Silver Bell Mine, Pima County, Arizona | Professor Emeritus Spencer R. Titley (center) of the University of Arizona lecturing on supergene enrichment - supergene ore processes occur near the Earth's surface - at the Silver Bell Mine as part of a US Geological Survey field trip for Senior Geologists of the Geological Survey of the People’s Republic of China. (Photo by Bob Kamilli, USGS)
Editor's note: Dr. Titley is internationally renowned for his work on the origin of porphyry copper deposits.
