Description:
In 2011, the Arizona Wilderness Coalition, Sonoran Institute, and The Wilderness Society
proposed that the Federal Government designate approximately 1170 square miles (~750,000
acres) of Arizona public lands as Wilderness Area, National Conservation Area, and Special
Management Area (Marlow and Richins, 2012). These lands are located primarily west of
Phoenix in the low deserts of western Arizona. Enactment of this “Sonoran Desert Heritage”
proposal would prevent mining and quarrying on these lands, and would prohibit or impede
exploration activity such as drilling and geophysical surveying. This report is an evaluation of
the mineral resource potential of these lands, but is limited to the following types of deposits: (1)
sand and gravel (aggregates), which are used for concrete and asphalt, (2) porphyry copper
deposits, which are a specific type of large copper deposit that is common in Arizona (these
deposits 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, which are present
over a large area in western Arizona but, historically, have been too low grade for mining.
The geology of the lands included within the Sonoran Desert Heritage proposal is complex, and
varies greatly from range to range. Geologic mapping of almost the entire area, done by Arizona
Geological Survey geologists under the joint State-Federal STATEMAP and COGEOMAP
programs over the past 30 years, allows informed evaluation of mineral resource potential. This
report identifies areas that have high potential for future aggregate production, and areas
considered favorable for future discovery of porphyry copper deposits, precious-metal vein
deposits, and manganese deposits. One area encompassed by the Sonoran Desert Heritage
proposal, which includes the Harquahala and Big Horn Mountains, is identified as having high
potential for all of these deposit types. Other metallic mineral commodities such as uranium,
iron, and rare-earth elements, and industrial minerals such as clay and zeolite, were not
evaluated. This report also includes a brief review of USGS designations of proposed SDH lands
as “permissive” and “not permissive” for future discovery of several mineral commodities.
Finally, it should be noted that evaluations of potential for undiscovered mineral deposits are
inherently speculative, and are complicated by the fact that new technologies for mining and
quarrying, changing economic conditions, and changes in commodity demand may result in
some uneconomic deposits becoming economic, or in previously uninteresting deposits
becoming targets for exploration and mining. As a result, removal of even unattractive lands
from mining and quarrying can eventually have adverse economic consequences.