A collector's guide to Mission complex, USA: its geology, mining history and notable minerals, illustrated with the 24 specimens documented from this locality on EarthWonders.
Key facts
The Mission complex is a classic southern Arizona copper locality in two quite different senses. As an ore deposit it is a large, still-active copper-molybdenum operation on the eastern pediment of the Sierrita Mountains, south of Tucson, where Laramide porphyry-copper mineralization was imposed on a structurally dismembered package of Paleozoic carbonate and quartzite, Mesozoic sedimentary and volcanic rocks, and quartz monzonite porphyry. As a specimen locality, however, Mission’s fame rests on an unusually delicate byproduct of that industrial geology: bright, hairlike to dendritic native copper sealed inside clear to translucent gypsum, often cut or cleaved into “windowpane” slabs that show the copper as suspended red metallic ferns.
Mission is not a mine that collectors revere for great freestanding copper crystals like the Keweenaw, or for colorful cabinet masses of secondary copper minerals like Bisbee. Its best specimens are more intimate and more optical. The classic pieces are transparent selenite plates and cleavages, commonly a few centimeters to cabinet size, with micro-dendritic copper branching through the gypsum in arborescent sprays. Some slabs show penny-bright acicular copper; others have darker tarnished filaments, with green or brown alteration along fractures or edges. When the gypsum is clear, the effect is strikingly three-dimensional: copper appears to float within the sulfate rather than sit on it.
The geological setting explains why Mission specimens look so different from most Arizona copper classics. Much of the ore system is a porphyry/skarn environment rather than a simple oxidized vein locality. The main economic ore minerals are sulfides such as chalcopyrite and molybdenite, with skarn and tactite assemblages developed in carbonate host rocks. Later oxidation, sulfate mobility, and fracture/cavity growth produced the gypsum-copper association that collectors know. Massive gypsum and anhydrite occur in the mine, while free-growing colorless gypsum crystals and cleavages formed in cavities and fractures, locally enclosing native copper and, rarely, chalcotrichite.
Regional View
Country View
Historically, the district is important because the Pima-Mission ore bodies belong to the first generation of Arizona copper deposits found by modern geophysical exploration rather than by obvious surface showings alone. Magnetic work traced a magnetite-rich skarn belt between Mineral Hill and the Daisy Mine, leading to exploration beneath alluvium in the Santa Cruz Valley. The Mission deposit itself became a major ASARCO operation in the early 1960s, eventually absorbing or combining with neighboring Pima, Eisenhower, Mineral Hill, South San Xavier, and North San Xavier properties into the modern Mission complex.
Photo: Wikimedia Commons
Photo: Wikimedia Commons
Search for specimens: View all specimens from Mission complex, USA
The Mission complex lies near Sahuarita, in the San Xavier area of the Pima Mining District, Pima County, Arizona. In mining terms it is an open-pit copper-molybdenum complex made up of the Mission, Eisenhower, Pima, Mineral Hill, and South San Xavier properties, with the nearby North San Xavier mine also part of the operation. ASARCO describes the present pit as about 2.5 miles long, 1.5 miles wide, and 1,200 feet deep, set within a 20,000-acre property, with 40-foot benches. For collectors, those dimensions matter because the Mission locality name on a label may cover material from the Mission pit, Mission underground mine, Pima mine, Mineral Hill workings, San Xavier units, or the broader integrated complex rather than a single small shaft.
Geologically, the deposit is best treated as a porphyry copper system with strong skarn and replacement components. The ore system sits in a faulted mass of sedimentary, volcanic, and plutonic rocks along the eastern side of the Sierrita Mountains. Laramide quartz monzonite porphyries intruded Paleozoic and Mesozoic rocks, and mineralization is concentrated both in altered intrusive rocks and, very importantly, in calcareous strata converted to garnet-pyroxene tactite, hornfels, and calc-silicate rocks. The San Xavier low-angle fault and later normal faulting complicate the district geometry, and the broader Pima-Mission-Twin Buttes-San Xavier North area has long been discussed as a structurally rearranged suite of related porphyry/skarn deposits.
Ore at Mission is not simply a uniform porphyry stockwork. Published descriptions and MRDS records describe replacement, disseminated, lens-like, and veinlet mineralization controlled by tactite and hornfelsed sedimentary units, limy horizons, the contact of younger formations with underlying Paleozoic rocks, and low-angle faulting. Chalcopyrite is the central sulfide mineral in the copper ore, with molybdenite as the important molybdenum mineral. Bornite, chalcocite, pyrite, sphalerite, galena, magnetite, hematite, scheelite, and tennantite-group minerals are part of the broader ore and accessory assemblage. Supergene enrichment and oxidation added chalcocite, chrysocolla, malachite, native copper, iron oxides, clays, and sulfate minerals in upper and fault-controlled zones.
Mining history in the immediate area began with much smaller and richer targets than the modern open pit. The eastern flank of the Sierrita Mountains saw silver, lead-silver, and copper prospecting and underground mining from the Spanish and territorial periods into the late nineteenth century. In the early twentieth century the district included properties such as Mineral Hill, the Daisy shaft, the Azurite group, and the Pima workings. By the 1950s the focus shifted from narrow high-grade ore to large, disseminated copper bodies hidden beneath alluvium. The Pima Mining Company began active development in 1952 with the Alpha shaft, and early high-grade shipments came from oxidized and primary sulfide zones before large open-pit methods took over.
ASARCO discovered the Mission deposit in 1954 and began pre-mine development in 1959. The Mission mill began operating in 1961 at 15,000 tons per day, with expansions and mill upgrades following through the 1960s, 1980s, and 1990s. The Eisenhower property, between the Mission pit and South San Xavier pit, entered production in 1979 through a partnership with Anamax, and ASARCO later acquired the Anamax interest. The contiguous Pima mine was acquired in 1985, the Pima mill in 1989, and the complex was progressively consolidated into the large modern Mission operation. An underground mine was developed below the Mission pit and used block caving, with access by helical adit and 80-ton diesel haulers, but that underground operation was closed in 2003.
Specimen recovery appears to have been episodic rather than the result of continuous collecting. The pieces collectors recognize most quickly—clear gypsum or selenite with included native copper—are older material, often surfacing from long-held collections or dealer stock rather than from recent legal collecting. Named provenance in the specimen market includes collections such as Evan Jones, Bob Trimingham, Kay Robertson, John Kassionas, Harold Urish, and Consie Prince, indicating that much of the best material entered circulation decades ago. Dealer descriptions repeatedly emphasize that thick, transparent gypsum slices with rich internal copper “forests” are uncommon, and several documented examples were already considered old material when sold in the 2010s.
Collecting access today is not comparable to a public dump or fee locality. Mission is an active industrial mine on ASARCO property and related lands, with mine safety, operating, and property restrictions. The ASARCO Mineral Discovery Center offers public exhibits and ticketed mine tours, which are excellent for understanding the scale of the operation and copper processing, but a tour is not permission to collect specimens. Serious collectors should treat all legitimate Mission specimens as collection or dealer pieces unless accompanied by specific, credible documentation of authorized recovery.
Native copper from the Mission complex is prized above all as inclusions in gypsum: bright to tarnished reddish metallic filaments, hairs, needles, and micro-dendritic arborescent groups suspended in clear selenite cleavages or sawed gypsum slabs. Documented specimens range from thumbnails and miniatures around 3 cm to cabinet plates over 13 cm across, though the actual copper branches are typically fine and delicate rather than bulky. The best pieces come from the Mission Mine or Mission pit material and show dense, sharply visible copper “forests” within transparent gypsum, preferably with minimal bruising, edge whitening, or distracting saw marks; ordinary pieces have sparse or dark inclusions, cloudy gypsum, or broken cleavage surfaces that obscure the internal growth.
Gypsum at Mission occurs both as massive sulfate material and as colorless crystallized selenite associated with the copper-bearing zones; the collector pieces are generally transparent cleavages, slabs, or free-growing crystals from fractures, anhydrite-rich material, and cavities in marble or calc-silicate rock. Mission gypsum is rarely valued simply as gypsum—the locality’s appeal is the way clear, lustrous selenite acts as a natural display case for native copper and, more rarely, chalcotrichite or other copper minerals. Fine pieces are “windowpane” specimens: water-clear, thin enough to transmit light, thick enough to protect the inclusions, and cleanly split or cut so the dendritic copper is readable from both sides.
Beyond copper and gypsum, the Mission complex has a broad but mostly ore-geology-driven mineral list. Chalcopyrite, pyrite, molybdenite, chalcocite, bornite, covellite, cubanite, sphalerite, galena, magnetite, hematite, scheelite, fluorite, calcite, dolomite, anhydrite, quartz, and muscovite-sericite record the porphyry and alteration system. The skarn assemblage includes grossular or garnet-group material, diopside, wollastonite, tremolite, epidote, pyroxene-group minerals, and datolite. The oxidized and supergene suite includes cuprite, chalcotrichite, malachite, azurite, brochantite, rosasite, chrysocolla, tenorite, jarosite, hemimorphite, smithsonite, and wulfenite, while Mineral Hill and the Daisy shaft area add rare copper and lead-copper secondary species such as atacamite, gerhardtite, pseudoboleite, and sepiolite. Mission is not known as a mineral type locality; its importance lies instead in an unusual specimen habit—native copper enclosed in gypsum—within a major porphyry/skarn copper district.
The chief authenticity issue with Mission specimens is not usually outright faking, but imprecise labeling. Older pieces may be labeled “Mission Mine,” “Pima Mine,” “Pima Co.,” “Mission pit,” “San Xavier,” or simply “Sahuarita,” and not all of those labels distinguish the underground mine, the open pit, the Pima portion, or the integrated Mission complex. A good label should place the piece in the Mission complex, San Xavier, Pima Mining District, Sierrita Mountains, Pima County, Arizona, and should ideally preserve any older mine or collection name rather than modernizing it away.
No well-documented class of fake Mission copper-in-gypsum specimens stands out in the published collector record, but the material invites careless description. Many pieces are sawed, cleaved, or polished-looking because that is the practical way to reveal inclusions inside transparent gypsum; this is normal for the locality and should not automatically be treated as damage or enhancement. Conversely, a specimen advertised as a natural freestanding copper crystal specimen from Mission deserves scrutiny: the classic Mission style is micro-dendritic or acicular copper enclosed in selenite, not hefty native copper crystals perched on matrix.
Condition is central to value. Gypsum is soft, cleavable, and easily bruised, and transparent selenite shows scratches, saw haze, fingerprints, edge chips, and internal cracks mercilessly. Copper inclusions can be spectacular but visually lost if the gypsum is cloudy or abraded. Thin plates should be stored so they do not flex, and they should be kept away from high heat, prolonged direct sun, and very dry or rapidly changing environments that can worsen cleavage cracks. Do not wash these casually: gypsum is slightly water-soluble, and water can enter cleavage partings or fractures. Dusting with a soft brush or air bulb is safer than immersion.
Market availability is sporadic. Small included pieces appear from time to time, but the best cabinet-size transparent slabs with dense, bright copper are old-collection specimens and are much less common. Provenance matters: names such as Evan Jones, Kay Robertson, Consie Prince, Bob Trimingham, John Kassionas, Harold Urish, or a reputable Arizona dealer history add confidence and desirability. For advanced collectors, the ideal Mission specimen combines clear gypsum, three-dimensional copper branching, cabinet presence, and an older label that predates the recent online trade in generic “copper in selenite.”
One of the best stories in the Mission district is that the great ore body was found by looking for what could not be seen. The Santa Cruz Valley alluvium hid the disseminated copper bodies that would become Pima, Mission, and Twin Buttes, so the decisive clue was geophysical: magnetic surveys by United Geophysical Co. traced a magnetite-rich skarn zone from Mineral Hill toward the Daisy Mine. Mining engineers Walter E. Heinrichs Jr. and Robert E. Thurmond, backed by Herbert Hoover Jr., are credited with setting that exploration program in motion. The result was historic in Arizona mining: ore bodies discovered not by a prospector’s colorful outcrop or a lucky shaft, but by modern geophysics.
At Pima, the old and new eras overlapped. In 1952, the Pima Mining Co. sank the 600-foot Alpha shaft. Oxidized ore appeared at 209 feet, and primary sulfides followed at 255 feet. Over the next three years, 67,000 tons averaging 6 percent copper were shipped to ASARCO’s El Paso smelter—a grade that seems almost startling beside the later open-pit economics of the Mission deposit, where ore running around 0.67 percent copper justified huge tonnage, big mills, and relentless stripping rather than selective underground mining.
By 1958, ASARCO was developing the Mission Mine, then known as the East Pima Mine, with a 15,000-ton-per-day concentrating mill and open-pit overburden stripping. Commercial production began in 1961, and concentrates were shipped by rail to Hayden because there was no smelter at the mine itself. That contrast—low-grade rock moved on an immense scale, yet producing delicate copper dendrites in transparent gypsum as collector specimens—is part of Mission’s charm. The same operation that swallowed properties into a pit measured in miles also yielded fragile, hand-sized windows of selenite that collectors now hold up to the light.
A more mischievous Mission tale comes from J. David Lowell’s memoir, involving a claim man remembered as “Fraction Brown.” Around 1965, Brown identified a small claim fraction in the middle of the large Mission open pit. Arizona mining regulations required a six-by-six-by-eight-foot discovery pit. Brown reportedly rented a bulldozer, drove it to the ASARCO Mission gate, and was waved through because the guards assumed the machine belonged there. He then chugged down the spiral haul road among the mine’s large trucks, dug his discovery pit at the bottom, posted his notice and stakes, and drove back out. ASARCO’s lawyers challenged him, but Lowell’s version ends with the company having to “pay up”—a wonderfully Arizona episode in which a huge corporate pit was briefly outmaneuvered by a small fraction and a bulldozer.
