matter of economic and national security. If established there should be no additional “buffer areas” adjacent to the heritage area.