Pearl Copper Project
Mammoth Mining District, Pinal County, Arizona
- Porphyry copper exploration project immediately north of BHP’s past producing San Manuel-Kalamazoo mining complex
- 241 unpatented mining claims (4,983 acres) with numerous copper occurrences and historic workings displaying copper mineralization and propylitic alteration, as well as Laramide-age porphyry dikes
- Soil geochemical surveys have identified widespread elevated copper and other pathfinders and rock grab samples up to 7.3% copper
- ZTEM geophysical survey identified strongly conductive zone.
The Pearl project is located in the Sonoran Desert of southwestern Arizona near the town of Mammoth, approximately 25 miles northeast of Tucson. It sits immediately north of BHP’s San Manuel-Kalamazoo mine, one of the largest deposits in the North American porphyry copper province. Access is via state route 77 and a network of roads and tracks within Arizona State and U.S. Bureau of Land Management ground. Elevations range from 2900 to 4300 feet.
Zacapa acquired the Pearl project through claim staking and initiated exploration activities in 2021. Historic exploration in and around the area started in the late 1800’s and the Mammoth Mine was discovered in 1879. The Mammoth district reached peak activity in the late 1930’s, predominantly as a gold and silver producer from large veins. Later, during World War II, production shifted to molybdenum and vanadium and increased copper exploration resulted in the discovery of porphyry-style mineralization at San Manuel in 1944 (~600 meters SE of the Pearl claims). In 1955, the first ore was extracted from the San Manuel orebody. In 1967, J. David Lowell famously discovered the missing upper portion of the orebody [Kalamazoo], which ultimately lead to the development of the Lowell and Guilbert geological model for porphyry copper deposits, still in use today. Mining ceased at San Manuel-Kalamazoo in 1999 after the copper price dropped below $0.65/lb, and remediation began in 2003. Ultimate production totaled over 9.3 Blbs of copper and 146 Mlbs of molybdenum. Historic workings are visible throughout the Pearl project area but no information is available on modern exploration that has taken place outside the footprint of the San Manuel-Kalamazoo orebody.
The Pearl project occurs in the San Pedro trough within the Basin and Range province where Tertiary extension has resulted in complex faulting and tilting of pre-existing geology coincident with burial by syn-extensional alluvial deposits (e.g. Cloudburst Formation). Precambrian basement rocks consist of Pinal Schist and granitic plutons (Oracle Granite) overlain by Proterozoic sedimentary rocks all of which are intruded by diabase sills. These older units are unconformably overlain by Paleozoic carbonate and clastic units and Mesozoic volcaniclastic and clastic successions. Late Cretaceous to early Paleogene compression associated with the Laramide Orogeny produced folds and thrusts and introduced metaluminous plutons and dikes (e.g. quartz monzonite to granodiorite porphyry) and andesitic to rhyolitic volcanic rocks, and associated porphyry copper mineralization. Erosion stripped much of the Laramide volcanic cover and later arc magmatism introduced Oligocene felsic plutons and volcanic rocks that are locally associated with polymetallic vein deposits. In the Miocene, regional extension and faulting was associated with the deposition of extensive coarse-grained sedimentary rocks and mafic to intermediate volcanism.
The Pearl project area is dominated by Precambrian Oracle Granite cut by diabase, aplite, and pegmatite dikes. The Oracle Granite is also locally cut by dikes of 67-68 Ma San Manuel granodiorite porphyry that is associated with porphyry copper mineralization at the San Manuel-Kalamazoo deposit. On the southwestern third of the property and portions on the east side of the property the Oracle Granite is concealed by andesitic volcanic rocks and coarse-grained sedimentary rocks of the Miocene Cloudburst Formation. The area is cut by a number of large normal faults, including the San Manuel fault that also bisects the San Manuel orebody. Chlorite-epidote-carbonate ± silica-sericite (propylitic) alteration occurs along these faults as well as locally disseminated in rocks without apparent relationship to structures. In many instances this propylitic alteration forms halos around polymetallic vein mineralization and has been interpreted as related to hydrothermal fluid circulation during Oligocene to Miocene felsic intrusions. In some instances the alteration lacks a clear affiliation with young structures and may be related to Laramide age porphyry magmatic-hydrothermal systems.
Late Oligocene Au-Ag-Pb-Zn-Mo-V-(Cu) bearing minerals occur in numerous quartz veins systems such as those at the historic Tiger, Mohawk, Collins, and Mammoth-St. Anthony Mines. Quoted production from these vein-style occurrences includes 400 koz of gold, 1,000 koz of silver, 75 Mlbs of lead, 50 Mlbs of zinc, 6 Mlbs of molybdenum oxide, 3.5 Mlbs of copper, and 2.5 Mlbs of vanadium oxide. Twenty-six different occurrences of fault-associated copper oxides have been documented on the Pearl property, consisting predominantly of chrysocolla, malachite, and azurite occasionally in association with pyrite and galena. Psilomelane (manganese oxide) is also locally observed. The Pearl mine appears to be the most recently active and displays outcropping copper oxide minerals over more than 600 meter strike length within a larger propylitic alteration halo of more than 850 meters.
Exploration is targeting Laramide age porphyry copper deposits associated with the San Manuel granodiorite, akin to the San Manuel-Kalamazoo deposit. The porphyry deposit alteration patterns in the area have been complicated by extensional faulting and post-mineral cover, and are cross-cut by later Oligocene polymetallic vein systems with their own propylitic alteration halos. Some of the polymetallic veins have unusually high Cu, Mo, and V, which has been interpreted as an indication that they have incorporated metals from the underlying porphyry deposits. However, some of these veins could also be directly related to the porphyry deposits. Vein occurrences that are not definitively young must be carefully scrutinized as a potentially distal signature of an underlying porphyry.
Surface access for the project is via the state of Arizona with federal mineral rights governed by the U.S. Bureau of Land Management. Permits for surface exploration activities have been granted by the state of Arizona.
Ascarza, W., 2013, Tiger mines once thrived near today’s Mammoth: Arizona Daily Star, Tiger mines once thrived near today’s Mammoth (tucson.com).
Briggs, D.F., 2014, History of the San Manuel-Kalamazoo Mine, Pinal County, Arizona: Arizona Geological Survey Contributed Report, CR-14-1, 9 p.
Force, E.R. and Dickinson, W.R., 1994, Tucson Wash – An introduction to new work in the San Manuel and Mammoth Districts, Pinal County, Arizona, in Thorman, C.H. and Lane, D.E., eds., USGS Research on Mineral Resources – 1994, Part B – Guidebook for Field Trips: U.S. Geological Survey Circular 1103-B, p. 103-115.
Leveille, R.A. and Stegen, R.J., 2012, The Southwestern North America Porphyry Copper Province, in Hedenquist, J.W., Harris, M., and Camus, F., eds., Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe: Society of Economic Geologists Special Publication 16.
Spencer, J.E., Gootee, B.F., Richard, S.M., and Cook, J.P., 2018, Geologic map of the Mammoth 7½’ Quadrangle, Pinal County, Arizona: Arizona Geological Survey Digital Geologic Map DGM-67, scale 1:24k.
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