Copper mines and deposits stretch 20 miles from Green Valley to near Tucson, Arizona. There are just two known original deposits, but one of them got spread out like a fanned deck of cards. From south to north, these mines are Sierrita/Esperanza, Twin Buttes, Mission/Pima, San Xavier, and Saginaw Hill. The deposits were emplaced between 64- and 55 million years ago. Then things happened.
I will start off with the Sierrita-Esperanza deposit on the southwest end of the district and apparently the oldest of the deposits there. The ore zone is in the shape of an inverted teacup, the classic shape according to the Lowell and Guilbert porphyry copper model. That shape shows that the deposit is relatively intact from when it was formed. Sierrita is a relatively low-grade copper-molybdenum deposit emplaced into igneous rocks. The molybdenum contains rhenium and the mill at the mine site currently is the only U.S. source of rhenium although a second source may come on line in the next year or two in Utah. There is also minor gold, silver, lead, and zinc associated with the ore.
The Twin Buttes mine occurs about 5 miles northeast of Sierrita has mineralization similar to Sierrita. The mineralization was emplaced into limestones and quartzite wall-rocks. Twin Buttes was probably much larger than Sierrita and its mineralization extends to below sea level. However, subsequent tectonic activity sliced up the original deposit and spread it out like a deck of cards. The current understanding of the geology proposes that the Mission-Pima mine, 7 miles north of Twin Buttes, the San Xavier orebody, and even Saginaw Hill mineralization, some 20 miles north of Twin Buttes, were all originally part of the Twin Buttes deposit. A large, low-angle fault, the San Xavier fault, transported slices of Twin Buttes northward about 28 million years ago. Drilling has confirmed the existence of the fault beneath the Mission-Pima mine.
Shafiqullah and Langlois, who did extensive age-dating of rocks in the district propose the following sequence of events:
“The geologic complexity of the Pima district is largely due to superposition of two magmatic and three tectonic episodes within the last 65 million years. The first magmatic and tectonic episode occurred during Laramide [ca. 65-55 million years ago] when plutonic rocks…intruded Paleiozoic and Mesozoic sedimentary and volcanic sequences, as well as Precambrian granite. This…culminated in hydrothermal mineralization…from 64 to 57 million years ago. The second intrusive and tectonic episode occurred during the mid-Tertiary at which time the coarse-grained Helmet Fanglomerate was being deposited in tectonic basins 31 to 27 m.y. ago…The complex and imbricate San Xavier fault developed at the culmination of the mid-Tertiary magmatic-tectonic episode. A third tectonic episode, characterized by normal faulting, occurred during the past 12 million years…[as part of] the Basin and Range disturbance….” (Fanglomerate is poorly sorted and poorly stratified alluvial fan debris, slope wash, colluvium, and talus.)
Shafiqullah and Langlois also suggest that the Sierrita and Twin Buttes deposits, now 5 miles apart, where once adjacent.
All this structural complexity makes life interesting for the exploration geologist. The Pima-Mission orebody was completely covered by alluvium and discovered by experimental geophysical exploration by Walter Heinrichs, jr. and R.C. Thurmond in 1950. Heinrichs, a friend of mine, is on the north side of 90 years old and still active in Tucson geology circles.
Because of the structural complexity, one might wonder if there are more buried orebodies along the Twin Buttes-Pima Mine trend. I know of one such body. Just north of Twin Buttes, but just south of Duval Mine Road, several drill holes have intersected what may be a slice of the original Twin Buttes orebody that is now entrained within the Helmet Fanglomerate. If the mineralization is continuous between drill holes, there could be about 100 million tons grading 0.75% copper.
Finally, I must note that the University of Arizona’s San Xavier underground laboratory is built in an old mine just west of the Pima-Mission complex. (Not the same as the San Xavier open pit mine.)
Aiken, D. and West, R., 1978, Some geologic aspects of the Sierrita-Esperanza Copper-molybdenum deposit, Arizona Geological Society Digest Volume XI
Cooper, J.R., 1960, Some geologic features of the Pima mining district, Pima County, Arizona, USGS Bulletin 1112-C.
Langlois, J.D., 1978, Geology of the Cyprus Pima Mine, Arizona Geological Society Digest XI
Lowell, J. D. and Guilbert, J. M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits, Economic Geology , vol. 65, no. 4, pp. 373-408, 1970
Shafiqullah, M. and Langlois, J.D., 1978, The Pima Mining District – A geochronologic update, New Mexico Geological Society Guidebook 26th Field Conference.
For a general geologic history see: