Aerial view of the Mission Complex, looking southwest. Photo courtesy of http://www.asarco.com/
Visitors to Arizona traveling along I-19 south of Tucson usually are heading to Nogales, Mexico, and/or the “artist” community of Tubac, Arizona. I suppose many drivers glance to the west about 15-20 miles south along the road and sort of wonder about the seeming large “piles” of rock. Those travelers with a greater curiosity will discover that these features represent overburden coming from a really large open pit, copper porphyry mine known as the Mission Complex. The current mine is about 2.5 miles long, 1.5 miles wide and 1200 feet deep. At one time there were several smaller mines owned by different companies; however, the Mission, Pima, Mineral Hill and North and South San Xavier were acquired by ASARCO and now represent a single integrated mining operation. If travelers are interested in further exploring the world of copper the company offers a Discovery Center and mine tours---see http://www.asarco.com/
One of the early mines, the Pima, has produced a rather uncommon, or as the Handbook of Mineralogy (www.handbookofmineralogy.org) states, “an inconspicuous mineral” that is an iron copper sulfide magnesium aluminum hydroxide [(Fe++,Cu)4(Mg.Al)3S4(OH,O)6] known as valleriite. More than uncommon or inconspicuous, I would say just pretty weird and certainly tough to identify unless one is familiar with the mineral. For one thing, valleriite is an extremely soft mineral at about 1.0 (Mohs) and could be confused with graphite, the softest metal-like mineral on most rockhound’s radar. It is a dark gray to black mineral with a bronze sheen and a black streak. The luster is metallic, it is opaque and sort of looks like a lump of metal or coal; however, the softness and bronze sheen are the prime identifiers, at least for me. Most commonly valleriite is massive or nodular and appears to be slightly foliated. Tiny crystals may be present but are indistinguishable.
Valleriite is an alteration product of chalcopyrite at the Christmas Mine, Arizona, but probably of magnetite in the Pima Mine (Anthony and others, 1995). As best that I understand, the mineralization at the Pima Mine is in a skarn deposit located along the boundary between various Paleozoic carbonates and a Laramide quartz monzonite intrusion. The volatile elements such as aluminum, iron and magnesium were introduced sometime along the line and new minerals formed such as valleriite. This metasomatic or pyrometasomatic action occurs at relatively high temperature (up to ??5000C) but with fairly low pressure.
As a bit of trivia, I-19 is the 4th shortest primary interstate highway in the lower 48 states. It runs from Exit 0 Nogales, Arizona, (across the international border from Nogales, Mexico) north to south Tucson at Exit 99 and then merges with I-10. So far so good; however, I-19 is signed in kilometers and its actual length is about 67 miles. But, the speed limit is posted in MPH. This dual usage is a source of massive confusion for many first time visitors!
Anthony, J.W., S.A. Williams, R.A. Bideaux and R.W. Grant, 1995, Mineralogy of Arizona: The University of Arizona press, Tucson.